University of Southern California Dissertations and Theses

Integrated carbon dioxide capture and utilization: catalysis enabled carbon-neutral methanol synthesis and hydrogen generation

(USC Thesis Other)

Integrated carbon dioxide capture and utilization: catalysis enabled carbon-neutral methanol synthesis and hydrogen generation

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content INTEGRATED CARBON DIOXIDE CAPTURE AND UTILIZATION: CATALYSIS ENABLED CARBON-NEUTRAL METHANOL SYNTHESIS AND HYDROGEN GENERATION by Raktim Sen A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (CHEMISTRY) May 2023 Copyright 2023 Raktim Sen ii Dedication Dedicated to my parents iii Acknowledgements As my Ph.D. program comes towards a conclusion, it gives me immense pleasure to pen my research dissertation, which is essentially an account of my Ph.D. journey and shall stay forever as one of the most learning, memorable and proud chapters in my life. Thank you, God for blessing me with this opportunity. My thesis is incomplete without the present section, where I intend to recognize the importance and efforts of few of the numerous individuals, who played key roles in making this day a reality for me. Unfortunately, acknowledging every one of those is challenging in this short summary, but all are present in my thoughts and are thanked from the core of my heart. My journey at the Loker Hydrocarbon Research Institute of USC dates to the summer of 2016, when as a stroke of fortune, I had got an opportunity to travel from India and spend a couple of months here as a research intern. When I came back to USC in 2017 as a PhD researcher, I had found a new home in Loker and the best mentor in my life in Prof. Surya Prakash. In my opinion, Prof. Prakash’s personality reflects aptly those of an “ideal advisor” and his guidance in lab and life, is priceless. His consistent agility and dedication towards research, teaching and his group will continue to inspire me. I deeply cherish our coffee table conversations on science, philosophy, food, music and life during which he would often leave me with a piece of a paper containing a new research idea of his. As a true north star, he taught me like he did to all his students, how to be a humble scientist and a kind human being. I aspire to continue working along those lines and keep learning from him in the future. Thank you, Prof. Prakash, for everything! I am immensely privileged to be a part of the legacy of USC’s first Nobel Laurate, the late Prof. George Olah. I was quite fortunate to be in his presence during a few group meetings I iv attended during my internship. Unfortunately, he passed away a few months before I returned to USC in 2017. His towering personality, age-defying excitement and dedication for science and his notion for humanity to be humble will continue to motivate me. It is my honor to be able to walk a few steps in the research field of the “Methanol Economy”, one of Prof. Olah’s numerous revolutions in science. I find myself extremely lucky to be able to find a great mentor in Dr. Alain Goeppert. To every roadblock in my research in the lab, Alain was always right there with a solution ready. His inimitable skills as a chemist and chemical engineer and his exceptionally helpful nature made my PhD journey smooth and effortless. Thank you, Alain! I would also like to acknowledge Dr. Robert Aniszfeld, for his continued support and for providing with a commercial and entrepreneurial angle to my research. His questions and thoughts always added to my motivation and a provided a tangential perspective for my projects. Thank you, Robert! Special thanks to Dr. Thomas Matthew, Dr. Patrice Batamack for their support, mentorship and for being my lunch-time buddies. I would like to take this opportunity to acknowledge the members of the various committees during my PhD program for their time, generosity and evaluation: Prof. Sri Narayan, Prof. Ishwar Puri, Prof. Susumu Takahashi, Prof. Katherine Shing, Prof. Barry Thompson and Prof. Ralf Haiges. The research accomplishments would not have been possible without a great and unified team in our lab. I would like to thank Dr. Jotheeswari Kothandaraman and Dr. Sayan Kar for training me during my internship and initial phase of PhD, and for their continued mentorship. They readily welcomed me to work with them on their projects which eventually helped me to develop new projects in the lab. I would also like to thank Dr. Vicente Galvan, Dr. Nazanin v Entesari, C. J. Koch, Anushan Alagaratnam, Zohaib Suhail for being amazing collaborators, friends and for helping to bring our projects to completion. In addition, over the past five years, I got the opportunity to know a lot of the members of the Prakash group who became great friends, colleagues and mentors: Archith, Sahar, Colby, Xanath, Vinayak, Juan Pablo (JP), Ziyue, Daniel, Alex, Yijie, Matt, Sankar, Somesh, Laxman, Dean, Amanda, Fang, Huong, Kavita, Socrates, Antonio, Zsofi, Aisha, Dr. Suri, Bo among others. I would like to thank Anuj, Anju, Pratyusha and Shwetha, who joined the Ph.D. program with me for our memories as a group during those initial and otherwise, vulnerable days in Los Angeles. Thank you Abhirup and Sraddha, for being great friends during graduate school and several years before that back in India. Special thanks to my aunts in San Diego, Ishita Sen and Debalina Ganguli for their continuing support and for giving me those memorable vacation opportunities at their luxurious house on the hills, and with amazing food and hospitality. Thanks to all my friends in India, to name a few: Rishind, Vaishak, Abhishek and Bipul. The support of the Loker Institute and the Department of Chemistry, especially the staff including David, Michele, Magnolia, Claudia, Allan, Ralph, Carole and Jessy is earnestly appreciated. I always enjoyed chatting with Jessy, who never missed greeting me with a smile. Thank you, Jessy! Thank you, Gloria for our chats and your continuing efforts in keeping Loker tidy and presentable. I strongly believe that my late grandparents, who played important roles in the early days of my childhood with emphasis on my education and intellectual growth, continue to support me to this day from wherever they are, and all my accomplishments are dedicated to their blessings. Also, thanks to my aunts, uncles, cousins and other relatives who have always been there for me. Teachers have come in different forms and at different junctions of my life serving as those vi steppingstones to move forward in life. I would like to take this opportunity to remember and thank all my teachers and professors, to mention a few: Tapas Dutta, Mamta Rishi, Anjali Khunger, Prof. Arindam Mukherjee, Prof. Shashank Deep, Prof. M. S. Frank, and all my chemistry professors at St. Stephen’s College and IIT Delhi. Thank you, Shubhangi for walking all along with me in this nine-year long journey since the undergraduate days, from India to Los Angeles. Thank you for always standing strong by me through those difficult days and it was you, who motivated me from the first day of the application phase to the last day of my PhD. This would not have been possible if I had not met you. Finally, I am thankful to God for giving me the two most important people in my life, my mom and dad! All my accomplishments to date, including my PhD degree is truly because of them. Last, but not least, thank you for taking out time to read my thesis! Fight on! Raktim Sen vii Table of Contents Dedication ....................................................................................................................................... ii Acknowledgements ....................................................................................................................... iii List of Tables .................................................................................................................................. ix List of Figures ................................................................................................................................. xi Abstract ....................................................................................................................................... xxii Chapter 1. Introduction: The Renaissance of Low Temperature Catalysis in the Context of the Methanol Economy .......................................................................................................................... 1 1.1. Introduction .......................................................................................................................... 1 1.2. Methanol from Fossil and Renewable Sources .................................................................... 5 1.3. Hydrogenation of CO2 to Methanol ...................................................................................... 7 1.4. Direct CO2 to methanol using molecular catalysts ............................................................. 12 1.5. Hydrogenation of CO2 Derivatives to Methanol ................................................................ 33 1.6. Mechanistic Insights into Catalysis .................................................................................... 40 1.7. Status Quo ........................................................................................................................... 50 1.8. Conclusion and Outlook ..................................................................................................... 57 1.9. References .......................................................................................................................... 59 Chapter 2. Integrated Carbon Capture and Conversion to Methanol with Epoxide- functionalized Polyamines under Homogeneous Catalytic Conditions ........................................ 78 2.1. Introduction ........................................................................................................................ 78 2.2. Reaction Optimization and Investigations on CO2 Capture and Catalytic Conversion with Epoxide-Functionalized Amines ....................................................................................... 81 2.3. Conclusion .......................................................................................................................... 90 2.4. Experimental Methods ........................................................................................................ 91 2.5. References ........................................................................................................................ 101 viii Chapter 3. Tertiary Amine Based Integrated Post-Combustion Capture of CO2 and Hydrogenation to Methanol ......................................................................................................... 107 3.1. Introduction ...................................................................................................................... 107 3.2 Investigations on CO2 Capture and its Subsequent Hydrogenation .................................. 109 3.3. Conclusion ........................................................................................................................ 115 3.4. Experimental Methods ...................................................................................................... 116 3.5. References ........................................................................................................................ 122 Chapter 4. Hydroxide Based Integrated Direct Air Capture of CO2 and Conversion: Hydrogenation of Metal Carbonates to Methanol ....................................................................... 125 4.1. Introduction ...................................................................................................................... 125 4.2. Process Discovery and Optimization ................................................................................ 128 4.3. Conclusion ........................................................................................................................ 137 4.4. Experimental Methods ...................................................................................................... 138 4.5. References ........................................................................................................................ 147 Chapter 5. Glycol Assisted Conversion of CO2 and Metal Carbonates to Methanol with a Heterogeneous Copper Catalyst .................................................................................................. 152 5.1. Introduction ...................................................................................................................... 152 5.2. Reaction Optimization for Cu/ZnO/Al2O3 Catalysed Methanol Synthesis ...................... 155 5.3. Conclusions ...................................................................................................................... 167 5.4. Experimental Methods ...................................................................................................... 168 5.5. References ........................................................................................................................ 188 Chapter 6. Low Temperature Catalytic Methanol Reforming for High Pressure H2 Generation and Integration with a Fuel Cell ............................................................................... 194 6.1. Introduction ...................................................................................................................... 194 6.2. Catalytic Reforming of Methanol under Alkaline Conditions ......................................... 198 6.3. Conclusion ........................................................................................................................ 213 6.4. Experimental Methods ...................................................................................................... 215 6.5. References ........................................................................................................................ 218 ix List of Tables Table 1.1. Selected examples of CO2 to methanol processes with molecular catalysts. .............. 52 Table 1.2. Selling prices of selected molecular catalysts and ligands .......................................... 55 Table 2.1. CO2 capture by aqueous solutions of epoxide-functionalized PEHA. ......................... 84 Table 2.2. Catalytic hydrogenation of CO2 captured by aqueous amine solutions. ...................... 86 Table 3.1 CO2 capture by tertiary amines in alcohol solvent. .................................................... 110 Table 3.2. Tandem hydrogenation of captured CO2 in ethylene glycol. ..................................... 112 Table 3.3. Regeneration of tertiary amine base after hydrogenation step. ................................. 114 Table 3.4. CO2 capture from simulated flue gas (10% CO2/N2) and conversion to CH3OH. ..... 114 Table 3.5. Amine loss during CO2 capture from simulated flue gas (10% CO2/N2). ................. 115 Table 4.1. CO2 capture by alcoholic hydroxide solutions .......................................................... 129 Table 4.2. Tandem hydrogenation of captured CO2 in ethylene glycol ...................................... 130 Table 4.3. Variation of hydrogen pressure in hydrogenation reactions ...................................... 131 Table 4.4. Formation of potassium glycolate (PG) during hydrogenation reactions .................. 132 Table 4.5. CO2 capture from ambient air and conversion to CH3OH ......................................... 134 Table 4.6. Hydrogenation of potassium bicarbonate to methanol in different solvents ............. 135 Table 5.1. Cu/ZnO/Al2O3 catalyzed hydrogenation of CO2 to CH3OH in ethylene glycol ........ 158 x Table 5.2. CO2 capture and hydrogenation to CH3OH in ethylene glycol ................................. 162 Table 5.3. Cu 0 crystallite size for various catalyst samples. ....................................................... 173 Table 5.4. Composition of activated catalyst (calculated for activated catalyst based on the composition provided by Alfa Aesar). ........................................................................................ 173 Table 5.5. Methanol yields calculated with the R-Gibbs model at the end reaction conditions . 175 Table 5.6. Methanol yields calculated at Pfinal and Pmax .............................................................. 175 Table 6.1. Control studies to validate the role of the base .......................................................... 200 Table 6.2. Attempted hydrogenation of the C-1 residues obtained from methanol reforming ... 211 xi List of Figures Figure 1.1. Sustainable production based on Power-to-Liquid (PtL) and Carbon Capture and Utilization (CCU). ........................................................................................................................... 2 Figure 1.2. a) Global methanol demand and production capacity and b) methanol usage by industrial sectors. Based on data from MMSA ................................................................................ 4 Figure 1.3. Selected examples of the first-generation homogeneous catalysts instrumental in developing CO2 hydrogenation to methanol. ................................................................................ 10 Figure 1.4. CO2 reduction to methanol: selected categories of H-source, catalysis and process covered by this review (framed). ................................................................................................... 12 Figure 1.5. Cascade catalysis of CO2 to methanol ........................................................................ 13 Figure 1.6. Ruthenium-Triphos catalysis for CO2 to methanol .................................................... 14 Figure 1.7. Biphasic system to separate methanol and catalyst. ................................................... 15 Figure 1.8. Comparative performances of Ru-4 and Ru-6 .......................................................... 16 Figure 1.9. Comparison of Ru catalysts based on neutral C-triphos, N-triphos and cationic Me N-triphos. ................................................................................................................................... 17 Figure 1.10. Amine assisted one-pot CO2-to-methanol system. ................................................... 18 Figure 1.11. Amine assisted sequential methanol synthesis from CO2 via N-formylation. ......... 18 Figure 1.12. Low pressure CO2-to-methanol route via oxazolidinone ......................................... 19 Figure 1.13. Amine assisted CO2 hydrogenation to methanol using Ru-Macho-BH (Ru-9). a) Recycling of reaction components with PEHA and CO2:H2 (1:9); b) Repeated pressure refill experiment with PEHA and CO2:H2 (1:3); c) Effect of amine molecular structures on methanol xii formation; and d) Effect of solvent volume on yields of methanol and intermediates with PEHA ............................................................................................................................................. 21 Figure 1.14. Pyrrolizidine assisted CO2 hydrogenation to methanol. ........................................... 22 Figure 1.15. Ruthenium catalysts for amine assisted CO2 to methanol. ....................................... 22 Figure 1.16. Co-production of glycol and methanol from epoxide and CO2. .............................. 24 Figure 1.17. MOF-encapsulated catalysts for one-pot CO2-to-methanol ..................................... 25 Figure 1.18. CO2 to methanol via formic acid disproportionation. .............................................. 26 Figure 1.19. Multinuclear Ir complexes for gas-solid phase hydrogenation of CO2 to methanol .... ....................................................................................................................................................... 27 Figure 1.20. Transfer hydrogenation of CO2 to methanol using ethanol as H-source. ................. 28 Figure 1.21. Solid supported amines for CO2 hydrogenation to methanol ................................... 29 Figure 1.22. Cobalt based catalysis with triphos-derived ligands. ............................................... 30 Figure 1.23. Sequential amine assisted CO2 to methanol using base metal catalysts. ................. 31 Figure 1.24. Mn catalyzed direct hydrogenation of CO2 to methanol. ......................................... 32 Figure 1.25. Fe-scorpionate catalysis for CO2 to methanol. ......................................................... 33 Figure 1.26. Indirect methanol synthesis from various derivatives and capture products of CO2 ............................................................................................................................................... 35 Figure 1.27. CO2 capture with amines and metal hydroxides. ..................................................... 36 Figure 1.28. Integrated carbon capture and conversion to methanol ............................................ 38 xiii Figure 1.29. Homogeneous catalytic systems for hydrogenation of CO to methanol. ................. 39 Figure 1.30. Modes of bond activation by metal-PN(H)P complexes. ......................................... 41 Figure 1.31. Proposed catalytic route with Ru-PN(H)P catalysts for amine assisted CO2 hydrogenation to methanol ............................................................................................................ 42 Figure 1.32. Mechanistic insights into the role of catalyst molecular structure on methanol formation. ...................................................................................................................................... 44 Figure 1.33. Addition of heterogeneous Lewis acid (ZnO) to enhance CO2 hydrogenation to methanol ........................................................................................................................................ 45 Figure 1.34. Modes of bond activation by aromatized Ru-PNP complexes. ................................ 46 Figure 1.35. Catalytic cycle for aromatized Ru-PNP/PNN complexes for hydrogenation of A) carbamic acid derivatives and B) CO2. .......................................................................................... 48 Figure 1.36. Mechanistic insights and catalytic cycle of Ru-triphos based CO2 to methanol ...... 50 Figure 1.37. Anthropogenic carbon cycle in the context of a circular methanol economy .......... 58 Figure 2.1. Amine based CO2 capture and hydrogenation. .......................................................... 80 Figure 2.2. Functionalization of PEHA with various epoxides. ................................................... 83 Figure 2.3. A-F) Investigations to probe the proposed mechanism of CO2 hydrogenation and the role of amine, G) Demonstration of biphasic system for CO2 capture and conversion, Reaction conditions: PEHA-1PO (1.0 mmol) in H2O (5 mL), CO2 captured (3.3 mmol), C-1 (40 μmol), 2-MeTHF (5 mL), H2 (80 bar), T = 145 °C, t = 48 h. Yields calculated relative to CO2 captured as determined by 1 H NMR. Calculation error ±5%. ............................................... 90 Figure 2.4. 13 C NMR spectra of CO2 captured by PEHA-1PO in water in D2O. ......................... 95 Figure 2.5. Representative 1 H NMR spectra of the reaction mixture after hydrogenation of CO2 loaded PEHA in water with THF as solvent in D2O and imidazole (Im) as internal standard .......................................................................................................................................... 95 xiv Figure 2.6. Representative 1 H NMR spectra of the reaction mixture after hydrogenation of CO2 loaded PEHA-1PO in water with THF as solvent in D2O and imidazole (Im) as internal standard. ......................................................................................................................................... 96 Figure 2.7. Representative 1 H NMR spectra of the reaction mixture after hydrogenation of CO2 loaded PEHA-1PO in water with triglyme as solvent in D2O and imidazole (Im) as internal standard. ........................................................................................................................... 96 Figure 2.8. Representative 1 H NMR spectra of the reaction mixture after hydrogenation of CO2/3H2 in presence of PEHA in triglyme in D2O and imidazole (Im) as internal standard. ...... 97 Figure 2.9. 1 H NMR spectra of the reaction mixture after hydrogenation of [HCOO-PEHA] in triglyme in D2O and imidazole (Im) as internal standard. ............................................................. 97 Figure 2.10. 1 H NMR spectra of the aqueous layer after hydrogenation of CO2 loaded PEHA- PO in water/2-MeTHF biphasic system in D2O and imidazole (Im) as internal standard. ........... 98 Figure 2.11. 1 H NMR spectra of the organic layer after hydrogenation of CO2 loaded PEHA- PO in water/2-MeTHF biphasic system in D2O and imidazole (Im) as internal standard. ........... 98 Figure 2.12. Typical GC spectra of the gas mixture after hydrogenation of CO2 capture solution. ......................................................................................................................................... 99 Figure 2.13. Typical GC spectra of the gas mixture after hydrogenation of CO2/H2 mixture. .... 99 Figure 2.14. Representative 1 H NMR of the product mixture from the reaction of PEHA with epoxide (PEHA + 1 eq. PO) in DMSO-d6. .................................................................................. 100 Figure 3.1. Previously reported integrated carbon capture and conversion to methanol systems compared to the present study. ....................................................................................... 108 Figure 3.2. Typical 13 C NMR spectra of ethylene glycol solution of TMEDA after CO2 capture (Table 3.1, Entry 1) in DMSO-d6. .................................................................................. 119 Figure 3.3. 13 C NMR spectra of aqueous solution of TMEDA after CO2 capture (Table 3.1, Entry 4) in DMSO-d6. .................................................................................................................. 119 xv Figure 3.4. Typical 1 H NMR spectra of reaction mixture after hydrogenation reaction of captured CO2 (Table 3.2, Entry 15) in DMSO-d6. ....................................................................... 120 Figure 3.5. Typical 13 C NMR spectra of reaction mixture after hydrogenation reaction of captured CO2 (Table 3.2, Entry 15) in DMSO-d6. ....................................................................... 120 Figure 3.6. Typical 13 C NMR spectra to observe CO2 captured by regenerated base in reaction mixture in DMSO-d6 (Table 3.3, Entry 1). .................................................................................. 121 Figure 4.1. Reported integrated CO2 capture and conversion systems compared to the present study ............................................................................................................................................. 126 Figure 4.2. Proof of Concept: Hydrogenation of bicarbonate and formate salts to methanol. Reaction conditions: substrate (2.5 mmol), ethylene glycol (5 mL), 70 bar H2, 140 °C, catalyst (0.5 mol%), 48 h. Yields determined by 1 H NMR ( t BuOH as internal standard). ...................... 128 Figure 4.3. Insights into the reaction sequences and base regeneration. .................................... 132 Figure 4.4. Control experiments to probe the fate of ethylene glycol and KOH. ....................... 133 Figure 4.5. Pressure and temperature profile with time for a control experiment similar to one in Figure 4.3B, Reaction conditions: KOH (10 mmol), ethylene glycol (20 mL), catalyst C-1 (0.25 mol%), H2 = 70 bar, 140 o C, 72 h. .............................................................................. 134 Figure 4.6. Hydrogenation of potassium bicarbonate to methanol at different temperatures. ... 136 Figure 4.7. Hydrogenation of potassium carbonate to methanol at different temperatures. ...... 136 Figure 4.8 Typical 1 H NMR spectra of hydrogenation reactions presented in Figure 4.2 (in DMSO-d6). ................................................................................................................................... 141 Figure 4.9 13 C NMR spectra of ethylene glycol solution after CO2 capture (Table 4.1) in DMSO-d6. .................................................................................................................................... 142 Figure 4.10 Typical 1 H spectra of reaction mixture after hydrogenation reaction of captured CO2 (Table 4.2) in DMSO-d6. ..................................................................................................... 142 xvi Figure 4.11 Typical 13 C spectra of reaction mixture after hydrogenation reaction of captured CO2 (Table 4.2) in DMSO-d6. ..................................................................................................... 143 Figure 4.12 Observation of alkyl carbonate after CO2 capture with active KOH in DMSO-d6. 143 Figure 4.13 Typical 1 H spectra of reaction mixture for control experiment as in Figure 4.3B in DMSO-d6. ................................................................................................................................ 144 Figure 4.14 Typical 13 C spectra of reaction mixture for control experiment as in Figure 4.3B in DMSO-d6. ................................................................................................................................ 144 Figure 4.15 Typical GC spectra of the unreacted gas mixture for control experiments as in Figure 4.3B. ................................................................................................................................. 145 Figure 4.16 Typical GC-MS spectra of the reaction mixture for control experiments as in Figure 4.3B. ................................................................................................................................. 145 Figure 4.17 13 C spectra of CO2 captured from ambient air in DMSO-d6 (Table 4.3). ............... 146 Figure 5.1. CO2 hydrogenation to CH3OH. a) equilibrium reactions involved in the methanol synthesis process; b) proposed concept: solvent assisted integrated CO2 capture and conversion to CH3OH. ................................................................................................................. 153 Figure 5.2. Effect of solvent on hydrogenation of CO2 to CH3OH. Reaction conditions: t = 72 h, a CO2 = 21.7 mmol (with no solvent), CH3OH yields calculated relative to CO2 as determined by 1 H/ 13 C NMR. Yield calculations error ±5%. ....................................................... 156 Figure 5.3. Effect of catalyst loading on MeOH production in ethylene glycol. Reaction conditions: total pressure = 74 bar at rt (CO2:H2 = 1:18), EG = 10 mL, t = 72 h, T = 200 °C (as in Table 1). The methanol production rate is calculated as gram of methanol formed per kilogram of catalyst per hour (g·kg -1 ·h -1 ). ................................................................................... 157 Figure 5.4. Effect of reaction time on MeOH production in ethylene glycol. Reaction conditions: total pressure = 74 bar at rt (CO2:H2 = 1:18), catalyst loading = 300 mg, EG = 10 mL, T = 200 °C (as in Table 1). The methanol production rate is calculated as gram of methanol formed per kilogram of catalyst per hour (g·kg -1 ·h -1 ). ................................................ 158 xvii Figure 5.5. Effect of CO2/H2 ratio on CH3OH production and comparison with calculated methanol yield at equilibrium. Reaction conditions: total pressure = 74 bar at rt, catalyst loading = 300 mg, EG = 10 mL, t = 72 h, T = 200 °C, CH3OH yields calculated relative to CO2 as determined by 1 H NMR. Yield calculations error ±5%. .................................................. 159 Figure 5.6. Effect of CO2 content in the feed gas on MeOH production in ethylene glycol (EG). Reaction conditions: total pressure = 74 bar at rt, catalyst loading = 300 mg, EG = 10 mL, t = 72 h, T = 200 °C. The methanol production rate is calculated as gram of methanol formed per kilogram of catalyst per hour (g·kg -1 ·h -1 ). ................................................................ 160 Figure 5.7. Effect of water content in ethylene glycol solvent on MeOH production. Reaction conditions: total pressure = 74 bar at rt (CO2:H2 = 1:18), solvent = 10 mL, t = 24 h, T = 200 °C, catalyst = 300 mg. ................................................................................................................. 160 Figure 5.8. Methanol production over multiple pressure refill cycles. Reaction conditions: total pressure = 74 bar at rt (CO2:H2 = 1:18), solvent = 10 mL, t = 24 h, T = 200 °C catalyst = 300 mg. ..................................................................................................................................... 161 Figure 5.9. A) Recycling of the catalyst with methanol formation. Reaction conditions: total pressure = 74 bar at rt, catalyst loading = 300 mg, EG = 10 mL, t = 72 h, T = 200 °C, CH3OH yields calculated relative to CO2 as determined by 1 H NMR. Yield calculations error ±5%. B) XRD spectra of the catalyst after 5th run in recycling studies as compared to the fresh preactivated catalyst. ................................................................................................................... 162 Figure 5.10. Conversion of potassium carbonate to methanol. Reaction conditions: K2CO3 (10 mmol), water or alcohol (10 mL), H2 = 70 bar (r.t.), catalyst loading = 300 mg, 24 h, 200 °C. CH3OH yields calculated relative to K2CO3 as determined by 1 H NMR. Yield calculations ± 5%. ........................................................................................................................ 164 Figure 5.11. Insights into the plausible reaction sequences during hydrogenation of CO2 in ethylene glycol. ............................................................................................................................ 166 Figure 5.12. 1 H NMR spectra of reaction mixture after hydrogenation reaction of CO2 with no solvent in DMSO-d6. .............................................................................................................. 176 Figure 5.14. Typical 13 C NMR spectra of reaction mixture after hydrogenation reaction of CO2 in triglyme in DMSO-d6. ..................................................................................................... 177 Figure 5.15. Typical 1 H NMR spectra of reaction mixture after hydrogenation reaction of CO2 in squalene in D2O. .............................................................................................................. 177 xviii Figure 5.16. Typical GC spectra of the gas mixture after hydrogenation in the absence of solvent. ......................................................................................................................................... 178 Figure 5.17. Typical GC spectra of the gas mixture after hydrogenation in ethylene glycol. .... 178 Figure 5.18. 1 H NMR spectra of reaction mixture after hydrogenation in 5 th run of recycling studies in DMSO-d6. .................................................................................................................... 179 Figure 5.19. 13 C NMR spectra of ethylene glycol solution of PEHA after CO2 capture in D2O. ..................................................................................................................................................... 179 Figure 5.20. 13 C NMR spectra of captured CO2 by KOH in ethylene glycol in DMSO-d6. ...... 180 Figure 5.21. 13 C NMR spectra of CO2 captured in ethylene glycol solution of PEHA from ambient air in DMSO-d6. ............................................................................................................. 180 Figure 5.22. 1 H NMR spectra of reaction mixture after hydrogenation of CO2 captured in ethylene glycol solution of PEHA from ambient air in DMSO-d6. ............................................. 181 Figure 5.23. 13 C NMR spectra of CO2 captured in ethylene glycol solution of KOH from ambient air in DMSO-d6 (Table 5.2, entry 2). ............................................................................. 181 Figure 5.24. 1 H NMR spectra of reaction mixture after hydrogenation of CO2 captured in ethylene glycol solution of KOH from ambient air in DMSO-d6 (Table 5.2, entry 2). ............... 182 Figure 5.25. 1 H NMR spectra of reaction mixture after hydrogenation of KHCO3 in ethylene glycol in DMSO-d6. ..................................................................................................................... 182 Figure 5.26. 1 H NMR spectra of reaction mixture after attempted hydrogenation of KHCO3 in triglyme in DMSO-d6. ............................................................................................................. 183 Figure 5.27. 1 H NMR spectra of reaction mixture after hydrogenation of HCOOK in ethylene glycol in DMSO-d6. ..................................................................................................................... 183 Figure 5.28. 1 H NMR spectra of reaction mixture after attempted hydrogenation of HCOOK in triglyme in DMSO-d6. ............................................................................................................. 184 xix Figure 5.29. 1 H NMR spectra of reaction mixture after hydrogenation of HCOOEt in triglyme in DMSO-d6. ................................................................................................................. 184 Figure 5.30. 1 H NMR spectra of reaction mixture after hydrogenation of PEHA-HCOOH adduct in ethylene glycol in DMSO-d6. ....................................................................................... 185 Figure 5.31. 1 H NMR spectra of reaction mixture after attempted hydrogenation of PEHA- HCOOH adduct in triglyme in DMSO-d6. .................................................................................. 185 Figure 5.32. GC-MS spectra of the formate ester in the reaction mixture extracted with diethyl ether. ................................................................................................................................ 186 Figure 5.33. 1 H NMR spectra of ethylene glycol monoformate in reaction mixture in DMSO-d6. .................................................................................................................................... 186 Figure 5.34. 13 C NMR spectra of ethylene glycol monoformate in reaction mixture in DMSO-d6. .................................................................................................................................... 187 Figure 5.35. XRD spectra of the catalyst under different conditions. ........................................ 187 Figure 6.1. Introduction. a) methanol as a hydrogen carrier; b) conventional routes for CO2- to-methanol and methanol reforming; and c) overall theme of the present study: low- temperature methanol reforming for emission-free hydrogen generation. .................................. 196 Figure 6.2. Methanol reforming in closed system. a) screening of Ru-based molecular catalysts. reaction conditions: CH3OH/H2O (10 mL, 9:1), KOH (80 mmol), catalyst loading (20 ppm.), stirring (800 rpm), T = 100 °C, t = 24 h; b) effect of reaction temperature. reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), t = 24 h; c) effect of altering the base. reaction conditions: CH3OH/H2O (30 mL, 9:1), OH - (240 mmol), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C t = 24 h; d) effect of KOH content. reaction conditions: CH3OH/H2O (30 mL, 9:1), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 24 h; e) effect of externally introducing H2 pressure. reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 24 h. Pressures measured with a piezoelectric pressure transducer. Calculation error ±5%. ................................................................................................................ 199 Figure 6.3. A typical gas chromatography (GC) spectra of the gas mixture produced during alkaline methanol reforming, showing the sole presence of H2 and no CO/CO2. ....................... 201 xx Figure 6.4. Methanol reforming at 100 °C over extended period. reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), t = 90 h. .... 202 Figure 6.5. Effect of reaction volume on H2 generation. reaction conditions: CH3OH/H2O (9:1), KOH (8 M), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 90 h. .............................. 203 Figure 6.6. Effect of catalyst loading on H2 generation. reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 24 h. ....... 203 Figure 6.7. Effect on methanol content in the CH3OH/H2O reforming solution on H2 generation: total volume (10 mL), KOH (80 mmol), C-4 (20 ppm.), stirring (800 rpm), T = 100 °C, t = 24 h. .................................................................................................................... 204 a Figure 6.8. a)Parr pressure reactors used for H2 generation under pressure. b) photograph of pressure gauge depicting the maximum H2 pressure reached in lab-scale experiment at 140 °C and c) at rt. ................................................................................................................................... 205 Figure 6.9. Generation of H2 at ultra-high pressures. Reaction conditions: CH3OH/H2O (9:1), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 24 h, pressures measured with piezoelectric pressure transducer. Calculation error ±5%. ............................................................................... 205 Figure 6.10. Realistic methanol reformer setup. a) design schematic; b) photograph of actual lab-scale prototype with two reactor modifications. ................................................................... 207 Figure 6.11. Continuous hydrogen generation at ambient pressures. a) methanol reforming at 100 °C (Tset). reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), Tset = 100 °C, t = 12 h; b) effect of reaction temperature (Tset). reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), t = 12 h; c) methanol reforming for extended period (50 h) reaction conditions: CH3OH(400 mL), H2O (50 mL), KOH (200 g), C-4 (50 μmol), stirring (800 rpm), Tset = 110-125 °C, t = 50 h; d) polarization curves with reformed methanol and commercial H2 e) constant current hold of 2.5 A with reformed methanol and commercial H2, Fuel cell parameters: membrane (Nafion 211), electrode (4 cm 2 ), anode (Pt on teflonized carbon paper, 0.5 mg/cm 2 ), Cathode (Pt on teflonized carbon paper with microporous layer, 0.5 mg/cm 2 ), gases humidified at 100 % RH, H2 flow rate (40 ml/min), air flow rate (400 mL/min). Flow and volume measured with calibrated mass flow meter. Calculation error ±5%. ....................... 210 Figure 6.12. Representative 1 H NMR spectra of reaction mixture after hydrogenation of C-1 residues of methanol reforming in D2O. ...................................................................................... 212 xxi Figure 6.13. Catalytic dehydrogenation of different alcohols for H2 generation. reaction conditions: alcohol/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 20 h. For equimolar, a correction factor was incorporated to assume equal moles of ethylene glycol and ethanol as compared to methanol (667 mmol). ............................ 213 Figure 6.14. Image of the actual reformer-fuel cell setup (I-RMFC) used in this study. ........... 217 xxii Abstract With the objective of achieving a net-zero and sustainable economy, recent years have witnessed the emergence of carbon dioxide capture and utilization models (CCU), wherein waste carbon dioxide is identified as a C-1 building block and upcycled to value-added chemicals and materials, including methanol, which has a fast-growing market of >100 Mt/yr. This framework is at the core of the methanol economy concept championed by the late Nobel Laureate George A. Olah with Prof. G. K. Surya Prakash. In addition to being a primary chemical feedstock, methanol is also being widely used as a convenient liquid fuel and energy carrier. The energy stored in methanol can be efficiently utilized in the form of hydrogen via its catalytic reforming owing to its remarkable H2 content of 12.6 wt%. In a long run, the methanol economy has the potential to liberate us from our dependence on finite reserves of fossil fuels and pave the way for a sustainable future. In this direction, my doctoral dissertation titled “Integrated Carbon Dioxide Capture and Utilization: Catalysis Enabled Carbon-Neutral Methanol Synthesis and Hydrogen Generation” comprises of projects, discussed vide infra, which aim to illustrate the vital components of the proposed carbon neutral cycle. Chapter 1, titled “Introduction: The Renaissance of Low Temperature Catalysis in the Context of the Methanol Economy”, discusses the importance and development of low-carbon routes for methanol synthesis with special emphasis on low-temperature homogeneous catalysis. Thereafter, an in-depth account of the rapidly growing field of homogeneous catalysis for methanol synthesis from carbon dioxide (CO2), carbon monoxide (CO) and their derivatives is presented, since its resurgence in 2011. Based on the critical assessment of the progress thus far, the present key xxiii challenges in this field have been highlighted and potential directions have been suggested for practically viable applications. 1 In Chapter 2, titled “Integrated Carbon Capture and Conversion to Methanol with Epoxide- functionalized Polyamines under Homogeneous Catalytic Conditions”, a novel amine based integrated process is developed for CO2 capture and utilization (CCU) to synthesize methanol. Reaction of pentaethylenehexamine (PEHA) with selected epoxides led to a library of functionalized polyamines. In aqueous medium, the modified amines were efficient in capturing CO2 under ambient conditions. Subsequently, the CO2 loaded solutions were directly subjected to catalytic hydrogenation to CH3OH in presence of molecular H2 and Ru-PNP based molecular catalysts. While the epoxide-modified amines exhibited CO2 capture efficiencies comparable to those of unmodified PEHA, they displayed a notable enhancement in the hydrogenation step resulting in higher methanol yields of up to 93%. Furthermore, the integrated process was demonstrated in a water and 2-methyl-THF based biphasic solvent system. We believe that the improved amine-based system for CO2 capture and hydrogenation to methanol is key in advancing low temperature and renewable methanol synthesis processes. 2 In Chapter 3, titled “Tertiary Amine Based Integrated Post-Combustion Capture of CO2 and Hydrogenation to Methanol”, carbon dioxide capture using tertiary amines in ethylene glycol solvent was performed under ambient conditions. Subsequently, the CO2 captured as alkyl carbonate salts was successfully hydrogenated to methanol, in the presence of H2 gas and Ru- Macho-BH catalyst. A comprehensive series of tertiary amines were selected for the integrated capture and conversion process. While most of these amines were effective for CO2 capture, tetramethylethylenediamine (TMEDA) and tetramethylbutanediamine (TMBDA) provided the best CH3OH yields. Deactivation of the base due to side reactions was significantly minimized and xxiv substantial base regeneration was observed. The proposed system was also highly efficient for CO2 capture from a gas mixture containing 10 % CO2, as found in flue gases, followed by tandem conversion to CH3OH. We postulate that such high boiling tertiary amine-glycol systems as dual capture and hydrogenation solvents are promising for the realization of a sustainable and carbon- neutral methanol economy in a scalable process. 3 The first example of an alkali hydroxide-based system for CO2 capture and conversion to methanol is reported in Chapter 4, titled “Hydroxide Based Integrated Direct Air Capture of CO2 and Conversion: Hydrogenation of Metal Carbonates to Methanol”. In an integrated one-pot system, CO2 was efficiently captured by an ethylene glycol solution of the base and subsequently hydrogenated to CH3OH at relatively mild temperatures (100–140 °C) using Ru-PNP catalysts. This further led to the discovery of a fundamental chemical reactivity: direct hydrogenation of metal carbonates to methanol. The produced methanol could be easily separated by distillation. Hydroxide base regeneration at low temperatures was observed for the first time. Finally, CO2 capture from ambient air and hydrogenation to CH3OH was demonstrated. We postulate that the high capture efficiency and stability of hydroxide bases make them superior to existing amine- based routes for direct air capture and conversion to methanol in a scalable process. 4 In Chapter 5, titled “Glycol Assisted Conversion of CO2 and Metal Carbonates to Methanol with a Heterogeneous Copper Catalyst”, a highly effective liquid phase system for hydrogenation of CO2 to methanol using a heterogeneous Cu/ZnO/Al2O3 catalyst was developed under batch conditions. Among the screened solvents, glycols were found to have a marked promoting effect on methanol formation at a relatively low temperature range of 170–200 °C using molecular H2. Relative to the solventless system, ethylene glycol enhanced the CO2 conversion values by up to 120% which is close to the calculated equilibrium limit. CH3OH yields of up to 90% were xxv achieved. The catalyst was remarkably stable and recyclable over multiple hydrogenation cycles. Furthermore, CO2 captured by alkali hydroxides as well as amines were successfully hydrogenated to CH3OH with the Cu/ZnO/Al2O3 catalyst for the first time with >90% yields. Hydrogenation of metal carbonates, bicarbonates and formates were demonstrated for the first time under heterogeneous catalytic conditions. The catalytic process and the plausible reaction pathways were evaluated by control experiments, which suggest that the hydrogenation in the presence of an alcohol proceeds through the formation of formate ester as an intermediate. Finally, the integration of direct air capture (DAC) and hydrogenation of CO2 was demonstrated efficiently as a novel methanol synthesis process using the combination of heterogeneous catalysis and air as a renewable carbon source. Such scalable processes have considerable potential for synthesis of renewable methanol in an efficient and relatively cost-effective approach. 5 Methanol, a renewable and green C-1 feedstock is a highly promising liquid hydrogen carrier (H2 loading of 12.6 wt%). The state-of-the-art methanol reformers operate under harsh and energy- intensive conditions and produce impure H2 which is contaminated with CO2 and CO. Chapter 6 titled, “Low Temperature Catalytic Methanol Reforming for High Pressure H2 Generation and Integration with a Fuel Cell”, presents a realistic and emission-free methanol reformer that operates efficiently under mild reaction temperatures (100-140 °C) enabled by a highly active Ru- PNP molecular catalyst. The novel alkaline methanol reforming system has been demonstrated to operate under open as well as closed conditions, producing H2 rapidly in extremely pure form (>99.9%) with no detectable CO2/CO from a mixture of methanol, alkali hydroxide and water. When the reformer is operated as a closed unit, the catalytic reaction proceeds with fast kinetics to generate H2 under pressure. This led to the demonstration of the first example of methanol based H2 generator under pressures ranging from high (100-150 bar) to ultra-high pressures (>450 bar). xxvi Parallelly, the reactor was modified to operate under open conditions wherein >140 L of clean H2 was produced on-site and directly fed to a H2/air fuel cell without any further purification. The first prototype of an emission-free and low temperature integrated reformed methanol fuel cell (I- RMFC) was established. While the fuel cell showed consistent performance, the catalytic efficiencies of the reformer showed a cumulative TON of > 1.2 X 10 5 and a maximum TOF of >5400 h -1 . Finally, the C-1 residues of the catalytic reforming or the “H2 release” step were subjected to hydrogenation or “H2 loading” step, and successfully converted back to methanol, demonstrating the potential of the present methanol based H2 carrier as a novel “rechargeable H2 battery”. In conclusion, my dissertation work is geared towards understanding and addressing the key mechanistic and systemic challenges in developing novel and viable catalytic systems relevant to the notion of trapping CO2, the greenhouse gas and its further recycling to obtain renewable energy via the proposed carbon neutral cycle, and hence, closing the loop. In addition, several other research projects were pursued during my doctoral studies; discussion on which is, however, beyond the scope of this thesis. References 1. R. Sen, A. Goeppert, G. K. S. Prakash, Homogeneous Hydrogenation of CO2 and CO to Methanol: The Renaissance of Low Temperature Catalysis in the Context of the Methanol Economy, Angew. Chem. Int. Ed., 2022, 61, e202207278. 2. R. Sen, A. Goeppert, G. K. S. Prakash, Integrated Carbon Capture and Utilization to Methanol with Epoxide-functionalized Polyamines under Homogeneous Catalytic Conditions, J. Organomet. Chem., 2022, 965-966, 122331. 3. R. Sen, C. J. Koch, A. Goeppert, G. K. S. Prakash, Tertiary Amine-Ethylene Glycol Based Tandem CO2 Capture and Hydrogenation to Methanol: Direct Utilization of Post-Combustion CO2, ChemSusChem, 2020, 13, 6318-6322 xxvii 4. R. Sen, A. Goeppert, S. Kar, G. K. S. Prakash, Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol, J. Am. Chem. Soc., 2020, 142, 4544-4549. (highlighted on JACS front cover, featured in JACS Spotlights and C&E News) 5. R. Sen, C. J. Koch, V. Galvan, N. Entesari, A. Goeppert, G. K. S. Prakash, Glycol Assisted Efficient Conversion of CO2 Captured from Air to Methanol with a Heterogeneous Cu/ZnO/Al2O3 Catalyst, J. CO2 Util., 2021, 54, 101762. 6. M. Chassé, R. Sen, A. Goeppert, G. K. S. Prakash, N. Vasdev, Polyamine Based Solid CO2 Adsorbents for [ 11 C]CO2 Purification and Radiosynthesis, J. CO2 Util., 2022, 64, 102137. 7. R. Sen, A. Goeppert, G. K. S. Prakash, Carbon Dioxide Capture and Recycling to Methanol: Building a Carbon-Neutral Methanol economy, Aldrichimica Acta, 2020, 53, 39-56. 8. S. Kar, R. Sen, J. Kothandaraman, A. Goeppert, R. Chowdhury, S. B. Munoz, R. Haiges, G. K. S. Prakash, Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol, J. Am. Chem. Soc., 2019, 141, 3160– 3170. 9. A. Goeppert, H. Zhang, R. Sen, H. Dang, G. K. S. Prakash, Efficient, Stable, Oxidation Resistant and Cost Effective Epoxide Modified Polyamine Adsorbents for CO2 Capture from Various Sources Including Air, ChemSusChem, 2019, 12, 1712-1723. 10. J. Kothandaraman, S. Kar, A. Goeppert, R. Sen, G. K. S. Prakash, Advances in Homogeneous Catalysis for Low Temperature Methanol Reforming in the Context of the Methanol Economy, Top. in Catal., 2018, 61, 542-559. 11. S. Kar, R. Sen, A. Goeppert, G. K. S. Prakash, Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy, J. Am. Chem. Soc., 2018, 140, 1580–1583. 12. S. Kar, A. Goeppert, R. Sen, J. Kothandaraman, G. K. S. Prakash, Regioselective Deuteration of Alcohols in D2O Catalyzed by Homogeneous Manganese and Iron Pincer Complexes, Green Chem., 2018, 20, 2706-2710. 13. J. Kothandaraman, S. Kar, R. Sen, A. Goeppert, G. A. Olah, G. K. S. Prakash, Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol, J. Am. Chem. Soc., 2017, 139, 2549-2552 (highlighted in Nature Reviews). 1 Chapter 1. Introduction: The Renaissance of Low Temperature Catalysis in the Context of the Methanol Economy This dissertation chapter is based on a recent review article from our group published in Angewandte Chemie (Sen, R.; Goeppert, A.; Prakash, G. K. S., Angew. Chem. Int. Ed. 2022, 61, e202207278; Angew. Chem. 2022, 134, e202207278). Part of the article is reprinted by permission of John Wiley and Sons. Copyright 2022. John Wiley and Sons. 1.1. Introduction Since the 19 th century, we have witnessed unparalleled industrial and technological progress. Concurrently, the world population has grown rapidly, and so has the demand for energy, which has increased by at least a factor of ten over the past century. These energy requirements have come at the expense of exploiting our natural reserves of fossilized sunshine (fossil fuels), in an unaccountable fashion. 1 As a natural consequence, this has been followed by the inevitable and unchecked emissions of CO2, among other greenhouse gases (GHGs) to the atmosphere, outpacing the natural carbon cycle. 2 Early signs of these repercussions include rise in average global temperatures by ~0.8 °C over the past century, more unpredictable and extreme climate changes, rising of sea levels, ocean acidification, increasing wildfires and ongoing loss of biodiversity. 3-5 In response, there have been increasing efforts towards a more sustainable approach to development and adoption of a circular economy model. 6-9 These ongoing efforts range from policymaking, government regulations and incentives, to industrial implementation of greener technologies and academic research worldwide. 10-13 2 Within this broad framework, one of the key goals is to gradually substitute conventional feedstocks and processes with more sustainable and greener alternatives. 13-16 In this context, technologies such as Power-to-Liquid (PtL) and Carbon Capture and Utilization (CCU) have emerged with growing interests (Figure 1.1). 17-22 In such processes, renewable liquid hydrocarbons such as methanol, oxymethylene ethers (OME) and Fischer-Tropsch products can be produced using renewable electricity and feedstocks, mainly CO2 and water. 23-27 Methanol is one of the most attractive and fastest growing primary chemicals, which has been revolutionizing the chemical and energy sectors over the past several decades. 28 Methanol, when produced renewably can cut CO2 emissions by up to 95% compared to conventional fuels. 29 Sustainable production of methanol, preferably from renewable sources is at the crux of the Methanol Economy, envisioned by the late Nobel Laureate Prof. G. A. Olah, and our group. 30-31 Many aspects of this framework have been described in a monograph co-authored by Olah, Goeppert and Prakash. 32 Renewable methanol (green methanol), in due course, has the potential to liberate us from our long-standing dependence on the finite reserves of fossil fuels and address the carbon conundrum facing humankind. 33 Figure 1.1. Sustainable production based on Power-to-Liquid (PtL) and Carbon Capture and Utilization (CCU). Catalysis holds a central role in the roadmap to achieve a circular economy and sustainable manufacturing. 34-36 Industrial processes powered by catalytic technologies have allowed rapid and 3 mass production of chemicals and materials with increased reaction rates and energy efficiencies. Simultaneously, catalysis is an important tool for renewable energy production, storage and utilization, for instance, in PtL and CCU processes. In this context, hydrogenation catalysts are of special interest as they offer direct routes to infuse renewable energy (in form of hydrogen atoms) into chemicals to produce value-added fuels and feedstocks. 36 The stored renewable energy can later be utilized directly via combustion or by release of the stored hydrogen content (dehydrogenation). 13, 37 Catalytic hydrogenation is practiced in several industrial processes for production of both fine and bulk chemicals, using heterogeneous as well as molecular catalysts. 38- 39 1.1.1 Methanol: The versatile fuel and feedstock Methanol or methyl alcohol (CH3OH) is the simplest member among alcohols. It is a colorless liquid at room temperature (b.p. 64.6 °C) that is easy to store, transport and dispense. Methanol is water-soluble and readily biodegradable. It is one of the most critical building blocks in the chemical industry. The annual global demand has already reached about 107 million tonnes (Mt), almost doubling over the past decade largely driven by the expansion of the methanol-to-olefin (MTO) process and emerging energy applications (Figure 1.2). 29, 31, 40 4 Figure 1.2. a) Global methanol demand and production capacity and b) methanol usage by industrial sectors. Based on data from MMSA. [29, 41] Methanol is a primary feedstock and carbon source which is used to produce a myriad of chemicals, polymers, paints, adhesives, construction materials, pharmaceuticals and others. 32 Production of formaldehyde, another key platform chemical accounts for one of the largest shares (~23%) of the current methanol demand. Other chemical derivatives of methanol include acetic acid, methyl-tert-butyl ether, methyl methacrylate, dimethyl ether, etc. Methanol can also be catalytically converted to a variety of olefins such as ethylene and propylene as well as gasoline. The methanol-to-olefin (MTO) process has witnessed a tremendous growth as a substitute to the more traditional petrochemical routes. From almost no production in 2010, the MTO industry now accounts for about 31% of the global methanol usage. 29, 42 Apart from its application as a feedstock, methanol is an attractive fuel and energy carrier. It is a direct drop-in fuel for internal combustion engines (ICE) and direct methanol fuel cells (DMFC). 32 Methanol’s role has grown rapidly in the energy sector, currently accounting for around 28% of its consumption. 29 As an alternate and clean fuel, it is being widely adopted by a number of countries as gasoline and diesel fuel substitute (M100) or as gasoline blend (M15, M85) for vehicles, ships, boilers as well as cooking. 30 Lately, methanol has also been recognized as a 5 promising liquid organic hydrogen carrier (LOHC). Methanol’s significant H2 content of 12.6 wt% can be extracted through catalytic reforming and utilized in hydrogen fuel cells. 13, 43-49 1.2. Methanol from Fossil and Renewable Sources Methanol can be found naturally in fruits, vegetables, beverages, the atmosphere and even in space. Until the 1920s, methanol was manufactured solely from wood via destructive distillation, hence it is historically referred to as wood alcohol. Since then, methanol has been predominantly derived from fossil fuels, mainly from natural gas (65%) and coal (35%) at present. 29 Worldwide, over 100 million tonnes of methanol is produced yearly in over 90 methanol plants. 50 1.2.1 Traditional Methanol Synthesis from Syngas In conventional commercial processes, methanol is produced from a feed gas mixture, referred to as synthesis gas or syngas that typically comprises carbon monoxide (CO), carbon dioxide (CO2) and hydrogen (H2). 51 Syngas is mainly obtained via coal gasification and natural gas (or shale gas) reforming; and the produced methanol is tagged accordingly as “brown” or “grey”. In comparison to the typical methane reforming routes (steam/dry/tri-reforming), steam and dry reforming when combined as bi-reforming, results in an exclusive 1:2 ratio for CO/H2, coined as metgas by Olah and Prakash that is well suited for methanol synthesis. 52-54 After conditioning and purification, syngas is subjected to catalytic conversion at elevated temperatures of 200-300 °C and pressures of 50-100 bar. The obtained crude methanol is distilled to remove the formed water and other minor by-products. The commercial catalyst is generally based on copper, zinc oxide and alumina (Cu/ZnO/Al2O3). 55-56 Hydrogenation of CO and CO2 to methanol are exothermic (eq. 1.1-1.2). Hence, syngas to methanol proceeds with a thermodynamic conversion 6 limit of about 30% per cycle at such high operating temperatures. The unreacted syngas needs to be recycled continuously over the catalyst to enhance the overall conversion. It is widely agreed that the syngas-to-methanol process proceeds significantly through the transformation of CO to CO2 via the water gas shift (WGS) reaction (eq. 1.3). CO + 2H2 ⇌ CH3OH ΔH298K = −21.7 kcal mol −1 (1.1) CO2 + 3H2 ⇌ CH3OH + H2O ΔH298K = −11.9 kcal mol −1 (1.2) CO + H2O ⇌ CO2 + H2 ΔH298K = -9.8 kcal mol −1 (1.3) 1.2.2 Renewable and Low Carbon Methanol With the rapidly growing demand for methanol projected to surpass 120 Mt by 2025 and 500 Mt by 2050, the production capacity is also rising sharply worldwide. 29 Given the high fossil dependency and carbon emissions associated with the methanol production at present, there is a growing urge to decarbonize this industry. Steps in this direction include the use of power and feedstocks (H2 and carbon) that are generated with reduced carbon footprint, and preferably, using renewable resources. 21, 57-59 In parallel, low carbon methanol (LCM) can be achieved through recycling of the CO2, which would be emitted to the atmosphere otherwise, to synthesize methanol. 18, 60 Waste CO2 is produced at various stages within the methanol industry, for instance, during coal gasification, syngas conditioning and during heat generation. 61-62 CO2 from these and other emission sources, can be fed back into the methanol synthesis loop, to produce low carbon methanol (LCM). Further, carbon from renewable sources, such as geothermal vents, biomass and air, can be recycled to afford net carbon-neutral methanol (e-methanol and bio- methanol). 29 Air-to-methanol in 7 particular, is a distinct proposition, given that air has no geopolitical constraints, and is a near inexhaustible source of renewable carbon. 63-64 1.3. Hydrogenation of CO2 to Methanol Similar to hydrogenation of CO to methanol (refer to Section 1.2.1), CO2 can be activated by the same class of copper-based commercial catalysts for its hydrogenation to methanol under comparable reaction conditions (230-300 °C and 50-75 bar). 65 The catalyst activities decrease gradually due to water induced sintering and deactivation of active sites. High reaction temperatures favor the reverse water gas shift (RWGS) reaction resulting in formation of water. CO2 hydrogenation proceeds with a high selectivity of >99% for methanol using the standard Cu/ZnO/Al2O3 catalysts. 66 Although, the CO2 conversion is severely limited by thermodynamics similar to the syngas-to-methanol process. Over the past few years, significant research has taken place to develop improved heterogeneous catalytic systems with higher conversions and stability and operating under milder conditions. These include diverse modifications of the Cu/ZnO/Al2O3 catalyst, as well as designing entirely new compositions such as Ni-Ga, Pd, Pt, Re, and In2O3 based catalysts, among others. 56, 67-68 During 1993-95, a series of reports of CO2 hydrogenation to methanol via reverse water gas shift reaction were presented by Tominaga and co-workers. 69-70 A Ru3(CO)12 complex was shown to catalyze the hydrogenation with KI as an additive in N-methylpyrrolidone (NMP) solvent. Methanol formation was observed at high temperatures (160-240 °C) and a net pressure of 80 bar (CO2:3H2). It was proposed that CO2 initially converts to CO via the reverse water gas shift reaction, which undergoes further reduction to methanol. CO and CH4 were observed as side products, whose formation increased significantly at temperatures above 240 °C. The addition of an iodide was crucial to stabilize the in situ formed Ru carbonyl clusters, which act as the active 8 catalytic species for hydrogenation. Although the system demonstrated the first examples of homogeneous CO2 to methanol process, this method suffered from significant challenges including low yields, poor selectivity and high operating temperatures that was comparable to existing heterogeneous conditions. Following this work, the field of homogeneous catalysis for CO2 hydrogenation to methanol remained mostly unexplored for almost two decades. 1.3.1 Low Temperature Methanol Synthesis using Homogeneous Catalysts Since CO2 hydrogenation to methanol is exothermic in nature (eq. 2, ΔH298K = −11.9 kcal mol −1 ), the reaction would be thermodynamically favored at lower temperatures. Similarly, milder operating temperatures can significantly reduce the overall energy and capital inputs. Despite the exoergicity, a minimum temperature threshold is often required to maintain activity of the catalyst and enhance the overall kinetics of the reaction. Along these lines, in addition to the exploration in the field of heterogeneous catalysis, there has been a continuous search for alternate catalytic systems that offer selective routes to methanol, especially at lower temperatures. 56 In general, designing of novel catalysts should take into account desirable features such as being robust, relatively inexpensive, the ease of separation and recycling, among others. Additionally, high catalytic activity in combination with exclusive selectivity for the desired product is ideal for development of a viable methanol synthesis technology. In this context, molecular metal complexes are a promising class of catalysts, which can activate CO2 and H2 for methanol synthesis under relatively mild conditions. 35, 71 Operating at relatively low temperatures and in liquid phase, such catalysts also offer high selectivity for methanol by inhibiting side reactions. Additionally, homogeneous systems provide significant opportunity for better understanding of the reaction pathways and catalytic mechanisms at a molecular level. Based on such mechanistic insights as well as computational predictions, the 9 catalytic frameworks can be rationally tuned to improve the efficiency and selectivity of the reaction. 72 Desirable homogeneous catalysts should exhibit enhanced robustness and thermal stability over long runs under operating conditions along with substantial turnovers (both TONs and TOFs). Most of the molecular catalysts initially selected for CO2 hydrogenation were strongly inspired from their abilities to hydrogenate other carbonyl-based substrates and carboxylic acid derivatives such as amides, esters, ketones and others. 73-78 While several of these catalysts are successful in hydrogenation of CO2, the reduction is often arrested at the formic acid/formate stage. 8, 79-85 While thermodynamically favored, further hydrogenation to methanol is kinetically challenging, primarily due to the lower electrophilicity (hydride affinity) of the carbonyl moiety in these molecules. Hence, catalysts which form metal hydrides with higher hydricity (or ease of hydride transfer) are required for methanol synthesis reactions, compared to other carbonyl hydrogenation reactions. Additionally, formate species act as strong ligands to the metal center, blocking the active site for further hydrogenation. 