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Integrated capture and conversion of carbon dioxide from air into methanol and other C1 products
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Content
Integrated Capture and Conversion of Carbon Dioxide from
Air into Methanol and Other C1 Products
by
Sayan Kar
A dissertation submitted in partial fulfillment
of the requirements for the degree of
Doctor in Philosophy
(Chemistry)
in the University of Southern California
August 2019
FACULTY OF THE USC GRADUATE SCHOOL IN DOCTORAL COMMITTEE
Prof. G. K. Surya Prakash
Prof. S. R. Narayan
Prof. Kyung Jung
Dr. Ralf Haiges
Prof. Katherine Shing
© Sayan Kar 2019
To my Grandmother
1
TABLE OF CONTENTS
ACKNOWLEDGEMENT .............................................................................................................. 3
ABSTRACT .................................................................................................................................... 5
CHAPTER 1: INTRODUCTION
1.1. CARBON RECYCLING IN EARLY TWENTY-FIRST CENTURY ..................................... 9
1.2. METHANOL— A PERFECT SUBSTITUTE OF NATURAL OIL ..................................... 10
1.3. METHANOL ECONOMY— CARBON NEUTRAL CYCLE .............................................. 11
1.4. HETEROGENEOUS CATALYSIS FOR CO 2 TO METHANOL ......................................... 12
1.5. HOMOGENEOUS CATALYSIS— AN ALTERNATIVE ................................................... 13
1.6. REFERENCES ....................................................................................................................... 18
CHAPTER 2: MECHANISTIC INSIGHTS INTO RUTHENIUM-PINCER-CATALYZED
AMINE-ASSISTED HOMOGENEOUS HYDROGENATION OF CO2 TO METHANOL ...... 23
2.1. INTRODUCTION: AMINE-ASSISTED CO 2 TO METHANOL .......................................... 23
2.2. EFFECT OF THE CATALYST’S MOLECULAR STRUCTURE ........................................ 25
2.3. EFFECT OF THE AMINE’S MOLECULAR STRUCTURE................................................ 28
2.4. MECHANISTIC INVESTIGATIONS ................................................................................... 30
2.5. PROPOSED MECHANISM ................................................................................................... 40
2.6. CONCLUSION ....................................................................................................................... 42
2.7. EXPERIMENTAL PROCEDURES ....................................................................................... 42
2. 8. REFERENCES ...................................................................................................................... 48
CHAPTER 3: INTEGRATIVE CO2 CAPTURE AND HYDROGENATION TO METHANOL
WITH REUSABLE CATALYST AND AMINE ......................................................................... 51
3.1. INTRODUCTION: NEED FOR CATALYST AND AMINE RECYLING .......................... 51
3.2. RECYCLING SCHEME ........................................................................................................ 52
3.3. CO 2 CAPTURE BY VARIOUS AMINES ............................................................................. 53
3.4. HYDROGENATION OF CAPTURED CO 2 TO METHANOL ............................................ 56
3.5. RECYCLING OF CATALYST AND AMINE ...................................................................... 60
3.6. CONCLUSION ....................................................................................................................... 62
3.7. EXPERIMENTAL PROCEDURES ....................................................................................... 62
3.8. REFERENCES ....................................................................................................................... 67
CHAPTER 4: COMBINED CO2 CAPTURE AND HYDROGENATION TO METHANOL:
AMINE IMMOBILIZATION ENABLES EASY RECYCLING OF ACTIVE ELEMENTS .... 70
4.1. INTRODUCTION: AMINE-ASSISTED CO 2 TO METHANOL .......................................... 70
2
4.2. RECYCLING SCHEME BASED ON AMINE IMMOBILIZATION ................................... 71
4.3. FINDING THE OPTIMAL SOLID SUPPORTED AMINE .................................................. 72
4.4. SCREENING OF DIFFERENT CATALYSTS ..................................................................... 78
4.5. COMBINED CAPTURE AND HYDROGENATION WITH CATALYST AND AMINE
RECYCLING ................................................................................................................................. 80
4.6. CONCLUSION ....................................................................................................................... 83
4.7. EXPERIMENTAL PROCEDURES ....................................................................................... 83
4.8. REFERENCES ....................................................................................................................... 89
CHAPTER 5: MANGANESE-CATALYZED SEQUENTIAL HYDROGENATION OF CO2
TO METHANOL VIA FORMAMIDE ........................................................................................ 94
5.1. INTRODUCTION: CO 2 TO METHANOL ............................................................................ 94
5.2. FIRST ROW TRANSITION METAL CATALYSIS ............................................................. 95
5.3. OPTIMIZATION OF FORMYLATION REACTION ........................................................... 96
5.4. FORMYLATION OF DIFFERENT AMINES ....................................................................... 98
5.5. SEQUENTIAL HYDROGENATION TO METHANOL ...................................................... 99
5.6. MECHANISTIC STUDIES .................................................................................................. 100
5.7. ATTEMPTS AT DIRECT ONE-POT CO 2 TO METHANOL ............................................. 103
5.8. SCALE UP REACTION ....................................................................................................... 103
5.9. CONCLUSION ..................................................................................................................... 104
5.10. EXPERIMENTAL PROCEDURES ................................................................................... 104
5.11. REFERENCES ................................................................................................................... 116
CHAPTER 6: INTEGRATIVE CO2 CAPTURE AND HYDROGENATION TO FORMATE
WITH LOW TEMPERATURE REGENERATION OF SODIUM HYDROXIDE .................. 121
6.1. INTRODUCTION: CO 2 CAPTURE WITH HYDROXIDES AND UTILIZATION .......... 121
6.2. COMPARISON BETWEEN INTEGRATED SYSTEMS WITH AMINES AND
HYDROXIDE BASES ................................................................................................................ 122
6.3. CO 2 CAPTURE WITH DIFFERENT HYDROXIDE BASES ............................................ 123
6.4. SUBSEQUENT HYDROGENATION OF CAPTURED CO 2 TO FORMATE .................. 124
6.5. RECYCLING OF THE HYDROGENATION CATALYST IN BIPHASIC SYSTEM ...... 128
6.6. REGENERATION OF THE HYDROXIDE BASE IN FUEL CELL .................................. 129
6.7. CATALYTIC RESTING STATE AND CO 2 FROM AIR TO FORMATE ......................... 134
6.8. CONCLUSION ..................................................................................................................... 136
6.9. EXPERIMENTAL PROCEDURES ..................................................................................... 137
6.10. REFERENCS 146
3
Acknowledgement
The list of people without whose contributions this dissertation would not be complete is
far larger than what would be described in the following two pages. Part of is due to space
constraint, but the major reason is perhaps due to the complex and often incomprehensible ways
human beings influence each other. Nonetheless, I will try my best to mention everyone whose
influence is consciously present in my mind. The list is by no means exhaustive, and if your name
is unjustly not here, I apologize in advance.
First and foremost, my sincere gratitude goes to my PhD supervisor, Prof. G. K. Surya
Prakash. Human beings are much like plants who need proper environment to flourish. The
freedom of exploration that I have consistently enjoyed in your lab has not only fueled my curiosity
to venture deep into the projects of my liking, but also taught me the responsibility that comes with
freedom. The discussions we had in chemistry and as well in philosophy has been very
enlightening and interesting; and without your inspiration, my world outlook may not have been
so optimistic.
This would be an appropriate time to acknowledge Prof. George Olah, who unfortunately
passed away in 2017. Despite limited interactions, Prof. Olah has influenced us greatly by his
enthusiastic passion for knowledge. My research work is based upon the idea of methanol
economy, so it wouldn’t be farfetched if I say that this dissertation wouldn’t have been possible
without him.
I would like to thank other committee members, Prof. Narayan, Prof. Jung, Prof. Shing,
and Prof. Haiges for their time during the screening and qualifying exam, as well as for the
stimulating discussion we had during them and beyond.
The contribution of Dr. Alain Goeppert to all the projects I describe in this thesis is
immense. Thank you, Alain, for constantly providing feedback regarding project ideas, for
answering all my queries regarding minute details of methanol economy, and most importantly,
for carefully proofreading the plethora of documents I emailed you over the years!
A special thanks goes to Dr. Jotheeswari Kothandaraman and Dr. Sankarganesh
Krishnamoorthy for patiently teaching me laboratory techniques of organometallic chemistry and
synthetic organic chemistry, respectively.
4
To all my other co-authors, Raktim, Vincente, Socrates, Hang, Ryan, Justin, it was
wonderful to work with you guys in different projects. The cheerful attitude displayed by you have
certainly made our collaborations enjoyable and fruitful!
My gratitude to the staffs of LHI and the chemistry department, Dr. Robert Aniszfeld, Jessy
May, Carole Phillips, David Hunter, Michele Dea, and Magnolia Benitez for helping me to get my
chemicals and pay checks in a timely manner. Also, the help of technical staffs, Allan Kershaw
and Ralph Pan is greatly appreciated. Special thanks to Gloria Canada for all the discussion and
laughter we had in the bistro.
I would like to thank all senior research scientist and post-doctoral students in the Prakash
group, Dr. Golam Rasul, Dr. Thomas Matthew, Dr. Patrice Batamack, Dr. Miklos Czaun, Dr.
Nazanin Entesary for stimulating scientific discussion and encouraging words.
I have been lucky to work with fellow PhD students in lab who are blessed with the funny
jar. Here is the space for all the countless laughs we had, Sahar, Vinayak, Archith, Fang, Huong,
Adam, Kavita, Alex, Colby, Eugene, Xanath, that we enjoyed over last four years. Also, outside
my own research group, thank you Buddhinie, Jitendra, Robert, Cay, Yiying, Bob, Thomas,
Antonina, Dibyendu, Amir for all the merriment. A big thank you to my roommates, Rakesh,
Tushar, Tirhendu and Prakarsh, and the Bengali community here at USC for making all weekends
a sort of festival. Also, credit also goes to the friends back in India, Bijoy, Apurba, Soumyajit,
Subhankar, who made the annual home trip a month-long enjoyment.
Among close family relatives, special thanks to my uncle and aunt for their unrelenting
faith on me. Finally, I am deeply indebted to my parents for everything they have provided
throughout. They have provided me with ample freedom to always follow my own, mostly stupid,
life decisions. A thank you to my sister for listening to the consequences of those stupid decisions
and to my grandmother, to whom I dedicate this dissertation, for providing post-facto wisdom on
their obvious stupidity!
5
Abstract
As the title suggest, the central theme of this dissertation is utilization of CO2 from air to produce
methanol or other C1 products. Such utilization of atmospheric CO2 is becoming increasingly
important in view of the rapid increase in CO2 concentration in air due to anthropogenic activities.
In the first chapter, an overview on the recent “carbon conundrum” is provided. Specially worrying
is the rapidly increasing CO2 concentration in the air and the associated global warming. The idea
of methanol economy based on carbon neutral cycle is introduced. A brief overview of the reported
homogeneous CO2 hydrogenation to methanol systems are provided that can produce methanol at
lower temperatures as compared to industrially utilized heterogeneous catalysts.
1,2
In the second chapter, mechanistic insights into the amine-assisted CO2 to methanol process is
reported. In this amine-assisted process, CO2 is converted into methanol in presence of an amine
through formamide formation. We describe the effect of catalyst and amine molecular structure on
the methanol yield. Also, we identify a catalyst deactivation pathway based on carbonylation of
the metal center leading to the formation of catalytically inactive ruthenium biscarbonyl
complexes. The electronic influence of pincer ligand on these biscarbonyl complexes are
described, providing insights on developing second generation CO2 to methanol catalysts.
3
In the third chapter, a recyclable system is described where CO2 is captured and converted to
methanol for multiple cycles in high yields. The CO2 capture is carried out in an aqueous amine
solution. After capture, the resulting carbamate and bicarbonate salts are hydrogenated to produce
methanol and regenerate the amine. The recycling of the catalyst and amine is achieved by using
a biphasic solvent system (2-methyltetrahydrofuran/water) that allows convenient separation of
the catalyst and amine after each hydrogenation cycle.
4
In the fourth chapter, an alternate recycling scheme based on amine immobilization onto solid
supports is described. CO2 is captured using the solid supported amines without requiring any
solvent. In the next step, the CO2 loaded amines are placed inside a parr reactor with high H2
pressure in presence of an active hydrogenation catalyst to produce methanol. After the reaction,
the solid amines are filtered and reused for next cycle of capture. We explored different preparation
6
methods to find the most suitable solid amine for this purpose. Covalently attached polyamines
were found optimal for repeated use without significant decrease in methanol yields in multiple
cycles.
5
In chapter 5, manganese based catalysts for amine assisted CO2 to methanol process is described.
The previous methods for this process used ruthenium based catalysts. However, to scale up the
reaction, use of cheaper earth abundant base-metal-based catalysts are necessary. A methanol
turnover number of 36 was obtained using manganese pincer catalyst, along with CO 2 to
formamide turnover of 840.
6
In chapter 6, a method for CO2 capture using hydroxide bases and its subsequent conversion to
formate salts is described. The initial capture produces bicarbonate salts which under
hydrogenation conditions produce the formate. The hydrogenation catalyst (ruthenium/iron
complex) is recycled in a biphasic system. More importantly, the hydroxide base (e.g. NaOH) was
regenerated in an unprecedented low temperature of 80
o
C in a cation conducting direct formate
fuel cell. The previous methods of hydroxide generation required a series of steps and high
temperatures (>750
o
C).
7
Additionally, my research activities over last four years also included investigations on first-row
transition metal catalyzed regioselective deuteration of alcohols,
8
CO2 capture from air using solid
supported amines,
9
amine promoted reforming of methanol,
10
and synthetic organofluorine
chemistry (especially, difluoromethylation of aromatic thiols and aldehydes using TMS-CF3)
11-12
which are not discussed here.
References
1. Sayan Kar, Jotheeswari Kothandaraman, Alain Goeppert, G. K. Surya Prakash, Advances
in Catalytic Homogeneous Hydrogenation of Carbon Dioxide to Methanol. J. CO2 Util.
2018, 23, 212-218. (HTML)
7
2. Jotheeswari Kothandaraman, Sayan Kar, Alain Goeppert, Raktim Sen, G. K. Surya Prakash
Advances in Homogeneous Catalysis for Low Temperature Methanol Reforming in the
Context of the Methanol Economy. Top. Catal. 2018, 61 (7), 542-559. (HTML)
3. Sayan Kar, Raktim Sen, Jotheeswari Kothandaraman, Alain Goeppert, Ryan Chowdhury,
Socrates B. Munoz, Ralf Haiges, G. K. Surya Prakash, Mechanistic Insights into
Ruthenium-pincer-catalyzed Amine Assisted Homogeneous Hydrogenation of CO2 to
Methanol. J. Am. Chem. Soc. 2019, 141, 3160-3170. (HTML)
4. Sayan Kar, Raktim Sen, Alain Goeppert, G. K. Surya 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. (HTML)
5. Sayan Kar, Alain Goeppert, G. K. Surya Prakash, Combined CO2 Capture and
Hydrogenation to Methanol: Amine Immobilization Enables Easy Recycling of Active
Elements. ChemSusChem 2019, doi:10.1002/cssc.201900324. (PDF)
6. Sayan Kar, Alain Goeppert, Jotheeswari Kothandaraman, G. K. Surya Prakash,
Manganese-Catalyzed Sequential Hydrogenation of CO2 to Methanol via Formamide. ACS
Catal. 2017, 7, 6347-6351. (HTML)
7. Sayan Kar, Alain Goeppert, Vicente Galvan, Ryan Chowdhury, Justin Olah, G. K. Surya
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. (HTML) (Article on the cover; issue 49, vol. 140)
8. Sayan Kar, Alain Goeppert, Raktim Sen, Jotheeswari Kothandaraman, G. K. Surya
Prakash, Regioselective Deuteration of Alcohols in D2O Catalysed by Homogeneous
Manganese and Iron Pincer Complexes. Green Chem. 2018, 20, 2706-2710. (HTML)
9. Hang Zhang, Alain Goeppert, Sayan Kar, G. K. Surya Prakash, Structural Parameters to
Consider in Selecting Silica Supports for Polyethylenimine based CO2 Solid Adsorbents:
Importance of Pore Size. J. CO2 Util. 2018, 26, 246-253. (HTML)
10. Jotheeswari Kothandaraman, Sayan Kar, Raktim Sen, Alain Goeppert, George A. Olah, G.
K. Surya Prakash, Efficient Reversible Hydrogen Carrier System Based on Amine
8
Reforming of Methanol. J. Am. Chem. Soc. 2017, 139, 2549-2552. (HTML) (highlighted in
Nature Reviews Chemistry; several mentions in the media)
11. Sankarganesh Krishnamoorthy, Sayan Kar, Jotheeswari Kothandaraman, G. K. Surya
Prakash, Nucleophilic Difluoromethylation of Aromatic Aldehydes using
Trimethyl(trifluoromethyl)silane (TMSCF3). J. Fluor. Chem. 2018, 208, 10-14. (HTML)
12. G. K. Surya Prakash, Sankarganesh Krishnamoorthy, Sayan Kar, George A. Olah, Direct
S-difluoromethylation of Thiols using the Ruppert–Prakash Reagent. J. Fluor. Chem. 2015,
180, 186-191. (HTML)
9
CHAPTER 1
Introduction
1.1. Carbon recycling in early twenty-first century
Humankind is addicted to fossil fuels. In the present twenty-first century, fossil fuels not
only provide us with abundant energy (for producing electricity, heating, cooking, in the
transportation sector, etc.), but are also used for manufacturing varied chemical products and
materials. As we enjoy the euphoria of our addiction, a debilitating disease has meanwhile started
to take hold of our planet. The extravagant amount of CO2 being emitted into the atmosphere has
increased the CO2 concentration in air to an unprecedented 414 ppm (as of April 14, 2019) (Figure
1).
1
In comparison, the atmospheric CO2 concentration in the pre-industrial years around 1750 was
close to 250 ppm. The net effect of this increased CO2 concentration is not pleasing. The Earth’s
surface temperature is increasing and unless CO2 emissions are reduced, the Intergovernmental
Panel on Climate Change (IPCC) has predicted an increase of up to 4.8
o
C in the Earth’s
temperature by 2100.
2
If these changes happen, it is expected that many low-laying countries and
territories will end up under water due to increased sea levels, along with other widespread
problems such as ocean acidification, weather pattern changes, epidemics, refugee crisis, and
ecological destabilization of marine ecosystems.
3-5
In this context, restriction in CO2 emissions to
the atmosphere is urgently required, along with capture of CO2 from air, if possible.
6-8
Figure 1. The rising CO2 concentration. Copyright Scripps Institution of Oceanography.
10
Moreover, while fossil fuels were formed inside Earth’s crust over eons, they are being
used up rather quickly. Even with recent developments in extraction technologies such as fracking
and enhanced oil recovery, fossil fuels would most probably not last for more than a century. Their
ever increasing usage, due in large part to the increasing population and improvement of living
conditions in populous developing countries, is one of the main reasons behind this impendent
depletion. As non-renewable sources of energies are being exhausted, humankind will need to turn
progressively to renewable sources such as solar, wind, geothermal energies (perhaps also non-
renewable nuclear) for its needs. Due to the intermittent and fluctuating nature of the most scalable
of these renewable energy sources, namely solar and wind, ways to store them efficiently and
economically on a large scale is of paramount importance. Furthermore, we need to develop
convenient systems to incorporate renewable energies into the transportation sector as well as in
chemical industry.
9-11
1.2. Methanol— a perfect substitute of natural oil
While battery technologies offer a way to store electricity, they have a number of
drawbacks such as, among others, cost, scalability, charging speed, and limited cycle life and
limited lifetime. Another possibility for storing electrical energy is in the form of a chemical bond
in a chemical energy carrier. One of the most versatile energy carriers is methanol. Methanol can
be used as a fuel in internal combustion engines, can be blended with gasoline as a fuel additive,
or alternatively be utilized in a direct methanol fuel cell to produce electrical power at room
temperature. Furthermore, methanol can also serve as a chemical feedstock to produce all the
value-added products currently obtained from fossil fuels (ethylene, propylene, etc) (Figure 2).
Moreover, methanol can be synthesized by combining carbon dioxide and hydrogen, thus
providing an opportunity to tackle both problems of CO2 utilization and energy carrier in a single
solution. Methanol, being a safe and easy to handle liquid at room temperature, can use the existing
infrastructure currently dedicated to fossil fuels transportation and distribution. Thus, it would only
require limited efforts to switch from petroleum oil based fuels to methanol in the transportation
sector. In fact, a number of countries including China, Sweden and Israel have started using
methanol as a drop-in fuel. In China especially, methanol already covers about 9% of the
transportation fuel needs. However, it should be noted that the methanol in these cases is not
11
synthesized from renewable sources, but rather from synthesis gas derived from fossil fuels such
as coal.
12
Figure 2. Multifaceted utility of methanol (methanol use statistics in 2015)
1.3. Methanol Economy— Carbon neutral cycle
The idea of a Methanol Economy as a sustainable model for a future energy cycle was
advanced by our former colleague and Nobel Laureate, the late Prof. George A. Olah (Figure 3).
13-
16
At the heart of the methanol economy is a carbon neutral cycle. The electricity produced from
renewable energy is used to electrolyze water and form hydrogen. The resulting hydrogen is then
combined with CO2 to form methanol. The methanol can then be used for any application or as a
synthetic raw material. The CO2 emitted from burning the methanol is captured back and used for
methanol synthesis, thus closing the loop for a carbon neutral cycle.
12
Figure 3. Carbon neutral cycle and methanol economy. Reproduced with permission from
reference 30. Copyright 2018, Elsevier.
The above-mentioned idea has already been realized, on a humble scale, in Carbon
Recycling International’s (CRI) George Olah Renewable Methanol plant in Svartsengi, Iceland.
17
This plant recycles annually 5500 tonnes of CO2 from geothermal steam emissions, which would
otherwise be released to the atmosphere, to produce more than 5 million liters of renewable
methanol. The hydrogen necessary for the conversion of CO2 to methanol is produced locally
through electrochemical water splitting with electricity from the Icelandic grid, generated from
hydro and geothermal energy. The produced methanol is trademarked and commercially sold as
Vulcanol and is mostly used as a fuel or fuel-additive.
1.4. Heterogeneous catalysis for CO2 to methanol
As mentioned above, methanol is currently produced industrially from synthesis gas (CO
+ 2H2). Catalysts similar to the ones that catalyze methanol synthesis from CO are also effective
for CO2 hydrogenation to methanol. This is not surprising as it is generally agreed upon that the
CO to methanol process likely proceeds through the formation of CO2 from CO in the water gas
shift reaction (Scheme 1).
