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Carbon dioxide capture using silica supported organoamine adsorbents

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Carbon dioxide capture using silica supported organoamine adsorbents

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CARBON DIOXIDE CAPTURE USING SILICA SUPPORTED ORGANOAMINE
ADSORBENTS

By
Hang Zhang

__________________________________________

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(Chemistry)

May 2016

Copyright 2016 Hang Zhang

ii

To the well-being of our planet

iii

ACKNOWLEDGEMENTS
I would like to thank my advisor Professor G. K. Surya Prakash for the opportunity
to study under his supervision. Professor Prakash is a leader in the field. His knowledge,
insight, and influence set a high bar for the research conducted in our group. I am extremely
grateful for his guidance and support through the past years in research. In addition to his
successful career in advancing science and technology, Professor Prakash is the kindest
person I have ever met. He cares about his students and everyone else. Despite of his busy
schedule, Professor Prakash has willingly spent time to take care the issues I encounter so
that I can focus on research, meet my goals, and have the best outcomes. Although I might
not express it outwardly all the time, I have always been grateful for everything he has
done for me.
Professor George Olah is an exemplary role model. His pursuits for good science
and meaningful application always inspire me. It is a privilege to follow this legend. Dr.
Robert Aniszfeld impressed me for his skills in business and negotiation. He also has very
good management skills, making Loker an awesome place to work at.
I would also like to extend my appreciation to my dissertation committee members:
Professor Sri Narayan and Professor Katherine Shing, and to my qualification exam
committee members Professor Richard Brutchey and Professor Chao Zhang.
I want to give my special thanks to Dr. Alain Goeppert, who worked with me
closely in the past years. A lot of work for this dissertation is built on his previous research.
I could not have made it this far without him. I would also like to acknowledge the
following people for their constructive opinion, voluntary help, pleasant companion and so
much more: Dr. Miklos Czaun, Dr. Hema Krisman, Huong Dang, Dr. Arjun Narayanan,

iv

Dr. Patrice Batamack, Dr. Laxman Gurung, Jotheeswari Kothandaraman, Dr. Thomas
Mathew, Dr. Zhe Zhang, Kavita Belligund, Sankarganesh Krishnamoorthy, Dr. Socrates
Munoz, Dr. Bo Yang, Dean Glass, Marc Iuliucci, and Ralph Pan. I sincerely thank the
financial support provided by Loker Hydrocarbon Research Institute, Department of
Chemistry, and Morris S. Smith Endowed Fellowship.
I would not be able to do this without the support from my family. I want to thank
my parents, Jing Li and Yuansheng Zhang for the unconditional love and endless support.
They have given me their best to make sure that I could get the best education, live a
burden-free life, see the world, and be myself. I hope I have made you proud. And to my
husband Dr. Zhishan Guo, thank you for loving me and guiding me, for always being there
when I needed you.

v

TABLE OF CONTENTS
DEDICATION ii
ACKNOWLEDGEMENTS iii
TABLE OF CONTENTS v
LIST OF TABLES x
LIST OF FIGURES xi
LIST OF SCHEMES xvii
ABSTRACT xviii
Chapter 1. Introduction – CO2 capture and the development of silica supported
organoamine adsorbents 1
1.1 Introduction: CO2 capture: background and motivation 1
1.1.1 Increase in atmospheric CO2 concentration and associated problems 1
1.1.2 Carbon capture and sequestration 2
1.1.3 Direct air capture 6
1.1.4 CO2 as an important feedstock (Methanol Economy) 7
1.2 Types of solid adsorbents for CO2 capture 10
1.3 Silica supported organoamine adsorbents for CO2 capture 13
1.3.1 Early development of silica supported organoamine adsorbents 13
1.3.2 Classes of the silica supported organoamine material 14
1.4 Conclusion 17
1.5 References 20

vi

Chapter 2. Linear polyethylenimine supported on fumed silica as a CO2
adsorbent and its remarkable desorption kinetics 24
2.1 Introduction 24
2.2 Results and Discussion 25
2.2.1 Heat of CO2 adsorption/desorption 25
2.2.2 Comparison of linear with branched PEI 27
2.2.3 Effect of temperature and CO2 concentration on the CO2 adsorption 33
2.2.4 Effect of LPEI molecular weight 36
2.2.5 Effect of LPEI loading 40
2.2.6 Adsorption of CO2 from the air 43
2.2.7 Adsorption/desorption capacity of FS-LPEI over numerous cycles 45
2.2.8 Stability of the adsorbents based on linear PEI 47
2.3 Conclusion 51
2.4 Experimental 53
2.4.1 Chemicals 53
2.4.2 Preparation of the adsorbents 53
2.4.3 Equipment and methods 54
2.5 References 56

vii

Chapter 3. Mesocellular silica foam with expanded pore volume as an
efficient support for CO2 capture 58
3.1 Introduction 58
3.2 Results and Discussion 61
3.2.1 Preparation of MCF supports: screening of synthesis conditions 61
3.2.2 Preparation of CO2 adsorbents based on MCF and PEI 69
3.2.3 CO2 adsorption capacity measurements 76
3.3 Conclusion 92
3.4 Experimental 93
3.4.1 Chemicals 93
3.4.2 Preparation of adsorbent supports 94
3.4.3 Preparation of adsorbents 95
3.4.4 Measurement of CO2 adsorption capacity 95
3.4.5 Characterization 96
3.5 References 98
Chapter 4. Applicability of fumed silica supported branched polyethylenimine
as an adsorbent for direct air capture 101
4.1 Introduction 101
4.2 Results and Discussion 102
4.2.1 Effect of the particle size of the adsorbent 103

viii

4.2.2 Influence of PEI loading 107
4.2.3 Effect of temperature on the adsorption 112
4.2.4 Effect of flow rate on the adsorption 115
4.2.5 Desorption and adsorption cycling 118
4.2.6 Heat (enthalpy) of adsorption and desorption of CO2 on FS-PEI 121
4.2.7 Effect of the molecular weight of PEI on the adsorption 125
4.3 Conclusion 128
4.4 Experimental 130
4.4.1 Chemicals 130
4.4.2 Preparation of adsorbents 131
4.4.3 Characterization 131
4.4.4 Measurement of CO2 adsorption and desorption capacity 132
4.5 References 134
Chapter 5. Criteria of selecting an application-specific supported amine
adsorbent based on CO2 capture conditions-a literature review 138
5.1 Introduction 138
5.2 Selecting a solid support 140
5.3 Selecting the amine based on CO2 concentration 143
5.3.1 CO2 capture from concentrated sources 143
5.3.2 CO2 capture from near ambient air concentration 144

ix

5.4 Selecting the amine based on capture temperature 146
5.4.1 Adsorption Stage: thermodynamics vs diffusion control 147
5.4.2 Desorption Stage: energy vs kinetics 148
5.4.3 The problem with amine leaching and thermal degradation 149
5.5 Selecting the amine based on humidity level 150
5.6 Selecting the amine based the type of gas components 152
5.6.1 Oxidative degradation 152
5.6.2 CO2-induced degradation 154
5.6.3 Other gases of concern (H2S, SOx, and NOx) 155
5.7 Conclusion 158
5.8 References 161
BIBLIOGRAPHY 165

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LIST OF TABLES
Table 2-1 Stability of the adsorbents based on FS-LPEI(5000) ....................................... 48
Table 3-1 Physical data for supports prepared with various NH4F concentrations .......... 63
Table 3-2 Effect of heating rate on the physical characteristics of MCFs ........................ 65
Table 3-3 Effect of the addition of trimethylbenzeneand equilibration time on the
structure of MCFs ...................................................................................................... 66
Table 3-4 Characteristics of the supports used in the study of the effect of pore
volume and pore diameter on the preparation of PEI-based adsorbents and
their CO2 adsorption capacity .................................................................................... 70
Table 3-5 Aspect of the adsorbents prepared from MCF and PEI25k .............................. 73
Table 3-6 Surface area of the adsorbents prepared from MCF and PEI25k ..................... 74
Table 3-7 Total pore volume of the adsorbents prepared from MCF and PEI25k ........... 74
Table 3-8 CO2 adsorption capacity on MCF/PEI25k with various PEI loading ............... 77
Table 4-1 Effect of PEI loading on the CO2 adsorption characteristics at 25 °C ........... 108
Table 4-2 Temperature dependence of adsorption characteristics of FS-PEI-33 ........... 114
Table 4-3 Temperature dependence of adsorption characteristics of FS-PEI-50 ........... 115
Table 4-4 Temperature dependence of adsorption characteristics of FS-PEI-33
and FS-PEI-50 ......................................................................................................... 117
Table 4-5 Effect of PEI molecular weight on the adsorption of CO2 from the air ......... 128
Table 4-6 Comparison of FS-PEI with other solid adsorbents based on PEI for
CO2 adsorption from the air ..................................................................................... 128
Table 5-1 Gas stream conditions of representative applications .................................... 139
Table 5-2 CO2 levels and the effects of exposure on humans ........................................ 145

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LIST OF FIGURES
Figure 1-1 Atmospheric CO2 concentration measured at the Mauna Loa
Observatory .................................................................................................................. 2
Figure 1-2 Illustration of carbon capture and sequestration ............................................... 3
Figure 1-3 Major CCS approaches for CO2 capture from point sources ............................ 5
Figure 1-4 The Methanol Economy .................................................................................... 9
Figure 1-5 George Olah renewable methanol plant in Iceland ......................................... 10
Figure 1-6 Classes of amine-inorganic hybrid CO2 adsorbents and examples of
adsorbents .................................................................................................................. 15
Figure 1-7 Structures of common polyamines used in Class 1 adsorbents ....................... 16
Figure 1-8 Structures of common aminosilanes used in Class 2 adsorbent
preparation ................................................................................................................. 17
Figure 1-9 Structures of monomers used in the preparation of Class 3 adsorbents .......... 17
Figure 2-1 Heat of adsorption for aqueous amine systems ............................................... 26
Figure 2-2 Heat of CO2 adsorption at various temperatures (a) as a function of
molecular weight. (b) as a function of LPEI(25000) weight loading in FS-
LPEI(25000) .............................................................................................................. 27
Figure 2-3 CO2 adsorption kinetics on linear and branched PEIs at 25, 55, and 85 °C
................................................................................................................................... 29
Figure 2-4 CO2 desorption kinetics on linear and branched PEIs at 55, 70, and 85 °C
................................................................................................................................... 30
Figure 2-5 CO2 desorption on FS-LPEI(5000) at temperatures from 30 to 60 °C ........... 32

xii

Figure 2-6 Ln (CO2 desorbed) versus time at various temperatures on FS-
LPEI(5000)-48 ........................................................................................................... 32
Figure 2-7 Arrhenius plot for the CO2 desorption on FS-LPEI(5000)-48 ........................ 33
Figure 2-8 Effect of temperature on the adsorption capacity on FS-LPEI(5000)-47.8
as a function of CO2 concentration ............................................................................ 34
Figure 2-9 Effect of CO2 concentration on the adsorption capacity on FS-
LPEI(5000)-47.8 as a function of temperature .......................................................... 36
Figure 2-10 CO2 adsorption from a gas mixture containing 95% CO2 in N2 on FS-
LPEI with various Mw as a function of temperature .................................................. 37
Figure 2-11 CO2 adsorption from a gas mixture containing 10% CO2 in N2 on FS-
LPEI with various Mw as a function of temperature .................................................. 38
Figure 2-12 Effect of LPEI molecular weight on the adsorption capacity with 10%
CO2 ............................................................................................................................ 39
Figure 2-13 Adsorption/desorption cycles at 85 °C on FS-LPEI. Effect of LPEI
molecular weight. Adsorption under 95% CO2 for 15 min, desorption under
pure N2 for 25 min. Insert: Percentage of the adsorption capacity after 15 min
compared to 3 h adsorption at 85 °C ......................................................................... 39
Figure 2-14 Effect of the LPEI loading on the CO2 adsorption capacity on FS-
LPEI(25000). Adsorption under 95% CO2 ................................................................ 40
Figure 2-15 Efficiency of PEI utilization on FS-LPEI(25000) with various LPEI
loadings ...................................................................................................................... 42
Figure 2-16 Adsorption/desorption cycles at 85 °C on FS-LPEI(25000). Effect of
LPEI loading. Adsorption under 95% CO2 for 15 min, desorption under pure