80 Hence, a mediating component such as an alcohol or amine is often required to detach the formate species and stabilize it in the form of formate esters or formamides, enhancing the rates of hydrogenation further towards methanol. For methanol synthesis from CO2, homogeneous catalysts should be able to split H2 for its insertion into CO2 to afford formate species and have a high enough hydricity to reduce formate to methanol. 73, 78, 86 In addition, these catalysts should retain activity and be compatible to any additive, reaction pH and side products such as CO. Some of the first and most widely explored catalysts for such transformations include metal-pincer PNN and PNP complexes including pyridine-based complexes developed in Milstein’s group as well as the PN(H)P type catalysts which can split H2 via metal-ligand cooperation (MLC) to afford the hydrogenation of a carbonyl 10 moiety (Figure 1.3). 72 On the other hand, notable catalysts including ones bearing tripodal- phosphine and bipyridine based ligands performing under acidic-neutral conditions have also gained special significance in the field of CO2 hydrogenation. The early examples of CO2-to- methanol homogeneous catalysts are based on precious transition metals, mostly with ruthenium and a few with iridium. Lately, analogues of these catalysts with earth abundant metals, namely iron, manganese and cobalt are being developed as well. Many of these state-of-the-art catalysts as well as the ligand frameworks are becoming commercially available with increasing demand and research interests due to the rapid growth in the overall homogeneous hydrogenation and dehydrogenation catalysis fields. 38, 87-97 Figure 1.3. Selected examples of the first-generation homogeneous catalysts instrumental in developing CO2 hydrogenation to methanol. 1.3.2 Hydrogenative Routes for CO2 to Methanol Hydrogenation of CO2 to methanol with H2 can proceed through various intermediates and derivatives of CO2. 98 In direct hydrogenation of CO2, the reaction may proceed through HCOOH intermediate as in the cases of additive free systems. Additionally, other favorable intermediates such as formate esters and formamides observed in presence of an alcohol and amine additive, respectively, have been shown to facilitate the hydrogenation of CO2 and CO to methanol. 11 Invariably, formaldehyde or dihydroxymethane is the immediate species hypothesized to precede methanol formation. However, the high reactivity poses challenges in detection or isolation of such species. Further, CO2 hydrogenation may proceed through CO in reverse water gas shift reaction. However, it is a minor route under mild homogeneous conditions. 99 Concurrently, methanol can be accessed via hydrogenation of various derivatives of CO2 or carbonic acid as well as ionic capture products of CO2. 72 The derivative or intermediate formed is also governed by the pH of the system: acidic, neutral or alkaline. Organic derivatives including organic carbonates (acyclic and cyclic), urea and carbamate derivatives can be readily obtained from CO2 via known processes without any reduction step. On the other hand, CO2, which is a weak acid can be chemically captured in presence of a base under ambient conditions. In alkaline medium, amines or alkali metal hydroxides react with CO2 to form ammonium or metal salts of carbamate, bicarbonate or carbonates. 100-101 Such molecules can be catalytically hydrogenated to obtain methanol with three equivalents of H2. While reduction of CO2 to methanol have also been achieved using other hydrogen-sources including metal hydrides, boranes, silanes, water and others; molecular H2 stands out as the most efficient, atom-economical, cost-effective and industrially viable reductant, provided that appropriate infrastructure is available to handle H2 under high pressures (Figure 1.4). In addition, H2 is also a direct and green carrier of renewable energy. In parallel to the thermocatalytic processes for CO2 hydrogenation, there have been significant advances in achieving such transformations through alternate routes involving photocatalysis, electrocatalysis, organocatalysis and biocatalysis. However, these subjects are outside the focus and scope of this review. 102-115 12 Figure 1.4. CO2 reduction to methanol: selected categories of H-source, catalysis and process covered by this review (framed). 1.4. Direct CO2 to methanol using molecular catalysts 1.4.1 Precious Metal Based Catalytic Systems 1.4.1.1 Cascade Route for CO2 to Methanol In 2011, Sanford and co-workers achieved a breakthrough in synthesizing methanol directly from CO2 with homogeneous catalysts at low temperatures (Figure 1.5). 116 Using a cascade of three different catalysts, the reaction proceeded through the sequence: Step A) CO2 hydrogenation to formic acid with a Ru-phosphine catalyst (Ru-1); Step B) esterification with CD3OH in presence of Sc(OTf)3, a Lewis acid catalyst to generate methyl formate; and Step C) hydrogenation of formate ester to methanol using a Ru-PNN catalyst (Ru-2). The broad class of Ru-PNN catalysts had been developed previously by the group of Milstein and have been utilized extensively for numerous (de)hydrogenation transformations. When the tri-catalytic hydrogenation was attempted with a 1:3 CO2:H2 mixture (40 bar) in one-pot at 135 °C, methanol was formed, albeit with a very low TON of 2.5. Possible deactivation of the Ru-2 by Sc(OTf)3 was suggested to be a key challenge. To circumvent the catalyst decomposition, the cross-reactive catalysts were physically compartmentalized within the reactor which enhanced the catalytic activities significantly with a TON of 21 for methanol over 16 h. While the turnovers were modest 13 for the cascade catalysis, this approach served as one of the primary reports in the development of homogeneous catalysis for direct hydrogenation of CO2 to methanol. Figure 1.5. Cascade catalysis of CO2 to methanol. Based on ref. 116 and 117. Goldberg and co-workers further evaluated the individual steps in the cascade catalysis previously reported by Sanford and modified the system accordingly (Figure 1.5). 117 Using ethanol (EtOH) as the solvent, the rationally selected catalyst triad involving Ru-3, Sc(OTf)3 and Ir-1 showed considerable improvement in the net catalytic efficiency. A notable TONMeOH of 428 over 40 h was achieved partly due to the use of higher reaction temperatures (155 °C) and more active Ir metal. Ir-1 was also more stable than the previously employed Ru-2 by Sanford under high temperature and acidic conditions. Additionally, it was noted that Ru-3 required preactivation with H + in step A, which is generated in situ by the triflate salt. Interestingly, a substantial amount of diethyl ether was also observed due to self-condensation of EtOH under acidic conditions. A notable limitation of this system is the generation of CO as a side product due to ethyl formate decomposition, which inhibited the catalytic activities of both the Ru-3 and Ir-1 catalysts. 14 1.4.1.2 Tripodal Phosphine Based Catalytic Systems In 2012, Leitner, Klankermayer and co-workers developed a single molecular catalyst for hydrogenation of CO2 to methanol (Figure 1.6). 118 The catalyst was based on ruthenium metal decorated with a tripodal and tridentate phosphine ligand, commonly abbreviated as Triphos (1,1,1-tris(diphenylphosphinomethyl)ethane). The hydrogenation catalysis required the presence of an organic acid co-catalyst such as methanesulfonic acid (MSA) or bis(trifluoromethane)sulfonimide (HNTf2). Similar to the catalysis reported by Sanford and Goldberg, the present reaction was also assisted by an alcohol additive. The metal-triphos complex was either formed in situ from the precursors Ru(acac)3 and Triphos ligand (Ru-5); or was introduced as the isolated Ru-triphos species (Ru-4). Using ethanol and Ru-5, authors achieved a TON of 221 for methanol under the optimized reaction conditions (CO2:3H2 = 80 bar, 140 °C, 24 h). Figure 1.6. Ruthenium-Triphos catalysis for CO2 to methanol. Based on ref. 118. Continuing on their initial findings, the same group of authors later demonstrated the catalytic hydrogenation even in the absence of an alcohol additive. 119 Using Ru-4 in combination with HNTf2 in THF, a promising TON of 603 was reported over 48 h. The catalysis was active for methanol even at low temperatures (80-100°C) albeit with lower activities. Further, a biphasic system (2-MeTHF and water) was developed for efficient catalyst separation and recycling. After 15 four cycles, a TON of 769 was achieved and about half of the initial catalytic activity was retained (Figure 1.7). Figure 1.7. Biphasic system to separate methanol and catalyst. Adapted from ref. 119 with permission from the Royal Society of Chemistry. In a 2020 report, Klankermayer and co-workers tuned the original catalytic system by introducing a tdppcy (1,3,5-tris(diphenylphosphino)cyclohexane) ligand in place of the triphos ligand (Figure 1.8). 120 The Ru-tdppcy (Ru-6) showed up to four-fold enhancement in the TON for hydrogenation of CO2 to methanol, compared to Ru-triphos (Ru-4), even under additive free conditions. In presence of ethanol, a remarkable TON of 2148 was achieved with Ru-6 and Al(OTf)3 as a co-catalyst. The authors suggested that the increased activity was possibly due to higher rigidity of the tdppcy ligand with a cyclohexyl ring structure when compared to the more flexible triphos ligand. CO2/H2 CH3OH H2O H2O phase 2-MeTHF phase Ru-4 CH3OH CH3OH + H2O CO2 + 3 H2 Ru-4 2-MeTHF - CO2/H2 H2O distil. 16 Figure 1.8. Comparative performances of Ru-4 and Ru-6. Adapted from ref. 120 with permission from the American Chemical Society. Around the same time, another modification to the Ru-triphos system was reported by Wiedner et al. by modulating the bridgehead moiety of the tripodal ligand (Figure 1.9). 121 N- triphos (Ru-7) and cationic Me N-triphos (Ru-8) complexes were studied in comparison to the previously explored C-triphos complex (Ru-4). The authors were able to study the reaction profile via operando 1 H NMR spectroscopy. While the formation rates of formate ester were similar for all the studied catalysts, the corresponding methanol production was in the order Ru-8 ( Me N- triphos)<Ru-7 (N-triphos)<Ru-4 (C-triphos) in ethanol. The relative rate constants showed that Ru-8 displayed a 12-fold enhancement compared to Ru-4 in rate of methanol formation from the transient formaldehyde intermediate. The relative electron deficiency of the N-triphos ligand was suggested to make the catalyst more active towards methanol than the formaldehyde acetal (dialkoxymethane). 17 Figure 1.9. Comparison of Ru catalysts based on neutral C-triphos, N-triphos and cationic Me N- triphos. 1.4.1.3 Amine Assisted Catalysis The catalytic systems described previously in Sections 1.4.1.1 and 1.4.1.2 operated in neutral or slightly acidic medium. In addition to such pathways under alcohol assisted or additive free conditions, there has been significant progress in CO2 reduction in basic medium. In 2015, Sanford and co-workers reported an amine promoted approach for the transformation of CO2 to methanol (Figure 1.10). 122 In the amine assisted route, CO2 reacts with an amine moiety to form the corresponding ammonium carbamate. The carbamate species can directly undergo hydrogenation to formamides and then further to methanol. Alternatively, the carbamate adduct can reversibly release CO2 gas at mild operating temperatures. The free CO2 can be catalytically hydrogenated to ammonium formate. This is followed by the formation of the key formamide intermediate via condensation, for subsequent reduction to methanol. Based on these hypotheses and other preliminary investigations, authors demonstrated a one-pot temperature ramp strategy (95 °C to 155 °C) to hydrogenate a gaseous mixture of CO2 (2.5 bar) and H2 (50 bar) to methanol with a TON of 550 after 54 h. A ruthenium metal complex with PNP based pincer ligand, trademarked as Ru-Macho-BH (Ru-9) was found promising to catalyze this transformation efficiently in presence of dimethylamine, a model amine additive. 18 Figure 1.10. Amine assisted one-pot CO2-to-methanol system. Shortly after, Ding and co-workers accomplished a sequential route to synthesize methanol from CO2 as an application of the N-formylation of amines developed in the same report (Figure 1.11). 123 In the first step of the one-pot sequence, a 1:1 mixture of CO2 and H2 at 70 bar was reacted at 120 °C for 40 h in presence of Ru-Macho catalyst (Ru-10) and morpholine, a model amine, to afford the corresponding formamides. In the subsequent step, the resulting crude reaction mixture containing the active catalyst was subjected to 50 bar of H2 at a higher temperature of 160 °C to obtain methanol in 36% yield within 1 h. Figure 1.11. Amine assisted sequential methanol synthesis from CO2 via N-formylation. It is also worth highlighting here that the preliminary reports by Sanford and Ding provided key insights into the amine-assisted CO2-to-methanol sequence: a) while CO2 hydrogenation with one equivalent of H2 to formate/formamides proceeds at relatively low temperatures, further hydrogenation to methanol (with two equivalents of H2) required elevated temperatures indicating that this step is relatively more challenging; b) identification of the Ru-Macho based catalysts, a family of metal-PNP pincer complexes effectively catalyzing the CO2-to-methanol process under 19 basic conditions. These molecular catalysts had previously been explored for related (de)hydrogenative transformations. 8, 75 In 2015, another sequential approach for conversion of CO2 under near-ambient pressures to methanol was demonstrated by Milstein et al. using amino-alcohols (Figure 1.12). 124 First, CO2 (1-3 bar) was heated to 150 °C in presence of an amino-alcohol (N-methylethanolamine or Valinol) and Cs2CO3 catalyst in DMSO to form the corresponding oxazolidinone. Here, the higher operating temperatures and catalytic conditions allowed for a different reactive pathway between amino-alcohols and CO2 in contrast to the formation of carbamate salts observed at lower temperature. In the second step, the Ru-PNN catalyst (Ru-11) and a co-catalyst t BuOK were introduced to the reaction mixture following evacuation of CO2 and the mixture was subjected to hydrogenation at 135 °C with 60 bar of H2 to produce methanol with up to 53% yield. Notably, the Ru-PNN catalyst (Ru-11) showed higher efficiency for the hydrogenation step as compared to Ru-Macho (Ru-9) under similar conditions. Additionally, small amounts of formate species were also detected as possible intermediates, though further hydrogenation to methanol could proceed through either formamides or formate esters. Figure 1.12. Low pressure CO2-to-methanol route via oxazolidinone. Based on ref. 124. During this period, Olah, Prakash and co-workers also reported a one-step methanol synthesis process directly from CO2 using pentaethylenehexamine (PEHA), a commercially available and high boiling polyamine (Figure 13). 125 Among the pincer catalysts screened, Ru- Macho-BH (Ru-9) and Ru-Macho (Ru-10) showed the most promising activities. The 20 hydrogenation was carried out with 75 bar of 1:9 CO2:H2 in presence of PEHA in triglyme solvent to afford methanol with high TONs of 1200 (CH3OH yield = 65%) and 985 (CH3OH yield = 54%) at temperatures of 145 °C and 125 °C, respectively, over an extended reaction time of 200 h. The reaction system was shown to be quite robust and stable allowing efficient recycling of the catalyst, amine and solvent over multiple cycles. The products formed (methanol and H2O) were distilled out after each cycle, and a total TON of 2150 could be achieved after five hydrogenation cycles (Figure 1.13a). Similarly, a repeated pressure refill experiment led to an accumulation of 4 mL (98 mmol) of CH3OH isolated via distillation (Figure 1.13b). Also, trace amounts of CO were detected in the unreacted gas mixture due to RWGS reaction and possible decomposition of formaldehyde, a transient intermediate species. Another key highlight of this study was the demonstration of methanol synthesis using air as the carbon source (discussed in Section 1.5.2). Following up on their initial work, Prakash et al. reported a detailed and systematic investigation of the amine-assisted CO2 hydrogenation to methanol. 99 The effect of the amine’s molecular structure was evaluated as amines play multiple key roles in the catalytic CO2-to- methanol pathway (Figure 1.13c). Upon screening a library of amines with varied molecular structures, authors observed that monoamines afforded only trace methanol and mostly formate/formamide species, whereas diamines with primary and secondary amino groups were significantly active for methanol synthesis. Polyamines with higher N contents such as diethylenetriamine (DETA) or PEHA showed further enhancements. In contrast, presence of a tertiary amino group completely arrested the methanol formation. Additionally, the solvent volume had a notable effect, as the methanol yield doubled when the volume of triglyme was increased from 5 mL to 10 mL (Figure 1.13d). An opposite effect on the accumulation of formamide intermediate was observed, suggesting that increased amounts of solvent enhances the dissolution 21 and hence, conversion of these intermediates. Finally, running the CO2 hydrogenation for prolonged time (10 days) led to a remarkable TON of 9900 using a combination of Ru-9 as catalyst and PEHA as the amine at 145 °C. In a similar approach, Gademann demonstrated a CO2-to- methanol system, with pyrrolizidine-based diamine and Ru-9 as the catalyst, although the selected amine demonstrated a modest promoting effect leading to a TON of 28 for methanol over 134 h (Figure 1.14). 126 Figure 1.13. Amine assisted CO2 hydrogenation to methanol using Ru-Macho-BH (Ru-9). a) Recycling of reaction components with PEHA and CO2:H2 (1:9); b) Repeated pressure refill experiment with PEHA and CO2:H2 (1:3); c) Effect of amine molecular structures on methanol formation; and d) Effect of solvent volume on yields of methanol and intermediates with PEHA. Adapted from ref. 99 and 125 with permission from the American Chemical Society. 22 Figure 1.14. Pyrrolizidine assisted CO2 hydrogenation to methanol. In 2017, Everett and Wass studied a catalytic system with a series of Ru complexes bearing bidentate P-N ligands and relatively low-boiling amines (Figure 15). [127] While the hydrogenation occurred under a relatively harsh temperature of 180 °C, the applied H2 pressure was relatively lower (30 bar). Increasing the steric bulk of the amine enhanced the rate and selectivity of the formamide to methanol step. Using di-isopropylamine for instance, a much higher TON of 2300 was obtained as compared to that with dimethylamine (TON = 110). Interestingly, modifying the N substituent on the catalyst had profound effect on the methanol productivity. When the NH2 moiety on the ligand (Ru-12) was mono-methylated (Ru-13), a higher TON of 4000 was achieved. However, further methylation (Ru-14) led to a complete loss of catalytic activity. This is strongly indicative of a metal-ligand cooperation involving the outer sphere. Finally, lowering the catalyst loading resulted in a notable TON of 8900 within 2 h. Figure 1.15. Ruthenium catalysts for amine assisted CO2 to methanol. Zhou and co-workers presented a novel tetradentate bipyridine based Ru catalyst (Ru-15) for amine assisted hydrogenation of CO2 (Figure 1.15). 128 Primarily, the catalysis was demonstrated separately involving two steps via formamides, similar to the prior study by Ding et al. Direct hydrogenation of CO2 in the presence of excess H2 (CO2/H2 = 2.5/50 bar) was also 23 achieved using dimethylamine in isopropanol solvent to realize a net CO2 conversion of 84% to CH3OH with a TON of 2100. In the same direction, Kayaki et al. in 2019 caried out methanol synthesis in presence of high molecular weight polyethylenimines catalyzed by Ru-9. 129 Increasing temperatures as well as decreasing CO2:H2 ratio had enhancing effect on methanol formation. Most linear and branched PEIs had comparable TONs for methanol (203-362) except for BPEI10K (TON = 102). The highlight of this report was the demonstration of the intended catalytic role of amines in the CO2- to-methanol process as an amine turnover of 3 was achieved. A similar amine turnover was also achieved by Prakash and co-workers in a very recent study using PEHA over an extended period of 96 h. 130 1.4.1.4 Miscellaneous CO2-to-Methanol Systems 1.4.1.4.1 Co-production of Methanol and Glycol. In 2020, Kothandaraman and Heldebrant demonstrated a process for the co-production of methanol and glycol from CO2, H2 and an epoxide in an atom efficient approach converging the industrial OMEGA process and methanol synthesis under homogeneous hydrogenation conditions (Figure 1.16). 131 At 140 °C in presence of an amine promoter (PEI600), CO2 (30 bar) and epoxide were reacted to form the corresponding cyclic carbonate. Next, the carbonates were hydrogenated with Ru-Macho (Ru-10) to co-produce methanol (yield = 84%) and glycol (yield > 99%). Interestingly, the methanol formation was consistently lower as compared to the glycol, which could in part be explained by the detection of CO and CO2 as unreacted species. However, authors did not observe any formate or formamide intermediates. While attempts to carry out both processes in one-step lowered the product yields, the sequential addition of CO2 and H2 to the reactor (containing amine, Ru-10 and epoxide) successfully produced methanol and glycol in high 24 yields. Overall, the proposed process avoided the formation of an equivalent amount of water, which would be produced in the conventional CO2 to methanol systems. Figure 1.16. Co-production of glycol and methanol from epoxide and CO2. 1.4.1.4.2 MOF-encapsulated Molecular Catalysts In a novel direction for CO2-to-methanol process, Byers, Tsung and co-workers disclosed in 2020, a highly recyclable MOF-encapsulated catalytic system (Figure 1.17). 132 Inspired by the cascade pathway via formate ester previously reported by Sanford and Goldberg, authors identified two molecular catalysts: a Ru-PNP catalyst (Ru-16) for the CO2 to formate step and a Ru-PNN complex (Ru-11) for the hydrogenation of formate ester. These complexes were encapsulated in a zirconium-based metal organic framework, trademarked as UiO-66, selected based on a good fit of the catalyst into the pores of the MOF. Besides, the ZrO2 nodes in the MOF acted as Lewis acid catalysts to promote the esterification step. Using trifluoroethanol (TFE) as the optimized alcohol additive, Ru-16@MOF and Ru-11 catalysts, an impressive TON of 6600 was achieved for methanol within 16 h at significantly mild temperature of 70 °C. Notably, authors used a very low CO2:H2 ratio of about 1:12 in the feed gas at 40 bar. Upon interchanging the catalyst in the MOF, Ru-11@MOF with Ru-16 gave a comparable TON of 5700. Using both catalysts in encapsulated form (Ru-11@MOF + Ru-16@MOF) or as co-encapsulated ([Ru-11+Ru-16]@MOF) gave relatively lower TONs of 3500 and 4300, respectively. However, when the catalytic components (Ru-11, Ru-16 and UiO-66) were introduced together directly in solution without any prior encapsulation, no hydrogenation activity was observed. Authors believed that this could be a result O CO 2 O O O H 2 O OH OH CO 2 + cat. cat. O CO 2 O O O H 2 OH OH CH 3 OH + amine Ru-10 Shell Omega process Heldebrant, 2020 25 of possible bimolecular decomposition pathways between catalysts Ru-11 and Ru-16 which establishes the importance of their mutual isolation in form of encapsulation. Overall, the catalytic setups demonstrated excellent recyclability over five cycles affording remarkable TONs of 17,500 and 21,000 for Ru-11@MOF + Ru-16@MOF and [Ru-11+Ru-16]@MOF, respectively. Figure 1.17. MOF-encapsulated catalysts for one-pot CO2-to-methanol. Adapted from ref. 132 and 133 with permission from Elsevier Inc. and American Chemical Society. The same group later studied the effect of systematically manipulating the functionality of the UiO-66 MOF and found that methanol productivity was significantly enhanced by a NH3 + functionality closely followed by NH2 (Figure 1.17). 133 Authors suggested that the ammonium group, when in close proximity to Ru-16, promoted the CO2 to HCOOH reduction step by detaching the formate species from the metal center. Finally, recycling of the catalytic system ([Ru-11]@MOF) + ([Ru-16]@MOF-NH3 + ) over ten runs led to a striking TON of 10 5 . N, N’ dimethylformamide (DMF) was used as a solvent for hydrogenation and authors confirmed via 26 control experiments, that DMF did not undergo hydrogenation to methanol under these reaction conditions. 1.4.1.4.3 Aqueous Phase Catalysis CO2 hydrogenation to methanol may proceed through formic acid that can undergo further hydrogenation without its derivatization to either formamides or formate ester. However, such a route has rarely been achieved in the domain of homogeneous catalysis. The field encompassing formic acid disproportionation under aqueous conditions to CH3OH and CO2 has been explored previously. [134] In 2016, this concept was extended by Himeda, Laurenczy and co-workers to establish a novel CO2-to-methanol process using an iridium based complex (Ir-2) which could catalyze both the hydrogenation (of CO2) and disproportionation (of HCOOH) at a notably low temperature of 70 °C (Figure 1.18). 135 The transformation required highly acidic medium (2.5 m H2SO4) leading to a TON of about 8 for methanol after 50 h. With a wide scope for further improvement in the catalytic activities, the Ir based system is the only viable neat aqueous process reported to date for CO2 to methanol. Figure 1.18. CO2 to methanol via formic acid disproportionation. 1.4.1.4.4 Solvent-free Catalysis Very recently, Onishi, Himeda and co-workers envisioned a gas-solid phase hydrogenation of CO2 to methanol using molecular catalysts under solvent and additive free conditions (Figure 1.19). 136 A series of dinuclear (Ir-4/ortho, Ir-5/meta, Ir-6/para) and trinuclear (Ir-7) complexes based on a monomeric picolinamide based catalyst (Ir-3) were developed for this transformation. Among 27 these catalysts, only the Ir-5 and Ir-7 catalysts were active for methanol synthesis with similar TONs of 9 and 11, respectively over 165 h using a 40 bar gas mixture of CO2:3H2. Further, the catalyst Ir-5 was recycled over five hydrogenation cycles of 336 h to achieve a cumulative TON of 113. While the kinetics of the reaction were rather slow, it is worth noting that the system was active even at 30 °C (TON = 2). Authors proposed that the multinuclear template prevented the liberation of the formate species from the metal center, pushing towards a concerted reduction mechanism via multiple intramolecular hydride transfers on the catalyst. Figure 1.19. Multinuclear Ir complexes for gas-solid phase hydrogenation of CO2 to methanol. 1.4.1.4.5 Transfer Hydrogenation of CO2 to Methanol Catalytic transfer hydrogenation of CO2 has been explored previously. The primary advantage of such systems is avoiding the use of high pressure H2. Instead, abundant and renewable alcohols can act as the source of H2 (H-source). However, these transformations have been mostly limited to afford formates from CO2. Further hydrogenation to methanol has been a major challenge in the absence of molecular hydrogen under pressure. In a rare example, Klankermayer and co-workers successfully demonstrated a transfer hydrogenation system for 28 CO2-to-methanol in one-step, using alcohols as the H-donor and Ru- catalysts with triphos ligand (Ru-4) and substituted equivalents (Figure 1.20). 137 Although pressurized H2 gas was avoided, the reactions were performed with a relatively high CO2 pressure of 50 bar. Among the screened alcohols as H-source, ethanol performed most efficiently with TONs of 96-121 for CH3OH at 160 °C. Apart from CH3OH, ethyl formate was obtained as a result of partial hydrogenation followed by condensation. H2 was generated in-situ by the dehydrogenation of ethanol to ethyl acetate. The reaction mixture, after transfer hydrogenation, was subjected to 80 bar H2 to regenerate the H- source (ethanol) from its H-lean form (ethyl acetate). The catalyst showed comparable activity for a second cycle of methanol synthesis, indicating that the system could be promising in the development of a recyclable ethanol-based system for methanol synthesis. Figure 1.20. Transfer hydrogenation of CO2 to methanol using ethanol as H-source. 1.4.1.4.6 CO2-to-methanol Using Solid-supported Amines Alternate to liquid-based amine systems, solid-supported amines (SSAs) have also gained much interest as convenient and reusable matrix for varied purposes including CO2 adsorption (Figure 1.21). 138 Yet, their use as heterogeneous amine components in CO2 hydrogenation systems remain limited. In a study by Prakash and co-workers in 2019, 139 linear and branched polyethylenimines anchored (physically or covalently) on silica supports were screened for amine- assisted homogeneous CO2 hydrogenation reaction with Ru-Macho-BH (Ru-9). Results indicated that most of the SSAs screened were able to assist in methanol formation with TONs up to 520. However, leaching of the amine moieties into the solvent still remain a major challenge; though 29 chemically grafted SSAs suffered from much less degradation, when compared to the physically impregnated adsorbents. Figure 1.21. Solid supported amines for CO2 hydrogenation to methanol. Reproduced from ref. 138 with permission from John Wiley and Sons. 1.4.2 Base Metal Based Catalytic Systems 1.4.2.1 Cobalt Based Catalysts In early 2017, Beller et al. developed the first example of a base metal based molecular catalyst for homogeneous CO2 to methanol system (Figure 1.22). 140 Extending on the previously studied ruthenium-triphos catalysts, this work involved Co(acac)3 as the most active cobalt precursor along with the triphos ligand and HNTf2 as the acid additive (Co-1). Interestingly, unlike the ruthenium-based analogues (Ru-5), the cobalt catalyst did not show a drop in activity when the reaction temperatures were reduced from 140 °C to 100 °C. The authors observed a marked induction period of 6-8 h during which only trace amount of methanol was produced due to the slow formation of the active cationic cobalt-triphos species. In the presence of ethanol, a maximum TON of 78 for methanol was achieved with 20 bar of CO2 and 70 bar of H2 after 96 h at 100 °C. 30 Later, the same group was able to improve on their previously reported TONs by modifying the P-substituents on the tripodal phosphine-cobalt catalyst (Co-2 to Co-5). 141 When o-methyl and p-methyl substituted aryl groups (xylyl, tolyl, anisyl) were used in place of phenyl as in the parent triphos ligand, the TONs increased up to 125. Authors surmised that the effect could be due to a) increased electron density on the metal center accelerating the H2 activation, and b) minimized Co-dimer formation. However, the o-tolyl substituent completely inhibited the catalytic activity indicating a strong steric effect. Additionally, other base metal-based catalysts did not show any promising activity under the reported reaction conditions. Figure 1.22. Cobalt based catalysis with triphos-derived ligands. 1.4.2.2 Manganese Based Catalysts The first manganese-based catalyst for CO2-to-methanol conversion was developed in late 2017 by Prakash and co-workers (Figure 1.23). 142 The Mn catalyst decorated with a PNP iPr ligand (Mn- 1), was similar to the well-studied ruthenium analogues for (de)hydrogenation catalysis. In presence of morpholine as an amine additive, CO2 reductively condensed to form N-formyl morpholine under 70 bar of 1:1 CO2/H2 mixture at 110 °C. Further hydrogenation to methanol was, however, ineffective. Rather, a sequential protocol similar to that developed by Ding et al. was found productive wherein the in situ generated formamides were hydrogenated in the same pot containing the active catalyst in the presence of 80 bar H2. At 150 °C, TONs of 840 and 36 were achieved for the formamide synthesis and methanol formation steps, respectively. Authors 31 were able to detect a manganese-formate species as the resting state during the formylation step. The complex when isolated was active for hydrogenation of formamides to methanol only at high H2 pressures. It was proposed that H2 pressure enhances the conversion of the Mn-formate complex to form the catalytically active Mn-hydride species. On the other hand, the presence of CO2 during the reaction favors the formation of the formate complex, which made a one-step CO2-to-methanol system challenging under the reported reaction conditions. Figure 1.23. Sequential amine assisted CO2 to methanol using base metal catalysts. Very recently, a potential route to the one-step CO2-to-methanol reaction with a Mn catalyst was suggested by Leitner et al. (Figure 1.24). 143 To unlock the Mn-formate resting state, authors proposed that in place of an amine, a combination of alcohol and acid additives could detach the formate ligand via esterification and exchange it with the counter anion (from the acid additive). A Lewis acid, Ti(O i Pr)4 in the presence of ethanol, activated the Mn catalyst for a TON of 19 for methanol directly from CO2 and H2 in one step. Additionally, the authors noted that replacing the i Pr substituent on the catalyst (Mn-1) with phenyl (Mn-2) led to increased catalytic activity. Finally, using methanol as the alcohol, 5 bar of 13 CO2 and a significantly high pressure of H2 (160 bar at rt), the reaction afforded a TON of 160 for methanol ( 13 C labelled). 32 Figure 1.24. Mn catalyzed direct hydrogenation of CO2 to methanol. 1.4.2.3 Iron Based Catalysts Following the Mn-PNP catalyzed sequential hydrogenation system, Hazari, Bernskoetter et al. developed an Fe-PNP (Fe-1) based catalysis, proceeding via the formamide intermediate in the presence of an amine additive (Figure 1.23). 144 Similar to the system developed by Prakash and co-workers previously, the catalyst was unable to turnover to produce methanol in the presence of CO2 gas, owing to the formation of the Fe-formate resting state. However, in a two-step manner, notable TON of 590 for methanol was achieved at 100 °C with CO2:H2 (250:1150 psi) and morpholine as a model amine. When the authors employed 3Å molecular sieves during formylation step and LiOTf and DBU in the methanol synthesis step, enhanced productivities were observed. Interestingly, authors observed an increase in product yields for both the steps by lowering the temperature from 120 to 100 °C. Martins, Pombeiro et al. developed another example of iron based homogeneous catalyst for hydrogenation of CO2 to methanol (Figure 1.25). 145 Distinct from the pincer or tripodal phosphine lead complexes documented for such transformations, this system involved a phosphine-free pyrazole-based Fe(II) C-scorpionate complex, Fe-2 [FeCl2(κ 3 -HC(pz)3)] (pz = pyrazol-1-yl). The hydrogenation reaction proceeded efficiently in the presence of an amine such as PEHA or tetramethylguanidine (TMG). However, an amine-free system was also demonstrated 33 using acetonitrile solvent. Methanol was produced at a much lower temperature of 80 °C with a TON of 2335 at 75 bar of CO2/3H2 gas mixture. While the catalytic system presented a novel and highly promising CO2-to-methanol process at low temperatures, there has yet to be any further development using this catalytic system, mostly due to the unusual and difficult to obtain Fe precursor (FeCl2.2H2O) needed to synthesize this complex. Figure 1.25. Fe-scorpionate catalysis for CO2 to methanol. 1.5. Hydrogenation of CO2 Derivatives to Methanol 1.5.1 Hydrogenation of Neutral Carbamic Acid Derivatives Simultaneous to direct hydrogenation of CO2 gas, reduction of various carbonic acid derivatives has also been developed over the years as indirect routes to access methanol from CO2 (Figure 1.26). In 2011, Milstein and co-workers developed the first catalytic systems for efficient hydrogenation of organic carbonates, carbamates, and later, that of urea derivatives under relatively mild homogeneous conditions. 146-147 At 110 °C, the hydrogenation of dimethyl carbonate proceeded effectively to afford TONs as high as 4400, whereas higher catalyst loadings of 1-2 mol% were needed to achieve high conversions for the relatively challenging carbamate (TONmax = 97) and urea derivatives (TONmax = 61). Since this report, this family of catalysts have contributed significantly to the further exploration of homogeneous hydrogenation chemistry, including direct CO2-to-methanol systems, described in previous sections. 34 In a similar approach, Ding et al. demonstrated the hydrogenation of cyclic carbonates to methanol and corresponding diol in 2012. 148 Using Ru-Macho catalysts (Ru-9/Ru-10), the hydrogenation of different cyclic carbonate substrates was achieved up to near quantitative yields and impressive TONs of up to 84,000 for methanol were reported. Later, Leitner, Klankermayer and co-workers used the Ru-triphos system (Ru-4) for methanol synthesis from organic carbonates and urea derivatives. 149 Furthermore, the research groups of Cavallo, El-Sepelgy and Rueping; 150 Leitner; 151 Milstein; 152-153 and Beller 154 have independently reported different manganese and cobalt-based catalytic systems as alternatives to the precious metal-based catalysts, for the hydrogenation of carbonate, carbamate and urea derivatives with notable methanol yields (>95%) in the temperature range of 110-140 °C. Hydrogenation of oxazolidinones, another derivative of CO2 was demonstrated by Milstein (refer to discussion in Section 1.4.1.3). 155 Apart from the widely explored transformations using molecular H2, Hong et al. reported the transfer hydrogenation of cyclic carbonates using Ru-10 and isopropanol (H-source) at 140 °C with up to 99% methanol yield. 156 It is worthwhile to note here that the reductive transformation of the organic derivatives of CO2 is of significant conceptual and practical impact. While molecular catalysts were majorly instrumental in the discovery of these small molecule transformations, the catalytic systems further inspired the development of several hydrogenative depolymerization of polycarbonates, polyurethanes and polyurea derivatives to methanol and the co-monomers, demonstrating a green chemical process for waste upcycling. 8, 148, 150, 157-160 1.5.2 Hydrogenation of CO2 Capture Products In contrast to neutral organic derivatives of carbonic acids, CO2 can also be derivatized as ionic molecules. This is achieved via the reaction between CO2, a weak acid (pKa of 6.37 in H2O) with a basic moiety, most common being ammonia, amines, amino acids and metal hydroxides. 35 These reactions are broadly referred to as CO2 capture, and many of such capture processes are being practiced at industrial scales. 161 When compared to the synthesis of organic carbamates, carbonates and urea derivatives from CO2 which often require high temperatures or high CO2 pressures (or both), the ionic salts of carbonic acids (carbamate/carbonate/bicarbonate) can be readily obtained at ambient pressures and temperatures by exposing CO2 to a basic medium (Figure 1.27). Another major advantage is that many of these basic media are capable of chemically absorbing CO2 from sources as dilute as ambient air (containing about 420 ppm CO2). 162 Direct air capture (DAC) is an emerging field over the past few years and is considered as a prime tool to counter growing anthropogenic CO2 emissions. 163-167 Figure 1.26. Indirect methanol synthesis from various derivatives and capture products of CO2. In the context of chemical recycling of CO2, a convergent system is desired wherein the CO2 capture products are directly upgraded to products with added value, without any purification 36 or separation (Figure 1.28). 27, 168-169 The highlight of such integrated carbon capture and utilization (ICCU) processes is the elimination of the considerable energy and capital inputs for the desorption and compression steps required to extract pure CO2 in conventional processes. 80 Hydrogenation of CO2 capture products has been explored widely for synthesis of formate salts. However, further hydrogenation to methanol is a significant challenge. To note, ICCU to methanol has been developed predominantly as solvent based systems using homogeneous catalysts, in particular Ru- Macho catalysts (Ru-9 and Ru-10), enabling the hydrogenation at relatively mild temperatures. 80, 98, 125 Figure 1.27. CO2 capture with amines and metal hydroxides. The initial work in the field of integrated capture and hydrogenation of methanol was inspired by the amine assisted CO2 hydrogenation systems. Amines are also attractive CO2 capturing agents (Figure 1.27). 170-171 Primary and secondary amine functionalities react with CO2 to form the corresponding ammonium carbamates. In presence of water, they can also form ammonium bicarbonates and carbonates. 172 In 2015, Sanford et al. demonstrated methanol synthesis from dimethylammonium dimethylcarbamate (obtained from reaction between dimethyl amine and CO2) using Ru-Macho-BH (Ru-9) and molecular H2 as a novel approach to obtain methanol with 50% yield and a TON of 306 at 155 °C. 122 Later, Olah, Prakash and co-workers revealed an integrated process using polyamine solutions containing primary and secondary amino 37 groups, into which CO2 was captured to form the corresponding ammonium carbamates as well as ammonium bicarbonates in presence of water. These capture species were effectively hydrogenated in situ to afford high yields of methanol (up to 95%). Further, using a biphasic water/2-MeTHF system, effective recycling of both the catalyst and the amine was demonstrated to achieve cumulative TONs of about 800. 173 Methanol synthesis was also demonstrated by capturing CO2 directly from air, which can be considered as a renewable carbon source. In contrast to other forms of amine, tertiary amines capture CO2 only in presence of water or a protic solvent (Figure 1.27). Prakash et al. recently developed a tertiary amine based integrated system wherein a series of tertiary amines and polyamines in the presence of an alcohol captured CO2 to form ammonium alkyl carbonate and were further hydrogenated to methanol in the presence of H2. 174 While, the amine-alcohol system was inefficient in capturing CO2 from air, it was able to remove CO2 from a simulated flue gas stream (10% CO2 in N2) and the captured species were thereafter converted to methanol. Instead of amines, CO2 can also be captured with alkali metal hydroxide solution to form carbonates, bicarbonates, and alkyl carbonates. 175-177 However, owing to the extremely low electrophilicity of these ions, their hydrogenation to methanol remains a major challenge. 63, 178 In a rare example, Prakash and co-workers recently studied a hydroxide based integrated system in which CO2 captured by alkali metal hydroxides were hydrogenated in situ to methanol in presence of ethylene glycol, which performed a dual role of solvent as well as an alcohol mediator. 179 The hydrogenation was relatively fast with quantitative methanol yields within 20 h and the catalysis was found to be active at temperatures as low as 100 °C. It is important to note here that irrespective of the nature of the captured products (alkali or ammonium based), the corresponding formate salts 38 were found to be a key intermediate in either and undergoing further hydrogenation to afford methanol. Figure 1.28. Integrated carbon capture and conversion to methanol. Based on ref. 179. 1.5.3. Homogeneous Hydrogenation of CO to Methanol Hydrogenation of CO over heterogeneous catalysts is one of the most developed process to produce methanol. 180 However, there has been relatively limited progress in the field of homogeneous catalysis. One of the primary challenges of using soluble metal salts or complexes is the high affinity of CO for metal center as a ligand forming metal carbonyls and possibly inhibiting the catalysis. 99 One of the first reports is by Bradley and co-workers from Exxon in 1979 using soluble Ru(acac)3 in THF, although at extremely high pressure (1300 bar) of CO and H2 and a high temperature of 268 °C. 181 The Ru precursor eventually coordinates with CO in solution to give different ruthenium carbonyl clusters, as detected by the authors. Shortly after, Dombek from Union Carbide Corp. demonstrated a similar approach directly using a Ru-carbonyl complex, Ru3(CO)12 which afforded methanol under 240 bar of H2:CO. 182 Later, a series of studies including those by the groups of Mahajan, Marchionna, Ohyama and Jiang independently presented the use of nickel salts in combination with alkoxide salts to activate CO and H2 to methanol. 183-186 While the operating conditions of 100-120 °C and 20-40 bar were significantly milder as to the preceding reports, the addition or in situ formation of highly toxic and flammable Ni carbonyl complexes posed significant limitation. Alternatively, Cu acetate salts were employed by Li and Jens to 39 catalyze the hydrogenation of CO to methanol, albeit in the presence of stoichiometric alkali metal hydride reagents. 187-188 Figure 1.29. Homogeneous catalytic systems for hydrogenation of CO to methanol. Lately, there has been a resurgence in low temperature CO-to-methanol studies, significantly aided by the development of robust homogeneous catalysts for the similar CO2-to- methanol routes (Figure 1.29). To note, CO is also an intermediate in CO2 hydrogenation and often detected in trace quantities in the homogeneous CO2 hydrogenation systems. With this background, Prakash et al. utilized the amine-assisted route to access CH3OH from CO. 189 Using a high-boiling amine diethylenetriamine (DETA), the reaction proceeded at 140-145 °C in two steps: 1) K3PO4 catalyzed anchoring of CO onto the amine as formamide; and 2) hydrogenation of formamides to CH3OH in the presence of H2 and Ru-PNP catalyst (Ru-9/Ru-10). The reaction steps were efficiently demonstrated as a two-step sequential process as well as a one-pot direct process in a toluene/ethanol solvent system. Using a CO:H2 mix (10:70 bar) and Ru-Macho-BH (Ru-9), a TON of 539 for methanol was achieved over 168 h. 40 Shortly after, Beller, Checinski and co-workers reported a similar methanol synthesis system using Mn-PNP catalyst (Mn-1), K3PO4 and a N-promoter. 190 Among the N-promoters screened, pyrrole-based N-heterocycles were found to be the most promising with TONs of 546 (indole), 3170 (Scatole) and 2550 (pyrrole). The catalytic nature of the amines was also highlighted in the report. While the reaction pathway proceeded via N-formyl intermediates, authors also observed the formation of methyl formate suggesting autocatalysis by the methanol product. In 2021, Leitner et al. also presented a Mn-PNP (Mn-1) catalyzed system via an alternate alcohol assisted pathway. 191 Interestingly, using ethanol under the reaction conditions of CO:H2 (5:50 bar) at 150 °C, authors observed similar activities with Ru-9, Ru-10 and Mn-1 catalysts, although the P-substituents in the PNP ligands were not identical. High TON of >4000 for methanol were achieved upon condition optimizations. Finally, the reaction was also demonstrated as a potential “autocatalytic” system using methanol as the solvent and 13 C labelled CO. 1.6. Mechanistic Insights into Catalysis 1.6.1 Catalysts with Bifunctional PN(H)P Pincer Ligands As shown in the previous sections, molecular catalysts with tridentate pincer ligands having a non-innocent -NH moiety (Noyori-type catalysts) have been employed extensively for the transformation of CO2 as well as its derivatives to methanol. Broadly, two modes of bond activation have been proposed for this group of catalysts. 73 First, they can operate through an amino/amido based metal-ligand cooperation (MLC), for heterolytic H2 activation and hydride/proton transfer to an acceptor molecule (Figure 1.30). This is the primary catalytic pathway assigned to addition of H2 across a carbonyl bond for reduction of CO2 derived molecules including carbamate, urea, carbonate. formate, formamide, formaldehyde. In addition to that, direct activation of CO2 is also possible via a proposed inner-sphere mechanism involving insertion 41 of CO2 to a metal-hydride (M-H) bond as a “2+2 interaction”, resulting in a metal-formate complex. Figure 1.30. Modes of bond activation by metal-PN(H)P complexes. Based on a series of control experiments and trapping of catalytic species, a catalytic cycle was proposed by Prakash et al. (Figure 1.31). 99 wherein; 1) the formation of Ru-dihydride via H2 cleavage is followed by the insertion of CO2 to generate Ru-formate species; 2) the formate is detached from the metal by an amine, as the corresponding ammonium formate and further condensed to formamides; 3) formamide is hydrogenated to hemiaminal via reversible amino- amido cycle based on MLC mechanism; and 4) hemiaminal dissociates to formaldehyde (HCHO), which is reduced to methanol by another amino-amido cycle. As a side reaction, a small amount of HCHO decompose to produce CO. The role of the amine additive was also found to be crucial in the catalysis. In addition to increasing availability of CO2 in solution, amine assists in cleaving the metal-formate bond, a key step to move the catalyst from the resting state. In the absence of an amine, the hydrogenation of CO2 is completely arrested, even to formate. Moreover, amine with an -NH proton is required to afford a formamide, which is necessary for methanol formation. 42 Figure 1.31. Proposed catalytic route with Ru-PN(H)P catalysts for amine assisted CO2 hydrogenation to methanol. Adapted from ref. 80 with permission from the American Chemical Society. Among the PN(H)P catalysts, the Macho type Ru-9 and Ru-10 have been found to be most effective in hydrogenation of CO2 and derived molecules to methanol. Consequently, Prakash et al. carried out an in-depth investigation to gain insights into catalysis of the amine assisted CO2 hydrogenation (Figure 1.32). First, the effect of varying the ligand functionalities in the catalyst was studied. Replacing the CO spectator ligand by an N-heterocyclic carbene moiety resulted in comparable activities. In contrast, as the P-substituents were swapped from Ph (phenyl) to i Pr (iso- propyl), Cy (cyclohexyl) and t Bu (tert-butyl), the CO2 hydrogenation reaction produced only marginal amounts of methanol. This trend in catalyst activity could be attributed to the in situ formation of deactivating Ru-biscarbonyl complexes during the reaction, which were detected and 43 characterized by the authors. The biscarbonyl complex could be converted to the active di-hydride species, the rate of which indirectly correlated to the nature of the P-substituents. In case of electron rich phosphines (iPr, Cy, tBu), the electron density on the metal center was enhanced, which further strengthened the back-bonding between the axial CO and metal. Consequently, the rate of reversal of the Ru-carbonyl species was lower. On the other hand, phenyl as the P-substituent enhances the lability of the CO ligand and hence the reversibility to the catalytically active dihydride species. Hence, Ru-Macho-BH (Ru-9) and Ru-Macho (Ru-10) were the most active catalysts for one-step hydrogenation of CO2 to CH3OH. In order to suppress the formation of dormant metal-formate complex, Bai, Sels et al. suggested the addition of a Lewis acid co-catalyst to accelerate the formate-to-formamide step and increase the amount of active metal-dihydride species. 192 Among various Lewis acid promoters, ZnO was found to enhance both CO2 conversion and methanol yields by additional TONs of about 300 and 100, respectively (Figure 1.33). Operando IR studies showed that the formation of Ru- formate complex gradually disappeared upon addition of ZnO. Similarly, recent efforts on investigating the catalytic species of these PNP complexes and their possible degradation as well as further stabilization have been performed by several groups, including those by Schaub, Keith and Chianese. 193-195 44 Figure 1.32. Mechanistic insights into the role of catalyst molecular structure on methanol formation. Adapted from ref. 80 and 99 with permission from the American Chemical Society. There has also been some progress on the front of computational modelling of the catalytic pathways. 196 Pathak and co-workers presented several DFT studies supporting the previously proposed reaction pathways, for morpholine assisted CO2 to methanol. 197-198 Authors highlighted the high exergonicity of the N-formylation step (-10.4 kcal·mol -1 for morpholine) which enhances the overall reactivity. Two parallel pathways were modelled for formamide to methanol, based on the priority in C=O and C-N bond hydrogenation, although both the routes afforded comparable free energy barrier. The energy barrier for CO2 to HCOOH step was found highest for Mn-PNP, 45 followed by that of Fe and Ru. The energy for hydride transfer also followed the same trend. However, the amide hydrogenation step was energetically similar for the different metal complexes. Further, Yang et al. performed calculations focusing on the role of N-H in the PN(H)P catalysts. 199 Notably, the authors found that the hydrogenation of the carbamic acids with corresponding amines to be more favorable than direct CO2 insertion. Moreover, the proposed reaction sequences in the report departed significantly from the otherwise proposed mechanism. In particular, the N-H functionality was suggested to be partly innocent in the hydrogenation sequence. This argument was also supported in a report by Gordon and co-workers suggesting inner sphere mechanism, although it was shown for ketone hydrogenation. 200 Figure 1.33. Addition of heterogeneous Lewis acid (ZnO) to enhance CO2 hydrogenation to methanol. Adapted from ref. 192 with permission from the American Chemical Society. 1.6.2 Aromatized Metal-PNP Catalysts One of the key classes of hydrogenation catalysts discussed in the previous sections include metal-PNP or PNN complexes involving N-heterocycle derived pincer ligands (Figure 1.34). Such catalysts operate via aromatization-dearomatization based MLC. 201 Heterolytic cleavage of H2 occurs between the side arm C-H proton and the metal center to transform the enamide to the imine form, reversibly and further activating an unsaturated bond. The N-heterocyclic backbone includes those such as pyridine, bipyridine as well as acridine, providing added rigidity to the metal 46 complex. The other ligand functionalities are different innocent ligands including phosphines and tertiary amines. 73 This broad group of catalysts were introduced by Milstein and co-workers for hydrogenation of numerous CO2-derived carbonyl compounds, including organic carbamates, carbonates, urea and formates, as discussed in Section 1.5.1. Authors proposed a general catalytic cycle for the hydrogenation of these molecules (Figure 1.35a). 146 First, the dearomatized complex activates H2 and transforms into the aromatized dihydride form. Subsequently, the hydride is transferred to the carbonyl group of the carbonate/urea/carbamate substrate. This can occur directly via hydride attack on the carbonyl or by coordination of the carbonyl O to the metal center on the site vacated by the pyridyl arm. Next, one of the carbonyl substituents (alkoxy or amino group) leaves with one of the benzylic protons, and results in the dearomatized catalytic form bearing the formate/formamide ligand. Repetitions of a similar cycle leads to the formation of formaldehyde, and finally methanol while regenerating the pre-catalyst. Figure 1.34. Modes of bond activation by aromatized Ru-PNP complexes. Apart from the carbamic acid and formic acid derivatives, these catalysts have been studied for direct CO2 hydrogenation as well. The groups of Sanford 79 and Pidko 202 have independently investigated the activation of CO2 and H2 using Milstein’s pyridine-PNP and PNN catalysts under 47 alkaline conditions, wherein the hydrogenation proceeded up to formate (Figure 1.35b). 203 Later, Saouma et al. performed a detailed thermodynamic analysis of the catalytic process and its relation to the catalyst performance. 204 Calculations showed that the Ru-formate resting state was favored by four orders of magnitude as compared to the active Ru-hydride form. To drive the hydrogenation forward, authors used sub-stoichiometric amount of a strong base such as t BuOK or LiDMC, to produce methanol from dimethylammonium dimethylcarbamate, used as a CO2 surrogate. A maximum turnover of 40 was achieved, and a catalyst deactivation pathway was suggested in presence of methanol and/or water. Among several computational studies relevant to this class of catalysts, 205 Yang presented DFT calculations in support of the proposed mechanism for Ru-PN(Py)N catalyzed dimethyl carbonate hydrogenation to methanol. 206 The essential role of MLC with reversible gain/loss of aromaticity was also highlighted. In a similar study, Wang and co-workers noted that the hydrogenation route through carbonyl insertion into the Ru-H bond is energetically disfavored when comparing to a stepwise hydrogen-transfer route. 207 Another pathway was suggested in the reports by Hasanayn et al. involving direct ion-pair mediated metathesis. 208-209 As per the proposed model, the reaction can proceed via a) hydride transfer from Ru-dihydride to dimethyl carbonate forming an ion-pair of cationic metal complex and a anionic dimethyl orthoformate; b) rearrangement and coordination of a methoxy group to the metal followed by cleavage of the methoxy group from the carbonyl substrate to yield methyl formate and Ru-methoxy catalytic species. 48 Figure 1.35. Catalytic cycle for aromatized Ru-PNP/PNN complexes for hydrogenation of A) carbamic acid derivatives and B) CO2. Adapted from ref. 146 and 79 with permission from the Springer Nature and American Chemical Society. 1.6.3 Metal-Triphos and Related Catalysts While the pincer-based metal catalysts discussed in Sections 1.6.1 and 1.6.2 have been exploited widely owing to their high activities in CO2 hydrogenation reactions, these catalysts predominantly operate under neutral to alkaline conditions. In contrast, the triphos ligand based molecular catalytic system developed by Leitner, Klankermayer and co-workers have been key in allowing CO2 hydrogenation under neutral, and preferably, acidic conditions. 118-120 The tridentate ligands in these catalysts are innocent and are not involved in MLC. Hence, these catalysts are 49 monofunctional and H2 activation and hydride transfer steps takes place on the metal center through an inner sphere mechanism. Leitner, Klankermayer et al. provided a comprehensive mechanistic investigation into the Ru-triphos catalyzed CO2 to methanol system (Figure 1.36). 119 The Ru-formate species with formate acting as a bidentate ligand was identified in solution and assigned to be the catalyst resting state. Since, the formate complex could not be isolated in solid state, the analogous Ru-acetate complex was isolated which was able to catalyze the hydrogenation to produce methanol even in the absence of any acid or alcohol additive (TON = 165). Hence, an additive-free pathway was achieved as an alternate to the route via formate ester, which proceeds via three stages; 1) formate/formic acid, followed by 2) hydroxymethanolate/formaldehyde and ultimately 3) methanolate/methanol species. These transformations were suggested to occur via multiple hydride transfer-protonolysis steps within the coordination sphere of a cationic Ru-triphos complex, enabled by the favorable facial coordination of the tripodal ligand. The proposed catalytic route and the hydride transfers were also supported by DFT calculations by the authors. Further, the barriers for proton transfer steps were significantly lowered in a protic solvent medium. 50 Figure 1.36. Mechanistic insights and catalytic cycle of Ru-triphos based CO2 to methanol. Adapted from ref. 119 with permission from the Royal Society of Chemistry. 1.7. Status Quo As described previously, the current production of methanol is almost exclusively based on syngas conversion. Even so, there has been a paradigm shift over the past decade, towards realizing CO2-to-methanol processes at commercial scales. 29 The first commercial CO2-to- methanol plant was established in 2011, at the George Olah Renewable Methanol Plant in Svartsengi, Iceland by Carbon Recycling International (CRI). 210 The plant holds an annual capacity of recycling 5500 Mt of CO2 which is captured from the flue gases released by an adjacent geothermal powerplant. Parallelly, the required H2 is sourced through water electrolysis using local hydrothermal and geothermal energy sources. The renewable CO2 and H2 are subjected to direct hydrogenation to produce methanol, rather than converting them first to syngas via RWGS. The plant produces around 5 million liters of e-methanol per year, trademarked as Vulcanol. This brand 51 of green methanol cuts CO2 emissions by 90% compared to gasoline or diesel, well to wheel. More recently, the Dalian Institute of Chemical Physics, China developed a demonstration plant for e- methanol with an annual capacity of 1000 tonnes. In addition, several commercial CO2-to- methanol plants are being developed across the globe with capacitates ranging from 8000 to 180,000 tonnes per year, with a projection that e-methanol, especially obtained via CO2 recycling can cater to a substantial fraction of the methanol demand in the foreseeable future. 29 Meanwhile, over the past decade, scientific research in homogeneous catalysis with the goal of developing low temperature processes for methanol synthesis has progressed significantly (Table 1.1). Alcohol assisted pathways operating under mild acidic to neutral conditions and enabled by the cooperation of three catalysts have been identified. The modest TONs achieved in the first report (TON = 21) for this route have been remarkably improved to TONs of up to 100,000 when the catalysts were caged inside MOFs. 116, 133 Similarly, single molecular catalysts decorated with tripodal ligands have not only demonstrated their ability to catalyze alcohol assisted methanol synthesis but have also shown the possibility of an additive free hydrogenation pathway. In addition, reports of this family of catalysts supports a fundamental lesson: rational tweaking of the core ligand framework has considerable potential in enhancing catalytic activity. Overall, continued developments in catalysis for CO2 hydrogenation under acidic condition are crucial in the pursuit of a feasible low temperature CO2-to-methanol process. On the other hand, methanol synthesis under alkaline conditions, especially with amines has gained momentum. To date, these transformations have been mostly enabled by a handful of Ru-PNP and Ru-PNN catalysts. While high TONs and efficiencies have proven the power of these catalysts, development of catalysts for hydrogenation under high alkaline conditions have been a challenging avenue. Furthermore, in the library of 30+ catalysts developed for CO2-to-methanol; examples of earth abundant base-metal 52 catalysts remain extremely limited. Especially, development of Fe based catalyst is worthwhile, as iron is significantly more abundant when compared to Mn and Co. Alongside, a highly active ligand framework will be required to achieve activities comparable to existing Ru based catalysts. In-depth mechanistic investigations are also required in parallel to catalyst design for a broader understanding of this space. Table 1.1. Selected examples of CO2 to methanol processes with molecular catalysts. Entry Catalyst CO2:H2 (bar) [a] T (°C) / Time Solvent / Additive TON / Yield Ref. Precious metal catalysts (Ru, Ir) 1 Ru-1 + Ru-2 + Sc(Otf)3 10:30 75 (1 h)-135 (15 h) methanol, dioxane 21 116 2 Ru-3 + Ir-2 + Sc(Otf)3 10:80 155 (40 h) ethanol 428 117 3 Ru-4/Ru-5 + CH3SO3H/HNTf2 20:60 140 (24 h) ethanol, THF 221 118 4 Ru-4 + HNTf2 20:60 140 (48 h) THF 603 119 53 5 Ru-6 + Al(OTf)3 30:90 120 (20 h) ethanol/THF 2148 120 6 Ru-8 + Al(OTf)3 15:45 [b] 90 (16 h) ethanol, THF 240 121 7 Ru-9 + K3PO4 2.5:50 95 (18 h) – 155 (36 h) NHMe2, THF 550 122 8 Ru-10 + KO t Bu Step 1: 35:35, Step 2: n/a:50 Step 1: 120 (40 h), Step 2: 160 (1 h) morpholine, THF 36% 123 9 Step 1: Cs2CO3, Step 2: Ru-11 + KO t Bu Step 1: 1:n/a, Step 2: n/a:60 Step 1: 150 (24 h), Step 2: 135 (72 h) valinol, DMSO 45% 124 10 Ru-9 19:57 145 (244 h) PEHA, triglyme 9900 125, 99 11 Ru-12/Ru-13 + NaOEt 10:30 180 (2 h) i Pr2NH, toluene 8900 127 12 Ru-15 2.5:50 90 (48 h) - 170 (72 h) Me2NH, i PrOH 2100 128 13 Ru-9 1:3 (80) 150 (20 h) B-PEI10k, toluene 362 129 14 Ru-10 Step1: 20:n/a, Step 2: n/a:50 140 (16 h + 16 h) B-PEI600, PO, THF 82% 131 15 Ru-11@MOF + Ru- 16@MOF-NH3 + 3:37 70 (16 h) trifluoroethanol, M.S. (3 Å) 19000 132, 133 16 Ir-2 20:60 70 (50 h) water, H2SO4 ~8 134 17 Ir-5/ Ir-7 10:30 60 (336 h) n/a 23 136 18 Ru-4 + Zn(NTf2)2 50:n/a 160 (22 h) ethanol 96 137 19 Ru-9 20:60 145 (40 h) THF, silica supported LPEI25k 520 139 Base metal catalysts (Co, Mn, Fe) 20 Co-1 + HNTf2 20:70 100 (96 h) THF, ethanol 78 140 21 Co-3 + HNTf2 20:70 100 (24 h) THF, ethanol 125 141 22 Mn-1 + KO t Bu Step 1: 30:30, Step 2: n/a:80 Step 1: 110 (36 h), Step 2: 150 (36 h) THF, morpholine 36 142 54 23 Mn-2 + Ti(O i Pr)4 5:160 150 (68 h) methanol, 1,4- dioxane 160 143 24 Fe-1 Step 1: 17:78, Step 2: n/a:78 Step 1: 100(16 h), Step 2: 100 (16 h) THF, morpholine 590 144 25 Fe-2 + K3PO4 19:57 80 (24 h) PEHA 2335 145 [a] pressures at room temperature [b] pressures at reaction temperature, PEHA = pentaethylenehexamine, B-PEI = branched polyethylenimine, PO = propylene oxide Reasonably, the industrial methanol synthesis continues to utilize heterogeneous catalysts, which have gained much more maturity as compared to the homogeneous catalysts. To date, production of heterogeneous catalysts is less capital- and labor-intensive. Additionally, they offer simplified operating conditions in gas-solid phase under flow conditions, and ease of separation and recycling, making them more appealing to industrial setups. On the other hand, the field of molecular catalysis for methanol synthesis is still in an early stage of development. This provides a wide scope for identifying limitations in the present systems and proposing possible improvements and directions. (1) A majority of the molecular catalysts developed for CO2-to-methanol are cost intensive as well as challenging to prepare and handle at an industrial scale, when compared to heterogeneous catalysts. Along with the metal component, which is often a precious metal like Ru or Ir, the exotic and specialized ligands contribute substantially to the cost of these catalysts (Table 2). Hence, it is equally important to design ligands which are easy-to-synthesize, scalable, and stable to light, O2 and water; as is to develop catalysts out of earth abundant metals (such as iron, manganese, and cobalt). In this context, replacing the phosphine-based ligands with amine-based alternates is a possible direction, especially given the fact that amines also facilitate CO2 capture 55 and conversion. Together, addressing these issues will potentially lead to the discovery of molecular catalysts which are efficient and sustainable as well as cost-effective and scalable. Table 1.2. Selling prices of selected molecular catalysts and ligands Entry Catalyst Catalyst price (USD) [a] Ligand price (USD) [a] Ligand (wt.%) 1 Milstein’s Ru-PNN (Ru-16) 99 (100 mg) 83 (100 mg) 66 2 Ru-Macho-BH (Ru-9) 53 (250 mg) 50 (500 mg) 75 [a] Prices in the US catalogue of Strem Chemicals. 211 (2) The recycling ability of the catalysts plays a key role in process development as the life cycle of the catalyst affect the cost of the overall process. Hence, studies focused on recycling, long term stability and understanding of possible deactivation pathways of the molecular catalysts are necessary. (3) While impressive TONs have been demonstrated using several catalysts, there is substantial opportunity to improve on the kinetics of the reactions and hence, the TOFs. Similarly, in parallel to using TONs as a standard parameter to compare catalytic systems, it is also important to present the overall CO2 conversions and methanol selectivities, which are often useful industrial standards. (4) A synergistic relationship between the realms of homogeneous and heterogeneous catalysis has been evolving over the past several years. Some properties of a heterogeneous catalyst can be mimicked using molecular catalysis via, for instance, tethering of the catalyst on a solid support or a fluidized bed, encapsulation of the catalyst inside insoluble frameworks, or design of polymeric ligand structures around the metal center. 212-214 It is important that the active sites of the catalyst must remain unperturbed. The MOF-encapsulated catalysis developed by Tsung, Byers and co-workers for CO2 hydrogenation provide a promising example in this direction. 132 56 Conversely, lessons from homogeneous catalytic systems have aided developments in the field of heterogeneous catalysts. For example, the indirect CO2-to-methanol systems demonstrated using molecular catalysts have been successfully translated with heterogeneous catalysts. Recent studies, independently by the groups of Heldebrant and Prakash showed that amine and alcohol mediated processes can enable hydrogenation of CO2 as well as its derivatives to methanol using the commercial Cu/ZnO/Al2O3 catalyst at much lower temperatures (140-200 °C) with up to 90% conversion, which is rarely accomplished under conventional heterogeneous setups. 215-219 Hence, homogeneous catalysis has long-term value as a go-to tool to discover and test novel processes for methanol synthesis rapidly at small scales using easy-to-operate setups. (5) To date, all the homogeneous methanol synthesis investigations have been studied under batch conditions and at extremely small scales. While batch processes are practiced at industrial scale, continuous flow processes offer significant advantages for the massive production scale of methanol. [38] Hence, engineering of flow systems using homogeneous catalysts geared towards a pilot demonstration is desired. One of the plausible ways is to opt for solvent-free conditions as demonstrated by the preliminary report by Onishi and Himeda by converting a mononuclear molecular catalyst to its multinuclear forms. 136 Alternatively, liquid-phase methanol synthesis (LPMEOH) processes can also be operated as a flow process. In fact, such a process was developed by the Air Products and Chemicals in association with the US Department of Energy in a demonstration plant. 