18-20
The formation of CO2 from CO also removes any water present
locally from the catalyst surface. The presence of water inhibits the hydrogenation reaction. It has
13
been reported that in the absence of any moisture or CO2, CO hydrogenation to methanol hardly
takes place, suggesting that methanol production proceeds through intermediate CO2 formation.
Scheme 1.
Most heterogeneous systems for CO2 hydrogenation to methanol utilize Cu/ZnO/Al2O3
catalysts and modifications thereof.
21-23
The hydrogenation is typically carried out around 230-300
o
C and at a CO2/3H2 pressure of 50-75 bars. At this temperature and pressure, methanol formation
is highly selective with more than 99% selectivity. On the other hand, CO2 to methanol being an
exothermic reaction, the conversion of CO2 at high temperature decreases according to Le
Chatelier’s principle (Scheme 1). Thus, under the conditions typically employed, CO2 conversion
hovers around 30%. To achieve higher conversions, the unreacted gas mixture is recycled.
Recently, alternative catalysts based on Ni-Ga have also been reported for methanol synthesis.
24-
26
These catalysts displayed similar catalytic activities as compared to traditional Cu/ZnO/Al 2O3
catalysts, along with much lower CO formation. Indium oxide supported on zirconium oxide has
also been reported for heterogeneous CO2 to methanol synthesis.
27-28
1.5. Homogeneous catalysis— An alternative
1.5.1. Benefits of homogeneous catalysis
Although the heterogeneous catalysts for CO2 hydrogenation to methanol are robust and
offer high selectivity, new catalysts that can hydrogenate CO2 at lower temperature are desired. In
this regard, homogeneous catalysis is studied extensively in last 10 years and hydrogenation of
CO2 to methanol at a temperature as low as 100
o
C is reported.
29-31
Operating at low temperatures,
homogeneous systems allow for a higher possible conversion of CO2 to methanol. Furthermore,
homogeneous complexes can be rationally designed on a molecular level and improved based on
previous experimental results to increase their efficiency and selectivity for methanol.
14
There is another aspect in which homogeneous systems holds advantage over
heterogeneous catalysis. In a typical heterogeneous CO2 to methanol system a CO2/3H2 gas
mixture is flowed through a column containing the catalyst at high temperature and pressure. For
this purpose, relatively pure and concentrated CO2 is required. Thus, if the CO2 is obtained from
sources like flue gas or from diffused sources like ambient air, it is necessary after CO 2
absorption/adsorption to desorb the captured CO2 and compress it to the desired pressure. The
desorption step is highly energy intensive (especially when aqueous solutions of amines or metal
hydroxides are used for the capture) and adds a significant energy penalty to the entire capture
process. On the other hand, with homogeneous catalysts, the CO2 capture products can be directly
hydrogenated to methanol, eliminating the CO2 desorption and compression step in an integrated
capture and conversion system as will be discussed below. After the reaction, the capturing
materials are recovered and can be re-utilized for next cycle.
1.5.2. CO2 hydrogenation to methanol through different routes
Figure 4. Different routes of obtaining methanol from CO2
15
As shown in Figure 4, CO2 can be reduced to methanol through various routes including
carbonates, ureas, carbamates, formate esters, formamides, formic acid, and carbon monoxide.
Among these, the routes involving formic acid and carbon monoxide do not require any additives.
In all other approaches, the use of an amine or alcohol additive is required. Notably, the pathways
involving carbonates, urea derivatives and carbamates do not require any reduction in the first step,
whereas in the second step, the resulting intermediate is hydrogenated to methanol by 3 equivalents
of H2. On the other hand, pathways involving formate esters, formamides, formic acid and CO
involve two consecutive hydrogenation steps with one and two equivalents of hydrogen,
respectively.
1.5.3. Homogenous CO2 to methanol processes
In 1995, Tominaga et al used Ru3(CO)12 to convert CO2 to methanol via the formation of
CO, although high temperature (>200 °C) was required, and concomitant formation of CO and
CH4 was observed (Scheme 2).
32-33
Scheme 2
Huff and Sanford in their 2011 article, demonstrated an elegant sequential reduction of
CO2 to methanol, via the formation of formic acid and methyl formate as intermediates, using three
different catalysts for three distinct steps in the same pot (Scheme 3).
34
16
Scheme 3. Cascade CO2 to methanol by Huff and Sanford
More recently, Leitner and coworkers have used (triphos)Ru(TMM), (TMM-
trimethylenemethane) complex as a catalyst precursor in presence of NHTf2 to obtain methanol
with a TON of 769.
35-36
Beller et al. reported a similar system with cobalt-based homogeneous
catalysts in 2017(Scheme 4).
37
Scheme 4. CO2 to methanol system reported by Leitner and coworkers
In 2015, Sanford and co-workers reported the hydrogenation of CO2 to methanol in the
presence of dimethylamine proceeding through the formation of a formamide intermediate
(Scheme 5).
38
17
Scheme 5. Amine-assisted CO2 to methanol via formamide
The ability of an amine to assist in the CO2 hydrogenation to methanol opened up the vista
for integration of CO2 capture with conversion to methanol. Considering that amines are widely
used as CO2 scrubbing agents, amine assisted CO2 to methanol systems are ideal for integrated
CO2 capture and conversion and bypass the energy intensive CO2 desorption and compression
steps. In 2015, our group demonstrated the feasibility of such a one pot capture and conversion by
hydrogenating CO2 directly captured from air into methanol (Scheme 6).
39
Scheme 6. Integrated carbon capture and utilization cycle
18
Similarly, in 2015, Milstein and co-workers demonstrated sequential capture of CO2 by
ethanolamines to oxazolidinones and same pot conversion to CH3OH by Milstein’s PNN catalyst
(Scheme 7).
40
Scheme 7. CO2 capture as oxazolidinone and conversion to methanol
In the meantime, other studies employing earth abundant metal-based complexes as
catalysts have been reported by us and others.
41,42
Notably, in 2017, we reported manganese based
homogneous catalysts for the amine-assited CO2 to methanol process which will be discussed in
chapter 5. The easy recyclability of the catalyst as well as the amine through the use of a biphasic
system or amine immobilization has also recently been disclosed by our group, as described in
chapter 3 and chapter 4, respectively. Also, in 2019, we have reported a catalytic deactivation
pathway through carbonylation during amine-assisted CO2 to methanol process, which will be
discussed in the subsequent chapter.
1.6. References
1. https://www.co2.earth/ Accessed on April 14, 2019
2. IPCC Fifth Assessment Report: Climate Change 2014. Synthesis Report. Summary for
Policymaker (2014). Cambridge University Press, Cambridge, U.K.
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39. Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.; Prakash, G. K. S., Conversion
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23
CHAPTER 2
Mechanistic Insights into Ruthenium-Pincer-Catalyzed
Amine-Assisted Homogeneous Hydrogenation of CO
2
to
Methanol
This dissertation chapter is based on a recent research article from our group published on the
Journal of the American Chemical Society (Kar, S.; Sen, R.; Kothandaraman, J.; Goeppert, A.;
Chowdhury, R.; Munoz, S. B.; Haiges, R.; Prakash, G. K. S., J. Am. Chem. Soc. 2019, 141, 3160-
3170). Part of the article is reprinted by permission of the American Chemical Society. Copyright
2019. American Chemical Society
2.1. Introduction: Amine-assisted CO2 to methanol
As mentioned in the previous champer, in 2015, Sanford and co-workers reported the
hydrogenation of CO2 to methanol in the presence of dimethylamine proceeding through the
formation of a formamide intermediate (Scheme 1).
1
Scheme 1. Amine assisted CO2 hydrogenation to MeOH
The ability of an amine to assist in the CO2 hydrogenation to methanol opened up the vista
for integration of CO2 capture with conversion to methanol. Considering that amines are widely
used as CO2 scrubbing agents, amine assisted CO2 to methanol systems are ideal for integrated
CO2 capture and conversion and bypass the energy intensive CO2 desorption and compression
24
steps. In 2015, our group demonstrated the feasibility of such a one pot capture and conversion by
hydrogenating CO2 directly captured from air into methanol (Scheme 2).
2
Scheme 2. Integrated CO2 capture and conversion to methanol
The easy recyclability of the catalyst as well as the amine through the use of a biphasic
system has also recently been disclosed by our group.
4
In the meantime, other studies employing
earth abundant metal-based complexes as catalysts have been reported by us and others.
5,6
A list
of selected reports of amine-assisted CO2 to CH3OH systems is provided in Table 1.
Table 1. Selected examples of reported amine assisted CO2 to methanol systems
However, despite the immense practical utility of the amine assisted CO2 to methanol
process, a systematic study describing the influence of catalyst/ amine molecular structure on
methanol production had not been undertaken yet. Given that the highest TONs reported to date
for this process is only 9000, there is clearly a practical need to improve upon the catalytic
efficiency by a judicious choice of catalyst/amine pairing, in order to make the process
Sanford and coworkers
(2015)
1
Prakash and
coworkers (2015)
2
Pombeiro and
coworkers (2017)
6
Wass and
coworkers (2017)
7
Catalyst
Amine Me2NH PEHA PEHA Me2NH/ i -Pr2NH
Solvent THF Triglyme - Toluene
T (
o
C) 95 to 155 155 80 180
TONmax 550 2150 2283 9000
Cat.
Loading
1.7 µmol 10 µmol 10 µmol 50 nmol
25
economically viable. In this study we explored the relation between catalyst/ amine molecular
structure and methanol yield and the results of our investigations are detailed below.
2.2. Effect of the catalyst’s molecular structure
2.2.1. Stark effect of P substituents
In previous reports, our group and others demonstrated the ability of Ru-Macho-BH (C-1; Table
2) to convert CO2 into CH3OH in the presence of an amine (Me2NH or PEHA).
1-2
To probe for the
effect of structural changes in the ligand framework on methanol yield, several RuPNP
R
pincer
complexes with variable substitutions in the phosphine ligands were screened (R= Ph (C-2), i-Pr
(C-3), R= Cy (C-4), R= t-Bu (C-5). To our surprise, the methanol yield dropped drastically as the
R groups were changed from Ph to i-Pr/Cy/t-Bu, when the reaction was performed with PEHA
(5.1 mmol) in 10 mL triglyme and C-1 (10 µmol) as the catalyst under 75 bar of CO2:3H2 mixture,
the formation of 10.5 mmol of CH3OH was observed (TON = 1050) after 40 h at 145 °C along
with 8.0 mmol of formamide and 1.2 mmol of formate (Table 2, entry 1). Catalyst C-2 (Ru-Macho)
formed a similar amount of MeOH under the same reaction conditions, although an additional base
(K3PO4) was required for its initial activation (entry 2). All subsequent catalysts from C-3 to C-8
were screened in the presence of K3PO4 (1 mmol). When the P substituent was changed from Ph
to i-Pr (complex C-3, RuHClPNP
iPr
(CO)), a stark decrease in MeOH formation was observed with
only 3.2 mmol of MeOH obtained (TON = 320) (entry 3). Instead, a large accumulation of
formamide products (22.6 mmol, 80% w.r.t the amine content) was noticed, indicating that the
formamide reduction step to MeOH is the most challenging under these reaction conditions. At a
lower temperature of 125
o
C, in the presence of C-3, an even greater amount of formamide products
(93%) was detected after 40 h, but no appreciable amount of methanol was observed as analyzed
through
1
H or
13
C NMR, indicating the elevated temperature required for the methanol synthesis
(entry 4). Catalyst C-4, with its Cy group attached to phosphorus atoms, afforded a meager 0.5
mmol of methanol (TON = 50), and 14.7 mmol of formamide (entry 5). Similarly, catalyst C-5,
Ru-PNP
tBu
led to 17.5 mmol of formamide products, but no CH3OH (entry 6).
26
Table 2. Effect of catalyst molecular structures on MeOH yield
Reaction conditions: PEHA (5.1 mmol), Cat. (10 µmol), K3PO4 (1 mmol), triglyme (10 mL), CO2:3H2 (75
bar), 145 °C, 40 h.
a
yields were determined from
1
H NMR spectra with 1,3,5-trimethoxybenzene (TMB) as
an internal standard.
b
CO detection limit - 0.099%.
c
In the absence of K3PO4.
d
T = 125 °C. TONMeOH = mol
of CH3OH formed per mol of cat. Yield calculation error ±5%
2.2.2. The observed selectivity is surprising
The inability of catalyst C-3, C-4, and C-5 and the exclusive ability of C-1 and C-2 for
effective CO2 to CH3OH conversion is somewhat surprising. Ding and coworkers, in their study
on N-formylation of amines with CO2 and H2 catalyzed by ruthenium pincer complexes, reported
similar activities for C-2 – C-5 in the N-formylation of morpholine.
8
Similarly, we found in this
Entry Catalyst formate
(mmol)
a
formamide
(mmol)
a
methanol
(mmol)
a
CO
(%)
b
TON
formate+formamide
TON
MeOH
1
c
C-1 1.2 8.0 10.5 0.21 920 1050
2 C-2 1.6 8.1 10.4 0.22 970 1040
3 C-3 1.1 22.6 3.2 0 2370 320
4
d
C-3 2.3 28.4 0 0 3270 0
5 C-4 1.0 14.7 0.5 0 1570 50
6 C-5 1.6 17.5 0 0 1910 0
7 C-6 1.3 7.0 6.8 0.1 830 680
8 C-7 0.4 18.4 0 0 1880 0
9 C-8 0.7 11.0 0 0 1170 0
27
study that under a H2 pressure of 20 bar, C-2, C-3, C-4 are able to hydrogenate N-formylpiperidine
(F-1) very efficiently (but not C-5, probably due to its bulky t-Bu groups) to methanol and the
corresponding amine (Table 3).
9
In fact, RuPNP
iPr
and RuPNP
Cy
were even more active than
RuPNP
Ph
, as observed from the completion time of the hydrogenation reactions. The fact that the
MeOH formation is not dependent on the effectiveness of the parent catalysts to catalyze the two
steps of this sequential reaction could indicate the formation of deactivating catalytic
intermediate(s) during the reaction. We will explore these intermediates later while discussing the
reaction mechanism. It should also be noted here that although in the present manuscript we
concern ourselves with ruthenium pincer complexes, very similar observations are reported in
literature by Bernskoetter et al. and our group with regards to iron pincer complexes.
2, 10-12
Table 3. Formamide reduction by Ru-pincer complexes
Reaction conditions: N-formylpiperidine (20 mmol), H2 (20 bar), Catalyst (10
µmol), K3PO4 (1 mmol), triglyme (10 mL)
a
yields were determined from
1
H NMR
spectra with TMB as an internal standard. Reaction times were determined
based on cessation of pressure decrease. Yield calculation error ±5%.
2.2.3. Effect of spectator ligand
Going back to Table 2, among other ruthenium complexes, the dichloride pincer NHC
complex C-6, with a PNP
Ph
pincer ligand and NHC as the spectator ligand (as opposed to CO in
other complexes) was able to reduce CO2 to MeOH in the presence of PEHA (entry 7). After 40
h, 6.8 mmol of CH3OH (TON = 680) was observed through
1
H NMR. A continuous pressure-drop
inside the reaction vessel until the reaction termination indicated the active nature of the catalyst
throughout the reaction. However, with both Milstein’s PNN pincer complex (C-7) and the PNP
entry catalyst Time
(min)
formamide
(%)
a
amine
(%)
a
MeOH
(%)
a
TONMeOH
1 C-2 440 26 67 70 1400
2 C-3 110 26 79 79 1580
3 C-4 270 18 80 87 1740
4 C-5 480 86 4 5 100
28
acridine pincer complex C-8, only intermediate formamide products were observed (18.4 and 11.0
mmol, respectively) with no observable methanol formation through
1
H and
13
C NMR. Thus, it
seems that the presence PNP
Ph
as the pincer ligand is essential to obtain good methanol yields
directly from CO2 via amine assisted process. In contrast, other structural features such as varying
spectator ligand do not influence methanol yield to the same degree.
2.3. Effect of the amine’s molecular structure
2.3.1. Different roles of amine during catalysis
The amine plays multiple roles during the catalytic reaction. First, it helps to dissolve CO2
in the organic solution and thus effectively increasing the CO2 concentration in the solution.
Second, during the initial CO2 hydrogenation to form alkylammonium formate salts, the amine
assists in the detachment of the formate ligand from the ruthenium center (Figure 1; top). In the
absence of amine, no CO2 hydrogenation takes place (even to formic acid) as the catalyst is
kinetically trapped in the formate form (C-1B) (Figure 1; bottom, eq. 1). Similarly, in the absence
of amine, formamide reduction by CO2:3H2 gas mixture is not viable because of the formation of
C-1B, which is unable to revert to the dihydride species (C-1A) under the reaction conditions
(Figure 1 bottom, eq. 2). Third, the amine forms formamide from the ammonium formate salt via
condensation reaction, which is a crucial reaction step for obtaining methanol (Scheme 1).
Figure 1. (Top) Interconversion of C-1A and C-1B in presence of amine; (Bottom) Failed
hydrogenations in the absence of amine
29
2.3.2. The crucial primary/secondary diamino moiety
When diamines were employed for the reaction (since the polyamine PEHA is known to
be able to assist in CH3OH production), varying MeOH yields were obtained (Figure 2). The
nature of primary or secondary amine had a very slight effect on methanol formation, with
secondary amines being marginally more efficient. With ethylenediamine (1), containing two
primary amino groups, 4.3 mmol of MeOH was observed after 20 hours, whereas an 18% higher
yield was obtained (5.1 mmol of methanol) in case of N,N’-dimethylethylenediamine (2)
containing two secondary amino groups (Figure 2). N-methylethylenediamine 3, with one primary
and one secondary amino group provided a CH3OH yield, which was in between 1 and 2.
Surprisingly, in presence of mixed primary-tertiary and secondary-tertiary amines, 4 and 5
respectively, no MeOH was produced, but only formamide intermediates were observed. The
effect of the presence of hydroxyl groups was then explored using N-(2-
hydroxyethyl)ethylenediamine (6) and N,N′-bis(2-hydroxyethyl)ethylenediamine (7) for the
reaction. A rapid decrease in reaction pressure was observed during the initial hours of these
experiments; and 3.7 mmol and 4.8 mmol MeOH were obtained after 20 hours with 6 and 7,
respectively. In comparison, the polyamines DETA (9) and PEHA (10) provided 5.8 and 6.2 mmol
of methanol formation, respectively. In the case of piperidine (8), 92% of the amino groups were
formylated after 20 h (18.4 mmol), along with only 3% of methanol formation.
The 1,2-diamines or polyamines with a 1,2-diamine substructure, where both amine
functional groups are either primary or secondary provide the best methanol yields (as in case of
1, 2, 3, 9, 10). The hydroxyl groups present in the amine, in case of 6 and 7, have a deactivating
effect, providing somewhat inferior methanol yield. More interestingly, when one of the amino
group of the diamine was tertiary, as in 4 and 5, methanol formation completely subsided. Thus,
1,2-diamines with primary/secondary amines were the unique structural motifs that were able to
assist in the formation of methanol in high yields. The reason of this correlation between amine
structure and methanol formation was investigated further through mechanistic studies presented
hereafter.
30
Figure 2. Methanol and formamide yield with different amines. Reaction conditions: C-1 (10
µmol), triglyme (10 mL), CO2: 3H2 (75 bar) 145 °C, 20 h. amine functionality content = 30.6 mmol
(PEHA 5.1 mmol, DETA (9) 10.2 mmol, all diamines 15.3 mmol,). Yields were determined by
1
H
NMR with TMB as an internal standard. With piperidine, 20 mmol was used. Yield calculation
error ±5%.
2.4. Mechanistic Investigations
2.4.1. Catalytic deactivation: Biscarbonyl complexes
To obtain a better understanding of the reaction system, catalytic species present in the
reaction were monitored through various spectroscopic techniques. Catalysts C-1 – C-5 are
reported in the literature to form ruthenium formate species (similar to C-1B) in the presence of
4.3
5.1
5
0 0
3.7
4.8
0.6
5.8
6.2
0
1
2
3
4
5
6
7
1 2 3 4 5 6 7 8 9 10
0
5
10
15
20
Methanol (mmol)
Amine
Formamide (mmol)
MeOH Formamide
31
H2 and CO2 (see Figure 1). However, under the reaction conditions of our mechanistic
investigation (12 mg C-1, 10 mg PEHA in THF-d8, 145
o
C for 40 h), a completely different species
was observed in the solution. The
1
H spectra showed the presence of a triplet (J = 16.4 Hz) at -6.3
ppm (Figure 3) along with a minor triplet at -5.6 ppm in a 20:1 ratio.
Figure 3. Hydride signals observed after hydrogenation reaction
The strong low field shift of this hydride peak signifies a strong trans effect from the
opposite axial ligand. The corresponding
31
P peak was observed at 58.5 ppm (major) and 57.4 ppm
(minor), while in the
13
C NMR spectra, two different carbonyl peaks for the major isomer were
observed at 200.2 (t; J = 10.5 Hz) and 191.9 ppm in a 1:1 ratio (Figure 4). The coupling between
the carbonyl ligands and the hydride was further observed from the proton coupled
13
C NMR
(Figure 4).
32
Figure 4. Observation of two different carbonyl peaks in
13
C NMR (top:
1
H decoupled; bottom:
1
H coupled)
Similarly, in the ATR-IR spectra, two different CO stretches were observed at 2052 and
1964 cm
-1
. In the ESI-MS, the molecular fragment [RuHPNP
Ph
(CO)2]
+
corresponding to M/Z=
600.08 was also observed. Thus the cationic structure of [RuHPNP
Ph
(CO)2]
+
(C-1D) was assigned
to the catalytic resting state (structure shown in Figure 4). Finally, the cationic structure was
confirmed through single crystal X-ray diffraction, and the presence of bicarbonate as the counter
anion was observed (Figure 5).
Figure 5. Single crystal X-ray structure of the cation in C-1D cation moiety. ORTEP diagrams
plotted at 50% probability level. Selected hydrogen atoms have been omitted for clarity.
1
H coupled
33
2.4.2 How are the biscarbonyl complexes formed?
The second carbonyl ligand is surmised to come from the in situ generated CO gas. Indeed,
the presence of CO gas was confirmed through ATR-IR analysis of the reaction gas mixture inside
the reactor (Figure 6). Further, a correlation between the presence of CH3OH and CO gas in the
reaction mixture was observed (Table 2), indicating that the CH3OH formation pathway is the
primary source for CO formation. However, when an experiment was conducted in the presence
of 2 mmol of
13
CH3OH, no enhanced carbonyl peak of the biscarbonyl complex C-1D was
observed by
13
C NMR. This indicates that CH3OH decarbonylation is unlikely to be the pathway
for the generation of CO. Rather, it is most likely that the CO forms through the decomposition of
in situ generated formaldehyde intermediate during the reaction.