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N2 for 25 min. Insert: Percentage of the adsorption capacity after 15 min
compared to 3 h adsorption at 85 °C ......................................................................... 43
Figure 2-17 CO2 adsorption from simulated air at 25 °C on FS-LPEI with various
molecular weights ...................................................................................................... 45
Figure 2-18 Adsorption capacity of FS-LPEI(25000) over numerous ads/des cycles
................................................................................................................................... 46
Figure 2-19 TGA plot of 180 ads/des cycles on FS-LPEI(25000) performed at 55 °C
................................................................................................................................... 47
Figure 2-20 FS-LPEI(5000) sample weight vs time under N2, air, or CO2 at 70 and
100 °C ........................................................................................................................ 48
Figure 3-1 Influence of the concentration of ammonium fluoride on (a) the surface
area of MCF (BET), (b) the average pore diameter of MCF, and (c) the total
pore volume of MCF. ................................................................................................ 64
Figure 3-2 TEM pictures of supports prepared with and without TMB and after
various equilibration times ........................................................................................ 68
Figure 3-3 N2 adsorption/desorption isotherm for MCF S1 to S5. ................................... 71
Figure 3-4 Pore size distribution of MCF S1 to S5 .......................................................... 71
Figure 3-5 Cumulative pore volume as a function of pore diameter of MCF S1 to
S5 ............................................................................................................................... 72
Figure 3-6 Total pore volume of adsorbents based on MCFs as a function of PEI25k
loading in the adsorbent ............................................................................................. 74
Figure 3-7 Pore Distribution of S5 MCF impregnated with PEI in various
concentration .............................................................................................................. 75

xiv

Figure 3-8 CO2 adsorption capacity on Sx-PEI25k as a function of PEI loading. (a)
25 °C, (b) 55 °C, and (c) 85 °C. ................................................................................. 78
Figure 3-9 Effect of the pore volume of the support on CO2 adsorption capacity of
adsorbents prepared with various concentrations of PEI25k (a) 25 °C, (b) 55 °C,
and (c) 85 °C. ............................................................................................................. 79
Figure 3-10 Effectiveness of PEI utilization. CO2 Adsorption capacity in mg CO2/g
PEI in the adsorbent measured at various temperatures on S5-PEI25k as a
function of PEI loading. ............................................................................................. 81
Figure 3-11 Desorption completion at 85 °C as a function of time of adsorbents
based on S5 containing PEI25k loadings from 50% to 83% ..................................... 82
Figure 3-12 Effect of the Mw of PEI on the adsorption capacity at various
temperatures ............................................................................................................... 84
Figure 3-13 TGA plot of 100 Adsorption/desorption cycles performed at 75 °C on
(a) S5-PEI800-80, (b) S5-PEI1800-80, and (c) S5-PEI25k-80 ................................. 87
Figure 3-14 Stability of S5-PEIx sorbents over 100 adsorption/desorption cycles at
75 °C .......................................................................................................................... 88
Figure 3-15 First five adsorption/desorption cycles on S5-PEI-80 containing PEI
with various Mw. ........................................................................................................ 89
Figure 3-16 Desorption capacity as a function of time for adsorbents containing PEI
with various Mw and concentrations .......................................................................... 90
Figure 3-17 Adsorption capacity as a function of time for the adsorbents containing
PEI with various Mw and PEI concentrations of 50% and 80% ................................ 91

xv

Figure 3-18 Adsorption capacity in short adsorption/desorption cycles at 75 °C as
a function of PEI Mw and concentration .................................................................... 92
Figure 4-1 Effect of the particle size of FS-PEI-50 on the CO2 adsorption from air
at 25 °C .................................................................................................................... 105
Figure 4-2 Total CO2 adsorption and CO2 adsorption under 10 ppm as a function
of FS-PEI-50 particle size ........................................................................................ 106
Figure 4-3 Effect of the particle size of FS-PEI-50 on CO2 desorption at 85 °C ........... 107
Figure 4-4 Effect of PEI loading in FS-PEI on CO2 adsorption from air at 25 °C ......... 109
Figure 4-5 Total CO2 adsorption and CO2 adsorption under 10 ppm as a function
of PEI loading in FS-PEI ......................................................................................... 109
Figure 4-6 Effect of PEI loading on the pore size distribution of FS-PEI ...................... 109
Figure 4-7 Efficiency of PEI utilization for CO2 adsorption from air at 25 °C as a
function of PEI loading in FS-PEI ........................................................................... 111
Figure 4-8 CO2 adsorption from air as a function of time on FS-PEI-33 at various
temperatures ............................................................................................................. 113
Figure 4-9 Adsorption as a function of temperature on FS-PEI-33 ................................ 113
Figure 4-10 CO2 adsorption from air as a function of time on FS-PEI-50 at various
temperatures ............................................................................................................. 114
Figure 4-11 CO2 adsorption as a function of temperature on FS-PEI-50 ....................... 114
Figure 4-12 Effect of air flow rate on the CO2adsorption as a function of time on
FS-PEI-33 ................................................................................................................ 116
Figure 4-13 Effect of air flow rate on the CO2 adsorption as a function of time on
FS-PEI-50 ................................................................................................................ 117

xvi

Figure 4-14 Effect of temperature on the CO2 desorption as a function of time on
FS-PEI-50 ................................................................................................................ 119
Figure 4-15 CO2 desorption under air on FS-PEI-50 at various temperatures.
Desorption conducted after CO2 adsorption from air3 (400 ppm) at 25 °C. ........... 120
Figure 4-16 Effect of flow rate on the CO2 desorption at 85 °C as a function of time
on FS-PEI-50 ........................................................................................................... 120
Figure 4-17 Adsorption/desorption cycles for FS-PEI-33 and FS-PEI-50 ..................... 121
Figure 4-18 Typical heat of adsorption graph obtained on the Perkin Elmer DSC 7
for FS-PEI-33. Isotherm 85 °C for 20 min .............................................................. 122
Figure 4-19 Ln of the CO2 concentration as a function of time at various
temperatures on FS-PEI-50 ...................................................................................... 125
Figure 4-20 Arrhenius plot for the CO2 desorption on FS-PEI-50 ................................. 125
Figure 4-21 Effect of PEI molecular weight on CO2 adsorption from the air. (a) On
FS-PEI(x)-33, (b) On FS-PEI(x)-50 ........................................................................ 127
Figure 5-1 Reaction of grafted DMAPS with CO2 in presence of water ........................ 151
Figure 5-2 Degradation species for branched and linear PEIs ........................................ 153
Figure 5-3 Sayari et al. proposed CO2-induced degradation mechanisms ..................... 155

xvii

LIST OF SCHEMES
Scheme 1-1 Carbonation and calcination processes for calcium oxide ............................ 11
Scheme 1-2 Reaction of amine and CO2 under dry and humid conditions ...................... 12
Scheme 2-1 Reaction of linear PEI with CO2 under dry and humid conditions ............... 24
Scheme 2-2 Oxidative degradation of FS-LPEI ............................................................... 50
Scheme 4-1 Reaction of CO2 with PEI ........................................................................... 103

xviii

ABSTRACT
The rapid increase in the atmospheric carbon dioxide (CO2) concentration is
broadly acknowledged as the main cause for the currently observed global warming and
climate change. Concepts such as carbon capture and sequestration (CCS) and direct air
capture (DAC) are proposed to mitigate the problems associated with excessive
anthropogenic carbon emissions. Interest in capturing CO2 from large point sources as well
as from the air is rising in scientific and engineering communities. Among many options
studied, silica supported organoamine materials have emerged as a promising type of CO2
adsorbent. Due to the complex nature of the reaction between CO2 and the silica supported
organoamine, as it often involves several elementary steps, more fundamental research is
needed to understand the reaction process and optimize the adsorbents' structures for
capturing CO2 from various point sources and for diverse purposes.
This dissertation describes the development of silica supported organoamine
materials and their practical use as carbon dioxide (CO2) adsorbents. A particular focus of
the dissertation is to understand the structural influence of the organoamine and the silica
support on the CO2 capture characteristics, including CO2 uptake, adsorption and
desorption kinetics, material stability, and its response towards various CO2 concentration,
temperature and relative humidity.
The first chapter introduces the background and motivation for CO2 capture. CCS
and DAC concepts are explained in summary to provide the context for this dissertation.
The development of various adsorbents and capture systems are described, with an
emphasis on amine-inorganic oxides hybrid material, especially silica supported

xix

organoamine adsorbents. Chapter 1 also summarizes the Olah/Prakash Group's efforts in
exploiting materials of similar kinds.
The organoamine and silica support are two major components in this type of
materials, and therefore are discussed in detail in Chapter 2 and Chapter 3, respectively. In
Chapter 2, a remarkably fast desorption kinetics was observed with the adsorbent based on
linear polyethylenimine and fumed silica. Compared to its branched counterpart, linear
polyethylenimine exhibited similar adsorption capacity and kinetics under test conditions,
but was able to desorb CO2 at lower temperatures and faster rates. It offers significant
energy saving in the regeneration step as well as faster adsorption/desorption cycling.
Aspects on the effect of temperature, humidity, stability, and the applicability for capturing
CO2 at concentration of 400 ppm were also examined in detail.
Chapter 3 describes the synthesis of an ultra-large pore volume mesocelluar foam
(MCF) silica support through a sol-gel approach with a triblock copolymer surfactant and
trimethylbenzene (TMB) as a swelling agent. A pore volume of 4.17 cm
3
/g obtained with
some of the MCF was among the highest reported in the literature. To study the effect of
support structure on CO2 capture, MCFs with a wide range of pore volumes (0.98 - 4.17
cm
3
/g) were prepared by varying synthesis parameters such as ammonium fluoride
concentration, equilibration time of the swelling agent, and calcination heating rate. The
obtained MCFs were impregnated with branched polyethylenimine. Higher amine loading
was achieved with larger pore volume MCF. The excess of pore volume also enabled better
distribution of PEI in the pores, assisted the CO2 diffusion and its ability to reach to the
active amine sites, and consequently improved adsorption capacity and kinetics. The

xx

highest adsorption capacity at 265 mg CO2/g (6 mmol CO2/g) was realized at 85 °C using
MCF-4.17 and PEI800.
In Chapter 4, a type of adsorbent based on fumed silica supported branched
polyethylenimine is evaluated for its applicability in various real life conditions. They were
exposed to exhaustive conditions, including an array of CO2 concentrations,
adsorption/desorption temperature, relative humidity, and others. Additionally, the capture
of CO2 from a gas mixture containing only 400 ppm CO2 (ambient CO2 concentration) was
also studied.
Finally, the research on the development of supported organoamine materials for
CO2 capture is concluded in Chapter 5. Due to the variety and complexity of the CO2
capture applications, a list of criteria is given on selecting adsorbents based on the CO2
concentration, temperature, humidity level and other gas components in the streams. The
list is summarized from our previous experience and other literature on supported
organoamine adsorbents. At the end, future work and direction toward the ongoing
development of amine functionalized solid adsorbents is provided.

1

Chapter 1. Introduction - CO2 capture and the development of silica supported
organoamine adsorbents
1.1 Introduction: CO2 capture: background and motivation
1.1.1 Increase in atmospheric CO2 concentration and associated problems
Carbon dioxide (CO2) is a naturally occurring gas in the Earth's atmosphere. Its
concentration had been regulated by the nature through the absorption from carbon sinks
on land and in the oceans, and by the respiration effect of the plants and animals.
Unfortunately, this natural equilibrium has been disrupted by humankind. Since the
Industrial Revolution, fossil fuels have been powering up human activities, but this was
inevitably accompanied by CO2 emissions. The anthropogenic CO2 emission has been
outpacing nature's carbon cycles, leaving the excess CO2 in the atmosphere. In recent years,
the burning of fossil fuel resulted in enormous amount CO2 release exceeding 30 Gt in
2008, and approximately 43% of the annual CO2 emissions remained in the atmosphere.
1

In the preindustrial era, the atmospheric CO2 level was 280 ppm. As it continued to increase,
in 2006, the CO2 concentration was 381 ppm and today it is at 402 ppm.
2
The obvious
increasing trend is shown Figure 1-1, measured at the Mauna Loa Observatory in Hawaii.
According to information obtained from ice cores, the present CO2 concentration
represents the highest level in the past 65000 years.
3

As fossil fuels continue to serve as the main energy supply for human activities, the
continual increase in CO2 concentration is inevitable. CO2 is a well acknowledged
greenhouse gas (GHG) and one of the largest contributors to the greenhouse effect, which
keeps earth’s temperature at an acceptable level to sustain life. The ever increasing CO2

2

level is associated with the currently observed global warming and climate changes. Other
environmental consequences include rising sea levels, ocean acidification, increased risk
of drought and floods, and loss of biodiversity.