220 Overall, such demonstrations will require collaborative efforts which include system modelling, reactor engineering, process intensification, heat and mass transport analysis, techno-economic and life-cycle analysis, among others. (6) Although, methanol synthesis is exothermic per se, the overall processes are significantly energy-intensive to operate. First, energy is required to recycle CO2 from waste 57 sources, and for the production of H2. Next, circulation of these gases under high pressure and heating the reactor to high temperatures require energy inputs. The required energy comes at a considerable cost, both in terms of capital as well as carbon footprint. Hence, there is a paradigm shift towards use of low-carbon and renewable energy for such processes and for the production of H2 via water electrolysis. Nevertheless, enhancing energy efficiency of the process is a long- term goal in the field of methanol synthesis. 1.8. Conclusion and Outlook Within a short span of ten years, homogeneous catalysis has played a dominant role in deciphering novel routes to synthesize methanol from CO2 and in achieving this at temperatures significantly lower than traditional heterogeneous catalysis. This has primarily been enabled by the highly active metal sites of molecular catalysts with rigid spatial and electronic environments leading to high activities and selectivities. Under solvent assisted conditions, key alternate routes have been realized using alcohol and amine additives which offer energetically favorable pathways for CO2 to methanol by forming adducts with the formate intermediate. These include early examples of Milstein’s pioneering homogeneous catalysts for hydrogenation of various CO2 derivatives, Sanford’s amine and alcohol assisted routes and several other homogeneous systems that have shown excellent activities in the mild temperature range of 100-140 °C. A few catalysts have shown turnovers even at temperatures below 100 °C and as low as 60-70°C. The challenging additive-free CO2 to methanol reaction has been achieved in rare cases (Leitner, Klankermayer et al.) and with water as a medium (Himeda et al.), in contrast to more commonly used organic solvents. These initial studies have been followed by extensive mechanistic investigations and DFT modelling to help understand the catalytic routes involved in the hydrogenation processes and have further led to rational tuning of the molecular catalysts. In recent years, integrated CO2 58 capture and utilization have been realized using homogeneous catalysts. Prakash et al. have demonstrated several amine assisted capture and conversion systems to recycle CO2 from air into carbon-neutral methanol. The first example of a hydroxide based integrated system was also realized recently by the same group. In the broad context, homogeneous catalysis has led to key breakthroughs in green and energy efficient transformations including that of CO2 and CO to methanol; and will continue to play a vital role in realizing sustainable and green processes in a carbon neutral framework (Figure 1.37). Figure 1.37. Anthropogenic carbon cycle in the context of a circular methanol economy. Reproduced from ref. 43 with permission from Springer Nature. 59 1.9. References [1] A. Y. Hoekstra, T. O. Wiedmann, Humanity’s unsustainable environmental footprint. Science 2014, 344, 1114-1117. [2] H. D. Matthews, N. P. Gillett, P. A. Stott, K. Zickfeld, The proportionality of global warming to cumulative carbon emissions. Nature 2009, 459, 829-832. [3] R. Rehan, M. Nehdi, Carbon dioxide emissions and climate change: policy implications for the cement industry. Environ. Sci. Policy 2005, 8, 105-114. [4] J. Hansen, M. Sato, R. Ruedy, K. Lo, D. W. Lea, M. Medina-Elizade, Global temperature change. Proc. Natl. Acad. Sci. U S A 2006, 103, 14288-14293. [5] N. S. Diffenbaugh, D. Singh, J. S. Mankin, D. E. Horton, D. L. Swain, D. Touma, A. Charland, Y. Liu, M. Haugen, M. Tsiang, B. Rajaratnam, Quantifying the influence of global warming on unprecedented extreme climate events. Proc. Natl. Acad. Sci. U S A 2017, 114, 4881-4886. [6] J. H. A. Krug, Accounting of GHG emissions and removals from forest management: a long road from Kyoto to Paris. Carbon Balance Manage. 2018, 13, 1. [7] G. A. Olah, G. K. S. Prakash, A. Goeppert, Anthropogenic chemical carbon cycle for a sustainable future. J. Am. Chem. Soc. 2011, 133, 12881-12898. [8] A. Kumar, C. Gao, Homogeneous (De)hydrogenative Catalysis for Circular Chemistry – Using Waste as a Resource. ChemCatChem 2021, 13, 1105-1134. [9] R. Meys, A. Kätelhön, M. Bachmann, B. Winter, C. Zibunas, S. Suh, A. Bardow, Achieving net-zero greenhouse gas emission plastics by a circular carbon economy. Science 2021, 374, 71- 76. [10] C. B. Field, V. R. Barros, Intergovernmental Panel on Climate Change. Working Group II, Climate change 2014 : Impacts, adaptation, and vulnerability : Working Group II contribution to the fifth assessment report of the Intergovernmental Panel on Climate Change, Cambridge University Press, New York, NY, 2014. [11] A. Katelhon, R. Meys, S. Deutz, S. Suh, A. Bardow, Climate change mitigation potential of carbon capture and utilization in the chemical industry. Proc. Natl. Acad. Sci. U S A 2019, 116, 11187-11194. [12] B. Obama, Science 2017, The irreversible momentum of clean energy. 355, 126-129. 60 [13] A. Kumar, P. Daw, D. Milstein, Homogeneous Catalysis for Sustainable Energy: Hydrogen and Methanol Economies, Fuels from Biomass, and Related Topics. Chem. Rev. 2022, 122, 385- 441. [14] T. Keijer, V. Bakker, J. C. Slootweg, Circular chemistry to enable a circular economy. Nat. Chem. 2019, 11, 190-195. [15] A. D. Kreuder, T. House-Knight, J. Whitford, E. Ponnusamy, P. Miller, N. Jesse, R. Rodenborn, S. Sayag, M. Gebel, I. Aped, I. Sharfstein, E. Manaster, I. Ergaz, A. Harris, L. N. Grice,Grice, A Method for Assessing Greener Alternatives between Chemical Products Following the 12 Principles of Green Chemistry. ACS Sustain. Chem. Eng. 2017, 5(4), 2927- 2935. [16] R. J. Giraud, P. A. Williams, A. Sehgal, E. Ponnusamy, A. K. Phillips, J. B. Manley, Implementing Green Chemistry in Chemical Manufacturing: A Survey Report. ACS Sustain. Chem. Eng. 2014, 2(10), 2237-2242. [17] V. Dieterich, A. Buttler, A. Hanel, H. Spliethoff, S. Fendt, Power-to-liquid via synthesis of methanol, DME or Fischer–Tropsch-fuels: a review. Energy Environ. Sci. 2020, 13, 3207-3252. [18] K. M. K. Yu, I. Curcic, J. Gabriel, S. C. E. Tsang, Recent Advances in CO2 Capture and Utilization. ChemSusChem 2008, 1, 893-899. [19] E. A. Quadrelli, G. Centi, Green Carbon Dioxide. ChemSusChem 2011, 4, 1179-1181. [20] E. A. Quadrelli, 25 years of energy and green chemistry: saving, storing, distributing and using energy responsibly. Green Chem. 2016, 18, 328-330. [21] B. Lee, H. Lee, D. Lim, B. Brigljevic, W. Cho, H. S. Cho, C. H. Kim, H. Lim,Lim, Renewable methanol synthesis from renewable H-2 and captured CO2: How can power-to-liquid technology be economically feasible? Appl. Energy 2020, 279, 115827. [22] C. Chen, A. D. Yang, Power-to-methanol: The role of process flexibility in the integration of variable renewable energy into chemical production. Energy Convers. Manag. 2021, 228, 113673. [23] G. A. Olah, A. Goeppert, G. K. S. Prakash, Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons. J. Org. Chem. 2009, 74, 487-498. [24] W. M. Verdegaal, S. Becker, C. von Olshausen, Power-to-Liquids: Synthetic Crude Oil from CO2, Water, and Sunshine. Chem. Ing. Tech. 2015, 87(4), 340-346. 61 [25] P. Schmidt, W. Weindorf, A. Roth, V. Batteiger, F. Riegel, Power-to-Liquids Potentials and Prespectives for the Future Supply of Renewable Aviation Fuel, German Environmental Agency, can be found under https://www.umweltbundesamt.de/sites/default/files/medien/377/publikationen/161005_uba_hi ntergrund_ptl_barrierrefrei.pdf, 2016. [26] Q. Liu, L. Wu, R. Jackstell, M. Beller, Using carbon dioxide as a building block in organic synthesis. Nat. Commun. 2015, 6, 5933. [27] J. Leclaire, D. J. Heldebrant, A call to (green) arms: a rallying cry for green chemistry and engineering for CO2 capture, utilisation and storage. Green Chem. 2018, 20, 5058-5081. [28] T. Vass, P. Levi, A. Gouy, H. Mandová, Chemicals, International Energy Agency, can be found under https://www.iea.org/reports/chemicals, 2021. [29] S. Kang, F. Boshell, A. Goeppert, G. K. S. Prakash, I. Landälv, Innovation Outlook. Renewable Methanol, IRENA and Methanol Institute, can be found under https://www.irena.org/publications/2021/Jan/Innovation-Outlook-Renewable-Methanol, 2021. [30] A. Goeppert, G. A. Olah, G. K. S. Prakash, in Green Chemistry (Eds.: B. Török, T. Dransfield), Elsevier, 2018, pp. 919-962. [31] G. A. Olah, Beyond Oil and Gas: The Methanol Economy. Angew. Chem. Int. Ed. 2005, 44, 2636-2639. [32] G. A. Olah, A. Goeppert, G. K. S. Prakash, Beyond Oil and Gas: The Methanol Economy, 3rd ed., Wiley-VCH, 2018. [33] G. A. Olah, Towards Oil Independence Through Renewable Methanol Chemistry. Angew. Chem. Int. Ed. 2013, 52, 104-107. [34] J. Artz, T. E. Müller, K. Thenert, J. Kleinekorte, R. Meys, A. Sternberg, A. Bardow, W. Leitner, Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment. Chem. Rev. 2017, 118, 434-504. [35] D. W. Stephan, Catalysis: A step closer to a methanol economy. Nature 2013, 495, 54-55. [36] G. Centi, E. A. Quadrelli, S. Perathoner, Catalysis for CO2 conversion: a key technology for rapid introduction of renewable energy in the value chain of chemical industries. Energy Environ. Sci. 2013, 6, 1711-1731. [37] M. Nielsen, in Hydrogen Production and Remediation of Carbon and Pollutants (Eds.: E. Lichtfouse, J. Schwarzbauer, D. Robert), Springer, 2015, pp. 1-60. 62 [38] V. Sivakumar, R. A. Watile, T. J. Colacot, Organometallics in Process Chemistry: An Historical Snapshot. Top. Organometal. Chem. 2019, 65, 1-29. [39] M. A. Stoffels, F. J. R. Klauck, T. Hamadi, F. Glorius, J. Leker, Technology Trends of Catalysts in Hydrogenation Reactions: A Patent Landscape Analysis. Adv. Synth. Catal. 2020, 362, 1258-1274. [40] M. Bertau, H. Offermanns, L. Plass, F. Schmidt, H.-J. r. Wernicke, H., Methanol: The Basic Chemical and Energy Feedstock of the Future. Asinger's Vision Today, Springer-Verlag GmbH, 2016. [41] MMSA (Methanol Market Services Asia), Methanol Institute, can be found under https://www.methanol.org/methanol-price-supply-demand, 2022. [42] P. Tian, Y. Wei, M. Ye, Z. Liu, Methanol to Olefins (MTO): From Fundamentals to Commercialization. ACS Catal. 2015, 5, 1922-1938. [43] J. Kothandaraman, S. Kar, A. Goeppert, R. Sen, G. K. S. Prakash, Top. Catal.Advances in Homogeneous Catalysis for Low Temperature Methanol Reforming in the Context of the Methanol Economy. Top Catal 2018, 61, 542-559. [44] P. Preuster, C. Papp, P. Wasserscheid, Liquid Organic Hydrogen Carriers (LOHCs): Toward a Hydrogen-free Hydrogen Economy. Acc. Chem. Res. 2016, 50, 74-85. [45] M. Nielsen, E. Alberico, W. Baumann, H.-J. Drexler, H. Junge, S. Gladiali, M. Beller, Low- temperature aqueous-phase methanol dehydrogenation to hydrogen and carbon dioxide. Nature 2013, 495, 85-89. [46] V. Yadav, G. Sivakumar, V. Gupta, E. Balaraman, Recent Advances in Liquid Organic Hydrogen Carriers: An Alcohol-Based Hydrogen Economy. ACS Catal. 2021, 11, 14712-14726. [47] D. R. Palo, R. A. Dagle, J. D. Holladay, Methanol Steam Reforming for Hydrogen Production. Chem. Rev. 2007, 107, 3992-4021. [48] R. E. Rodríguez-Lugo, M. Trincado, M. Vogt, F. Tewes, G. Santiso-Quinones, H. Grützmacher, A homogeneous transition metal complex for clean hydrogen production from methanol–water mixtures. Nat. Chem. 2013, 5, 342-347. [49] P. Hu, Y. Diskin-Posner, Y. Ben-David, D. Milstein, Reusable Homogeneous Catalytic System for Hydrogen Production from Methanol and Water. ACS Catal. 2014, 4, 2649-2652. [50] Methanol Institute: The Methanol Industry, https://www.methanol.org/the-methanol- industry/. 63 [51] W. C. Liu, J. Baek, G. A. Somorjai, The Methanol Economy: Methane and Carbon Dioxide Conversion. Top. Catal. 2018, 61, 530-541. [52] G. A. Olah, G. K. S. Prakash, US Pat., 7,906,559, 2011. [53] G. A. Olah, A. Goeppert, M. Czaun, G. K. S. Prakash, Bi-reforming of Methane from Any Source with Steam and Carbon Dioxide Exclusively to Metgas (CO-2H2) for Methanol and Hydrocarbon Synthesis. J. Am. Chem. Soc. 2013, 135(2), 648-650. [54] G. A. Olah, A. Goeppert, M. Czaun, T. Mathew, R. B. May, G. K. S. Prakash, Single Step Bi- reforming and Oxidative Bi-reforming of Methane (Natural Gas) with Steam and Carbon Dioxide to Metgas (CO-2H2) for Methanol Synthesis: Self-Sufficient Effective and Exclusive Oxygenation of Methane to Methanol with Oxygen. J. Am. Chem. Soc. 2015, 137, 8720-8729. [55] A. Goeppert, M. Czaun, J. P. Jones, G. K. S. Prakash, G. A. Olah, Recycling of carbon dioxide to methanol and derived products - closing the loop. Chem. Soc. Rev. 2014, 43, 7995-8048. [56] S.-T. Bai, G. De Smet, Y. Liao, R. Sun, C. Zhou, M. Beller, B. U. W. Maes, B. F. Sels,Sels, Homogeneous and heterogeneous catalysts for hydrogenation of CO2 to methanol under mild conditions. Chem. Soc. Rev. 2021, 50, 4259-4298. [57] P. Gautam, Neha, S. N. Upadhyay, S. K. Dubey, Bio-methanol as a renewable fuel from waste biomass: Current trends and future perspective. Fuel 2020, 273, 117783. [58] M. Pehl, A. Arvesen, F. Humpenoder, A. Popp, E. G. Hertwich, G. Luderer, Understanding future emissions from low-carbon power systems by integration of life-cycle assessment and integrated energy modelling. Nat. Energy 2017, 2, 939-945. [59] W. Leitner, E. A. Quadrelli, R. Schlögl, Harvesting renewable energy with chemistry. Green Chem. 2017, 19, 2307-2308. [60] K. Beydoun, J. Klankermayer, in Organometallics for Green Catalysis (Eds.: P. Dixneuf, J.- F. Soulé), Springer, 2019 pp. 39-76. [61] E. A. Quadrelli, G. Centi, J.-L. Duplan, S. Perathoner, Carbon Dioxide Recycling: Emerging Large-Scale Technologies with Industrial Potential. ChemSusChem 2011, 4, 1194-1215. [62] P. Styring, E. A. Quadrelli, K. Armstrong, Carbon Dioxide Utilisation: Closing the Carbon Cycle, Elsevier, 2015. [63] J. Pelley, One-pot process converts CO2 from air into methanol. Chem. Eng. News 2020, 98(10), 9. 64 [64] S. Mbatha, R. C. Everson, N. M. Musyoka, H. W. Langmi, A. Lanzini, W. Brilman, Power- to-methanol process: a review of electrolysis, methanol catalysts, kinetics, reactor designs and modelling, process integration, optimisation, and techno-economics. Sustain. Energy Fuels 2021, 5, 3490-3569. [65] M. Behrens, F. Studt, I. Kasatkin, S. Kühl, M. Hävecker, F. Abild-Pedersen, S. Zander, F. Girgsdies, P. Kurr, B.-L. Kniep, M. Tovar, R. W. Fischer, J. K. Nørskov, R. Schlögl, The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3 Industrial Catalysts. Science 2012, 336, 893-897. [66] E. Kunkes, M. Behrens, in Chemical Energy Storage (Ed.: S. Robert), De Gruyter, 2012, pp. 413-442. [67] S. Dang, H. Yang, P. Gao, H. Wang, X. Li, W. Wei, Y. Sun, A review of research progress on heterogeneous catalysts for methanol synthesis from carbon dioxide hydrogenation. Catal. Today 2019, 330, 61-75. [68] X. Jiang, X. Nie, X. Guo, C. Song, J. G. Chen, Recent Advances in Carbon Dioxide Hydrogenation to Methanol via Heterogeneous Catalysis. Chem. Rev. 2020, 120, 7984-8034. [69] K.-i. Tominaga, Y. Sasaki, M. Kawai, T. Watanabe, M. Saito, Ruthenium complex catalysed hydrogenation of carbon dioxide to carbon monoxide, methanol and methane. J. Chem. Soc. 1993, 629-631. [70] T. Ken-ichi, S. Yoshiyuki, W. Taiki, S. Masahiro, Homogeneous Hydrogenation of Carbon Dioxide to Methanol Catalyzed by Ruthenium Cluster Anions in the Presence of Halide Anions. Bull. Chem. Soc. Jpn. 1995, 68, 2837-2842. [71] N. Onishi, Y. Himeda, CO2 hydrogenation to methanol by organometallic catalysts. Chem Catal. 2022, 2, 242-252. [72] S. Kar, J. Kothandaraman, A. Goeppert, G. K. S. Prakash, Advances in catalytic homogeneous hydrogenation of carbon dioxide to methanol. J. CO2 Util. 2018, 23, 212-218. [73] C. Gunanathan, D. Milstein, Bond Activation and Catalysis by Ruthenium Pincer Complexes. Chem. Rev. 2014, 114, 12024-12087. [74] K. Natte, H. Neumann, M. Beller, R. V. Jagadeesh, Transition-Metal-Catalyzed Utilization of Methanol as a C1 Source in Organic Synthesis. Angew. Chem. Int. Ed. 2017, 56, 6384-6394. [75] J. R. Cabrero-Antonino, R. Adam, V. Papa, M. Beller, Homogeneous and heterogeneous catalytic reduction of amides and related compounds using molecular hydrogen. Nat. Commun. 2020, 11, 3893. 65 [76] M. F. Hertrich, M. Beller, in Organometallics for Green Catalysis, (Eds.: P. Dixneuf, J.-F. Soulé), Springer, 2019, pp. 1-16. [77] J. Pritchard, G. A. Filonenko, R. van Putten, E. J. M. Hensen, E. A. Pidko, Heterogeneous and homogeneous catalysis for the hydrogenation of carboxylic acid derivatives: history, advances and future directions. Chem. Soc. Rev. 2015, 44, 3808-3833. [78] P. G. Jessop, T. Ikariya, R. Noyori, Homogeneous Hydrogenation of Carbon Dioxide. Chem. Rev. 1995, 95, 259-272. [79] C. A. Huff, M. S. Sanford, Catalytic CO2 Hydrogenation to Formate by a Ruthenium Pincer Complex. ACS Catal. 2013, 3, 2412-2416. [80] S. Kar, A. Goeppert, G. K. S. Prakash, Integrated CO2 Capture and Conversion to Formate and Methanol: Connecting Two Threads. Acc. Chem. Res. 2019, 52, 2892-2903. [81] F. Zhu, L. Zhu-Ge, G. Yang, S. Zhou, Iron-catalyzed hydrogenation of bicarbonates and carbon dioxide to formates. ChemSusChem 2015, 8, 609-612. [82] Y. Zhang, A. D. MacIntosh, J. L. Wong, E. A. Bielinski, P. G. Williard, B. Q. Mercado, N. Hazari, W. H. Bernskoetter, Iron catalyzed CO2 hydrogenation to formate enhanced by Lewis acid co-catalysts. Chem.Chem. Sci. 2015, 6, 4291-4299. [83] J. Kothandaraman, A. Goeppert, M. Czaun, G. A. Olah, G. K. S. Prakash, CO2 capture by amines in aqueous media and its subsequent conversion to formate with reusable ruthenium and iron catalysts. Green Chem. 2016, 18, 5831-5838. [84] Z. J. Dai, Q. Luo, H. J. Cong, J. Zhang, T. Y. Peng, New Ru(II) N ' NN '-type pincer complexes: synthesis, characterization and the catalytic hydrogenation of CO2 or bicarbonates to formate salts. New J. Chem. 2017, 41, 3055-3060. [85] S. Coufourier, S. Gaillard, G. Clet, C. Serre, M. Daturi, J.-L. Renaud, A MOF-assisted phosphine free bifunctional iron complex for the hydrogenation of carbon dioxide, sodium bicarbonate and carbonate to formate. Chem. Commun. 2019, 55, 4977-4980. [86] J. Klankermayer, S. Wesselbaum, K. Beydoun, W. Leitner, Selective Catalytic Synthesis Using the Combination of Carbon Dioxide and Hydrogen: Catalytic Chess at the Interface of Energy and Chemistry. Angew. Chem. Int. Ed. 2016, 55, 7296-7343. [87] E. A. Bielinski, M. Förster, Y. Zhang, W. H. Bernskoetter, N. Hazari, M. C. Holthausen, Base-Free Methanol Dehydrogenation Using a Pincer-Supported Iron Compound and Lewis Acid Co-catalyst. ACS Catal. 2015, 5, 2404-2415. 66 [88] Z. Shao, Y. Li, C. Liu, W. Ai, S.-P. Luo, Q. Liu, Reversible interconversion between methanol-diamine and diamide for hydrogen storage based on manganese catalyzed (de)hydrogenation. Nat. Commun. 2020, 11, 591. [89] L. Piccirilli, D. Lobo Justo Pinheiro, M. Nielsen, Recent Progress with Pincer Transition Metal Catalysts for Sustainability. Catalysts 2020, 10, 773. [90] J. Kothandaraman, S. Kar, R. Sen, A. Goeppert, G. A. Olah, G. K. S. Prakash, Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol. J. Am. Chem. Soc. 2017, 139, 2549-2552. [91] Y. Xie, P. Hu, Y. Ben-David, D. Milstein, A Reversible Liquid Organic Hydrogen Carrier System Based on Methanol-Ethylenediamine and Ethylene Urea. Angew. Chem. Int. Ed. 2019, 58, 5105-5109. [92] S. Kar, A. Goeppert, R. Sen, J. Kothandaraman, G. K. S. Prakash, Regioselective deuteration of alcohols in D2O catalysed by homogeneous manganese and iron pincer complexes. Green Chem. 2018, 20, 2706-2710. [93] P. Hu, Y. Ben-David, D. Milstein, Rechargeable Hydrogen Storage System Based on the Dehydrogenative Coupling of Ethylenediamine with Ethanol. Angew. Chem. Int. Ed. 2016, 55, 1061-1064. [94] V. Cherepakhin, T. J. Williams, Iridium Catalysts for Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Scope and Mechanism. ACS Catal. 2018, 8, 3754-3763. [95] W. H. Bernskoetter, N. Hazari, Reversible Hydrogenation of Carbon Dioxide to Formic Acid and Methanol: Lewis Acid Enhancement of Base Metal Catalysts. Acc. Chem. Res. 2017, 50, 1049-1058. [96] E. Balaraman, E. Khaskin, G. Leitus, D. Milstein, Catalytic transformation of alcohols to carboxylic acid salts and H2 using water as the oxygen atom source. Nat. Chem. 2013, 5, 122. [97] E. Alberico, M. Nielsen, Towards a methanol economy based on homogeneous catalysis: methanol to H2 and CO2 to methanol. Chem. Commun. 2015, 51, 6714-6725. [98] R. Sen, A. Goeppert, G. K. S. Prakash, Carbon Dioxide Capture and Recycling to Methanol: Building a Carbon–Neutral Methanol Economy. Aldrichimica Acta 2020, 53, 39-56. [99] S. Kar, R. Sen, J. Kothandaraman, A. Goeppert, R. Chowdhury, S. B. Munoz, R. Haiges, G. K. S. Prakash, J.Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2019, 141, 3160-3170. 67 [100] S. Kar, A. Goeppert, G. K. S Prakash, in CO2 Hydrogenation Catalysis (Ed: Y. Himeda), Wiley-VCH, 2021, pp. 89-112. [101] D. Wei, R. Sang, A. Moazezbarabadi, H. Junge, M. Beller, Homogeneous Carbon Capture and Catalytic Hydrogenation: Toward a Chemical Hydrogen Battery System. JACS Au, 2022, 2, 1020-1031. [102] W.-H. Wang, Y. Himeda, J. T. Muckerman, G. F. Manbeck, E. Fujita, CO2 Hydrogenation to Formate and Methanol as an Alternative to Photo- and Electrochemical CO2 Reduction. Chem. Rev. 2015, 115, 12936-12973. [103] J. Wu, Y. Huang, W. Ye, Y. Li, CO2 Reduction: From the Electrochemical to Photochemical Approach. Adv. Sci. 2017, 4, 1700194. [104] S. N. Riduan, Y. Zhang, J. Y. Ying, Conversion of Carbon Dioxide into Methanol with Silanes over N-Heterocyclic Carbene Catalysts. Angew. Chem. Int. Ed. 2009, 48, 3322-3325. [105] P. K. Giesbrecht, D. E. Herbert, Electrochemical Reduction of Carbon Dioxide to Methanol in the Presence of Benzannulated Dihydropyridine Additives. ACS Energy Lett. 2017, 2, 549- 555. [106] S. Ohya, S. Kaneco, H. Katsumata, T. Suzuki, K. Ohta, Electrochemical reduction of CO2 in methanol with aid of CuO and Cu2O. Catal. Today 2009, 148, 329-334. [107] R. Obert, B. C. Dave, Enzymatic Conversion of Carbon Dioxide to Methanol: Enhanced Methanol Production in Silica Sol−Gel Matrices. J. Am. Chem. Soc. 1999, 121, 12192-12193. [108] M.-A. Courtemanche, M.-A. Légaré, L. Maron, F.-G. Fontaine, A Highly Active Phosphine– Borane Organocatalyst for the Reduction of CO2 to Methanol Using Hydroboranes. J. Am. Chem. Soc. 2013, 135, 9326-9329. [109] T. Kobayashi, H. Takahashi, Novel CO2 Electrochemical Reduction to Methanol for H2 Storage. Energy Fuels 2004, 18, 285-286. [110] B. El-Zahab, D. Donnelly, P. Wang, Particle-tethered NADH for production of methanol from CO2 catalyzed by coimmobilized enzymes. Biotechnol. Bioeng. 2008, 99, 508-514. [111] A. Yutaka, W. Tomoe, Photochemical and Enzymatic Synthesis of Methanol from HCO3 − with Dehydrogenases and Zinc Porphyrin. Chem. Lett. 2004, 33, 1544-1545. [112] D. J. Boston, C. Xu, D. W. Armstrong, F. M. MacDonnell, Photochemical Reduction of Carbon Dioxide to Methanol and Formate in a Homogeneous System with Pyridinium Catalysts. J. Am. Chem. Soc. 2013, 135, 16252-16255. 68 [113] S. Kuwabata, K. Nishida, R. Tsuda, H. Inoue, H. Yoneyama, Photochemical Reduction of Carbon Dioxide to Methanol Using ZnS Microcrystallite as a Photocatalyst in the Presence of Methanol Dehydrogenase. J. Electrochem. Soc. 1994, 141, 1498-1503. [114] S. C. Sau, R. Bhattacharjee, P. K. Vardhanapu, G. Vijaykumar, A. Datta, S. K. Mandal, Metal-Free Reduction of CO2 to Methoxyborane under Ambient Conditions through Borondiformate Formation. Angew. Chem. Int. Ed. 2016, 55, 15147-15151. [115] S. A. Rawool, R. Belgamwar, R. Jana, A. Maity, A. Bhumla, N. Yigit, A. Datta, G. Rupprechter, V. Polshettiwar, Direct CO2 capture and conversion to fuels on magnesium nanoparticles under ambient conditions simply using water. Chem. Sci. 2021, 12, 5774-5786. [116] C. A. Huff, M. S. Sanford, Cascade catalysis for the homogeneous hydrogenation of CO2 to methanol. J. Am. Chem. Soc. 2011, 133, 18122-18125. [117] W.-Y. Chu, Z. Culakova, B. T. Wang, K. I. Goldberg, Acid-Assisted Hydrogenation of CO2 to Methanol in a Homogeneous Catalytic Cascade System. ACS Catal. 2019, 9, 9317-9326. [118] S. Wesselbaum, T. vom Stein, J. Klankermayer, W. Leitner, Hydrogenation of Carbon Dioxide to Methanol by Using a Homogeneous Ruthenium–Phosphine Catalyst. Angew. Chem. 2012, 51, 7499-7502. [119] S. Wesselbaum, V. Moha, M. Meuresch, S. Brosinski, K. M. Thenert, J. Kothe, T. v. Stein, U. Englert, M. Hölscher, J. Klankermayer, W. Leitner, Hydrogenation of carbon dioxide to methanol using a homogeneous ruthenium–Triphos catalyst: from mechanistic investigations to multiphase catalysis. Leitner, Chem. Sci. 2015, 6, 693-704. [120] B. G. Schieweck, P. Jürling-Will, J. Klankermayer, Structurally Versatile Ligand System for the Ruthenium Catalyzed One-Pot Hydrogenation of CO2 to Methanol. ACS Catal. 2020, 10, 3890-3894. [121] J. D. Erickson, A. Z. Preston, J. C. Linehan, E. S. Wiedner, Enhanced Hydrogenation of Carbon Dioxide to Methanol by a Ruthenium Complex with a Charged Outer-Coordination Sphere. ACS Catal. 2020, 10, 7419-7423. [122] N. M. Rezayee, C. A. Huff, M. S. Sanford, Tandem Amine and Ruthenium-Catalyzed Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2015, 137, 1028-1031. [123] L. Zhang, Z. Han, X. Zhao, Z. Wang, K. Ding, Highly Efficient Ruthenium-Catalyzed N- Formylation of Amines with H2 and CO2. Angew. Chem. Int. Ed. 2015, 54, 6186-6189. [124] J. R. Khusnutdinova, J. A. Garg, D. Milstein, Combining Low-Pressure CO2 Capture and Hydrogenation To Form Methanol. ACS Catal. 2015, 5, 2416-2422. 69 [125] J. Kothandaraman, A. Goeppert, M. Czaun, G. A. Olah, G. K. S. Prakash, Conversion of CO2 from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst. J. Am. Chem. Soc. 2016, 138, 778-781. [126] J. M. Hanusch, I. P. Kerschgens, F. Huber, M. Neuburger, K. Gademann, Pyrrolizidines for direct air capture and CO2 conversion. Chem. Commun. 2019, 55, 949-952. [127] M. Everett, D. F. Wass, Highly productive CO2 hydrogenation to methanol – a tandem catalytic approach via amide intermediates. Chem. Commun. 2017, 53, 9502-9504. [128] F.-H. Zhang, C. Liu, W. Li, G.-L. Tian, J.-H. Xie, Q.-L. Zhou, An Efficient Ruthenium Catalyst Bearing Tetradentate Ligand for Hydrogenations of Carbon Dioxide. Chin. J. Chem. 2018, 36, 1000-1002. [129] A. Yoshimura, R. Watari, S. Kuwata, Y. Kayaki, Poly(ethyleneimine)-Mediated Consecutive Hydrogenation of Carbon Dioxide to Methanol with Ru Catalysts. Eur. J. Inorg. Chem. 2019, 2375-2380. [130] R. Sen, A. Goeppert, G. K. S. Prakash, Integrated carbon capture and utilization to methanol with epoxide-functionalized polyamines under homogeneous catalytic conditions. J. Organomet. Chem. 2022, 965-966, 122331. [131] J. Kothandaraman, D. J. Heldebrant, Catalytic coproduction of methanol and glycol in one pot from epoxide, CO2, and H2. RSC Adv. 2020, 10, 42557-42563. [132] T. M. Rayder, E. H. Adillon, J. A. Byers, C.-K. Tsung, A Bioinspired Multicomponent Catalytic System for Converting Carbon Dioxide into Methanol Autocatalytically. Chem 2020, 6, 1742-1754. [133] T. M. Rayder, A. T. Bensalah, B. Li, J. A. Byers, C.-K. Tsung, J.Engineering Second Sphere Interactions in a Host–Guest Multicomponent Catalyst System for the Hydrogenation of Carbon Dioxide to Methanol. J. Am. Chem. Soc. 2021, 143, 1630-1640. [134] E. Alberico, T. Leischner, H. Junge, A. Kammer, R. Sang, J. Seifert, W. Baumann, A. Spannenberg, K. Junge, M. Beller, HCOOH disproportionation to MeOH promoted by molybdenum PNP complexes. Chem. Sci. 2021, 12, 13101-13119. [135] K. Sordakis, A. Tsurusaki, M. Iguchi, H. Kawanami, Y. Himeda, G. Laurenczy, Carbon Dioxide to Methanol: The Aqueous Catalytic Way at Room Temperature. Chem. Eur. J. 2016, 22, 15605-15608. 70 [136] R. Kanega, N. Onishi, S. Tanaka, H. Kishimoto, Y. Himeda, Catalytic Hydrogenation of CO2 to Methanol Using Multinuclear Iridium Complexes in a Gas–Solid Phase Reaction. J. Am. Chem. Soc. 2021, 143, 1570-1576. [137] N. Westhues, J. Klankermayer, Transfer Hydrogenation of Carbon Dioxide to Methanol Using a Molecular Ruthenium‐Phosphine Catalyst. ChemCatChem 2019, 11, 3371-3375. [138] A. Goeppert, H. Zhang, R. Sen, H. Dang, G. K. S. Prakash, Oxidation-Resistant, Cost- Effective Epoxide-Modified Polyamine Adsorbents for CO2 Capture from Various Sources Including Air. ChemSusChem 2019, 12, 1712-1723. [139] S. Kar, A. Goeppert, G. K. S. Prakash, Combined CO2 Capture and Hydrogenation to Methanol: Amine Immobilization Enables Easy Recycling of Active Elements. ChemSusChem 2019, 12, 3172-3177. [140] J. Schneidewind, R. Adam, W. Baumann, R. Jackstell, M. Beller, Low-Temperature Hydrogenation of Carbon Dioxide to Methanol with a Homogeneous Cobalt Catalyst. Angew. Chem. Int. Ed. 2017, 56, 1890-1893. [141] F. K. Scharnagl, M. F. Hertrich, G. Neitzel, R. Jackstell, M. Beller, Homogeneous Catalytic Hydrogenation of CO2 to Methanol – Improvements with Tailored Ligands. Adv. Synth. Catal. 2019, 361, 374-379. [142] S. Kar, A. Goeppert, J. Kothandaraman, G. K. S. Prakash, Manganese-Catalyzed Sequential Hydrogenation of CO2 to Methanol via Formamide. ACS Catal. 2017, 7, 6347-6351. [143] D. A. Kuß, M. Hölscher, W. Leitner, Hydrogenation of CO2 to Methanol with Mn-PNP- Pincer Complexes in the Presence of Lewis Acids: the Formate Resting State Unleashed. ChemCatChem 2021, 13, 3319-3323. [144] E. M. Lane, Y. Zhang, N. Hazari, W. H. Bernskoetter, Sequential Hydrogenation of CO2 to Methanol Using a Pincer Iron Catalyst. Organometallics 2019, 38, 3084-3091. [145] A. P. C. Ribeiro, L. M. D. R. S. Martins, A. J. L. Pombeiro, Carbon dioxide-to-methanol single-pot conversion using a C-scorpionate iron(ii) catalyst. Green Chem. 2017, 19, 4811-4815. [146] E. Balaraman, C. Gunanathan, J. Zhang, L. J. W. Shimon, D. Milstein, Efficient hydrogenation of organic carbonates, carbamates and formates indicates alternative routes to methanol based on CO2 and CO. Nat. Chem. 2011, 3, 609. [147] E. Balaraman, Y. Ben-David, D. Milstein, Unprecedented Catalytic Hydrogenation of Urea Derivatives to Amines and Methanol. Angew. Chem. Int. Ed. 2011, 50, 11702-11705. 71 [148] Z. Han, L. Rong, J. Wu, L. Zhang, Z. Wang, K. Ding, Catalytic Hydrogenation of Cyclic Carbonates: A Practical Approach from CO2 and Epoxides to Methanol and Diols. Angew. Chem. Int. Ed. 2012, 51, 13041-13045. [149] T. vom Stein, M. Meuresch, D. Limper, M. Schmitz, M. Hölscher, J. Coetzee, D. J. Cole- Hamilton, J. Klankermayer, W. Leitner, J.Leitner, Highly Versatile Catalytic Hydrogenation of Carboxylic and Carbonic Acid Derivatives using a Ru-Triphos Complex: Molecular Control over Selectivity and Substrate Scope. J. Am. Chem. Soc. 2014, 136, 13217-13225. [150] V. Zubar, Y. Lebedev, L. M. Azofra, L. Cavallo, O. El-Sepelgy, M. Rueping, Hydrogenation of CO2-Derived Carbonates and Polycarbonates to Methanol and Diols by Metal–Ligand Cooperative Manganese Catalysis. Angew. Chem. Int. Ed. 2018, 57, 13439-13443. [151] A. Kaithal, M. Hölscher, W. Leitner, Catalytic Hydrogenation of Cyclic Carbonates using Manganese Complexes. Angew. Chem. Int. Ed. 2018, 130, 13637-13641. [152] U. K. Das, A. Kumar, Y. Ben-David, M. A. Iron, D. Milstein, Manganese Catalyzed Hydrogenation of Carbamates and Urea Derivatives. J. Am. Chem. Soc. 2019, 141, 12962-12966. [153] A. Kumar, T. Janes, N. A. Espinosa-Jalapa, D. Milstein, Manganese Catalyzed Hydrogenation of Organic Carbonates to Methanol and Alcohols. Angew. Chem. Int. Ed. 2018, 57, 12076-12080. [154] F. Ferretti, F. K. Scharnagl, A. Dall'Anese, R. Jackstell, S. Dastgir, M. Beller, Additive-free cobalt-catalysed hydrogenation of carbonates to methanol and alcohols. Catal. Sci. Technol. 2019, 9, 3548-3553. [155] R. Langer, Y. Diskin-Posner, G. Leitus, L. J. Shimon, Y. Ben-David, D. Milstein, Low- pressure hydrogenation of carbon dioxide catalyzed by an iron pincer complex exhibiting noble metal activity. Angew. Chem. Int. Ed. 2011, 50, 9948-9952. [156] S. H. Kim, S. H. Hong, Transfer Hydrogenation of Organic Formates and Cyclic Carbonates: An Alternative Route to Methanol from Carbon Dioxide. ACS Catal. 2014, 4, 3630-3636. [157] E. M. Krall, T. W. Klein, R. J. Andersen, A. J. Nett, R. W. Glasgow, D. S. Reader, B. C. Dauphinais, S. P. Mc Ilrath, A. A. Fischer, M. J. Carney, D. J. Hudson, N. J. Robertson, Robertson, Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(ii) PNN pincer complexes. Chem. Commun. 2014, 50, 4884-4887. [158] S. Westhues, J. Idel, J. Klankermayer, Molecular catalyst systems as key enablers for tailored polyesters and polycarbonate recycling concepts. Sci. Adv. 2018, 4, eaat9669. 72 [159] W. Zhou, P. Neumann, M. Al Batal, F. Rominger, A. S. K. Hashmi, T. Schaub, Depolymerization of Technical-Grade Polyamide 66 and Polyurethane Materials through Hydrogenation. ChemSusChem 2021, 14, 4176-4180. [160] A. Kumar, J. Luk, Catalytic Hydrogenation of Urea Derivatives and Polyureas. Eur. J. Org. Chem. 2021, 2021, 4546-4550. [161] A. S. Bhown, B. C. Freeman, Analysis and status of post-combustion carbon dioxide capture technologies. Environ. Sci. Technol. 2011, 45, 8624-8632. [162] K. S. Lackner, Capture of carbon dioxide from ambient air. Eur. Phys. J. Spec. Top. 2009, 176, 93-106. [163] A. Goeppert, M. Czaun, G. K. S. Prakash, G. A. Olah, Air as the renewable carbon source of the future: an overview of CO2 capture from the atmosphere. Energy Environ. Sci. 2012, 5, 7833-7853. [164] Carbon Engineering Ltd., https://carbonengineering.com. [165] Climeworks, http://www.climeworks.com/. [166] E. S. Sanz-Pérez, C. R. Murdock, S. A. Didas, C. W. Jones, Direct Capture of CO2 from Ambient Air. Chem. Rev. 2016, 116, 11840-11876. [167] D. W. Keith, G. Holmes, D. S. Angelo, K. Heidel, A Process for Capturing CO2 from the Atmosphere. Joule 2018, 2, 1573-1594. [168] W. D. Jones, Carbon Capture and Conversion. J. Am. Chem. Soc. 2020, 142, 4955-4957. [169] H.-C. Fu, F. You, H.-R. Li, L.-N. He, CO2 Capture and in situ Catalytic Transformation. Front. Chem. 2019, 7. [170] T. Wang, J. Hovland, K. J. Jens, Amine reclaiming technologies in post-combustion carbon dioxide capture. J. Environ. Sci. 2015, 27, 276-289. [171] A. Goeppert, M. Czaun, R. B. May, G. K. S. Prakash, G. A. Olah, S. R. Narayanan, Carbon Dioxide Capture from the Air Using a Polyamine Based Regenerable Solid Adsorbent. J. Am. Chem. Soc. 2011, 133, 20164-20167. [172] D. J. Heldebrant, J. Kothandaraman, in Carbon Capture and Storage (Eds.: M. Bui, N. Mac Dowell), The Royal Society of Chemistry, 2020, pp. 36-68. 73 [173] S. Kar, R. Sen, A. Goeppert, G. K. S. Prakash, Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy. J. Am. Chem. Soc. 2018, 140, 1580-1583. [174] R. Sen, C. J. Koch, A. Goeppert, G. K. S. Prakash, Tertiary Amine-Ethylene Glycol Based Tandem CO2 Capture and Hydrogenation to Methanol: Direct Utilization of Post-Combustion CO2. ChemSusChem 2020, 13, 6318-6322. [175] J. K. Stolaroff, D. W. Keith, G. V. Lowry, Carbon Dioxide Capture from Atmospheric Air Using Sodium Hydroxide Spray. Environ. Sci. Technol. 2008, 42, 2728-2735. [176] S.-J. Han, J.-H. Wee, Carbon Dioxide Fixation by Combined Method of Physical Absorption and Carbonation in NaOH-Dissolved Methanol. Energy Fuels 2017, 31, 1747-1755. [177] S.-J. Han, J.-H. Wee, Carbon Dioxide Fixation via the Synthesis of Sodium Ethyl Carbonate in NaOH-Dissolved Ethanol. Ind. Eng. Chem. Res. 2016, 55, 12111-12118. [178] S. Kar, A. Goeppert, V. Galvan, R. Chowdhury, J. Olah, G. K. S. Prakash, A Carbon-Neutral CO2 Capture, Conversion, and Utilization Cycle with Low-Temperature Regeneration of Sodium Hydroxide. J. Am. Chem. Soc. 2018, 140, 16873-16876. [179] R. Sen, A. Goeppert, S. Kar, G. K. S. Prakash, Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol. J. Am. Chem. Soc. 2020, 142, 4544-4549. [180] G. A. Somorjai, The Catalytic Hydrogenation of Carbon Monoxide. The Formation of C1Hydrocarbons. Catal. Rev. Sci. Eng. 2006, 23, 189-202. [181] J. S. Bradley, Homogeneous carbon monoxide hydrogenation to methanol catalyzed by soluble ruthenium complexes. J. Am. Chem. Soc. 1979, 101, 7419-7421. [182] B. D. Dombek, Hydrogenation of carbon monoxide to methanol and ethylene glycol by homogeneous ruthenium catalysts. J. Am. Chem. Soc. 1980, 102, 6855-6857. [183] D. Mahajan, Atom-economical reduction of carbon monoxide to methanol catalyzed by soluble transition metal complexes at low temperatures. Top. Catal. 2005, 32, 209-214. [184] D. Mahajan, V. Krisdhasima, R. D. Sproull, Kinetic modeling of homogeneous methanol synthesis catalyzed by base-promoted nickel complexes. Can. J. Chem. 2001, 79, 848-853. [185] S. Ohyama, Low-temperature methanol synthesis in catalytic systems composed of nickel compounds and alkali alkoxides in liquid phases. Appl. Catal. A: Gen. 1999, 180, 217-225. 74 [186] M. Marchionna, L. Basini, A. Aragno, M. Lami, F. Ancillotti, Mechanistic studies on the homogeneous nickel-catalyzed low temperature methanol synthesis. J. Mol. Catal. 1992, 75, 147-151. [187] B. Li, K. J. Jens, Liquid-Phase Low-Temperature and Low-Pressure Methanol Synthesis Catalyzed by a Raney Copper-Alkoxide System. Top. Catal. 2013, 56, 725-729. [188] B. Li, K.-J. Jens, Low-Temperature and Low-Pressure Methanol Synthesis in the Liquid Phase Catalyzed by Copper Alkoxide Systems. Ind. Eng. Chem. Res. 2013, 53, 1735-1740. [189] S. Kar, A. Goeppert, G. K. S. Prakash, Catalytic Homogeneous Hydrogenation of CO to Methanol via Formamide. J. Am. Chem. Soc. 2019, 141, 12518-12521. [190] P. Ryabchuk, K. Stier, K. Junge, M. P. Checinski, M. Beller, Molecularly Defined Manganese Catalyst for Low-Temperature Hydrogenation of Carbon Monoxide to Methanol. J. Am. Chem. Soc. 2019, 141, 16923-16929. [191] A. Kaithal, C. Werlé, W. Leitner, Alcohol-Assisted Hydrogenation of Carbon Monoxide to Methanol Using Molecular Manganese Catalysts. JACS Au 2021, 1, 130-136. [192] S.-T. Bai, C. Zhou, X. Wu, R. Sun, B. Sels, Suppressing Dormant Ru States in the Presence of Conventional Metal Oxides Promotes the Ru-MACHO-BH-Catalyzed Integration of CO2 Capture and Hydrogenation to Methanol. ACS Catal. 2021, 11, 12682-12691. [193] T. He, J. C. Buttner, E. F. Reynolds, J. Pham, J. C. Malek, J. M. Keith, A. R. Chianese, Dehydroalkylative Activation of CNN- and PNN-Pincer Ruthenium Catalysts for Ester Hydrogenation. J. Am. Chem. Soc. 2019, 141, 17404-17413. [194] V. Avasare, S. Virani, D. Das, S. Pal, Computational Exploration of the Efficacy of Fe(II)PNN Versus Fe(II)NNN Pincer Complexes in the Hydrogenation of Carbon Dioxide to Methanol. J. Phys. Chem. C 2021, 125, 24350-24362. [195] A. Anaby, M. Schelwies, J. Schwaben, F. Rominger, A. S. K. Hashmi, T. Schaub, Study of Precatalyst Degradation Leading to the Discovery of a New Ru 0 Precatalyst for Hydrogenation and Dehydrogenation. Organometallics 2018, 37, 2193-2201. [196] Y. Lu, Z. Liu, J. Guo, S. Qu, R. Zhao, Z.-X. Wang, A DFT study unveils the secret of how H2 is activated in the N-formylation of amines with CO2 and H2 catalyzed by Ru(II) pincer complexes in the absence of exogenous additives. Chem. Commun. 2017, 53, 12148-12151. [197] S. C. Mandal, B. Pathak, Identifying the preferential pathways of CO2 capture and hydrogenation to methanol over an Mn(I)-PNP catalyst: a computational study. Dalton Trans. 2021, 50, 9598-9609. 75 [198] S. C. Mandal, K. S. Rawat, S. Nandi, B. Pathak, Theoretical insights into CO2 hydrogenation to methanol by a Mn–PNP complex. Catal. Sci. Technol. 2019, 9, 1867-1878. [199] J. Yang, A. J. Pell, N. Hedin, A. Lyubartsev, Computational insight into the hydrogenation of CO2 and carbamic acids to methanol by a ruthenium(II)-based catalyst: The role of amino (NH) ligand group. Mol. Catal. 2021, 506, 111544. [200] P. A. Dub, B. L. Scott, J. C. Gordon, Why Does Alkylation of the N–H Functionality within M/NH Bifunctional Noyori-Type Catalysts Lead to Turnover? J. Am. Chem. Soc. 2017, 139, 1245-1260. [201] T. Shimbayashi, K.-i. Fujita, Recent Advances in Homogeneous Catalysis via Metal–Ligand Cooperation Involving Aromatization and Dearomatization. Catalysts 2020, 10, 635. [202] G. A. Filonenko, E. J. M. Hensen, E. A. Pidko, Mechanism of CO2 hydrogenation to formates by homogeneous Ru-PNP pincer catalyst: from a theoretical description to performance optimization. Catal. Sci. Technol. 2014, 4, 3474-3485. [203] G. A. Filonenko, M. P. Conley, C. Copéret, M. Lutz, E. J. M. Hensen, E. A. Pidko, The impact of Metal–Ligand Cooperation in Hydrogenation of Carbon Dioxide Catalyzed by Ruthenium PNP Pincer. ACS Catal. 2013, 3, 2522-2526. [204] C. L. Mathis, J. Geary, Y. Ardon, M. S. Reese, M. A. Philliber, R. T. VanderLinden, C. T. Saouma, J.Thermodynamic Analysis of Metal–Ligand Cooperativity of PNP Ru Complexes: Implications for CO2 Hydrogenation to Methanol and Catalyst Inhibition. J. Am. Chem. Soc. 2019, 141, 14317-14328. [205] J. Pham, C. E. Jarczyk, E. F. Reynolds, S. E. Kelly, T. Kim, T. He, J. M. Keith, A. R. Chianese, The key role of the latent N–H group in Milstein's catalyst for ester hydrogenation. Chem. Sci. 2021, 12, 8477-8492. [206] X. Yang, Metal Hydride and Ligand Proton Transfer Mechanism for the Hydrogenation of Dimethyl Carbonate to Methanol Catalyzed by a Pincer Ruthenium Complex. ACS Catal. 2012, 2, 964-970. [207] H. Li, M. Wen, Z.-X. Wang, Computational Mechanistic Study of the Hydrogenation of Carbonate to Methanol Catalyzed by the Ru II PNN Complex. Inorg. Chem. 2012, 51, 5716-5727. [208] F. Hasanayn, A. Baroudi, Direct H/OR and OR/OR' Metathesis Pathways in Ester Hydrogenation and Transesterification by Milstein's Catalyst. Organometallics 2013, 32, 2493- 2496. 76 [209] F. Hasanayn, A. Baroudi, A. A. Bengali, A. S. Goldman, Hydrogenation of Dimethyl Carbonate to Methanol by trans-[Ru(H)2(PNN)(CO)] Catalysts: DFT Evidence for Ion-Pair- Mediated Metathesis Paths for C-OMe Bond Cleavage. Organometallics 2013, 32, 6969-6985. [210] Carbon Recycling International, http://www.carbonrecycling.is/george-olah/ [211] Strem Chemicals: Product Catalog, https://www.strem.com/catalog/. [212] S. Padmanaban, G. H. Gunasekar, S. Yoon, Direct Heterogenization of the Ru-Macho Catalyst for the Chemoselective Hydrogenation of α,β-Unsaturated Carbonyl Compounds. Inorg. Chem. 2021, 60, 6881-6888. [213] W. Gao, S. Liang, R. Wang, Q. Jiang, Y. Zhang, Q. Zheng, B. Xie, C. Y. Toe, X. Zhu, J. Wang, L. Huang, Y. Gao, Z. Wang, C. Jo, Q. Wang, L. Wang, Y. Liu, B. Louis, J. Scott, A.-C. Roger, R. Amal, H. He, S.-E. Park, Industrial carbon dioxide capture and utilization: state of the art and future challenges. Chem. Soc. Rev. 2020, 49, 8584-8686. [214] M. Haumann, P. Wasserscheid, in Applied Homogeneous Catalysis with Organometallic Compounds (Eds.: B. Cornils, W. A. Herrmann, M. Beller, R. Paciello), Wiley-VCH, 2017, pp. 999-1068. [215] A. Álvarez, A. Bansode, A. Urakawa, A. V. Bavykina, T. A. Wezendonk, M. Makkee, J. Gascon, F. Kapteijn, Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes. Chem. Rev. 2017, 117, 9804-9838. [216] J. Kothandaraman, R. A. Dagle, V. L. Dagle, S. D. Davidson, E. D. Walter, S. D. Burton, D. W. Hoyt, D. J. Heldebrant, Condensed-phase low temperature heterogeneous hydrogenation of CO2 to methanol. Catal. Sci. Technol. 2018, 8, 5098-5103. [217] S. Xie, W. Zhang, C. Jia, S. S. G. Ong, C. Zhang, S. Zhang, H. Lin,Lin, Eliminating carbon dioxide emissions at the source by the integration of carbon dioxide capture and utilization over noble metals in the liquid phase. J. Catal. 2020, 389, 247-258. [218] R. Sen, C. J. Koch, V. Galvan, N. Entesari, A. Goeppert, G. K. S. Prakash, Glycol assisted efficient conversion of CO2 captured from air to methanol with a heterogeneous Cu/ZnO/Al2O3 catalyst. J. CO2 Util. 2021, 54, 101762. [219] J. Kothandaraman, D. J. Heldebrant, Towards environmentally benign capture and conversion: heterogeneous metal catalyzed CO2 hydrogenation in CO2 capture solvents. Green Chem. 2020, 22, 828-834. 77 [220] E. C. Heydorn, B. W. Diamond, R. D. Lilly, Commercial-Scale Demonstration of the Liquid Phase Methanol (LPMEOH TM ) Process, can be found under https://www.osti.gov/servlets/purl/823132, 2003. 78 Chapter 2. Integrated Carbon Capture and Conversion to Methanol with Epoxide-functionalized Polyamines under Homogeneous Catalytic Conditions This dissertation chapter is based on a recent research article from our group published in the Journal of Organometallic Chemistry (Sen, R.; Goeppert, A.; Prakash, G. K. S., J. Organomet. Chem. 2022, 965-966, 122331). Part of the article is reprinted by permission of Elsevier. Copyright 2022. Elsevier. 2.1. Introduction With the objectives of achieving carbon neutrality and a circular economy, recent years have witnessed the emergence of CO2 capture and recycling or utilization models (CCR or CCU), wherein CO2 released from various anthropogenic sources is captured and directed downstream as a C-1 building block for the production of a multitude of value-added chemicals and materials. 1-3 Distinct from the more conventional carbon capture and storage (CCS) technologies, CCU processes can collectively address the issues and challenges relevant to increasing CO2 emissions as well as offer renewable carbon sources for fuels and feedstocks with low-carbon footprints. 4-6 Among the various chemicals accessible from CO2, methanol (CH3OH) is one of the most attractive products. 7 It is the simplest alcohol, a convenient one-carbon liquid at room temperature that is easy to store, transport and dispense. It is a prominent building block to synthesize various commodity chemicals and materials such as acetic acid, formaldehyde, methyl-tert-butyl ether (MTBE), various polymers and pharmaceuticals. Industrially, methanol can also be catalytically 79 converted to a variety of hydrocarbons and olefins such as ethylene, propylene, gasoline etc. via the methanol-to-olefin (MTO) and methanol-to-gasoline (MTG) processes. 8 Methanol from CO2 is an excellent green substitute as a direct drop-in fuel (M100) and as an additive to gasoline (M15, M85). In essence, most of the hydrocarbon-based chemicals and fuels presently obtained in an unsustainable manner can be derived from methanol. The “methanol economy” when coupled with renewable methanol synthesis through CO2 recycling, has the power to pave the way for a sustainable and fossil-independent future. 9 Methanol is already produced at a scale of over 100 million MT per year to supply the vast and rapidly growing global market. 5 At present, methanol is majorly derived from fossil fuel resources (natural gas and coal) via an industrial catalytic process through syngas which is mainly comprised of CO and H2. However, novel routes to access methanol from renewable carbon sources including biomass, biogas and carbon dioxide are desired. Particularly, methanol can be synthesized from CO2 in the presence of high pressures of H2 and copper based heterogeneous catalysts at high temperatures of 250 to 300 °C, although with a relatively low conversion of about 30% per cycle due to intrinsic thermodynamic limitations of the reaction. 10-11 Over the past decade, there has been growing interest in the search for alternate low temperature CO2 to methanol processes, with notable advances in hydrogenation systems enabled by highly efficient and selective molecular catalysts. 12-14 Operating under mild homogeneous conditions, the catalysis has been shown to proceed in the presence of alcohol or amine additives, as well as in some cases under additive free conditions. 15-19 80 Figure 2.1. Amine based CO2 capture and hydrogenation. Significant advances have been achieved in the field of amine-assisted routes for CO2 to methanol (Figure 2.1) initiated by seminal studies including those by the groups of Milstein, Sanford, Ding and Olah/Prakash. 20-30 Operating under basic conditions, the catalytic hydrogenation proceeds through key intermediates with the corresponding amine: ammonium carbamates/bicarbonates, ammonium formates and formamides (Figure 2.1B). In addition, amine- based adsorbents/absorbents are also being widely deployed for state-of-the-art CO2 capture technologies, hence making such systems potentially useful in developing CCU processes. 31-34 In this direction, our group as well as others have demonstrated initial examples of an integrated capture and conversion strategy. 25, 35-43 The advantage of a one-pot integrated CCU process is that it essentially eliminates the desorption and compression steps. These two steps are otherwise necessary to strip off the absorbed CO2 and store it under pressure for further use in conventional non-integrated processes. Hence, the proposed process not only combines two processes, but is also step-economical. There are limited examples of amine assisted CO2 capture and hydrogenation to methanol to date. While these systems have contributed to the fundamental understanding of the catalytic pathways, the amines used in such studies have relatively low boiling points and significantly high vapor pressures. 40 Hence, they often suffer from volatility issues as well as degradation over several cycles under the reaction conditions. 44 As part of our enduring research to develop stable, H 2 O H O R 2 NH 2 CO 2 -H 2 O R 2 N H O 2H 2 CH 3 OH A. CO 2 capture using amines B. Amine assisted CO 2 hydrogenation to CH 3 OH O NR 2 O R 2 NH 2 CO 2 H 2 O 2 R 2 NH O OH O R 2 NH 2 - R 2 NH R 2 NH - R 2 NH 81 efficient and cost-effective amine based systems, we report herein, a focused array of easy-to- prepare epoxide-functionalized polyamines and demonstrate their utility in an integrated CO2 capture and hydrogenation system to synthesize CH3OH. The catalysis is enabled by highly efficient and commercially available ruthenium based PNP pincer catalysts under relatively mild conditions of 140-145 °C. 2.2. Reaction Optimization and Investigations on CO2 Capture and Catalytic Conversion with Epoxide-Functionalized Amines Amine based CO2 scrubbing technologies are well established and commonly practiced on a large scale to strip CO2 from effluent gas streams in industrial processes. 45 In a typical “capture” reaction of CO2 with amines, primary and secondary amine functionalities can form the corresponding ammonium carbamate ion pairs with CO2 with a maximum amine efficiency (moles of CO2 absorbed per mole of amine moiety) of 50% (Figure 2.1A). Introducing water either as a solvent or humidity in the gas stream can enhance the CO2 capture efficiency through the formation of the corresponding ammonium bicarbonate with a 1:1 stoichiometric ratio between amine and CO2. In contrast to primary and secondary amines, tertiary amino groups lacking -NH proton only react with CO2 in the presence of water. The use of amines with high boiling points, low volatility and rich in amine content is highly desired. In this regard, branched and linear polyethylenimines (B/L-PEI) have been the focus of numerous CO2 capture studies. 46-48 While such polyamines are commercially available in bulk, their complex mode of production leads to significantly high prices and hence, restricting the scale-up of PEI based technologies. 49 Additionally, in our previous study, we found that PEIs were inefficient for methanol synthesis in an integrated CCU system. 41 As an alternative, shorter polyamine chains such as pentaethylenehexamine (PEHA) or tetraethylenepentamine (TEPA) that are produced at much lower bulk price have been shown to 82 be efficient in both CO2 capture as well as CO2 hydrogenation processes. 25, 50-51 However, even these high boiling amines pose major challenges in terms of volatility, atmospheric contamination as well as degradation under thermal and oxidative conditions. 52-53 Recently, Goeppert, Prakash et al. developed solid supported adsorbents using PEHA while effectively addressing these drawbacks. 54 In that study, amines were treated with epoxides such as propylene oxide (PO) and butylene oxide (BO) increasing their molecular weight and sharply decreasing their vapor pressure. The epoxide-functionalized amines demonstrated high capture efficiencies and regenerability. Most importantly, relative to their unmodified equivalents these materials displayed remarkable stability even under harsh operating conditions with negligible loss of capture efficiencies over multiple adsorption-desorption cycles. However, application of epoxide-modified polyamines in liquid phase for CO2 capture as well as for CO2 hydrogenation has yet to be explored. In the present study, we selected PEHA as a model polyamine with high content of primary and secondary amino groups. The modification of PEHA with various epoxides was carried out by a simple and scalable process at room temperature in aqueous phase. 55 Water is the ideal reaction solvent, being environmentally benign and economic. The selected epoxides are liquids at room temperature and can be handled conveniently. In general, ring opening reaction of the oxirane with amine leads to formation of the corresponding β-amino-alcohol species. Hence, the modification of PEHA with an epoxide essentially converts a fraction of the amino groups to the corresponding amino-alcohols (Figure 2.2). For a molar ratio of 1:1 for PEHA and epoxide, a statistical distribution of products is obtained with a major probability of reaction products with 1:1 PEHA/epoxide (a mixture of regio-isomers), although minor fractions with multiple epoxide functionalization may also be present. 54 Further, the content of secondary and tertiary amino groups in PEHA is increased as a result of functionalization. 83 Figure 2.2. Functionalization of PEHA with various epoxides. For CO2 capture, a polar protic solvent with high solubility for the amines as well as the capture products is desired. Hence, water was selected as the capture solvent in the present study, also owing to its key role in enhancing the CO2 capture efficiency for amines, as stated vide supra. When an aqueous solution of PEHA-1PO (reaction product of PEHA with one equivalent propylene oxide) was subjected to CO2 at ambient temperature and atmospheric pressure, 3.3 mmol of CO2 was captured per mol of PEHA-1PO with a high amine efficiency of 0.55 CO2/N (Table 2.1, entry 2). 13 C NMR of the CO2 loaded amine solution showed the presence of carbamate and bicarbonate ion pairs supporting our observations from previous studies with aqueous amines. 41 The activity of PEHA-1PO was comparable to that of unmodified PEHA in water (CO2/N = 0.58, entry 1). With PEHA-2PO, the capture efficiencies decreased slightly to 3.1 mmol of CO2 (CO2/N = 0.52, entry 3), which can be accounted for by the increased bulkiness of the polyamine as well as higher content of tertiary amino groups, which have kinetically lower efficiencies for CO2 capture. Apart from PO, the effect of other epoxides was explored for PEHA functionalization. PEHA-1BO (BO = 1,2 butylene oxide) and PEHA-1TFPO (TFPO = 3,3,3- trifuoropropylene oxide) both exhibited CO2 capture efficiencies of 0.50 CO2/N, possibly due to minor stereoelectronic and steric effects of the substituents (entries 4-5). However, in case of H 2 N H N N H H N N H NH 2 O R H 2 N H N N H H N N H H N H 2 N H N N H H N N NH 2 R OH NH 2 H N N H N R OH R OH N H H 2 N O CH 3 O CH 2 CH 3 O CH 2 Cl O CF 3 stirred at rt 24 h pentaethylenehexamine (PEHA) Epoxides 1,2-Propylene oxide (PO) 1,2-Butylene oxide (BO) 3,3,3-Trifluoropropylene oxide (TFPO) 3-Chloropropylene oxide (CPO) possible product distribution of PEHA-1PO 84 PEHA-1CPO (CPO = 3-chloropropyleneoxide or epichlorohydrin), a drastic decrease in the CO2 capture activity was observed (CO2/N = 0.37, entry 6). Table 2.1. CO2 capture by aqueous solutions of epoxide-functionalized PEHA. Entry [a] Amine CO2 captured (mmol) [b] CO2/N [c] 1 PEHA 3.5 0.58 2 PEHA-1PO 3.3 0.55 3 PEHA-2PO 3.1 0.52 4 PEHA-1TFPO 3.0 0.50 5 PEHA-1BO 3.0 0.50 6 PEHA-1CPO 2.2 0.37 [a] Capture conditions: PEHA or PEHA-epoxide (1.0 mmol), water (5 mL), stirring (800 rpm.), rt, t = 1 h. [b] Captured CO2 amounts determined gravimetrically. [c] moles of CO2 captured per mole of nitrogen. Calculation error ±5%. To integrate CO2 capture and its subsequent conversion, the CO2 loaded amine solutions were directly subjected to hydrogenation in the presence of a Ru based molecular catalyst and H2 gas. An efficient protocol for reduction of CO2 capture species with amines to methanol was developed by our group previously. 25,41 It is important to note here that while water is an ideal solvent for the CO2 capture step, the Ru-complexes used in the catalysis step have minimal solubility in water. Hence, a compatible co-solvent was introduced to form a homogeneous reaction system. The molecular catalysts selected in the present study have been extensively employed for reduction of carbonyl compounds and more recently, for CO2 to CH3OH catalytic systems. 13, 56-60 Using THF as the co-solvent and Ru-Macho-BH (C-1) as the catalyst, the aqueous solution of CO2 loaded PEHA (3.6 mmol CO2) was subjected to hydrogenation at 140 °C under 70 bar of H2 (Table 2.2, entry 1). After 20 h, 1.1 mmol CH3OH was obtained corresponding to 31% yield with respect to the captured CO2. When CO2 captured in aqueous PEHA-1PO solution was 85 subjected to a similar reaction condition, the methanol yield increased significantly to 47% (entry 2). Increasing the volume of THF had no distinct effect on the methanol production (entry 3). With ethylene glycol as a co-solvent, methanol yield decreased to 28% (entry 4). After the reaction completion, the catalyst was found to be only partly soluble in the highly polar water-ethylene glycol solvent system. When the co-solvent was switched to triglyme, a high boiling and green ethereal solvent, methanol yield improved markedly to 59% in 20 h (entry 5). Similar to C-1, Ru- Macho (C-2) was comparably active for hydrogenation with a 56% yield for CH3OH (entry 6). However, a negligible CH3OH yield of 9% was obtained with C-3 as the catalyst (entry 7). The observed trend in the catalytic activities is in strong correlation with our recent insights into the mechanism of amine assisted CO2-to-CH3OH in which the P-substituent in the PNP ligand of the catalyst was found to influence the CH3OH yield significantly. 27 Finally, with a slight manipulation of the reaction parameters to 145 °C and 80 bar of H2, the methanol yield could be enhanced to 72% in 20 h and further to a high yield of 91% within 48 h (entries 8-9). Apart from CH3OH, formate (HCOO - ) and trace formamide (N–CHO) intermediates were also accumulated during the hydrogenation reactions. 86 Table 2.2. Catalytic hydrogenation of CO2 captured by aqueous amine solutions. Entry [a] Amine CO2 captured (mmol) catalyst (mol%) co-solvent (mL) H2 (bar) t (h) formate (mmol) CH3OH (mmol) CH3OH (%) 1 PEHA 3.5 C-1(0.5) THF(5) 70 20 0.8 1.1 31 2 PEHA- 1PO 3.3 C-1(0.5) THF (5) 70 20 0.6 1.4 42 3 PEHA- 1PO 3.3 C-1(0.5) THF (10) 70 20 0.6 1.4 42 4 PEHA- 1PO 3.3 C-1(0.5) ethylene glycol (5) 70 20 0.3 0.7 21 5 PEHA- 1PO 3.3 C-1(0.5) triglyme (5) 70 20 0.4 1.9 58 6 PEHA- 1PO 3.3 C-2(0.5) triglyme (5) 70 20 0.9 1.7 52 7 PEHA- 1PO 3.3 C-3(0.5) triglyme (5) 70 20 0.9 0.4 12 8 [b] PEHA- 1PO 3.3 C-1(0.5) triglyme (5) 80 20 0.4 2.2 67 9 [b] PEHA- 1PO 3.3 C-1(0.5) triglyme (5) 80 48 0.1 3.0 91 10 [b] PEHA- 2PO 3.1 C-1(0.5) triglyme (5) 80 48 0.3 2.4 77 11 [b] PEHA- 1BO 3.0 C-1(0.5) triglyme (5) 80 48 0.2 2.5 83 12 [b] PEHA- 1CPO 2.2 C-1(0.5) triglyme (5) 80 48 0.2 0.9 41 13 [b] PEHA- 1TFPO 3.0 C-1(0.5) triglyme (5) 80 48 0.2 2.8 93 [a] Reaction conditions: solutions from Table 1 (as specified) were directly hydrogenated, stirring at 800 rpm, T = 140 °C, [b] T = 145 °C (entry 8-13). pressures at rt. Yields calculated relative to CO2 captured as determined by 1 H NMR. Calculation error ±5%. Ru P CO N P H H Ph Ph Ph Ph H Ru P CO N P H H Ph Ph Ph Ph Cl BH 3 Ru P CO N P H H i Pr i Pr i Pr i Pr Cl H 2 O (5 mL) CO 2 CH 3 OH + H 2 O + Amine + CO 2 capture in situ hydrogenation Ru CO 2 captured as carbamate and bicarbonate salts Amine 1.0 mmol H 2 , T (°C), t(h) C-1 C-2 C-3 Ru-Macho-BH Ru-Macho 87 Under these optimized conditions for hydrogenation of CO2 to CH3OH with PEHA-1PO, other CO2 loaded polyamines were also subjected to the catalytic reduction. With PEHA-2PO, the hydrogenation efficiency dropped to a certain extent with a CH3OH yield of 77% (entry 10). A similar effect was observed in the case of PEHA-1BO affording a CH3OH yield of 83% (entry 11). However, a prominent decrease in CH3OH production was observed in the hydrogenation of CO2 captured with PEHA-1CPO and a moderate yield of 41% was achieved for CH3OH (entry 12). Interestingly, the catalytic hydrogenation of captured CO2 was quite efficient in presence of PEHA-1TFPO leading to a high CH3OH yield of 93% (entry 13). In-depth understanding of the overall reaction pathways in the catalytic hydrogenation of CO2 to methanol is key for the further advancement of this process. Specifically, the role of the amine in the hydrogenation sequence has been studied previously by us as well as others (Figure 2.3F). 27, 29, 61 In an integrated CO2-to-methanol process, amine plays dual roles in the CO2 capture step as well as in the hydrogenation step. The modes of amine reactivity with CO2 in the capture step has been discussed vide supra. In the hydrogenation step, amines mediate multiple key transformations during the catalytic cycle. First, amines promote dissolution of CO2 gas, thus enhancing its abundance in the solution phase where the catalyst operates. Next, after the first reductive step of CO2 to formate species, amine assists in detaching the formate anion (as ammonium formate) attached to the Ru-center as a ligand. This hypothesis is supported by our observation in a previous related study, 27 in which CO2 hydrogenation even to formate, was completely absent without an amine. Finally, amines with -NH protons undergo condensation with the ammonium formate to give a formamide intermediate and producing an equivalent of H2O. This step is crucial in further reduction of the formate salt to CH3OH. 88 To further validate the proposed reaction sequences and to evaluate the role of amine in the integrated system, a series of experiments were performed. First, we observed that the components of the reaction system including the Ru catalyst and amine can efficiently catalyze the hydrogenation of a gas mixture of CO2/3H2 to methanol (Figure 2.3A). Over 48 h, 24.2 mmol CH3OH were produced with a TON of 1350 along with 1.4 mmol of formates and 7.1 mmol of formamides. To note, the amount of CH3OH produced is around 100% relative to the total amine content in the reaction system. Similar results have been reported previously where the proposed catalytic role of amine could not be established (TON with respect to the amine <2) possibly because of large excess amine being used in most cases. 12, 20, 23, 25, 27, 29, 58 This observation was also raised in a cautionary tale by Barteau et al. 62 To probe this, CO2 hydrogenation was carried out with a lower amine loading of 0.8 mmol PEHA (Figure 2.3B). Gratifyingly, the reaction produced 15 mmol of CH3OH over a prolonged reaction time of 96 h. Relative to the total amino groups present (4.8 mmol), a 313% yield of methanol was achieved ascertaining that amines turn over in catalytic CO2 hydrogenation to CH3OH. A comparable CH3OH/amine of 302% was observed by Kayaki and co-workers with a similar catalytic system using linear PEI as the amine. 30 It is also worth pointing out here that the calculated amine TON is essentially an averaged representation of all the amino groups in the system. Although, in the cases of polyamines where the type and chemical nature of all the amino groups are not uniform, the actual TON of an individual N group will vary. Further, while amines can turn over during a hydrogenation reaction, this feature is not relevant to an integrated capture and conversion process where amines are a stoichiometric component of the hydrogenation substrate (ammonium carbamate/bicarbonate). When the amine and catalyst were physically separated from each other within the reactor (please see experimental section for details), only trace amount of methanol was produced by the 89 catalytic hydrogenation of CO2, highlighting the cooperative roles of the catalyst and amine in the reaction (Figure 2.3C). Similarly, when CO2-loaded amine was placed in the inner vessel with additional amine and catalyst in the outer vessel, 1.4 mmol of CH3OH was obtained (Figure 2.3D). The CH3OH obtained in this case is less than 50% of that in the case where no physical compartmentalization exists. This suggest that desorption of the CO2 captured by amine is possible during the reaction, however it is thermodynamically unfavorable under pressurized conditions. During hydrogenation of CO2 capture species, formates were observed to be the most abundant C- 1 species, apart from CH3OH. To confirm that such species are intermediates in CO2-to-CH3OH reaction, PEHA-formate adduct (formed by reaction of PEHA with HCOOH) was subjected to similar hydrogenation conditions which resulted in an 89% conversion to CH3OH in 48 h (Figure 2.3E). Finally, the integrated capture and conversion was demonstrated in a biphasic solvent system (Figure 2.3G). Previously, our group developed such a system with water and 2-methyl tetrahydrofuran (2-MeTHF) wherein, CO2 is captured in the aqueous phase containing the amine. 41 Next, the catalyst containing organic (2Me-THF) phase is introduced and the biphasic system is subjected to hydrogenation. To our advantage, the utilized catalysts are sparingly soluble in water. Hence, following hydrogenation the aqueous layer and organic layer can be easily separated. The formed methanol can be isolated by simple distillation as methanol does not form azeotrope with water. With CO2-loaded aqueous PEHA-1PO and Ru-Macho-BH in 2-MeTHF, an overall CH3OH yield of 82% was achieved upon hydrogenation of the biphasic reaction mixture. The aqueous layer (containing the amine) and organic layer (with the catalyst) can be reused in subsequent cycles. 90 Figure 2.3. A-F) Investigations to probe the proposed mechanism of CO2 hydrogenation and the role of amine, G) Demonstration of biphasic system for CO2 capture and conversion, Reaction conditions: PEHA-1PO (1.0 mmol) in H2O (5 mL), CO2 captured (3.3 mmol), C-1 (40 μmol), 2- MeTHF (5 mL), H2 (80 bar), T = 145 °C, t = 48 h. Yields calculated relative to CO2 captured as determined by 1 H NMR. Calculation error ±5%. 2.3. Conclusion The present study describes an amine assisted integrated carbon capture and utilization (CCU) process to efficiently produce methanol from CO2. Epoxide-functionalization of PEHA was achieved in an easy, scalable and cost-effective manner leading to formation of amino-alcohol functionalities while increasing the overall molecular weight. The CO2 capture efficiencies of most of the modified polyamines were found to be significantly high (CO2/N = 0.50–0.55) and comparable to unmodified PEHA. During hydrogenation of these CO2-loaded amine solutions, the methanol yields were markedly increased for PO-functionalized PEHA compared to the 91 unmodified counterpart. With modifications in some reaction parameters and using triglyme as the solvent, good to high methanol yields were achieved in most cases, with higher than 90% yields with PEHA-1PO and PEHA-1TFPO. The role of the amine in CO2 hydrogenation as well as in an integrated capture and conversion system was validated through a series of control experiments. The highly cooperative role of the catalyst and amine was critical for the hydrogenation to proceed. The hypothesized turnover of amine in CO2-to-methanol reaction was also ascertained. Furthermore, the integrated system was demonstrated in a biphasic solvent system. In the long term, the present system has potential applications in integration of the already existing CO2 scrubbing industries with the methanol synthesis plants toward carbon neutral and renewable methanol synthesis. 2.4. Experimental Methods 2.4.1. Materials and methods All experiments were carried out under an inert atmosphere (with N2 or Ar) using standard Schlenk techniques with the exclusion of moisture unless otherwise stated. Complexes Ru-Macho- BH (C-1, Strem Chemicals, 98%), Ru-Macho (C-2, Strem Chemicals, 98%) and RuHClPNP iPr (CO) (C-3, Strem Chemicals, 98%) were used as received without further purification. All catalysts were weighed inside an argon filled glove box. Tetrahydrofuran (Drisolv, Merck), ethylene glycol (anhydrous, Sigma Aldrich), triglyme (Alfa Aesar), 2- methyltetrahydrofuran (Alfa Aesar) and water (deionized) were sparged with N2 for 1 h prior to use. Pentaethylenehexamine (technical grade, Sigma Aldrich), propylene oxide (99.5%, Sigma Aldrich), 1,2-epoxybutane (99%, Sigma Aldrich), (±)-epichlorohydrin (99%, Sigma Aldrich) and 2-(trifluoromethyl)oxirane (98%, Synquest) were used without further purification. D2O (CIL, D- 92 99.5%), imidazole (Fischer, 99.5%) and 1,3,5-trimethoxybenzene (99%, Sigma Aldrich) were used as received. 1 H and 13 C NMR spectra were recorded on 400, 500 or 600 MHz, Varian NMR spectrometers. 1 H and 13 C NMR chemical shifts were determined relative to the residual solvent signals. The gas mixtures were analyzed using a Thermo Finnigan gas chromatograph (column: Supelco, Carboxen 1010 plot, 30 m X 0.53 mm) equipped with a TCD detector (CO detection limit: 0.099 v/v%). CO2 (Gilmore, instrument grade), 1:3 CO2:H2 mix (Airgas, certified standard- spec grade) and H2 (Gilmore, ultra-high pure grade 5.0) were used without further purification. Caution: Reactions are associated with H2 gas. They should be carefully handled inside proper fume hoods without any flame, spark or static electricity sources nearby. 2.4.2 Preparation of epoxide-functionalized amine in water The reaction of PEHA with epoxides was carried out following a protocol previously established by our group and others. To a stirred solution of PEHA (2 g) in water (15 mL), the required amount of epoxide was added dropwise at room temperature. The reaction was allowed to stir at room temperature for 24 h. The aqueous solution was used as stock for the required amount in CO2 capture experiments. 2.4.3. Standard procedure for CO2 capture by amine in water A known amount of amine (PEHA or PEHA-epoxide) was dissolved in water (total volume = 5 mL) in a vial with a magnetic stir bar. The gases inside the vial were then removed under vacuum. CO2 was subsequently added while stirring the solution at 800 rpm for 1 h and maintaining the CO2 pressure inside the reactor at 1 psi above atmospheric pressure. The amounts of CO2 captured were calculated through gravimetric analysis of the solutions before and after the capture. 