13-15
Figure 6. Presence of CO gas in the reaction mixture.
2.4.3. Two isomers of biscarbonyl complexes
From
1
H NOE spectra, no spatial correlation between the hydride peak of the major isomer
and the N-H peak was observed. Thus, the structure of anti C-1D, where the Ru-H and N-H are
trans to each other, was assigned to the major isomer. Notably, the same anti isomer was also
observed in the single X-ray crystal structure. Thus, the other minor peak in the
1
H and
31
P was
assigned to the other isomer, syn C-1D (Figure 7).
34
Figure 7. Structure of anti C-1D (major) and syn C-1D (minor)
2.4.4. Biscarbonyl complexes can catalyze the first CO2 to formamide step
When the mechanistic studies were conducted with catalyst C-3 (R= i-Pr); C-4 (R= Cy)
and C-5 (R = t-Bu) instead of C-1, similar biscarbonyl species (C-3D; C-4D, and C-5D,
respectively) were observed in the reaction mixture, along with minute amounts of CH3OH, and
CO in the gas mixture. The presence of similar pincer biscarbonyl iron complexes has been
reported recently by Hazari et al.
10
At the same time, they reported the catalytic activity of this
biscarbonyl complex for N-formylation reaction.
2.4.5. Biscarbonyl complexes convert to active dihydride complex at high H2
pressure
In our study, the crystal structures of the biscarbonyl complexes showed a higher bond
length for the newly formed axial Ru-CO bond, similar to the previously reported aforementioned
iron complex. A compilation of the metal carbonyl bond lengths of manganese, iron and ruthenium
biscarbonyl complexes is shown in Table 4.
Table 4. Metal carbonyl bond lengths in various pincer biscarbonyl complexes
Complex M-COeq (Å) M-COax (Å) Reference
MnBrPNP
iPr
(CO)2 1.7810(17) 1.7507(16)
16
MnBrPNP
Cy
(CO)2 1.787(2) 1.754(3)
16
[FeHPNP
iPr
(CO)2]
+
1.7457(3) 1.8141(3)
10
[RuHPNP
Ph
(CO)2]
+
1.870(3) 1.980(3) This work
[RuHPNP
tBu
(CO)2]
+
1.875(5) 1.971(5) This work
35
The second axial carbonyl ligand in these (iron and ruthenium) biscarbonyl complexes is
surmised to be labile,
17-18
and depending on CO and H2 pressure can detach from the metal center
to form dihydride species (Figure 6 and S7). The formation of the dihydride species should be
favored at high H2 pressure and low CO pressure. The two plausible routes for the formation of
the dihydride catalytic species C-XA from the biscarbonyl species (C-XD) is shown in Figure 8.
Figure 8. Proposed routes of conversion of ruthenium biscarbonyl to dihydride species
Accordingly, when the biscarbonyl complex C-4D was treated with 60 bar H2 for 40 h at
145 °C, the two CO stretches of the biscarbonyl complex (2033 and 1966 cm
-1
) disappeared in the
ATR-IR spectra. Instead, a new carbonyl peak was observed at 1912 cm
-1
corresponding to the
monocarbonyl dihydride species (C-4A) (Figure 9).
36
Figure 9. In blue, solution containing C-4D biscarbonyl complex obtained from mechanistic
reaction; in red, after hydrogenation of the solution with 60 bar H2 for 40 h at 145
o
C. The loss of
one carbonyl stretch signifies the formation of ruthenium dihydride monocarbonyl species.
2.4.6. Biscarbonyl complexes cannot catalyze formamide hydrogenation to
methanol
The biscarbonyl complexes with R= Ph (C-1D), and i-Pr (C-3D) were able to hydrogenate
formamide (N, N’-bisformyldimethylethylenediamine (F-2)) in pure H2 at a pressure of 60 bar
(Table 5), proceeding through the formation of the dihydride complex (C-XA). However, when a
CO pressure was introduced in the system (5 bar, R = Ph), the formation of dihydride species was
inhibited and the formamide reduction completely stopped. Thus, the biscarbonyl complex is
unable to catalyze the formamide reduction step by itself and acts as a deactivated catalytic species
under the reaction conditions.
Table 5. Hydrogenation of formamides using biscarbonyl complexes
60
65
70
75
80
85
90
95
100
500 1000 1500 2000 2500 3000 3500 4000
Absorbance
Wavenumber (cm
-1
)
1912
37
Reaction conditions: N-CHO (10 mmol), Cat. (10 µmol), triglyme (10 mL), H2 (60 bar), 145 °C, 20h.
Percentage values in the table represent CH3OH yields as observed by
1
H NMR. Yield calculation error
±5%.
2.4.7. Lability trend of axial CO
The lability of the axial carbonyl group in the biscarbonyl complex can be expected to
follow a trend based on the substitution on the P atoms of the ligand. In theory, as the electron
donating ability of the PNP ligand increases with substitutions (t-Bu>i-Pr>Ph), the electron density
at the metal center increases. As a result, the metal-carbonyl back bonding also increases in an
attempt to diffuse the high electron density from Ru center. Due to this increased back bonding,
the metal carbonyl bond gets stronger as the ligands become increasingly electron donating (t-
Bu>i-Pr>Ph), resulting in decreased lability of the second carbonyl ligand (lability: t-Bu< i-Pr<
Ph). This decrease in lability can be conveniently monitored through CO stretching frequencies of
the carbonyl ligands, as the CO bond strength decreases as metal carbonyl bond strengthens. Thus,
while in the parent monocarbonyl complex-es C-2, C-3 and C-5, the equatorial CO stretching
frequencies were found to be similar (Table 6), in the ATR-IR spectra of the biscarbonyl
complexes, the axial CO stretches showed increasing frequencies as the ligand was changed from
t-Bu to i-Pr to Ph (Table 6, Figure 10).
38% 46%
4% 55%
38
Table 6. CO stretching frequencies of monocarbonyl and biscarbonyl complexes
Catalyst CO stretch wavenumber (cm
-1
)
RuHClPNP
Ph
(CO) [C-2] 1902
RuHClPNP
iPr
(CO) [C-3] 1906
RuHClPNP
Cy
(CO) [C-4] 1908
RuHClPNP
tBu
(CO) [C-5] 1894
[RuHPNP
Ph
(CO)2]
+
[C-1D] 2052, 1964
[RuHPNP
iPr
(CO)2]
+
[C-3D] 2033, 1965
[RuHPNP
cy
(CO)2]
+
[C-4D] 2033, 1966
[RuHPNP
tBu
(CO)2]
+
[C-5D] 1996, 1942
This signifies that as the ligand is changed from PNP
tBu
to PNP
iPr
to PNP
Ph
, the bond
strength of the second axial CO and metal center decreases (hence stronger CO stretching
frequency), and thus increasing its lability.
Wavenumber (cm
-1
)
Figure 10. CO stretching frequencies of biscarbonyl complexes as observed in ATR-IR
spectroscopy.
39
2.4.8. Rationalizing the ligand effect
Based on these observations, a mechanistic explanation for the drastic effects of ligand
substitution as observed in Table 2 can be provided. The main deactivating pathway for the
catalytic species is through biscarbonyl monohydride ruthenium species (C-XD), that forms in the
presence of in situ generated CO gas. The CO concentration continuously increases during the
reaction along with increasing CH3OH production. The second axial carbonyl ligand in these
biscarbonyl complexes displays a higher lability than the parent equatorial one. While the
biscarbonyl complexes are able to hydrogenate CO2 to formate salts or formamides, they are
unable to catalyze the formamide hydrogenation to methanol and amine. In the case of PNP
Ph
pincer ligand, the axial CO ligand of the biscarbonyl complex is most labile, due to the electronic
effect of the ligand. As a result, even in the presence of 0.1% CO it can revert back to the dihydride
species, which catalyzes the formamide reduction. Thus, with catalyst C-2, a continuous methanol
production was observed for 40 h (Table 2, entry 2). On the other hand, for Ru-PNP
iPr
(C-3), the
lability of axial CO is lower; meaning that if the CO concentration increase above a certain level,
the biscarbonyl species is unable to revert back to dihydride, stopping the methanol formation. For
Ru-PNP
tBu
(C-5), the threshold CO level is even lower, and the formamide hydrogenation stops
even before any visible CH3OH formation through
1
H NMR.
2.4.9. Observing the effect of amine
Having explored how the reaction mechanism is dependent on the catalyst’s molecular
structure, the effect of amine structure was explored next. As mentioned earlier, none of the
primary and secondary monoamines screened was able to effectively produce methanol under the
reaction conditions with C-1, and the reduction stopped at the formamide stage. To probe the
reason behind this, the hydrogenation of two representative amides, N, N’-
bisformyldimethylethylenediamine (F-2) and N-formylpiperidine (F-1) was tried using the
biscarbonyl complexes C-1D and C-3D. We observed that while the biscarbonyl complex with R
= i-Pr (C-3D) was effective for the hydrogenation of both formamides, R= Ph (C-1D) selectively
hydrogenated the diamide only (Table 4). The reason behind this observed selectivity only with
C-1D is not clear and further experimental and theoretical investigations are needed for its
elucidation. However, the absence of CH3OH with monoamines can be surmised due to this
inability of C-1D to hydrogenate their corresponding formamides.
40
2.5. Proposed Mechanism
Taking these observations together, a plausible mechanism for the sequential CO2
hydrogenation to CH3OH can be proposed as depicted in Figure 11. Needless to say, detailed
investigations dedicated to separate mechanistic steps need to be undertaken in future to properly
understand each individual reaction steps. According to our proposed mechanism, in the first stage,
in presence of a base and H2, complex C-2 to C-5 can form the dihydride species C-XA (C-1, Ru-
Macho-BH can form the dihydride species even in the absence of a base under thermal activation,
hence catalysis with C-1 does not require an activating base). The dihydride species, in the
presence of CO2, forms the formate complex (C-XB) through CO2 insertion into the Ru-H bond.
19
Subsequently, in the presence of an amine, the formate ligand gets detached from the complex to
form a pentacoordinated species C-XF along with an alkylammonium formate salt. In the presence
of H2 gas, the pentacoordiated species can afford back the dihydride species (C-XA) via C-XE to
complete the catalytic cycle. Importantly, during this catalytic cycle to produce alkylammonium
formate salts from CO2, the N-H moiety of PNP ligand does not actively take part in the reaction
mechanism. Catalyst RuHClPNMeP
Ph
(CO) (C-9), in which the N-H was replaced with N-Me, was
also able to form formate salts at a rate similar to C-2.
20-21
Next, the alkylammonium formate salt
forms the formamide product through a condensation reaction. The formamides are amenable to
hydrogenation under the pressure of hydrogen, and after the first hydrogenation by the dihydride
species (C-XA), an amino alcohol is produced, along with the ruthenium amido complex C-XC,
which forms back dihydride complex C-XA in the presence of H2.
22-23
The amino alcohol quickly
decomposes to formaldehyde and amine under the reaction conditions as its presence wasn’t
detected in the reaction mixture. Formaldehyde gets further hydrogenated quickly by C-XA to
produce methanol.
24
Also, due to formaldehyde decomposition,
14-15
minute amounts of CO are
produced in the system, which coordinates with the amido complex C-XC to form the catalytic
resting state C-XD. As mentioned earlier, the biscarbonyl complex is reverted back eventually to
the dihydride complex (C-XA) in the active catalytic cycle (see Figure 6). Notably, in this second
formamide reduction step, N-H moiety does actively take part in the reaction mechanism and the
N-Me analogue (C-9) does not produce any CH3OH.
2
Hence, amido complex CX-C is postulated
to participate at this stage.
41
Figure 11. Proposed mechanistic cycle for amine assisted CO2 hydrogenation to methanol
42
2.6. Conclusion
In conclusion, correlations between methanol yield with catalyst and amine molecular
structure has been established for the process of amine-assisted methanol synthesis through CO2
hydrogenation. Among various ruthenium pincer hydrogenation catalysts, complexes with the
PNP
Ph
ligand were most efficient in methanol production (C-1/C-2/C-6). The observed reactivity
and efficiency were ascribed to the high lability of the axial carbonyl ligand in the in situ formed
ruthenium biscarbonyl deactivated complex. The high lability of axial CO ligand was conveniently
monitored through ATR-IR spectroscopy. Among various amines, diamines or polyamines with
primary/secondary diamine units were most efficient in CH3OH production. The reason for the
high methanol yields using these amine structures was due to the ability of the aforementioned
biscarbonyl complex of C-1 (C-1D) to selectively hydrogenate the corresponding formamides of
diamine/polyamines. A main deactivation pathway has been elucidated and a ligand-dependent
reactivity profile has been identified for several catalysts. With these new findings, our next focus
in this context is towards developing second generation pincer complexes with improved CO 2
hydrogenation to methanol turnover numbers.
2.7. Experimental Procedures
2.7.1. Materials and methods
All experiments were carried out under inert atmosphere using standard Schlenk techniques
with the exclusion of moisture. Ru-Macho-BH (C-1, Strem, 98%), Ru-Macho (C-2, Strem, 98%),
RuHClPNP
ipr
(CO) (C-3, Strem, 98%), RuHClPNP
Cy
(CO) (C-4, Strem, 98%), RuHClPNP
tBu
(CO)
(C-5, Strem, 98%), Carbonylhydrido[6-(di-t-butylphosphinomethylene)-2-(N,N-
diethylaminomethyl)-1,6-dihydropyridine]ruthenium(II) (C-7, Strem, 98%), Milstein acridine
complex (C-8, Strem, 98%) were used without purification. RuCl2PNP
Ph
(NHC) (C-6) and
RuHClPN MeP
Ph
(C-9) were prepared following reported procedures.
25,26
All solvents, triglyme
(Alfa Aesar) and water (DI) were degassed for 1 h prior to use. Ethylenediamine (1, Combi-Blocks,
98%), N,N’-dimethylethylenediamine (2, Combi-Blocks, 97%), N-methylethylenediamine (3,
Combi-Blocks, 98%), N,N-dimethylethylenediamine (4, Combi-Blocks, 97%), N,N,N’-
trimethylethylenediamine (5, Combi-Blocks, 98%), N-ethylhydroxyethylenediamine (6, Combi-
Blocks, 95%), N,N’-diethylhydroxyethylenediamne (7, TCI America) , piperidine (8, Sigma-
Aldrich), diethylenetriamine (9, Sigma-Aldrich, 98%) were sparged with N2 for 1 h prior to use.
43
Pentaethylenehexamine, (PEHA, 10, Sigma-Aldrich, 98%) was treated with vacuum for 1 h before
use. All other chemicals were purchased from commercial vendors and used without further
purification.
1
H and
13
C NMR spectra were recorded on 400 MHz, 500 MHz, and 600 MHz Varian
NMR spectrometers.
1
H and
13
C NMR chemical shifts were determined relative to the residual
solvent signals (dmso-d6) or internal standard (TMB). The gas mixtures were analyzed using a
Thermo 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%).1:3 CO2:H2 (Airgas, certified standard-spec
grade) was used without further purification. ATR-IR spectra was recorded on Jasco FT/IR-4600
spectrometer.
2.7.2. Standard procedure for hydrogenation reactions
The procedure as reported previously by our group for this reaction was followed.
2
Catalyst
C-1 to C-14, K3PO4, amine (1-17) and solvent (triglyme) were added in a nitrogen chamber to a
125 mL Monel Parr reactor equipped with a magnetic stir bar, thermocouple and piezoelectric
pressure transducer. After pressurizing the reactor with a CO2/3H2 mixture, the LabVIEW 8.6
software was used to monitor and record the internal temperature and pressure of the reactor. The
reaction mixture was stirred at room temperature (RT) for 30 min and then heated in a pre-heated
oil bath to an internal temperature of 145 °C After heating for a given amount of time, the reactor
was cooled to RT and the gas mixture analyzed by GC. A biphasic reaction mixture containing a
white oily material (lower layer) and a pale-yellow solution (upper layer) was obtained. Water was
added to the above mixture until all the oily material was dissolved resulting in a homogeneous
solution. 100 mg of 1,3,5-trimethoxybenzene was added as an internal standard to the reaction
mixture. This mixture was then analyzed by
1
H and
13
C NMR with a few drops of D2O to lock the
signals.
44
Figure 12.
1
H (A) and
13
C (B) NMR of the reaction mixture after hydrogenation
45
Figure 13. Typical GC spectra of the gas mixture after hydrogenation reaction
2.7.3. Procedure for mechanistic reactions
Catalyst C-1/ C-2/ C-3/ C-4/ C-5 (12 mg) was added to PEHA (10 mg), t-BuOK (22 mg)
and THF-d8 (1 ml) in a nitrogen chamber to a 125 mL Parr reactor equipped with a magnetic stir
bar, thermocouple and piezoelectric pressure transducer. After pressurizing the reactor with 75 bar
of a 1:3 CO2/H2 mixture, the LabVIEW 8.6 software was used to monitor and record the internal
temperature and pressure of the reactor. The reaction mixture was stirred at RT for 30 min and
then heated in an oil bath directly to 145 °C. After heating the reactor for 40 h, the reactor was
cooled to room temperature and the gas mixture was analyzed by ATR-IR. A pale-yellow solution
was obtained upon opening the reaction vessel. The pale-yellow solution was transferred into a J.
Young NMR tube under N2 atmosphere and analyzed by
1
H,
13
C and
31
P NMR.
2.7.4. Procedure for hydrogenation reactions using biscarbonyl complexes
Reaction solution obtained from the previously mentioned mechanistic studies (procedure
detailed under section 8.1), containing the biscarbonyl complexes were used directly for the
subsequent formamide hydrogenation reactions of Table 4. K3PO4, formamide (F-1/F-2), solvent
(triglyme), and reaction solution containing the biscarbonyl complex were added in a nitrogen
chamber to a 125 mL Monel Parr reactor equipped with a magnetic stir bar, thermocouple and
piezoelectric pressure transducer. After pressurizing the reactor with H 2, the LabVIEW 8.6
software was used to monitor and record the internal temperature and pressure of the reactor. The
46
reaction mixture was stirred at room temperature (RT) for 30 min and then heated in a pre-heated
oil bath to an internal temperature of 145 °C After heating for a given amount of time, the reactor
was cooled to RT and the gas mixture analyzed by GC. Upon opening the reaction vessel, a clear
solution was obtained (colorless or pale yellow). 100 mg of 1,3,5-trimethoxybenzene was added
as an internal standard to the reaction mixture. This mixture was then analyzed by
1
H and
13
C
NMR with a few drops of D2O to lock the signals.
2.7.5. X-ray crystallography
The X-ray intensity data were measured on a Bruker APEX DUO 3-circle platform
diffractometer equipped with a APEX II CCD detector, using MoKα radiation from a fine-focus
tube (λ = 0.71073 Å) monochromatized by a TRIUMPH curved-crystals monochromator. The
frames were integrated using a Bruker SAINT V8.18C algorithm. Data were corrected for
absorption effects using multi-scan method (SADABS). The structures were solved by intrinsic
phasing and refined using Bruker SHELXTL Software Package.
Selected bond lengths (Å) and angles (
o
): Ru(1)-P(1) 2.3952(15), Ru(1)-P(2) 2.3851(16), Ru(1)-
N(1) 2.180(4), Ru(1)-C(1) 1.870(5), Ru(1)-C(2) 1.971(5), P(1)-Ru(1)-P(2) 159.51(5), C(1)-
Ru(1)-C(2) 94.0(2), C(2)-Ru(1)-N(1) 97.42(19)
C-5D.HCO 3.3H 2O
Chemical formula C23H53NO8P2Ru
Formula weight 634.67 g/mol
Temperature 100(2) K
Wavelength 0.71073 Å
47
Crystal size 0.036 x 0.209 x 0.311 mm
Crystal habit clear colourless plate
Crystal system orthorhombic
Space group P b c a
Unit cell dimensions a = 12.463(7) Å α = 90°
b = 15.073(8) Å β = 90°
c = 32.221(17) Å γ = 90°
Volume 6053.(5) Å
3
Z 8
Density (calculated) 1.393 g/cm
3
Absorption coefficient 0.666 mm
-1
F(000) 2688
Selected bond lengths (Å) and angles (
o
): Ru(1)-P(1) 2.3351(9), Ru(1)-P(2) 2.3474(9), Ru(1)-
N(1) 2.187(3), Ru(1)-C(1) 1.870(3), Ru(1)-C(2) 1.980(3), P(1)-Ru(1)-P(2) 161.70(3), C(1)-
Ru(1)-C(2) 94.58(13), C(2)-Ru(1)-N(1) 93.67(12)
C-2D.HCO 3
Chemical formula C31H31NO5P2Ru
Formula weight 660.58 g/mol
Temperature 100(2) K
Wavelength 0.71073 Å
Crystal size 0.116 x 0.177 x 0.377 mm
48
Crystal habit clear colourless prism
Crystal system triclinic
Space group P -1
Unit cell dimensions a = 8.6522(17) Å α = 76.100(3)°
b = 12.368(2) Å β = 83.249(3)°
c = 16.043(3) Å γ = 79.846(3)°
Volume 1635.3(6) Å
3
Z 2
Density (calculated) 1.342 g/cm
3
Absorption coefficient 0.614 mm
-1
F(000) 676
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51
CHAPTER 3
Integrative CO
2
Capture and Hydrogenation to Methanol
with Reusable Catalyst and Amine
This dissertation chapter is based on a recent research article from our group published on the
Journal of the American Chemical Society (Kar, S.; Sen, R.; Goeppert, A.; Prakash, G. K. S., J.
Am. Chem. Soc. 2018, 140, 1580-1583. Part of the article is reprinted by permission of the
American Chemical Society. Copyright 2018. American Chemical Society
3.1. Introduction: Need for catalyst and amine recyling
Development of integrated CO2 capture and utilization (CCU) systems, wherein the
captured CO2 can be directly converted to value added products (in this case CH3OH), is an area
of enormous interest as it can bypass the otherwise intermediary and energy intensive desorption
and compression steps to produce pure CO2. For practical implementation, the separation and
recycling of the catalyst (generally the most expensive component) and capture material are also
essential to keep the entire process cost-effective. Traditional catalysts for CO2 hydrogenation to
CH3OH are heterogeneous and require high temperatures and pressures. This has led researchers,
in recent times, to look for homogeneous alternatives able to hydrogenate CO2 under much milder
conditions.