Figure 1-1 Atmospheric CO2concentration measured at the Mauna Loa Observatory
4

To prevent the further increase in CO2 concentrations beyond the hazardous level
of 450 ppm and avoid unwanted climate change, there is an urgency in controlling
anthropogenic CO2 emissions and the atmospheric CO2 levels.
1.1.2 Carbon capture and sequestration
Carbon dioxide (CO2) emissions from large point sources make up to 60% in the
global emission portfolio. Such large point sources include fossil fuel burning power plants,
cement production facilities, refineries, iron, steel and aluminum industries, petrochemical
companies, oil and gas processing sites, and biomass industry. To reduce the emissions
from these large point sources, carbon dioxide capture and sequestration (or storage, CCS)
is proposed. It is described as a set of technologies consisting CO2 separation from the flue
gases, transportation of the captured and compressed CO2 and secure storage in depleted

3

gas and oil fields, saline formations, deep aquifers and in the ocean or utilization in other
industrial processes (Figure 1-2).

Figure 1-2 Illustration of carbon capture and sequestration (courtesy of CO2CRC)
5

The carbon dioxide capture at fossil fuel burning sites is achieved by one of the
three approaches: pre-combustion capture, post-combustion capture, and oxyfuel
combustion (Figure 1-3).
In the pre-combustion configuration, an air separation unit is required to first isolate
the oxygen from the air. Fuel is injected with the oxygen and water into a gasifier to
produce synthesis gas (syngas), containing mostly hydrogen and carbon monoxide (CO).
Subsequently, CO is converted to CO2 in a water-gas shift reactor. The CO2 concentration
in the gas stream ranges from 15% to 50%, is removed and compressed for storage or
utilization. The process is also known as integrated gasification combined cycle (IGCC).

4

It not only reduces CO2 emission, maximizes power output, but also ensures a pollutant-
free exhaust. IGCC is recommended for inefficient but highly polluting pulverized coal
burning processes.
The post-combustion system separates CO2 in the flue gases produced by
combustion of fossil fuels with the air. Depending on the sources of the fuel, flue gases
often contain 3-15% CO2 with the rest being primarily nitrogen, the major constituent in
the air. The CO2 concentration in post-combustion capture is much more dilute than in the
pre-combustion capture. An advantage of this process is that a capture unit can be
retrofitted into the pre-existing infrastructure.
The oxy-fuel combustion system is similar to the conventional combustion except
that pure oxygen is used, instead of air, to burn the fuel. The oxygen-rich atmosphere allows
for higher CO2 concentration in the exhaust gas leading to comparatively easier purification.
Its flue gas contains water vapor and highly concentrated CO2 steam (up to 80%).
6
In some
cases, oxyfuel combustion flue gas is considered sequestration ready. Challenges in
operating the oxyfuel approach lie in the required costly oxygen separation. Conventional
cryogenic air separation technology is expensive. Nevertheless, with the advancements in
oxygen-selective membrane materials,
7
oxyfuel combustion capture is becoming more and
more promising.
At large point sources like fossil fuel burning sites, 85-90% of the total CO2 can be
captured through one of the above mentioned technologies. Although many studies have
conducted to analyze the technical and economic feasibility of these technologies, due to
the lack of experience in operation, there is not one preferred approach. The conditions of
post-combustion capture are most commonly applied while testing the performance of the

5

adsorbents because it is believed that the capture unit can be easily retrofitted to the existing
plants.
CCS in other industrial-related processes is also of interest to society to reduce
carbon emissions. In the oil and natural gas industry, a gas sweetening procedure is applied
to the raw gas to remove acidic gas components such as CO2 and hydrogen sulfide (H2S),
which could be highly corrosive to the pipeline in contact with water. According to
International Energy Agency, cement manufacturing accounts for 5% of the global carbon
dioxide emission. The cement production requires heating clay and limestone up to 1400 °C
in kilns, which releases CO2 both directly and indirectly. The decomposition of calcium
carbonate produces CO2 in 1 to 1 molar ratio. And the heating to such high temperature
requires intensive energy input through fossil fuel burning, accompanied by large amounts
of CO2. The CO2 emissions in the other industrial fields also need to be addressed, and
CCS provides a possible solution.

Figure 1-3 Major CCS approaches for CO2 capture from point sources (adapted from
IPCC report)
8

6

A few representative CO2 adsorbents and capture systems are discussed in the next
sections (1.2 and 1.3). Also discussed are their advantages and disadvantages. The key
factor in realizing any of the CCS approaches lies in finding efficient material and
technology for CO2 capture. Thus, searching for an economical feasible adsorbent is the
first step. Also, towards the commercialization of CCS, collaboration among scientists,
engineers and policymakers are key to make this strategy succeed on a large scale.
1.1.3 Direct air capture
In addition to conventional CCS, capturing CO2 from ambient air, commonly
referred to as direct air capture (DAC), has recently attracted increasing interest.
Approximately, 50% of the anthropogenic CO2 emissions originates from small and
distributed sources such as cars, airplanes, home and office HVACs. Collecting CO 2
directly at the emission source from millions, even billions of the small units is difficult
and essentially impractical and not economical. Therefore, direct air capture offers a great
advantage due to its flexibility. The infrastructure for capturing CO2 can be located
anywhere, preferably close to CO2 storage sites or CO2 recycling centers, and where it has
limited impact on the environment and human activities. The atmosphere would essentially
serve as a conveyor belt for CO2 with limited need for an infrastructure to transport CO2
over long distances to sequestration or recycling sites. Lackner et al. suggested the concept
of air capture in 1999. Over time, the group of Lackner updated with economic calculation,
suggesting DAC is a viable solution of reducing global CO2 concentration.
9

With respect to the challenges, the biggest difference between DAC and CCS is the
CO2 concentration in the gas streams. In DAC, atmospheric level of CO2 is used and the
current level is about 400 ppm, while CO2 concentrations in flue gases, depending on the

7

sources, are in the range of 3 to 15%. In other words, CO2 concentration in flue gases with
10% CO2 concentration is 250 times higher than the atmospheric level. The considerably
low concentration of CO2 in ambient air poses considerable technical challenges. First,
CO2 represents only 1 out of 2500 molecules in the air. In order to adsorb 1 CO2 molecule,
the rest of the 2500 molecules also need to pass through the adsorbent system. The process
is expensive because of the force needed to drive vast amounts of air flow. Secondly, the
current system used for pre- or post-combustion capture are intended for CO2 adsorption
from flue gases or more concentrated sources. Much less attention has been given to CO 2
capture from the air, for which available data is still limited and has only attracted
increasing interest recently. Whereas practical applications have been developed for
essential removal of CO2 from submarines and spacecrafts, the separation and recovery of
CO2 from ambient air on a larger scale is still in its infancy. Adsorbents based on strong
bases, such as Ca(OH)2, NaOH, and combination of thereof, have been proposed, but their
use is energy intensive because of the high temperatures required for the regeneration step.
Theoretically, DAC has more demanding requirements for the capturing technology and
engineering design than conventional CCS, but scrubbing CO2 from the sky has many
advantages and would be the only solution to achieve negative carbon emission.
1.1.4 CO2 as an important feedstock (Methanol Economy)
Carbon dioxide is not merely a waste product. Many have the vision for it to be an
important, abundant, and cheap feedstock for fuels and raw industrial materials.
10-11
In
synthetic chemistry, CO2 is used as a starting material and neutralizer for alkaline effluents.
It is also the primary material for producing urea fertilizer. CO2 is also used to manufacture
inorganic carbonates, organic monomers, polycarbonates and polyurethanes. In addition,

8

it is known that fuel containing one carbon such as methane (CH4), methanol (CH3OH) and
formic acid (HCOOH) can be obtained by converting CO2 with hydrogen. Specifically, the
concept of Methanol Economy is proposed by the Olah/Prakash group suggesting using
methanol as an energy storage medium.
12
This approach promises to liberate humankind
from the dependence on oil and gas and mitigate the global warming problem. Methanol
can be obtained from CO2 by catalytic conversion using hydrogen, or by electrochemical
reduction methods. The anthropogenic chemical carbon cycle based on CO2 and methanol
is presented in Figure 1-4, providing a feasible alternative to nature's photosynthesis.
13
First,
the burning of fuels supplies energy for various human activities, such as industrial
production manufacture, transportation, cooking and heating. The released CO2 is captured
from concentrated point sources or directly from the air. Then, part of the CO2 is
catalytically, photochemically, or electrochemically converted to fuels including methanol,
dimethyl ether, synthetic hydrocarbon and others. In this process, renewable energy
sources (solar, winder, geothermal) and water which are readily available on most parts of
the Earth are used. The unused portion of CO2 can be stored in depleted oil and natural gas
fields and deep underground or in aquifers for future use. To complete the cycle, methanol

9

is utilized as a fuel and energy storage medium, and transformed to other synthetic
hydrocarbons, materials and other products.

Figure 1-4 The Methanol Economy
To date, the largest CO2 based methanol plant, the George Olah renewable
methanol plant (Figure 1-5) was built by Carbon Recycling International in Iceland. In

10

2012, it started production of about 2 million liters of methanol per year. The plant has now
an annual capacity of 5 million liters of methanol from 5500 tonnes of CO2.

Figure 1-5 George Olah renewable methanol plant in Iceland
14

1.2 Types of solid adsorbents for CO2 capture
Among the existing CO2 separation technologies,
15-16
amine scrubbing, used
industrially for many decades, is considered one of the most suitable methods for
separating CO2 from high volume flue gas streams. In a typical process, 20 to 30% aqueous
solutions of alkanolamines or proprietary hindered amines, such as monoethanolamine
(MEA), diethanolamine (DEA) and KS-1 are used as absorbents.
11,17-18
CO2 desorption is
achieved by heating the aqueous solution to a higher temperature, typically 100 – 140 °C.
Although it is technologically feasible, there are several acknowledged drawbacks
associated with aqueous amine systems, with the demanding energy supply for the
regeneration step being the primary one. Other problems are amine degradation, corrosion
and slow absorption/desorption kinetics.
7

11

In order to reduce the energy requirement of the desorption step and avoid heating
large amounts of water as well as overcome some of the other drawbacks of using liquid
amines, many research groups have proposed using solid adsorbents as an alternative.
19-21

A variety of solid adsorbents have been examined for CO2 capture, and they can be roughly
divided into inorganic and organic sorbents based on the presence of organic materials in
the adsorbent. Inorganic sorbents, including zeolite, activated carbon, alkali and earth alkali
metal oxides are widely considered due to their traditional roles in gas separations. For
example, zeolites are a category of porous crystalline aluminosilicate materials that are
known to adsorb acidic gas molecules like CO2. With zeolites, the majority of the CO2 is
physically adsorbed on the surface or trapped in the pore structure of the adsorbent.
Therefore, they belong to the physisorbent category of the inorganic adsorbent. They have
been used in some industrial applications for CO2 separation and showed high adsorption
capacity for pure CO2. However, they suffer from low selectivity due to the fact that CO2
is only physically adsorbed on the surface. These physical adsorbents are therefore
generally considered impractical for the separation of gases at low concentrations. Calcium
oxide (CaO) represents another subset of inorganic adsorbent. The adsorption happens due
to the chemical reaction between calcium oxide and CO2 yielding calcium carbonate
(CaCO3), a process known as carbonation (Scheme 1-1). Theoretically, the adsorption
capacity of CaO materials is 17.8 mmol/g based on the 1 to 1 stoichiometric ratio. CaO is
a widely available raw material from limestones and dolomites. Yet, there are drawbacks
limiting the applicability of CaO only to processes involving high temperatures. The first
drawback is the slow adsorption reaction kinetics controlled by CO2 diffusion. Due to
inadequate pore volume and channels in the product, the reaction rate is significantly