93 2.4.4. Standard procedure for hydrogenation of CO2 loaded amine solutions The capture of CO2 in aqueous amine solutions were performed prior to hydrogenation reactions. The solution was then purged under N2 flow to remove any physically absorbed CO2. In a nitrogen-filled chamber, the CO2 loaded amine solution, catalyst and co-solvent were added to a 125 mL Monel Parr reactor equipped with a magnetic stir bar, thermocouple and piezoelectric pressure transducer. The vessel was then filled with H2 to the desired pressure (70-80 bar). The reaction mixture was then stirred with a magnetic stirrer for 5 minutes (800 rpm) and subsequently placed in a preheated aluminum block with stirring at 800 rpm. After heating for a given period, the reactor was cooled to room temperature. The vessel was then cooled in an ice bath for 30 minutes. Afterwards, the gases inside the vessel were partly collected in a gas sampling bag for GC analysis whereas the remaining gas was slowly released. Upon opening the reaction vessel, a known amount of internal standard was added to the homogeneous solution which was then analyzed by 1 H and 13 C NMR with a deuterated solvent. Yields were determined through 1 H NMR from integration ratios. 2.4.5. Standard procedure for control studies In a nitrogen-filled chamber, the catalyst, solvents and substrate (amine, CO2-loaded amine or amine-formic acid adduct) were added to a 125 mL Monel Parr reactor equipped with a magnetic stir bar, thermocouple and piezoelectric pressure transducer. In the case of compartmentalization of the reaction components, a 2 dram borosilicate vial was used as the inner vessel. The vessel was then filled with H2 or CO2/3H2 to the desired pressure (70-80 bar). The reaction mixture was then stirred with a magnetic stirrer for 5 minutes (800 rpm) and subsequently placed in a preheated aluminum block with stirring at 800 rpm. After heating for a given period, the reactor was cooled to room temperature. The vessel was then cooled in an ice bath for 30 minutes. Afterwards, the 94 gases inside the vessel were partly collected in a gas sampling bag for GC analysis whereas the remaining gas was slowly released. Upon opening the reaction vessel, a known amount of internal standard was added to the homogeneous solution which was then analysed by 1 H and 13 C NMR with a deuterated solvent. Yields were determined through 1 H NMR from integration ratios. 2.4.6. Standard procedure for CO2 hydrogenation in a biphasic system In a nitrogen-filled chamber, the catalyst, 2-MeTHF and aqueous solution of CO2-loaded amine were added to a 125 mL Monel Parr reactor equipped with a magnetic stir bar, thermocouple and piezoelectric pressure transducer. The vessel was then filled with H2 to the desired pressure (80 bar). The reaction mixture was then stirred with a magnetic stirrer for 5 minutes (800 rpm) and subsequently placed in a preheated aluminum block with stirring at 800 rpm. After heating for a given period, the reactor was cooled to room temperature. The vessel was then cooled in an ice bath for 30 minutes and the gases inside were slowly released. Upon opening the reaction vessel, a biphasic solution was obtained. The entire solution was transferred to a 15 mL centrifuge tube and the layers were separated carefully. A known amount of TMB was added as internal standard to the organic layer and was then analyzed by 1 H and 13 C NMR with DMSO-d6 as deuterated solvent. A known amount of imidazole was added as internal standard to the aqueous layer and was then analyzed by 1 H and 13 C NMR with D2O as deuterated solvent. Yields were determined through 1 H NMR from integration ratios. 95 2.4.7 Representative Spectra Figure 2.4. 13 C NMR spectra of CO2 captured by PEHA-1PO in water in D2O. Figure 2.5. Representative 1 H NMR spectra of the reaction mixture after hydrogenation of CO2 loaded PEHA in water with THF as solvent in D2O and imidazole (Im) as internal standard. - 1 0 1 0 3 0 5 0 7 0 9 0 1 1 0 1 3 0 1 5 0 1 7 0 1 9 0 2 1 0 2 3 0 f 1 ( p p m ) 0 . 5 1 . 0 1 . 5 2. 0 2 . 5 3 . 0 3. 5 4. 0 4. 5 5. 0 5. 5 6. 0 6 . 5 7 . 0 7 . 5 8 . 0 8 . 5 9 . 0 9 . 5 f 1 ( p p m ) 1 . 1 4 1 . 0 0 0 . 4 9 0 . 0 5 0 . 2 8 carbamate (–NCOO - ) bicarbonate (HCO3 - ) PEHA-1PO formate/ formamide Im Im CH3OH THF THF 96 Figure 2.6. Representative 1 H NMR spectra of the reaction mixture after hydrogenation of CO2 loaded PEHA-1PO in water with THF as solvent in D2O and imidazole (Im) as internal standard. Figure 2.7. Representative 1 H NMR spectra of the reaction mixture after hydrogenation of CO2 loaded PEHA-1PO in water with triglyme as solvent in D2O and imidazole (Im) as internal standard. 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 4 . 0 4 . 5 5 . 0 5 . 5 6 . 0 6 . 5 7 . 0 7 . 5 8 . 0 8 . 5 9 . 0 9 . 5 f 1 ( p p m ) 1 . 4 0 1 . 0 0 0 . 5 3 0 . 1 9 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 4 . 0 4 . 5 5 . 0 5 . 5 6 . 0 6 . 5 7 . 0 7 . 5 8 . 0 8 . 5 9 . 0 9 . 5 f 1 ( p p m ) 1 . 9 5 1 . 0 0 0 . 5 0 0 . 1 3 3 . 0 0 3 . 0 5 3 . 1 0 3 . 1 5 3 . 2 0 3 . 2 5 3 . 3 0 3 . 3 5 3 . 4 0 f 1 ( p p m ) formate Im Im CH3OH THF THF formate Im Im CH3OH triglyme triglyme CH3OH 97 Figure 2.8. Representative 1 H NMR spectra of the reaction mixture after hydrogenation of CO2/3H2 in presence of PEHA in triglyme in D2O and imidazole (Im) as internal standard. Figure 2.9. 1 H NMR spectra of the reaction mixture after hydrogenation of [HCOO-PEHA] in triglyme in D2O and imidazole (Im) as internal standard. 0 . 0 1 . 0 2 . 0 3 . 0 4 . 0 5 . 0 6 . 0 7 . 0 8 . 0 9 . 0 1 0 . 0 f 1 ( p p m ) 2 4 . 7 6 1 . 0 0 0 . 5 0 2 . 4 0 0 . 4 6 0 . 0 1 . 0 2 . 0 3 . 0 4 . 0 5 . 0 6 . 0 7 . 0 8 . 0 9 . 0 1 0 . 0 f 1 ( p p m ) 3 . 1 9 1 . 0 0 0 . 4 9 0 . 1 4 formate/ formamide Im Im CH3OH triglyme formate/ formamide Im Im CH3OH triglyme 98 Figure 2.10. 1 H NMR spectra of the aqueous layer after hydrogenation of CO2 loaded PEHA-PO in water/2-MeTHF biphasic system in D2O and imidazole (Im) as internal standard. Figure 2.11. 1 H NMR spectra of the organic layer after hydrogenation of CO2 loaded PEHA-PO in water/2-MeTHF biphasic system in D2O and imidazole (Im) as internal standard. 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 4 . 0 4 . 5 5 . 0 5 . 5 6 . 0 6 . 5 7 . 0 7 . 5 8 . 0 8 . 5 9 . 0 9 . 5 f 1 ( p p m ) 2 . 6 2 1 . 0 0 0 . 4 9 0 . 2 2 0 . 5 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 4 . 0 4 . 5 5 . 0 5 . 5 6 . 0 6 . 5 7 . 0 7 . 5 f 1 ( p p m ) 0 . 4 7 3 . 1 5 1 . 0 0 formate/ formamide Im Im CH3OH 2-MeTHF 2-MeTHF TMB CH3OH 2-MeTHF 2-MeTHF 2-MeTHF 2-MeTHF 99 Figure 2.12. Typical GC spectra of the gas mixture after hydrogenation of CO2 capture solution. Figure 2.13. Typical GC spectra of the gas mixture after hydrogenation of CO2/H2 mixture. C:\Xcalibur\data\Alain\raktim\rs-3-113-A 12/03/2021 02:04:42 PM RT: 0.05 - 10.73 1 2 3 4 5 6 7 8 9 10 Time (min) 0 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000 220000 240000 260000 280000 300000 320000 340000 360000 380000 400000 420000 440000 460000 480000 500000 520000 540000 560000 580000 600000 620000 640000 660000 680000 700000 720000 740000 760000 780000 800000 820000 840000 Counts 2.26 9.02 8.81 8.46 7.94 9.08 7.17 2.34 10.22 2.55 3.00 0.57 0.97 3.71 4.44 4.70 6.78 5.56 6.39 NL: 8.24E7 TCD Analog 2 rs-3-113-A C:\Xcalibur\...\Alain\raktim\rs-3-128-sa RT: 0.14 - 11.44 1 2 3 4 5 6 7 8 9 10 11 Time (min) 0 100000 200000 300000 400000 500000 600000 700000 800000 900000 1000000 1100000 1200000 1300000 1400000 Counts 8.34 2.29 8.57 8.99 2.58 8.17 7.44 9.41 10.96 7.04 2.65 1.65 0.69 5.75 4.14 4.61 NL: 8.75E7 TCD Analog 2 rs-3-128-sa RT: 0.00 - 14.00 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Time (min) 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 Counts 2.83 6.56 7.69 5.39 6.25 9.35 3.64 5.33 7.07 1.97 1.47 0.47 10.19 8.34 13.73 10.39 12.07 NL: 2.35E4 FID Analog rs-3-128-sa H2 N2 H2 N2 CO2 CO 100 Figure 2.14. Representative 1 H NMR of the product mixture from the reaction of PEHA with epoxide (PEHA + 1 eq. PO) in DMSO-d6. 0 . 5 1 . 0 1 . 5 2. 0 2 . 5 3. 0 3. 5 4 . 0 4 . 5 5. 0 5. 5 6 . 0 6 . 5 7 . 0 7 . 5 8 . 0 8 . 5 9 . 0 9 . 5 f 1 ( p p m ) 3 . 1 1 2 2 . 45 1 . 0 0 H 2 N H N N H H N N H H N CH 3 OH + other isomers/products –N-CH2– –CH3 –CH 101 2.5. References [1] Jones, W. D., Carbon Capture and Conversion. J. Am. Chem. Soc. 2020, 142, 4955-4957. [2] Bui, M.; Adjiman, C. S.; Bardow, A.; Anthony, E. J.; Boston, A.; Brown, S.; Fennell, P. S.; Fuss, S.; Galindo, A.; Hackett, L. A.; Hallett, J. P.; Herzog, H. J.; Jackson, G.; Kemper, J.; Krevor, S.; Maitland, G. C.; Matuszewski, M.; Metcalfe, I. S.; Petit, C.; Puxty, G.; Reimer, J.; Reiner, D. M.; Rubin, E. S.; Scott, S. A.; Shah, N.; Smit, B.; Trusler, J. P. M.; Webley, P.; Wilcox, J.; Mac Dowell, N., Carbon capture and storage (CCS): the way forward. Energy Environ. Sci. 2018, 11, 1062-1176. [3] Liu, Q.; Wu, L.; Jackstell, R.; Beller, M., Using carbon dioxide as a building block in organic synthesis. Nat. Commun. 2015, 6, 5933. [4] Alberico, E.; Nielsen, M., Towards a methanol economy based on homogeneous catalysis: methanol to H2 and CO2 to methanol. Chem. Commun. 2015, 51, 6714-6725. [5] Kang, S.; Boshell, F.; Goeppert, A.; Prakash, G. K. S.; Landälv, I., Innovation Outlook: Renewable Methanol. IRENA and Methanol Institute 2021. https://www.irena.org/- /media/Files/IRENA/Agency/Publication/2021/Jan/IRENA_Innovation_Renewable_Methanol_2 021.pdf. [6] Goeppert, A.; Czaun, M.; Prakash, G. S.; Olah, G. A., CO2 capture and recycling to fuels and materials: towards a sustainable future. In An Introduction to Green Chemistry Methods, pp 132- 146. [7] Olah, G. A., Beyond Oil and Gas: The Methanol Economy. Angew. Chem. Int. Ed. 2005, 44, 2636-2639. [8] Goeppert, A.; Olah, G. A.; Surya Prakash, G. K., Chapter 3.26 - Toward a Sustainable Carbon Cycle: The Methanol Economy. In Green Chemistry, Török, B.; Dransfield, T., Eds. Elsevier: 2018; pp 919-962. [9] Olah, G. A.; Goeppert, A.; Prakash, G. K. S., Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons. J. Org. Chem. 2009, 74, 487-498. [10] Carbon Recycling International. http://www.carbonrecycling.is/george-olah/ [11] Gao, W.; Liang, S.; Wang, R.; Jiang, Q.; Zhang, Y.; Zheng, Q.; Xie, B.; Toe, C. Y.; Zhu, X.; Wang, J.; Huang, L.; Gao, Y.; Wang, Z.; Jo, C.; Wang, Q.; Wang, L.; Liu, Y.; Louis, B.; Scott, J.; Roger, A.-C.; Amal, R.; He, H.; Park, S.-E., Industrial carbon dioxide capture and utilization: state of the art and future challenges. Chem. Soc. Rev. 2020, 49, 8584-8686. 102 [12] Kar, S.; Kothandaraman, J.; Goeppert, A.; Prakash, G. K. S., Advances in catalytic homogeneous hydrogenation of carbon dioxide to methanol. J. CO2 Util . 2018, 23, 212-218. [13] Bai, S.-T.; De Smet, G.; Liao, Y.; Sun, R.; Zhou, C.; Beller, M.; Maes, B. U. W.; Sels, B. F., Homogeneous and heterogeneous catalysts for hydrogenation of CO2 to methanol under mild conditions. Chem. Soc. Rev. 2021, 50, 4259-4298. [14] Li, Y.-N.; Ma, R.; He, L.-N.; Diao, Z.-F., Homogeneous hydrogenation of carbon dioxide to methanol. Catal. Sci. Technol. 2014, 4, 1498-1512. [15] Sen, R.; Goeppert, A.; Prakash, G., Carbon Dioxide Capture and Recycling to Methanol: Building a Carbon–Neutral Methanol Economy. Aldrichimica Acta 2020, 53, 39-56. [16] Wesselbaum, S.; Moha, V.; Meuresch, M.; Brosinski, S.; Thenert, K. M.; Kothe, J.; Stein, T. v.; Englert, U.; Hölscher, M.; Klankermayer, J.; Leitner, W., Hydrogenation of carbon dioxide to methanol using a homogeneous ruthenium–Triphos catalyst: from mechanistic investigations to multiphase catalysis. Chem. Sci. 2015, 6, 693-704. [17] Huff, C. A.; Sanford, M. S., Cascade catalysis for the homogeneous hydrogenation of CO2 to methanol. J. Am. Chem. Soc. 2011, 133, 18122-18125. [18] Rayder, T. M.; Adillon, E. H.; Byers, J. A.; Tsung, C.-K., A Bioinspired Multicomponent Catalytic System for Converting Carbon Dioxide into Methanol Autocatalytically. Chem 2020, 6, 1742-1754. [19] Sordakis, K.; Tsurusaki, A.; Iguchi, M.; Kawanami, H.; Himeda, Y.; Laurenczy, G., Carbon Dioxide to Methanol: The Aqueous Catalytic Way at Room Temperature. Chem. Eur. J. 2016, 22, 15605-15608. [20] Khusnutdinova, J. R.; Garg, J. A.; Milstein, D., Combining Low-Pressure CO2 Capture and Hydrogenation To Form Methanol. ACS Catal. 2015, 5, 2416-2422. [21] Balaraman, E.; Gunanathan, C.; Zhang, J.; Shimon, L. J. W.; Milstein, D., Efficient hydrogenation of organic carbonates, carbamates and formates indicates alternative routes to methanol based on CO2 and CO. Nat. Chem. 2011, 3, 609. [22] Balaraman, E.; Ben-David, Y.; Milstein, D., Unprecedented Catalytic Hydrogenation of Urea Derivatives to Amines and Methanol. Angew. Chem. Int. Ed. 2011, 50, 11702-11705. [23] Rezayee, N. M.; Huff, C. A.; Sanford, M. S., Tandem Amine and Ruthenium-Catalyzed Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2015, 137, 1028-1031. 103 [24] Zhang, L.; Han, Z.; Zhao, X.; Wang, Z.; Ding, K., Highly Efficient Ruthenium-Catalyzed N- Formylation of Amines with H2 and CO2. Angew. Chem. Int. Ed. 2015, 54, 6186-6189. [25] Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. S., Conversion of CO2 from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst. J. Am. Chem. Soc. 2016, 138, 778-781. [26] Kar, S.; Goeppert, A.; Kothandaraman, J.; Prakash, G. K. S., Manganese-Catalyzed Sequential Hydrogenation of CO2 to Methanol via Formamide. ACS Catal. 2017, 7, 6347-6351. [27] Kar, S.; Sen, R.; Kothandaraman, J.; Goeppert, A.; Chowdhury, R.; Munoz, S. B.; Haiges, R.; Prakash, G. K. S., Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2019, 141, 3160-3170. [28] Lane, E. M.; Zhang, Y.; Hazari, N.; Bernskoetter, W. H., Sequential Hydrogenation of CO2 to Methanol Using a Pincer Iron Catalyst. Organometallics 2019, 38, 3084-3091. [29] Everett, M.; Wass, D. F., Highly productive CO2 hydrogenation to methanol – a tandem catalytic approach via amide intermediates. Chem. Commun. 2017, 53, 9502-9504. [30] Yoshimura, A.; Watari, R.; Kuwata, S.; Kayaki, Y., Poly(ethyleneimine)-Mediated Consecutive Hydrogenation of Carbon Dioxide to Methanol with Ru Catalysts. Eur. J. Inorg. Chem. 2019, 2375-2380. [31] Goeppert, A.; Czaun, M.; Prakash, G. K. S.; Olah, G. A., Air as the renewable carbon source of the future: an overview of CO2 capture from the atmosphere. Energy Environ. Sci. 2012, 5, 7833-7853. [32] Lackner, K. S., Capture of carbon dioxide from ambient air. Eur. Phys. J. Spec. Top. 2009, 176, 93-106. [33] Goeppert, A.; Czaun, M.; May, R. B.; Prakash, G. K. S.; Olah, G. A.; Narayanan, S. R., Carbon Dioxide Capture from the Air Using a Polyamine Based Regenerable Solid Adsorbent. J. Am. Chem. Soc. 2011, 133, 20164-20167. [34] Sanz-Pérez, E. S.; Murdock, C. R.; Didas, S. A.; Jones, C. W., Direct Capture of CO2 from Ambient Air. Chem. Rev. 2016, 116, 11840-11876. [35] Kar, S.; Goeppert, A.; Galvan, V.; Chowdhury, R.; Olah, J.; Prakash, G. K. S., A Carbon- Neutral CO2 Capture, Conversion, and Utilization Cycle with Low-Temperature Regeneration of Sodium Hydroxide. J. Am. Chem. Soc. 2018, 140, 16873-16876. 104 [36] Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. S., CO2 capture by amines in aqueous media and its subsequent conversion to formate with reusable ruthenium and iron catalysts. Green Chem. 2016, 18, 5831-5838. [37] Kar, S.; Goeppert, A.; Prakash, G. K. S., Combined CO2 Capture and Hydrogenation to Methanol: Amine Immobilization Enables Easy Recycling of Active Elements. ChemSusChem 2019, 12, 3172-3177. [38] Sen, R.; Koch, C. J.; Galvan, V.; Entesari, N.; Goeppert, A.; Prakash, G. K. S., Glycol assisted efficient conversion of CO2 captured from air to methanol with a heterogeneous Cu/ZnO/Al2O3 catalyst. J. CO2 Util. 2021, 54. [39] Sen, R.; Goeppert, A.; Kar, S.; Prakash, G. K. S., Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol. J. Am. Chem. Soc. 2020, 142, 4544-4549. [40] Kar, S.; Goeppert, A.; Prakash, G. K. S., Integrated CO2 Capture and Conversion to Formate and Methanol: Connecting Two Threads. Acc. Chem. Res. 2019, 52, 2892-2903. [41] Kar, S.; Sen, R.; Goeppert, A.; Prakash, G. K. S., Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy. J. Am. Chem. Soc. 2018, 140, 1580-1583. [42] Sen, R.; Koch, C. J.; Goeppert, A.; Prakash, G. K. S., Tertiary Amine-Ethylene Glycol Based Tandem CO2 Capture and Hydrogenation to Methanol: Direct Utilization of Post-Combustion CO2. ChemSusChem 2020, 13, 6318-6322. [43] Wei, D.; Junge, H.; Beller, M., An amino acid based system for CO2 capture and catalytic utilization to produce formates. Chem. Sci. 2021, 12, 6020-6024. [44] Jahandar Lashaki, M.; Khiavi, S.; Sayari, A., Stability of amine-functionalized CO2 adsorbents: a multifaceted puzzle. Chem. Soc. Rev. 2019, 48, 3320-3405. [45] Heldebrant, D. J.; Kothandaraman, J., Chapter 3. Solvent-based Absorption. In Carbon Capture and Storage, The Royal Society of Chemistry: 2020; pp 36-68. [46] Xu, X.; Myers, M. B.; Versteeg, F. G.; Pejcic, B.; Heath, C.; Wood, C. D., Direct air capture (DAC) of CO2 using polyethylenimine (PEI) “snow”: a scalable strategy. Chem. Commun. 2020, 56, 7151-7154. [47] Goeppert, A.; Zhang, H.; Czaun, M.; May, R. B.; Prakash, G. K. S.; Olah, G. A.; Narayanan, S. R., Easily Regenerable Solid Adsorbents Based on Polyamines for Carbon Dioxide Capture from the Air. ChemSusChem 2014, 7, 1386-1397. 105 [48] Dutcher, B.; Fan, M.; Russell, A. G., Amine-Based CO2 Capture Technology Development from the Beginning of 2013—A Review. ACS Appl. Mater. Interfaces 2015, 7, 2137-2148. [49] Tauhardt, L.; Kempe, K.; Knop, K.; Altuntaş, E.; Jäger, M.; Schubert, S.; Fischer, D.; Schubert, U. S., Linear Polyethyleneimine: Optimized Synthesis and Characterization - On the Way to “Pharmagrade” Batches. Macromol. Chem. Phys. 2011, 212, 1918-1924. [50] Park, S.; Choi, K.; Yu, H. J.; Won, Y.-J.; Kim, C.; Choi, M.; Cho, S.-H.; Lee, J.-H.; Lee, S. Y.; Lee, J. S., Thermal Stability Enhanced Tetraethylenepentamine/Silica Adsorbents for High Performance CO2 Capture. Ind. Eng. Chem. Res. 2018, 57, 4632-4639. [51] Bui, T. Q.; Khokarale, S. G.; Shukla, S. K.; Mikkola, J.-P., Switchable Aqueous Pentaethylenehexamine System for CO2 Capture: An Alternative Technology with Industrial Potential. ACS Sustain. Chem. Eng. 2018, 6, 10395-10407. [52] Srikanth, C. S.; Chuang, S. S. C., Spectroscopic Investigation into Oxidative Degradation of Silica-Supported Amine Sorbents for CO2 Capture. ChemSusChem 2012, 5, 1435-1442. [53] Pang, S. H.; Lee, L.-C.; Sakwa-Novak, M. A.; Lively, R. P.; Jones, C. W., Design of Aminopolymer Structure to Enhance Performance and Stability of CO2 Sorbents: Poly(propylenimine) vs Poly(ethylenimine). J. Am. Chem. Soc. 2017, 139, 3627-3630. [54] Goeppert, A.; Zhang, H.; Sen, R.; Dang, H.; Prakash, G. K. S., Oxidation-Resistant, Cost- Effective Epoxide-Modified Polyamine Adsorbents for CO2 Capture from Various Sources Including Air. ChemSusChem 2019, 12, 1712-1723. [55] Azizi, N.; Saidi, M. R., Highly Chemoselective Addition of Amines to Epoxides in Water. Org. Lett. 2005, 7, 3649-3651. [56] Kothandaraman, J.; Heldebrant, D. J., Catalytic coproduction of methanol and glycol in one pot from epoxide, CO2, and H2. RSC Adv. 2020, 10, 42557-42563. [57] Gunanathan, C.; Milstein, D., Bond Activation and Catalysis by Ruthenium Pincer Complexes. Chem. Rev. 2014, 114, 12024-12087. [58] Bai, S.-T.; Zhou, C.; Wu, X.; Sun, R.; Sels, B., Suppressing Dormant Ru States in the Presence of Conventional Metal Oxides Promotes the Ru-MACHO-BH-Catalyzed Integration of CO2 Capture and Hydrogenation to Methanol. ACS Catal. 2021, 11, 12682-12691. [59] Kothandaraman, J.; Kar, S.; Sen, R.; Goeppert, A.; Olah, G. A.; Prakash, G. K. S., Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol. J. Am. Chem. Soc. 2017, 139, 2549-2552. 106 [60] Kothandaraman, J.; Kar, S.; Goeppert, A.; Sen, R.; Prakash, G. K. S., Advances in Homogeneous Catalysis for Low Temperature Methanol Reforming in the Context of the Methanol Economy. Top. Catal. 2018, 61, 542-559. [61] Bernskoetter, W. H.; Hazari, N., Reversible Hydrogenation of Carbon Dioxide to Formic Acid and Methanol: Lewis Acid Enhancement of Base Metal Catalysts. Acc. Chem. Res. 2017, 50, 1049- 1058. [62] Wilson, E. A. K.; Eady, S. C.; Silbaugh, T.; Thompson, L. T.; Barteau, M. A., Both sites must turn over in tandem catalysis: Lessons from one-pot CO2 capture and hydrogenation. J. Catal. 2021, 404, 977-984. 107 Chapter 3. Tertiary Amine Based Integrated Post-Combustion Capture of CO2 and Hydrogenation to Methanol This dissertation chapter is based on a recent research article from our group published in ChemSusChem (Sen, R.; Koch, C. J.; Goeppert, A.; Prakash, G. K. S., ChemSusChem. 2020, 13, 6318-6322). Part of the article is reprinted by permission of John Wiley and Sons. Copyright 2020. John Wiley and Sons. 3.1. Introduction Surging levels of anthropogenic CO2 emissions have resulted in escalating atmospheric CO2 concentrations over the past century. This phenomenon is already having severe impacts on the biosphere including rising temperatures and unforeseen weather-related catastrophes across the globe. 1 As a long-term solution to mitigate such environmental crises, renewable energy resources with low carbon-footprints are being widely adopted. 2 Parallelly, technologies are being developed to remove CO2 directly at their source of emissions as well as indirectly from diffused sources like ambient air. 3 Among these techniques, capturing CO2 from dilute gas streams like flue gases (5-15% CO2) in an energy-efficient and cost-effective way is highly valuable to various industrial sectors like fossil-fuel burning power plants, refineries, cement, iron and steel plants that contribute significantly to anthropogenic CO2 emissions to the atmosphere (> 34 Gt/year). 4 With increasing regulations and governmental policies to curb CO2 emitted by these industries, post- combustion CO2 capture is gaining considerable attention. 4a, 5 108 While CO2 is a potent greenhouse gas, it is also a highly valuable building block to produce a multitude of chemicals and polymers. 6 Among them, methanol is an attractive C-1 feedstock to synthesize various materials, which is already produced on a scale of over 100 billion liters per year. 7 Additionally, methanol has tremendous applications as an alternate fuel and H2 storage media. 8 Hence, integration of the two threads: CO2 capture and CO2 utilization; is particularly desired as a value-added and green route to access renewable methanol and other feedstocks, to address the issue of increasing CO2 emissions. 9 Such integrated processes also eliminate the high energy and cost penalty associated with the desorption and compression steps in conventional Carbon Capture and Storage or Utilization (CCS/CCU) processes. 10 Figure 3.1. Previously reported integrated carbon capture and conversion to methanol systems compared to the present study. In the context of CO2 capture and conversion to methanol, our group as well as others reported initial examples of integrated systems using primary and secondary amines (Figure 3.1a). 11 However, primary/secondary amines have been shown to suffer from volatility issues, oxidative degradation and leaching of active components over multiple cycles. 12 Very recently, we discovered an unprecedented strategy wherein alkali hydroxides in alcohol solvents captured CO2 in the form of metal alkyl carbonate salts, followed by in situ hydrogenation to methanol (Figure R H N R + CO 2 O N O R R R 2 NH 2 H 2 cat. R H N Previous works a) primary/secondary amine-assisted systems b) alkali hydroxide-alcohol assisted systems + CO 2 MO OR O H 2 cat. MOH CH 3 OH + R MOH -H 2 O This work + CH 3 OH ROH tertiary amine-alcohol assisted systems R N R R R 2 NH 2 N O R R 2H 2 cat. H 2H 2 cat. M + CO 2 O OR O H 2 cat. CH 3 OH + -H 2 O ROH 2H 2 cat. O H O R N R R R 3 NH R 3 NH O H O O H O -H 2 O -ROH -ROH -R 2 NH 109 3.1b). 13 Compared to previously reported amines, hydroxide based systems showed higher capture efficiency and facile hydrogenation with minimal volatility and stability issues. Yet, a major fraction of the base was deactivated as a result of parasitic side reactions, leading to incomplete regeneration of the base. As part of our ongoing research to develop a stable yet regenerable system, we report here the first example of a tertiary amine-based system. With considerably higher boiling points, tertiary amines have been shown to be stable under oxidative conditions and have lower vapor pressures relative to their secondary/primary amine analogues. 14 Lately, Heldebrant and co-workers reported CO2 hydrogenation to methanol in tertiary amine-alcohol solvents using CO2 under pressure and heterogeneous catalysts at temperatures of 170 °C and above. 15 In this report, we have investigated CO2 capture using ethylene glycol solutions of tertiary amines and subsequent hydrogenation with a homogeneous catalyst under mild temperatures as an integrated process. 3.2 Investigations on CO2 Capture and its Subsequent Hydrogenation Amines are frequently used for scrubbing CO2 from effluent gas streams from various industrial processes. Amines with primary and secondary amino groups are most commonly used. They can react with CO2 as nucleophiles to form ammonium carbamates, and additionally, act as bases in presence of water to form bicarbonate/carbonate salts. Unlike these functionalities, tertiary amines can act only as bases, necessitating the presence of a protic solvent like water to capture CO2. 16 In this context, CO2 capture studies have mostly been focused on aqueous systems while non-aqueous systems have not been well explored. 17 In advancing the existing CO2 capture technologies, a water-lean solvent system can offer added benefits, 18 since it requires significantly less energy to heat such a solvent during desorption and regeneration. For instance, an alternate protic solvent like ethylene glycol has a specific heat capacity of 2.4 J/g-°C compared to 4.2 J/g- 110 °C for water. 19 Further, higher boiling solvents are greatly desired for practical purposes to minimize significant evaporative solvent loss encountered over multiple CO2 sorption cycles. 19 In the present study, a broad library of tertiary amines with relatively high boiling points were selected. Their solutions in various alcohols were subjected to CO2 capture at ambient temperature and atmospheric pressure (Table 3.1). TMEDA was found to be active for CO2 capture in ethylene glycol (entry 1), with a loading of 0.37 mol CO2 per mole of amino group (0.37 CO2/N). This observation was comparable to aqueous TMEDA with a CO2 loading of 0.5 CO2/N (entry 4). When diethylene glycol was used as the solvent, the capture efficiency decreased drastically to 0.1 CO2/N (entry 2). Furthermore, ethanolic solution of TMEDA was found inactive for CO2 capture (entry 3), displaying the important role of solvent polarity in the reaction with CO2. In tertiary amine-alcohol capture solvents, CO2 was captured to form ammonium alkyl carbonates as detected by 13 C NMR. Table 3.1 CO2 capture by tertiary amines in alcohol solvent. Entry [a] Amine Solvent mmol CO2 captured [b] mmol CO2 per mmol amino group (CO2/N) [c] 1 TMEDA ethylene glycol 3.7 0.37 2 TMEDA diethylene glycol 1.0 0.1 3 TMEDA ethanol - - 4 TMEDA water 5.0 0.5 5 TEEDA ethylene glycol 4.3 0.43 N N N N N N OH OH HO HO N N OH OH HO HO N N N N N N N N OH HO OH N OH HO N OH N N N N N O N Bis[2-(Dimethylamino)ethyl] ether (DMAEE) bp = 189 °C Tetraethylethylenediamine (TEEDA) bp = 189-192 °C Tetrakis(2-hydroxyethyl) ethylenediamine (THEEDA) bp ~ 280 °C Tetrakis(2-hydroxypropyl) ethylenediamine (Quadrol ® ) bp ~ 392 °C Pentamethyldiethylenetriamine (PMDETA) bp ~ 198 °C Hexamethyltriethylenetetramine (HMTETA) bp ~ 264 °C Triethanolamine (TEA) bp = 360 °C N-Methyldiethanolamine (MDEA) bp = 246-248 °C N,N-Diethylethanolamine (DEEA) bp = 163 °C Tetramethyl-1,4-butanediamine (TMBDA) bp = 166-167 °C Tetramethyl-1,6-hexanediamine (TMHDA) bp = 209-210 °C Tetramethylethylenediamine (TMEDA) bp = 120-122 °C 111 6 THEEDA ethylene glycol 3.7 0.37 7 Quadrol ® ethylene glycol 3.6 0.36 8 PMDETA ethylene glycol 3.8 0.38 9 HMTETA ethylene glycol 3.2 0.32 10 TEA ethylene glycol 3.1 0.31 11 MDEA ethylene glycol 8.3 0.83 12 DEEA ethylene glycol 8.2 0.82 13 TMBDA ethylene glycol 6.0 0.6 14 TMHDA ethylene glycol 7.9 0.79 15 DMAEE ethylene glycol 6.5 0.65 [a] Capture conditions: amine (10 mmol. of amino groups), ethylene glycol (5 mL), stirring (800 rpm), rt, t = 3 h. [b] Captured CO2 amounts determined gravimetrically . [c] Calculations error ±5%. Simple tertiary diamine TEEDA and similar hydroxy derivatives THEEDA and Quadrol exhibited capture comparable to TMEDA, with CO2 loading of 0.43, 0.37 and 0.36 CO2/N, respectively (entry 5-7). Likewise, tertiary polyamines PMDETA and HMTETA absorbed 0.38 and 0.32 mol CO2 per mol of amine (entry 8-9). When high boiling tertiary alkanolamines were screened for CO2 capture, MDEA and DEEA showed high loading of > 0.8 CO2/N, although TEA displayed modest adsorption (CO2/N = 0.31) (entries 10-12). When the chain length between the two amino groups was increased from two C-atoms as in TMEDA, the capture efficiency improved with increasing chain length, as observed in the cases of TMBDA (four C-atoms) with 0.6 CO2/N, TMHDA (six C-atoms) with 0.79 CO2/N and the alkoxy-derivative DMAEE absorbing 0.65 CO2/N (entry 13-15). To the best of our knowledge, this is the first report studying the CO2 capture by various tertiary amines in the glycol solvent. Furthermore, ethylene glycol is an ideal solvent with a high boiling point, low vapor pressure and high dielectric constant allowing complete solubility of the amines as well as captured species. 112 Table 3.2. Tandem hydrogenation of captured CO2 in ethylene glycol. Entry [a] Amine mmol CO2 captured T (°C) Formate [mmol] CH3OH [mmol] Yield [CH3OH] (%) 1 TMEDA 3.7 140 0.9 2.6 70 2 TEEDA 4.3 140 0.7 1.4 33 3 THEEDA 3.7 140 0.8 1.7 46 4 Quadrol® 3.6 140 0.3 0.9 25 5 PMDETA 3.8 140 1.2 1.7 45 6 HMTETA 3.2 140 0.8 1.9 59 7 TEA 3.1 140 0.2 1.9 61 8 MDEA 8.3 140 1.5 3.5 42 9 DEEA 8.2 140 1.4 2.8 34 10 TMBDA 6.0 140 0 5.5 92 11 TMHDA 7.9 140 1.3 4.6 58 12 DMAEE 6.5 140 0.8 3.6 55 13 TMEDA 3.7 130 0.4 1.9 51 14 TMEDA 3.7 150 0.4 2.7 73 15 [b] TMEDA 3.7 140 0.2 3.4 92 [a] Hydrogenation conditions: solutions from Table 1 (as specified) were directly hydrogenated, H2 = 70 bar, 72 h, ethylene glycol (5 mL), Ru-Macho-BH (0.5 mol% of CO2 captured), [b] 1.0 mol% Ru-Macho-BH. Yields determined by 1 H NMR (imidazole as internal standard). Yield calculations error ±5%. Following the CO2 capture study, direct hydrogenation of the captured species to CH3OH was explored using Ru-Macho-BH (Table 3.2). This homogenous PNP catalyst was previously found to be highly efficient and recyclable for hydrogenation of CO2 to CH3OH and similar processes by our group and others. [20] First, CO2 captured by TMEDA in ethylene glycol was subjected to hydrogenation at 140 °C under 70 bar of H2 with 0.5 mol% catalyst. After 72 h, a net R N R R CO 2 O O O CH 3 OH + + H 2 O + R N R R R 3 NH HO OH OH HO OH CO 2 capture H 2 , in situ hydrogenation Ru T, t Ru P CO N P H H Ph Ph Ph Ph H BH 3 Ru-Macho-BH 113 conversion of 94% to CH3OH and formate was observed, with a CH3OH yield of 70% (entry 1). Subsequently, the CO2-loaded solutions of TEEDA, THEEDA and Quadrol were also hydrogenated with modest CH3OH yields (33%, 46% and 25%, respectively) (entry 2-4). When tertiary ethanolamine derivatives were screened, MDEA and DEEA produced 42% and 34% CH3OH, respectively, whereas the yield increased to 61% with TEA (entry 7-9). For polyamines PMDETA and HMTETA, CH3OH yields of 45% and 59% were obtained (entry 5-6). Surprisingly, longer C-chain length diamine TMBDA showed a promising CH3OH yield of 92%, while TMHDA and DMAEE gave moderate conversion of 58% and 55% to CH3OH, respectively (entry 10-12). With TMEDA, lowering the temperature to 130 °C decreased the CH3OH yield substantially to 51% (entry 13), from 70% at 140 °C. However, increasing the temperature to 150 °C had no distinct effect (entry 14). When the catalyst loading was increased to 1.0 mol% for CO2- loaded TMEDA solution, CH3OH yield markedly improved to 92% (entry 15). In our study using alkali hydroxides under similar conditions (discussed in Chapter 4), we observed that most of the base (>95%) was deactivated during hydrogenation due to a parasitic side reaction, leading to the dehydrogenation of ethylene glycol in the presence of the base and catalyst to glycolate salt. 13 In the present study, the formation of glycolate species was essentially suppressed. This significant improvement with the present system is possibly due to reduced basicity of tertiary amines as compared to hydroxides. No glycolate species was observed in any of the examples (Table 3.2), with the exception of TMBDA, where a small amount of ammonium glycolate was detected (Table 3.3, entry 2). 114 Table 3.3. Regeneration of tertiary amine base after hydrogenation step. Entry [a] Amine CO2 captured (Run 1) (mmol.) CH3OH (mmol.) Formate (mmol.) Glycolate (mmol.) CO2 captured (Run 2) (mmol.) 1 [b] TMEDA 7.4 6.8 0.4 0.0 6.7 2 TMBDA 12.0 11.0 0.0 4.0 8.9 [a] Capture conditions: same as in Table 1, amine = 10 mmol. (amino groups = 20 mmol.), ethylene glycol (10 mL). Hydrogenation conditions: same as in Table 2, [b] 1.0 mol% catalyst loading. Next, the best performing amines TMEDA and TMBDA were investigated for regeneration of the tertiary amine for subsequent capture (Table 3.3). When the reaction mixture of TMEDA (10 mmol) was subjected to CO2 capture after hydrogenation, the solution captured 6.7 mmol of CO2, showing a 91% retention in capture efficiency (entry 1). With TMBDA, a CO2 capture capacity of 75% was retained (entry 2). Table 3.4. CO2 capture from simulated flue gas (10% CO2/N2) and conversion to CH3OH. Entry [a] Amine CO2 capture Tandem hydrogenation CO2 captured (mmol.) CO2/N Formate (mmol.) CH3OH (mmol.) Yield CH3OH (%) 1 TMEDA 3.2 0.16 0.6 2.3 72 2 TMBDA 7.0 0.35 0.0 6.6 94 [a] Capture conditions: amine = 10 mmol. (amino groups = 20 mmol.), ethylene glycol (10 mL), rt, overnight. Hydrogenation conditions: same as in Table 2, 0.5 mol% Ru-Macho-BH. [b] Captured CO2 amounts determined 13 C NMR. Calculations error ±5%. With the promising results of the integrated system for CO2 capture and in situ hydrogenation to CH3OH, potential application of the system for post-combustion CO2 capture R N R R CO 2 CH 3 OH + R N R R HO OH CO 2 capture Run 1 H 2 , in situ hydrogenation Ru T, t + HCOO R 3 NH HO COO R 3 NH + CO 2 HO OH CO 2 capture Run 2 115 from dilute streams combined with downstream utilization was explored (Table 3.4). For this, 10% CO2 in N2 was bubbled through an ethylene glycol solution with 10.0 mmol TMEDA. After overnight stirring, 3.2 mmol of CO2 was captured; more than 40% of the capture efficiency obtained with pure CO2 (entry 1). When TMBDA was used, 7 mmol of CO2 was captured with an improved efficiency of >60% compared to capture with pure CO2 (entry 2). Subjecting these capture solutions to the tandem hydrogenation led to significant CH3OH yields. When the gas mixture was bubbled through a solution containing 10 mmol TMEDA in 10 mL ethylene glycol at a flow rate of 100 mL/min overnight, around 0.8 mmol TMEDA was collected in a cold trap (T = -20 °C) when the outlet gas was passed through it. Interestingly, we observed that upon reducing the flow rate to 10 mL/min, amine loss due to evaporation was essentially absent (no amine peaks detected by 1 H NMR). Nevertheless, the amount of CO2 captured at 10 mL/min (3.4 mmol captured overnight) was comparable to that at 100 mL/min (3.2 mmol captured overnight). Table 3.5. Amine loss during CO2 capture from simulated flue gas (10% CO2/N2). Entry Flow rate (mL/min) CO2 captured (mmol.) a Amine in cold trap (mmol.) a 1 100 3.2 0.8 2 10 3.4 0.0 [a] amounts determined by 1 H and 13 C NMR. 3.3. Conclusion In conclusion, a combined approach for CO2 capture and subsequent catalytic hydrogenation to methanol has been demonstrated using high boiling and relatively stable tertiary amine-glycol solvents. Various tertiary amines were screened and found to have significant CO2 capture efficiency. We found that the captured species were hydrogenated effectively to CH3OH 116 under H2 pressure in presence of Ru-Macho-BH. Among the screened amines, TMEDA and TMBDA were most efficient, with high CH3OH yields. Additionally, after reaction completion, the amine solutions were mostly regenerable for subsequent CO2 capture. Finally, the present system showed notable CO2 capture efficiency from dilute CO2 streams (10% CO2) and its effective conversion to CH3OH. We believe that the present system could play a key and promising role toward developing a direct route to efficiently utilize abundant post-combustion CO2 to produce value-added CH3OH in a green and sustainable manner. 21 3.4. Experimental Methods 3.4.1. Materials and methods All experiments were carried out under an inert atmosphere (with N2 or Ar) using standard Schlenk techniques with the exclusion of moisture unless otherwise stated. Catalyst Ru-Macho- BH (Strem Chemicals, 98%) was used as received without further purification. All catalysts were weighed inside an argon filled glove box. Tertiary amines TMEDA, TEEDA, THEEDA, Quadrol, PMDETA, HMTETA, TEA, MDEA, DEEA, TMBDA, TMHDA and DMAEE were purchased from commercial sources and used without further purification. Ethylene glycol (Sigma Aldrich), diethylene glycol (Fischer Scientific) and ethanol (Alfa Aesar) were sparged with N2 for 1 h prior to use. DMSO-d6 (CIL, D-99.9%) and imidazole (Fischer, 99.5%) were used as received. 1 H and 13 C NMR spectra were recorded on 400, 500 or 600 MHz Varian NMR spectrometers. 1 H and 13 C NMR chemical shifts were determined relative to the residual solvent signals (DMSO-d6). H2 (Gilmore, ultra-high pure grade 5.0) was used without further purification. Caution: Reactions are associated with H2 gas. They should be carefully handled inside proper fume hoods without any flame, spark or static electricity sources nearby. 117 3.4.2. Standard procedure for CO2 capture by alcohol solutions of tertiary amines Tertiary amines (10 mmol. amine content) were dissolved in the specified alcohol or water (5 mL) in a vial with a magnetic stir bar. The gases inside the vial were then removed under vacuum. CO2 was subsequently added while stirring the solution at 800 rpm and maintaining the CO2 pressure inside the reactor at 1 psi above atmospheric pressure. The amounts of CO2 captured were calculated through gravimetric analysis of the solutions before and after the capture. 3.4.3. Standard procedure for the hydrogenation reactions The capture of CO2 was performed prior to hydrogenation reactions. Upon completion of capture, in a nitrogen-filled chamber, the CO2-loaded amine solution, catalyst Ru-Macho-BH and an additional 5 mL of solvent were added to a 125 mL Monel Parr reactor. The vessel was then filled with H2 to the desired pressure (generally 70 bar unless otherwise stated). The reaction mixture was then stirred with a magnetic stirrer for 5 minutes (800 rpm) and subsequently placed in a preheated aluminum block with stirring at 800 rpm. After heating for a given period of time, the reactor was cooled to room temperature. The vessel was then cooled in an ice bath for 30 minutes and the gases inside the vessel were slowly released. Upon opening the reaction vessel, a known amount of imidazole (Im) was added as an internal standard to the homogeneous solution, which was then analyzed by 1 H and 13 C NMR with DMSO-d6 as the deuterated solvent. Yields were determined through 1 H NMR from integration ratios. 3.4.4. Standard procedure for probing the regeneration of the tertiary amine base CO2 capture was performed using 10 mmol of TMEDA or TMBDA in ethylene glycol (10 mL). The CO2-loaded solution, catalyst and additional solvent (10 mL) were added to a 125 mL Monel Parr reactor. The vessel was then filled with H2 to 70 bar. The reaction mixture was then 118 stirred with a magnetic stirrer for 5 minutes (800 rpm) and subsequently placed in a preheated aluminum block with stirring at 800 rpm. After heating for a given period of time, the reactor was cooled to room temperature. The vessel was then cooled in an ice bath for 30 minutes and the gases inside the vessel were slowly released. Afterwards, imidazole (Im) was added as an internal standard to a 0.5 mL aliquot of the homogeneous solution and analyzed by 1 H and 13 C NMR with DMSO-d6 as the deuterated solvent. The remaining solution was transferred to a 100 mL round- bottom flask. The gases inside the closed round bottom flask were then removed under vacuum. After that, CO2 was added while stirring the solution at 800 rpm and maintaining the CO2 pressure inside the reactor at 1 psi above atmospheric pressure. The amount of CO2 captured was calculated through gravimetric analysis of the solutions before and after the capture. 3.4.5. Standard procedure for CO2 capture from a simulated flue gas (10% CO2 in nitrogen) In a 30 mL vial, 10 mmol TMEDA or TMBDA was dissolved in 10 mL ethylene glycol. A simulated flue gas mixture containing 10% CO2 and 90% N2 was then bubbled overnight through the solution at a flowrate of 100 mL/min under stirring (800 rpm). The resulting solution was then sparged with N2 for 1 h. Afterwards, imidazole (Im) was added as an internal standard to a 0.5 mL aliquot of the homogeneous solution and analyzed by 1 H and 13 C NMR with DMSO-d6 as the deuterated solvent. The amount of CO2 captured was calculated through 13 C NMR analysis (relaxation delay = 20 seconds). The remaining solution was used for hydrogenation. 119 3.4.6. Representative Spectra Figure 3.2. Typical 13 C NMR spectra of ethylene glycol solution of TMEDA after CO2 capture (Table 3.1, Entry 1) in DMSO-d6. Figure 3.3. 13 C NMR spectra of aqueous solution of TMEDA after CO2 capture (Table 3.1, Entry 4) in DMSO-d6. - 10 0 1 0 2 0 3 0 4 0 5 0 6 0 70 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 f 1 ( p p m ) - 1 0 0 1 0 2 0 3 0 40 5 0 6 0 7 0 80 9 0 1 0 0 1 1 0 1 20 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0 f 1 ( p p m ) alkyl carbonate CO2 alkyl carbonate ethylene glycol TMEDA TMEDA bicarbonate CO2 TMEDA TMEDA 120 Figure 3.4. Typical 1 H NMR spectra of reaction mixture after hydrogenation reaction of captured CO2 (Table 3.2, Entry 15) in DMSO-d6. Figure 3.5. Typical 13 C NMR spectra of reaction mixture after hydrogenation reaction of captured CO2 (Table 3.2, Entry 15) in DMSO-d6. - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 1 . 6 0 1 . 0 0 0 . 5 0 0 . 0 3 - 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 f 1 ( p p m ) HCOO - Im Im TMEDA MeOH alkyl carbonate Im ethylene glycol TMEDA TMEDA HCOO - Im MeOH 121 Figure 3.6. Typical 13 C NMR spectra to observe CO2 captured by regenerated base in reaction mixture in DMSO-d6 (Table 3.3, Entry 1). Figure 3.7. Typical 13 C NMR spectra of CO2 captured from 10% CO2/N2 in DMSO-d6 (Table 3.4, Entry 1). - 1 0 0 1 0 20 3 0 4 0 5 0 60 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0 f 1 ( p p m ) - 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0 f 1 ( p p m ) 1 . 0 0 0 . 5 0 0 . 4 7 alkyl carbonate ethylene glycol TMEDA TMEDA HCOO - MeOH CO2 alkyl carbonate Im ethylene glycol TMEDA TMEDA Im 122 3.5. References [1] C. B. Field, V. R. Barros, Intergovernmental Panel on Climate Change. Working Group II, Climate change 2014 : Impacts, adaptation, and vulnerability : Working Group II contribution to the fifth assessment report of the Intergovernmental Panel on Climate Change, Cambridge University Press, New York, NY, 2014. [2] a) B. Obama, The irreversible momentum of clean energy. Science 2017, 355, 126-129; b) M. Pehl, A. Arvesen, F. Humpenoder, A. Popp, E. G. Hertwich, G. Luderer, Understanding future emissions from low-carbon power systems by integration of life-cycle assessment and integrated energy modelling. Nat. Energy 2017, 2, 939-945. [3] A. Goeppert, M. Czaun, G. K. S. Prakash, G. A. Olah, Air as the renewable carbon source of the future: an overview of CO2 capture from the atmosphere. Energy Environ. Sci. 2012, 5, 7833- 7853. [4] a) Y. Wang, L. Zhao, A. Otto, M. Robinius, D. Stolten, A Review of Post-combustion CO2 Capture Technologies from Coal-fired Power Plants Energy Procedia 2017, 114, 650-665; b) J. G. J. Oliver, J. A. H. W. Peters, Trends in Global CO2 And Total Greenhouse Gas Emissions, PBL Netherlands Environmental Assessment Agency, The Hague, 2019. [5] a) B. Adderley, J. Carey, J. Gibbins, M. Lucquiaud, R. Smith, Post-combustion carbon dioxide capture cost reduction to 2030 and beyond. Faraday Discuss. 2016, 192, 27-35; b) D. Bhattacharyya, D. C. Miller, Post-combustion CO2 capture technologies — a review of processes for solvent-based and sorbent-based CO2 capture. Curr. Opin. Chem. Eng. 2017, 17, 78-92; c) A. S. Bhown, B. C. Freeman, Analysis and Status of Post-Combustion Carbon Dioxide Capture Technologies. Environ. Sci. Technol. 2011, 45, 8624-8632; d) D. Leaf, H. J. H. Verolme, W. F. Hunt Jr., Overview of regulatory/policy/economic issues related to carbon dioxide. Environ. Int. 2003, 29, 303-310; e) R. Rehan, M. Nehdi, Carbon dioxide emissions and climate change: policy implications for the cement industry. Environ. Sci. Policy 2005, 8, 105-114. [6] a) Q. Liu, L. Wu, R. Jackstell, M. Beller, Using carbon dioxide as a building block in organic synthesis. Nat. Commun. 2015, 6, 5933; b) A. Goeppert, M. Czaun, J. P. Jones, G. K. S. Prakash, G. A. Olah, Recycling of carbon dioxide to methanol and derived products – closing the loop. Chem. Soc. Rev. 2014, 43, 7995-8048. [7] a) M. Bertau, H. Offermanns, L. Plass, F. Schmidt, H.-J. r. Wernicke, H. Springer-Verlag GmbH, Methanol: The Basic Chemical and Energy Feedstock of the Future Asinger's Vision Today, 2016; b) G. A. Olah, A. Goeppert, G. K. S. Prakash, V. C. H. Wiley, Beyond Oil and Gas: The Methanol Economy, 2018. [8] a) J. Kothandaraman, S. Kar, A. Goeppert, R. Sen, G. K. S. Prakash, Advances in Homogeneous Catalysis for Low Temperature Methanol Reforming in the Context of the Methanol Economy. Top. Catal. 2018, 61, 542-559; b) J. Kothandaraman, S. Kar, R. Sen, A. Goeppert, G. A. Olah, G. 123 K. S. Prakash, Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol. J. Am. Chem. Soc. 2017, 139, 2549-2552. [9] a) S. Kar, A. Goeppert, G. K. S. Prakash, Integrated CO2 Capture and Conversion to Formate and Methanol: Connecting Two Threads. Acc. Chem. Res. 2019, 52, 2892-2903; b) W. D. Jones, Carbon Capture and Conversion. J. Am. Chem. Soc. 2020, 142, 4955-4957. [10] a) K. Z. House, C. F. Harvey, M. J. Aziz, D. P. Schrag, The energy penalty of post-combustion CO2 capture & storage and its implications for retrofitting the U.S. installed base. Energy Environ. Sci. 2009, 2, 193-205; b) J. Leclaire, D. J. Heldebrant, A call to (green) arms: a rallying cry for green chemistry and engineering for CO2 capture, utilisation and storage. Green Chem. 2018, 20, 5058-5081. [11] a) S. Kar, R. Sen, A. Goeppert, G. K. S. Prakash, Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy. J. Am. Chem. Soc. 2018, 140, 1580-1583; b) S. Kar, R. Sen, J. Kothandaraman, A. Goeppert, R. Chowdhury, S. B. Munoz, R. Haiges, G. K. S. Prakash, Mechanistic Insights into Ruthenium- Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol J. Am. Chem. Soc. 2019, 141, 3160-3170; c) J. R. Khusnutdinova, J. A. Garg, D. Milstein, Combining Low-Pressure CO2 Capture and Hydrogenation To Form Methanol. ACS Catal. 2015, 5, 2416- 2422; d) J. Kothandaraman, A. Goeppert, M. Czaun, G. A. Olah, G. K. S. Prakash, Conversion of CO2 from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst. J. Am. Chem. Soc. 2016, 138, 778-781. [12] a) M. Jahandar Lashaki, S. Khiavi, A. Sayari, Stability of amine-functionalized CO2 adsorbents: a multifaceted puzzle. Chem. Soc. Rev. 2019, 48, 3320-3405; b) A. Goeppert, H. Zhang, R. Sen, H. Dang, G. K. S. Prakash, Oxidation-Resistant, Cost-Effective Epoxide-Modified Polyamine Adsorbents for CO2 Capture from Various Sources Including Air. ChemSusChem 2019, 12, 1712-1723. [13] R. Sen, A. Goeppert, S. Kar, G. K. S. Prakash, Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol. J. Am. Chem. Soc. 2020, 142, 4544-4549. [14] a) A. Heydari-Gorji, Y. Belmabkhout, A. Sayari, Degradation of amine-supported CO2 adsorbents in the presence of oxygen-containing gases. Microporous Mesoporous Mater. 2011, 145, 146-149; b) P. Bollini, S. Choi, J. H. Drese, C. W. Jones, Oxidative Degradation of Aminosilica Adsorbents Relevant to Postcombustion CO2 Capture. Energy Fuels 2011, 25, 2416- 2425. [15] a) J. Kothandaraman, R. A. Dagle, V. L. Dagle, S. D. Davidson, E. D. Walter, S. D. Burton, D. W. Hoyt, D. J. Heldebrant, Condensed-phase low temperature heterogeneous hydrogenation of CO2 to methanol. Catal. Sci. Technol. 2018, 8, 5098-5103; b) J. Kothandaraman, D. J. Heldebrant, Towards environmentally benign capture and conversion: heterogeneous metal catalyzed CO2 hydrogenation in CO2 capture solvents. Green Chem. 2020, 22, 828-834. 124 [16] D. J. Heldebrant, J. Kothandaraman, in Carbon Capture and Storage, The Royal Society of Chemistry, 2020, pp. 36-68. [17] J. E. Rainbolt, P. K. Koech, C. R. Yonker, F. Zheng, D. Main, M. L. Weaver, J. C. Linehan, D. J. Heldebrant, Anhydrous tertiary alkanolamines as hybrid chemical and physical CO2 capture reagents with pressure-swing regeneration. Energy Environ. Sci. 2011, 4, 480-484. [18] S. Xie, W. Zhang, C. Jia, S. S. G. Ong, C. Zhang, S. Zhang, H. Lin, Eliminating carbon dioxide emissions at the source by the integration of carbon dioxide capture and utilization over noble metals in the liquid phase. J. Catal. 2020, 389, 247-258. [19] D. J. Heldebrant, P. K. Koech, V. A. Glezakou, R. Rousseau, D. Malhotra, D. C. Cantu, Water- Lean Solvents for Post-Combustion CO2 Capture: Fundamentals, Uncertainties, Opportunities, and Outlook. Chem. Rev. 2017, 117, 9594-9624. [20] a) S. Kar, J. Kothandaraman, A. Goeppert, G. K. S. Prakash, Advances in catalytic homogeneous hydrogenation of carbon dioxide to methanol. J. CO2 Util. 2018, 23, 212-218; b) N. M. Rezayee, C. A. Huff, M. S. Sanford, Tandem Amine and Ruthenium-Catalyzed Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2015, 137, 1028-1031; c) L. Piccirilli, D. Lobo Justo Pinheiro, M. Nielsen, Recent Progress with Pincer Transition Metal Catalysts for Sustainability. Catalysts 2020, 10, 773; d) C. Gunanathan, D. Milstein, Bond Activation and Catalysis by Ruthenium Pincer Complexes. Chem. Rev. 2014, 114, 12024-12087. [21] G. A. Olah, G. K. S. Prakash, A. Goeppert, Anthropogenic Chemical Carbon Cycle for a Sustainable Future. J. Am. Chem. Soc. 2011, 133, 12881-12898. 125 Chapter 4. Hydroxide Based Integrated Direct Air Capture of CO2 and Conversion: Hydrogenation of Metal Carbonates to Methanol This dissertation chapter is based on a recent research article from our group published in the Journal of the American Chemical Society (Sen, R.; Goeppert, A.; Kar, S.; Prakash, G. K. S., J. Am. Chem. Soc. 2020, 142, 4544-4549). Part of the article is reprinted by permission of the American Chemical Society. Copyright 2020. American Chemical Society. 4.1. Introduction Carbon dioxide, a potent greenhouse gas is the main component of industrial effluents and automobile emissions. 1 Over the past two centuries we witnessed a major rise of anthropogenic CO2 emissions leading to an exponential increase in atmospheric CO2 concentration from about 280 ppm to more than 415 ppm, currently. 2 This phenomenon has already led to a substantial average global temperature rise of ~0.8 °C compared to pre-industrial levels. Related problems such as global sea rise, ocean acidification, more extreme and unpredictable weather patterns have also emerged. 3 In a united effort to curb global warming, the Kyoto Protocol followed by the Paris Agreement were signed by most countries with the aim of curbing greenhouse gas emissions and transition to renewable energy alternatives. 4 Among various proposed solutions, direct air capture of CO2 (DAC) has recently attracted momentous attention. This process allows the point of capture to be independent from the emission sources. Hence, it enables capture stations to be free of geographical constraints and allows the capture of CO2 from small and dispersed point sources responsible for about half of total 126 emissions. 5 Once CO2 is captured, it can be desorbed, pressurized and sequestered under earth’s surface in what is known as Carbon Capture and Sequestration (CCS). 6 An alternate and value- added route is the utilization of the captured CO2 (Carbon Capture and Utilization, CCU) as a C1 source by conversion into various products including chemical feedstocks and polymers. 7 CO2 can be hydrogenated with H2 generated through H2O electrolysis using renewable energy. 8 Among possible hydrogenation products, methanol is one of the most attractive. With a current annual global production of over 100 billion liters, CH3OH is already a C-1 feedstock for a multitude of chemicals and products and a superior fuel for internal combustion engines and fuel cells. CH3OH is also an attractive liquid hydrogen carrier (12.6 wt% of H2). 9 Figure 4.1. Reported integrated CO2 capture and conversion systems compared to the present study Recently, the integration of CO2 capture from air and subsequent conversion to methanol was spearheaded by us as well as others (Figure 4.1). 10 Until now, such systems used amine as R H N R + CO 2 O N O R R R 2 NH 2 H 2 cat. O H O R 2 NH 2 R H N R + CO 2 O N O R R R 2 NH 2 3 H 2 cat. R H N + CO 2 MO OH O H 2 cat. Previous work Amine-assisted systems Alkali hydroxide-assisted systems MOH MO H O + H 2 O + CO 2 MO OR O 3 H 2 cat. MOH CH 3 OH + ROH + R MOH -H 2 O M = Na, K bicarbonate/ alkyl carbonate CO 2 / Air M + RCO 3 - MOH H 2 , 100-140 o C CH 3 OH MOH Ru CH 3 OH a) integrated capture and conversion by hydroxide and alcohol to methanol b) schematic representation of the present “one-pot” system This work = ethylene glycol + H 2 O + CH 3 OH ROH 127 capturing agents. These amine systems can suffer from volatility issues, oxidative degradation and notable toxicity, making them less suitable for DAC in a scaled-up process. 11 Alternate CO2 scrubbing agents used on a large scale are alkali hydroxide solutions. Hydroxides have higher capture efficiency from dilute CO2 sources than amines. 5c However, recycling of the base through a conventional causticization-calcination-slaking process leads to a high energy penalty with required temperatures of 700 °C and above. 5c, 12 On the other hand, practical utilization of metal carbonates as C1-building blocks is not well established. 13 In our previous work on alkali hydroxide-based capture and conversion, we captured CO2 from air and synthesized formate salts. However, further hydrogenation of the formate to CH3OH was found ineffective. 13b, 14 Alcohol-assisted CO2 hydrogenation to methanol via formate ester has been studied in great details. 10h, 15 Hence, we speculated that alcohols should be able to mediate the hydrogenation pathway from formate salt to methanol through the key formate ester intermediate. It was also shown that formamides are essential intermediates for amine assisted hydrogenation of CO2 to CH3OH (Figure 1). 10f Herein, we report a catalytic system for CO2 capture and conversion to CH3OH involving alkali-metal hydroxides (capturing agent) and alcohol (formate ester facilitator). This process has numerous advantages, including: (1) alkali hydroxides have high efficiency for DAC of CO2; (2) widespread availability of the hydroxide bases; (3) low toxicity, volatility and higher stability over amines; (4) hydrogenation of ester intermediates is more facile than formamide intermediates. 15d Additionally, for the first time, this system offers an easy integration with already existing hydroxide-based CO2 scrubbing industries to utilize the captured CO2 to produce value-added methanol. 128 4.2. Process Discovery and Optimization 4.2.1. Integrated CO2 capture and catalytic conversion To validate our proposed alcohol assisted pathway, we initially explored the yet unreported hydrogenation of formate salt to methanol. 13a, 15g Ethylene glycol was chosen as the alcohol solvent for its high boiling point and relative non-toxicity. In accordance with our hypothesis, quantitative conversion of HCOOK to CH3OH was observed under 70 bar of H2 at 140 °C in the presence of catalyst Ru-Macho-BH (C-1). Similarly, KHCO3 under identical conditions formed CH3OH with 92% yield as observed by 1 H NMR (Figure 4.2). The formation of ester could not be detected by NMR after reaction completion, suggesting that the formate salt is thermodynamically favored, which is typical under alkaline conditions. Figure 4.2. Proof of Concept: Hydrogenation of bicarbonate and formate salts to methanol. Reaction conditions: substrate (2.5 mmol), ethylene glycol (5 mL), 70 bar H2, 140 °C, catalyst (0.5 mol%), 48 h. Yields determined by 1 H NMR ( t BuOH as internal standard). Analogous to metal hydrogen (bi) carbonates, metal alkyl carbonates are formed when CO2 reacts with metal alkoxides. While this concept is well established, its role in the field of carbon capture has not been well explored. 16 In 2016, Wee et al. demonstrated that CO2 capture in an ethanolic NaOH solution formed sodium ethyl carbonate having minimal solubility in ethanol (Table 4.1, entry 1). 17 For the present study, the alcohol should have the following properties: (1) H O OK HO O OK CH 3 OH Yield = 92% Yield = 100% Ru-Macho-BH H 2 HO OH bicarbonate formate 129 high boiling point for easy separation of methanol by distillation, and (2) high dielectric constant enabling facile dissolution of the hydroxide base and the formed metal alkyl carbonate for efficient hydrogenation. Hence, ethylene glycol (b.p. = 197.6 °C, ε= 37) was selected for our one pot CCU system. Interestingly, a solution of NaOH in ethylene glycol captured CO2 quantitatively and the obtained carbonates were completely soluble (entry 2). The capture capacity of a KOH solution was similar to that of NaOH (entry 3). With Ca(OH)2, the CO2 capture was significantly lower. Table 4.1. CO2 capture by alcoholic hydroxide solutions Entry a Base CO2 captured (mmol) b CO2/ OH c Solubility of carbonate 1 d NaOH 2.5 ~1 insoluble 2 NaOH 2.5 ~1 soluble 3 KOH 2.5 ~1 soluble 4 e Ca(OH)2 1.5 ~0.6 insoluble a Capture conditions: base (2.5 mmol), ethylene glycol (5 mL), stirring (800 rpm), rt, t = 3 h. b Captured CO2 amounts determined gravimetrically. c Moles of CO2 captured per mole of hydroxide. d Ethylene glycol replaced by EtOH. e 1.25 mmol of Ca(OH)2 was used. Calculations error ±5%. Following the capture, the CO2-loaded solutions were hydrogenated under 70 bar H2 in presence of Ru-Macho-BH (C-1) (Table 4.2). This catalyst was already well-studied by our group and others for CO2 hydrogenation to methanol. KOH was chosen as the model alkali hydroxide due to fast dissolution in ethylene glycol. At 140 °C, with 0.5 mol% catalyst loading, full conversion of the captured CO2 was achieved with a methanol yield of 80% within 10 h. When the reaction time was extended to 20 h, quantitative yield of CH3OH was obtained. Ru-Macho (C-2) displayed an activity similar to C-1 with 92% CH3OH yield. When the P-substituents in the pincer catalyst were replaced from R=Ph to R= i Pr, t Bu or Cy (C-3 to C-5), the CH3OH yield dropped drastically. Similarly, a meager 13% CH3OH yield was observed when the N-H moiety of C-2 was 130 replaced with N-Me in C-6 suggesting the involvement of a Ru-N cooperative mechanism. With NaOH as the base, CH3OH was produced with 93% yield. CO2-loaded Ca(OH)2 solution was not hydrogenated efficiently. When the C-1 catalyst loading was lowered to 0.25 and 0.1 mol%, the CH3OH yield decreased to 90% (TON = 360) and 48% (TON = 480), respectively. Upon lowering the temperature to 120 °C CH3OH was obtained with 99% yield after 72 h. Further lowering the temperature to 100 °C resulted in a CH3OH yield of 31% and 60% at 0.5 and 1 mol% catalyst loading, respectively. From these results, it is clear that the glycol mediated Ru-PNP based catalytic system is significantly active for CH3OH synthesis even at relatively low temperatures. The effect of H2 pressure on the hydrogenation reaction was also evaluated and was found to follow an expected trend for methanol production (Table 4.3). Table 4.2. Tandem hydrogenation of captured CO2 in ethylene glycol Entry a MOH Cat (mol%) T ( o C) Time (h) Formate (%) b MeOH (%) b TONMeOH 1 KOH C-1 (0.5) 140 10 20 80 160 2 KOH C-1 (0.5) 140 20 0 100 200 3 KOH C-2 (0.5) 140 20 8 92 184 4 KOH C-3 (0.5) 140 20 90 10 20 5 KOH C-4 (0.5) 140 20 100 0 0 6 KOH C-5 (0.5) 140 20 76 6 12 7 KOH C-6 (0.5) 140 20 87 13 26 8 NaOH C-1 (0.5) 140 20 1 93 186 O O M O - H 2 O CO 2 70 bar H 2 CH 3 OH + MOH + HO HO OH MOH HO OH + CO 2 capture in situ hydrogenation Ru T, t Ru P CO N P H H Ph Ph Ph Ph H Ru P CO N P H H Ph Ph Ph Ph Cl BH 3 Ru P CO N P H H i Pr i Pr i Pr i Pr Cl Ru P CO N P H H Cy Cy Cy Cy Cl Ru P CO N P H H t Bu t Bu t Bu t Bu Cl C-1 C-2 C-3 C-4 C-5 Ru P CO N P Me H Ph Ph Ph Ph Cl C-6 Ru-Macho-BH Ru-Macho 131 9 c Ca(OH)2 C-1 (0.5) 140 20 5 20 24 10 KOH C-1 (0.25) 140 72 10 90 360 11 KOH C-1 (0.1) 140 72 52 48 480 12 KOH C-1 (0.5) 120 72 1 99 198 13 KOH C-1 (0.5) 100 72 69 31 62 14 KOH C-1 (1) 100 72 40 60 60 a Reaction conditions: solutions from Table 4.1 (as specified) were directly hydrogenated. Captured CO2 = 2.5 mmol, H2 = 70 bar. b Yields determined by 1 H NMR ( t BuOH as internal standard). c Captured CO2 = 1.5 mmol. Yield calculations error ±5%. TON = moles of methanol per mole of catalyst. Table 4.3. Variation of hydrogen pressure in hydrogenation reactions Entry a H2 (bar) Formate (%) b MeOH (%) b TONMeOH 1 10 77 23 46 2 40 29 71 142 3 70 0 100 200 a Reaction conditions: capture solution: 2.5 mmol KOH in 5 mL ethylene glycol. Captured CO2 = 2.5 mmol. T = 140 o C, t = 20 h, Catalyst (C-1, 0.5 mol%). b Yields were determined based on 1 H NMR with t BuOH as internal standard. Yield calculations error ±5%. TON = moles of methanol/moles of catalyst. 4.2.2. Investigating base regeneration and parasitic reaction pathways During the hydrogenation reactions, formation of potassium glycolate (PG) was observed in addition to CH3OH (Table 4.4). To investigate this unforeseen observation, a set of control experiments were performed (Figure 4.3B). It was revealed that under the hydrogenation conditions, a solution of KOH in ethylene glycol was converted to PG. This phenomenon was not observed in the absence of catalyst. Dehydrogenation of alcohols using such catalytic systems have been well documented. 18 Yet, our observation of dehydrogenation at such high H2 pressure is entirely unprecedented. The formation of PG during the hydrogenation reactions indicates that only a fraction of the base remained available for subsequent CO2 capture (Figure 4.3A). To confirm the partial regeneration of the base, a scaled-up reaction was performed with 20 mmol of 132 captured CO2 and catalyst loading of 0.1% (Figure 4.3C). 9.6 mmol CH3OH, 10.4 mmol HCOOK and 3.3 mmol PG were produced after 72 h. When this reaction solution was directly subjected to subsequent CO2 capture, only about 5 mmol of CO2 was captured by the regenerated base. Table 4.4. Formation of potassium glycolate (PG) during hydrogenation reactions Entry a Catalyst loading (mol%) T ( o C) Time (h) MeOH (mmol) PG (mmol) Deactivated base (%) b 1 0.5 140 10 2.0 1.4 70 2 0.5 140 20 2.5 2.4 96 3 0.25 140 72 2.25 2.2 97 4 0.1 140 72 1.2 0.4 33 5 0.5 120 72 2.45 2.4 98 a Reaction conditions: capture solution: 2.5 mmol KOH in 5 mL ethylene glycol. Captured CO2 = 2.5 mmol, H2 = 70 bar. Catalyst (C-1). b Yields were determined based on 1 H NMR with t BuOH as internal standard. Yield calculations error ±5%. b Moles of potassium glycolate/theoretical yield of KOH (= moles of MeOH). Figure 4.3. Insights into the reaction sequences and base regeneration. RO O K O H 2 , H O K O - ROH H OR O CH 3 OH metal alkyl carbonate potassium formate alkyl formate 2 H 2 , - ROH Ru Ru A. Plausible reaction sequences during hydrogenation of captured CO 2 ROH Pathway 1: Base Regeneration Ru - 2H 2 HO COOK Pathway 2: Base Deactivation KOH R = HO-CH 2 -CH 2 - HO OH HO OH + KOH 140 o C, t = 20 h H 2 (70 bar) HO COOK + 2H 2 B. Control experiments to probe base deactivation potassium glycolate (PG) catalyst PG (%) Ru-Macho-BH (0.5 mol%) 99 - 0 5 mL 2.5 mmol potassium glycolate (PG) C. Detection of regenerated hydroxide base O O K O HCOOK + CH 3 OH + KOH + HO HO COOK H 2 (70 bar) 140 o C - H 2 O CO 2 KOH HO OH + C-1 (0.1 mol%) 72 h 20 mmol CO 2 5 mmol CO 2 captured 9.6 mmol 3.3 mmol 10.4 mmol 133 In order to investigate the formation of potassium glycolate during the hydrogenation reactions, a set of control experiments were carried out. It was observed that ethylene glycol in the presence of KOH and catalyst C-1 was dehydrogenated to potassium glycolate at 70 bar H2. However, dehydrogenation was not observed in absence of either the base or the catalyst. The formation of dihydrogen was also observed as additional pressure buildup in the reactor (Figure S9). The gas mixture was pure H2 as analyzed using gas chromatography. Moreover, no additional products were detected using GC-MS. Figure 4.4. Control experiments to probe the fate of ethylene glycol and KOH. When a solution of potassium glycolate in ethylene glycol was subjected to hydrogenation (70 bar H2, 140 °C) in the presence of catalyst C-1, less than 1% conversion to ethylene glycol was observed. Also, no other products were detected by NMR ( 1 H and 13 C) and GC-MS analysis of the reaction mixtures. Further, no gaseous side products were observed after analyzing the unreacted gas mixture using gas chromatography. It is evident that under the given conditions, the equilibrium is predominantly towards potassium glycolate formation. It should be noted, however, that further mechanistic and thermodynamic evidence are required to completely understand the phenomenon. HO OH + KOH HO COOK + 2H 2 ∼ 99% C-1 (0.5 mol%) H 2 (70 bar), 140 o C, 20 h HO OH + KOH no conversion HO OH H 2 (70 bar), 140 o C, 20 h C-1 (0.5 mol%) H 2 (70 bar), 140 o C, 20 h no conversion HO OH + KOH HO COOK + 2H 2 ∼ 99% ∼ 1% at 70 bar H 2 , 140 o C 134 Figure 4.5. Pressure and temperature profile with time for a control experiment similar to one in Figure 4.3B, Reaction conditions: KOH (10 mmol), ethylene glycol (20 mL), catalyst C-1 (0.25 mol%), H2 = 70 bar, 140 o C, 72 h. Finally, the efficacy of the system was tested for DAC of CO2 and conversion to CH3OH. Following our reported protocol, 13b indoor air was bubbled through a solution of 5 mmol KOH in 10 mL ethylene glycol. After 48 h, 3.3 mmol of CO2 was captured in the form of carbonate and alkyl carbonate salts (Table 4.5). Additionally, there was an accumulation of around 3 mL of H2O from moisture in air. When the resulting solution was hydrogenated, CH3OH was obtained with 25% yield after 20 h. When the reaction was extended to 72 h, the captured CO2 was completely converted to CH3OH. Table 4.5. CO2 capture from ambient air and conversion to CH3OH Entry a Captured CO2 (mmol) Time (h) Formate (%) b MeOH (%) b 1 3.3 20 75 25 2 3.3 72 0 100 a Reaction conditions: 140 °C. b Yields determined by 1 H NMR ( t BuOH as internal standard). Yield calculations error ±5%. 0 50 100 150 200 250 300 350 400 450 500 800 900 1000 1100 1200 1300 1400 1500 0 50000 100000 150000 200000 250000 300000 350000 400000 Temperature ( o C) Pressure (psi.) time (sec.) Hydrogen pressure Temperature P initial = 1056 psi. P final = 1110 psi. ∆P = 54 psi. (corresponds to 18 mmol H 2 ) Air + KOH r.t., 48 h. CO 2 captured as carbonate/ alkyl carbonate salts (~ 415 ppm CO 2 ) CH 3 OH in situ hydrogenation direct air capture of CO 2 5 mmol. ethylene glycol (10 mL) C-1 (0.5 mol%) H 2 (70 bar) 135 4.2.3. Investigating hydrogenation of metal carbonates and bicarbonates to methanol The novel chemical reactivity of metal carbonates and bicarbonates for their catalytic conversion to methanol was briefly explored. Hydrogenation of potassium bicarbonate was performed in presence of a library of solvents. As shown in Table 4.6, the substrate (KHCO3) was readily converted in all the solvents screened, however only up to the formate stage. Further hydrogenation to methanol was only achieved efficiently in selected solvents, which were primary alcohols. Among these, ethylene glycol provided the highest methanol yield. Table 4.6. Hydrogenation of potassium bicarbonate to methanol in different solvents Entry a Solvent Conv. (%) MeOH (%) HCOO- (%) 1 methanol 98 16 84 2 ethanol 72 34 38 3 trifluoroethanol 87 15 72 4 2-methoxyethanol 100 53 47 5 ethylene glycol 100 99 1 6 1,2-propylene glycol 100 85 15 7 isopropanol 42 8 34 8 2,3-butane diol 73 13 60 9 water 82 5 77 a solvent = 10 mL. b Yields determined by 1 H NMR ( t BuOH as internal standard). Yield calculations error ±5%. Further, the catalytic conversion of carbonate/bicarbonate salts to methanol was investigated at different temperatures. As shown in Figure 4.6 and Figure 4.7, hydrogenation of bicarbonate (KHCO3) as well carbonate (K2CO3) was significantly active at temperatures lower than 140 °C. Even at 100 °C, methanol yields of 39% and 54% were achieved from K2CO3 and KO O OH + 3 H 2 solvent 140 °C, H 2 (70 bar), 20 h Ru-Macho-BH (0.5 mol%) CH 3 OH + KOH + H 2 O 2.5 mmol 136 KHCO3, respectively within 20 h. Additionally, the parasitic side reaction of glycolate formation could be significantly arrested by lowering the reaction temperature. In fact, when the hydrogenations were performed for 60 h and with 1 mol% catalyst loading, methanol formation was observed even near room temperatures (30-40 °C), although the yields were low and will require further optimization. Figure 4.6. Hydrogenation of potassium bicarbonate to methanol at different temperatures. Figure 4.7. Hydrogenation of potassium carbonate to methanol at different temperatures. 140 120 100 methanol 98.9 70.0 53.6 formate 1.1 30.0 46.4 glycolate 90.4 19.0 13.3 base regeneration 9.6 81.0 86.7 0.0 20.0 40.0 60.0 80.0 100.0 Product Yield (%) temp. (°C) HO OH + KOH Ru-Macho-BH HO OK O + 2 H 2 140 120 100 methanol 100.0 74.1 39.0 formate 0.0 25.9 60.3 glycolate 67.1 23.4 7.7 base regeneration 32.9 76.6 92.3 0.0 20.0 40.0 60.0 80.0 100.0 Product Yield (%) temp. (°C) HO OH + KOH Ru-Macho-BH HO OK O + 2 H 2 137 4.3. Conclusion In conclusion, we have developed a novel amine-free system for integrated CO2 capture and conversion to methanol. The alkali hydroxide captured CO2 quantitatively in ethylene glycol, forming alkyl carbonate salts, which were efficiently hydrogenated using Ru-PNP catalysts. With Ru-Macho-BH, quantitative conversion to CH3OH was achieved within 20 h at 140 °C. The catalyst showed significant activity for CH3OH synthesis at temperatures as low as 100 °C (yield = 60%). Additionally, to the best of our knowledge, the present report is the first example demonstrating hydrogenation of bicarbonate and formate salts to CH3OH; as well as low temperature regeneration of the hydroxide base. Insights into the reaction pathways revealed that a fraction of the base produced was rendered inactive by the formation of carboxylates through in situ dehydrogenation of ethylene glycol. The current system offers various advantages: (1) high efficiency with minimal stability issues for CO2 capture (2) facile hydrogenation to CH3OH via an ethylene glycol mediated pathway (3) highly promising for direct air capture. For large scale deployment of the CCU process revealed in this study, recycling of the active elements will play a key role. Our ongoing efforts are therefore geared towards retaining complete efficiency of the capturing species and develop a flow system to achieve continuous CO2 capture and methanol production. This report is essentially the first key step towards employing hydroxide-based CO2 scrubbing and economical regeneration of the base in addition to the production of value-added methanol with the goal of achieving a sustainable and carbon neutral cycle. 