1-4
Over the last decade, with the appearance of new hydrogenation catalysts, significant
progresses have been made in both indirect and direct one-pot homogeneous CO2 to CH3OH
synthesis.
5-12
In 2015, Sanford et al. demonstrated a direct CO2 hydrogenation system to CH3OH
under basic reaction condition in the presence of dimethylamine.
13
The presence of an amine in
this system provides the opportunity to capture and hydrogenate CO2 in tandem, as was
demonstrated by our group (Figure 1).
14
Since the initial report by Sanford et al., multiple studies
have been published using various metal complexes for amine assisted hydrogenation of pure CO2
to CH3OH.
14-19
However, the integration of CO2 capture with subsequent hydrogenation to
CH3OH with easy recycling of the active elements has not been explored despite the importance
of such a process. These considerations have led us to search for a system where CO2 can be easily
52
and repeatedly captured and hydrogenated to CH3OH and the catalyst and capture materials can
be conveniently separated and reused.
Figure 1. CO2 capture from air and conversion to methanol
3.2. Recycling scheme
Most of the reported metal complexes that catalyze CO2 hydrogenation to CH3OH are air
sensitive and soluble in organic solvents, whereas the capturing amines are readily soluble in
aqueous medium. Aqueous solutions of amines and amino alcohols have long been utilized for
scrubbing CO2 from industrial gas streams.
20-25
Water is a desirable solvent for CO2 capture due to
its benign nature and its ability to enhance the absorption capacity of amines. Based on these facts
we envisioned a biphasic reaction solvent system, where after the hydrogenation step, the
regenerated amine and catalyst can be easily separated and recycled. The aqueous layer, containing
the amine, can be reused for CO2 capture (including from the air), whereas the organic layer,
containing the catalyst, can be reused for subsequent hydrogenation (Figure 2). The formed
CH3OH can be extracted from the reaction mixture through simple or vacuum distillation, due to
its volatile nature. Similar biphasic systems have been demonstrated recently by our group and
Leitner et al. for integrated CO2 capture and conversion to formate salts with reusable catalysts.
26-
27
53
Figure 2. Recycling scheme using biphasic solvent system
3.3. CO2 capture by various amines
For the purpose of CO2 capture in aqueous amine solutions, the employment of amines
with low vapor pressures is desirable to avoid atmospheric amine contamination. Keeping that in
mind, we decided to use high boiling polyamines for capture. In addition, two ethanolamines were
also tested (Table 1). The capture studies were performed by stirring the amine solutions under a
CO2 atmosphere (see footnote).
Table 1. CO2 capture by aqueous amine solutions
Entry Amine $/kg
a
CO 2 (mmol)/g
b
CO 2/N
c
1 PEHA 105 (S) 11.0 0.43
2 BPEI800 352 (S) 10.2 0.46
3 BPEI25k 320 (S) 10.4 0.47
54
4
d
PAA10k 7533 (P) 6.2 0.36
5
e
LPEI2.5k 56,000 (P) 5.5 0.25
6
e
LPEI100k 24,500 (P) 6.1 0.28
7 MEA 35 (S) 11.7 0.71
8 DEEDA 820 (T) 7.2 0.53
Capture conditions: Amine (1g), water (3 mL), stirring (800 rpm), rt. The aqueous amine
solutions were stirred in a CO2 atmosphere while maintaining the CO2 pressure at 1 psi.
Captured CO2 amounts were calculated through gravimetric analysis. Calculations error
±5%.
a
Prices from Sigma-Aldrich (S), Polysciences (P) or TCI America (T), as of Nov 14,
2017
b
CO2 captured per gram of amine
c
mols of CO2 captured per mol of nitrogen
d
commercial 15 wt% PAA10k aqueous solution was directly used.
e
10 mL water was used,
capture done at 70 °C
Among various polyamines, pentaethylenehexamine (PEHA), and branched
polyethyleneimines (BPEI) were found to be efficient for the CO2 capture. For example, under the
capture conditions, the aqueous PEHA solution captured a total of 11.0 mmol of CO 2 per g of
PEHA after 4 h, corresponding to 0.43 mol of CO2 captured per mol of amino group (Table 1,
entry 1). The
13
C NMR of the CO2 loaded aqueous PEHA solution revealed the presence of
carbamate and bicarbonate salts in the solution (Figure 3).
55
Figure 3.
1
H (top) and
13
C (bottom) NMR spectra of aqueous PEHA solution after CO2 capture.
Reaction conditions: PEHA (1g), H2O (3 mL).
Similar to PEHA, BPEI800 and BPEI25k captured 0.46 and 0.47 mol of CO2, respectively,
per mol of amino group (10.2 and 10.4 mmol of CO2/g, respectively) (entry 2-3). On the other
hand, CO2 capture by aqueous PAA10k solution was found to be slower, as after 4 h only 6.2 mmol/g
of CO2 had been captured, corresponding to 0.36 mol of CO2 per amino group (entry 4). The linear
polyethyleneimines (LPEI2.5k, LPEI100k) are costlier and display limited solubility in water at room
temperature, making them less convenient for CO2 capture in aqueous solutions. Nonetheless, in
this study, we decided to screen these LPEIs, mainly to investigate their abilities to assist
hydrogenation of the captured CO2 in the next step. With a large amount of water being used (10
mL), and at a higher temperature of 70 °C, LPEI2.5k and LPEI100k captured 5.5 and 6.1 mmol of
CO2/g, respectively. On the other hand, among the amines tested, monoethanolamine (MEA),
which has long been used industrially for scrubbing CO2 and H2S from flue gases, was found to
be most effective for CO2 capture both by mass and by efficiency of amine utilization (11.7
mmol/g; 0.71 CO2/N) (entry 7). Similarly, 7.2 mmol/g CO2 was captured by
56
diethanolethylenediamine (DEEDA) corresponding to 0.53 mol of CO2 per mol of amino group
(entry 8).
3.4. Hydrogenation of captured CO2 to methanol
Table 2. Tandem homogeneous hydrogenation of CO2 captured by aqueous amine solutions
Entry Amine Captured
CO 2 (mmol)
Catalyst
(µmol)
Formate
(%)
a
Formamide
(%)
a
MeOH
(mmol)
a
Yield
(%)
a
P MeOH
b
TON
1 PEHA 11.0 C-1(10) 11 10 5.2 47 0.16 520
2 PEHA 11.0 C-1(20) 5 2 8.7 79 0.17 435
3 BPEI800 10.2 C-1(20) 16 13 4.5 45 0.18 175
4 BPEI25k 10.4 C-1(20) 10 7 5.2 50 0.13 245
5 LPEI2.5k 5.5 C-1(20) 30 15 0.9 16 0.11 45
6 LPEI100k 6.1 C-1(20) 21 44 0.9 15 0.11 45
7 PAA10k 6.2 C-1(20) 32 38 0.1 2 0 5
8 MEA 11.7 C-1(20) 26 17 0 0 nd 0
9 DEEDA 7.2 C-1(20) 15 6 3.3 46 0.14 150
10
c
PEHA 11.0 C-2(20) 6 13 7.4 67 0.15 370
11
c
PEHA 11.0 C-3(20) 15 20 0.5 5 0 25
12
c
PEHA 11.0 C-4(20) 18 19 0.5 5 0 25
13
c
PEHA 11.0 C-5(20) 20 18 0.0 0 nd 0
14
d
PEHA 11.0 C-1(20) 12 11 5.7 52 0 285
15
e
PEHA 11.0 C-1(20) 9 6 6.3 57 0 315
16
f
PEHA 11.0 C-1(50) 3 0 10.4 95 0.17 208
17
g
PEHA 5.4 C-1(50) 5 0 4.8 89 0.11 96
Reaction conditions: Solutions from Table 1 (as specified) were directly hydrogenated after the addition of organic
solvent and the catalyst. 2-MTHF (5 mL), H2 (70 bar), 145 °C, 72 h.
a
Yields were determined based on
1
H NMR with
1,3,5-trimethoxybenzene (TMB) and imidazole (Im) as the internal standard for organic and aqueous layer
57
respectively.
b
PMeOH = methanol in organic layer/ methanol in aqueous layer
c
K3PO4 (1 mmol) was added
d
CPME was
used as organic solvent.
e
P-xylene was used as organic solvent
f
H2 (80 bar).
g
CO2 was captured from simulated air
(CO2 concentration: 408 ppm) with 0.79g PEHA. Yield calculations error ±5%. TON = mols of methanol formed per
mol of catalyst. nd = non-definable
3.4.1. Effect of different amines
Following the capture, the formed aqueous solutions were hydrogenated at 145 °C in the
presence of homogeneous catalysts and 70 bars of H2, after the addition of 5 mL 2-MTHF as an
additional solvent (Table 2). When Ru-MACHO-BH (C-1) (10 µmol) was used as the catalyst
along with PEHA as the capture material, a CH3OH yield of 47% (5.2 mmol) was observed after
72 h (entry 1). No concomitant CO/CH4 formation was observed through GC analysis of the
reaction gas mixture.
1
H and
13
C NMR revealed that 14% of formed CH3OH was present in the
upper organic layer, whereas the remaining CH3OH along with PEHA, formamide and formate
intermediates, was observed in the bottom aqueous layer (Figure 4).
58
Figure 4.
1
H spectra of organic (top) and aqueous (bottom) layers after hydrogenation reaction of
captured CO2 with aqueous PEHA solution (Table 2, entry 2) Reaction conditions: PEHA (1g),
H2O (3 mL), captured CO2 (11 mmol), C-1 (20 µmol), 2-MTHF (5 mL), H2 (70 bar), T = 145 °C,
72 h
The catalyst remained in the upper organic layer as observed by
31
P NMR (Figure 5),
indicating the possibility of easy catalyst separation from the biphasic mixture.
59
Figure 5.
31
P spectra of organic (top, A) and aqueous (bottom, B) layers after hydrogenation
reaction of captured CO2 with aqueous PEHA solution (Table 2, entry 1) Reaction conditions:
PEHA (1g), H2O (3 mL), captured CO2 (11 mmol), C-1 (10 µmol), 2-MTHF (5 mL), H2 (70 bar),
T = 145 °C, 72 h
Increasing the catalyst loading to 20 µmol increased the methanol yield to 79% (8.7 mmol),
along with decreased amounts of the intermediates (Table 2, entry 2). Next, various amine
solutions after CO2 capture (from Table 1) were hydrogenated in order to determine the most
promising amine for an integrated system for CO2 capture and hydrogenation (Table 2). Changing
the amine from PEHA to BEPI800 or BEPI25k decreased both the amounts of CH3OH formed (4.5
mmol and 5.2 mmol, respectively) and the hydrogenation yield (45% and 50%) (entry 3-4). For
LPEI2.5k, LPEI25k, and PAA10k, CH3OH yields decreased even more significantly to 0.9, 0.9, and
0.1 mmol, respectively (entry 5-7). Surprisingly, in the case of MEA, no CH3OH formation was
observed (entry 8). Instead, an increased amount of formamide and formate intermediates were
seen. We surmise that in the presence of a primary amine, such as PAA10k and MEA, the second
hydrogenation step of formamide to methanol becomes more challenging. Indeed, when the amine
was changed to a secondary analogue of MEA, DEEDA, methanol formation was observed in 46%
yield after 72 h (3.3 mmol; entry 9). Thus, among various amines, PEHA was found to be the most
60
efficient for the overall CO2 capture and conversion to CH3OH. The low vapor pressure and easy
availability of inexpensive PEHA (Table 1) make it promising for large scale CCU process to
methanol.
3.4.2. Effect of different hydrogenation catalysts
Next, we decided to screen different known homogeneous hydrogenation catalysts to
investigate their efficiency in this carbon capture and hydrogenation system (Table 2). Ru-
MACHO (C-2), expectedly, was found to be almost equally effective to Ru-MACHO-BH, in the
presence of an additional base K3PO4 (entry 10). The P -substituent in the PNP ligand was found
to heavily influence the CH3OH yield. When RuHClPNP
iPr
(CO) (C-3) was used as catalyst for the
hydrogenation, a meager 5% methanol yield was observed after 72 h (entry 11). Complex
MnBrPNP
iPr
(CO)2 (C-4), recently reported by our group to be active for sequential CO 2 to
methanol hydrogenation, was only capable of producing CH3OH in 5% yield (0.5 mmol; entry 12).
No methanol formed when FeHBrPNP
iPr
(CO) (C-5) was used as the catalyst (entry 13). An
accumulation of formamide and formate intermediates in the case of C-3 to C-5 suggests a low
activity of these catalysts for the effective hydrogenation of formamides and formates to CH 3OH
under the present reaction conditions.
3.4.3. Effect of different organic co-solvents
The most suitable choice of organic solvent for the biphasic hydrogenation reaction was
subsequently explored (Table 2). A slight decrease in methanol formation was observed when the
organic solvent was changed from 2-MTHF to cyclopentyl methyl ether (CPME) or p-xylene (52%
and 57%, respectively) (entry 14-15). Also, in case of the latter solvents, their higher
hydrophobicity compared to 2-MTHF resulted in an accumulation of the produced methanol
exclusively in the bottom aqueous layer. However, the lower solubility of C-1 in these solvents at
room temperature caused some of the catalyst to precipitate out from the solution during work-up,
making its complete recycling challenging. Hence, 2-MTHF was identified as the most convenient
solvent for repeated capture and utilization study. Using 2-MTHF, a CH3OH yield as high as 95%
was obtained with a higher C-1 loading of 50 µmol (entry 16). Finally, we showed that even CO2
from air can be captured and hydrogenated to CH3OH in high yields following this protocol (entry
17).
61
3.5. Recycling of catalyst and amine
With the optimized selection of the capturing amine (PEHA), catalyst (C-1), and organic
solvent (2-MTHF), two recycling studies were conducted to determine the recycling abilities of
the system. In the first study, only the catalyst was recovered from the organic layer and reused for
successive hydrogenation cycles (see section 3.7.5 for details). After four cycles, 95% of the
catalytic efficiency of the initial cycle of C-1 was retained, with a total of 40.5 mmol of CH3OH
formation, demonstrating the high recyclability of the catalyst, enabled by this biphasic system
(Figure 6, A). In a second study (Figure 3, B), both the catalyst and capturing amine were
recovered from the organic and aqueous layer, respectively, and reused for successive cycles. 89%
of the CO2 capture efficiency of the amine was retained in the third cycle, along with 87% of
methanol productivity. The slight loss in capture is most probably due to the presence of formate
species after the reaction and the loss of amine while transferring PEHA solution between
glassware.
Figure 6. Methanol formation with catalyst recycling (A) and catalyst and amine recycling (B).
Reaction conditions: After capture with 1 g PEHA in 3 mL water, H2 (80 bar), C-1 (50 µmol), 2-
MTHF (10 mL), 145 °C, 72 h. Methanol yields are calculated from
1
H NMR with Ph-CH3 and Im
(A)/ t-BuOH (B) as internal standards for organic and aqueous layers, respectively. Error in yield
calculations ±5%.
62
3.6. Conclusion
In conclusion, a tandem system for CO2 capture in aqueous amine solution and subsequent
hydrogenation to methanol is described where the catalyst and amine can be recycled multiple
times without significant loss in effectiveness. Among the catalysts tested, a well-defined and
commercially available complex, Ru-MACHO-BH (C-1) was found most effective. Among
various amines tested, a high boiling polyamine, PEHA, provided the best CH3OH yields. Our
next focus in the context of integrated CO2 capture and hydrogenation will be towards developing
a continuous CO2 to CH3OH conversion flow system.
3.7. Experimental Procedures
3.7.1. Materials and methods
All hydrogenation experiments were carried out under an inert atmosphere (with N2 or Ar)
using standard Schlenk techniques. Complexes Ru-MACHO-BH (C-1, Strem chemicals, 98%),
Ru-MACHO (C-2, Strem chemicals, 98%), and RuHClPNP
iPr
(CO) (C-3, Strem chemicals, 97%)
were used as received without further purification. Complexes MnBrPNP
iPr
(CO)2 (C-4) and
FeHBrPNP
iPr
(CO) (C-5) were prepared by previously reported methods. All catalysts and K3PO4
(Sigma-Aldrich) were weighed inside argon filled glove box. Pentaethylenehexamine (PEHA,
Sigma-Aldrich), BPEI800 (Sigma-Aldrich), BPEI25k (Sigma-Aldrich), LPEI2.5k (Polysciences),
LPEI100k (Polysciences), Poly(allylamine) (PAA10k, 15% in water, Polysciences), and
diethanolethylenediamine (DEEDA, TCI America) were used without further purification. 2-
Methyltetrahydrofuran (2-MTHF, BTC), cyclopentyl methyl ether (CPME, Sigma-Aldrich), p-
xylene (Sigma-Aldrich), water (deionized), toluene (drisolv.), and t-BuOH (Sigma-Aldrich), and
monoethanolamine (MEA, Fischer Scientific) were sparged with N2 for 1 h prior to use. 1,3,5-
trimethoxybenzene (TMB) (Sigma-Aldrich, >99%), imidazole (Fischer Scientific, 99.7%),
DMSO-d6 (CIL, D-99.9%), and D2O (CIL, D-99.5%) were used as received.
1
H and
13
C NMR
spectra were recorded on 400 MHz, Varian NMR spectrometers.
1
H and
13
C NMR chemical shifts
were determined relative to the residual solvent signals (DMSO-d6, D2O) or internal standard
(TMB/Im). The gas mixtures were analyzed using a Thermo 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 4.0), H2 (Gilmore, ultra-high pure grade 5.0) were used
without further purification.
63
3.7.2. Standard procedure for CO2 capture by aqueous amine solutions
Amines (1 g) were dissolved in water (3 mL) in a 30 mL vial equipped with a magnetic stir
bar. CO2 was added into the vial while stirring the aqueous solution at 800 rpm and maintaining
CO2 pressure inside the vial at 1 psi. The amounts of CO2 captured after 4 h were calculated
through gravimetric analysis of the aqueous solutions before and after the capture. For LPEI2.5k
and LPEI100k, 10 mL water was used with the vial placed in a water bath (70 °C) during CO2
capture. For PAA, the commercial 15% aqueous solution was directly used for capture.
3.7.3. Standard procedure for the hydrogenation reactions
The capture of CO2 in aqueous amine solutions were performed prior to hydrogenation
reactions. Upon completion of capture, in a nitrogen-filled chamber, the CO2 loaded amine
solution, catalyst C-1/C-2/C-3/C-4/C-5, K3PO4 (0 or 1 mmol), and solvent (5 mL) (2-MTHF,
CPME, or p-xylene) 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. The
internal temperature of the reaction mixture (145 °C), and pressure inside the reactor were
monitored and recorded through the LabVIEW 8.6 software. After heating for a given period of
time (72 h), 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 solution
was obtained with two distinct layers. After transferring the whole solution in a 15 mL centrifuge
tube, the two layers were separated. A known amount of TMB was added as an internal standard
to the top 2-MTHF layer which was then analyzed by
1
H and
13
C NMR with DMSO-d6 as the
deuterated solvent. A known amount of imidazole was added to bottom aqueous layer as internal
standard, followed by its analysis by
1
H and
13
C NMR with D2O as the deuterated solvent. Yields
were determined through
1
H NMR from integration ratios.
3.7.4. Standard Procedure of CO2 capture from air
A similar procedure to the one reported by our group previously was followed.
3
In a 30
mL vial, 0.79 g PEHA was dissolved in 15 mL water. Simulated air (N2/O2 = 80:20) containing
408 ppm CO2 was then bubbled through the solution at a flowrate 200 mL/min for 64 h. 5 mL of
additional water was added after 40 h. After the completion of CO2 capture, the final solution had
64
~5 mL of water. The resulting pale-yellow solution was then sparged with N2 for 30 mins.
Afterwards, 243 µL of THF was added to the solution as an internal standard and the amount of
CO2 captured was calculated through
13
C NMR. Another similar but separate capture experiment
was conducted, which was used for the hydrogenation reaction.
3.7.5. Procedure for recycling studies
3.7.5.1. Catalyst recycling study
The capture of CO2 in aqueous amine solutions were performed prior to hydrogenation
reactions. Upon completion of capture, in a nitrogen-filled chamber, the CO2 loaded PEHA
solution, catalyst C-1 (50 µmol), and 2-MTHF (10 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 (80 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. The internal temperature of the reaction mixture (145 °C), and pressure inside
the reactor were monitored and recorded through the LabVIEW 8.6 software. After heating for a
given period of time (72 h), the reactor was cooled to room temperature. The vessel was then
cooled in an ice bath for 30 minutes and the gas inside were slowly released in a N 2 atmosphere.
Upon opening the reaction vessel inside a N2 chamber, a solution was obtained with two distinct
layers. After transferring the whole solution in a 15 mL centrifuge tube, the two layers were
separated. The bottom layer was washed with additional 2 mL of 2-MTHF, and the combined
organic layer was transferred to a Schlenk flask. A known amount of toluene was added to the
organic layer, after which 0.1mL of organic solution was analyzed with
1
H and
13
C NMR with
DMSO-d6 as the deuterated solvent. The catalyst was recovered by removing the solvent, methanol
and toluene in vacuo. The recovered catalyst was used for the next cycle. Meanwhile, a known
amount of imidazole was added to bottom aqueous layer as internal standard, followed by its
analysis by
1
H and
13
C NMR with D2O as the deuterated solvent. Yields were determined through
1
H NMR from integration ratios of toluene and imidazole aromatic peaks with methanol proton
peak.
65
Figure 7. TON of C-1 in each successive cycle along with total TON in catalyst recycle study
The decrease in methanol yield (%) in successive reaction cycles is relatively limited and
most probably due to incomplete recycling. Between each cycle, ~1% catalyst was taken out from
reaction solution for NMR analysis resulting in a slight decrease in catalyst amount for the next
cycle. Furthermore, minor catalyst losses can happen during the transfer of reaction solutions
between glassware. The measurement of the amount of retrieved catalyst after first cycle was
attempted, however, the presence of traces of PEHA in the organic solution makes such
quantification challenging. The ruthenium presence in the aqueous solution due to catalyst
leaching was also investigated through
31
P NMR of the aqueous layer, which displayed no
31
P peak
in the spectra, signifying that the catalyst leaching was probably not a significant factor in catalyst
loss.