12

hindered after the initial formation of CaCO3. As already known, high temperature
promotes gas diffusion, therefore, CaO is more applicable to CO2 adsorption at high
temperatures to counter the limitation from slow diffusion. Secondly, CaO’s regenerability
and adsorption capacity decreases over multiple adsorption-desorption cycles. Studies have
shown that the volume of the pore decreases over time possibly through pore collapse and
blockage.
Scheme 1-1 Carbonation and calcination processes for calcium oxide
Adsorption (carbonation): CaO + CO2 CaCO3
Desorption (calcination): CaCO3 CaO + CO2
In order to obtain a solid adsorbent with higher CO2 selectivity, better regenerability
and wider applicability, a type of organic-inorganic hybrid material has emerged. Built on
the existing knowledge on liquid amine systems and some of the solid adsorbent systems,
it was logical and creative to think about merging the two systems. The organic component
of the system provides selectivity while the solid system affords a water-free setting
reducing the energy cost in the regeneration step.
Results show that the reaction mechanism between CO2 and the supported
organoamine are similar to the ones observed in liquid amine system.
22
Under dry
conditions, primary and secondary amines react with CO2 to form carbamate salts or
carbamic acids (Scheme 1-2). Theoretically, two amine groups are needed to stabilize each
CO2 molecule. Tertiary amines do not react with CO2 under dry conditions. In the presence
of water, only one amino group is needed for each CO2, forming bicarbonate species. Here,
water acts like a free base. Tertiary amines are also capable of stabilizing CO2 under humid
conditions. Thus, water improves the amine efficiency in capturing CO2. The theoretical

13

adsorption capacity is also governed by the availability of the CO2 molecules at the reaction
sites, which is limited by gas diffusion into the pore channels.
Scheme 1-2 Reaction of amine and CO2 under dry and humid conditions

1.3 Silica supported organoamine adsorbents for CO2 capture
1.3.1 Early development of silica supported organoamine adsorbents
As early as the 1990s, Tsuda et al. reported a suspension of polyethylenimines and
silica gel in water or DMF, which could absorb and desorb CO2 effectively.
23
However, it
was not until 2002 that silica supported organoamine material arouse broad interest for CO2
capture applications. Based on the study performed by Xu et al., a synergetic effect, with
respect to adsorption capacity and sorption/desorption kinetics, between PEI and the
inorganic support was observed. Ordered mesoporous silica materials such as MCM-41
and SBA-15 were used, and the concept was named “molecular basket”.
24-27
Separately,
MCM-41 adsorbed 8.6 mg CO2/g and pure PEI adsorbed 109 mg CO2/g. However, when
50 wt% PEI was supported on the MCM-41, the adsorbent captured 112 mg CO2/g, almost
twice higher than the combined capacity of individual MCM-41 and PEI (8.6/2+109/2=59
mg CO2/g). The desorption capacity (%) for the PEI was also increased from 56.4% to
99.8%. The enhanced adsorption/desorption characteristics are due to the porous support
providing an open and accessible backbone for the stabilization of a large number of amine
adsorption sites. Meanwhile, the field of synthesizing ordered mesoporous silica positively

14

influenced the development of silica supported organoamine adsorbents.
28-29
This type of
material belongs to the Class 1 adsorbents, which is described in the next section.
With respect to the organic content, polymeric amines with low volatility are
preferred in this approach because they do not suffer from leaching problems encountered
with most of the lower molecular weight amines.
30-36
Polyethylenimine (PEI) in particular
has been a popular choice for CO2 capture studies of Class 1 adsorbents due to its
widespread availability, relatively low cost and high CO2 adsorption capacity.
1.3.2 Classes of the silica supported organoamine material
Adsorbents based on silica supported organoamine have attracted particular interest.
Based on the interaction between the amine and the support as well as the approach used
in their preparation, such materials can be divided into three classes (Figure 1-6).
19,37-38
It
is noteworthy that this classification also applies to the general category of inorganic oxides

15

supported organoamines. In the later cases, porous alumina (Al2O3),
39
titanium oxide
(TiO2),
40
and other inorganic oxides serve as the supports.

Figure 1-6 Classes of amine-inorganic hybrid CO2 adsorbents and examples of
adsorbents
In Class 1 adsorbents, monomeric, oligomeric, and polymeric amines are physically
deposited on the support materials, usually through wet impregnation.
24
This type of

16

adsorbents are the easiest to prepare, making them potentially the most applicable to large
scale separation of CO2. They can be generated by simple mixing of the support with the
amine, either neat, or in an aqueous or organic solvent, followed by drying. For Class 1
adsorbents, polyamines or amine oligomers are frequently used. Figure 1-7 presents the
most studied amines for Class 1 adsorbents. The branched polyethylenimines vary in
molecular weights.

Figure 1-7 Structures of common polyamines used in Class 1 adsorbents
In Class 2 adsorbents, the amines are chemically grafted to the support through
covalent bonding (via a propyl linker or hydrolysis condensation
41
), preventing the possible
leaching problem.
23
Figure 1-8 shows the chemical structures of commonly used
aminosilanes in the preparation of Class 2 adsorbents. Among them, APS, MAPS and
DMAPS contain only primary, secondary and tertiary amino groups, respectively. Due to
their relatively simple structures, they are usually used for theoretical studies to understand
the fundamentals of the amine-inorganic oxide capture characteristics.
42-44

17

Figure 1-8 Structures of common aminosilanes used in Class 2 adsorbent preparation
Class 3 adsorbents consist of an inorganic support and a chemically grafted
polyamine which is prepared by in situ polymerization of an amine-containing monomer
such as aziridine.
19,45

Figure 1-9 Structures of monomers used in the preparation of Class 3 adsorbents
1.4 Conclusion
The ever-increasing atmospheric CO2 concentration is associated with the currently
observed global warming and climate change. To avoid more unforeseen problems,
concepts such as CCS and DAC are proposed to tackle our carbon conundrum. Over the
past decades, CO2 capture technologies based on liquid amine systems, physical and
chemical adsorbents have been studied extensively. Among them, silica supported
organoamine adsorbents have shown great promises. As efficient adsorbents, the material

18

should (a) be selective towards CO2, (b) have relatively high adsorption capacities under
many applications, (c) have fast adsorption/desorption kinetics, (d) be regenerable in
adsorption/desorption cycles, (e) be stable over numerous adsorption/desorption cycles,
and (f) be tolerant to other gaseous components in the gas stream. In the process of
developing new silica supported organoamine adsorbents, these parameters are studied to
certain degree. Among them, the adsorption capacity, kinetics, and regenerability are the
most covered. Although other parameters such as the desorption kinetics and stability over
many cycles, are of equal importance, they were less studied. In this dissertation, the
remarkably improved desorption kinetics observed using linear polyethylenimine is
reported, along with its applicability for capture CO2 under various gas stream conditions,
the effect of molecular weight and polyamine loading, and its stability and degradation
mechanism (Chapter 2). Also, a novel mesocellular silica foam support with ultra-large
pore volume was synthesized. An established method was used for preparing a series of
silica supports with pore volume hierarchy, and the supports were loaded with polyamine
to understand the effect of pore volume on the performance of CO2 capture (Chapter 3).
Then, a comprehensive study on an adsorbent of great potential in large scale applications,
consisting of fumed silica and branched polyethylenimine, both commercially available
and inexpensive, is presented. Practical aspects including the effect of particle size,
polyamine molecular weight and loading, adsorption temperature, flow rate of the feed gas,
and adsorption/desorption regeneration and cycling were evaluated (Chapter 4).
Despite the growing interest in developing new silica supported organoamine CO2
adsorbents, these materials are not yet designed to be application-specific. Due to the
diverse gas feed conditions and capture requirements, adsorbents suited for one application

19

may suffer from loss of performance in another one. In Chapter 5, based on our previous
experience and examples in the literature, tradeoffs are discussed for selecting application-
specific adsorbents with regard to the feed gas conditions. A list of criteria is also provided
for future reference.

20

1.5 References
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P.; Conway, T. J.; Doney, S. C.; Feely, R. A.; Foster, P.; Friedlingstein, P.; Gurney, K.;
Houghton, R. A.; House, J. I.; Huntingford, C.; Levy, P. E.; Lomas, M. R.; Majkut, J.;
Metzl, N.; Ometto, J. P.; Peters, G. P.; Prentice, I. C.; Randerson, J. T.; Running, S. W.;
Sarmiento, J. L.; Schuster, U.; Sitch, S.; Takahashi, T.; Viovy, N.; van der Werf, G. R.;
Woodward, F. I. Nat. Geosci. 2009, 2, 831.
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T.; Ciais, P.; Conway, T. J.; Gillett, N. P.; Houghton, R. A.; Marland, G. Proc. Natl. Acad.
Sci. 2007, 104, 18866.
(3) Siegenthaler, U.; Stocker T. F.; Monnin, E.; Luthi, D.; Schwander, J.;
Stauffer, B.; Raynaud, D.; Barnola, J. M.; Fischer, H.; Masson-Delmotte, V.; Jouzel, J.
Science 2005, 310, 1313.
(4) National Oceanic and Atmospheric Administration, Full Mauna Loa CO2
Record http://www.esrl.noaa.gov/gmd/ccgg/trends/full.html
(5) CO2CRC, http://www.co2crc.com.au/imagelibrary/
(6) D'Alessandro, D. M.; Smit, B.; Long, J. R. Angew. Chem. Intl. Ed. 2010, 49,
6058.
(7) Zeng, Q.; Zuo, Y.; Fan, C.; Chen, C., J. Membr. Sci. 2009, 335, 140.
(8) IPCC Carbon Dioxide Capture and Storage Special Report. Cambridge
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(10) Chueh, W. C.; Falter C.; Abbott, M.; Scipio, D.; Furler, P.; Haile, S. M.;
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(11) Goeppert, A.; Czaun, M.; Prakash, G. K. S.; Olah, G. A. Energ. Environ.
Sci. 2012, 5, 7833.
(12) Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The
Methanol Economy; 2nd ed.; WILEY-VCH: Weinheim, 2011.
(13) Olah, G. A.; Prakash, G. K. S.; Goeppert, A. J. Am. Chem. Soc. 2011, 133,
12881.
(14) CRI. World's Largest CO2 Methanol Plant
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=en
(15) MacDowell, N.; Florin, N.; Buchard, A.; Hallett, J.; Galindo, A.; Jackson,
G.; Adjiman, C. S.; Williams, C. K.; Shah, N.; Fennell, P. Energ. Environ. Sci. 2010, 3,
1645.
(16) Hedin, N.; Chen, L.; Laaksonen, A. Nanoscale 2010, 2, 1819.
(17) Kohl, A.; Nielsen, R. Gas Purification; 5th ed.; Gulf Publishing Company:
Houston, Texas, 1997.
(18) Rochelle, G. T. Science 2009, 325, 1652.
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Jones, C. W. Adv. Funct. Mater. 2009, 19, 3821.
(20) Wang, Q.; Luo, J.; Zhong, Z.; Borgna, A. Energ. Environ. Sci. 2011, 4, 42.
(21) D'alessandro, D. M.; Smit, B.; Long, J. R. Angew. Chem. Int. Ed. 2010, 49,
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(22) Serna-Guerrero, R.; Da'na, E.; Sayari, A. Ind. Eng. Chem. Res. 2008, 47,
9406.
(23) Tsuda, T.; Fujiwara, T. J. Chem. Soc. Chem. Commun. 1992, 1659.
(24) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Energ.
Fuels 2002, 16, 1463.
(25) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Micropor.
Mesopor. Mater. 2003, 62, 29.
(26) Xu, X.; Song, C.; Miller, B. G.; Scaroni, A. W. Fuel Process. Technol. 2005,
86, 1457.
(27) Xu, X. C.; Song, C. S.; Miller, B. G.; Scaroni, A. W. Ind. Eng. Chem. Res.
2005, 44, 8113.
(28) Schmidt-Winkel, P.; Lukens, W. W. J.; Zhao, D.; Yang, P.; Chmelka, B. F.;
Stucky, G. D. J. Am. Chem. Soc. 1999, 121, 254.
(29) Schmidt-Winkel, P.; Lukens, W. W.; Yang, P.; Margolese, D. I.; Lettow, J.
S.; Ying, J. Y.; Stucky, G. D. Chem. Mater. 2000, 12, 686.
(30) Goeppert, A.; Meth, S.; Prakash, G. K. S.; Olah, G. A. Energ. Environ. Sci.
2010, 3, 1949.
(31) Qi, G.; Fu, L.; Choi, B. H.; Giannelis, E. P. Energ. Environ. Sci. 2012, 5,
7368.
(32) Liu, S.-H.; Hsiao, W.-C.; Sie, W.-H. Adsorption 2012, 18, 431.
(33) Yan, W.; Tang, J.; Bian, Z.; Liu, H. Ind. Eng. Chem. Res. 2012, 51, 3653.
(34) Qi, G.; Wang, Y. B.; Estevez, L.; Duan, X. N.; Anako, N.; Park, A. H. A.;
Li, W.; Jones, C. W.; Giannelis, E. P. Energ. Environ. Sci. 2011, 4, 444.