138 4.4. Experimental Methods 4.4.1 Materials and methods All experiments were carried out under an inert atmosphere (with N2 or Ar) using standard Schlenk techniques with the exclusion of moisture unless otherwise stated. Complexes Ru-Macho- BH (C-1, Strem Chemicals, 98%), Ru-Macho (C-2, Strem Chemicals, 98%), RuHClPNP iPr (CO) (C-3, Strem Chemicals, 98%), RuHClPNP tBu (CO) (C-4, Strem Chemicals, 98%) and RuHClPNP Cy (CO) (C-5, Strem Chemicals, 98%) were used as received without further purification. RuHClPNMeP Ph (CO) (C-6) was prepared following a reported procedure. 1 All catalysts were weighed inside an argon filled glove box. Sodium hydroxide, potassium hydroxide, calcium hydroxide, glycolic acid, potassium formate and potassium bicarbonate were purchased from commercial sources and used without further purification. Ethylene glycol (anhydrous, Sigma Aldrich) and ethanol (anhydrous, Alfa Aesar) were sparged with N2 for 1 h prior to use. DMSO-d6 (CIL, D-99.9%), imidazole (Fischer, 99.5%) and t BuOH (99%, Sigma Aldrich) were used as received. 1 H and 13 C NMR spectra were recorded on 400 or 500 MHz, Varian NMR spectrometers. 1 H and 13 C NMR chemical shifts were determined relative to the residual solvent signals (DMSO-d6). H2 (Gilmore, ultra-high pure grade 5.0) was used without further purification. Caution: Reactions are associated with H2 gas. They should be carefully handled inside proper fume hoods without any flame, spark or static electricity sources nearby. 4.4.2 Standard procedure for the hydrogenation of KHCO3 and HCOOK In a nitrogen-filled chamber, 2.5 mmol of the substrate, catalyst C-1 and ethylene glycol (5 mL) were added to a 125 mL Monel Parr reactor equipped with a magnetic stir bar, thermocouple and piezoelectric pressure transducer. The vessel was then filled with H2 to the desired pressure 139 (70 bar). The reaction mixture was then stirred with a magnetic stirrer for 5 minutes (800 rpm) and subsequently placed in a preheated oil bath with stirring at 800 rpm. After heating for a given period of time, the reactor was cooled to room temperature. The vessel was then cooled in an ice bath for 30 minutes and the gas inside was slowly released. Upon opening the reaction vessel, a known amount of t BuOH was added as an internal standard to the homogeneous solution, which was then analyzed by 1 H and 13 C NMR with DMSO-d6 as the deuterated solvent. Yields were determined through 1 H NMR from integration ratios. 4.4.3 Standard procedure for CO2 capture by ethylene glycol solutions of alkali hydroxides Metal hydroxide bases (2.5 mmol) were dissolved in ethylene glycol (5 mL) in a vial with a magnetic stir bar. The gases inside the vial were then removed under vacuum. CO2 was subsequently added while stirring the solution at 800 rpm and maintaining the CO2 pressure inside the reactor at 1 psi above atmospheric pressure. The amounts of CO2 captured were calculated through gravimetric analysis of the solutions before and after the capture. 4.4.4 Standard procedure for the hydrogenation reactions The capture of CO2 was performed in the same reactor used subsequently for hydrogenation reactions. Upon completion of capture, in a nitrogen-filled chamber, catalyst C-1/C- 2/C-3/C-4/C-5 or C-6 was added to the solution. The vessel was then filled with H2 to the desired pressure (generally 70 bar unless otherwise stated). The reaction mixture was then stirred with a magnetic stirrer for 5 minutes (800 rpm) and subsequently placed in a preheated oil bath with stirring at 800 rpm. After heating for a given period of time, the reactor was cooled to room temperature. The vessel was then cooled in an ice bath for 30 minutes and the gases inside were slowly released. Upon opening the reaction vessel, a known amount of t BuOH was added as an 140 internal standard to the homogeneous solution which was then analyzed by 1 H and 13 C NMR with DMSO-d6 as the deuterated solvent. Yields were determined through 1 H NMR from integration ratios. 4.4.5 Standard procedure for probing base regeneration After reaction workup, the solution was transferred to a 30 mL vial. The gases inside the vial were then removed under vacuum. CO2 was then added while stirring the solution at 800 rpm and maintaining the CO2 pressure inside the reactor at 1 psi above atmospheric pressure. The amount of CO2 captured was calculated through gravimetric analysis of the solutions before and after the capture. 4.4.6 Standard procedure for the control experiments In a nitrogen-filled chamber, 2.5 mmol of KOH, catalyst C-1 (0 or 0.5 mol%) and ethylene glycol (5 mL) were added to a 125 mL Monel Parr reactor equipped with a magnetic stir bar, thermocouple and piezoelectric pressure transducer. The vessel was then filled with H2 to the desired pressure (70 bar). The reaction mixture was then stirred with a magnetic stirrer for 5 minutes (800 rpm) and subsequently placed in a preheated oil bath with stirring at 800 rpm. After heating for a given period, the reactor was cooled to room temperature. The vessel was then cooled in an ice bath for 30 minutes and the gases inside were slowly released. Upon opening the reaction vessel, a known amount of imidazole was added as an internal standard to the homogeneous solution which was then analyzed by 1 H and 13 C NMR with DMSO-d6 as the deuterated solvent. Yields were determined through 1 H NMR from integration ratios. 141 4.4.7 Standard procedure of CO2 capture from air In a 30 mL vial, 5 mmol KOH was dissolved in 10 mL ethylene glycol. Atmospheric air containing ~415 ppm CO2 was then bubbled through the solution at a flowrate of 200 mL/min for 48 h using a pump. The gas outlet was connected to an IR based CO2 detector to monitor the CO2 saturation of the solution. After completion of the CO2 capture, the solution volume was ~13.3 mL due to accumulation of moisture from air. The resulting solution was then sparged with N2 for 1 h. Afterwards, imidazole (Im) was added as an internal standard to 0.5 mL aliquot of the homogeneous solution and analyzed by 1 H and 13 C NMR with DMSO-d6 as the deuterated solvent. The amount of CO2 captured was calculated through 13 C NMR analysis (relaxation delay = 25 seconds). The remaining solution was used for hydrogenation. 4.4.8 Representative Spectra Figure 4.8 Typical 1 H NMR spectra of hydrogenation reactions presented in Figure 4.2 (in DMSO- d6). - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 f 1 ( p p m ) 1 . 0 0 0 . 8 6 MeOH t BuOH PG 142 Figure 4.9 13 C NMR spectra of ethylene glycol solution after CO2 capture (Table 4.1) in DMSO- d6. Figure 4.10 Typical 1 H spectra of reaction mixture after hydrogenation reaction of captured CO2 (Table 4.2) in DMSO-d6. - 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 11 0 1 20 1 3 0 14 0 1 50 1 6 0 1 7 0 1 8 0 1 90 2 0 0 21 0 2 2 0 2 3 0 f 1 ( p p m ) 1 . 0 0 0 . 9 9 1 . 0 0 6 1 . 7 7 6 7 . 7 4 1 5 9 . 8 1 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 f 1 ( p p m ) 1 . 0 0 0 . 8 5 0 . 5 5 MeOH t BuOH PG O O K O HO 1 3 2 C(1) C(3) C(2) 143 Figure 4.11 Typical 13 C spectra of reaction mixture after hydrogenation reaction of captured CO2 (Table 4.2) in DMSO-d6. Figure 4.12 Observation of alkyl carbonate after CO2 capture with active KOH in DMSO-d6. - 1 0 1 0 3 0 5 0 7 0 9 0 1 1 0 1 3 0 1 5 0 1 7 0 1 9 0 2 1 0 2 3 0 f 1 ( p p m ) 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0 2 1 0 2 2 0 2 3 0 f 1 ( p p m ) Im Im HCOOK HO-CH2-COOK alkyl carbonate (CO2 capture by regenerated KOH) MeOH t BuOH PG t BuOH PG 144 Figure 4.13 Typical 1 H spectra of reaction mixture for control experiment as in Figure 4.3B in DMSO-d6. Figure 4.14 Typical 13 C spectra of reaction mixture for control experiment as in Figure 4.3B in DMSO-d6. - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 f 1 ( p p m ) 1 . 6 5 1 . 0 0 0 . 5 0 3 . 6 0 3 . 3 3 . 4 3 . 5 3 . 6 3 . 7 3 . 8 f 1 ( p p m ) 3 . 6 0 - 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0 2 1 0 2 2 0 2 3 0 f 1 ( p p m ) 6 2 . 1 7 1 7 7 . 2 6 6 2 . 0 6 2 . 5 6 3 . 0 6 3 . 5 6 4 . 0 6 4 . 5 6 5 . 0 f 1 ( p p m ) 6 2 . 1 7 HO-CH2-COOK HO-CH2-COOK HO-CH2-COOK Im Im HO-CH2-COOK 145 Figure 4.15 Typical GC spectra of the unreacted gas mixture for control experiments as in Figure 4.3B. Figure 4.16 Typical GC-MS spectra of the reaction mixture for control experiments as in Figure 4.3B. C:\Xcalibur\...\rs-11-132_200113132629 01/13/2020 01:26:29 PM RT: 1.48 - 3.07 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 Retention Time (min) -3.55 -3.50 -3.45 -3.40 -3.35 -3.30 -3.25 -3.20 -3.15 -3.10 -3.05 -3.00 -2.95 -2.90 -2.85 -2.80 -2.75 -2.70 -2.65 -2.60 -2.55 -2.50 -2.45 -2.40 -2.35 -2.30 -2.25 -2.20 -2.15 -2.10 -2.05 -2.00 -1.95 -1.90 Relative Intensity NL: 4.66E6 TCD Analog 2 rs-11- 132_20011 3132629 H 2 N 2 No CO Relative Abundance Relative Abundance 146 Figure 4.17 13 C spectra of CO2 captured from ambient air in DMSO-d6 (Table 4.3). - 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 13 0 1 4 0 1 5 0 1 60 1 7 0 1 8 0 1 9 0 2 0 0 2 1 0 2 2 0 23 0 f 1 ( p p m ) 1 . 0 0 0 . 5 0 0 . 5 1 0 . 5 9 alkyl carbonate CO3 2- Im Im alkyl carbonate 147 4.5. References [1] (a) Edenhofer, O.; Pichs-Madruga, R.; Sokona, Y.; Agrawala, S.; Bashmakov, I. A.; Blanco, G.; Broome, J.; Bruckner, T.; Brunner, S.; Bustamante, M.; Clarke, L.; Creutzig, F.; Dhakal, S.; Dubash, N. K.; Eickemeier, P.; Farahani, E.; Fischedick, M.; Fleurbaey, M.; Gerlagh, R.; Gomez- Echeverri, L.; Gupta, S.; Harnisch, J.; Jiang, K. J.; Kadner, S.; Kartha, S.; Klasen, S.; Kolstad, C.; Krey, V.; Kunreuther, H.; Lucon, O.; Masera, O.; Minx, J.; Mulugetta, Y.; Patt, A.; Ravindranath, N. H.; Riahi, K.; Roy, J.; Schaeffer, R.; Schlomer, S.; Seto, K.; Seyboth, K.; Sims, R.; Skea, J.; Smith, P.; Somanathan, E.; Stavins, R.; von Stechow, C.; Sterner, T.; Sugiyama, T.; Suh, S.; Urama, K. C.; Urge-Vorsatz, D.; Victor, D. G.; Zhou, D. D.; Zou, J.; Zwickel, T.; Baiocchi, G.; Chum, H.; Fuglestvedt, J.; Haberl, H.; Hertwich, E.; Kriegler, E.; Rogelj, J.; Rogner, H. H.; Schaeffer, M.; Smith, S. J.; van Vuuren, D.; Wiser, R., Summary for Policymakers. In Climate Change 2014: Mitigation of Climate Change, 2014; pp 1-30; (b) Perera, F., Pollution from Fossil- Fuel Combustion is the Leading Environmental Threat to Global Pediatric Health and Equity: Solutions Exist. Int. J. Environ. Res. Public Health 2018, 15; (c) Diffenbaugh, N. S.; Singh, D.; Mankin, J. S.; Horton, D. E.; Swain, D. L.; Touma, D.; Charland, A.; Liu, Y.; Haugen, M.; Tsiang, M.; Rajaratnam, B., Quantifying the influence of global warming on unprecedented extreme climate events. Proc. Natl. Acad. Sci. U S A 2017, 114, 4881-4886. [2] Le Quéré, C.; Andrew, R. M.; Friedlingstein, P.; Sitch, S.; Hauck, J.; Pongratz, J.; Pickers, P. A.; Korsbakken, J. I.; Peters, G. P.; Canadell, J. G.; Arneth, A.; Arora, V. K.; Barbero, L.; Bastos, A.; Bopp, L.; Chevallier, F.; Chini, L. P.; Ciais, P.; Doney, S. C.; Gkritzalis, T.; Goll, D. S.; Harris, I.; Haverd, V.; Hoffman, F. M.; Hoppema, M.; Houghton, R. A.; Hurtt, G.; Ilyina, T.; Jain, A. K.; Johannessen, T.; Jones, C. D.; Kato, E.; Keeling, R. F.; Goldewijk, K. K.; Landschutzer, P.; Lefevre, N.; Lienert, S.; Liu, Z.; Lombardozzi, D.; Metzl, N.; Munro, D. R.; Nabel, J. E. M. S.; Nakaoka, S.; Neill, C.; Olsen, A.; Ono, T.; Patra, P.; Peregon, A.; Peters, W.; Peylin, P.; Pfeil, B.; Pierrot, D.; Poulter, B.; Rehder, G.; Resplandy, L.; Robertson, E.; Rocher, M.; Rodenbeck, C.; Schuster, U.; Schwinger, J.; Seferian, R.; Skjelvan, I.; Steinhoff, T.; Sutton, A.; Tans, P. P.; Tian, H. Q.; Tilbrook, B.; Tubiello, F. N.; van der Laan-Luijkx, I. T.; van der Werf, G. R.; Viovy, N.; Walker, A. P.; Wiltshire, A. J.; Wright, R.; Zaehle, S.; Zheng, B., Global Carbon Budget 2018. Earth Syst. Sci. Data 2018, 10, 2141-2194. [3] Hansen, J.; Sato, M.; Ruedy, R.; Lo, K.; Lea, D. W.; Medina-Elizade, M., Global temperature change. Proc. Natl. Acad. Sci. U S A 2006, 103, 14288-14293. [4] (a) Krug, J. H. A., Accounting of GHG emissions and removals from forest management: a long road from Kyoto to Paris. Carbon Balance Manage. 2018, 13, 1; (b) Vicedo-Cabrera, A. M.; Guo, Y.; Sera, F.; Huber, V.; Schleussner, C. F.; Mitchell, D.; Tong, S.; de Sousa Zanotti Stagliorio Coelho, M.; Saldiva, P. H. N.; Lavigne, E.; Correa, P. M.; Ortega, N. V.; Kan, H.; Osorio, S.; Kysely, J.; Urban, A.; Jaakkola, J. J. K.; Ryti, N. R. I.; Pascal, M.; Goodman, P. G.; Zeka, A.; Michelozzi, P.; Scortichini, M.; Hashizume, M.; Honda, Y.; Hurtado-Diaz, M.; Cruz, J.; Seposo, X.; Kim, H.; Tobias, A.; Iniguez, C.; Forsberg, B.; Astrom, D. O.; Ragettli, M. S.; Roosli, M.; Guo, Y. L.; Wu, C. F.; Zanobetti, A.; Schwartz, J.; Bell, M. L.; Dang, T. N.; Van, D. D.; Heaviside, C.; Vardoulakis, S.; Hajat, S.; Haines, A.; Armstrong, B.; Ebi, K. L.; Gasparrini, A., Temperature- related mortality impacts under and beyond Paris Agreement climate change scenarios. Clim. 148 Change 2018, 150, 391-402; (c) Nolan, C.; Overpeck, J. T.; Allen, J. R. M.; Anderson, P. M.; Betancourt, J. L.; Binney, H. A.; Brewer, S.; Bush, M. B.; Chase, B. M.; Cheddadi, R.; Djamali, M.; Dodson, J.; Edwards, M. E.; Gosling, W. D.; Haberle, S.; Hotchkiss, S. C.; Huntley, B.; Ivory, S. J.; Kershaw, A. P.; Kim, S. H.; Latorre, C.; Leydet, M.; Lezine, A. M.; Liu, K. B.; Liu, Y.; Lozhkin, A. V.; McGlone, M. S.; Marchant, R. A.; Momohara, A.; Moreno, P. I.; Muller, S.; Otto- Bliesner, B. L.; Shen, C.; Stevenson, J.; Takahara, H.; Tarasov, P. E.; Tipton, J.; Vincens, A.; Weng, C.; Xu, Q.; Zheng, Z.; Jackson, S. T., Past and future global transformation of terrestrial ecosystems under climate change. Science 2018, 361, 920-923. [5] (a) Chalmers, H., Fundamentals point to carbon capture. Nat. Clim. Change 2019, 9, 348-348; (b) Goeppert, A.; Czaun, M.; Prakash, G. K. S.; Olah, G. A., Air as the renewable carbon source of the future: an overview of CO2 capture from the atmosphere. Energy Environ. Sci. 2012, 5, 7833-7853; (c) Sanz-Pérez, E. S.; Murdock, C. R.; Didas, S. A.; Jones, C. W., Direct Capture of CO2 from Ambient Air. Chem. Rev. 2016, 116, 11840-11876. [6] (a) White, C. M.; Strazisar, B. R.; Granite, E. J.; Hoffman, J. S.; Pennline, H. W., Separation and capture of CO2 from large stationary sources and sequestration in geological formations- coalbeds and deep saline aquifers. J. Air Waste Manag. Assoc. 2003, 53, 645-715; (b) Bui, M.; Adjiman, C. S.; Bardow, A.; Anthony, E. J.; Boston, A.; Brown, S.; Fennell, P. S.; Fuss, S.; Galindo, A.; Hackett, L. A.; Hallett, J. P.; Herzog, H. J.; Jackson, G.; Kemper, J.; Krevor, S.; Maitland, G. C.; Matuszewski, M.; Metcalfe, I. S.; Petit, C.; Puxty, G.; Reimer, J.; Reiner, D. M.; Rubin, E. S.; Scott, S. A.; Shah, N.; Smit, B.; Trusler, J. P. M.; Webley, P.; Wilcox, J.; Mac Dowell, N., Carbon capture and storage (CCS): the way forward. Energy Environ. Sci. 2018, 11, 1062- 1176. [7] (a) Olah, G. A.; Prakash, G. K. S.; Goeppert, A., Anthropogenic chemical carbon cycle for a sustainable future. J. Am. Chem. Soc. 2011, 133, 12881-12898; (b) Katelhon, A.; Meys, R.; Deutz, S.; Suh, S.; Bardow, A., Climate change mitigation potential of carbon capture and utilization in the chemical industry. Proc. Natl. Acad. Sci. U S A 2019, 116, 11187-11194; (c) Liu, Q.; Wu, L.; Jackstell, R.; Beller, M., Using carbon dioxide as a building block in organic synthesis. Nat. Commun. 2015, 6, 5933. [8] (a) Goeppert, A.; Czaun, M.; Jones, J. P.; Prakash, G. K. S.; Olah, G. A., Recycling of carbon dioxide to methanol and derived products - closing the loop. Chem. Soc. Rev. 2014, 43, 7995-8048; (b) Olah, G. A.; Goeppert, A.; Prakash, G. K. S., Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons. J. Org. Chem. 2009, 74, 487-498. [9] (a) Olah, G. A.; Goeppert, A.; Prakash, G. K. S.; Wiley, V. C. H., Beyond Oil and Gas: The Methanol Economy. 2018; (b) Kothandaraman, J.; Kar, S.; Sen, R.; Goeppert, A.; Olah, G. A.; Prakash, G. K. S., Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol. J. Am. Chem. Soc. 2017, 139, 2549-2552; (c) Heim, L. E.; Thiel, D.; Gedig, C.; Deska, J.; Prechtl, M. H., Bioinduced Room-Temperature Methanol Reforming. Angew. Chem. Int. Ed. Engl. 2015, 54, 10308-10312; (d) Stephan, D. W., Catalysis: A step closer to a methanol economy. Nature 2013, 495, 54-55; (e) Alberico, E.; Nielsen, M., Towards a methanol economy based on 149 homogeneous catalysis: methanol to H2 and CO2 to methanol. Chem. Commun. 2015, 51, 6714- 6725; (f) Liu, W. C.; Baek, J.; Somorjai, G. A., The Methanol Economy: Methane and Carbon Dioxide Conversion. Top. Catal. 2018, 61, 530-541; (g) Kothandaraman, J.; Kar, S.; Goeppert, A.; Sen, R.; Prakash, G. K. S., Advances in Homogeneous Catalysis for Low Temperature Methanol Reforming in the Context of the Methanol Economy. Top. Catal. 2018, 61, 542-559; (h) Bertau, M.; Offermanns, H.; Plass, L.; Schmidt, F.; Wernicke, H.-J. r.; Springer-Verlag Gmb, H., Methanol: The Basic Chemical and Energy Feedstock of the Future Asinger's Vision Today. 2016. [10] (a) Yoshimura, A.; Watari, R.; Kuwata, S.; Kayaki, Y., Poly(ethyleneimine)-Mediated Consecutive Hydrogenation of Carbon Dioxide to Methanol with Ru Catalysts. Eur. J. Inorg. Chem. 2019, 2375-2380; (b) Rezayee, N. M.; Huff, C. A.; Sanford, M. S., Tandem Amine and Ruthenium-Catalyzed Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2015, 137, 1028- 1031; (c) Khusnutdinova, J. R.; Garg, J. A.; Milstein, D., Combining Low-Pressure CO2 Capture and Hydrogenation To Form Methanol. ACS Catal. 2015, 5, 2416-2422; (d) Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. S., CO2 capture by amines in aqueous media and its subsequent conversion to formate with reusable ruthenium and iron catalysts. Green Chem. 2016, 18, 5831-5838; (e) Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. S., Conversion of CO2 from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst. J. Am. Chem. Soc. 2016, 138, 778-781; (f) Kar, S.; Sen, R.; Kothandaraman, J.; Goeppert, A.; Chowdhury, R.; Munoz, S. B.; Haiges, R.; Prakash, G. K. S., Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2019, 141, 3160-3170; (g) Kar, S.; Sen, R.; Goeppert, A.; Prakash, G. K. S., Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy. J. Am. Chem. Soc. 2018, 140, 1580-1583; (h) Kar, S.; Kothandaraman, J.; Goeppert, A.; Prakash, G. K. S., Advances in catalytic homogeneous hydrogenation of carbon dioxide to methanol. J. CO2 Util. 2018, 23, 212-218. [11] (a) Goeppert, A.; Zhang, H.; Sen, R.; Dang, H.; Prakash, G. K. S., Oxidation-Resistant, Cost- Effective Epoxide-Modified Polyamine Adsorbents for CO2 Capture from Various Sources Including Air. ChemSusChem 2019, 12, 1712-1723; (b) Jahandar Lashaki, M.; Khiavi, S.; Sayari, A., Stability of amine-functionalized CO2 adsorbents: a multifaceted puzzle. Chem. Soc. Rev. 2019, 48, 3320-3405. [12] (a) Carbon Engineering Ltd. https://carbonengineering.com; (b) Keith, D. W.; Holmes, G.; Angelo, D. S.; Heidel, K., A Process for Capturing CO2 from the Atmosphere. Joule 2018, 2, 1573- 1594; (c) Fasihi, M.; Efimova, O.; Breyer, C., Techno-economic assessment of CO2 direct air capture plants. J. Cleaner Prod. 2019, 224, 957-980. [13] (a) Kar, S.; Goeppert, A.; Prakash, G. K. S., Integrated CO2 Capture and Conversion to Formate and Methanol: Connecting Two Threads. Acc. Chem. Res. 2019, 52, 2892-2903; (b) Kar, S.; Goeppert, A.; Galvan, V.; Chowdhury, R.; Olah, J.; Prakash, G. K. S., A Carbon-Neutral CO2 Capture, Conversion, and Utilization Cycle with Low-Temperature Regeneration of Sodium Hydroxide. J. Am. Chem. Soc. 2018, 140, 16873-16876. 150 [14] (a) Marcos, R.; Xue, L. Q.; Sanchez-de-Armas, R.; Ahlquist, M. S. G., Bicarbonate Hydrogenation Catalyzed by Iron: How the Choice of Solvent Can Reverse the Reaction. ACS Catal. 2016, 6, 2923-2929; (b) Bertini, F.; Mellone, I.; Ienco, A.; Peruzzini, M.; Gonsalvi, L., Iron(II) Complexes of the Linear rac-Tetraphos-1 Ligand as Efficient Homogeneous Catalysts for Sodium Bicarbonate Hydrogenation and Formic Acid Dehydrogenation. Acs Catal. 2015, 5, 1254- 1265; (c) Dai, Z. J.; Luo, Q.; Cong, H. J.; Zhang, J.; Peng, T. Y., New Ru(II) N'NN'-type pincer complexes: synthesis, characterization and the catalytic hydrogenation of CO2 or bicarbonates to formate salts. New J. Chem. 2017, 41, 3055-3060; (d) Kothandaraman, J.; Czaun, M.; Goeppert, A.; Haiges, R.; Jones, J. P.; May, R. B.; Prakash, G. K. S.; Olah, G. A., Amine-Free Reversible Hydrogen Storage in Formate Salts Catalyzed by Ruthenium Pincer Complex without pH Control or Solvent Change. ChemSusChem 2015, 8, 1442-1451; (e) Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J.; Ben-David, Y.; Milstein, D., Low-pressure hydrogenation of carbon dioxide catalyzed by an iron pincer complex exhibiting noble metal activity. Angew. Chem. Int. Ed. Engl. 2011, 50, 9948-9952; (f) Ziebart, C.; Federsel, C.; Anbarasan, P.; Jackstell, R.; Baumann, W.; Spannenberg, A.; Beller, M., Well-defined iron catalyst for improved hydrogenation of carbon dioxide and bicarbonate. J. Am. Chem. Soc. 2012, 134, 20701-20704; (g) Zhu, F.; Zhu-Ge, L.; Yang, G.; Zhou, S., Iron-catalyzed hydrogenation of bicarbonates and carbon dioxide to formates. ChemSusChem 2015, 8, 609-612; (h) Zhang, Y.; MacIntosh, A. D.; Wong, J. L.; Bielinski, E. A.; Williard, P. G.; Mercado, B. Q.; Hazari, N.; Bernskoetter, W. H., Iron catalyzed CO2 hydrogenation to formate enhanced by Lewis acid co-catalysts. Chem. Sci. 2015, 6, 4291-4299; (i) Coufourier, S.; Gaillard, S.; Clet, G.; Serre, C.; Daturi, M.; Renaud, J.-L., A MOF-assisted phosphine free bifunctional iron complex for the hydrogenation of carbon dioxide, sodium bicarbonate and carbonate to formate. Chem. Commun. 2019, 55, 4977-4980. [15] (a) Balaraman, E.; Gunanathan, C.; Zhang, J.; Shimon, L. J. W.; Milstein, D., Efficient hydrogenation of organic carbonates, carbamates and formates indicates alternative routes to methanol based on CO2 and CO. Nat. Chem. 2011, 3, 609; (b) Wesselbaum, S.; vom Stein, T.; Klankermayer, J.; Leitner, W., Hydrogenation of Carbon Dioxide to Methanol by Using a Homogeneous Ruthenium–Phosphine Catalyst. Angew. Chem. 2012, 124, 7617-7620; (c) Han, Z.; Rong, L.; Wu, J.; Zhang, L.; Wang, Z.; Ding, K., Catalytic Hydrogenation of Cyclic Carbonates: A Practical Approach from CO2 and Epoxides to Methanol and Diols. Angew. Chem. Int. Ed. 2012, 51, 13041-13045; (d) Li, Y.-N.; Ma, R.; He, L.-N.; Diao, Z.-F., Homogeneous hydrogenation of carbon dioxide to methanol. Catal. Sci. Technol. 2014, 4, 1498-1512; (e) Wesselbaum, S.; Vom Stein, T.; Klankermayer, J.; Leitner, W., Hydrogenation of carbon dioxide to methanol by using a homogeneous ruthenium-phosphine catalyst. Angew. Chem. Int. Ed. Engl. 2012, 51, 7499-7502; (f) Huff, C. A.; Sanford, M. S., Cascade catalysis for the homogeneous hydrogenation of CO2 to methanol. J. Am. Chem. Soc. 2011, 133, 18122-18125; (g) Bernskoetter, W. H.; Hazari, N., Reversible Hydrogenation of Carbon Dioxide to Formic Acid and Methanol: Lewis Acid Enhancement of Base Metal Catalysts. Acc. Chem. Res. 2017, 50, 1049-1058. [16] (a) Suerbaev, K. A.; Chepaikin, E. G.; Kanapieva, F. M.; Seitenova, G. Z., Carboxylation of organic compounds with metal alkyl carbonates (review). Pet. Chem. 2009, 49, 265-273; (b) Ichiro, H.; Taketoshi, K.; Takatomo, F.; Taketoshi, M.; Katsuyuki, U., Carboxylation of Phenol Derivatives. XXII. Formation of Alkali Alkyl Carbonate by the O-Carboxylation of Alcohol in the Presence of an Alkali Salt of a Weak Acid. Bull. Chem. Soc. Jpn. 1976, 49, 2775-2779; (c) Kothandaraman, J.; Dagle, R. A.; Dagle, V. L.; Davidson, S. D.; Walter, E. D.; Burton, S. D.; 151 Hoyt, D. W.; Heldebrant, D. J., Condensed-phase low temperature heterogeneous hydrogenation of CO2 to methanol. Catal. Sci. Technol. 2018, 8, 5098-5103; (d) Huang, S.; Yan, B.; Wang, S.; Ma, X., Recent advances in dialkyl carbonates synthesis and applications. Chem. Soc. Rev. 2015, 44, 3079-3116; (e) Han, S.-J.; Wee, J.-H., Carbon Dioxide Fixation by Combined Method of Physical Absorption and Carbonation in NaOH-Dissolved Methanol. Energy Fuels 2017, 31, 1747-1755. [17] Han, S.-J.; Wee, J.-H., Carbon Dioxide Fixation via the Synthesis of Sodium Ethyl Carbonate in NaOH-Dissolved Ethanol. Ind. Eng. Chem. Res. 2016, 55, 12111-12118. [18] (a) Cherepakhin, V.; Williams, T. J., Iridium Catalysts for Acceptorless Dehydrogenation of Alcohols to Carboxylic Acids: Scope and Mechanism. ACS Catal. 2018, 8, 3754-3763; (b) Balaraman, E.; Khaskin, E.; Leitus, G.; Milstein, D., Catalytic transformation of alcohols to carboxylic acid salts and H2 using water as the oxygen atom source. Nat. Chem. 2013, 5, 122; (c) Kar, S.; Goeppert, A.; Sen, R.; Kothandaraman, J.; Prakash, G. K. S., Regioselective deuteration of alcohols in D2O catalysed by homogeneous manganese and iron pincer complexes. Green Chem. 2018, 20, 2706-2710; (d) Hu, P.; Milstein, D., Conversion of alcohols to carboxylates using water and base with H2 liberation. In Organometallics for Green Catalysis, 2019; Vol. 63, pp 175- 192. 152 Chapter 5. Glycol Assisted Conversion of CO2 and Metal Carbonates to Methanol with a Heterogeneous Copper Catalyst This dissertation chapter is based on a recent research article from our group published in the Journal of CO2 Utilization (Sen, R.; Koch, C. J.; Galvan, V.; Entesari, N.; Goeppert, A.; Prakash, G. K. S., J. CO2 Util. 2021, 54, 101762). Part of the article is reprinted by permission of Elsevier. Copyright 2021. Elsevier. 5.1. Introduction Carbon capture and storage (CCS) technologies are considered prime solutions to address escalating atmospheric CO2 levels. 1 Recently, a superior and value-added alternative to CCS, carbon capture and utilization (CCU) has emerged with paramount potential. 2 In CCU processes, CO2 is captured from various emission sources and utilized downstream as a renewable carbon source to produce value-added materials and chemical feedstocks. 3 While a variety of useful organic molecules can be synthesized from the captured CO2, its hydrogenative conversion can provide access to value-added fuels and feedstocks, and a promising way of storing renewable energy in chemical bonds. 3e Among such products, methanol is an attractive chemical currently produced at an annual rate of over 100 billion liters. 4 It is a one carbon alcohol and liquid at room temperature that is easy to store, transport and dispense. Methanol is an essential building block to produce a plethora of hydrocarbon-based chemicals, polymers, paints, adhesives, construction materials and pharmaceuticals. Additionally, it is an excellent clean burning drop in fuel and a convenient liquid organic hydrogen carrier (LOHC) with a high H2 storage capacity of 12.6 wt%. 5 153 Traditionally, methanol is produced by the catalytic conversion of syngas, which is a mixture of mainly CO and H2 (Figure 5.1a), using commercial Cu/ZnO/Al2O3 based heterogeneous catalysts at temperatures of 250 to 300 °C and pressures of 50-100 bar. 4a, 6 Alternatively, CO2 can be activated by the same class of catalyst to synthesize methanol under similar operating conditions in the presence of H2. Such a CO2 to methanol process is already operating commercially at the George Olah Renewable Methanol Plant by Carbon Recycling International in Iceland. Over the years, a number of pilot and demonstration plants have also been constructed and operated by companies, research institutes and universities including Mitsui Chemicals in Japan, the Korean Institute of Science and Technology (KIST) in South Korea (CAMERE process) and more recently the Dalian Institute of Chemical Physics in China. Numerous commercial plants are also being planned in countries including Sweden, Australia, Norway, Germany and Canada to produce “blue” and “green” methanol with a lower carbon footprint than traditional coal and natural gas based “brown” and “grey” methanol. 7 Figure 5.1. CO2 hydrogenation to CH3OH. a) equilibrium reactions involved in the methanol synthesis process; 4c b) proposed concept: solvent assisted integrated CO2 capture and conversion to CH3OH. While the current catalytic process has a high selectivity of >99% for methanol in a flow reactor, high reaction temperatures result in low CO2 conversion of about 30% per pass due to the exothermic nature of the reaction. 4c, 8 To achieve higher conversions, the unreacted gas mixture after separation from the products (methanol and water), is constantly recycled back to the reactor. CO 2 (!) + 3H 2 (!) catalyst CO 2 / Air (!) solvent CO 2 loaded solution CH 3 OH (") + H 2 O (") CO (!) + 2H 2 (!) CH 3 OH (") ∆H° 298K = -11.9 kcal mol -1 ∆H° 298K = -21.7 kcal mol -1 CO 2 (!) + H 2 (!) CO (!) + H 2 O (") ∆H° 298K = 9.8 kcal mol -1 a) b) CH 3 OH (") + H 2 O (") CO 2 capture direct utilization H 2 (!) 154 Furthermore, the competing reverse water gas shift (RWGS) reaction is favored at high temperatures, typically above 250 °C, forming CO and H2O, which have detrimental effects on the catalyst. To overcome the limitations in the methanol synthesis process in gas phase, novel catalytic systems and solvent assisted liquid-phase processes have been studied at lower reaction temperatures. 9 Notably, alcohol based solvents were shown to promote the hydrogenation process as it proceeds through an energetically favorable pathway via formate ester intermediates. 10 While homogeneous catalysts in liquid phase have been shown to improve the reaction conditions effectively, 9g, 11 there are limited reports of solvent assisted systems with heterogeneous catalysts. Still, the major challenge of low CO2 conversions calls for significant improvements. 9a, 9b, 12 In the scope of CCU, coupling of two threads: a) CO2 capture and b) CO2 hydrogenation to methanol, in an integrated one-pot process is highly desirable (Figure 5.1b). 13 The highlight of such systems is the reduction or elimination of the large energy and cost penalties associated with the desorption and compression steps in conventional CCS/CCU processes, which could make them economically and practically attractive. In such tandem processes, CO2 can be captured in solution using a capturing agent (amines or alkali hydroxides) under ambient conditions from relatively concentrated emission point sources as well as dilute sources such as air. The CO2 loaded solutions, without any purification and separation, are directly hydrogenated to form methanol. 14 Recently, such integrated capture and conversion processes were realized by our group and others, using homogeneous catalysts. 11a, 15 However, similar integrated systems with heterogeneous catalysts have yet to be explored. For industrial applications, heterogeneous catalysts offer major advantages in terms of ease of separation and recycling at large scale. Further, employing the non- noble metal based commercial Cu/ZnO/Al2O3 catalyst can significantly lower the operating costs. 2c, 9e, 9h 155 As part of our ongoing research endeavor to develop practically viable and efficiently integrated CCU systems, we studied herein a liquid phase batch process using a commercial Cu/ZnO/Al2O3 catalyst for methanol synthesis where the use of a glycol solvent remarkably enhanced the CO2 conversion at relatively low temperatures and with high recyclability. Furthermore, we have demonstrated as a proof of concept, the feasibility of directly converting the CO2 captured from air to CH3OH with a heterogeneous catalyst. Direct air capture (DAC) is valuable as air is the ultimate sink for all anthropogenic CO2 emissions and a sustainable carbon source. This is of particular interest for developing a circular economy based on CO2 recycling using H2 obtained from renewable energy sources. 16 5.2. Reaction Optimization for Cu/ZnO/Al2O3 Catalysed Methanol Synthesis In our initial exploration for a suitable solvent for methanol synthesis, we studied the hydrogenation of CO2 (4 bar) with 70 bar H2 at 200 °C in a batch reactor (Figure 5.2). With ethylene glycol, a high boiling and relatively polar alcohol, 18 mmol of CH3OH were obtained, corresponding to a 90% yield relative to CO2. It showed a marked increase of 92% from the reaction without any solvent that afforded only a 47% CH3OH yield. With di-/tri-/tetra-ethylene glycol, slightly lower yields in the range of 87-82% were obtained. Polyethylene glycol (PEG400) exhibited a relatively mild enhancement with 74% CH3OH yield. Overall, a decreasing trend in the amount of methanol with increasing chain length of the glycol solvent was thus observed, owing to a combined effect of primarily a) solvent polarity and b) ability of the alcohol to stabilize the formate species via esterification. Reactions with other high boiling alcohols such as 1,2- propanediol and 2-methoxyethanol gave significant, yet lower, CH3OH yields (78-79%). The promoting effect of alcohols in the reaction became evident as reactions in triglyme and squalene showed negligible enhancement in methanol formation when compared to a reaction without any 156 solvent. Notably, the catalyst was found to be stable under these reaction conditions as indicated by the powder X-ray diffraction (XRD) studies (Figure 5.35). CuO peaks were not observed. The Cu 0 crystallite size (12 nm) of the used catalyst was similar to that of a freshly activated catalyst. Figure 5.2. Effect of solvent on hydrogenation of CO2 to CH3OH. Reaction conditions: t = 72 h, a CO2 = 21.7 mmol (with no solvent), CH3OH yields calculated relative to CO2 as determined by 1 H/ 13 C NMR. Yield calculations error ±5%. Based on the encouraging results with ethylene glycol (Table 5.1, entry 1), we briefly investigated key parameters in the hydrogenation reaction. When the catalyst loading was halved to 150 mg, the CH3OH yield decreased to 78%, while increasing the loading to 450 mg had only minor effects (entry 2-3 and Figure 5.3). Since our reaction system was active for methanol synthesis at 200 °C which is much lower than the current gas phase commercial processes, we decided to study the reaction at an even lower temperature. At 170 °C, the catalytic system still afforded a considerable CH3OH yield of 60% (entry 4). Further, when the reaction time was shortened from the initial 72 h (entry 6-7), it was observed that 80% of the net conversion to CH3OH was achieved within 24 h (yield = 72%) with a methanol production rate of 63.6 gMeOH·kgcat -1 ·h -1 (Figure 5.4). The yield increased to 80% in 48 h. However, running the reaction 47 90 87 84 82 74 78 79 56 50 0 10 20 30 40 50 60 70 80 90 100 no solvent ethylene glycol diethylene glycol triethylene glcyol tetraethylene glycol PEG400 1,2-propylene glycol 2-methoxyethanol triglyme squalene CH 3 OH Yield (%) solvent a CO 2 (!) + H 2 (!) CH 3 OH (") + H 2 O (") Cu/ZnO/Al 2 O 3 (300 mg) solvent (10 mL), 200 °C 20 mmol 70 bar 157 for 120 h gave a CH3OH synthesis comparable to the one obtained with 72 h (entry 5), indicating that an equilibrium had been achieved. Lowering the H2 pressure to 50 bar afforded CH3OH with 71% yield (entry 8). The gas phase after reaction showed only trace amounts of CO and CH4, along with unreacted H2 and CO2 (Figure 5.16-5.17), indicating that the RWGS reaction was significantly suppressed under our reaction conditions. When water is introduced in the ethylene glycol (EG) solvent, the methanol yield decreased from 72% to 60% at (EG:water = 1:3). Over the course of two pressure refill cycles (after the initial pressure fill), the methanol yield per run is reduced because of a shift in equilibrium due to product accumulation (thermodynamic effect) or/and a possible deactivation of the catalyst (kinetic effect). Figure 5.3. Effect of catalyst loading on MeOH production in ethylene glycol. Reaction conditions: total pressure = 74 bar at rt (CO2:H2 = 1:18), EG = 10 mL, t = 72 h, T = 200 °C (as in Table 1). The methanol production rate is calculated as gram of methanol formed per kilogram of catalyst per hour (g·kg -1 ·h -1 ). 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 100 200 300 400 500 MeOH production rate (g.kg -1 h -1 ) MeOH yield (%) catalyst loading (mg) MeOH production vs. catalyst loading 158 Figure 5.4. Effect of reaction time on MeOH production in ethylene glycol. Reaction conditions: total pressure = 74 bar at rt (CO2:H2 = 1:18), catalyst loading = 300 mg, EG = 10 mL, T = 200 °C (as in Table 1). The methanol production rate is calculated as gram of methanol formed per kilogram of catalyst per hour (g·kg -1 ·h -1 ). Table 5.1. Cu/ZnO/Al2O3 catalyzed hydrogenation of CO2 to CH3OH in ethylene glycol Entry a H2 (bar) cat. (mg) T ( o C) Time (h) MeOH (mmol) MeOH (%) b 1 70 300 200 72 18.0 90 2 70 150 200 72 15.5 78 3 70 450 200 72 18.1 91 4 70 300 170 72 12.0 60 5 70 300 200 120 17.8 89 6 70 300 200 48 16.0 80 7 70 300 200 24 14.3 72 8 50 300 200 72 14.2 71 a Reaction conditions: CO2 = 4 bar (20 mmol), pressures at rt, b CH3OH yields calculated relative to CO2 as determined by 1 H NMR. Yield calculations error ±5%. 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0 20 40 60 80 100 120 140 MeOH production rate (g.kg -1 h -1 ) MeOH yield (%) reaction time (h) MeOH production vs. reaction time CO 2 (!) + H 2 (!) CH 3 OH (") + H 2 O (") Cu/ZnO/Al 2 O 3 ethylene glycol (10 mL) 20 mmol 159 Figure 5.5. Effect of CO2/H2 ratio on CH3OH production and comparison with calculated methanol yield at equilibrium. Reaction conditions: total pressure = 74 bar at rt, catalyst loading = 300 mg, EG = 10 mL, t = 72 h, T = 200 °C, CH3OH yields calculated relative to CO2 as determined by 1 H NMR. Yield calculations error ±5%. CO2 hydrogenation to methanol is an equilibrium reaction and the conversions in such processes are limited by the thermodynamics. To determine the thermodynamic limit for CH3OH synthesis under our reaction conditions, equilibrium CO2 conversions were calculated by the Gibbs energy minimization models in Aspen Plus using Soave-Redlich-Kwong (SRK) equations of state (details in Section 5.4.9). 18 Remarkably, the estimated equilibrium value of 92% for the CH3OH yield by the model (CO2:H2 = 4:70 bar), was in great agreement with the highest value obtained experimentally. It is therefore a strong indication that the presence of ethylene glycol efficiently shifts the methanol yields towards the equilibrium values for CO2 conversion. As shown in Figure 5.5, similar solvent induced trends were also observed for higher CO2 partial pressure in the reactant gas feed. For CO2:H2 ratio of 1:6, 65% of the CO2 was converted to methanol in ethylene glycol with a 110% rise from the reaction without solvent. When the CO2:H2 was further increased to the stoichiometric ratio of 1:3, methanol was obtained in the glycol solvent with a 48% yield corresponding to a noteworthy rise of 120% relative to solventless conditions. The corresponding methanol production rate of 65.2 gMeOH·kgcat -1 ·h -1 is comparable to the highest values reported 47 31 22 90 65 48 92 67 47 0 10 20 30 40 50 60 70 80 90 100 1:18 1:6 1:3 CH 3 OH yield (%) CO 2 :H 2 ratio no solvent ethylene glycol equilibrium limit (calculated) 160 previously for similar alcohol promoted batch processes (Figure 5.6). 9a While a decreasing trend in the methanol yield relative to CO2 is noted with increasing initial CO2 content in the gas mixture, the promoting effect of the glycol solvent is prominent irrespective of the CO2:H2 ratio, shifting the reaction forward toward the thermodynamic limit. Figure 5.6. Effect of CO2 content in the feed gas on MeOH production in ethylene glycol (EG). Reaction conditions: total pressure = 74 bar at rt, catalyst loading = 300 mg, EG = 10 mL, t = 72 h, T = 200 °C. The methanol production rate is calculated as gram of methanol formed per kilogram of catalyst per hour (g·kg -1 ·h -1 ). Figure 5.7. Effect of water content in ethylene glycol solvent on MeOH production. Reaction conditions: total pressure = 74 bar at rt (CO2:H2 = 1:18), solvent = 10 mL, t = 24 h, T = 200 °C, catalyst = 300 mg. 0 10 20 30 40 50 60 70 0 1 2 3 4 5 0 5 10 15 20 25 30 MeOH production rate (g.kg -1 h -1 ) MeOH (mol/L) CO 2 (% of feed gas) MeOH production vs. CO 2 content 50 55 60 65 70 75 0 25 50 75 100 MeOH yield (%) water content (vol % of total solvent) MeOH production vs. water content in solvent 161 Figure 5.8. Methanol production over multiple pressure refill cycles. Reaction conditions: total pressure = 74 bar at rt (CO2:H2 = 1:18), solvent = 10 mL, t = 24 h, T = 200 °C catalyst = 300 mg. Subsequently, recycling of the catalyst was investigated under the optimized reaction conditions (Figure 5.9A). Effective recycling of the catalyst is considered to be a key feature for the large scale application of the process. To recycle the Cu/ZnO/Al2O3 catalyst, it was recovered from the reaction mixture and reused for successive hydrogenation cycles. Demonstrated over five cycles, the catalyst was found to be highly stable, and its catalytic efficiency was entirely retained with an average methanol yield of 90%. No deactivating species were detected via XRD even after the 5 th cycle (Figure 5.9B). Furthermore, the average crystallite size of the recycled catalyst was similar to that of the freshly activated one, indicating minimal or no sintering. 14.9 24.9 33.2 0 5 10 15 20 25 30 35 0 10 20 30 40 50 60 70 80 Run 1 Run 2 Run 3 MeOH (mmol.) MeOH yield (%) Repeated Pressure Refill Studies cycle yield accumulated MeOH (total) 162 Figure 5.9. A) Recycling of the catalyst with methanol formation. Reaction conditions: total pressure = 74 bar at rt, catalyst loading = 300 mg, EG = 10 mL, t = 72 h, T = 200 °C, CH3OH yields calculated relative to CO2 as determined by 1 H NMR. Yield calculations error ±5%. B) XRD spectra of the catalyst after 5th run in recycling studies as compared to the fresh preactivated catalyst. Table 5.2. CO2 capture and hydrogenation to CH3OH in ethylene glycol Entry a Base (mmol) CO2 source CO2 captured b (mmol) CH3OH (mmol) CH3OH yield (%) b 1 b PEHA (8.6) pure CO2 14.9 13.5 91 2 indoor air 7.0 6.2 89 3 b KOH (10) pure CO2 10.0 10.0 >99 4 indoor air 6.6 6.4 97 a Capture conditions: solvent = 10 mL, rt, t = 72 h, stirring at 800 rpm, b t = 3h. Hydrogenation conditions: H2 = 70 bar at rt, catalyst loading = 300 mg, T = 200 °C, t = 72 h, CH3OH yields calculated relative to CO2 captured. Yield calculations error ±5%. The effect of water on the reaction was investigated as well. The addition of water to the ethylene glycol solvent led to a decline in methanol yield (Figure 5.7). A drop of about 6% and 18% in methanol production was observed with a 3:1 and 1:3 ethylene glycol/water solution, respectively. Similarly, the effect of accumulation of products (CH3OH and H2O) on the catalyst performance was studied (Figure 5.8). Over multiple pressure refill studies, the catalyst was found to be active for CO2 hydrogenation to CH3OH, but with decreasing yields in each subsequent cycles. 17 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Intensity (a.u) 2θ( degree) 10.0 12.0 14.0 16.0 18.0 20.0 50 60 70 80 90 100 1 2 3 4 5 CH 3 OH (mmol) CH 3 OH yield (%) cycle CO 2 + H 2 CH 3 OH + H 2 O Cu/ZnO/Al 2 O 3 (300 mg) ethylene glycol (10 mL) 200 °C, 72 h 20 mmol 70 bar Cu 0 size = 12 nm (fresh sample) Cu 0 size = 13 nm (after 5 cycles) A B Cu 0 Cu 0 Cu 0 ZnO ZnO ZnO ZnO ZnO ZnO ZnO ZnO CO 2 (!) + H 2 (!) CH 3 OH (") + H 2 O (") Cu/ZnO/Al 2 O 3 (300 mg) ethylene glycol (10 mL) 200 °C, 72 h 20 mmol 70 bar capture agent CO 2 absorbed in solution CH 3 OH (!) in situ hydrogenation CO 2 capture ethylene glycol H 2 (") CO 2 / Air (") Cu/ZnO/Al 2 O 3 163 Next, the catalytic system developed here was examined for its primary application in integrated capture and conversion processes. Among various sources for CO2 capture, capture from air is considered to be thermodynamically challenging, due to its extremely low CO2 concentration (currently about 420 ppm). 19 In this regard, our group and others have previously demonstrated solution-based DAC systems with various CO2 capture agents. 13, 20 Notably, pentaethylenehexamine (PEHA), a high boiling polyamine with low vapor pressure is highly effective for CO2 capture. Alternatively, alkali hydroxides are also attractive as amine-free CO2 adsorbents. In these studies, ethylene glycol was identified as an effective solvent for CO2 capture because of its high polarity, high boiling point and particular ability to dissolve bases as well as CO2 capture species. 15g, 15l, 21 Ethylene glycol is a green and environmentally benign solvent which can be produced from renewable feedstocks. 22 Owing to its compatibility with both DAC of CO2 and Cu/ZnO/Al2O3 catalyzed hydrogenation, an ethylene glycol assisted integrated system is highly desirable. To study the efficacy of the proposed integrated system, initially CO2 was captured in solution from concentrated CO2 stream and further hydrogenated with H2 and the heterogeneous catalyst (Table 5.2). First, using amine as the base, 14.9 mmol of CO2 was captured by PEHA (2g) in ethylene glycol as ammonium-carbamate/alkyl carbonate salts (entry 1). When the resulting solution was directly subjected to hydrogenation under the optimized reaction conditions, the captured CO2 efficiently converted to CH3OH with a yield of 91%. Similarly, KOH (10 mmol), a model alkali hydroxide in ethylene glycol captured a molar equivalent of CO2 as potassium alkyl carbonate (entry 3). The CO2 loaded solution was directly hydrogenated to afford a quantitative methanol yield. 164 Finally, direct air capture was performed by bubbling indoor air through ethylene glycol solution of the capturing agents. With PEHA, 7.0 mmol of CO2 was captured from air and efficiently converted to CH3OH with 89% yield (entry 2). Using similar protocol, 6.6 mmol CO2 was captured by a KOH solution from air and its hydrogenation afforded a high methanol yield of 97% (entry 4). Figure 5.10. Conversion of potassium carbonate to methanol. Reaction conditions: K2CO3 (10 mmol), water or alcohol (10 mL), H2 = 70 bar (r.t.), catalyst loading = 300 mg, 24 h, 200 °C. CH3OH yields calculated relative to K2CO3 as determined by 1 H NMR. Yield calculations ± 5%. Similar to Chapter 4, the novel chemical reactivity of hydrogenation of metal carbonates to methanol revealed in the present dissertation thesis was also explored here using the heterogeneous Cu/ZnO/Al2O3 catalyst and was found in close correlation with the previous observations. In absence of any solvent, when potassium carbonate (K2CO3, 10 mmol.) was contacted with Cu/ZnO/Al2O3 catalyst and H2 gas at 200 °C, no conversion was observed (Figure 5.10). When water (10 mL) was introduced into the reaction system, only a small amount of CH3OH (1.1 mmol, 11% yield) was formed over 24 h. Trials with alcohols such as ethanol and iso-propanol also resulted in minor amounts of methanol. However, a significantly higher conversion was observed using ethylene glycol as the medium, with a methanol yield of 24 % within 24 h of reaction. Under prolonged reaction time, the methanol yield improved to 99 % in 72 h. Stirring of the solution in presence of the catalyst had adverse effect due to plausible 0 10 20 30 40 50 60 none water ethanol 2-propanol ethylene glycol CH 3 OH yield (%) K 2 CO 3 + 3 H 2 CH 3 OH + 2 KOH Cu/ZnO/Al 2 O 3 200 °C, 70 bar 165 agglomeration of the catalyst particles and was hence avoided. Overall, the reduction of K2CO3 proceeded with high selectivity for methanol. The reaction pathways and the catalytic activity for methanol synthesis from CO2 have been investigated extensively for the commercial Cu/ZnO/Al2O3 catalyst. 9, 10, 12 In the cases of alcohol promoted processes, the hydrogenation is facilitated by the formation of formate ester as an intermediate with the alcohol additive. 10 Further hydrogenation of the formate ester is energetically favorable leading to efficient conversion to methanol. Under our reaction conditions, trace amounts of ethylene glycol monoformate, the formate ester with ethylene glycol were detected during the analysis of the post-reaction mixture (Figure 5.11). This further validates the high promoting effects observed with ethylene glycol. In comparison to methanol synthesis from CO2 in the gas phase under pressure, investigations focusing on hydrogenation of captured CO2 species in solution using Cu/ZnO/Al2O3 and their plausible reaction pathways are rare. 12b In this context, hydrogenation of alkali hydroxide based CO2 capture species (carbonates, alkyl/bi-carbonate salts) to methanol have been extremely challenging to date across the fields of homogeneous and heterogeneous catalysis. 13, 15g Hence, investigations to elucidate the plausible routes for such reaction are fundamental to further develop and improve such processes. 166 Figure 5.11. Insights into the plausible reaction sequences during hydrogenation of CO2 in ethylene glycol. Analogous to potassium alkyl carbonate, potassium bicarbonate was also successfully hydrogenated to methanol under similar conditions in the presence of ethylene glycol (Figure 5.11B). Notably, only trace methanol and potassium formate (intermediate) were detected when the reaction was carried out in the absence of the alcohol solvent (Figure 5.11C). Formate salts have been synthesized previously from bicarbonates. However, further hydrogenation of alkali formates to methanol is significantly difficult. 13, 15g Similar to KHCO3, the intermediate HCOOK was also active for hydrogenation to methanol only in the presence of ethylene glycol. These results suggest that the glycol possibly facilitates the conversion of HCOOK to formate ester in substrate CH 3 OH formed with ethylene glycol without ethylene glycol KHCO 3 ✓ ✗ HCOOK ✓ ✗ HCOOEt - ✓ [HCOO][PEHA] ✓ ✗ CO 2 H O O H OH O HO OH 2 H 2 CH 3 OH H 2 H 2 O OH HO OH formate ester (key intermediate) A. Ethylene glycol assisted hydrogenation of CO 2 gas B. Ethylene glycol assisted hydrogenation of captured CO 2 H O O HO OH 2 H 2 CH 3 OH H 2 , cat. MOH OH HO OH formate ester RO O M O bicarbonate/ alkyl carbonate H O M O metal formate - ROH 1. with alkali hydroxides 2. with PEHA (amine) H O O HO OH 2 H 2 CH 3 OH H 2 , cat. RNH 2 + H 2 O OH HO OH formate ester RO O RNH 3 O H O RNH 3 O ammonium formate - ROH RHN O RNH 3 O alkyl carbonate/ carbamate C. Control studies: hydrogenation of substrates and intermediates cat. cat. cat. cat. 167 situ, which allows for further hydrogenation to methanol. When ethyl formate, a representative formate ester was subjected to hydrogenation in the absence of any additional alcohol, full conversion to methanol was observed within 6 h, which indicates that the hydrogenation of formate esters is significantly more facile. The high susceptibility of a formate ester to undergo hydrogenation could also possibly explain the inability to detect the formate ester during reaction work up as such intermediate species might only be present briefly and not accumulate over the course of the reaction. Also, it is important to note that the intermediate species formed during CO2 hydrogenation in the gas phase may possibly be present under alcohol assisted conditions as well. Even in the case of amine-based system, the formate species is a possible intermediate. Similar to HCOOK, the ammonium formate [HCOO] - [PEHA] + could be hydrogenated to methanol only in the presence of ethylene glycol. This indicates that the formate ester route is the major pathway to methanol, and not the formamide based route. Additionally, CO2 sorption by amines is a reversible process and a mild temperature swing can lead to desorption of the captured CO2. 19a Hence, an alternate reaction pathway involving the hydrogenation of CO2 gas desorbed by the amine during the reaction is also viable. 5.3. Conclusions In conclusion, this report reveals an efficient solvent assisted process for hydrogenation of CO2 to methanol using a Cu/ZnO/Al2O3 catalyst operating at relatively low temperatures (170-200 °C). Compared to solventless conditions, glycol solvents displayed a remarkable increase in the CO2 conversions to methanol with a rise of up to 120%. Ethylene glycol was found to be the most efficient solvent to shift the conversions toward the equilibrium limit and a methanol yield of up to 90%. The hydrogenation reaction proceeded via the formate ester intermediate in presence of 168 the corresponding alcohol. Notably, the catalyst was stable under the reaction conditions and was recyclable over multiple hydrogenation cycles. Furthermore, to the best of our knowledge, the first example of integrated direct air capture and hetero-catalytic conversion of CO2 to methanol was effectively demonstrated here. The catalytic conversion of the captured CO2 was found to be facile, with significantly high methanol yields. The first evidence of Cu/ZnO/Al2O3 catalyzed hydrogenation of metal carbonate to methanol was achieved. The importance of an alcohol solvent for the hydrogenation of various CO2 capture species was validated. We believe that the present batch system when operated under flow conditions could potentially be implemented at commercial scale to enhance the industrial methanol synthesis process. Additionally, catalyst modifications, in terms of composition and method of preparation could also enhance the present system based on the commercial methanol synthesis catalyst. 23 In a worthwhile direction for the future, this system offers an easy integration with already existing CO2 scrubbing units toward a renewable and carbon neutral methanol economy. 5.4. Experimental Methods 5.4.1. Materials and methods All experiments were carried out under an inert atmosphere (with N2 or Ar) using standard Schlenk techniques with the exclusion of moisture unless otherwise stated. Commercial Cu/ZnO/Al2O3 catalyst was purchased from Alfa Aesar. The catalyst composition (63.5 wt% CuO, 24.7 wt% ZnO, 10.1 wt% Al2O3, 1.3 wt% MgO fume) 24 provided by the manufacturer was verified via elemental analysis (Table 5.4) and the average surface area of the activated catalyst was measured to be 79.6 m 2 ·g -1 cat. 25 Potassium hydroxide and pentaethylenehexamine (PEHA) were 169 purchased from Millipore Sigma and used without further purification. All solvents were purchased from commercial sources and were sparged with N2 for 1 h prior to use. DMSO-d6 (CIL, D-99.9%), D2O (CIL, D-99.9%), imidazole (Fischer, 99.5%) and t BuOH (99%, Sigma Aldrich) were used as received. 1 H and 13 C NMR spectra were recorded on 400, 500 or 600 MHz, Varian NMR spectrometers. 1 H and 13 C NMR chemical shifts were determined relative to the residual solvent signals. The gas mixtures were analyzed using a Thermo Finnigan gas chromatograph (column: Supelco, Carboxen 1010 plot, 30 m X 0.53 mm) equipped with a TCD detector (CO detection limit: 0.099 v/v%). CO2 (Gilmore, instrument grade), 1:3 CO2:H2 mix (Airgas, certified standard-spec grade) and H2 (Gilmore, ultra-high pure grade 5.0) were used without further purification. Caution: Reactions are associated with H2 gas. They should be carefully handled inside proper fume hoods without any flame, spark or static electricity sources nearby. 5.4.2. Standard procedure for CO2 hydrogenation reactions For activating the catalyst, the catalyst pellets were ground and sieved. Next, it was subjected to a flow of N2 (100 mL/min) at 120 °C for 1 h, then ramped up to 270 °C (10 °C /min) with a H2 flow (35 mL/min) in N2 (100 mL/min) at 1 atm for 5 h and stored afterwards under Ar for further use. 17a In a nitrogen-filled chamber, the pre-activated catalyst (in a borosilicate glass vial) and the solvent (if any) were introduced to a 125 mL Monel Parr reactor. The sealed vessel was then filled to the desired pressure and composition with CO2 and H2. The reactor was placed in a preheated aluminum block and heated to the desired temperature. After heating for a given reaction period, the reactor was cooled to room temperature. The vessel was then cooled in an ice bath for 30 minutes. Afterwards, the gases inside the vessel were partly collected in a gas sampling bag for GC analysis whereas the remaining gas was slowly released. Upon opening the reaction vessel, an aliquot of the liquid sample was collected, and a known amount of internal standard 170 (imidazole or t BuOH) was added to it and passed through a PTFE syringe filter to avoid any trace metal impurities. The sample was then analyzed by 1 H and 13 C NMR with a deuterated solvent. Yields were determined through 1 H or 13 C NMR from integration ratios. The carbon and hydrogen are balanced between the reactants (CO2 and H2) and the products (CH3OH and H2O) in liquid phase. The side products (CO and CH4) if any, were observed only in trace amounts (<0.1% of the unreacted gas mixtures) and cannot be quantified accurately because of the large excess of unreacted H2. For solventless conditions, a known amount of H2O was used to dissolve the produced liquid products before analysis. For the reactions in squalene, the formed methanol was extracted from the reaction mixture with a known amount of D2O and a known amount of imidazole (Im) was added for 1 H NMR analysis. The volume of the reactor headspace was measured to be approximately 130 mL (without solvent) and 120 mL (with 10 mL solvent). 5.4.3. Standard procedure for recycling studies In a nitrogen-filled chamber, the pre-activated catalyst (in a borosilicate glass vial) and ethylene glycol (10 mL) were introduced to a 125 mL Monel Parr reactor. The sealed vessel was then filled with CO2 (4 bar) and H2 (70 bar). The reactor was placed in a preheated aluminum block and heated to the desired temperature. After heating for 72 h, the reactor was allowed to cool to room temperature. The vessel was then further cooled in an ice bath for 30 minutes and the gases inside partly collected in a gas sampling bag for GC analysis. The rest of the gas was slowly released and vented. Upon opening the reaction vessel, an aliquot of the liquid sample was collected, and a known amount of internal standard was added to it and passed through a PTFE syringe filter to avoid any trace metal impurities. The sample was then analyzed by 1 H and 13 C NMR with DMSO-d6 as the deuterated solvent. Yields were determined through 1 H NMR from 171 integration ratios. The catalyst was recovered by removing the solvent, methanol and water in vacuo. The recovered catalyst was used for subsequent hydrogenation cycle. 5.4.4. Standard procedure for CO2 capture by alkali hydroxide/amine in ethylene glycol A known amount of alkali hydroxide (KOH) or amine (PEHA) were dissolved in ethylene glycol (10 mL) in a vial with a magnetic stir bar. The gases inside the vial were then removed under vacuum. CO2 was subsequently added while stirring the solution at 800 rpm for 3 h and maintaining the CO2 pressure inside the reactor at 1 psi above atmospheric pressure. The amounts of CO2 captured were calculated through gravimetric analysis of the solutions before and after the capture. 5.4.5. Standard procedure of CO2 capture from air In a 30 mL vial, a specific amount of PEHA or KOH was dissolved in 10 mL ethylene glycol. Atmospheric air containing ~420 ppm CO2 was then bubbled through the solution at a flowrate of 200 mL/min for 72 h using a pump. After completion of the CO2 capture, the solution volume increased by around 2 mL due to accumulation of moisture from air. The resulting solution was then sparged with N2 for 2 h. Afterwards, imidazole (Im) was added as an internal standard to 0.5 mL aliquot of the homogeneous solution that was analyzed by 1 H and 13 C NMR with DMSO- d6 as the deuterated solvent. The amount of CO2 captured was calculated through 13 C NMR analysis. The remaining solution was used for hydrogenation. 5.4.6. Standard Procedure for control studies In a nitrogen-filled chamber, 300 mg pre-activated catalyst (in a borosilicate glass vial) and a 10 mmol substrate were introduced to a 125 mL Monel Parr reactor. 10 mL solvent (ethylene glycol or triglyme) was added to the reactor. The sealed vessel was then filled with H2 (70 bar). The 172 reactor was placed in a preheated aluminum block and heated to 200 °C. The reactions were stopped after 16 h, shorter time than the general hydrogenation conditions in attempt to detect any possible short lived intermediate. After venting the gases, the reaction mixture was analyzed by 1 H and 13 C NMR with a deuterated solvent. PEHA-HCOOH adduct was prepared in situ by adding a diluted solution of 97% aqueous HCOOH in the reaction solvent to the solution of PEHA in the reaction solvent cooled in ice. 5.4.7. Detection of ethylene glycol monoformate The formate ester with ethylene glycol was not observed by NMR analysis of the reaction mixtures for the hydrogenation experiments. However, ethylene glycol monoformate was detected via GC-MS and was identified by matching with a reference spectrum in MS library. When a hydrogenation reaction was performed using excess CO2 (CO2:H2 = 1:1), substantial formate ester was produced and observed by 1 H and 13 C NMR. 5.4.8. Catalyst characterization studies X-ray diffraction studies. X-ray diffraction (XRD) patterns were obtained using a Rigaku Ultima IV X-Ray Diffractometer with a Cu-Kα (0.154nm) radiation source and a scan rate of 4° min -1 from a 2θ value of 10° to 80°. Peaks were analyzed and fit using the MDI Jade 9 software. Catalyst crystallite size was calculated using the Scherrer equation based off the broadening of the peak at 43.5°. Dashed lines correspond to Cu 0 peaks, solid lines correspond to ZnO and β peaks correspond to CuO. 3 173 Table 5.3. Cu 0 crystallite size for various catalyst samples. Entry Size (nm.) Pretreated catalyst 12 Spent – No Solvent 12 Spent - EG 12 Spent – EG + PEHA 12 Spent – EG + KOH 13 After 5 cycles 13 Surface Analysis (BET). Surface properties of the commercial Cu/ZnO/Al2O3 catalyst were characterized by nitrogen adsorption, using a Quantachrome NOVA 2200e instrument. First, the catalyst was degassed under vacuum at 110 °C for 3 h to remove the pre-adsorbed moisture and gases. Nitrogen adsorption and desorption isotherms were measured at 77K. The specific surface area was calculated by the multi-point Brunauer-Emmett-Teller (MBET) method using the adsorption branch of the physisorption isotherm. Average surface area was measured to be 79.6 m 2 /g. The measured value is in close correlation with values reported in literature. 9 Elemental Analysis (EDS). Energy Dispersive X-Ray Analysis (EDS) was peformed on a JEOL JSM-7001 electron microscope with an accelerating voltage of 20 keV. The elemental analysis of the activated catalyst is described in Table x. Table 5.4. Composition of activated catalyst (calculated for activated catalyst based on the composition provided by Alfa Aesar). Element Composition (wt%) Calculated Measured O 11.8 11.97 Al 6.1 10.04 Cu 58.1 56.23 Zn 22.8 21.77 174 5.4.9. Calculations for hydrogenation of CO2 to methanol at equilibrium At the equilibrium, Gibbs-Free-Energy as stated in Eq. 5.1 is at its minimum. In this equation, 𝑁 is the number of components and the total Gibbs-Free-Energy (𝐺 ! ) is sum of chemical potentials of all the components with 𝑛 being the number of moles. Chemical potential (𝜇) is then defined based on the standard Gibbs-Free-Energy of formation for each component (𝐺 "# $ ), constant of gases (R), temperature (𝑇) and fugacity (𝑓 # ) as shown in Eq. 5.2. In this work, R-Gibbs reactor model in Aspen Plus along with Soave-Redlich-Kwong equations of state (Eq. 5.3-5.5) were used to minimize Gibbs-Free-Energy of the hydrogenation unit. In addition to the main components involved in the hydrogenation and water gas shift reactions, CH4 was also included in the model to account for potential side reactions. 𝐺 ! =*𝑛 # 𝜇 # % #&' (Eq. 5.1) 𝜇 # =𝐺 "# $ +𝑅𝑇 𝑙𝑛/ 𝑓 # 𝑓 # $ 0 (Eq. 5.2) 𝑃 = 𝑅𝑇 (𝑉+𝑐−𝑏) − 𝑎𝛼(𝑇) (𝑉+𝑐)(𝑉+𝑐+𝑏) (Eq. 5.3) 𝑐 =0.40768 𝑅𝑇 (,# 𝑃 (,# /0.29441− 𝑃 (,# 𝑉 (,# 𝑅𝑇 (,# 0 (Eq. 5.4) 𝑏 =0.08664 𝑅𝑇 (,# 𝑃 (,# (Eq. 5.5) All the input parameters to the reactor model as well as the calculated properties at its outlet stream for the end reaction conditions are listed in Table S3. Three different scenarios with CO2:H2 ratios of 1:18, 1:6 and 1:3 were modeled to mimic the experimental studies in this work. For each of these scenarios, methanol yields have been calculated for the pressure (Pfinal) when the reaction 175 reaches steady state as experimentally observed under batch conditions. Table 5.6 compares the calculated methanol yields at Pfinal with the ones calculated at the highest pressures (Pmax) measured during the course of the reaction for each of these three scenarios. Table 5.5. Methanol yields calculated with the R-Gibbs model at the end reaction conditions CO2:H2 1:18 1:6 1:3 Inlet Outlet Inlet Outlet Inlet Outlet Temperature (°C) 200 200 200 200 200 200 Pressure, Pfinal (bar) 88 88 75 75 74 74 Vapor Fraction 1 1 1 1 1 1 Mass Flow (kg/hr) 412 412 831 831 1251 1251 Mole Flow (kmol/hr) Total 100 90.7 0 0 100 77 CH4 0 0.0 0 10.2 0 0 H2O 0 4.6 15 4.8 0 11.9 CO2 5 0.4 0 0.1 25 13.1 CO 0 0.0 85 54.7 0 0.2 H2 95 81.2 0 10.1 75 39.8 MeOH 0 4.6 0 0 0 11.7 MeOH Yield (%) 92.1 67.1 46.6 Table 5.6. Methanol yields calculated at Pfinal and Pmax CO2:H2 1:18 1:6 1:3 Temperature (°C) 200 200 200 Pfinal (bar) 88 75 74 Yield at Pfinal (%) 92.1 67.1 46.6 Pmax (bar) 107 102 95 Yield at Pmax (%) 94.6 77.9 54.8 176 5.4.10. Representative Spectra Figure 5.12. 1 H NMR spectra of reaction mixture after hydrogenation reaction of CO2 with no solvent in DMSO-d6. Figure 5.13. Typical 1 H NMR spectra of reaction mixture after hydrogenation reaction of CO2 in ethylene glycol (EG) in DMSO-d6. - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 2 . 0 6 1 . 0 0 0 . 5 0 - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 1 . 4 5 1 . 0 0 0 . 5 0 Im Im CH3OH Im Im CH3OH EG 177 Figure 5.14. Typical 13 C NMR spectra of reaction mixture after hydrogenation reaction of CO2 in triglyme in DMSO-d6. Figure 5.15. Typical 1 H NMR spectra of reaction mixture after hydrogenation reaction of CO2 in squalene in D2O. - 1 0 1 0 3 0 5 0 7 0 9 0 1 1 0 1 3 0 1 5 0 1 7 0 1 9 0 2 1 0 2 3 0 f 1 ( p p m ) 0 . 3 7 1 . 0 0 0 . 4 9 - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 5 . 1 3 1 . 0 0 0 . 5 0 Im Im CH3OH Triglyme Im Im CH3OH 178 Figure 5.16. Typical GC spectra of the gas mixture after hydrogenation in the absence of solvent. Figure 5.17. Typical GC spectra of the gas mixture after hydrogenation in ethylene glycol. C:\Xcalibur\data\Alain\raktim\rs-2-241 RT: 0.20 - 11.06 1 2 3 4 5 6 7 8 9 10 11 Time (min) 0 50000 100000 150000 200000 250000 300000 350000 400000 450000 500000 550000 600000 650000 700000 750000 800000 850000 900000 Counts 2.27 8.36 4.40 8.96 8.19 7.22 9.07 10.29 2.39 2.61 4.53 1.63 3.63 6.21 5.48 0.68 NL: 1.06E8 TCD Analog 2 rs-2-241 RT: 0.00 - 13.98 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Time (min) 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 Counts 3.60 0.34 5.28 2.12 1.60 3.32 4.45 5.61 7.21 8.02 9.71 10.09 8.26 11.65 13.59 12.64 NL: 3.09E4 FID Analog rs-2-241 C:\Xcalibur\data\Alain\raktim\rs-2-228 RT: 0.33 - 10.75 1 2 3 4 5 6 7 8 9 10 Time (min) 50000 100000 150000 200000 250000 300000 350000 400000 450000 500000 550000 600000 650000 700000 750000 800000 850000 900000 Counts 2.27 8.36 8.46 9.03 4.40 8.26 7.34 7.04 2.39 10.04 2.61 4.52 1.63 6.60 5.37 3.43 0.67 NL: 1.01E8 TCD Analog 2 rs-2-228 RT: 0.00 - 13.99 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Time (min) 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 32000 34000 36000 Counts 12.23 13.90 13.10 11.89 11.07 9.47 1.08 0.24 3.90 8.89 6.67 5.24 1.53 7.46 6.29 2.35 3.21 5.03 NL: 3.71E4 FID Analog rs-2-228 H2 N2 CO CO2 CH4 H2 N2 CO CO2 CH4 179 Figure 5.18. 1 H NMR spectra of reaction mixture after hydrogenation in 5 th run of recycling studies in DMSO-d6. Figure 5.19. 13 C NMR spectra of ethylene glycol solution of PEHA after CO2 capture in D2O. - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 1 . 8 2 1 . 0 0 0 . 5 0 - 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0 2 1 0 2 2 0 2 3 0 f 1 ( p p m ) Im Im CH3OH EG carbamate/alkyl carbonate PEHA 180 Figure 5.20. 13 C NMR spectra of captured CO2 by KOH in ethylene glycol in DMSO-d6. Figure 5.21. 13 C NMR spectra of CO2 captured in ethylene glycol solution of PEHA from ambient air in DMSO-d6. - 1 0 0 1 0 2 0 3 0 4 0 50 6 0 7 0 8 0 9 0 1 0 0 11 0 1 2 0 1 3 0 1 4 0 1 50 1 6 0 17 0 1 8 0 1 90 2 0 0 21 0 2 2 0 2 3 0 f 1 ( p p m ) 1 . 0 0 0 . 9 9 1 . 0 0 6 1 . 7 7 6 7 . 7 4 1 5 9 . 8 1 - 1 0 1 0 3 0 5 0 7 0 9 0 1 1 0 1 3 0 1 5 0 1 7 0 1 9 0 2 1 0 2 3 0 f 1 ( p p m ) 1 . 0 0 0 . 5 0 0 . 0 1 0 . 8 4 Im Im PEHA EG carbamate/alkyl carbonate O O K O HO 1 3 2 C(1) C(3) C(2) 181 Figure 5.22. 1 H NMR spectra of reaction mixture after hydrogenation of CO2 captured in ethylene glycol solution of PEHA from ambient air in DMSO-d6. Figure 5.23. 13 C NMR spectra of CO2 captured in ethylene glycol solution of KOH from ambient air in DMSO-d6 (Table 5.2, entry 2). - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 0 . 4 2 1 . 0 0 0 . 5 0 - 1 0 1 0 3 0 5 0 7 0 9 0 1 1 0 1 3 0 1 5 0 1 7 0 1 9 0 2 1 0 2 3 0 f 1 ( p p m ) 0 . 2 1 0 . 2 0 1 . 0 0 0 . 5 0 0 . 2 0 0 . 2 7 Im Im EG carbonate/alkyl carbonate alkyl carbonate Im Im CH3OH EG PEHA 182 Figure 5.24. 1 H NMR spectra of reaction mixture after hydrogenation of CO2 captured in ethylene glycol solution of KOH from ambient air in DMSO-d6 (Table 5.2, entry 2). Figure 5.25. 1 H NMR spectra of reaction mixture after hydrogenation of KHCO3 in ethylene glycol in DMSO-d6. - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 0 . 5 5 1 . 0 0 0 . 5 0 - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 1 . 0 0 1 . 1 6 Im Im CH3OH EG CH3OH t BuOH EG 183 Figure 5.26. 1 H NMR spectra of reaction mixture after attempted hydrogenation of KHCO3 in triglyme in DMSO-d6. Figure 5.27. 1 H NMR spectra of reaction mixture after hydrogenation of HCOOK in ethylene glycol in DMSO-d6. - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 1 . 0 0 0 . 4 4 0 . 0 4 t BuOH triglyme CH3OH t BuOH EG HCOOK 184 Figure 5.28. 1 H NMR spectra of reaction mixture after attempted hydrogenation of HCOOK in triglyme in DMSO-d6. Figure 5.29. 