3.7.5.2. For catalyst and amine recycling study
The capture of CO2 in aqueous amine solution was performed prior to hydrogenation
reaction. Upon completion of the capture step, in a nitrogen-filled chamber, the CO2 loaded PEHA
solution, catalyst C-1 (50 µmol), and 2-MTHF (10 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 (80 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. The internal temperature of the reaction mixture (145 °C), and pressure inside
208 204 200 198
412
612
810
0
100
200
300
400
500
600
700
800
900
1 2 3 4
TON
Cycle
TON(Cycle) TON(Total)
66
the reactor were monitored and recorded through the LabVIEW 8.6 software. After heating for a
given period of time (72 h), 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 inside a N2 chamber, a solution was obtained with two distinct layers. After
transferring the whole solution in a 15 mL centrifuge tube, the two layers were separated. The
bottom layer was washed with additional 2 mL of 2-MTHF, and the combined organic layer was
transferred to a Schlenk flask. A known amount of toluene was added to the organic layer, after
which 0.1mL of organic solution was analyzed with
1
H and
13
C NMR with DMSO-d6 as the
deuterated solvent. The catalyst was recovered by removing the solvent, methanol and toluene in
vacuo. The recovered catalyst was used for the next cycle. Meanwhile, a known amount of t-BuOH
was added to bottom aqueous layer as internal standard, followed by its analysis by
1
H and
13
C
NMR with D2O as the deuterated solvent. The amine was then recovered from the solution by
removing MeOH, water, and t-BuOH in vacuo. The recovered amine was then dissolved in 3 mL
water and was further used in CO2 capture for next cycle. Yields were determined through
1
H
NMR signal integration ratios.
Figure 8. TON of C-1 in each successive cycle along with total TON in catalyst and amine
recycle study
208
194
180
402
582
0
100
200
300
400
500
600
700
1 2 3
TON
Cycle
TON(Cycle) TON(Total)
67
3.8. References
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Ed. 2012, 51, 7499-7502.
9. 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.
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68
11. 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.
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Carbon Dioxide to Methanol: The Aqueous Catalytic Way at Room Temperature. Chem. Eur. J.
2016, 22, 15605-15608.
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Hydrogenation of CO2 to Methanol. J. Am. Chem. Soc. 2015, 137, 1028-1031.
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Am. Chem. Soc. 2016, 138, 778-781.
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N-Formylation of Amines with H2 and CO2. Angew. Chem. Int. Ed. 2015, 54, 6186-6189.
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Sequential Hydrogenation of CO2 to Methanol via Formamide. ACS Catal. 2017, 7, 6347-6351.
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single-pot conversion using a C-scorpionate iron(II) catalyst. Green Chem. 2017, 1-5.
18. Everett, M.; Wass, D. F., Highly productive CO2 hydrogenation to methanol - a tandem
catalytic approach via amide intermediates. Chem. Commun. 2017, 53, 9502-9504.
19. Sordakis, K.; Tang, C.; Vogt, L. K.; Junge, H.; Dyson, P. J.; Beller, M.; Laurenczy, G.,
Homogeneous Catalysis for Sustainable Hydrogen Storage in Formic Acid and Alcohols. Chem.
Rev. 2017, 118, 372-433.
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Capture by Aqueous Amines. Ind. Eng. Chem. Res. 2006, 45, 2457-2464.
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Distinct Amines Important for the Absorption of CO2 and Regeneration in Aqueous Solution. Ind.
Eng. Chem. Res. 2003, 42, 3179-3184.
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25. Yu, C.-h.; Huang, C.-h.; Tan, C.-s., A Review of CO2 Capture by Absorption and
Adsorption. Aerosol Air Qual. Res. 2012, 12, 745-769.
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capture by amines in aqueous media and its subsequent conversion to formate with reusable
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70
CHAPTER 4
Combined CO
2
Capture and Hydrogenation to Methanol:
Amine Immobilization Enables Easy Recycling of Active
Elements
This dissertation chapter is based on a recent research article from our group published on
ChemSusChem (Kar, S.; Goeppert, A.; Prakash, G. K. S., ChemSusChem 2019, 0,
doi:10.1002/cssc.201900324). Part of the article is reprinted by permission of the John Wiley and
Sons. Copyright 2019. John Wiley and Sons.
4.1. Introduction: Amine-assisted CO2 to methanol
The development of technologies for the capture of CO2 from the atmosphere and point
sources is of paramount importance in view of the continued increase in atmospheric CO 2 level
and its negative effect on the biosphere.
4-8
Once captured, it would be desirable to utilize the CO2
as a feedstock for the production of value added materials.
9-11
One such important value-added
product is methanol, which can itself be used as fuel, fuel additive, or as C1 source in organic
synthesis.
12-15
Traditionally, CO2 hydrogenation to CH3OH is performed with a Cu/ZnO/Al2O3
based heterogeneous catalyst at high temperatures and pressures.
16-20
However, in recent years,
homogeneous molecular catalysts have also been developed to hydrogenate CO2 under milder
conditions.
1-3, 19, 21-32
In 2015, the Sanford group reported the synthesis of CH3OH from CO2 with molecular H2
catalyzed by Ru-Macho-BH in the presence of dimethylamine.
1
The reaction proceeded through
the formation of a formamide as an intermediate, whose subsequent reduction produced CH3OH
(Figure 1a). The presence of an amine in this reaction system allowed for a tandem CO2 capture
and conversion to CH3OH. This was demonstrated by our group using a high boiling polyamine
PEHA as the CO2 capturing agent, followed by hydrogenation with a Ru-PNP catalyst.
2
With this
method, CO2 captured from air was successfully converted to CH3OH (Figure 1b). The
71
recyclability of the catalyst and amine in the system, necessary for any large-scale industrial
implementation, was also recently demonstrated by our group using a biphasic 2-MTHF/water
solvent system which has been discussed in the previous chapter.
3
Figure 1. (a) amine assisted CO2 hydrogenation to CH3OH (b) combined CO2 capture and
conversion
4.2. Recycling scheme based on amine immobilization
Apart from the usage of biphasic systems, the immobilization of catalyst and/or other
homogeneous components onto solid supports have been a popular and fruitful approach to
combine the advantages of homogeneous reactions (high selectivity, rational design) with that of
heterogeneous reactions that enable easy separation of the components.
33-35
Herein, we report such
a system for amine assisted CO2 capture and conversion to CH3OH, where easy recycling of both
the catalyst and amine is achieved through immobilization of the amine component onto a solid
support. Solid supported amines (SSA) have been used extensively in recent years as possible
alternatives to traditional aqueous amine solutions for CO2 scrubbing.
36-43
Their highly reversible
nature for CO2 absorption and desorption, along with the absence of solvent required for capture,
makes their use less energy intensive and more environment friendly as compared to traditional
systems. In this study, we demonstrate that these solid supported amines can also be employed for
tandem CO2 capture and utilization. The CO2 loaded solid amines were placed inside a vessel
pressurized with H2 in presence of an active catalyst and solvent to produce CH3OH. The amines
72
being tethered to the solid support, can subsequently be easily separated from the reaction solution
by filtration and reused for the next CO2 capture cycle. Meanwhile, the catalyst present in the
filtrate can be recovered by simply removing the solvent and the formed CH3OH under vacuum
and reused for the next hydrogenation cycle. A schematic diagram for the above-mentioned system
is shown in Figure 2.
Figure 2. Schematic diagram for combined CO2 capture and conversion to methanol with SSA
(solid supported amine) and catalyst recycling.
4.3. Finding the optimal solid supported amine
In the initial stage of this study, we focused on the suitability of different SSAs for the
amine-assisted CO2 hydrogenation to methanol process. To this extent, two parameters are of
major importance for the use of SSAs through multiple cycles: their ability to assist in the CH3OH
formation (for high yield) and leaching of amines from the support under the hydrogenation
conditions (with minimal leaching desired for continuous use). Eight solid supported amines were
screened, as part of three different preparation methods (Figure 3). SSA sample 1, 2, and 3 were
prepared through physical impregnation of BPEI (Mw: 25K), LPEI (Mw: 25K), and LPEI
(Mw:2.5K), respectively, on fumed silica (class 1; see section 4.7.2. for the detailed preparation
procedures of all SSAs).
38, 44-48
On the other hand, 4 was prepared by covalently binding
polyethyleneimine on the solid support (class 2).
49-52
Solid supported amines 5 and 6 also
represented covalently attached amine (diethylenetriamine, and tris(aminoethyl)amine,
respectively) onto a solid polymer support. For 7 and 8, pentaethylenehexamine (PEHA) was
crosslinked with glyoxal and glycerol diglycidyl ether, respectively, in an attempt to obtain SSAs
73
similarly stable as 1-3, while using an inexpensive polyamine PEHA (class 3).
53-54
The
crosslinking was performed in-situ, in the presence of the solid support (fumed silica). Using these
various supported amines, amine assisted CO2 hydrogenation was performed and the amount of
CH3OH formed was measured through
1
H NMR using an internal standard. The organic contents
of the solid amines were measured through thermogravimetric analysis before and after the
reaction to calculate the extent of leaching. The hydrogenation was performed in a CO2: 3H2 (80
bar) gas mixture with Ru-Macho-BH (C-1) as catalyst, THF as the solvent and at a reaction
temperature of 145
o
C (Table 1).
Figure 3. Solid supported amines used in this study. The numbers in parenthesis for 1-4 represents
the percentage of organic content
74
4.3.1. Methanol formation with different SSAs
As can be seen in Table 1, all SSAs were able to assist in the hydrogenation of CO2 to
CH3OH with varying degrees of success. The highest amount of CH3OH was observed with
physically impregnated silica supported amines (entry 1-3). The highest TON of 520 was observed
when SSA 3 (FS-LPEI2.5K) was used. However, the resulting
1
H spectra also revealed the presence
of some leached amines in the solution (Figure S2). The leaching was also verified through TGA
by measuring the organic content of the adsorbent before and after the reaction (vide infra). For
covalently grafted amines, lower CH3OH yields were in general observed (entry 4-6). Excitingly,
the
1
H NMR spectra of the filtrate after the reaction did not show any traces of amine in the
solution. For in situ crosslinked solid supported amines, 7 provided a higher methanol yield as
compared to 8 (4.5 and 1.0 mmol, respectively) (entry 7-8). Control studies revealed that both the
presence of catalyst and SSA are required for formamide or methanol formation (entry 9-10).
Table 1. SSA screening for CO2 hydrogenation
Entry SSA mmol N/g
SSA
SSA used
(g)
Formamide
(mmol)
[a]
CH 3OH
(mmol)
[a]
TON MeOH
1 1 10.5 0.49 0.3 5.1 510
2 2 10.5 0.49 0.4 2.9 290
3 3 9.8 0.52 0.2 5.2 520
4 4 10.0 0.50 - 0.9 90
5 5 4.7 1.11 - 1.4 140
75
Reaction conditions. SSA (as specified; amine functionality present during reaction ~ 5 mmol), Cat. (10
µmol), THF (5 mL), CO2: 3H2 (80 bar), 145
o
C, 40h.
[a]
Yields were calculated from
1
H NMR spectra with
1,3,5-Trimethoxybenzene (TMB) as an internal standard.
[b]
in the absence of catalyst. Yield calculations
error: ±5%. TON = moles of methanol produced per mole of catalyst
4.3.2. Amine leaching in different SSAs during the reaction
Through TGA of the filtered solids after the reaction, amine leaching was observed in the
case of physically impregnated solid supported amines. An organic content loss of 11%, 18%, and
17% was measured for 1, 2, and 3, respectively (Figure 4).
55
6 6 4.2 1.19 0.1 0.7 70
7 7 9.0 0.57 0.8 4.5 450
8 8 8.0 0.63 0.4 1.0 100
9 - - - 0 0 0
10
[b]
4 10.0 0.50 0 0 -
76
Figure 4. Percent of organic content in the SSAs before (blue) and after (red) the hydrogenation
reaction as observed through TGA.
In the
1
H NMR of the filtrate, the leached amines were observed along with traces of
formamide products (Figure 5).
Figure 5.
1
H spectra of the reaction mixture filtrate with SSA-2 (Table 1, entry 2). The formamide
products and the amine present in solution due to leaching are shown in the inserts.
77
Figure 6.
1
H spectra of the reaction mixture filtrate with SSA-4 (Table 1, entry 4). The absence of
free amine in the solution is highlighted in the insert.
On the other hand, with covalently grafted solid adsorbent 4, no leached amine was
detected by
1
H NMR (Figure 6). The TGA showed the presence of 49% organic content in the
filtered solid as compared to 46% organic content measured before the reaction. The increased
organic content is due to partial formylation of existing amino groups, which is also an
intermediate to methanol synthesis (Figure 1a). Indeed, the formyl groups were detected in the IR
spectra of the collected solid (Figure 7B). When this collected solid of 4 after the hydrogenation
reaction was treated with 80 bar H2 at 145 ℃ for 24 h, 0.4 mmol of CH3OH was observed due to
the hydrogenation of the formamide groups present on the surface. This filtered solid-amine
contained 45% of organic content, and in the IR spectra the formamide carbonyl stretches had
disappeared, signifying their successful hydrogenation (Figure 7C). Also, no leaching was
detected by
1
H NMR analysis of the filtrate.
78
Figure 7. Observation of formamide intermediates through ATR-IR and their subsequent
hydrogenation
No amine leaching was observed with either covalently grafted polymer supported amine
5 and 6 though
1
H NMR. However, due to the organic nature of the supporting polymer, TGA was
not the most suitable technique for the measurement of leaching in these cases. Among the in situ
polymerized tethered amines, glycerol diglycidyl ether was found to be a good linker as the
resultant SSA 8 suffered from only 9% loss in organic content (Figure 4). On the other hand,
drastic leaching (66%) was observed for 7 where glyoxal was used as the crosslinking agent. It
should be noted here that the high methanol yield for physically impregnated SSAs and 7 are most
probably due to free amine in the reaction solution resulting from leaching, as a positive correlation
between the extent of leaching and methanol yield can be observed from Table 1 and Figure 4.
4.4. Screening of different catalysts
Owing to the minimal leaching observed, covalently attached solid amines were
subsequently selected for further experiments. First, we decided to screen different hydrogenation
catalysts for the solid amine supported CO2 hydrogenation to methanol. SSA 4 was identified as a
suitable solid supported amine due to its high leaching resistance and high amine content. As can
be seen in Figure 8, under the reaction conditions, Ru-Macho-BH (C-1) provided the best CH3OH
79
yield with 0.9 mmol CH3OH observed after 40 h (TON = 90). With RuHClPNP
Ph
(CO) (Ru-Macho,
C-2), a similar TON (80) was obtained in the presence of 0.5 mmol of K3PO4 as an initial activator.
On the other hand, the CH3OH yield decreased significantly with RuHClPNP
iPr
(CO) catalyst (C-
3, TON = 15). Thus, the Ph substitution at the P atom in C-1 and C-2 plays an important role in
methanol formation. The manganese-based catalyst MnBrPNP
iPr
(CO)2 (C-4) also provided less
methanol (0.08 mmol; TON = 8) compared to C-1 and C-2.
Figure 8. Catalyst screening for CH3OH formation. Reaction conditions: 4 (500 mg), Cat. (10
µmol), K3PO4 (0.5 mmol), THF (5 mL), CO2:3H2 (80bar), 145
o
C, 40 h. Reaction with C-1 was
carried out in the absence of K3PO4. Yield calculations error: ±5%
80
4.5. Combined capture and hydrogenation with catalyst and amine
recycling
From the above catalyst and amine screening studies, C-1 was clearly identified as the best
hydrogenation catalyst, along with covalently attached solid supported amines (4, 5, 6) as the most
suitable immobilized amines due to the minimal leaching under hydrogenation conditions. The
CO2 capture ability of 4, 5 and 6 was subsequently explored and 4 was found to have significantly
higher capture efficiency for CO2 than 5 and 6 (Table 2). This is to be expected as 4 contains a
much higher number of nitrogen atoms per unit weight (~10 mmol/g) compared to 5 and 6 (4.7
and 4.2 mmol/g, respectively). Therefore, C-1 and 4 were selected for combined capture and
hydrogenation studies.
Table 2. CO2 capture efficiency of different covalently attached solid supported amine
SSA
CO 2 captured
(mg CO 2/g SSA)
CO 2/N
a
4
b
87 ~0.2
5 18 ~0.09
6 14 ~0.076
Capture conditions: SSA (5 g) in a 30 mL vial, CO2 pressure (1.07 atm absolute
pressure), CO2 was injected at a flow-rate of 30 mL.min
-1
when the inside pressure
decreased below 1 psi above atmospheric pressure.
a
moles of CO2 captured per
mole of nitrogen atoms
b
capture done at 85
o
C
CO2 capture was performed at 85
o
C under pure CO2. As shown in Table 3, 4 captured 87
mg of CO2 per g SSA in the first cycle. When the resulting CO2 loaded 4 was hydrogenated at 145
o
C and 80 bar H2 pressure in the presence of C-1 (50 µmol), 1.9 mmol (yield = 20%) of CH3OH
was observed after 40 h. The solid was collected from the reaction mixture through filtration and
reused for CO2 capture in the next cycle. The catalyst was recovered from the liquid filtrate by
removing the solvent in vacuo. TGA studies of the collected solid showed minimal loss in organic
content. Surface analysis by N2 adsorption/desorption isotherm measurements revealed no
significant changes in surface area and pore volume (Table 3); although the collected solid became
more powdery as compared to larger solid aggregates before the reaction. The organic content,
surface area and pore volume of the collected SSA after each hydrogenation cycle is shown in
81
Table 3. The collected solid after the first cycle was again employed for CO2 capture. The capture
efficiency decreased somewhat to 57mg CO2/g SSA, which could be due to changes in the
macrostructure of the adsorbent. Upon hydrogenation of the captured CO2 a methanol yield similar
to the one in the first cycle was observed (1.9 mmol, 32%). Again, surface analysis did not show
any significant change in the microstructure of the solid (Figure 9).
Table 3. Physical properties of the solid adsorbent and methanol yield after each hydrogenation
cycle
Cycle 1 Cycle 2 Cycle 3 After Cycle
3
Organic content
before
[a]
(%)
46 45 42 42
Avg. surface area
[b]
(m
2
/g)
52.4 46.4 55.8 56.3
Avg. pore volume
[c]
(cc/g)
0.385 0.372 0.437 0.404
CO 2 capture
[d]
(mg/g) 87 58 46 -
Methanol yield
[e]
(mmol)
1.9 1.9 1.4 -
Methanol yield
[f]
(%) 20 32 34 -
Organic content
after
[a]
(%)
45 42 42 -
Hydrogenation conditions: After CO2 capture with 5g of 4 at 85
o
C, H2 (80 bar), THF (40 mL), Cat (50
µmol), 145
o
C, 40 h
[a]
measured through TGA
[b]
measured through multi point MBET method
[c]
measured through BJH desorption method
[d]
measured from gravimetric analysis
[e]
measured from
1
H NMR spectra
[f]
percentage methanol yield is based on the total CO2 captured by the SSA in that
cycle. Yield calculations error: ±5%
82
Figure 9. Pore size distribution of solid supported amine 4 after each hydrogenation cycle
The CO2 capture efficiency decreased less from the subsequent cycle onwards, while the
methanol yield remained similar. A comparable experiment was also conducted where, instead of
pure CO2, CO2 was captured from air at room temperature with 4 and hydrogenated to methanol.
As expected, the intake capacity of 4 decreased when CO2 form air (at r.t.) was captured (36mg
and 28mg CO2/g SSA in the 1
st
and 2
nd
cycle, respectively) as compared to pure CO2 at 85
o
C;
however, after hydrogenation of the captured CO2, similar methanol yields were observed (24%
and 29% in the 1
st
and 2
nd
cycle, respectively) (Figure 10).
83
Figure 10. Methanol formation in different cycles for combined CO2 capture from air and
hydrogenation. Reaction conditions: After capture with 10 g SSA-4, H2 (80 bar), C-1 (50 µmol),
THF (40 mL), 145 °C, 40 h. Yields are calculated from the
1
H NMR with toluene as an internal
standard. Error in yield calculations ±5%.
4.6. Conclusion
In conclusion, we have demonstrated that solid supported amines can be used for tandem
CO2 capture and hydrogenation to CH3OH catalyzed by the homogeneous ruthenium pincer
complex Ru-Macho-BH. The immobilization of the amine enables convenient separation of the
amine and catalyst after the reaction, resulting in their easy reusability. One significant challenge
for the proper implementation of this process is the possible leaching of the amine under the high
reaction temperature. This was minimized by using covalently bonded solid supported amines.
With such amines, the catalyst and SSA were successfully recycled over multiple cycles. Our
group is currently working on improving the stability of the SSA under the reaction conditions as
well as methanol yields, and the findings will be reported in due course.
4.7. Experimental Procedures
4.7.1. Materials and methods
All experiments were carried out under inert atmosphere using standard Schlenk techniques
with the exclusion of moisture. Ru-Macho-BH (C-1, Strem, 98%), Ru-Macho (C-2, Strem, 98%),
RuHClPNP
ipr
(CO) (C-3, Strem, 97%) were used without purification. MnBrPNP
iPr
(CO)2 (C-4)
84
was prepared following a reported procedure.
56
Anhydrous THF (Drisolv) was degassed for 1 h
prior to use. Branched polyethylenimine BPEI25K (molecular weight (Mw) ca. 25000), linear
polyethylenimine LPEI2.5K with a Mw of 2500, pentaethylenehexamine (PEHA) and glycerol
diglycidyl ether were purchased from Sigma-Aldrich. LPEI with a Mw of 25000 (LPEI25K) was
purchased from Alfa Aesar. Fumed silica Aerosil® 380 (average primary particle size 7 nm) was
obtained from Evonik. The synthesis of the SSAs is reported in the next section.
Diethylenetriamine, polymer-bound (PS-DETA, with a N loading of 4.7 mmol/g) SSA-5 and
tris(2-aminoethyl)amine, polymer-bound (PS-TREN, with a N loading of 4.2 mmol/g) SSA-6 were
purchased from Sigma-Aldrich and used without purification. Trimethoxysilylpropyl-
polyethyleneimine (50% in iPrOH) was obtained from Gelest. All other chemicals were purchased
from commercial vendors and used without further purification.
1
H and
13
C NMR spectra were
recorded on 400 MHz, 500 MHz, or 600 MHz Varian NMR spectrometers.
1
H and
13
C NMR
chemical shifts were determined relative to the residual solvent signals (DMSO-d6) or internal
standard (TMB). 1:3 CO2:H2 (Airgas, certified standard-spec grade) and H2 (Gilmore, ultra-pure)
were used without further purification. ATR-IR spectra was recorded on a Jasco FT/IR-4600
spectrometer.