23

(35) Wang, X.; Li, H.; Liu, H.; Hou, X. Micropor. Mesopor. Mater. 2011, 142,
564.
(36) Chen, C.; Yang, S.-T.; Ahn, W.-S.; Ryoo, R. Chem. Commun. 2009, 3627.
(37) Bollini, P.; Didas, S. A.; Jones, C. W. J. Mater. Chem. 2011, 21, 15100.
(38) Li, W.; Choi, S.; Drese, J. H.; Hornbostel, M.; Krishnan, G.; Eisenberger,
P. M.; Jones, C. W. ChemSusChem 2010, 3, 899.
(39) Bali, S.; Chen, T. T.; Chaikittisilp, W.; Jones, C. W. Energ. Fuels 2013, 27,
1547.
(40) Jiang, G.; Huang, Q.; Kenarsari, S. D.; Hu, X.; Russell, A. G.; Fan, M.;
Shen, X. Applied Energy 2015, 147, 214.
(41) Tsuda, T.; Fujiwara, T.; Taketani, Y.; Saegusa, T. Chem. Lett. 1992, 2161.
(42) Didas, S. A.; Kulkarni, A.; Sholl, D. S.; Jones, C. W. ChemSusChem 2012,
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Am. Chem. Soc. 2008, 130, 2902.

24

Chapter 2. Linear polyethylenimine supported on fumed silica as a CO2 adsorbent
and its remarkable desorption kinetics
2.1 Introduction
As mentioned in the previous Chapter, Class 1 adsorbents are the easiest to
prepare, making them potentially the most applicable materials for large scale separation
of CO2. The support and the amine can be mixed, with or without a solvent to generate an
amine impregnated hybrid (organic/inorganic) solid. Polymeric amines with low
volatility are preferred in this approach because they do not suffer from leaching
problems encountered with most of the lower molecular weight amines.
1-7
Specifically,
branched polyethylenimine (PEI) has been a popular choice for CO2 capture studies of
Class 1 adsorbents due to its widespread availability, relatively low cost, and high CO 2
adsorption capacity.
In our earlier studies, our group has focused on branched polyethylenimines
(BPEIs) impregnated on precipitated and fumed silica (PS and FS, respectively) for the
capture of CO2 from various sources including the air.
1,8-11
BPEIs contain primary,
secondary, and tertiary amines. While primary amines are very efficient in capturing CO2,
it is generally reported that they also require more energy than secondary amines to
release the adsorbed CO2 during the regeneration step. The use of a polymer containing
only secondary amines, which bind CO2 less strongly, could be therefore advantageous,
necessitating a lower energy input for the endothermic desorption process.
We present here our work on such a polymer containing only secondary amines,
namely linear polyethylenimine (LPEI) impregnated on fumed silica. In the presence of

25

CO2, LPEI can form carbamates, which can further react to form bicarbonates in the
presence of water (Scheme 2-1). Various aspects including the effect of the LPEIs
molecular weight, loading, adsorption and desorption temperature on the adsorption
capacity for CO2 capture from various sources including the air are investigated and
discussed.
Scheme 2-1 Reaction of linear PEI with CO2 under dry and humid conditions

2.2 Results and Discussion
2.2.1 Heat of CO2 adsorption/desorption
One of the main purposes of using secondary amines is their lower reported heat
of CO2 adsorption compared to primary amines, which would allow desorption of CO2
under milder conditions. In aqueous solutions, the widely used primary amine
monoethanolamine (MEA) has a heat of adsorption around -85 kJ/mol CO2 while the heat
of adsorption for the secondary aminediethanolamine (DEA) is about -72 kJ/mol CO2.
12

A tertiary amine such as triethanolamine, which forms weaker bonds with CO2 but only
in the presence of water, has a heat of adsorption of -48 kJ/mol CO2 (Figure 2-1). Also, it
is generally well acknowledged that primary amines bind CO2 more strongly than
secondary amines do. Inspired by these data, we surmised that a polyamine rich in

26

secondary amine content would have a lower heat of adsorption that could result in
energy saving in the regeneration step.

Figure 2-1 Heat of adsorption for aqueous amine systems
The heat of CO2 adsorption and desorption of FS-LPEI adsorbents were
determined by direct isothermal calorimetry by DSC. The heat of reaction was
determined at 55, 70, and 85 °C, respectively for adsorbents containing LPEIs with
molecular weights (Mw) going from 2500 to 25000 g/mol (with similar LPEI loadings).
The heat (enthalpy) of adsorption were in a range between -42 and -53 kJ/mol CO2.
Compared to the previously reported FS-BPEI (-83 kJ/mol CO2), containing primary,
secondary and tertiary amine in a ratio of 44:33:23, FS-LPEI adsorbents have
substantially lower values.
10,13
The enthalpy of desorption were all in a range from 40 to
49 kJ/mol CO2. The values for the adsorption/desorption heats did not depend much on
the molecular weight of the LPEIs. A slight increase in the heat of adsorption was
observed when the Mw went from 2500 to 10000 followed by a decrease for the adsorbent
prepared with LPEI(25000) (Figure 2-2). The heat of desorption followed a similar
pattern. The effect of the adsorbent’s LPEI(25000) loading on the adsorption enthalpy
was also studied. At 55 °C, when going from a concentration of 32% to 51% LPEI, a
noticeable increase in the heat of adsorption from -43 to -53 kJ/mol CO2 was observed as
can be seen in Figure 2-2. At 85 °C, an inverse trend was noticed with a decrease from -
49 to -43 kJ/mol CO2. Considering these results, the use of a polymer containing only

27

secondary amines (like LPEI), could be beneficial in CO2 capture systems compared to
other polyamines such as BPEI containing also primary amines. The substantially
lowered enthalpy of desorption in the regeneration step could be translated to milder
reaction condition and significant energy savings. Further experiments on evaluating the
applicability of FS-LPEI adsorbents are provided in the following sections.

Figure 2-2 Heat of CO2 adsorption at various temperatures (a) as a function of molecular
weight. (b) as a function of LPEI(25000) weight loading in FS-LPEI(25000)
2.2.2 Comparison of linear with branched PEI
Due to the positive information gained on the reaction enthalpy from the LPEI
based adsorbent. The adsorption and desorption behaviors between linear and branched
0
10
20
30
40
50
60
20 25 30 35 40 45 50 55 60
Heat of adsorption (kJ/mol CO
2
)
LPEI(25000) loading in FS-LPEI(25000) (%)
55 °C
70 °C
85 °C
(b)
0
10
20
30
40
50
60
2500 5000 10000 25000
Heat of adsorption (kJ/mol CO
2
)
LPEI molecular weight (g∙mol
-1
)
55 °C
70 °C
85 °C

28

PEIs were compared. Adsorbents with a similar PEI loading on fumed silica
(LPEI(25000)-44.5 and BPEI(25000)-44.8) were prepared.
2.2.2.1 Adsorption
For CO2 adsorption experiments, a gas stream containing 95% CO2 and 5% N2
was passed over the adsorbents for 3 h in TGA. As shown in Figure 2-3, the adsorption
was relatively fast with both linear and branched PEIs. At 25 °C, FS-LPEI(25000) and
FS-BPEI(25000) had similar CO2 uptake at around 87 mg CO2/g. At 55 and 85 °C, FS-
LPEI(25000) adsorbed a little more than FS-BPEI(25000) did. This is somewhat
surprising considering that for a same weight, BPEI has a lower number of theoretically
available amino groups than LPEI. LPEI has only secondary amino groups, whereas
BPEI contains primary, secondary and tertiary amino groups in a ratio of about
44:33:23
14
and of which only the secondary and primary ones are active for CO 2 capture
under dry conditions. As pointed out vide supra, at 25 °C LPEI is a solid. This could
hamper its adsorption capabilities by impeding the access to active amino sites. As the
temperature increased, LPEI impregnated on the support melted, making the previously
inaccessible amino groups available. While the access to the active amino sites was
rendered easier at higher temperatures, the desorption of CO2 was also increasingly
facilitated by this increase in temperature.
With respect to the adsorption kinetics, both materials adsorbed faster at higher
temperatures. For example, at 85 °C, both FS-LPEI(25000) and FS-BPEI(25000) reached
close to their maximum adsorption capacity in about 3 min. At 55 °C, the adsorbent
containing LPEI reached 90% of its 3 h adsorption capacity in about 18 min whereas it
took 30 min for FS-BPEI(25000). At 25 °C, the kinetics after the initial adsorption was

29

slower and 90% of the 3 h adsorption capacity was reached after 78 and 81 min, for FS-
LPEI(25000) and FS-BPEI(25000), respectively.

Figure 2-3 CO2 adsorption kinetics on linear and branched PEIs at 25, 55, and 85 °C
0
20
40
60
80
100
120
140
0
0.5
1
1.5
2
2.5
3
3.5
0 30 60 90 120 150 180
25 C
FS-LPEI(25000)
FS-BPEI(25000)
0
20
40
60
80
100
120
140
0
0.5
1
1.5
2
2.5
3
3.5
0 30 60 90 120 150 180
Adsorption capacity (mg CO
2
∙g
-1
)
Adsorption capacity (mmol ∙g
-1
)
55 C
0
20
40
60
80
100
120
140
0
0.5
1
1.5
2
2.5
3
3.5
0 30 60 90 120 150 180
Time (min)
85 C

30

2.2.2.2 Desorption
To compare the desorption kinetics, FS-LPEI(25000) and FS-BPEI(25000) were
first adsorbed in a CO2 atmosphere for 3 h. The desorption experiments were conducted
at 85, 70, and 55 °C under N2 (Figure 2-4).