1 H NMR spectra of reaction mixture after hydrogenation of HCOOEt in triglyme in DMSO-d6. - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 0 . 2 8 1 . 0 0 0 . 3 1 t BuOH triglyme CH3OH t BuOH triglyme EtOH 185 Figure 5.30. 1 H NMR spectra of reaction mixture after hydrogenation of PEHA-HCOOH adduct in ethylene glycol in DMSO-d6. Figure 5.31. 1 H NMR spectra of reaction mixture after attempted hydrogenation of PEHA- HCOOH adduct in triglyme in DMSO-d6. - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 1 . 0 0 0 . 1 7 - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) CH3OH t BuOH EG PEHA t BuOH triglyme PEHA 186 Figure 5.32. GC-MS spectra of the formate ester in the reaction mixture extracted with diethyl ether. Figure 5.33. 1 H NMR spectra of ethylene glycol monoformate in reaction mixture in DMSO-d6. - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 1 . 0 0 0 . 0 5 0 . 0 4 0 . 0 2 t BuOH EG formate O OH H O m/z: 90.03 187 Figure 5.34. 13 C NMR spectra of ethylene glycol monoformate in reaction mixture in DMSO-d6. Figure 5.35. XRD spectra of the catalyst under different conditions. - 10 0 1 0 2 0 3 0 4 0 5 0 60 7 0 8 0 90 1 0 0 1 1 0 1 2 0 1 3 0 14 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0 2 1 0 2 2 0 2 3 0 f 1 ( p p m ) 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Intensity (a.u) 2θ( degree) Before Pretreatment Spent - EG + KOH Spent - EG + PEHA β β β β β β O OH H O C1 C2 C3 C3 C1 C2 t BuOH EG 188 5.5. References [1] Bui, M.; Adjiman, C. S.; Bardow, A.; Anthony, E. J.; Boston, A.; Brown, S.; Fennell, P. S.; Fuss, S.; Galindo, A.; Hackett, L. A.; Hallett, J. P.; Herzog, H. J.; Jackson, G.; Kemper, J.; Krevor, S.; Maitland, G. C.; Matuszewski, M.; Metcalfe, I. S.; Petit, C.; Puxty, G.; Reimer, J.; Reiner, D. M.; Rubin, E. S.; Scott, S. A.; Shah, N.; Smit, B.; Trusler, J. P. M.; Webley, P.; Wilcox, J.; Mac Dowell, N., Carbon capture and storage (CCS): the way forward. Energy Environ. Sci. 2018, 11, 1062-1176. [2] (a) Leclaire, J.; Heldebrant, D. J., A call to (green) arms: a rallying cry for green chemistry and engineering for CO2 capture, utilisation and storage. Green Chem. 2018, 20, 5058-5081; (b) Jones, W. D., Carbon Capture and Conversion. J. Am. Chem. Soc. 2020, 142, 4955-4957; (c) Gao, W.; Liang, S.; Wang, R.; Jiang, Q.; Zhang, Y.; Zheng, Q.; Xie, B.; Toe, C. Y.; Zhu, X.; Wang, J.; Huang, L.; Gao, Y.; Wang, Z.; Jo, C.; Wang, Q.; Wang, L.; Liu, Y.; Louis, B.; Scott, J.; Roger, A.-C.; Amal, R.; He, H.; Park, S. E., Industrial carbon dioxide capture and utilization: state of the art and future challenges. Chem. Soc. Rev. 2020, 49, 8584-8686. [3] (a) Olah, G. A.; Prakash, G. K. S.; Goeppert, A., Anthropogenic chemical carbon cycle for a sustainable future. J. Am. Chem. Soc. 2011, 133, 12881-12898; (b) Goeppert, A.; Czaun, M.; Prakash, G. K. S.; Olah, G. A., CO2 capture and recycling to fuels and materials: towards a sustainable future. In An Introduction to Green Chemistry Methods, pp 132-146; (c) Yu, K. M. K.; Curcic, I.; Gabriel, J.; Tsang, S. C. E., Recent Advances in CO2 Capture and Utilization. ChemSusChem 2008, 1, 893-899; (d) Liu, Q.; Wu, L.; Jackstell, R.; Beller, M., Using carbon dioxide as a building block in organic synthesis. Nat. Commun. 2015, 6, 5933; (e) Fu, H. C.; You, F.; Li, H. R.; He, L. N., CO2 Capture and in situ Catalytic Transformation. Front. Chem. 2019, 7, 525. [4] (a) Olah, G. A.; Goeppert, A.; Prakash, G. K. S.; Wiley, V. C. H., Beyond Oil and Gas: The Methanol Economy. 3rd ed.; 2018; (b) Olah, G. A.; Goeppert, A.; Prakash, G. K. S., Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons. J. Org. Chem. 2009, 74, 487- 498; (c) Goeppert, A.; Czaun, M.; Jones, J. P.; Prakash, G. K. S.; Olah, G. A., Recycling of carbon dioxide to methanol and derived products - closing the loop. Chem. Soc. Rev. 2014, 43, 7995-8048; (d) Methanol to Olefins (MTO) and Methanol to Gasoline (MTG). In Zeolites and Catalysis, 2010; pp 687-711. [5] (a) Goeppert, A.; Olah, G. A.; Prakash, G. K. S., Chapter 3.26 - Toward a Sustainable Carbon Cycle: The Methanol Economy. In Green Chemistry, Török, B.; Dransfield, T., Eds. Elsevier: 2018; pp 919-962; (b) Zhen, X.; Wang, Y., An overview of methanol as an internal combustion engine fuel. Renewable Sustainable Energy Rev. 2015, 52, 477-493; (c) Kothandaraman, J.; Kar, S.; Goeppert, A.; Sen, R.; Prakash, G. K. S., Advances in Homogeneous Catalysis for Low Temperature Methanol Reforming in the Context of the Methanol Economy. Top. Catal. 2018, 61, 542-559; (d) Kothandaraman, J.; Kar, S.; Sen, R.; Goeppert, A.; Olah, G. A.; Prakash, G. K. S., 189 Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol. J. Am. Chem. Soc. 2017, 139, 2549-2552. [6] Liu, W. C.; Baek, J.; Somorjai, G. A., The Methanol Economy: Methane and Carbon Dioxide Conversion. Top. Catal. 2018, 61, 530-541. [7] (a) Joo, O. S.; Jung, K. D.; Moon, I.; Rozovskii, A. Y.; Lin, G. I.; Han, S. H.; Uhm, S. J., Carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction (the CAMERE process). Ind. Eng. Chem. Res. 1999, 38, 1808-1812; (b) Carbon Recycling International. http://www.carbonrecycling.is/george-olah/ ; (c) Mitsui Chemicals. https://jp.mitsuichemicals.com/en/release/2008/080825e.htm; (d) González-Garay, A.; Frei, M. S.; Al-Qahtani, A.; Mondelli, C.; Guillén-Gosálbez, G.; Pérez-Ramírez, J., Plant-to-planet analysis of CO2-based methanol processes. Energy Environ. Sci. 2019, 12, 3425-3436; (e) Kang, S.; Boshell, F.; Goeppert, A.; Prakash, G. K. S.; Landälv, I., Innovation Outlook: Renewable Methanol. IRENA and Methanol Institute 2021. https://www.irena.org/- /media/Files/IRENA/Agency/Publication/2021/Jan/IRENA_Innovation_Renewable_Methanol_2 021.pdf. [8] Lunkenbein, T.; Girgsdies, F.; Kandemir, T.; Thomas, N.; Behrens, M.; Schlogl, R.; Frei, E., Bridging the Time Gap: A Copper/Zinc Oxide/Aluminum Oxide Catalyst for Methanol Synthesis Studied under Industrially Relevant Conditions and Time Scales. Angew. Chem. Int. Ed. 2016, 55, 12708-12712. [9] (a) Xie, S.; Zhang, W.; Lan, X.; Lin, H., CO2 Reduction to Methanol in the Liquid Phase: A Review. ChemSusChem 2020, 13, 6141-6159; (b) Heydorn, E. C.; Diamond, B. W.; Lilly, R. D., Commerical-Scale Demonstration of the Liquid Phase Methanol (LPMEOH™) Process 2003; (c) Lee, S. G.; Sardesai, A., Liquid phase methanol and dimethyl ether synthesis from syngas. Top. Catal. 2005, 32, 197-207; (d) Behrens, M., Promoting the Synthesis of Methanol: Understanding the Requirements for an Industrial Catalyst for the Conversion of CO2. Angew. Chem. Int. Ed. 2016, 55, 14906-14908; (e) Zhong, J.; Yang, X.; Wu, Z.; Liang, B.; Huang, Y.; Zhang, T., State of the art and perspectives in heterogeneous catalysis of CO2 hydrogenation to methanol. Chem. Soc. Rev. 2020, 49, 1385-1413; (f) Studt, F.; Sharafutdinov, I.; Abild-Pedersen, F.; Elkjær, C. F.; Hummelshøj, J. S.; Dahl, S.; Chorkendorff, I.; Nørskov, J. K., Discovery of a Ni-Ga catalyst for carbon dioxide reduction to methanol. Nat. Chem. 2014, 6, 320-324; (g) Bai, S. T.; De Smet, G.; Liao, Y.; Sun, R.; Zhou, C.; Beller, M.; Maes, B. U. W.; Sels, B. F., Homogeneous and heterogeneous catalysts for hydrogenation of CO2 to methanol under mild conditions. Chem. Soc. Rev. 2021, 50, 4259-4298; (h) Jiang, X.; Nie, X.; Guo, X.; Song, C.; Chen, J. G., Recent Advances in Carbon Dioxide Hydrogenation to Methanol via Heterogeneous Catalysis. Chem. Rev. 2020, 120, 7984-8034; (i) Xie, S.; Zhang, W.; Jia, C.; Ong, S. S. G.; Zhang, C.; Zhang, S.; Lin, H., Eliminating carbon dioxide emissions at the source by the integration of carbon dioxide capture and utilization over noble metals in the liquid phase. J. Catal. 2020, 389, 247-258; (j) Reller, C.; Pöge, M.; Lißner, A.; Mertens, F. O. R. L., Methanol from CO2 by Organo-Cocatalysis: CO2 Capture and Hydrogenation in One Process Step. Environ. Sci. Technol. 2014, 48, 14799-14804. 190 [10] (a) Yang, R.; Fu, Y.; Zhang, Y.; Tsubaki, N., In situ DRIFT study of low-temperature methanol synthesis mechanism on Cu/ZnO catalysts from CO-containing syngas using ethanol promoter. J. Catal. 2004, 228, 23-35; (b) Fan, L.; Sakaiya, Y.; Fujimoto, K., Low-temperature methanol synthesis from carbon dioxide and hydrogen via formic ester. Appl. Catal., A 1999, 180, L11-L13. [11] (a) Kar, S.; Kothandaraman, J.; Goeppert, A.; Prakash, G. K. S., Advances in catalytic homogeneous hydrogenation of carbon dioxide to methanol. J. CO2 Util. 2018, 23, 212-218; (b) Rayder, T. M.; Adillon, E. H.; Byers, J. A.; Tsung, C.-K., A Bioinspired Multicomponent Catalytic System for Converting Carbon Dioxide into Methanol Autocatalytically. Chem 2020, 6, 1742- 1754; (c) Li, Y.-N.; Ma, R.; He, L.-N.; Diao, Z.-F., Homogeneous hydrogenation of carbon dioxide to methanol. Catal. Sci. Technol. 2014, 4, 1498-1512; (d) Wesselbaum, S.; vom Stein, T.; Klankermayer, J.; Leitner, W., Hydrogenation of Carbon Dioxide to Methanol by Using a Homogeneous Ruthenium–Phosphine Catalyst. Angew. Chem. 2012, 124, 7617-7620; (e) Schneidewind, J.; Adam, R.; Baumann, W.; Jackstell, R.; Beller, M., Low-Temperature Hydrogenation of Carbon Dioxide to Methanol with a Homogeneous Cobalt Catalyst. Angew. Chem. Int. Ed. 2017, 56, 1890-1893; (f) Huff, C. A.; Sanford, M. S., Cascade catalysis for the homogeneous hydrogenation of CO2 to methanol. J. Am. Chem. Soc. 2011, 133, 18122-18125; (g) Alberico, E.; Nielsen, M., Towards a methanol economy based on homogeneous catalysis: methanol to H2 and CO2 to methanol. Chem. Commun. 2015, 51, 6714-6725; (h) Kothandaraman, J.; Heldebrant, D. J., Catalytic coproduction of methanol and glycol in one pot from epoxide, CO2, and H2. RSC Adv. 2020, 10, 42557-42563; (i) Kar, S.; Goeppert, A.; Prakash, G. K. S., Catalytic Homogeneous Hydrogenation of CO2 to Methanol. In CO2 Hydrogenation Catalysis, 2021; pp 89- 112; (j) Sordakis, K.; Tsurusaki, A.; Iguchi, M.; Kawanami, H.; Himeda, Y.; Laurenczy, G., Carbon Dioxide to Methanol: The Aqueous Catalytic Way at Room Temperature. Chem. Eur. J. 2016, 22, 15605-15608; (k) Kanega, R.; Onishi, N.; Tanaka, S.; Kishimoto, H.; Himeda, Y., Catalytic Hydrogenation of CO2 to Methanol Using Multinuclear Iridium Complexes in a Gas– Solid Phase Reaction. J. Am. Chem. Soc. 2021, 143, 1570-1576. [12] (a) Nieminen, H.; Givirovskiy, G.; Laari, A.; Koiranen, T., Alcohol promoted methanol synthesis enhanced by adsorption of water and dual catalysts, J. CO2 Util. 2018, 24, 180-189; (b) Kothandaraman, J.; Dagle, R. A.; Dagle, V. L.; Davidson, S. D.; Walter, E. D.; Burton, S. D.; Hoyt, D. W.; Heldebrant, D. J., Condensed-phase low temperature heterogeneous hydrogenation of CO2 to methanol. Catal. Sci. Technol. 2018, 8, 5098-5103; (c) Reubroycharoen, P.; Yamagami, T.; Vitidsant, T.; Yoneyama, Y.; Ito, M.; Tsubaki, N., Continuous Low-Temperature Methanol Synthesis from Syngas Using Alcohol Promoters. Energy Fuels 2003, 17, 817-821; (d) Yang, R.; Zhang, Y.; Tsubaki, N., Dual catalysis mechanism of alcohol solvent and Cu catalyst for a new methanol synthesis method. Catal. Commun. 2005, 6, 275-279; (e) Chen, Y.; Choi, S.; Thompson, L. T., Low-Temperature CO2 Hydrogenation to Liquid Products via a Heterogeneous Cascade Catalytic System. ACS Catal. 2015, 5, 1717-1725; (f) Zeng, J.; Fujimoto, K.; Tsubaki, N., A New Low-Temperature Synthesis Route of Methanol: Catalytic Effect of the Alcoholic Solvent. Energy Fuels 2001, 16, 83-86; (g) Zeng, J.-Q.; Tsubaki, N.; Fujimoto, K., The promoting effect of alcohols in a new process of low-temperature synthesis of methanol from CO/CO2/H2. Fuel 2002, 81, 125-127; (h) Kothandaraman, J.; Heldebrant, D. J., Towards environmentally benign capture and conversion: heterogeneous metal catalyzed CO2 hydrogenation in CO2 capture solvents. Green Chem. 2020, 22, 828-834. 191 [13] Kar, S.; Goeppert, A.; Prakash, G. K. S., Integrated CO2 Capture and Conversion to Formate and Methanol: Connecting Two Threads. Acc. Chem. Res. 2019, 52, 2892-2903. [14] Sen, R.; Goeppert, A.; Prakash, G. K. S., Carbon Dioxide Capture and Recycling to Methanol: Building a Carbon–Neutral Methanol Economy. Aldrichimica Acta 2020, 53, 39-56. [15] (a) Kar, S.; Goeppert, A.; Galvan, V.; Chowdhury, R.; Olah, J.; Prakash, G. K. S., A Carbon- Neutral CO2 Capture, Conversion, and Utilization Cycle with Low-Temperature Regeneration of Sodium Hydroxide. J. Am. Chem. Soc. 2018, 140, 16873-16876; (b) Fu, H.-C.; You, F.; Li, H.- R.; He, L.-N., CO2 Capture and in situ Catalytic Transformation. Front. Chem. 2019, 7; (c) Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. S., CO2 capture by amines in aqueous media and its subsequent conversion to formate with reusable ruthenium and iron catalysts. Green Chem. 2016, 18, 5831-5838; (d) Kar, S.; Goeppert, A.; Prakash, G. K. S., Combined CO2 Capture and Hydrogenation to Methanol: Amine Immobilization Enables Easy Recycling of Active Elements. ChemSusChem 2019, 12, 3172-3177; (e) Khusnutdinova, J. R.; Garg, J. A.; Milstein, D., Combining Low-Pressure CO2 Capture and Hydrogenation To Form Methanol. ACS Catal. 2015, 5, 2416-2422; (f) Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. S., Conversion of CO2 from Air into Methanol Using a Polyamine and a Homogeneous Ruthenium Catalyst. J. Am. Chem. Soc. 2016, 138, 778-781; (g) Sen, R.; Goeppert, A.; Kar, S.; Prakash, G. K. S., Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol. J. Am. Chem. Soc. 2020, 142, 4544-4549; (h) Kar, S.; Sen, R.; Goeppert, A.; Prakash, G. K. S., Integrative CO2 Capture and Hydrogenation to Methanol with Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol Economy. J. Am. Chem. Soc. 2018, 140, 1580-1583; (i) Kar, S.; Sen, R.; Kothandaraman, J.; Goeppert, A.; Chowdhury, R.; Munoz, S. B.; Haiges, R.; Prakash, G. K. S., Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2019, 141, 3160-3170; (j) Lao, D. B.; Galan, B. R.; Linehan, J. C.; Heldebrant, D. J., The steps of activating a prospective CO2 hydrogenation catalyst with combined CO2 capture and reduction. Green Chem. 2016, 18, 4871-4874; (k) Rezayee, N. M.; Huff, C. A.; Sanford, M. S., Tandem Amine and Ruthenium- Catalyzed Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2015, 137, 1028-1031; (l) Sen, R.; Koch, C. J.; Goeppert, A.; Prakash, G. K. S., Tertiary Amine-Ethylene Glycol Based Tandem CO2 Capture and Hydrogenation to Methanol: Direct Utilization of Post-Combustion CO2. ChemSusChem 2020, 13, 6318-6322; (m) Wei, D.; Junge, H.; Beller, M., An amino acid based system for CO2 capture and catalytic utilization to produce formates. Chem. Sci. 2021, 12, 6020- 6024; (n) Liu, A.-H.; Ma, R.; Song, C.; Yang, Z.-Z.; Yu, A.; Cai, Y.; He, L.-N.; Zhao, Y.-N.; Yu, B.; Song, Q.-W., Equimolar CO2 Capture by N-Substituted Amino Acid Salts and Subsequent Conversion. Angew. Chem. Int. Ed. 2012, 51, 11306-11310; (o) Yang, Z.-Z.; Zhao, Y.-N.; He, L.- N., CO2 chemistry: task-specific ionic liquids for CO2 capture/activation and subsequent conversion. RSC Adv. 2011, 1, 545-567; (p) Yang, Z.-Z.; He, L.-N.; Zhao, Y.-N.; Li, B.; Yu, B., CO2 capture and activation by superbase/polyethylene glycol and its subsequent conversion. Energy Environ. Sci. 2011, 4, 3971-3975; (q) Li, Y.-N.; He, L.-N.; Lang, X.-D.; Liu, X.-F.; Zhang, S., An integrated process of CO2 capture and in situ hydrogenation to formate using a tunable ethoxyl-functionalized amidine and Rh/bisphosphine system. RSC Adv. 2014, 4, 49995-50002; (r) Zhang, S.; Li, Y.-N.; Zhang, Y.-W.; He, L.-N.; Yu, B.; Song, Q.-W.; Lang, X.-D., Equimolar Carbon Absorption by Potassium Phthalimide and In Situ Catalytic Conversion Under Mild Conditions. ChemSusChem 2014, 7, 1484-1489; (s) Li, Y.-N.; He, L.-N.; Liu, A.-H.; Lang, X.- 192 D.; Yang, Z.-Z.; Yu, B.; Luan, C.-R., In situ hydrogenation of captured CO2 to formate with polyethyleneimine and Rh/monophosphine system. Green Chem. 2013, 15, 2825-2829. [16] Kumar, A.; Gao, C., Homogeneous (De)hydrogenative Catalysis for Circular Chemistry – Using Waste as a Resource. ChemCatChem 2021, 13, 1105-1134. [17] (a) Liang, B.; Ma, J.; Su, X.; Yang, C.; Duan, H.; Zhou, H.; Deng, S.; Li, L.; Huang, Y., Investigation on Deactivation of Cu/ZnO/Al2O3 Catalyst for CO2 Hydrogenation to Methanol. Ind. Eng. Chem. Res. 2019, 58, 9030-9037; (b) Fichtl, M. B.; Schlereth, D.; Jacobsen, N.; Kasatkin, I.; Schumann, J.; Behrens, M.; Schlögl, R.; Hinrichsen, O., Kinetics of deactivation on Cu/ZnO/Al2O3 methanol synthesis catalysts. Appl. Catal., A 2015, 502, 262-270. [18] (a) Entesari, N.; Goeppert, A.; Prakash, G. K. S., Renewable Methanol Synthesis through Single Step Bi-reforming of Biogas. Ind Eng Chem Res 2020, 59, 10532-10541; (b) Hernández, B.; Martín, M., Optimal Process Operation for Biogas Reforming to Methanol: Effects of Dry Reforming and Biogas Composition. Ind. Eng. Chem. Res. 2016, 55, 6677-6685. [19] (a) Goeppert, A.; Czaun, M.; Prakash, G. K. S.; Olah, G. A., Air as the renewable carbon source of the future: an overview of CO2 capture from the atmosphere. Energy Environ. Sci. 2012, 5, 7833-7853; (b) Goeppert, A.; Czaun, M.; May, R. B.; Prakash, G. K. S.; Olah, G. A.; Narayanan, S. R., Carbon Dioxide Capture from the Air Using a Polyamine Based Regenerable Solid Adsorbent. J. Am. Chem. Soc. 2011, 133, 20164-20167; (c) Sanz-Pérez, E. S.; Murdock, C. R.; Didas, S. A.; Jones, C. W., Direct Capture of CO2 from Ambient Air. Chem. Rev. 2016, 116, 11840-11876; (d) Keith, D. W.; Holmes, G.; St. Angelo, D.; Heidel, K., A Process for Capturing CO2 from the Atmosphere. Joule 2018, 2, 1573-1594; (e) D'Alessandro, D. M.; Smit, B.; Long, J. R., Carbon Dioxide Capture: Prospects for New Materials. Angew. Chem. Int. Ed. 2010, 49, 6058- 6082; (f) Goeppert, A.; Zhang, H.; Sen, R.; Dang, H.; Prakash, G. K. S., Oxidation-Resistant, Cost- Effective Epoxide-Modified Polyamine Adsorbents for CO2 Capture from Various Sources Including Air. ChemSusChem 2019, 12, 1712-1723. [20] Heldebrant, D. J.; Kothandaraman, J., Chapter 3 Solvent-based Absorption. In Carbon Capture and Storage, The Royal Society of Chemistry: 2020; pp 36-68. [21] Heldebrant, D. J.; Koech, P. K.; Glezakou, V. A.; Rousseau, R.; Malhotra, D.; Cantu, D. C., Water-Lean Solvents for Post-Combustion CO2 Capture: Fundamentals, Uncertainties, Opportunities, and Outlook. Chem. Rev. 2017, 117, 9594-9624. [22] Jiang, Y.-Q.; Li, J.; Feng, Z.-W.; Xu, G.-Q.; Shi, X.; Ding, Q.-J.; Li, W.; Ma, C.-H.; Yu, B., Ethylene Glycol: A Green Solvent for Visible Light-Promoted Aerobic Transition Metal-Free Cascade Sulfonation/Cyclization Reaction. Adv. Synth. Catal. 2020, 362, 2609-2614. [23] Behrens, M.; Schlögl, R., How to Prepare a Good Cu/ZnO Catalyst or the Role of Solid State Chemistry for the Synthesis of Nanostructured Catalysts. Z. anorg. allg. Chem. 2013, 639, 2683- 2695. 193 [24] https://alfaaesar.com/certs/certs4/45776-C18W019.pdf. . [25] Zhu, J.; Ciolca, D.; Liu, L.; Parastaev, A.; Kosinov, N.; Hensen, E. J. M., Flame Synthesis of Cu/ZnO–CeO2 Catalysts: Synergistic Metal–Support Interactions Promote CH3OH Selectivity in CO2 Hydrogenation. ACS Catal. 2021, 11, 4880-4892. 194 Chapter 6. Low Temperature Catalytic Methanol Reforming for High Pressure H2 Generation and Integration with a Fuel Cell 6.1. Introduction Over the past century, unparalleled industrial growth and an ever-increasing global population have led to an exponential surge in energy demands. At present, the world’s energy consumption has surpassed 1.7 X 10 5 TWh annually, around 80% of which is still being supplied by burning fossil fuels such as coal, petroleum and natural gas. 1 In addition to the long-term risk of exhausting these relatively vast, yet limited natural reserves, their combustions have led to the inevitable emission of CO2 and other greenhouse gases to the atmosphere in an unaccountable fashion. The increasing concentration of CO2 has by far outpaced the natural carbon cycle by far, leading to repercussions such as global warming, more unpredictable and extreme weather events, rise of sea levels, ocean acidification and increasing loss of biodiversity among many others. 2-3 In an effort to reach an united solution to carbon emissions and related challenges originating from the use of fossil energy, there is a rapid transition to carbon-neutral and renewable energy sources as well as the implementation of sustainable and circular technologies is advocated. 4-7 However, renewable energy sources such as solar and wind often suffer from intermittency and hence, development of energy storage/carrier technologies is vital for their wide-scale deployment. Drawing inspiration from nature and photosynthesis, the storage of renewable energy in chemical bonds has been investigated extensively for the development of energy carriers. 8 Hydrogen (H2), which holds the highest gravimetric energy density of 120 MJ kg -1 , is considered a clean fuel producing only water upon combustion (Figure 6.1a). H2 generated with renewable energy by electrochemical water splitting is generally referred to as “green H2”. 9 Even though H2 195 is often recognized as an ideal renewable fuel candidate, it suffers from intrinsic challenges pertaining to its low volumetric energy density. 10 Limited industrial modes of H2 storage mostly include physical compression under very high pressures of 700 bar or at cryogenic temperatures. Due to its highly flammable and explosive nature, storage and transportation of neat H2 is extremely challenging in terms of safety hazards as well and high capital intensity. Hence, chemical hydrogen carriers have been proposed as practical and efficient alternatives because they can facilitate a progressive transition to a feasible “hydrogen economy” by utilizing the already existing fuel infrastructures. 11 In this regard, formic acid and methanol are of particular interest as both can propel the development of CO2 recycling technologies being products of the same, in parallel to the H2 economy. 12-16 The use of formic acid (HCOOH) has been well explored for its catalytic decomposition to equivalents of H2 and CO2 with favorable energetics (ΔG o = −6.9 kcal mol –1 ). 17-22 Yet, due to a low H2 storage capacity in formic acid (4.4 wt%), methanol (CH3OH) is more promising with a significantly higher H2 content of 12.6 wt% (higher than that of water). 23-25 In addition, methanol is a non-corrosive and easy-to-handle liquid that is convenient to store and transport. The H2 stored in methanol can be catalytically released (reforming) and further coupled with a fuel cell, referred to as reformed methanol fuel cell (RMFC). 23 However, conventional steam reforming of methanol is active only at high temperatures (>250 °C) and high pressures due to relatively unfavorable thermodynamics (ΔG o = +0.6 kJ mol –1 ) (Figure 6.1b). Furthermore, the H2 produced is a mixture with CO2 and small amounts of CO, which is not tolerated by the catalyst in the proton exchange membrane (PEM) based fuel cell catalysts even at very low concentrations of 10 ppm. 19, 26 196 Figure 6.1. Introduction. a) methanol as a hydrogen carrier; b) conventional routes for CO2-to- methanol and methanol reforming; and c) overall theme of the present study: low-temperature methanol reforming for emission-free hydrogen generation. The desirable criteria that a viable methanol reformer should meet, at least in part, include: 1) rapid H2 generation on-demand at moderate temperatures; 2) clean and emission-free production of H2 (free of any CO/CO2); 3) safe and cost-effective; 4) easy to operate at scale; 4) highly efficient catalysis with high activities under low loading; and 5) meet the gravimetric H2 storage capacity recommended by US Department of Energy (4.5 wt%). 8 In 2013, a novel catalytic process was identified independently in seminal studies by the groups of Beller and Grützmacher for low- - low volumetric energy density (0.0108 MJ L -1 ) - highly flammable and explosive - expensive transportation & storage H 2 renewable fuel - obtained by water electrolysis - high energy content (120 MJ kg -1 ) - clean burning fuel Methanol (H 2 carrier) convenient liquid (b.p. 64.7 °C) high H 2 content (12.6 wt %) carbon neutral CH 3 OH + H 2 O > 250 °C 20-50 bar 3 H 2 + CO 2 + CO cat. Steam Reforming of Methanol CH 3 OH + H 2 O 230-300 °C 50-75 bar CO 2 + 3 H 2 cat. CO 2 Recycling to Methanol renewable and green fuel discharging Ultra-high Pressure H 2 generator CH 3 OH + KOH Integrated reformed methanol fuel cell (I-RMFC) Water Electrolysis H 2 recharging Reformer Unit H 2 Renewable Energy C-1 residuals (CO 3 2- /HCO 2 - ) + a) b) c) - low volumetric energy density (0.0108 MJ L -1 ) - highly flammable and explosive - expensive transportation & storage H 2 renewable fuel - obtained by water electrolysis - high energy content (120 MJ kg -1 ) - clean burning fuel Methanol (H 2 carrier) convenient liquid (b.p. 64.7 °C) high H 2 content (12.6 wt %) carbon neutral CH 3 OH + H 2 O > 250 °C 20-50 bar 3 H 2 + CO 2 + CO cat. Steam Reforming of Methanol CH 3 OH + H 2 O 230-300 °C 50-75 bar CO 2 + 3 H 2 cat. CO 2 Recycling to Methanol renewable and green fuel discharging Ultra-high Pressure H 2 generator CH 3 OH + KOH Integrated reformed methanol fuel cell (I-RMFC) Water Electrolysis H 2 recharging Reformer Unit H 2 Renewable Energy C-1 residuals (CO 3 2- /HCO 2 - ) + a) b) c) 197 temperature methanol dehydrogenation to H2/3CO2. 27-28 These reactions were enabled by highly efficient Ru-based molecular catalysts under mild conditions (65-95 °C). Aqueous phase methanol dehydrogenation has since, been explored further to understand the plausible mechanistic pathways, develop new catalysts and unveil more favorable kinetics. 29 Notably, addition of bases such as amines or metal hydroxides to the aqueous methanol solution proved effective in lowering the CO2 content significantly in the evolved gas mixtures which was captured in situ by the base. 8, 29-31 A base-assisted methanol reformer which can produce a continuous stream of pure H2 and be directly fed into a PEM fuel cell is highly desired. 25, 32 Yet, to the best of our knowledge, an integrated low-temperature RMFC has yet to be demonstrated. Moreover, generation of clean H2 under high pressures has not been reported to date, from methanol or any alternate H2 carrier. 22, 33- 36 On-site H2 pressure generators or “chemical compressors” have wide applicability for the safer handling of H2 in H2-fuelled vehicles, stationery power units, H2 refueling stations and others. Currently, H2 compression is primarily achieved using mechanical compressors. 37-38 Inspired by the foundations of homogeneous methanol dehydrogenation and our long-term vision for a functional methanol economy, we present herein, a highly efficient and realistic low- temperature methanol reformer for emission-free H2 generation (Figure 6.1c). The two key reactants in the reformer: methanol and alkali hydroxide readily generate H2 in presence of a Ru- pincer based molecular catalyst at 100-140 °C. The system is emission-free and the H2 produced is of extremely high purity (>99.9 %). Under closed conditions, the catalytic system is efficient in generating H2 readily up to pressures of 144 bar. Under modified conditions, the H2 generation was effectively driven to remarkably high pressures of more than 450 bar, demonstrating a novel chemical compression of H2. Furthermore, the integration of a low-temperature methanol reformer 198 and PEM fuel cell was achieved for the first time. Under open conditions, our engineered reformer unit was able to produce a notably high volumes of H2 (up to 140 L) with promising TONs of 1.2 x 10 5 and a maximum TOF of 5,400 h -1 . The clean H2 generated was simultaneously fed, without any purification, to the PEM fuel cell, which showed stable and continuous production of electric current for an extended period of over 27 h. Finally, the C1-residues (carbonate/formate) of the reforming reaction were successfully converted reversibly to obtain methanol back, indicating the potential application of the present system as a reversible H2 battery. 6.2. Catalytic Reforming of Methanol under Alkaline Conditions 6.2.1. Methanol reforming in a closed system For H2 generation under pressures, the reaction was performed in a pressure vessel (Parr reactor) simply using a mixture of CH3OH/H2O (9:1) and KOH (a model metal hydroxide) in presence of a catalyst. The pressure generated in the headspace was monitored throughout the reaction and for reasonable comparison, the reported pressures are the ones after the reactor was cooled back to rt post reaction, unless mentioned otherwise. Efficient reforming of methanol or any other alcohol in a closed system is particularly challenging given that the accumulation of gaseous products in the reactive space can inhibit the forward reaction and arrest the catalyst in a reversible state. 29 However, we speculated that the addition of a strong base such as an alkali hydroxide might drive the reaction thermodynamically (i.e., increased exothermicity). In addition, our observation of a parasitic reaction in our recent hydrogenation reports, where trace amounts of a glycol solvent oxidized in the presence of a base even under 80 bar of H2, drove us to study the yet unexplored chemistry of methanol reforming in a closed system. 39-41 199 Figure 6.2. Methanol reforming in closed system. a) screening of Ru-based molecular catalysts. reaction conditions: CH3OH/H2O (10 mL, 9:1), KOH (80 mmol), catalyst loading (20 ppm.), stirring (800 rpm), T = 100 °C, t = 24 h; b) effect of reaction temperature. reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), t = 24 h; c) effect of altering the base. reaction conditions: CH3OH/H2O (30 mL, 9:1), OH - (240 mmol), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C t = 24 h; d) effect of KOH content. reaction conditions: CH3OH/H2O (30 mL, 9:1), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 24 h; e) effect of externally introducing H2 pressure. reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 24 h. Pressures measured with a piezoelectric pressure transducer. Calculation error ±5%. Ru P CO N P H H Ph Ph Ph Ph H Ru P CO N P H H Ph Ph Ph Ph Cl BH 3 Ru P CO N P H H i Pr i Pr i Pr i Pr Cl Ru P CO N P H H Cy Cy Cy Cy Cl Ru P CO N P H H t Bu t Bu t Bu t Bu Cl P(psi) 40 46 4 158 108 80 C-1 C-2 Ru P CO N P Me H Ph Ph Ph Ph Cl C-3 C-4 C-5 C-6 CH 3 OH + KOH 100 °C, 24 h H 2 + C 1 residuals cat. (20 ppm.) T (°C) H 2 (psi) a H 2 (bar) a H 2 (L) % reaction in 2 h rate 2h [psi/min] 100 485 33 3.3 17 0.7 120 1320 90 9 22 2.4 140 1470 100 10 74 9.1 160 1556 106 10.6 88 11.4 0 1 2 3 4 5 6 0 400 800 1200 1600 LiOH NaOH KOH Ca(OH)₂ net H 2 content (wt %) H 2 (psi) Alkali Hydroxide H 2 (psi) H 2 (bar) H 2 (L) H 2 content (wt%) Weight Efficiency LiOH 1350 92 9.2 5.7 1.5 NaOH 1370 93 9.3 4.5 1.2 KOH 1470 100 10 3.9 1.0 Ca(OH) 2 345 23.5 2.4 1.2 0.3 60 70 80 90 100 110 120 130 140 150 160 900 1100 1300 1500 1700 1900 2100 2300 140 180 220 260 300 340 380 H 2 pressure (bar) H 2 pressure (psi) KOH (mmol) 900 1100 1300 1500 1700 1900 2100 2300 60 70 80 90 100 110 120 130 140 150 160 0 10 20 30 50 H 2 Pressure (bar) H 2 Pressure (bar) Initial H Pressure (bar) P (net) P (generated) a) b) c) d) e) 0 200 400 600 800 1000 1200 1400 1600 1800 0 4 8 12 16 20 24 H 2 (psi) time (h) 100 °C 120 °C 140 °C 160 °C 200 Among the different molecular catalysts identified for dehydrogenation of methanol and other alcohols, the family of PNP-pincer based metal complexes were found to be most promising, especially when the pH of the system is neutral or basic. 42-45 Under our preliminary reaction condition at 100 °C, a library of catalysts was screened (Figure 6.2a). To our delight, the Ru-PNP catalysts (C-1 to C-6) were indeed effective in generating pressures (50-150 psi) from a solution of KOH in CH3OH/H2O due to gases generated inside the reactor. When the gas phase was analyzed for composition via gas chromatography (GC), only a single peak for H2 was detected, indicating that the gas phase was composed of H2 with a very high purity of >99.9% (Figure 6.3). Notably, no CO and CO2 peaks were detected suggesting that none of the oxidized C-1 products were present in the gas phase. When a scaled-up reaction (30 mL) was performed, Ru-PNP iPr (C- 4) catalyzed the reaction to generate a notable pressure of 485 psi (33 bar) corresponding to 3.3 L of clean H2. As depicted in Table 6.1, the base plays two essential roles: a) a CO2 scavenger, making the system emission-free; and b) an effective promoter, driving the rate of H2 generation by a factor of more than 100 when compared to a reaction in absence of the base. Table 6.1. Control studies to validate the role of the base Entry a KOH (mmol) Pressure (psi) b H2 purity (%) Carbon emissions 1 0.24 (catalytic) 5 < 90 CO2 detected 2 240 mmol 485 > 99.9 no CO or CO2 detected a Reaction conditions: CH3OH/H2O (30 mL, 9:1), C-4 (50 µmol), stirring (800 rpm), t = 24 h. b measured with piezoelectric pressure transducer. Calculation error ±5%. In the light of these encouraging initial results, the reaction profile at different temperatures was explored (Figure 6.2b). While the maximum achievable internal temperatures in the previous 201 open system studies were the reflux temperatures of the liquid mixture (generally below 100 °C), a closed system appeared advantageous to increase the internal temperatures to higher ranges and obtain better performance. Indeed, when the reaction temperature was increased from 100 °C to 120 °C, the catalysis improved significantly and a high H2 pressure of 90 bar was achieved corresponding to 9 L of H2, almost tripling from the 3.3 L at 100 °C. Ramping the temperatures further led to a relatively small increase with the final pressures of 100 bar for a reaction at 140 °C and 106 bar for 160 °C respectively. The saturation in the pressures point toward the limited availability of the base and methanol in these cases. Expectedly, varying the temperature had a prominent effect on the reaction rates, as evident in Figure 6.2b. While 17% of the reaction was complete in the first 2 h of heating at 100 °C (rate2h = 0.7 psi min -1 ), the yield rose sharply to 74% at 140 °C with a rate of 9.1 psi min -1 . In fact, 75% of the final H2 pressure was generated within the first hour of reaction at 160 °C. To note, a similar high H2 pressure of 84 bar was obtained even at 100 °C, albeit over an extended period of 90 h (Figure 6.4). Interestingly, analysis of the headspace prior to heating the reactor also showed a clean H2 peak in GC, suggesting that the catalysis was already active at rt. although the rate was too slow for quantification. Figure 6.3. A typical gas chromatography (GC) spectra of the gas mixture produced during alkaline methanol reforming, showing the sole presence of H2 and no CO/CO2. C:\Xcalibur\...\raktim\rs-190-2h30min RT: 0.00 - 13.99 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Time (min) 0 10000000 20000000 30000000 40000000 50000000 60000000 70000000 80000000 Counts 1.76 2.26 NL: 8.93E7 TCD Analog 2 rs-190- 2h30min RT: 0.00 - 13.98 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Time (min) 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 Counts 2.74 7.20 9.11 1.31 13.09 10.10 2.79 7.00 0.46 2.02 8.19 10.29 3.52 6.47 7.42 9.90 0.72 12.67 8.56 5.54 5.20 11.13 3.78 NL: 2.23E4 FID Analog rs-190- 2h30min C:\Xcalibur\...\raktim\rs-190-2h30min 02/12/2021 03:44:35 PM RT: 0.21 - 12.19 1 2 3 4 5 6 7 8 9 10 11 12 Time (min) -70000 -60000 -50000 -40000 -30000 -20000 -10000 0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000 110000 120000 130000 140000 150000 Counts NL: 8.93E7 TCD Analog 2 rs-190- 2h30min CO CO 2 H 2 N 2 /O 2 202 Figure 6.4. Methanol reforming at 100 °C over extended period. reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), t = 90 h. Next, the effect of altering the base on the methanol reforming reaction was investigated (Figure 6.2c). With LiOH and NaOH, H2 pressures of 92 bar and 93 bar were achieved, respectively, which were comparable to the ones with KOH. Even Ca(OH)2 was active in H2 generation but led to a much lower pressure of 23.5 bar. Importantly, LiOH (23.9 g mol -1 ) and NaOH (40.0 g mol -1 ) have lower equivalent masses than KOH (56.1 g mol -1 ) and hence, provide significant gravimetric advantages in developing a viable H2 storage system. With LiOH, a significant working H2 capacity of 5.7 wt% was achieved with a weight efficiency increased by a factor of 1.5 relative to that with KOH. On the other hand, a NaOH-based system can provide cost- effectiveness due to its substantially lower commercial prices. Furthermore, the base content in the reformer had a pronounced impact on the H2 pressure generated. As shown in Figure 6.2d, the H2 pressure grew rapidly from 79 bar with 180 mmol of KOH to 146 bar with 360 mmol KOH with an overall linear trend. These observations underline the chemical compression of H2 achieved practically with the present system where ~15 L of H2 can be generated from a working volume of 30 mL which is less than 2% of the volume of the gas generated and hence, achieve a compression factor of 540. To further explore the factors effecting the H2 generation in closed system, H2 was introduced in the reactor at different pressures before the start of the reforming 0 400 800 1200 1600 0 20 40 60 80 100 0 20 40 60 80 H 2 pressure (psi) H 2 pressure (bar) time (h) at rt 203 reaction (Figure 6.2e). Interestingly, addition of external pressure had minimal effect on the pressure generated by the reformer which remained stable even upon introducing 50 bar of H2 (net pressure = 148 bar). This is a strong indication that a) the catalysis is equally efficient at high pressures and b) the present reformers have potential as multiple stacked units to generate a much higher cumulative H2 pressure. Other parameters including CH3OH/H2O ratios (Figure 6.7), reaction volume (Figure 6.5) and catalyst loading (Figure 6.6) were also investigated. Figure 6.5. Effect of reaction volume on H2 generation. reaction conditions: CH3OH/H2O (9:1), KOH (8 M), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 90 h. Figure 6.6. Effect of catalyst loading on H2 generation. reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 24 h. 0 4 8 12 16 20 0 20 40 60 80 100 120 140 160 5 15 25 35 45 H 2 volume (L) H 2 pressure (bar) scale (mL) H₂ Pressure (bar) H₂ volume (L) Catalyst loading (µmol) H 2 (psi) H 2 (bar) H 2 (mmol) TON 10 1260 86 357 35714 25 1325 90 376 15023 50 1470 100 417 8333 75 1540 105 437 5820 0 200 400 600 800 1000 1200 1400 1600 1800 0 5 10 15 20 H 2 pressure (psi) time (h) 10 µmol 25 µmol 50 µmol 75 µmol 204 Figure 6.7. Effect on methanol content in the CH3OH/H2O reforming solution on H2 generation: total volume (10 mL), KOH (80 mmol), C-4 (20 ppm.), stirring (800 rpm), T = 100 °C, t = 24 h. 6.2.2. Ultra-high pressure (UHP) generation. The methanol reformer demonstrated vide supra can efficiently generate pure H2 in the pressure range of 100-150 bar which is at par with the pressures generally available with commercial H2 cylinders and satisfy the requirements of regular applications, with a major advantage of safe and convenient transportation. In addition, certain applications such as storage and transportation in H2-fuelled vehicles and refueling stations use higher H2 pressures to obtain better volumetric energy densities. At present, these sectors use specialized carbon composite tanks to store H2 in the pressure range of 250-700 bar or higher. When compared to such high-risk storage modes over long period, alternate methods of generating the H2 pressure on-demand are highly desired. Hence, we explored further with the present methanol reformer to test its ability to generate H2 at even higher pressures. For this, the experimental setup was modified with a smaller volume reactor rated to hold higher pressures with a smaller volume (Figure 6.8). 0 20 40 60 80 100 120 140 160 180 0 2 4 6 8 10 H 2 pressure (psi) methanol content (mL) 205 a Figure 6.8. a)Parr pressure reactors used for H2 generation under pressure. b) photograph of pressure gauge depicting the maximum H2 pressure reached in lab-scale experiment at 140 °C and c) at rt. Figure 6.9. Generation of H2 at ultra-high pressures. Reaction conditions: CH3OH/H2O (9:1), C- 4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 24 h, pressures measured with piezoelectric pressure transducer. Calculation error ±5%. As discussed previously, we noted that the H2 pressures can be increased significantly by manipulating a couple of parameters; a) base content (Figure 6.2d) and b) relative headspace volume (Figure 6.5). When the reforming reaction with 30 mL CH3OH/H2O and 240 mmol KOH was repeated in the new reactor at 140 °C (Figure 6.9), the H2 pressure increased from 100 bar (with ~100 mL headspace) to 162 bar (with ~60 mL headspace). Progressively increasing the base Reactor 1: 135 mL, P limit = 3000 psi Reactor 2: 95 mL, P limit = 8500 psi a) b) c) 205 261 284 304 456 162 206 230 241 362 0 1000 2000 3000 4000 5000 6000 7000 0 100 200 300 400 500 30 mL, 240 mmol 30 mL, 300 mmol 30 mL, 360 mmol 30 mL, 420 mmol 40 mL, 550 mmol H 2 Pressure (bar) H 2 Pressure (bar) Reaction Volume, KOH content P (140 °C) P (rt) . 206 content to 420 mmol. resulted in a pressure generation of 241 bar (304 bar at 140 °C). Finally, a slight increase in the reaction scale (40 mL) drove the reaction to a promising ultra-high pressure (UHP) of 456 bar (6703 psi) at 140 °C and 362 bar (5321 psi) at rt. While the safety guidelines of our reactors and laboratory facilities limited us to the reported pressure ranges, our optimization studies allow us to reasonably project that the reformer can be further modified to achieve pressures of 10,000 psi or higher as per the application needs. 6.2.3. Methanol reforming at ambient pressures. Following the promising H2 generation achieved under closed conditions, the catalytic system was also assessed when operating under open conditions with a vision to obtain a continuous evolution of high purity H2 and tandem flow into a H2/air fuel cell. In 2013, Beller and co-workers found a stable homogeneous catalytic system which produced over 7 L of H2 from CH3OH under reflux over 24 days with a notable catalytic activity (TOFmax > 2500). 28 Since then, the catalysis for aqueous phase methanol dehydrogenation has been well studied under ambient pressures. However, development of these fundamental catalytic systems toward a realistic reformer at scale has remained unexplored at large. 8 Hence, in contrast to the conventionally used setup using glass vessels, condensers and burettes, a more safe, adjustable, easy-to-operate and industrially viable model was proposed. For this, a completely metallic unit was engineered as shown in Figure 6.10, by modifying the head of the original Parr reactor (135 mL) which was then connected to a metallic water condenser followed by a check valve set to 10 psi (to maintain positive flow). The H2 flows and volumes were conveniently monitored using a mass flow meter (MFM). 207 Figure 6.10. Realistic methanol reformer setup. a) design schematic; b) photograph of actual lab- scale prototype with two reactor modifications. At first, the reactor model was validated by heating a 30 mL solution of CH3OH/H2O and KOH in presence of 50 μmol (75 ppm.) C-4 (Figure 6.11a). While the hotplate was set to 100 °C (Tset), the temperature of the reaction mixture was within 90-95 °C (Tint). Simultaneously, a continuous generation of clean H2 was achieved, with a high purity of > 99.9% as verified via GC. Over an initial period of 2 h, more than 2.7 L of H2 was generated with a steady average flow of 22 ml min -1 . Overall, the reformer was run for 12 h which generated a cumulative volume of 11.5 L of H2 with notable catalytic turnovers (TON = 9494 and TOFavg = 1025 h -1 ). At this point, the reaction reached saturation with a negligible flow of < 1 mL min -1 and the Tint dropped to 75 °C. While the reaction extent remained unperturbed, the rates could be improved by moderate increase in the temperature (Tset). Compared to 2.7 L at 100 °C ,a significantly higher volume of ~4.4 L of H2 (flowavg = 37 mL min -1 ) was obtained at 110 °C within 2 h, which was further increased to 7.4 L H2 (flowavg = 62 mL min -1 ) at 120 °C with a notable TOF2h of 3020 h -1 (Figure 6.11b). When the catalyst loading was halved (38 ppm.) in the reaction at 120 °C, ~5 L of H2 was generated in 2h reactor condenser buffer check valve to GC or fuel cell mass flow meter water water a) b) 208 with a TOF2h of 4080. On the other hand, an increase in catalyst loading to 112 ppm. led to a comparable H2 volume (7.35 L, TOF2h = 1940). However, the open reforming system proved to be more dynamic than the closed system and hence, increasing the temperature further to 130-140 °C proved detrimental to the H2 generation, plausibly due to an increased fraction of methanol in the vapor phase. 6.2.4. A realistic methanol reformer at scale. Guided by our understanding of the methanol reforming system thus far, a scaled-up reforming reaction was operated for continuous generation of H2 over an extended period. This is key to establish the long-term stability and activity of the reforming catalyst as well as to demonstrate a steady integrated reformed methanol fuel cell (I-RMFC). For this, a 600 mL reactor was charged with 300 mL CH3OH, 40 mL H2O and 150 g KOH in the presence of 50 μmol C-4 (Figure 6.11c). When heated to Tset = 110 °C, the reforming proceeded with a very high rate of reaction (TOFmax > 5400)with an initial flow of up to 120 ml min -1 . For over 20 h, the H2 flow continued to be steady (> 40 mL min -1 ) as the Tset was gradually increased to 125 °C. A total H2 volume of 110 L was generated after 30 h. As the reactants were consumed, 100 mL CH3OH, 10 mL H2O and 50 g KOH was freshly introduced in the remaining solution containing the catalyst and the reaction was run for additional 20 h. Overall, the scaled-up reaction produced ~140 L of H2 and a remarkably stable catalytic performance resulted in one of the highest ever cumulative TON of > 115,000 and TOFavg of 2300 over 50 h for homogeneous methanol reforming. 6.2.5. Emission-free integrated reformed methanol fuel cell (I-RMFC). H2 fuel cells have been recognized as one of the most promising technologies in the overall development of clean energy and sustainable ecosystems. 46 In this regard, RMFCs have gained 209 much attention due to their high efficiencies and overall operating flexibilities. 47 Conventional RMFC units often include a steam reformer of methanol at high temperatures, and a H2 purifier preceding the fuel cell setup. With our goal of developing a novel low-temperature and emission- free RMFC, a PEM fuel cell unit was assembled and placed directly at the outlet of the methanol reformer. To demonstrate the tandem generation of H2 and electrical power, the open reforming system was chosen. As the proof-of-concept trial, the H2 generated from a small-scale (30 mL) reforming reaction was flowed at without any purification step first through a humidifier and then directly through the anode side of the fuel cell while the cathode side of the fuel cell was fed with either humified air or O2. The fuel cell performance was tested via polarization curve measurements (Figure 6.11d) using the H2 obtained by methanol reforming and was found analogous to the one with the benchmark ultra-high-purity commercial H2 as the anode feed. The H2/air fuel cell was also tested at a constant current (I = 2.5 A) and a stable cell voltage (0.55 V) was observed over 1 h. Following this, the I-RMFC was tested for its durability over extended duration. The scaled-up reforming unit (with 300 mL CH3OH) established earlier in this study with a steady H2 flow was highly suited to be coupled to the H2/air fuel cell. Hence in a separate extended run, the I-RMFC was operated continuously for over 27 h and a stable average voltage of 0.54 V was maintained (at constant I = 2.5 A) which was also very close to the values obtained in a run with commercial H2 (Figure 6.11e). During this period, ~65 L H2 (produced from methanol reforming) was consumed in the fuel cell, demonstrating efficient and continuous conversion of chemical energy to electricity. In addition to validating the robustness of the I-RMFC over an extended period, the consistent cell output also indicates that the cell continued to be mostly unaffected by any trace CO, if any, in the stream. 210 Figure 6.11. Continuous hydrogen generation at ambient pressures. a) methanol reforming at 100 °C (Tset). reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), Tset = 100 °C, t = 12 h; b) effect of reaction temperature (Tset). reaction conditions: CH3OH/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), t = 12 h; c) methanol reforming for extended period (50 h) reaction conditions: CH3OH(400 mL), H2O (50 mL), KOH (200 g), C-4 (50 μmol), stirring (800 rpm), Tset = 110-125 °C, t = 50 h; d) polarization curves with reformed methanol and commercial H2 e) constant current hold of 2.5 A with reformed methanol and commercial H2, Fuel cell parameters: membrane (Nafion 211), electrode (4 cm 2 ), anode (Pt on teflonized carbon paper, 0.5 mg/cm 2 ), Cathode (Pt on teflonized carbon paper with microporous layer, 0.5 mg/cm 2 ), gases humidified at 100 % RH, H2 flow rate (40 ml/min), air flow rate (400 mL/min). Flow and volume measured with calibrated mass flow meter. Calculation error ±5%. 0 1 2 3 4 5 6 7 8 0 0.5 1 1.5 2 H 2 volume (L) time (h) 100 °C 110 °C 120 °C 130 °C 140 °C 0 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 H 2 flow (mL min -1 ) H 2 volume (L) time (h) 0 0.2 0.4 0.6 0.8 1 0 5 10 15 20 25 Voltage (V) time (h) Reformed H₂ H₂ cylinder Series3 0 0.1 0.2 0.3 0.4 0.5 0 0.2 0.4 0.6 0.8 1 0 0.5 1 1.5 2 Power Density (W cm -2 ) Voltage (V) Current Density (A cm -2 ) H₂ cylinder Reform H₂ 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 160 0 10 20 30 40 50 H 2 fow (mL min -1 ) H 2 volume (mL) time (h) 0 1000 2000 3000 4000 5000 6000 0 20000 40000 60000 80000 100000 120000 140000 0 10 20 30 40 50 TOF (h -1 ) TON time (h) a) c) d) b) e) 211 6.2.6. Recharging the hydrogen battery. Once the H2 is generated, the spent or “H2-lean” species can potentially be converted (or hydrogenated) back to the “H2-rich” fuel, resulting in a rechargeable hydrogen storage media or an “H2 battery”. Such reversible systems are quite rare in literature, especially for methanol. 8, 13, 18, 30-31, 48 Methanol based reversible H2 storage was demonstrated by our group and later by Liu et al.; however, only when using amine as the base. To date, demonstrating a reversible alkaline methanol reforming system continues to be a highly challenging frontier. This is primarily due to the relative inertness of carbonate/formate salts toward catalytic hydrogenation and the thermodynamically uphill conversion to methanol. Recently, we developed a protocol where compounds analogous to metal carbonates were converted to methanol efficiently using ethylene glycol as the mediator and catalyst C-2. 39 Hence, within the limited scope of this study, we briefly investigated the possibility of hydrogenating the C-1 residues (primarily containing potassium formate and some potassium carbonate) obtained after the methanol reforming reaction is completed. A small fraction (1 g, carbon content ~11 mmol) of the dried residue was dissolved in ethylene glycol and subjected to hydrogenation with 70 bar of H2 and 50 μmol of C-2 (Table 6.2). After a reaction at 120 °C over 60 h, 10.1 mmol CH3OH was produced, substantiating that the present system is potentially rechargeable. Table 6.2. Attempted hydrogenation of the C-1 residues obtained from methanol reforming Entry a T (°C) t (h) HCOO - (mmol) b CH3OH (mmol) b 1 100 72 2.4 7.0 2 120 60 0.4 10.0 a Reaction conditions: reactant sample (1 g), C-2 (50 µmol), stirring (800 rpm), ethylene glycol (20 mL), H2 (70 bar). b yields determined via 1H NMR using an internal standard. Calculation error ±5%. 212 Figure 6.12. Representative 1 H NMR spectra of reaction mixture after hydrogenation of C-1 residues of methanol reforming in D2O. 6.2.7. Comparing reforming of methanol with other alcohols To validate the selection of methanol over other alcohols for reforming and H2 generation, the catalytic dehydrogenation of ethylene glycol and ethanol was performed and compared to that of methanol. For this, equal volume (27 mL) of the alcohol, 3 mL water and 240 mmol of KOH were activated in presence of C-4 (50 μmol). The generation of H2 under pressure was plotted as shown in Figure 6.13. It is quite evident that the rate of H2 pressures obtained with ethanol and ethylene glycol are comparable but significantly lower than that with methanol, even after including a correction factor to compare in equimolar terms. - 2 - 1 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 f 1 ( p p m ) 7 . 1 1 1 . 0 0 0 . 5 1 0 . 8 3 213 Figure 6.13. Catalytic dehydrogenation of different alcohols for H2 generation. reaction conditions: alcohol/H2O (30 mL, 9:1), KOH (240 mmol), C-4 (75 ppm.), stirring (800 rpm), T = 140 °C, t = 20 h. For equimolar, a correction factor was incorporated to assume equal moles of ethylene glycol and ethanol as compared to methanol (667 mmol). 6.3. Conclusion In conclusion, a realistic, highly efficient and low-temperature methanol reformer has been developed for rapid and clean H2 generation, based on the fundamental concept of catalytic methanol dehydrogenation. The reaction mixture is comprised of the key reactants: methanol and alkali hydroxide. In a novel closed reforming system, even a small-scale reaction generated high pressures of H2 up to 150 bar on-demand, which is in the range of most commercial H2 tanks and cylinders. Notably, almost 80% of the H2 pressure is reached within the first two hours of the reaction, exhibiting remarkable reaction rates, especially under thermodynamically unfavorable closed conditions. Furthermore, higher pressure requirements were achieved with minor modifications to the operating parameters and scale, which translated into a rare realization of ultra-high pressure H2 of more than 450 bar. These high pressures/volumes of H2 were generated from a working reaction volume which was as low as 2% of those, emphasizing on the major advantages of this methanol-based system as a “chemical compressor” of H2 in terms of convenience, safety as well as cost-effectiveness. 0 200 400 600 800 1000 1200 1400 1600 0 5 10 15 20 H 2 (psi) time (h) Equivolume ethanol ethylene glycol methanol 0 200 400 600 800 1000 1200 1400 1600 0 5 10 15 20 H 2 (psi) time (h) Equimolar (adjusted) ethanol ethylene glycol methanol 214 Parallelly, the low temperature methanol reformer can be operated in open conditions near atmospheric pressure. Rapid evolution of clean H2 with flow rates of 50-120 mL min -1 and a net volume of >11 L was observed with no traces of CO2 and CO as contaminants. The reformer was readily scaled-up to continuously produce clean H2 from 400 mL CH3OH and 200 g KOH with notably stable and high catalytic activities from a 5 ppm. catalyst loading. Hence, a cumulative H2 volume of 140 L, TON of 1.2 x 10 5 and a maximum TOF of >5400 h -1 were achieved, which was, to our knowledge, the highest TOF to date for homogeneous reforming at scale. These promising results prompted the first demonstration of a low temperature and emission-free “integrated RMFC” which showed continuous and stable performance (V = 0.55 V at A= 2.5 A) for over 27 h when the H2 from the reformer was directly fed to the fuel cell without any additional purification. Finally, the potential of the present methanol system as a reversible H2 carrier was validated by successfully converting back the C-1 residues from the reforming step to methanol by “recharging” with H2. Present research is geared towards developing more cost-effective and heterogenized catalysts as well as an ideal “one-pot H2 battery” as long-term prospect. We believe that the present system satisfies the key parameters of an ideal and working H2 carrier. In addition, it provides the fundamental steps to a functional and industrially viable “on-board” H2 generator which could stimulate the replacement of current hazardous H2 storage and transportation methods as well as the demand for safer and greener hydrogen-fueled vehicles. Moreover, the synergy of the present system with renewable and sustainable fuel technologies can also help in the long run, in reducing humankind’s carbon footprint across the globe. 215 6.4. Experimental Methods 6.4.1. Materials and methods All experiments were carried out under an inert atmosphere (with N2 or Ar) using standard Schlenk techniques with the exclusion of moisture unless otherwise stated. Complexes Ru-Macho (C-1, Strem Chemicals, 98%), Ru-Macho-BH (C-2, Strem Chemicals, 98%), RuHClPNP iPr (CO) (C-4, Strem Chemicals, 98%), RuHClPNP tBu (CO) (C-5, Strem Chemicals, 98%) and RuHClPNP Cy (CO) (C-5, Strem Chemicals, 98%) were used as received without further purification. RuHClPNMeP Ph (CO) (C-3) was prepared following a reported procedure. 49 All catalysts were weighed inside an argon filled glove box. Lithium hydroxide, sodium hydroxide, potassium hydroxide and calcium hydroxide were purchased from commercial sources and used without further purification. Methanol (DriSolv) and DI water were sparged with N2 for 1 h prior to use. 1 H and 13 C NMR spectra were recorded on 400 or 500 MHz, Varian NMR spectrometers. 1 H and 13 C NMR chemical shifts were determined relative to the residual solvent signals. H2 (Gilmore, ultra-high pure grade 5.0) was used without further purification. Caution: Reactions are associated with H2 gas including its generation under ultra-high pressures. They should be carefully handled inside proper fume hoods without any flame, spark or static electricity sources nearby. The pressure in the reactor should be continuously monitored and controlled well within the pressure limit of the reactor. 6.4.2. Standard procedure for methanol reforming in closed conditions In a nitrogen-filled chamber, the pre-weighed amount of base, catalyst, methanol and water were added to the Monel Parr reactor equipped with a magnetic stir bar, thermocouple and piezoelectric pressure transducer. The reaction mixture was then placed in a preheated aluminum 216 block and was stirred at 800 rpm. The internal temperature of the reaction mixture and pressure generated in the reactor were monitored through the LabVIEW software. After heating for a given amount of time, the reactor was cooled down to room temperature. The gas generated was partly collected in an airtight bag and was analyzed by GC whereas the remaining gas was slowly released. 6.4.3. Standard procedure for methanol reforming at ambient pressure The methanol reformer was assembled as depicted in Figure 6.10. In a nitrogen-filled chamber, the pre-weighed amount of base, catalyst, methanol and water were mixed and added to the Monel Parr reactor equipped with a magnetic stir bar. The reactor was then placed in a aluminum block and was stirred at 800 rpm. The reactor was attached to the water condenser and the acoompanying assembly. The hotplate was heated to the desired temperature (Tset) and the internal temperature (Tint) was monitored using a thermocouple inserted in the thermowell of the reactor. The pressure release valve was adjusted to 10 psi. The evolution of the H2 gas was monitored using a calibrated mass flow meter (MFM) connected to the LabVIEW software. A fraction of the evolved gas was analyzed by GC at regular intervals. 6.4.4. Membrane electrode assembly (MEA) fabrication Catalyst inks were made from 12 mg of Pt black, 3 mg of Vulcan XC72R, 750 mg of Millipore water, 75 mg of 5 wt% Nafion solution and 75 mg of isopropyl alcohol. The ink was sonicated and then painted onto a 4 cm 2 Toray Carbon Paper electrode (E-TEK, TGH-060, 10% wet proofing) for the anode and 4 cm 2 Toray Carbon Paper with a microporous layer (Sigracet 22BB) for the cathode. The catalyst inks were applied until a platinum loading of 0.5 mg/cm 2 was obtained. The electrodes were then dried in an oven at 110 ℃. The membrane electrode assembly 217 was prepared with Nafion-211 membrane and sandwiched between the cathode and anode and hot pressed at 140 ℃ for 5 min at 500 lbs. 6.4.5. Reformed Methanol Fuel Cell (RMFC) Measurements The fuel cell polarization measurements were performed using a Fuel Cell Test System 890B (Schribner Associated). Prior to experiments the MEA was subject to a break in procedure similar to previous report, where several constant voltage experiments were conducted at 80 ℃ with oxygen and at 40 ℃ with air. The MEAs were then tested at 60 ℃ after the break-in procedure. Humidified hydrogen (from cylinder or the coupled methanol reformer) was flowed through the anode compartment at 40 mL/min at 85 ℃ while humidified oxygen or air was flowed through the cathode compartment at 100 or 400 mL/min, respectively, at 85 ℃ with relative humidity of 100%. Figure 6.14. Image of the actual reformer-fuel cell setup (I-RMFC) used in this study. 218 6.5. References [1] Holechek, J. L.; Geli, H. M. E.; Sawalhah, M. N.; Valdez, R., A Global Assessment: Can Renewable Energy Replace Fossil Fuels by 2050? Sustainability 2022, 14, 4792. [2] Nolan, C.; Overpeck, J. T.; Allen, J. R. M.; Anderson, P. M.; Betancourt, J. L.; Binney, H. A.; Brewer, S.; Bush, M. B.; Chase, B. M.; Cheddadi, R.; Djamali, M.; Dodson, J.; Edwards, M. E.; Gosling, W. D.; Haberle, S.; Hotchkiss, S. C.; Huntley, B.; Ivory, S. J.; Kershaw, A. P.; Kim, S. H.; Latorre, C.; Leydet, M.; Lezine, A. M.; Liu, K. B.; Liu, Y.; Lozhkin, A. V.; McGlone, M. S.; Marchant, R. A.; Momohara, A.; Moreno, P. I.; Muller, S.; Otto-Bliesner, B. L.; Shen, C.; Stevenson, J.; Takahara, H.; Tarasov, P. E.; Tipton, J.; Vincens, A.; Weng, C.; Xu, Q.; Zheng, Z.; Jackson, S. T., Past and future global transformation of terrestrial ecosystems under climate change. Science 2018, 361, 920-923. [3] Hoekstra, A. Y.; Wiedmann, T. O., Humanity’s unsustainable environmental footprint. Science 2014, 344, 1114-1117. [4] Obama, B., The irreversible momentum of clean energy. Science 2017, 355, 126-129. [5] Meys, R.; Kätelhön, A.; Bachmann, M.; Winter, B.; Zibunas, C.; Suh, S.; Bardow, A., Achieving net-zero greenhouse gas emission plastics by a circular carbon economy. Science 2021, 374, 71-76. [6] Keijer, T.; Bakker, V.; Slootweg, J. C., Circular chemistry to enable a circular economy. Nat. Chem. 2019, 11, 190-195. [7] Taboada, S.; Clark, L.; Lindberg, J.; Tonjes, D. J.; Mahajan, D., Quantifying the Potential of Renewable Natural Gas to Support a Reformed Energy Landscape: Estimates for New York State. Energies 2021, 14, 3834. [8] Kumar, A.; Daw, P.; Milstein, D., Homogeneous Catalysis for Sustainable Energy: Hydrogen and Methanol Economies, Fuels from Biomass, and Related Topics. Chem. Rev. 2022, 122, 385- 441. [9] Zhu, B.; Wei, C., A Green Hydrogen Era: Hope or Hype? Environ. Sci. Technol. 2022, 56, 11107-11110. [10] Moradi, R.; Groth, K. M., Hydrogen storage and delivery: Review of the state of the art technologies and risk and reliability analysis. Int. J. Hydrog. Energy 2019, 44, 12254-12269. [11] Yadav, V.; Sivakumar, G.; Gupta, V.; Balaraman, E., Recent Advances in Liquid Organic Hydrogen Carriers: An Alcohol-Based Hydrogen Economy. ACS Catal. 2021, 11, 14712-14726. 219 [12] Preuster, P.; Papp, C.; Wasserscheid, P., Liquid Organic Hydrogen Carriers (LOHCs): Toward a Hydrogen-free Hydrogen Economy. Acc. Chem. Res. 2016, 50, 74-85. [13] Bernskoetter, W. H.; Hazari, N., Reversible Hydrogenation of Carbon Dioxide to Formic Acid and Methanol: Lewis Acid Enhancement of Base Metal Catalysts. Acc. Chem. Res. 2017, 50, 1049-1058. [14] Kar, S.; Goeppert, A.; Prakash, G. K. S., Integrated CO2 Capture and Conversion to Formate and Methanol: Connecting Two Threads. Acc. Chem. Res. 2019, 52, 2892-2903. [15] Sen, R.; Goeppert, A.; Prakash, G. K. S., Homogeneous Hydrogenation of CO2 and CO to Methanol: The Renaissance of Low‐Temperature Catalysis in the Context of the Methanol Economy. Angew. Chem. Int. Ed. 2022, 61, e202207278. [16] Sen, R.; Goeppert, A.; Prakash, G. K. S., Carbon Dioxide Capture and Recycling to Methanol: Building a Carbon–Neutral Methanol Economy. Aldrichimica Acta 2020, 53, 39-56. [17] Eppinger, J.; Huang, K.-W., Formic Acid as a Hydrogen Energy Carrier. ACS Energy Lett. 2017, 2, 188-195. [18] Wei, D.; Sang, R.; Sponholz, P.; Junge, H.; Beller, M., Reversible hydrogenation of carbon dioxide to formic acid using a Mn-pincer complex in the presence of lysine. Nat. Energy 2022, 7, 438-447. [19] Kar, S.; Rauch, M.; Leitus, G.; Ben-David, Y.; Milstein, D., Highly efficient additive-free dehydrogenation of neat formic acid. Nat. Catalysis 2021, 4, 193-201. [20] Czaun, M.; Goeppert, A.; Kothandaraman, J.; May, R. B.; Haiges, R.; Prakash, G. K. S.; Olah, G. A., Formic Acid As a Hydrogen Storage Medium: Ruthenium-Catalyzed Generation of Hydrogen from Formic Acid in Emulsions. ACS Catal. 2014, 4, 311-320. [21] Czaun, M.; Kothandaraman, J.; Goeppert, A.; Yang, B.; Greenberg, S.; May, R. B.; Olah, G. A.; Prakash, G. K. S., Iridium-Catalyzed Continuous Hydrogen Generation from Formic Acid and Its Subsequent Utilization in a Fuel Cell: Toward a Carbon Neutral Chemical Energy Storage. ACS Catal. 2016, 6, 7475-7484. [22] Czaun, M.; Goeppert, A.; May, R.; Haiges, R.; Prakash, G. K. S.; Olah, G. A., Hydrogen Generation from Formic Acid Decomposition by Ruthenium Carbonyl Complexes. Tetraruthenium Dodecacarbonyl Tetrahydride as an Active Intermediate. ChemSusChem 2011, 4, 1241-1248. [23] Palo, D. R.; Dagle, R. A.; Holladay, J. D., Methanol Steam Reforming for Hydrogen Production. Chem. Rev. 2007, 107, 3992-4021. 220 [24] Dalena, F.; Senatore, A.; Marino, A.; Gordano, A.; Basile, M.; Basile, A., Methanol Production and Applications: An Overview. In Methanol, 2018; pp 3-28. [25] Olah, G. A.; Goeppert, A.; Prakash, G. K. S.; Wiley, V. C. H., Beyond Oil and Gas: The Methanol Economy. 3rd ed.; 2018. [26] Sun, Z.; Sun, Z. Q., Hydrogen generation from methanol reforming for fuel cell applications: A review. J. Cent. South Univ. 2020, 27, 1074-1103. [27] Rodríguez-Lugo, R. E.; Trincado, M.; Vogt, M.; Tewes, F.; Santiso-Quinones, G.; Grützmacher, H., A homogeneous transition metal complex for clean hydrogen production from methanol–water mixtures. Nat. Chem. 2013, 5, 342-347. [28] Nielsen, M.; Alberico, E.; Baumann, W.; Drexler, H.-J.; Junge, H.; Gladiali, S.; Beller, M., Low-temperature aqueous-phase methanol dehydrogenation to hydrogen and carbon dioxide. Nature 2013, 495, 85-89. [29] Kothandaraman, J.; Kar, S.; Goeppert, A.; Sen, R.; Prakash, G. K. S., Advances in Homogeneous Catalysis for Low Temperature Methanol Reforming in the Context of the Methanol Economy. Top. Catal. 2018, 61, 542-559. [30] Kothandaraman, J.; Kar, S.; Sen, R.; Goeppert, A.; Olah, G. A.; Prakash, G. K. S., Efficient Reversible Hydrogen Carrier System Based on Amine Reforming of Methanol. J. Am. Chem. Soc. 2017, 139, 2549-2552. [31] Shao, Z.; Li, Y.; Liu, C.; Ai, W.; Luo, S.-P.; Liu, Q., Reversible interconversion between methanol-diamine and diamide for hydrogen storage based on manganese catalyzed (de)hydrogenation. Nat. Commun. 2020, 11, 591. [32] Simon Araya, S.; Liso, V.; Cui, X.; Li, N.; Zhu, J.; Sahlin, S. L.; Jensen, S. H.; Nielsen, M. P.; Kær, S. K., A Review of The Methanol Economy: The Fuel Cell Route. Energies 2020, 13, 596. [33] Cropley, C. C.; Norman, T. J. Low-Cost High-Pressure Hydrogen Generator; Giner Electrochemical Systems, LLC, Newton, MA: 2008. [34] Fellay, C.; Yan, N.; Dyson, P. J.; Laurenczy, G., Selective Formic Acid Decomposition for High-Pressure Hydrogen Generation: A Mechanistic Study. Eur. J. Chem. 2009, 15, 3752-3760. [35] Zhong, H.; Iguchi, M.; Song, F.-Z.; Chatterjee, M.; Ishizaka, T.; Nagao, I.; Xu, Q.; Kawanami, H., Automatic high-pressure hydrogen generation from formic acid in the presence of nano-Pd heterogeneous catalysts at mild temperatures. Sustain. Energy Fuels 2017, 1, 1049-1055. 221 [36] Iguchi, M.; Himeda, Y.; Manaka, Y.; Matsuoka, K.; Kawanami, H., Simple Continuous High- Pressure Hydrogen Production and Separation System from Formic Acid under Mild Temperatures. ChemCatChem 2016, 8, 874-874. [37] Li, M.; Bai, Y.; Zhang, C.; Song, Y.; Jiang, S.; Grouset, D.; Zhang, M., Review on the research of hydrogen storage system fast refueling in fuel cell vehicle. Int. J. of Hydrog. Energy 2019, 44, 10677-10693. [38] Wen, C.; Rogie, B.; Kærn, M. R.; Rothuizen, E., A first study of the potential of integrating an ejector in hydrogen fuelling stations for fuelling high pressure hydrogen vehicles. Appl. Energy 2020, 260, 113958. [39] Sen, R.; Goeppert, A.; Kar, S.; Prakash, G. K. S., Hydroxide Based Integrated CO2 Capture from Air and Conversion to Methanol. J. Am. Chem. Soc. 2020, 142, 4544-4549. [40] Sen, R.; Koch, C. J.; Goeppert, A.; Prakash, G. K. S., Tertiary Amine-Ethylene Glycol Based Tandem CO2 Capture and Hydrogenation to Methanol: Direct Utilization of Post-Combustion CO2. ChemSusChem 2020, 13, 6318-6322. [41] Sen, R.; Koch, C. J.; Galvan, V.; Entesari, N.; Goeppert, A.; Prakash, G. K. S., Glycol assisted efficient conversion of CO2 captured from air to methanol with a heterogeneous Cu/ZnO/Al2O3 catalyst. J. CO2 Util. 2021, 54, 101762. [42] Piccirilli, L.; Lobo Justo Pinheiro, D.; Nielsen, M., Recent Progress with Pincer Transition Metal Catalysts for Sustainability. Catalysts 2020, 10, 773. [43] Alberico, E.; Nielsen, M., Towards a methanol economy based on homogeneous catalysis: methanol to H2 and CO2 to methanol. Chem. Commun. 2015, 51, 6714-6725. [44] Crabtree, R. H., Homogeneous Transition Metal Catalysis of Acceptorless Dehydrogenative Alcohol Oxidation: Applications in Hydrogen Storage and to Heterocycle Synthesis. Chem. Rev. 2017, 117, 9228-9246. [45] Gunanathan, C.; Milstein, D., Bond Activation and Catalysis by Ruthenium Pincer Complexes. Chem. Rev. 2014, 114, 12024-12087. (46) Staffell, I.; Scamman, D.; Velazquez Abad, A.; Balcombe, P.; Dodds, P. E.; Ekins, P.; Shah, N.; Ward, K. R., The role of hydrogen and fuel cells in the global energy system. Energy Environ. Sci. 2019, 12, 463-491. [47] Eggert, A. R.; Friedman, D.; Badrinarayanan, P.; Ramaswamy, S.; Heinz-Hauer, K. In Characteristics of an indirect-methanol fuel cell system, Collection of Technical Papers. 35th 222 Intersociety Energy Conversion Engineering Conference and Exhibit (IECEC) (Cat. No.00CH37022), 24-28 July 2000; 2000; pp 1326-1332 vol.1322. [48] Xie, Y.; Hu, P.; Ben-David, Y.; Milstein, D., A Reversible Liquid Organic Hydrogen Carrier System Based on Methanol-Ethylenediamine and Ethylene Urea. Angew. Chem. Int. Ed. 2019, 58, 5105-5109. [49] Han, Z.; Rong, L.; Wu, J.; Zhang, L.; Wang, Z.; Ding, K., Catalytic Hydrogenation of Cyclic Carbonates: A Practical Approach from CO2 and Epoxides to Methanol and Diols. Angew. Chem. Int. Ed. 2012, 51, 13041-13045.