• Thermogravimetric analysis
Thermogravimetric measurements were carried out on a Shimadzu TGA-50
Thermogravimetric Analyzer under an air flow of 30 mL·min
-1
in a temperature range from 25 to
800
o
C with a heating rate of 10
o
C min
-1
(temperature holding time at 800
o
C = 10 min). The initial
weight loss observed below 150°C was attributed to the loss of adsorbed moisture and CO 2. The
weight loss from 150 to 800 °C was counted towards the loss of organic content.
• Pore structure analysis
Pore and surface properties of covalently tethered SSA-4 after each hydrogenation cycle were
characterized by nitrogen adsorption, using a Quantachrome NOVA 2200e instrument. First, the
adsorbents were degassed under vacuum at 85
o
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. The total pore volume (PV) was determined at
P/Po close to 1. Average pore diameters of the adsorbents were estimated with the surface area and
85
pore volume data, assuming all pores are cylindrical-shaped. Pore size distribution was derived
from the desorption branch of the nitrogen isotherm with the Barrett-Joyner-Halenda (BJH)
method.
4.7.2. Procedure for the preparation of solid supported amine (SSA)
adsorbents
Solid supported amines 1-3 [class 1]
For the preparation of SSA-1, 25 g of branched polyethyleneimine BPEI25K was dissolved
in 300 mL of methanol. This solution was then added dropwise under stirring to 25 g of fumed
silica (Aerosil-380) suspended in 800 mL methanol to ensure a good dispersion of the
polyethyleneimine on the support. The solution was then mixed for overnight. After that, the
solvent was removed from the mixture by heating at 50°C under vacuum on a rotavapor followed
by vacuum treatment overnight (< 1 mm Hg). The supported amine sorbent was obtained as a
white solid. The organic content was subsequently calculated through TGA. A similar procedure,
although on a smaller scale due to the cost of LPEI, was used for the preparation of SSA-2 with
LPEI25K and SSA-3 with LPEI2.5K.
Solid supported amine 4 [class 2]
For the preparation of SSA-4, a procedure reported previously by our group was
followed.
49
Aerosil-380 (7.0 g) was mixed in 280 mL toluene and N2 was bubbled through the
vigorously stirred suspension for 30 min. 19.95 mmol of silane coupling agent
(trimethoxysilylpropyl-polyethyleneimine, 50% in i-PrOH, was added dropwise and the reaction
mixture was stirred at room temperature for 10 min before heating at 110
o
C. The suspension was
stirred at this temperature under a nitrogen atmosphere for 12 h. The cold reaction mixture was
separated by centrifugation and the obtained solid was again resuspended in cold toluene. The
suspension was again separated by centrifugation. The above described purification steps were
repeated three times using cold methanol as a solvent. Finally, the solid was transferred to a round
bottom flask and the solvent was removed by heating at 50
o
C under vacuum followed by vacuum
treatment overnight.
Solid supported amines 7-8 [class 3]
86
For the preparation of SSA-7, PEHA (5 g) and fumed silica were mixed in 70 mL of
distilled water in a 250 mL round bottom flask. 1.25 g of glyoxal (as 40% aqueous solution) was
then added dropwise to the above mixture (PEHA: glyoxal = 1: 1). The resulting solution was left
overnight with stirring. Afterwards, the solution was heated to 60
o
C for 2 h while stirring
vigorously. Water was subsequently removed from the solution using a rotovapor to afford a white
solid. The white solid was dried further by vacuum treatment overnight and the organic content of
the solid adsorbent was measured through TGA.
For the preparation of SSA-8, PEHA (2.4 g) and fumed silica were mixed in 70 mL of
distilled water in a 250 mL round bottom flask. 2.1 g of glycerol diglycidyl ether (diluted in 15
mL water) was then added dropwise to the above mixture (PEHA: glycerol diglycidyl ether = 1:
1). The resulting solution was left overnight with stirring. Afterwards, the solution was heated to
60
o
C for 2 h while stirring vigorously. Water was subsequently removed from the solution using
a rotovapor to afford a white solid. The white solid was dried further with vacuum treatment
overnight and the organic content of the solid adsorbent was measured through TGA.
Figure 11. TGA curve of the solid supported amines. For the calculation of organic content, the
loss of weight beyond 150
o
C was considered.
0
20
40
60
80
100
0 100 200 300 400 500 600 700 800
Residual weight/%
Temperature/
o
C
SSA-2 SSA-3 SSA-4 SSA-5
SSA-6 SSA-7 SSA-8
87
4.7.3. Procedure for hydrogenation reactions
A procedure reported previously by our group for this reaction was followed.
2
The catalyst,
SSA and solvent (THF) were added in a nitrogen chamber to a 125 mL Monel Parr reactor
equipped with a magnetic stir bar, thermocouple and piezoelectric pressure transducer. After
pressurizing the reactor with a CO2/3H2 mixture, the LabVIEW 8.6 software was used to monitor
and record the internal temperature and pressure of the reactor. The reaction mixture was stirred
at room temperature (RT) for 30 min and then heated on a pre-heated oil bath directly to 145 °C.
After heating for a given amount of time, the reactor was cooled in an ice bath and the gases
released. A reaction mixture containing the solid SSA in a solution was obtained upon opening the
reaction vessel. 20 mg of 1,3,5-trimethoxybenzene was added as an internal standard to the
reaction mixture and stirred for 5 mins. A small portion (~ 0.1 mL) of the mixture was then filtered
through cotton and the filtrate was analyzed by
1
H and
13
C NMR with DMSO-d6 as the deuterated
solvent. Afterwards, all the solid supported amine was isolated through filtration and dried. A
sample of the collected solid was then analyzed through TGA to measure the organic content.
4.7.4. Procedure for combined CO2 capture and hydrogenation
5 g of solid supported amine was placed into a 30 mL vial. The SSA was then treated with
vacuum at 85
o
C for 10 min in a heated aluminum block. Afterwards, CO2 was introduced into the
vial while maintaining the temperature at 85
o
C and the CO2 pressure inside the vial at 1 psi above
the atmospheric pressure. After the intake of CO2 by the SSA stopped (typically about 10 min),
the vial was taken out of aluminum block and allowed to cool down for 30 min under an
atmosphere of CO2. The amount of CO2 captured was calculated through gravimetric analysis of
the SSA before and after the capture.
Subsequently, the CO2-loaded SSA was transferred to a Monel Parr reactor under N2
atmosphere along with the catalyst and THF solvent. The reactor was then pressurized with 80 bar
H2 and placed in an oil bath maintaining the internal temperature at 145
o
C for 40 h. After that, the
reactor was cooled down in an ice bath and the gases were released. The reactor was then opened
in a N2 atmosphere. 1 mL of toluene was added as an internal standard to the reaction mixture and
stirred for 5 mins. A small portion (~ 0.1 mL) of the mixture was then filtered through cotton and
the filtrate was analyzed by
1
H and
13
C NMR with DMSO-d6 as the deuterated solvent. Next, all
the solid supported amine was filtered, collected, dried and reused for the next CO2 capture cycle.
88
A sample of the collected solid was analyzed through TGA, BET and IR. The catalyst was
recovered from the filtrate by removing THF, methanol and toluene in vacuo and reused for the
next cycle of hydrogenation.
Figure 12.
1
H NMR spectra of the solution after combined CO2 capture and hydrogenation in the
first cycle
4.7.5. Procedure for combined CO2 capture from air and hydrogenation
10 g of SSA-4 was poured into a cylindrical column with an inside diameter of 1 cm. Cotton
was used on top and bottom of the SSA to prevent spills. Synthetic air (CO 2 level: 410 ppm) was
passed through the column with a flow-rate of 400 mL·min
-1
for 64 h at room temperature. The
amount of captured CO2 was calculated through gravimetric analysis. Subsequently, the CO2-
loaded SSA was transferred to a Monel Parr reactor under N2 atmosphere along with the catalyst
and THF solvent. The reactor was then pressurized with 80 bar H2 and placed in an oil bath to
maintain the internal temperature at 145
o
C for 40 h. After that, the reactor was cooled down in an
ice bath and the gases released. The reactor was opened in a N2 atmosphere. 0.5 mL of toluene was
added as an internal standard to the reaction mixture and stirred for 5 mins. A small portion (~ 0.1
mL) of the mixture was then filtered through cotton and the filtrate was analyzed by
1
H and
13
C
89
NMR with DMSO-d6 as the deuterated solvent. Next, all the solid supported amine was filtered,
collected, dried and reused for the next CO2 capture cycle. The catalyst was recovered from the
filtrate by removing THF, methanol and toluene in vacuo and reused for next cycle of
hydrogenation.
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94
CHAPTER 5
Manganese-Catalyzed Sequential Hydrogenation of CO
2
to
Methanol via Formamide
This dissertation chapter is based on a recent research article from our group published on ACS
Catalysis (Kar, S.; Goeppert, A.; Kothandaraman, J.; Prakash, G. K. S., ACS Catal. 2017, 7, 6347-
6351). Part of the article is reprinted by permission of the American Chemical Society. Copyright
2017. American Chemical Society
5.1. Introduction: CO2 to methanol
The rapid increase in atmospheric CO2 level due to human activities over the last three
centuries has prompted the development of carbon capture processes. While most capture efforts
have been focused on concentrated anthropogenic CO2 sources like flue gases and various
industrial activities, capture from diffuse sources like air has also gained attention.
1-6
Once
captured, the CO2 can be used as a feedstock for the synthesis of value added products such as
formic acid, methanol, methane and higher hydrocarbons.
7-9
Among these, methanol represents a
particularly attractive product as it can be used as a fuel (in direct methanol fuel cells, internal
combustion engines, etc.), drop-in fuel additive, C1 source in organic transformations, or as a
chemical precursor for the production of higher hydrocarbons.
10-14
Industrially, CH3OH is
produced from synthesis gas (CO, H2 and CO2) at high temperature and pressure over
Cu/ZnO/Al2O3 based heterogeneous catalysts.
15-16
CH3OH can also be produced from CO2
hydrogenation with similar heterogeneous catalysts.
17-19
In the last five years, homogeneous
catalysts, mainly based on noble metals like Ru and Ir, have also been reported for direct
20-26
and
indirect
27-29
CO2 reduction to CH3OH with molecular H2 at much lower temperatures and
pressures.
30-31
One of the promising pathways for CO2 to CH3OH reduction is via a formamide
intermediate in the presence of an amine, first developed by the Sanford group (Scheme 1).
25
The
95
presence of an amine in the system provides a unique opportunity for its integration with carbon
capture processes as was demonstrated by our group by converting directly CO2 captured from air
into CH3OH.
26
Such an integrated carbon capture and utilization process from diffused sources
can facilitate the installation of small CCU units in remote areas, isolated from large carbon
emission sources.
Scheme 1. CO2 hydrogenation to CH3OH via formamide
5.2. First row transition metal catalysis
The development of first row transition metal based catalysts is important because of the
high abundance and low cost of such metals compared to more widely studied catalysts based on
noble metals such as Pt, Ru and Ir. It should be noted however that the cost and accessibility of the
ligand should also be taken into consideration while designing such catalysts. Other crucial and
most desirable features include high activity, high selectivity, and high durability, along with the
ease of recyclability. Despite the growing attention that base metal catalysis has received in recent
years, surprisingly, only one study involving base metal catalysts has been reported for the
important process of CO2 to CH3OH reduction under homogeneous conditions.
32
That study,
published by the Beller group in early 2017, employed an in-situ generated Co based catalyst to
reduce CO2 to CH3OH with a TON of up to 78. Mn based catalysts have been explored extensively
for C-H activation.
33-34
In 2016, Beller et al. reported the catalytic hydrogenation of aldehydes,
ketones, and nitriles in the presence of Mn(I) complexes.
35
Since then, several studies have
successfully employed Mn(I) complexes for formic acid decomposition,
36
N-alkylation,
37
aqueous
methanol reforming,
38
N-formylation through CH3OH dehydrogenation,
39
ester hydrogenation,
40-
41
amide hydrogenation,
42
alcohol deoxygenation,
43
aminomethylation,
44
and transfer
hydrogenation.
45
Based on these results, we wondered whether such catalysts based on inexpensive
Mn(I) would be able to reduce CO2 to CH3OH. During the course of this study, two articles have
surfaced, reporting successful CO2 reduction to formate salts and formamide.
46-47
In the present
study, we show for the first time that CO2 can be sequentially reduced in the same pot by 3 eq. of
96
H2 to CH3OH in the presence of Mn(I) catalyst C-1 and an amine (Scheme 2).
48
A maximum TON
of 36 was achieved, which is comparable to the TON reported earlier with Co based homogeneous
catalysts.
Scheme 2. Mn(I) catalyzed reductions
5.3. Optimization of formylation reaction
In the initial stage of this study, we investigated the effectiveness of Mn-pincer complexes
C-1 and C-2 for the reduction of CO2 to formamide in the presence of morpholine (1) (Table 1).
Such Mn(I)-PNP pincer catalysts were previously reported, as mentioned earlier, as active for
aldehyde, ketone and nitrile reduction, formic acid decomposition, and for CH3OH
dehydrogenation. Satisfyingly, catalyst C-1 was also able to reduce CO2 in the presence of 1,
K3PO4 and molecular H2 at 120 °C in THF, and the corresponding product, 4-formylmorpholine
(1a), was observed through
1
H NMR in 65% yield after 24 h (Table 1, entry 1). Decreasing the
reaction temperature to 110 °C increased the yield to 77% (entry 2). However, decreasing the
temperature further was found to produce less 1a after 24 h (entry 3). A higher yield was observed
with a longer reaction time (entry 4), increasing catalyst loading (entry 5), or when the reaction
was performed at a higher pressure of 70 bar (entry 6). Catalyst C-2, MnBrPNP
Cy
(CO)2 was less
effective than C-1 for this reduction (entry 7). Among solvents, THF was found to be the most
suitable, while less polar solvents such as toluene, cyclopentyl methyl ether (CPME), and
97
cyclohexane provided lower yields (entry 8-10). t-BuOK was found to be similarly effective as
K3PO4 as an initial activator of catalyst C-1 (entry 11). Further, catalyst C-1 was reported in the
literature as being air stable even after 25 days, leading us to wonder whether the reaction could
be performed in the absence of an inert atmosphere. Selective N-formylation products were
observed (conversion 59%, yield 53%) even when the reaction was set up without using a glove
box with commercially bought chemicals (see section 5.10.4), but the yield was lower (entry 12;
compared to entry 5). Upon opening the reaction vessel, a brownish yellow solution was observed
instead of a yellow solution, generally obtained when reactions were performed under inert
atmosphere, indicating a probable catalyst decomposition taking place with the reaction set up in
air.
Table 1. Optimization of N-formylation of 1
Entry P (bar) T (°C) Time (h) Solv. Yield (%)
a
1 60 120 24 THF 65
2 60 110 24 THF 77
3 60 100 24 THF 53
4 60 110 36 THF 92
5
b
60 110 24 THF 93
6 70 110 24 THF 90
7
c
60 110 36 THF 66
8 60 110 36 Ph-CH3 65
9
b
60 110 24 CPME 43
10
b,d
60 110 24 Cy 32
11
b,d
60 110 24 THF 95
12
b,e
60 110 36 THF 53
98
Reaction conditions: Morpholine (2 mmol), cat. (0.5 mol %), K3PO4 (2.5 mol%), solvent (2 mL),
CO2: H2 (1:1)
a
yields were determined w.r.t amine from
1
H NMR spectra (N-CHO peaks) using
1,3,5-trimethoxybenzene (TMB) as an internal standard
b
morpholine (1 mmol), cat. (2 mol%),
K3PO4 (10 mol%)
c
catalyst C-2 (0.5 mol%) was used
d
t-BuOK (10 mol%) was used instead of
K3PO4
e
set up in bench-top condition, Na2CO3 (10 mol %) was used as t-BuOK and K3PO4 are too
hygroscopic to store under non-inert atmosphere. NMR yield calculations error = ±5%
5.4. Formylation of different amines
Among various amines, benzylamines reacted to afford formamide products in high yields.
For example, benzylamine (2) and N-methylbenzylamine (3) were converted to the corresponding
-NCHO products, 2a and 3a, after 24 h at 2 mol% catalyst loading and a CO2:H2 (1:1) pressure of
60 bar in 93% and 94% yields, respectively (Table 2, entry 2-3). On the other hand, alkyl amines,
such as primary amylamine (4) and secondary N,N’-dimethylethylenediamine (5) produced
moderate N-formyl yields even at higher pressures and catalyst loadings (entry 4-6).
Table 2. N-formylation of various amines
Entry Product C-1 (mol
%)
P
(bar)
Time
(h)
Yield (%)
a
1
2 60 36 95
2
2 60 24 93
3
2 60 24 94
4
2 60 24 25
5
b
4 70 36 53
6
b
4 60 24 43
Reaction conditions: amine (1 mmol), C-1 (2 mol%), t-BuOK (10 mol%), THF (2 mL), CO2: H2
(1:1), 110 °C
a
yields were determined w.r.t amines from
1
H NMR spectra (N-CHO peaks) using
99
TMB as an internal standard)
b
amine: 0.5 mmol, t-BuOK (20 mol%). NMR yield calculations error
= ±5%.
5.5. Sequential hydrogenation to methanol
Having explored the first reaction step in this sequential reduction process, we proceeded
to investigate the feasibility of in-situ reduction of the generated formamides. A study on catalyst
C-1’s effectiveness for formamide reduction showed that at 70 bar of H2 pressure, 1a can be
reduced to morpholine and CH3OH with a 64% CH3OH yield (TON = 128) at 0.5 mol% catalyst
loading in THF after 24 h (Table 3, entry 1). Expectedly, when we subjected the in-situ formamide
from the reaction mixture from Table 1, entry 4 (T1, E4) to 70 bar H2 pressure at 150 °C for 36 h,
reduction of the in-situ formed 1a was indeed observed with 11% of CH3OH formation (TON =
22) (Table 3, entry 2), although the TON was lower. 78% of un-reduced 1a was also observed. An
increase in the catalyst loading and H2 pressure to 2 mol % and 80 bar, respectively, increased both
CH3OH yield and TON (= 36), with only 6% of the in-situ generated 1a remaining in the reaction
mixture after 36 h (entry 3). Generation of CH3OH (~71%) and morpholine (~83%) were
confirmed through
1
H and
13
C spectra (Figure S6). Similarly, when the reaction mixture from
Table 2, entry 2 was used as the in-situ formamide, 46% CH3OH (TON = 23) was observed after
24 h at 80 bar and a C-1 loading of 2 mol % (entry 4). A higher CH3OH yield of 84% (TON = 21)
was obtained when 4 mol % C-1 was used (entry 5). Notably, no N-CH3 side-products were
observed through
1
H and
13
C NMR, and no trace of CO was detected in GC during hydrogenation
of the in-situ formed formamides.
Table 3. Hydrogenation of in-situ formed formamides
Entry Formamide
source
P
(bar)
Time
(h)
N-CHO
(%)
a
Amine
(%)
a
CH3OH
(%)
a,b
TON
1 Ex-situ 70 24 trace 94 64 128
2 T1, E4 70 36 78 15 11 22
3 T2, E1 80 36 6 83 71 36
100
4 T2, E2 80 24 52 # 46 23
5
c
T2, E2 80 36 5 # 84 21
Reaction conditions: Reaction mixtures from Table 1 or 2 (as specified) were subjected to H2
pressure after the release of previous CO2:H2 (1:1) gas mixture. T=150 °C.
a
amount of formamide,
amine and CH3OH were calculated w.r.t. amines from
1
H NMR spectra with TMB as internal
standard.
b
lower CH3OH yields are due to losses, while releasing gases after reaction.
c
4 mol% of
C-1 loading.
#
benzyl CH2 peaks overlapped with THF peaks making quantification challenging.
T1, E4 signifies Table 1, entry 4. NMR yield calculations error = ±5%. TON = mol of CH 3OH formed
per mol of C-1.
5.6. Mechanistic Studies
Next, mechanistic studies were performed to gain an understanding of the active catalytic
species formed in the reaction mixture. When 40 µmol of C-1 was treated with 60 bar of CO2: H2
(1:1) in the presence of t-BuOK (0.3 mmol) and 1 (0.2 mmol) in THF-d8 (1 mL) at 110 °C, a yellow
solution was obtained after 6 h. The
31
P NMR spectrum of the solution showed the presence of a
major peak at 87.4 ppm along with a minor peak due to catalyst C-1 at 82.3 ppm (Figure 1B). In
the
1
H NMR, one N-H triplet peak was observed at 7.1 ppm (J= 11.6 Hz) along with a singlet at
8.4 ppm (Figure 1A). Product 1a was also observed as a sharp singlet at 7.98 ppm. The new
1
H
and
31
P peaks were assigned to the complex Mn(OOCH)PNP
iPr
(CO)2 (C-3), which is formed in
the reaction mixture through CO2 insertion into in-situ formed complex MnHPNP
iPr
(CO)2 (C-4)
(Scheme 3, step 1).
101
Figure 1.
1
H (A) and
31
P (B) NMR spectra after 6 hours. Reaction conditions: 1 (0.2 mmol),
C-1 (40 µmol), t-BuOK (0.3 mmol), THF-d8 (1 mL), CO2: H2 (1:1) (60 bar), 110 °C, 6 h
102
Interestingly, the complex C-4 itself was not observed in the solution in either
1
H or
31
P
NMR spectra, which can be due to its high reactivity. Complex C-3 was found to be active for the
reduction of pure 1a to 1 and CH3OH. We surmise that the complex C-3, formed during initial N-
formylation step, gets decarboxylated during subsequent formamide reduction step at high H 2
pressure (70 bar) to form back active catalytic species C-4, which in turn reduces the in-situ formed
formamide (Scheme 3, step 2). Similar observations were recently reported by Gonsalvi et al with
another manganese PNP complex.
12b
The presence of t-butoxide manganese species (C-8) was also
detected after the hydrogenation of in-situ formed formamides (see Section 5.10.6).
Scheme 3. A Plausible Mechanism
103
5.7. Attempts at direct one-pot CO2 to methanol
Unfortunately, attempts of direct CH3OH synthesis in one step with low CO2 and high H2
pressure failed to produce CH3OH in the presence of C-1 (Table 4). When the reactor vessel was
charged with 5 bar CO2 and 85 bar H2 and heated at 150 °C for 24 h in the presence of morpholine
and C-1, only a minute amount (5%) of formamide was formed with no observable CH3OH (entry
1). When a stepwise temperature increase procedure was followed with an initial 24 h heating at
110 °C to allow the formation of 1a, followed by 36 h heating at 150 °C for subsequent formamide
hydrogenation, 38% of 1a was observed in the reaction mixture with only traces of CH3OH
formation (entry 2). We suggest that under these conditions (5 bar CO2, 85 bar H2), the rate of both
N-formylation and formamide reduction becomes sluggish.