Figure 2-4 CO2 desorption kinetics on linear and branched PEIs at 55, 70, and 85 °C
0
20
40
60
80
100
0 20 40 60 80
85 C
FS-LPEI(25000)
FS-BPEI(25000)
0
20
40
60
80
100
0 20 40 60 80
CO
2
desorbed (%)
70 C
0
20
40
60
80
100
0 20 40 60 80
Time (min)
55 C

31

At 85 °C, the desorption was much faster on the adsorbent containing LPEI
compared to BPEI. While on FS-BPEI(25000), it took more than 10 min to desorb all the
CO2, the desorption on FS-LPEI(25000) was essentially complete in less than 5 min on
stream. At lower temperatures (i.e. 70 to 55 ºC), this effect was even more pronounced.
At 70 °C, LPEI based material desorbed 90% of the adsorbed CO2 in about 8 min, while
BPEI based material needed 25 min. At 55 °C, FS-LPEI(25000) desorbed 90% of CO2 in
about 20 min. FS-BPEI(25000) could not completely desorb, and in 1.5 h, it only
desorbed 80% of the adsorbed CO2. This offer considerable advantage in the desorption
step if LPEI is used, meaning (1) desorption could take place at lower temperatures, and
less energy is needed; (2) the desorption step is shorter, and within a same period of time,
there is potential for more adsorption/desorption cycles. Both of these points can be
translated to cost savings in real world applications.
2.2.2.3 LPEI desorption kinetics and activation energy
To further explore the potential of the LPEI based adsorbent, it was exposed to
even lower desorption temperatures. And the apparent activation energy of the CO2
desorption reaction was also determined by TGA measurements. For these experiments,
the adsorption was first performed under 95% CO2 in N2 at 55 °C for 1 h on FS-
LPEI(5000)-47.8. The desorption was then conducted under pure N2 at temperatures
ranging from 30 °C to 60 °C in several adsorption/desorption cycles. The desorption as a
function of time at various temperatures is shown in Figure 2-5. The desorption followed
kinetics close to first order with respect to CO2 as shown in Figure 2-6. The measured
reaction rate constants obtained at various temperatures were entered in an Arrhenius plot
Figure 2-7. The apparent activation energy (Ea) for CO2 desorption on FS-LPEI(5000)

32

was found to be 94 kJ/mol CO2. Considering the ease with which CO2 could be desorbed
from the FS-LPEI adsorbents even at temperatures as low as 40 °C, this value seems
somewhat high. It is, however, close, although lower, to the apparent activation energy of
desorption that has been measured on FS-BPEI (97 kJ/mol CO2), which could indicate a
similar reaction pathway.

Figure 2-5 CO2 desorption on FS-LPEI(5000) at temperatures from 30 to 60 °C

Figure 2-6 Ln (CO2 desorbed) versus time at various temperatures on FS-LPEI(5000)-48
0
0.5
1
1.5
2
2.5
3
3.5
0 10 20 30 40 50 60 70 80
CO
2
adsorbed (mmol∙g
-1
)
Time (min)
60 °C
55 °C
50 °C
40 °C
45 °C
35 °C
30 °C
30 C y = -0.0278x - 5.669
R² = 0.9927
35°C y = -0.0535x - 5.3307
R² = 0.9973
40°C y = -0.0947x - 5.3912
R² = 0.9977
45°C y = -0.1667x - 5.5403
R² = 0.998
50°C y = -0.2756x - 5.4262
R² = 0.9967
55°C y = -0.5018x - 5.447
R² = 0.9984
60°C y = -0.8121x - 5.4962
R² = 0.9986
-9
-8.5
-8
-7.5
-7
-6.5
-6
0 10 20 30 40 50 60 70 80
Ln(mol CO 2 desorbed)
Time (min)
30C 35C 40C 45C 50 C 55C 60C

33

Figure 2-7 Arrhenius plot for the CO2 desorption on FS-LPEI(5000)-48
2.2.3 Effect of temperature and CO2 concentration on the CO2 adsorption
The CO2 adsorption capacities were measured on FS-LPEI(5000)-47.8 at various
temperatures and CO2 concentrations. Each 1h adsorption at a given temperature was
followed by desorption at 85 °C or at the temperature of the adsorption, if the adsorption
temperature was higher than 85 °C. Figure 2-8 shows the CO2 adsorption isotherms at
various temperatures for CO2 concentrations going from 5% to 95%. At 25 °C, the
adsorption capacities were essentially the same over the entire CO2 concentration range.
At 40 °C, the increase in CO2 adsorption capacities was very moderate when going from
5% CO2 to 95% CO2. However, with a further increase in temperature a progressive shift
in the maximum adsorption capacity to higher CO2 feed gas contents was observed. The
highest 1 h adsorption capacity was 155 mg CO2/g (3.53 mmol/g) obtained at 70 °C with
y = -11300x + 33.736
R² = 0.9996
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
0.00295 0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335
ln(-slope)
1/Temp (K
-1
)

34

95% CO2. At temperatures higher than 85 °C, the adsorption capacity decreased rapidly
and at 130 °C, essentially no CO2 adsorption was observable even at higher CO2
concentrations. The results obtained are comparable to the ones reported by Subagyono et
al. on LPEI(2500) impregnated on mesocellular foam.
15
The highest adsorption capacity
of 146 mg CO2/g they obtained was at 75 °C and a CO2 concentration of 50%. Their
LPEI loading was 68.5%, much higher than the 47.8% for FS-LPEI(5000) utilized in our
present work.

Figure 2-8 Effect of temperature on the adsorption capacity on FS-LPEI(5000)-47.8 as a
function of CO2 concentration
The adsorption isobars for various CO2 concentrations ranging from 5% to 95%
CO2 in the gas feed as a function of temperature are presented in Figure 2-9. It can be
observed that all isobars followed some kind of bell shaped curve. The adsorption
maxima are all in a narrow temperature region ranging from 55 °C for 5% CO2 to 70 °C
for 95% CO2. Below 55 °C, CO2 adsorption capacities were very similar, regardless of
0
20
40
60
80
100
120
140
160
0
0.5
1
1.5
2
2.5
3
3.5
0 10 20 30 40 50 60 70 80 90 100
Adsorption Capacity (mg CO
2
∙g
-1
)
Adsorption capacity (mmol∙g
-1
)
CO
2
Concentration (%)
25 C
40 C
55 C
70 C
85 C
100 C
115 C

35

the CO2 concentration in the feed gas. Above 55 °C, the adsorption capacities started to
progressively shift apart in accordance to the CO2 concentration. The largest difference in
adsorption capacity between a 5% and 95% CO2 concentration in the gas feed was
observed at 85 °C. As the temperature was further increased to 100 °C and then 115 °C,
the difference in adsorption capacities progressively diminished until becoming
nonexistent at 130 °C. Contrary to branched PEIs, which are viscous liquids at room
temperature, LPEIs are solids. The melting point of LPEI(5000) is 54-59 °C according to
the manufacturer. This could in part explain the observation that under 55 °C the
adsorption was similar for all CO2 concentrations. Only the amino groups present on the
surface of the FS-LPEI were readily available for reaction with CO2. Once those reacted,
the amino groups which are not in the immediate vicinity of the surface are therefore not
as accessible for reaction. Even when the adsorption time was increased to 10 h at 25 °C,
the difference in adsorption capacities between 5% and 95% CO2 increased only
moderately. Temperatures above the melting point of LPEI, could allow for a more “fluid”
like situation, making the previously non-available amino groups accessible to the CO2
gas feed.
Theoretically, according to Figure 2-8 and Figure 2-9, it should be possible to
adsorb approximately 130 mg CO2/g (2.95 mmol/g) from a gas feed containing 10% CO2
at 55 °C and release essentially all of it at a concentration of 95% or higher at 130 °C, i.e.
a working capacity of 130 mg CO2/g. In practice, as we will see vide infra, other
significant parameters such as the stability of the adsorbent have to be taken into account.

36

Figure 2-9 Effect of CO2 concentration on the adsorption capacity on FS-LPEI(5000)-
47.8 as a function of temperature
2.2.4 Effect of LPEI molecular weight
In the case of branched PEIs, the molecular weights have significant influence on
the adsorption characteristics of the adsorbents. The use of lower molecular weights
BPEIs generally led to higher adsorption capacities.
1
In the case of LPEI, the trend was
not as clear. Mostly independent from the molecular weight was the temperature for the
adsorption capacity apex, around 70 °C and 55 °C when using 95% CO2 and 10% CO2,
respectively (Figure 2-10, Figure 2-11 and Figure 2-12). The adsorption capacities at
70 °C for 95% CO2 were comparable to the ones obtained with BPEI supported on
precipitated silica.
1
The shape of the curves was similar at both feed gas CO2
concentrations with a much steeper decrease in adsorption capacity at higher temperature
for the gas containing only 10% CO2. At 25 °C, the adsorbent with the highest molecular
weight, FS-LPEI(25000) had the highest adsorption capacity at both CO2 concentrations;
0
20
40
60
80
100
120
140
160
0
0.5
1
1.5
2
2.5
3
3.5
25 40 55 70 85 100 115 130
Adsorption capacity (mg CO
2
∙g
-1
)
Adsorption capacity (mmol∙g
-1
)
Temperature ( C)
5%
10%
15%
20%
30%
40%
50%
60%
70%
80%
90%
100%
CO
2
concentration

37

almost double the one observed for FS-LPEI(5000). The differences in adsorption
capacity were much less pronounced at temperatures upward of 55 °C. With 10% CO2,
there was essentially no difference in adsorption capacity at various LPEI molecular
weights at temperatures of 85 °C and above. The adsorption capacity in mg CO2 adsorbed
per g of LPEI as well as mol CO2 per mol of N was also determined and up to 340 mg
CO2 per g LPEI were obtained.

Figure 2-10 CO2 adsorption from a gas mixture containing 95% CO2 in N2 on FS-LPEI
with various Mw as a function of temperature

0
20
40
60
80
100
120
140
160
0
0.5
1
1.5
2
2.5
3
3.5
4
10 25 40 55 70 85 100 115
Adsorption capacity (mg CO
2
∙g
-1
)
Adsorption Capacity (mmol∙g
-1
)
Temperature ( C)
LPEI(2500)
LPEI(5000)
LPEI(10000)
LPEI(25000)

38

Figure 2-11 CO2 adsorption from a gas mixture containing 10% CO2 in N2 on FS-LPEI
with various Mw as a function of temperature
In order to check for regenerability, all the adsorbents were subjected to 10 short
consecutive adsorption/desorption cycles at 85 °C isothermally. The adsorption was
conducted under 95% CO2 for 15 min. Each adsorption was then followed by desorption
under pure N2 for 25 min. The results presented in Figure 2-13 show that over 10 cycles
the adsorption capacity remained basically identical. It can also be noted that 15 min at
85 °C were sufficient to achieve about 98-99% of the adsorption capacity obtained after 3
h at the same temperature. The adsorption was thus very fast as was the desorption step.
0
20
40
60
80
100
120
140
0
0.5
1
1.5
2
2.5
3
3.5
10 25 40 55 70 85 100 115 130
Adsorption capacity (mg CO
2
∙g
-1
)
Adsorption capacity (mmol∙g
-1
)
Temperature ( C)
FS-LPEI(2500)
FS-LPEI(5000)
FS-LPEI(10000)
FS-LPEI(25000)

39

Figure 2-12 Effect of LPEI molecular weight on the adsorption capacity with 10% CO2

Figure 2-13 Adsorption/desorption cycles at 85 °C on FS-LPEI. Effect of LPEI
molecular weight. Adsorption under 95% CO2 for 15 min, desorption under pure N2 for
0
0.5
1
1.5
2
2.5
3
3.5
0 5000 10000 15000 20000 25000 30000
Adsorption Capacity (mmol∙g
-1
)
LPEI molecular weight (g∙mol
-1
)
25 °C
40 °C
55 °C
70 °C
85 °C
100 °C
115 °C
130 °C
0
20
40
60
80
100
120
140
160
0
0.5
1
1.5
2
2.5
3
3.5
4
0 2 4 6 8 10
Adsorption capacity (mg CO
2 ∙
g
-1
)
Adsorption capacity (mmol ∙g
-1
)
Cycle
LPEI(2500)
LPEI(5000)
LPEI(10000)
LPEI(25000)

40

25 min. Insert: Percentage of the adsorption capacity after 15 min (average over the 10
ads/des cycles) compared to 3 h adsorption at 85 °C
2.2.5 Effect of LPEI loading
The effect of LPEI loading was also investigated and Figure 2-14 shows that on
FS-LPEI(25000), the maximum adsorption capacity moved to higher temperatures as the
concentration of LPEI increased in the adsorbent from a maximum adsorption capacity at
25 °C for a LPEI(25000) content of 18.6% to 85 °C for the one containing 51.0%. The
highest overall capacity was obtained with the adsorbent containing 44.5% LPEI and not
the one with the highest LPEI concentration. At 25 °C, the highest adsorption capacity
was actually observed with the adsorbent containing only 32.1% LPEI. At this
temperature, the adsorption on the adsorbent containing 51.0% was only slightly higher
than the one on the adsorbent containing 18.6% LPEI.