Abstract (if available)

Abstract With the objective of achieving a net-zero and sustainable economy, recent years have witnessed the emergence of carbon dioxide capture and utilization models (CCU), wherein waste carbon dioxide can be upcycled as a C-1 building block to value-added downstream chemicals and materials, including methanol, which has a fast-growing market of >100 Mt/yr. Within this framework, this dissertation thesis focusses on the integration of the two steps in the CCU process: a) carbon dioxide capture, including direct air capture (DAC) and b) hydrogenation of carbon dioxide to methanol, into a convergent process. Carbon dioxide captured directly from air using an amine or metal hydroxide solution was efficiently converted to methanol (air-to-fuel) in presence of homogeneous and heterogeneous catalysts. This further led to the discovery of a fundamental chemical reactivity: direct hydrogenation of metal carbonates to methanol. Subsequently, the energy stored in methanol was utilized in the form of hydrogen via its catalytic reforming. A realistic alkaline methanol reformer was developed, which can generate hydrogen rapidly at ultra-high pressures at moderate temperatures. The produced hydrogen is clean and emission-free (no carbon monoxide and carbon dioxide are emitted). Overall, the low-carbon and energy-efficient processes discussed in this thesis synergize well with emerging renewable energy and low-carbon technologies.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

Integrated capture and conversion of carbon dioxide from air into methanol and other C1 products