Table 4. Attempts of direct hydrogenation of CO2 to CH3OH
Entry T (t) [°C (h)] N-CHO
(%)
a
RR’-NH (%)
a
MeOH (%)
a
1 150 (40) 7 86 0
2 110(24) –> 150(36) 38 49 trace
Reaction conditions: morpholine (1 mmol), C-1 (20 µmol), t-BuOK (0.1 mmol), THF (2 mL), CO2
(5 bar), H2 (85 bar).
a
yields were determined from
1
H NMR spectra (N-CHO peaks) using 1,3,5-
trimethoxybenzene (TMB) as an internal standard. NMR yield calculations error = ±5%
5.8. Scale up reaction
Finally, we decided to scale up the reaction with 10 mmol benzylamine at a low C-1 loading
(0.1 mol%) to observe the extent of catalytic efficiency of C-1. The first reduction step to
formamide proceeded with 84% yield (TONN-CHO = 840) after 48 hours at 70 bar pressure (H2:
CO2 = 1:1) and 110 °C. This is higher than the TON reported by Dubey et al for formamide
formation of secondary diethylamine using a Mn complex with 6,6’-dihydroxy-2,2’-bipyridine
ligand. A TONMeOH of 28 was observed for the subsequent hydrogenation of formed 2a at 85 bar
H2 for 48 h. Thus, we conclude that the TON of C-1 for CO2 hydrogenation to CH3OH is closer to
the other reported first row transition metal catalyst than the noble metal Ru based pincer catalysts.
104
5.9. Conclusion
In conclusion, CO2 was hydrogenated to CH3OH in the presence of an air-stable Mn(I)-
pincer catalyst. This sequential CO2 hydrogenation in the same pot produced CH3OH in good
yields with a maximum observed TON of 36. Our future efforts in this context will be towards
direct CO2 hydrogenation to CH3OH by base metal catalysts at lower pressures and temperatures.
5.10. Experimental Procedures
5.10.1. 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
MnBrPNP
iPr
(CO)2 (C-1) and MnBrPNP
Cy
(CO)2 (C-2) were prepared by previously reported
methods and weighed inside Ar filled glove box. t-BuOK (TCI, 97%), anhydrous K3PO4 (Aldrich,
97%) were weighed inside argon filled glove box without any further purification. Morpholine (1),
benzylamine (2), N-methylbenzylamine (3), amylamine (4), and N,N’-dimethylethylenediamine
(5) were commercially purchased and purified through distillation prior to use. Tetrahydrofuran
(DriSolv), toluene (DriSolv), cyclopentyl methyl ether (Aldrich, 99.9% anhydrous), and
cyclohexane (Aldrich, 99.5% anhydrous) were sparged with N2 for 1 h and stored in a Schlenk
flask over 3Å molecular sieves. Formic acid (Alfa Aesar, 97%), t-BuOH (Aldrich, 99.5%
anhydrous), and MeOH (DriSolv) were sparged with N2 for 1 h prior to use. 1,3,5-
trimethoxybenzene (TMB) (Aldrich, >99%), DMSO-d6 (CIL, D-99.9%), and THF-d8 (CIL, D-
99.5%), CDCl3 (CIL, D-99.8%), phosphoric acid-d3 solution (Aldrich; 85% in D2O, D-98%) were
used as received.
1
H and
13
C NMR spectra were recorded on 400 MHz, 500 MHz, and 600 MHz
Varian NMR spectrometers.
1
H and
13
C NMR chemical shifts were determined relative to the
residual solvent signals (DMSO-d6, CDCl3) or internal standard (TMB); and
31
P NMR chemical
shifts were determined relative to 85% phosphoric acid-d3 signal. The gas mixtures were analyzed
using a Thermo 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
4.0), H2 (Gilmore, ultra-high pure grade 5.0) were used without further purification.
105
5.10.2. Standard procedure for N-formylation reactions
In a nitrogen-filled chamber, amine (1-5) (0.5-2 mmol), catalyst C-1 or C-2 (10-20 µmol),
t-BuOK or K3PO4 (0.1-0.2 mmol), and solvent (2 mL) (THF, toluene, cyclohexane or CPME) were
added to a 134 mL Monel Parr reactor equipped with a magnetic stir bar, thermocouple and
piezoelectric pressure transducer. The vessel was then filled with CO2 to half of desired pressure
(30 bar or 35 bar), followed by H2 filling till the desired pressure was achieved (60 bar or 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. The internal temperature of
the reaction mixture (100-120 °C), and pressure inside the reactor were monitored and recorded
through the LabVIEW 8.6 software. After heating for a given period of time (24 or 36 h), the
reactor was cooled to room temperature. The vessel was then cooled in an ice bath for 30 minutes
and the gas inside were slowly released. Upon opening the reaction vessel, a yellow solution was
obtained with little powdery solid inside (K3PO4 or t-BuOK). For the results reported in Table 1,
the solvent was evaporated from this reaction mixture with a rotavapor and a known amount of
TMB was added as an internal standard. The reaction mixture was then analyzed by
1
H and
13
C
NMR with CDCl3 as the deuterated solvent. Yields were determined through
1
H NMR from
integration ratios between formyl peaks (~8ppm) and TMB aromatic proton peak (~6.12ppm). For
Table 2 (except entry 6), after opening the reaction vessel a known amount of TMB was added.
The resulting mixture was analyzed by
1
H and
13
C NMR with DMSO-d6 as the deuterated solvent.
Yields were determined through
1
H NMR from the integration ratios between formyl peaks
(~8ppm) and TMB aromatic proton peak (~6.12ppm).
106
Figure 2:
1
H NMR spectra of in-situ reduction of 4-formylmorpholine (1a) Reaction
conditions: after N-formylation of 1, H2 (70 bar), 150 °C, 36 h. (Table 3, entry 2)
5.10.3. Standard procedure for the in-situ formamide reduction reactions
The N-formylation reaction was performed before subsequent hydrogenation of the
reaction mixture. Upon completion of formylation reaction, the reaction vessel was cooled to room
temperature. Subsequently it was placed in an ice bath for 30 minutes and the gases inside were
then slowly released. The vessel was then warmed up to the r.t. and a fresh batch of K 3PO4 or t-
BuOK (0.1 or 0.2 mmol) was then added to the reaction mixture inside a N 2 chamber. The vessel
was then filled with H2 gas to the desired pressure and was placed in a preheated oil bath with
stirring (800 rpm). After the desired reaction time (24-36 hours), the vessel was cooled down to
room temperature and placed in a liquid N2 bath for half an hour. The H2 gas was then slowly
released. The reaction vessel was subsequently warmed up to RT and opened. A known amount
of TMB was added to the reaction mixture as an internal standard. The reaction mixture was then
analyzed by
1
H and
13
C NMR with DMSO-d6 as the deuterated solvent. Conversions were
determined through
1
H NMR from the integration ratios between formyl peaks (~8 ppm) and TMB
107
aromatic proton peak (~6.12 ppm). Methanol yields were determined from integration ratios
between CH3OH peak (~3.2 ppm) and TMB aromatic proton peak.
Figure 3.
1
H NMR spectra of in-situ reduction of 4-formylmorpholine (1a) Reaction
conditions: after N-formylation of 1, H2 (80 bar), 150 °C, 36 h. (Table 3, entry 3)
5.10.4. Procedure for bench top reaction (Table 1, entry 12)
Commercially available THF (2 mL) from Omnisolv (opened 2 months prior to use) was
used as solvent. The catalyst C-1 (20 µmol) was weighed inside an Ar glove box and kept outside
in contact of air overnight. Anhydrous Na2CO3 (VWR) was used (opened 3 years prior to use). In
a bench top, morpholine (1) (1 mmol), catalyst C-1 (20 µmol), Na2CO3 (0.1 mmol), and THF (2
mL) were added to a 134 mL Monel Parr reactor equipped with a magnetic stir bar, thermocouple
and piezoelectric pressure transducer. The vessel was then filled with CO2 to half of desired
pressure (30 bar), followed by H2 filling till the desire pressure was achieved inside (60 bar). The
reaction mixture was then stirred in a magnetic stirrer for 5 minutes (800 rpm) and subsequently
placed in a preheated oil bath with stirring at 800 rpm. The internal temperature of the reaction
108
mixture (110 °C), and pressure inside the reactor were monitored through the LabVIEW 8.6
software. After heating for a given amount of time (36 h), 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 and carefully released. Upon opening the reaction vessel, a brown-yellow solution was
obtained with little powdery solid inside (Na2CO3). A known amount of TMB was added. The
resultant liquid mixture was 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 between formyl peaks
(~8ppm) and TMB aromatic proton peak (~6.12ppm).
5.10.5. Procedure for unsuccessful attempts of one-step CO2 to CH3OH
synthesis
In a nitrogen-filled chamber, amine (1) (1 mmol), catalyst C-1 (20 µmol), t-BuOK (0.1
mmol), and solvent (2 mL) (THF) were added to a 134 mL Monel Parr reactor equipped with a
magnetic stir bar, thermocouple and piezoelectric pressure transducer. The vessel was then filled
with CO2 to desired CO2 partial pressure (5 bar), followed by H2 filling till a total pressure of 90
bar was achieved. 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. The internal
temperature of the reaction mixture and pressure inside the reactor were monitored and recorded
through the LabVIEW 8.6 software. 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 yellow solution was obtained. A known
amount of TMB was added to the reaction mixture. The resultant liquid mixture was 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 between formyl peaks (~8ppm) and TMB aromatic proton peak
(~6.12ppm).
5.10.6. Procedure for mechanistic studies
In a nitrogen-filled chamber, amine (1) (0.2 mmol), catalyst C-1 (40 µmol), t-BuOK (0.3
mmol), and THF-d8 (1 mL) (THF) were added to a 134 mL Monel Parr reactor equipped with a
magnetic stir bar, thermocouple and piezoelectric pressure transducer. The vessel was then filled
with CO2 to the desired CO2 partial pressure (30 bar), followed by H2 filling till the desired pressure
was achieved (60 bar). The reaction mixture was then stirred with a magnetic stirrer for 5 minutes
109
(800 rpm) and subsequently placed in a preheated oil bath with stirring at 800 rpm. The internal
temperature of the reaction mixture and pressure inside the reactor were monitored and recorded
through the LabVIEW 8.6 software. After heating for a given period (6 h), 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 inside N2 chamber, a yellow solution was
obtained, which was analyzed by
1
H and
31
P NMR.
110
Figure 4:
1
H (A) and
31
P (B) NMR spectra after 6 hours. Reaction conditions: 1 (0.2 mmol),
C-1 (40 µmol), t-BuOK (0.3 mmol), THF-d8 (1 mL), CO2: H2 (1:1) (60 bar), 110 °C, 6 h
To confirm the identity of C-3 as Mn(OOCH)PNP
iPr
(CO)2, the formate manganese species
Mn(OOCH)PNP
iPr
(CO)2 was prepared through the procedure reported by Boncella et al.
2
The
prepared formate complex, when added to the reaction mixture of Figure 4, didn’t produce any
new peak in
31
P in NMR spectrum; instead, an increased integration for the peak at 87.4 was
observed. (Figure 5)
111
Figure 5:
31
P spectra of (A) reaction mixture obtained in Figure 4, (B) prepared complex
Mn(OOCH)PNP
iPr
(CO)2 in THF-d8 (C) solution from A + solution from B
When the reaction mixture from Figure 4 was treated with 80 bar of H2 at 150 °C for 6
hours, a new
31
P peak was observed at 89.1 ppm (Figure 6B). A new proton triplet peak, highly
shifted to low field, was seen at 8.95 ppm (J = 11.0 Hz) (Figure 6A), indicating the presence of
hydrogen bonding.
112
Figure 6.
1
H (A) and
31
P (B) NMR spectra after 6 hours of hydrogenation of Figure S4 solution.
Reaction conditions: After Figure S9, H2 (80 bar), 150 °C, 6 h.
113
To characterize the unknown peak in the Figure 6B at 89.1 ppm, hydroxide, methoxide,
and t-butoxide manganese species (C-6, C-7 and C-8 respectively) were prepared by the respective
addition of H2O (10 eq), MeOH (10 eq), or t-BuOH (10 eq) to the in-situ formed amido complex
(through addition of 3 eq, NaOtBu to complex C-1 followed by 15 min of stirring) in benzene-d6
(Figure 7-9). The unknown peak at 89.1 ppm in Figure 6B was assigned to t-butoxide manganese
species since its addition to the solution didn’t produce any new visible peak (Figure 10).
Figure 7:
31
P NMR of in-situ prepared Mn(OCH3)PNP
iPr
(CO)2 (C-7) in benzene-d6
114
Figure 8.
31
P NMR of in-situ prepared Mn(OH)PNP
iPr
(CO)2 (C-6) in benzene-d6
115
Figure 9.
31
P NMR of in-situ prepared Mn(OtBu)PNP
iPr
(CO)2 (C-8) in benzene-d6
Figure 10.
31
P spectra of (A) reaction mixture obtained in Figure 6 after 2 months, (the C-3 peak
disappeared completely to give rise of the unknown peak) (B) in-situ formed complex C-8 in
benzene-d6 (C) solution from A + solution from B.
5.10.7. Mass spectrometric and IR spectroscopic data of the complexes
Table 5: Mass and IR data
Complex CO IR stretches (cm
-1
)
a
Mass Peak (M/Z)
b
Calculated [M]
+
Observed
C-1 1817, 1912 495.1 495.1 [M]
+
(CI); 416.1 [M-Br]
+
(ESI)
C-2
1825, 1912 655.2 576.3 [M-Br]
+
(ESI)
C-3 1813, 1905 461.2 416.1 [M-OOCH]
+
(ESI); 359.1 [M-
HCOOH-2CO]
+
(CI)
C-7
1814, 1907 447.2 447.1 [M]
+
(CI)
C-8 1821, 1911 489.2 416.1 [M-O tBu]
+
(ESI)
116
a
FTIR spectra were recorded on a Jasco FT/IR-4600 instrument.
b
Mass spectra were recorded on VG 70-
VSE(B) instrument at the Mass Spectrometry Laboratory of the School of Chemical Sciences, University
of Illinois.
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121
CHAPTER 6
Integrative CO
2
Capture and Hydrogenation to Formate
with Low Temperature Regeneration of Sodium Hydroxide
This dissertation chapter is based on a recent research article from our group published on the
Journal of the American Chemical Society (Kar, S.; Goeppert, A.; Galvan, V.; Chowdhury, R.;
Olah, J.; Prakash, G. K. S. J. Am. Chem. Soc. 2018, 140, 16873-16876). Part of the article is
reprinted by permission of the American Chemical Society. Copyright 2018. American Chemical
Society.
6.1. Introduction: CO2 capture with hydroxides and utilization
The recent increase in atmospheric CO2 concentration has prompted researchers to develop
processes for capturing CO2 from point and diffused sources. Various CO2 capture materials have
been reported including aqueous hydroxides, amine or amino alcohol solutions, metal organic
frameworks, and silica amine hybrid adsorbents.
1-7
Metal hydroxides have long been known to
capture atmospheric CO2 and were extensively studied with different capture methods such as
sprays, towers, pools etc.
8-11,12-13
The regeneration of hydroxides is challenging and generally
achieved through a series of steps (causticization, calcination, slaking) at high temperature (750
°C) (Scheme 1). The CO2 released in the process can be sequestered underground in a rock
formation. Alternatively, the captured CO2 can be converted into value-added products and fuels
such as formic acid, formate salts or methanol.
14-19
With the recent progress in low temperature
formic acid and CH3OH synthesis through homogeneous CO2 hydrogenation, carbon capture and
utilization (CCU) can be a powerful tool to utilize atmospheric CO2 as a chemical feedstock.
20-36
Scheme 1. NaOH regeneration from Na2CO3
122
6.2. Comparison between integrated systems with amines and
hydroxide bases
The integration of carbon capture with utilization is an area of immense importance.
Recently, our group and others have demonstrated the tandem capture and conversion of CO2 to
ammonium formate salts and CH3OH with amines as the capturing agent.
26, 37-39
However, given
the low CO2 concentration in the air, inorganic hydroxide bases provide a more effective system
for integrated CCU. Advantages of employing aqueous hydroxide solutions for CCU as compared
to previously reported amine systems include— (i) easy availability of the hydroxides via
electrolysis of aqueous salts (ii) higher carbon capture efficacy from ambient air (iii) negligible
vapor pressure thus avoiding atmospheric contamination (iv) non-toxicity and (v) regenerability
of the hydroxide bases. Furthermore, unlike the ammonium formate salts, the metal formate salts
can be utilized directly in fuel cells and achieve a carbon neutral cycle. Herein, we describe such
a carbon neutral amine-free integrated system that utilizes the captured CO2 as C1 source to
produce formate salts through hydrogenation. The formate salt is subsequently utilized directly
without any purification in a direct formate fuel cell (DFFC) to produce electricity and regenerate
sodium hydroxide (Figure 1). While the reduction of bicarbonate salts has been reported
previously,
40-49
its integration with CO2 capture, and the recyclability of the capturing hydroxide
base is achieved for the first time in this study in the context of integrated CCU systems.
123
Anode
Cathode
Na +
Na +
Na +
Figure 1. Previously reported integrated CCU systems compared to the present study
6.3. CO2 Capture with different hydroxide bases
Among different metal hydroxide bases screened for CO2 capture, NaOH, KOH and CsOH
were found most effective. Under the capture conditions (Table 1), an aqueous NaOH solution
captured a total 13.8 mmol of CO2 after 3h, corresponding to 1 mol CO2 per mol of OH
-
, (Table
1, entry 1). The formation of NaHCO3 in the solution was observed through
13
C NMR. In the KOH
solution, CO2 capture was completed within 2h, and KHCO3 formation was observed (entry 2).
On the other hand, LiOH captured only 0.5 mol of CO2 per hydroxide group, indicating the
formation of Li2CO3, which precipitated from the aqueous solution (entry 3). CsOH captured 1
CO2/OH
-
, similar to NaOH and KOH, with the formation of CsHCO3 (entry 4). Finally, with
Ca(OH)2, mainly the formation of CaCO3 was observed after 3h. Also, due to the low solubility of
Ca(OH)2 in water, its initial CO2 capture rate was significantly lower (entry 5).
50
124
Table 1. CO2 capture by aqueous hydroxide solutions
entry base CO 2 captured
(mmol)
a
CO 2/OH
b
CO 2/g
c
t (min)
1 NaOH 13.8 ~1 25.0 167
2 KOH 13.8 ~1 17.8 123
3 LiOH 6.9 ~0.5 20.8 161
4 CsOH 13.7 ~1 6.7 109
5
d
Ca(OH)2 9.0 ~0.65 13.5 186
Capture conditions: Base (13.8 mmol), water (10 mL), stirring (800 rpm), rt. See SI for details.
a
Calculated
through gravimetric analysis. Calculations error ±5%.
b
mols of CO2 captured per mol of hydroxide
c
mmols
of CO2 captured per g of base
d
6.9 mmol of Ca(OH)2 was used.
Figure 2. CO2 capture with time for different hydroxide bases
6.4. Subsequent hydrogenation of captured CO2 to formate
6.4.1. Effect of catalyst structures
Following CO2 capture, in-situ hydrogenation of the CO2 loaded aqueous solutions was
performed at 80 °C under 50 bar H2 pressure with different hydrogenation catalysts to obtain the
formate salts (Table 2). The reactions were conducted in a 2-MTHF/H2O biphasic system
(5/10mL), which allows for efficient recovery of the catalyst (Figure 3).
125
Figure 3. Biphasic solution obtained after hydrogenation. Catalyst in top organic layer, formate in
bottom aqueous layer
When the CO2 loaded NaOH solution (containing NaHCO3) was hydrogenated under these
conditions with 5 µmol Ru-Macho-BH (C-1), 93% yield in HCOONa (TON=2560) was observed
after 12h (Table 2, entry 1). The hydrogenation progress was followed by monitoring the pressure
decrease inside the reactor, and a reaction completion time of 132 min was observed (TOF=1164
h
-1
). Upon analyzing the reaction mixture, the aqueous layer contained HCOONa and the organic
layer contained the catalyst, thus allowing for easy recycling. A lower TOF was obtained with
catalyst RuHClPNP
Ph
(CO) (C-2, Ru-Macho, entry 2). The TOF improved slightly with complex
RuHClPNP
iPr
(CO) (C-3) and RuHClPNP
Cy
(CO) (C-4) to 1024 h
-1
and 811 h
-1
, respectively (entry
3-4). However, significant increase in TOF was observed (2698 h
-1
) when RuHClPNP
tBu
(CO) (C-
5) was used as the catalyst, and the reaction was completed within 62 mins (entry 5). Thus, the
substitution on the P atoms in the pincer ligand markedly affects the hydrogenation rate. Among
base-metal catalysts, manganese-based catalysts, MnBrPNP
iPr
(CO)2 (C-6) and MnBrPNP
Cy
(CO)2
126
(C-7) were not very effective and low TOFs of 31 and 33 h
-1
were observed, respectively (entry 6-
7). On the other hand, the iron-pincer complex, FeHBrPNP
iPr
(CO) (C-8), displayed high
hydrogenation rate with a TOF of 428 h
-1
(entry 8). Thus, C-8 could be a convenient and cost-
effective catalyst for a large-scale implementation of this process. Finally, complex C-9, the N-Me
analogue of C-2, was equally active in hydrogenation (entry 9), indicating that the N-H moiety
may not be actively taking part in the reaction mechanism.
Table 2. Hydrogenation of the captured CO2
entry M(OH) n CO 2
(mmol)
Cat. t (min) Yield (%)
a
TON
b
TOF (
h
-1
)
1 NaOH 13.8 C-1 132 93 2560 1164
2 NaOH 13.8 C-2 256 100 2771 649
3 NaOH 13.9 C-3 159 98 2714 1024
4 NaOH 13.8 C-4 188 92 2541 811
5 NaOH 13.7 C-5 62 102 2788 2698
6 NaOH 13.7 C-6 2419 45 1233 31
7 NaOH 13.8 C-7 4142 82 2258 33
8 NaOH 13.8 C-8 379 98 2706 428
9 NaOH 13.6 C-9 258 96 2612 607
10 KOH 13.8 C-5 <30 98 2710 >5420
127
11 CsOH 13.7 C-5 63 98 2676 2548
12 LiOH 6.9 C-5 - 0 0 0
13 Ca(OH)2 9.0 C-5 -720 2 32 3
14
c
NaOH 13.7 C-5 101 98 2685 1595
Reaction conditions: Solutions from Table 1 (as specified) were hydrogenated following the addition of
organic solvent and catalyst. Cat. (5 µmol), 2-MTHF (5 mL), H2 (50 bar), 80 °C, 12h.
a
Yields were
determined based on
1
H NMR with imidazole as the internal standard
b
TON = mols of formate produced
per mol catalyst
c
CPME (1 mL) was used as the organic solvent. Yield calculations error ±5%.