Figure 2-14 Effect of the LPEI loading on the CO2 adsorption capacity on FS-
LPEI(25000). Adsorption under 95% CO2
0
20
40
60
80
100
120
140
0
0.5
1
1.5
2
2.5
3
3.5
10 25 40 55 70 85 100 115
Adsorption capacity (mg CO
2
∙g
-1
)
Adsorption capacity (mmol∙g
-1
)
Temperature ( C)
18.6%
32.1%
36.5%
44.5%
51.0%

41

With the active ingredient for CO2 capture being LPEI, the determination of the
specific adsorption capacities for LPEI are also of interest and are presented in Figure
2-15 in mg CO2 adsorbed per g of LPEI. The adsorption capacity in mol CO2 per mol of
N is also indicated in the same figure. The results show an absolute maximum around 360
mg CO2/g PEI at 70 °C with the adsorbent containing 36.5% LPEI. A similar value has
been reported for branched PEI on silica, although at BPEI loadings under 30%.
1
The
comparatively low adsorption capacity for the adsorbent containing 18.6% LPEI could be
explained by the interaction between the amino moieties on the LPEI with the silanol
groups on the surface of the silica as was described by Wang et al. for PEI impregnated
on SBA-15 and Goeppert et al. on FS-BPEI.
16
Coating fused-silica capillaries, used in the
separation of amino acids and peptides, with PEI is actually a way to passivate the acidic
silanol moieties.
17
Due to this interaction, less amino groups would be available for CO 2
adsorption. Nevertheless, once the surface is saturated with PEI, the remaining amino
groups are free to react with CO2 and this effect should therefore be less pronounced with
increasing PEI concentration in the adsorbent.

42

Figure 2-15 Efficiency of PEI utilization on FS-LPEI(25000) with various LPEI loadings
When the adsorbents were subjected to 10 short adsorption/desorption cycles
under isothermal conditions (85 °C, Figure 2-16), the adsorption capacity followed the
same order as the one observed when the adsorption was conducted over 3 h (Figure
2-14). The adsorbent containing 44.5% LPEI allowed for the highest cycling adsorption
capacity at around 3 mmol/g. 15 min were enough to reach more than 98% of the
adsorption capacity achieved after 3 h at 85 °C. Desorption was also complete in less
than 25 min.

0
50
100
150
200
250
300
350
400
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
10 25 40 55 70 85 100 115
Adsorption capacity (mg CO
2
per g PEI)
Adsorption capacity (mol CO
2
per mol N)
Temperature ( C)
18.6%
32.1%
36.5%
44.5%
51.0%

43

Figure 2-16 Adsorption/desorption cycles at 85 °C on FS-LPEI (25000). Effect of LPEI
loading. Adsorption under 95% CO2 for 15 min, desorption under pure N2 for 25 min.
Insert: Percentage of the adsorption capacity after 15 min (average over the 10 ads/des
cycles) compared to 3 h adsorption at 85 °C
2.2.6 Adsorption of CO2 from the air
The adsorption of CO2 directly from the atmosphere is gaining interest as a way to
deal with the emissions of small and distributed point sources such as automobiles, planes,
and home and office heating units. The collection of CO2 from billions of small fossil
fuel burning units would be otherwise challenging and probably economically prohibitive.
Whereas technologies already exist to remove CO2 from closed environments such as
submarines and spaceships for life support, they are not well adapted for larger scale
applications. For such uses, amine based sorbents have been proposed by several groups
including ours. They possess many of the attributes expected from a suitable sorbent for
the capture of CO2 from the air, which include: (i) being able to capture CO2 selectively
0
20
40
60
80
100
120
140
0
0.5
1
1.5
2
2.5
3
3.5
0 2 4 6 8 10 12
Adsorption capacity (mg CO
2
∙g
-1
)
Adsorption capacity (mmol∙g
-1
)
Cycle
18.6%
32.1%
36.5%
44.5%
51.0%
% LPEI(25000) loading % ads. capacity at
15 min vs 3 h
18.6 94.4
32.1 97.4
36.5 98.7
44.5 98.6
51.0 93.6

44

at room temperature, (ii) having a high CO2 adsorption capacity, (iii) being able to desorb
CO2 under mild conditions, (iv) affordable, (v) easy to produce, and (vi) durable. The
challenges for direct air capture arise mainly from the fact that the concentration of CO2
in the atmosphere is only 400 ppm, which is about 1/250 to 1/375 of the concentration in
a typical fossil fuel power plant flue gas (10% to 15% CO2).
While BPEI has been used in numerous CO2 capture studies including some
describing the capture from the atmosphere, the characteristics of LPEI for this
application has garnered much less attention. The adsorption capacity of FS-LPEI with
various molecular weights and similar LPEI loading was therefore measured. A gas
mixture containing 400 ppm CO2 in air was passed over the adsorbents at 25 °C for 12 h.
From Figure 2-17, it can be observed that the adsorbent containing LPEI(25000) had the
highest adsorption capacity with about 53 mg CO2/g (1.21 mmol/g). This adsorption
capacity was essentially achieved after 2 h on stream. The time needed to reach the
maximum adsorption capacity was longer on the other adsorbents based on LPEI(2500),
LPEI(5000) and LPEI(10000). Adsorbents with lower molecular weights had also lower
adsorption capacities going from 32 mg CO2/g for LPEI(5000) to 48 mg CO2/g for
LPEI(2500) with no apparent direct correlation with the molecular weight of LPEI. This
trend was different from the one observed with BPEI where the adsorption capacity was
about the same for BPEI(1800) and BPEI(25000).
10
On PEIs with a Mw of 25000, the
adsorption capacity was higher on the adsorbent containing BPEI compared to LPEI (74
mg CO2/g vs. 54 mg CO2/g. Chaikittisilp et al. using LPEI(2500) impregnated on
mesocellular foam obtained an adsorption capacity of 33 and 46 mg CO2/g (0.75 and 1.05
mmol/g) on the adsorbent containing 42 and 49% LPEI, respectively (25 °C, 400 ppm

45

CO2 in air, 12h adsorption).
18
This value is close to the one we obtained with FS-
LPEI(2500) containing 45.5% LPEI in the present study.

Figure 2-17 CO2 adsorption from simulated air (400 ppm CO2, dry) at 25 °C on FS-LPEI
with various molecular weights
2.2.7 Adsorption/desorption capacity of FS-LPEI over numerous cycles
An important parameter for a CO2 adsorbent, besides its ability to adsorb
reasonable amount of CO2, is its ability to adsorb and desorb CO2 efficiently, repeatedly
and in an acceptable timeframe. In order to test this possibility, FS-LPEI(25000)-44.5
was submitted to short adsorption/desorption cycles. Each cycle included a 10 min
adsorption step under either 95% CO2 or 10% CO2 in N2 followed by a 15 min desorption
step under N2. Both steps were run isothermally. For each cycle, the CO2 adsorption
capacity was determined by the difference in weight between the lowest points during
each desorption and the highest point during the next adsorption phase. At 75 °C, under
95% CO2, no significant loss in adsorption capacity was observed after 100
0
10
20
30
40
50
60
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 2 4 6 8 10 12
Adsorption capacity (mg CO
2
∙g
-1
)
Adsorption capacity (mmol∙g
-1
)
Time (h)
LPEI(2500)
LPEI(5000)
LPEI(10000)
LPEI(25000)

46

adsorption/desorption cycles and the adsorption capacity remained at 135 mg CO2/g
(Figure 2-18). Lowering the temperature to 55 °C, resulted in a still stable but lower
cyclic adsorption capacity of 115 mg CO2/g over 180 cycles. The desorption time of only
15 min was, however, sufficient to desorb most of the CO2 present on the adsorbent. The
TGA plot for this experiment is presented in Figure 2-19. At this same temperature of
55 °C and under 10% CO2, the adsorption capacity of about 103 mg CO2/g was also
stable over 100 cycles. Considering these results, it seems that under these particular
conditions, the adsorbents based on FS-LPEI are relatively stable over time.

Figure 2-18 Adsorption capacity of FS-LPEI(25000) over numerous ads/des cycles
0
20
40
60
80
100
120
140
0
0.5
1
1.5
2
2.5
3
3.5
0 20 40 60 80 100 120 140 160 180 200
Adsorption capacity (mg CO
2
∙g
-1
)
Adsorption capacity (mmol∙g
-1
)
Cycle
75°C, 95% CO2
55°C, 10% CO2
55°C, 95% CO2

47

Figure 2-19 TGA plot of 180 ads/des cycles on FS-LPEI(25000) performed at 55 °C
2.2.8 Stability of the adsorbents based on linear PEI
Some of the concerns with regards to amine and polyamine based adsorbents are
thermal instability and O2 and CO2 induced degradation over numerous cycles, especially
during the desorption step (regeneration), which is usually conducted at a higher
temperature. The stability of the adsorbents based on linear PEI was therefore studied.
For these experiments, FS-LPEI(5000)-47.8 was used. Under nitrogen and air, no
weight loss was observed at 70 °C, as can be seen in Figure 2-20. At a temperature of
100 °C, however, a slight decrease in weight was noticed under N2. Under air, the weight
was stable for about 5 h. After that initial period, the weight decreased by about 3.2%
over 15 h. The exact same shape was obtained when the experiment was repeated with
another sample of fresh adsorbent. Under CO2, the weight of the adsorbent increased at
70 °C and remained stable at 100 °C.

98
100
102
104
106
108
110
112
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Weight (%)
Time (min)

48

Figure 2-20 FS-LPEI(5000)sample weight vs time under N2, air or CO2 at 70 and 100 °C
Table 2-1 Stability of the adsorbents based on FS-LPEI(5000)
Conditions CO2 adsorption capacity after vs. before treatment (%)
N2, 70 ºC 101.6
N2, 100 ºC 94.4
Air, 70 ºC 93.6
Air, 100 ºC 7.2
CO2, 70 ºC 103.6
CO2, 100 ºC 101.4

49

As shown in Table 2-1, at 70 ºC, the adsorbent maintained its adsorption capacity
under N2 by recovering 101.6% of the original capture capacities. At 100 ºC, the
adsorption capacity was reduced by about 5.6%, presumably due to the thermal instability
at elevated temperature leading to amine leaching. Repeating the 100 °C treatment under
N2 gave the same result.
Under air, the adsorbent suffered from a 6.4% capacity loss at 70 °C. When the
treatment temperature was increased to 100 °C, the capture capacity was reduced
drastically to only 7.2% of the original value, representing a notable 92.8% decrease. The
observed loss of adsorption capacity is probably mainly due to oxidative degradation.
Previous studies showed that secondary amines are susceptible to degradation in O 2
containing atmosphere, especially at high O2 concentrations and elevated temperatures.
According to Chuang et al.
19
who studied the oxidative degradation of silica supported
pentaethylenehexamine (PEHA), the methylene groups in immediate proximity to the
amino groups can undergo oxidation in the presence of oxygen to form mostly amide and
imide groups (Scheme 2-2). Sayari et al. studied oxidative degradation with amines
grafted on pore expended MCM-41 (PE-MCM-41) and also observed by IR the formation
of C=O species with an associated decrease in CO2 adsorption capacity.
20
They found
that primary monoamines were substantially more stable than secondary or mixed
(secondary-primary) amines towards oxygen. In a more recent study, the same group
studied the oxidative degradation of BPEI and LPEI impregnated on SBA-15 by NMR
and described the formation of a number of structural units containing C=O and –CH=N-
species.
21
Jones et al. also found that PEI/alumina based adsorbents containing
primary/secondary and tertiary amino groups was less stable under oxidative conditions

50

than adsorbents composed of poly(allylamine) (PAA) having only primary amino
groups.
22
PEI/Alumina lost about 70% of its adsorption capacity after treatment at 110 °C
for 10 h in a gas mixture containing 21% O2 in N2. PAA/alumina on the other hand lost
only about 10% of its original adsorption capacity under the same conditions. The linear
PEIs used here contained exclusively secondary amino groups (one end of the chain is a
methyl group while the other end is a hydroxyl group) and were, thus, more prone to
oxidative degradation. However, the formation of C=O species cannot explain the
observed weight loss at 100 °C under air, which could be due to some other degradation
process and/or leaching.
Scheme 2-2 Oxidative degradation of FS-LPEI. Adapted from ref.
19