PDF

Reforming of green-house gases: a step towards the sustainable methanol economy; and, One-step deoxygenative fluorination and trifluoromethylthiolation of carboxylic acids

PDF

Carbon dioxide capture using silica supported organoamine adsorbents

PDF

Design of durable and efficient catalysts for the electro-oxidation of methanol

PDF

Studies on direct oxidation formic acid fuel cells: advantages, limitations and potential

PDF

Design and modification of electrocatalysts for use in fuel cells and CO₂ reduction

PDF

Methanol synthesis in a membrane reactor

PDF

Novel methods for functional group interconversions in organic synthesis and structural characterization of new transition metal heterogeneous catalysts for potential carbon neutral hydrogen storage

PDF

New bifunctional catalysts for ammonia-borane dehydrogenation, nitrile reduction, formic acid dehydrogenation, lactic acid synthesis, and carbon dioxide reduction

PDF

Hydrogen energy system production and storage via iridium-based catalysts

PDF

Studies on direct methanol, formic acid and related fuel cells in conjunction with the electrochemical reduction of carbon dioxide

PDF

Electrochemical pathways for sustainable energy storage and energy conversion

PDF

PSSA-PVDF semi-IPN blends for direct methanol fuel cells

PDF

Carbon-hydrogen bond activation: radical methane functionalization; unactivated alkene coupling; saccharide degradation; and carbon dioxide hydrogenation

PDF

Development of new bifunctional iridium complexes for hydrogenation and dehydrogenation reactions

PDF

Singlet halofluorocarbenes: Modes of generation and their reactions with alkenes and various heteroatom nucleophiles

PDF

Methanol synthesis in the membrane reactor

PDF

Methanol synthesis in a contactor-type membrane reactor

PDF

Understanding the mechanism of oxygen reduction and oxygen evolution on transition metal oxide electrocatalysts and applications in iron-air rechargeable battery

PDF

Sustainable metal catalysis for organic and organofluorine small molecule library generation: a diversity oriented synthesis approach

Asset Metadata

Creator Sen, Raktim (author)

Core Title Integrated carbon dioxide capture and utilization: catalysis enabled carbon-neutral methanol synthesis and hydrogen generation

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Degree Conferral Date 2023-05

Publication Date 04/19/2024

Defense Date 03/27/2023

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag carbon capture and utilization,catalysis,catalytic reforming,direct air capture,hydrogen,methanol economy,OAI-PMH Harvest

Format theses (aat)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Prakash, Surya G.K. (committee chair), Narayan, Sri R. (committee member), Puri, Ishwar K. (committee member)

Creator Email raktimse@usc.edu,raktimsen101@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC113057153

Unique identifier UC113057153

Identifier etd-SenRaktim-11668.pdf (filename)

Legacy Identifier etd-SenRaktim-11668

Document Type Dissertation

Format theses (aat)

Rights Sen, Raktim

Internet Media Type application/pdf

Type texts

Source 20230421-usctheses-batch-1027 (batch), University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the author, as the original true and official version of the work, but does not grant the reader permission to use the work if the desired use is covered by copyright. It is the author, as rights holder, who must provide use permission if such use is covered by copyright. The original signature page accompanying the original submission of the work to the USC Libraries is retained by the USC Libraries and a copy of it may be obtained by authorized requesters contacting the repository e-mail address given.

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA

Repository Email cisadmin@lib.usc.edu