6.4.2. Effect of formate counter cation
Next, CO2 loaded solutions of different bases were hydrogenated with C-5 to identify the
most suitable base. Compared to HCOONa, the formation of HCOOK was faster and completed
within 30 mins (TOF >5420 h
-1
; entry 10). The improved rate can be explained by the higher
standard enthalpy of formation of HCOOK (-679.7 kJ/mol), compared to HCOONa (-666.5
kJ/mol), which facilitates the release of the formate group from the catalytic resting state (vide
infra). With CO2 loaded solution of CsOH, the hydrogenation also proceeded rapidly (~1h), and
94% HCOOCs yield was observed, giving a TOF of 2548 h
-1
(entry 11). Surprisingly, CO2 loaded
LiOH and Ca(OH)2 solutions weren’t hydrogenated efficiently under our reaction conditions, and
only traces of formate salt was observed in either cases (entry 12-13). This is likely due to the
electron rich nature of the carbonates, as compared to bicarbonate salts, and similar findings were
reported recently by Huang et al.
32
Besides 2-MTHF, cyclopentyl methyl ether (CPME) was also
a good co-solvent for the biphasic system, with 10% CPME (v/v) being sufficient for effective
hydrogenation (entry 14). From the hydrogenation yields and TOFs observed, catalyst C-5 is
clearly the most suitable for hydrogenation, with NaOH and KOH being the most judicious choice
of bases.
6.4.3. Correlation between TOF and catalyst molecular structure
As seen from the above NMR spectra, ruthenium formate species is observed under the
reaction conditions as the catalytic resting state. Thus, one of the key rate determining step for the
hydrogenation is the subsequent release of the formate from catalyst metal center (Scheme 2).
Hazari- Bernskoetter and others have previously reported extensively on the rate enhancement of
this step in the presence of Lewis base such as LiBF4 or LiOTf. Similarly, in this study we also
observed the dependence of rate on the metal ion present in the system. For example, when K+
was used instead of Na+, as in KHCO3, enhanced rate was observed due to higher heat of formation
of HCOOK.
128
Scheme 2. Plausible mechanism cycle of foramte production
With regards to the correlation between catalyst molecular structure and TOF observed, a
certain trend can be expected. Given the electron donating ability of PNP
tBu
is higher than PNP
iPr
,
followed by PNP
Ph
, the electron density at the metal center in case of R= t-Bu would be higher
than the other cases. Our working hypothesis is that as a result of this increased electron density,
the ejection of formate from the catalytic center becomes easier. As a result, the TOF increases
along with the increasing donating ability of the PNP ligand. It would be interesting to follow this
proposed effect of ligands through DFT studies in future.
6.5. Recycling of the hydrogenation catalyst in biphasic system
Subsequently, the recycling of the active elements was investigated. As mentioned earlier,
easy recycling of the catalyst and organic solvent can be achieved by utilizing a biphasic solvent
system, (Figure 4). Following this recycling protocol, catalyst C-5 was recycled for 5 cycles. A
catalytic activity similar to the first cycle was retained even in the fifth cycle, with over 90%
HCOONa yield in each cycle (Figure 5). Thus, catalyst C-5 is an efficient and robust catalyst for
this hydrogenation reaction.
129
Figure 4. A recycling scheme for the catalyst, 2-MTHF and hydroxide base
Figure 5. TOF in consecutive cycles of catalyst recycling
6.6. Regeneration of the hydroxide base in fuel cell
The regeneration of the hydroxide solution was explored next. In this regard, DFFC present
the most convenient way to harness the chemical energy of the formed HCOONa and regenerate
the hydroxide base.
51-54
Recently, He et al. described a sodium ion conducting DFFC, that produced
0
500
1000
1500
2000
2500
3000
1 2 3 4 5
TOF (h
-1
)
Run
130
electricity with simultaneous theoretical regeneration of NaOH.
54
A schematic diagram of the
proposed cation conducting direct formate fuel cell is shown in Figure 6.
Figure 6. Schematic diagram of a cation conduction DFFC
We improved upon this previously reported fuel cell by using Pt/C as the cathode catalyst
for O2 reduction instead of Pd/C to obtain a 1.5 fold increase in peak power density at 80 °C
(Figure 7).
0
10
20
30
40
50
60
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 50 100 150 200 250 300 350
Power Density / mW cm
-2
Potential / V
Current Density /mA cm
-2
60 ℃
80 ℃
Solid: Pt/C
Dash: Pd/C
131
Figure 7. Change in power density and polarization curve with different cathode catalysts. Anode
fuel: HCOONa, 1.0 M, 2.5 mL/min
-1
; Cathode oxidant: oxygen, 100 sccm, ambient pressure;
Anode electrode: Catalyst: Pd/C; Loading: 2.0 mgPd cm
-2
; Cathode electrode: Catalyst: Pd/C or
Pt/C; Loading: 2.0 mg cm
-2
; CEM: Nafion 211.
The aqueous layer obtained from the hydrogenation and containing HCOONa (~1 M) was directly
fed to the DFFC anode (Pd/C) without further purification (flow-rate 2.5 mL/min). As shown in
Figure 8B, at 10 mA constant current, a relatively steady voltage of about 0.7 V was observed for
more than 13h (Figure 8A represents the polarization curve). The pH of the collected cathodic
solution after 2 hours was found to be highly caustic (pH~ 12.6) compared to the parent formate
solution (pH~ 8.7), indicating the formation of NaOH; while at the anode, formation of NaHCO 3
was observed in the
13
C NMR (Figure S15).
132
Figure 8. (A) Polarization curve of the fuel cell used in this study (B) voltage vs time at a constant
current of 10 mA at 80 °C. See SI for details of the fuel cell
Minor amounts of HCOONa and Na2CO3 were also detected in the cathode solution due to
the crossover, which can be substantially mitigated using F-1850 sodium conducting membrane
(Table 3).
Table 3. Amounts of formate and carbonate crossover to cathode side with Nafion-211 and F-
1850 membranes
Formate (mmol) Carbonate (mmol)
Nafion-211 0.6 0.5
F-1850 0.05 0.3
Conditions: Anode fuel: HCOONa, 1.0 M, 2.5 mL/min-1; Cathode oxidant: oxygen, 100 mL min
-1
, ambient
pressure; Anode electrode: Catalyst: Pd/C; Loading: 2.0 mgPd cm
-2
; Cathode electrode: Catalyst: Pd/C or
Pt/C; Loading: 2.0 mg cm
-2
; discharge@ 10 mA, 80
o
C time = 13h; the amount of formate and carbonate
was calculated from
1
H and
13
C NMR spectra, respectively, with imidazole as internal standard.
133
Figure 9. Polarization (A) and voltage vs time (B) curve with F-1850 membrane. Anode fuel:
HCOONa, 1.0 M, 2.5 mL/min
-1
; Cathode oxidant: oxygen, 100 sccm, ambient pressure; Anode
electrode: Catalyst: Pd/C; Loading: 2.0 mgPd cm
-2
; Cathode electrode: Catalyst: Pd/C or Pt/C;
Loading: 2.0 mg cm
-2
; CEM: F-1850. F-1850 membrane was prepared by similar method as
Nafion 211.
The obtained cathode solution containing NaOH was able to capture CO2 again (Figure
20, Section 6.9.7). After 13h, a total of 2.1 mmol NaOH was collected, as quantified by the amount
of captured CO2, corresponding to an 43% current efficiency in NaOH regeneration. Importantly,
134
even though the previous report by He et al. predicted the theoretical formation of NaOH based on
the current and potential observed in the fuel cell, its presence in the cathodic feed was not directly
detected. Thus, this is the first study reporting the regeneration of NaOH as verified though pH
analysis and CO2 capture experiment. Consequently it is demonstrated that HCOONa can be
recycled in a DFFC to regenerate NaOH, unlike other available studies where NaOH was
converted, instead, to NaHCO3 through dehydrogenation.
42-43, 45-46
Alternatively, HCOONa can be
recycled to regenerate NaOH either by direct electrodialysis,
55-56
or through the acidification of
HCOONa with HCl to form HCOOH (which is a fuel), followed by the production of NaOH and
HCl from the resultant NaCl through the chlor-alkali process.
6.7. Catalytic resting state and CO2 from air to formate
The catalytic resting state was also investigated with catalyst C-2, and the presence of the
ruthenium formate species [RuH(OOCH)PNP
Ph
(CO)] was detected, indicating its role as a
catalytic resting state under the reaction conditions (Figure 10).
135
Figure 10. Observation of Ru formate species as catalytic resting state through
1
H (top) and
31
P
(bottom) NMR
The efficiency of the system for direct air capture and conversion of CO2 to formate was
also tested. CO2 was captured by bubbling ambient air through a 1 N NaOH (15 mL) solution using
a pump and after 64 h, 3.0 mmol of CO2 was captured in form of carbonate salt (Figure 11). After
hydrogenation of the resulting solution using C-5, 2.2 mmol of HCOONa was observed, giving a
73% formate yield.
136
Figure 11.
13
C spectra of CO2 captured from ambient air
6.8. Conclusion
In conclusion, an amine-free system was developed that captures CO2 from concentrated
or ultra-dilute sources and is easily integrated with a subsequent hydrogenation step to synthesize
formate salts. Easy availability, high CO2 capture efficiency, lower vapor pressure and their
regenerability makes the hydroxide base system a promising alternative to the formerly reported
amine-based systems. With a suitable choice of base (KOH) and catalyst (C-5), a TOF as high as
5420 h
-1
was obtained. The biphasic system also enabled effective recycling of the precious metal
catalyst and NaOH as well as convenient utilization of the produced formate salts in a DFFC. Our
next efforts in this context will be directed towards developing amine-free integrated CO2 capture
and conversion to methanol systems.
137
6.9. Experimental Procedures
6.9.1. Materials and methods
All hydrogenation experiments were carried out under an inert atmosphere (with N2 or Ar)
using standard Schlenk techniques. Complexes Ru-MACHO-BH (C-1, Strem Chemicals, 98%),
Ru-MACHO (C-2, Strem Chemicals, 98%), RuHClPNP
iPr
(CO) (C-3, Strem Chemicals, 97%),
RuHClPNP
Cy
(CO) (C-4, Strem Chemicals, 97%), RuHClPNP
tBu
(CO) (C-5, Strem Chemicals,
97%) were used as received without further purification. Complexes MnBrPNP
iPr
(CO)2 (C-6),
MnBrPNP
Cy
(CO)2 (C-7), FeHBrPNP
iPr
(CO) (C-8) and RuHClPNMeP
Ph
(CO) (C-9) were prepared
by previously reported methods. All catalysts were weighed inside an argon filled glove box.
Sodium hydroxide, potassium hydroxide, cesium hydroxide monohydrate, calcium hydroxide and
lithium hydroxide monohydrate were purchased from commercial sources and used without further
purification. 2-Methyltetrahydrofuran (2-MTHF, BTC), cyclopentyl methyl ether (CPME, Sigma-
Aldrich), water (deionized) were sparged with N2 for 1 h prior to use. 1,3,5-trimethoxybenzene
(TMB) (Sigma-Aldrich, >99%), imidazole (Fischer Scientific, 99.7%), DMSO-d6 (CIL, D-99.9%),
and D2O (CIL, D-99.5%) were used as received.
1
H and
13
C NMR spectra were recorded on 400
MHz, Varian NMR spectrometers.
1
H and
13
C NMR chemical shifts were determined relative to
the residual solvent signals (DMSO-d6, D2O) or internal standard (TMB/Im). CO2 (Gilmore,
instrument grade 4.0), and H2 (Gilmore, ultra-high pure grade 5.0) were used without further
purification.
6.9.2. Standard procedure for CO2 capture by aqueous hydroxide solutions
Metal hydroxide bases (13.8 mmol) were dissolved in water (10 mL) in a 30 mL vial
equipped with a magnetic stir bar. CO2 was added into the vial while stirring the aqueous solution
at 800 rpm and maintaining CO2 pressure inside the vial at 1 psi (Figure 12). The amounts of CO2
captured after 3 h were calculated through gravimetric analysis of the aqueous solutions before
and after the capture. For CsOH and LiOH, their commercially available monohydrates (13.8
mmol) were directly used for capture.
138
Figure 12. Image of CO2 capture station
139
Figure 13.
13
C NMR spectra of aqueous NaOH solution after CO2 capture. Reaction conditions:
NaOH (550 mg), H2O (10 mL).
6.9.3. Standard procedure for the hydrogenation reactions
The capture of CO2 in aqueous hydroxide solutions were performed prior to hydrogenation
reactions. Upon completion of capture, in a nitrogen-filled chamber, the CO2 loaded hydroxide
solution, catalyst C-1/C-2/C-3/C-4/C-5/C-6/C-7/C-8/C-9, and solvent (5 mL/1 mL) (2-MTHF,
CPME,) 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 (50 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. The
internal temperature of the reaction mixture (80 °C), and pressure inside the reactor were
monitored and recorded through the LabVIEW 8.6 software (Figure 14). After heating for a given
period of time (12 h), the reactor was cooled down to room temperature. The gas inside the vessel
was then slowly released. Upon opening the reaction vessel, a solution was obtained with two
distinct layers (Figure S5). After transferring the whole solution into a 15 mL centrifuge tube, the
two layers were separated. A known amount of TMB was added as an internal standard to the top
2-MTHF layer which was then analyzed by
1
H and
13
C NMR with DMSO-d6 as the deuterated
140
solvent. A known amount of imidazole was added to bottom aqueous layer as an internal standard,
followed by
1
H and
13
C NMR analysis with D2O as the deuterated solvent. Yields were determined
through
1
H NMR from integration ratios. The reaction completion time was assigned based on the
cessation in pressure decrease inside the reaction vessel during the reaction.
Figure 14. Pressure and temperature profile with time in a typical hydrogenation reaction with
catalyst C-2
141
Figure 15: Typical
1
H spectra of the top organic layer after hydrogenation reaction of captured
CO2 (Table 2)
Figure 16: Typical
1
H spectra of the bottom aqueous layer after hydrogenation reaction of captured
CO2 (Table 2)
142
6.9.4. Standard Procedure of CO2 capture from air
In a 30 mL vial, 0.6 g NaOH was dissolved in 15 mL water. Atmospheric air containing
408 ppm CO2 was then bubbled through the solution at a flow-rate 200 mL/min for 64h through a
pump. 5 mL of additional water was added after 40h. After the completion of CO2 capture, the
final solution had ~10 mL of water due to water evaporation while bubbling air. The resulting
solution was then sparged with N2 for 30 mins. Afterwards, 500 µL of THF was added to the
solution as an internal standard and the amount of CO2 captured was calculated through
13
C NMR
analysis. The remaining solution was used for hydrogenation with 5 mL THF as the organic
solvent.
6.9.5. Procedure for recycling studies
The capture of CO2 in aqueous amine solutions were carried out prior to hydrogenation
reactions. Upon completion of capture, in a nitrogen-filled chamber, the CO2 loaded NaOH
solution, catalyst C-5 (5 µmol), and 2-MTHF (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 (50 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. The internal temperature of the reaction mixture (80 °C), and pressure inside
the reactor were monitored and recorded through the LabVIEW 8.6 software. After heating for a
given period of time (12 h), the reactor was cooled to room temperature. The gases inside were
then slowly released under a N2 atmosphere. Upon opening the reaction vessel inside a N2
chamber, a solution was obtained with two distinct layers. After transferring the whole solution in
a 15 mL centrifuge tube, the two layers were separated in N2 atmosphere. The bottom layer was
washed with additional 2 mL of 2-MTHF, and the combined organic layer was transferred to a
Schlenk flask and used directly for the next cycle. Meanwhile, a known amount of imidazole was
added to bottom aqueous layer as internal standard, followed by its analysis by
1
H and
13
C NMR
with D2O as the deuterated solvent. Yields were determined through
1
H NMR from the integration
ratios of imidazole aromatic peaks with formate proton peak.
6.9.6. Observing the catalytic resting state
Procedure: In a nitrogen-filled chamber, sodium bicarbonate (0.1 mmol), catalyst C-1 (25
µmol), THF-d8 (1 mL) and water (0.2 mL) were added to a 125 mL Monel Parr reactor equipped
143
with a magnetic stir bar, thermocouple and piezoelectric pressure transducer. The vessel was then
filled with H2 to the desired pressure (50 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. The internal temperature of the reaction mixture (80 °C), and pressure inside
the reactor were monitored and recorded through the LabVIEW 8.6 software. After heating for a
given period of time (1 h), the reactor was cooled to room temperature. The gas inside were then
slowly released in a N2 atmosphere. Upon opening the reaction vessel inside a N2 chamber, a pale-
yellow solution was obtained. The solution was transferred inside a J. Young NMR tube and
subsequently was analyzed through
1
H and
31
P NMR spectroscopy. The chemical shifts of the
ruthenium formate species matches the value previously reported in the literature.
4
6.9.7. Fuel cell studies
Experimental procedure for the fabrication of the fuel cell
Carbon paper was sectioned into 4 cm
2
pieces and coated with catalysts. For the catalyst
preparation, catalyst powders (20%-Pd/C and 40%-Pt/C) were mixed with appropriate amounts of
ethanol and a neutral binder (Fluorinated Ethylene Propylene, 55% weight in water) and subject
to sonication for 10 minutes. The resulting catalyst suspensions were then hand-brush painted onto
teflonized carbon paper until 2.0 mg cm
-2
of Pd was added for the anode and either 2.0 mg cm
-2
of
Pt or Pd were painted onto teflonized carbon paper for the cathode. The membrane (Nafion 211;
the Na
+
membrane was prepared following previously reported procedure
5
) was then placed in
between the catalyst coated carbon paper and pressed at 130 °C for 5 minutes applying 500
kilograms of force.
The membrane electrode assembly was placed between graphite separators; the Direct
Formate Fuel Cell (DFFC) is depicted in Figure S12 and S13. The fuel cell measurements were
performed using a Fuel Cell Test System 890B (Scribner Associates) at a temperature of 80 °C.
Non heated ~1 M HCOONa solution obtained from the hydrogenation reaction was delivered
through the anode compartment at a flow rate of 2.5 mL min
-1
while humidified O2/air was passed
through the cathode compartment at 100 mL min
-1
heated to 85 °C.
144
Figure 17. Image of the fuel cell assembly used in this study
145
Figure 18. Schematic diagram of the fuel cell
Figure 19.
13
C NMR of the anode solution showing the formation of NaHCO3
1. Brass current collector
2. Bipolar graphite plate
3. Teflon gasket
4. Formate electrode (anode)
5. Cation exchange membrane
6. O 2 electrode (cathode)
146
Figure 20. Observation of sodium bicarbonate after CO2 capture with regenerated NaOH
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Abstract (if available)
Abstract
As the title suggest, the central theme of this dissertation is utilization of CO₂ from air to produce methanol or other C1 products. Such utilization of atmospheric CO₂ is becoming increasingly important in view of the rapid increase in CO₂ concentration in air due to anthropogenic activities. ❧ In the first chapter, an overview on the recent “carbon conundrum” is provided. Specially worrying is the rapidly increasing CO₂ concentration in the air and the associated global warming. The idea of methanol economy based on carbon neutral cycle is introduced. A brief overview of the reported homogeneous CO₂ hydrogenation to methanol systems are provided that can produce methanol at lower temperatures as compared to industrially utilized heterogeneous catalysts. ❧ In the second chapter, mechanistic insights into the amine-assisted CO₂ to methanol process is reported. In this amine-assisted process, CO₂ is converted into methanol in presence of an amine through formamide formation. We describe the effect of catalyst and amine molecular structure on the methanol yield. Also, we identify a catalyst deactivation pathway based on carbonylation of the metal center leading to the formation of catalytically inactive ruthenium biscarbonyl complexes. The electronic influence of pincer ligand on these biscarbonyl complexes are described, providing insights on developing second generation CO₂ to methanol catalysts. ❧ In the third chapter, a recyclable system is described where CO₂ is captured and converted to methanol for multiple cycles in high yields. The CO₂ capture is carried out in an aqueous amine solution. After capture, the resulting carbamate and bicarbonate salts are hydrogenated to produce methanol and regenerate the amine. The recycling of the catalyst and amine is achieved by using a biphasic solvent system (2-methyltetrahydrofuran/water) that allows convenient separation of the catalyst and amine after each hydrogenation cycle. ❧ In the fourth chapter, an alternate recycling scheme based on amine immobilization onto solid supports is described. CO₂ is captured using the solid supported amines without requiring any solvent. In the next step, the CO₂ loaded amines are placed inside a parr reactor with high H₂ pressure in presence of an active hydrogenation catalyst to produce methanol. After the reaction, the solid amines are filtered and reused for next cycle of capture. We explored different preparation methods to find the most suitable solid amine for this purpose. Covalently attached polyamines were found optimal for repeated use without significant decrease in methanol yields in multiple cycles. ❧ In chapter 5, manganese based catalysts for amine assisted CO₂ to methanol process is described. The previous methods for this process used ruthenium based catalysts. However, to scale up the reaction, use of cheaper earth abundant base-metal-based catalysts are necessary. A methanol turnover number of 36 was obtained using manganese pincer catalyst, along with CO₂ to formamide turnover of 840. ❧ In chapter 6, a method for CO₂ capture using hydroxide bases and its subsequent conversion to formate salts is described. The initial capture produces bicarbonate salts which under hydrogenation conditions produce the formate. The hydrogenation catalyst (ruthenium/iron complex) is recycled in a biphasic system. More importantly, the hydroxide base (e.g. NaOH) was regenerated in an unprecedented low temperature of 80℃ in a cation conducting direct formate fuel cell. The previous methods of hydroxide generation required a series of steps and high temperatures (>750℃). ❧ Additionally, my research activities over last four years also included investigations on first-row transition metal catalyzed regioselective deuteration of alcohols, CO₂ capture from air using solid supported amines, amine promoted reforming of methanol, and synthetic organofluorine chemistry (especially, difluoromethylation of aromatic thiols and aldehydes using TMS-CF₃) which are not discussed here.
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Kar, Sayan
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Integrated capture and conversion of carbon dioxide from air into methanol and other C1 products
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College of Letters, Arts and Sciences
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Doctor of Philosophy
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Chemistry
Publication Date
01/26/2020
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