When FS-LPEI(5000)-47.8 was exposed to CO2, no decrease in the adsorption
capacity could be observed even at 100 ºC. This result is in line with the study of Sayari
et al. showing a higher resistance of secondary amines towards CO2-induced
degradation.
23-24
Adsorbents containing grafted secondary monoamines (sMono) were
exceedingly resistant to CO2-induced deactivation. Primary and mixed (primary and
secondary) grafted amines as well as supported polyamines, branched PEI and PAA,
underwent significant deactivation in the presence of dry CO2. After 30 adsorption

51

(50 °C)/ desorption (130 °C) cycles under pure CO2, they lost between 62 and 72% of
their initial adsorption capacities. Under similar conditions LPEI lost only 50% of its
uptake capacity. According to Sayari,
23
under CO2, materials containing primary amines
are more prone to deactivation through an isocyanate intermediate followed by further
reaction with a primary or secondary amine to open chain urea or N-substituted
imidazolidinones. In the case of secondary amines (ethyleneamine units), the deactivation
under CO2 is a result of the formation of N ,N ’-disubstitutedimidazolidinones taking place
through the dehydration of ammonium carbamate or carbamic acid. When both amines
are secondary, the formation of an isocyanate intermediate is not possible and the
deactivation must involve the intramolecular dehydration of an ammonium carbamate,
which occurs less readily and could explain the higher resistance to deactivation of LPEI
containing adsorbents.
From the observations in the present work and previous studies, secondary amines,
compared to primary amine, are more vulnerable towards oxidative degradation, but
relatively more resistant to CO2-induced degradation.
2.3 Conclusion
Linear PEI supported on fumed silica are promising candidates for CO2 capture
from various sources including the air. These solid adsorbents are able to adsorb
selectively and reversibly CO2 with adsorption capacities of up to 155 mg CO2/g.
Desorption of the adsorbed CO2 occurs rapidly at mild temperatures (50 to 100 °C). The
adsorption/desorption process can be repeated for more than one hundred cycles with no
noticeable decrease in adsorption capacity. While the adsorption kinetics were similar for
linear and branched PEIs impregnated on fumed silica, the desorption rate was noticeably

52

faster with LPEI. This was somewhat surprising considering that the apparent activation
energy for the desorption step was only slightly lower on the LPEI based adsorbent, i.e.
94 kJ/mol CO2 compared to 97 kJ/mol CO2 for FS-BPEI. On the other hand, the heat of
adsorption (-42 to -53 kJ/mol CO2) and desorption (40 to 49 kJ/mol CO2) were much
lower on FS-LPEI than on FS-BPEI (-83 and -82 kJ/mol CO2, respectively).
The adsorption capacity from a mixture containing 10% CO2 had an apex at about
55 °C regardless of the molecular weights of LPEI. LPEI with a Mw of 25000 was the
most efficient at capturing CO2 from an ultra-dilute mixture containing only 400 ppm
CO2 (air concentration).
FS-LPEI was stable under CO2 and did not show any decrease in adsorption
capacity after treatment at both 70 °C and 100 °C. Under air, however, the adsorbent
showed a noticeable decrease in adsorption capacity after 20 h at 70 °C. After 20 h at
100 °C, this decrease was much more pronounced and only ~7% of the initial adsorption
capacity was retained. This loss in adsorption capacity could be problematic if these
adsorbents have to be used for CO2 capture from gas mixtures containing oxygen
especially at higher temperature (>70 °C). For the adsorption of CO2 from the air,
however, it should not pose a major challenge due to the lower adsorption temperatures
(generally ambient temperature between 10 and 35 °C). The desorption step, however,
should be conducted under conditions limiting exposure to oxygen and at lower
temperature, if possible.
To sum up, linear PEI based adsorbents adsorb selectively and repeatedly CO2
from various gas mixtures. Compared to branched PEI, they offer faster desorption
kinetics, which is advantageous for fast adsorption/desorption cycling. However, they

53

also suffer from some stability problems under oxidative environments, which could limit
their applicability.
2.4 Experimental
2.4.1 Chemicals
Linear polyethylenimines (LPEIs) with weight-average molecular weights (Mw)
of 2500, 5000 and 10000 g/mol were purchased from Sigma-Aldrich. LPEI with a Mw of
25000 was purchased from Alfa Aesar. They were labeled as LPEI(2500), LPEI(5000),
LPEI(10000) and LPEI(25000). Branched polyethylenimine (BPEI) with a M w of 25000
was purchased from Sigma-Aldrich. Aerosil
®
380 hydrophilic fumed silica was obtained
from Evonik. All chemicals were used as received, unless otherwise stated.
2.4.2 Preparation of the adsorbents
LPEI was coated on fumed silica by a wet impregnation method. Desired amounts
of LPEI and support were mixed in methanol at room temperature. After mixing for 24 h,
methanol was removed using a rotary evaporator. The prepared adsorbents were further
dried under high vacuum at room temperature overnight. The adsorbents were then stored
in closed glass vials. The adsorbent were named FS-LPEI(x) where x represented the Mw
of the LPEI.
The actual loading in PEI was determined by thermogravimetric analysis on a
Shimadzu TGA-50 instrument. The sample was heated under the flow of air from 25 °C
to 800 °C at a rate of 10 °C/min. The initial weight loss from 25 °C to about 130 °C was
attributed to the loss of CO2 and moisture adsorbed on the adsorbent. The weight loss

54

from about 130 °C to 800 °C was attributed to the loss of organic part (PEI) in the
adsorbent. The weight percentage of PEI was obtained from these measurements as the
proportion of weight loss due to the organic part and the weight of the adsorbent after
desorption of CO2 and water. When required, the adsorbents were named as FS-LPEI(x)-
y where y represented the content of LPEI in %.
2.4.3 Equipment and methods
2.4.3.1 CO2 adsorption and desorption measurements
CO2 adsorption and desorption were performed by thermogravimetric analysis on
a Shimadzu TGA-50 instrument. In a typical experiment, adsorbents were tested for CO2
adsorption capacities at 25, 40, 55, 70, 85, and 100 ºC using a 95% CO2/5% N2 gas
mixture. Typically, about 4 mg of solid adsorbent was loaded in a platinum pan. The
sample was first heated to 110 ºC under pure N2 atmosphere (flow of 60 mL/min) to
desorb water and CO2. After 30 min, the temperature was lowered to 25 ºC, and the
adsorbents was exposed to 95% CO2 (flow of 60 mL/min) for 3 h. The gas flow was
switched back to pure N2 accompanied by an increase of the temperature to 85 ºC. The
adsorbent was desorbed under these conditions for 90 min. Similar adsorption-desorption
cycles were carried out for the remaining temperatures.
2.4.3.2 Isobar and isotherm measurements
Adsorption of CO2 at concentrations going from 5% to 95% and temperatures
from 25 °C to 130 °C were performed using the TGA instrument. The gas flow rate to
obtain the desired CO2 concentrations were regulated with mass flow controllers from
Aalborg.

55

2.4.3.3 Heats of reaction measurements
The heats of adsorption and desorption of CO2 were measured by using a
PerkinElmer DSC 7. The heat of adsorption for the adsorption of CO2 was obtained at 55,
70, and 85 ºC under pure CO2. After the adsorption was completed, the heat of desorption
of the same sample was measured by heating under N2 at the same temperature as the
adsorption. The values presented here are the averages of 5 measurements.
2.4.3.4 Stability of the adsorbents based on linear PEIs
For the thermal stability studies, about 5 mg of FS-LPEI(5000) was treated under
N2, air or CO2 at 70 ºC and 100 ºC for 20 h in the TGA instrument. Adsorption capacity
at 85 ºC under 95% CO2 (60 mL/min) for 3 h was measured before and after the thermal
treatment and compared.

56

2.5 References
(1) Goeppert, A.; Meth, S.; Prakash, G. K. S.; Olah, G. A. Energ. Environ. Sci.
2010, 3, 1949.
(2) Qi, G.; Fu, L.; Choi, B. H.; Giannelis, E. P. Energ. Environ. Sci. 2012, 5,
7368.
(3) Liu, S.-H.; Hsiao, W.-C.; Sie, W.-H. Adsorption 2012, 18, 431.
(4) Yan, W.; Tang, J.; Bian, Z.; Liu, H. Ind. Eng. Chem. Res. 2012, 51, 3653.
(5) Qi, G.; Wang, Y. B.; Estevez, L.; Duan, X. N.; Anako, N.; Park, A. H. A.;
Li, W.; Jones, C. W.; Giannelis, E. P. Energ. Environ. Sci. 2011, 4, 444.
(6) Wang, X.; Li, H.; Liu, H.; Hou, X. Micropor. Mesopor. Mater. 2011, 142,
564.
(7) Chen, C.; Yang, S.-T.; Ahn, W.-S.; Ryoo, R. Chem. Commun. 2009, 3627.
(8) Meth, S.; Goeppert, A.; Prakash, G. K. S.; Olah, G. A. Energ. Fuel. 2012,
26, 3082.
(9) Goeppert, A.; Czaun, M.; May, R. B.; Prakash, G. K. S.; Olah, G. A.;
Narayanan, S. R. J. Am. Chem. Soc. 2011, 133, 20164.
(10) Goeppert, A.; Czaun, M.; Jones, J.-P.; Surya Prakash, G. K.; Olah, G. A.
Chem. Soc. Rev. 2014, 43, 7995.
(11) Zhang, H.; Goeppert, A.; Czaun, M.; Prakash, G. K. S.; Olah, G. A. RSC
Advances 2014, 4, 19403.
(12) Kohl, A.; Nielsen, R. Gas Purification; 5th ed.; Gulf Publishing Company:
Houston, Texas, 1997.

57

(13) Drese, J. H.; Choi, S.; Lively, R. P.; Koros, W. J.; Fauth, D. J.; Gray, M.
L.; Jones, C. W. Adv. Funct. Mater. 2009, 19, 3821.
(14) Fauth, D. J.; Gray, M. L.; Pennline, H. W.; Krutka, H. M.; Sjostrom, S.;
Ault, A. M. Energ. Fuels 2012, 26, 2483.
(15) Subagyono, D. J. N.; Marshall, M.; Knowles, G. P.; Chaffee, A. L.
Micropor. Mesopor. Mater. 2014, 186, 84.
(16) Wang, X.; Schwartz, V.; Clark, J. C.; Ma, X.; Overbury, S. H.; Xu, X.;
Song, C. J. Phys. Chem. C 2009, 113, 7260.
(17) Encyclopedia of Chromatography, 2nd edition; Cazes, J., Ed.; Taylor &
Francis, 2005.
(18) Chaikittisilp, W.; Khunsupat, R.; Chen, T. T.; Jones, C. W. Ind. Eng.
Chem. Res. 2011, 50, 14203.
(19) Srikanth, C. S.; Chuang, S. S. C. ChemSusChem 2012, 5, 1435.
(20) Heydari-Gorji, A.; Belmabkhout, Y.; Sayari, A. Micropor. Mesopor.
Mater. 2011, 145, 146.
(21) Ahmadalinezhad, A.; Sayari, A. Phys. Chem. Chem. Phys. 2014, 16, 1529.
(22) Bali, S.; Chen, T. T.; Chaikittisilp, W.; Jones, C. W. Energ. Fuels 2013,
27, 1547.
(23) Sayari, A.; Heydari-Gorji, A.; Yang, Y. J. Am. Chem. Soc. 2012, 134,
13834.
(24) Sayari, A.; Belmabkhout, Y.; Da’na, E. Langmuir 2012, 28, 4241.

58

Chapter 3. Mesocellular silica foam with expanded pore volume as an efficient
support for CO2 Capture.
3.1 Introduction
An important improvement from liquid amine to solid adsorbent system is using
porous inorganic oxides as the support for amines to replace the water. The heat capacity
of silica, an inorganic oxide support, is 0.75 J/g K (at about 300 K), significantly lower
than the heat capacity of water of 4.18 J/g K.
1
Thus, this approach successfully avoids
heating large amounts of water and remarkably reduces the energy requirement for the
regeneration step. In 2002, Xu et al. developed the so-called "molecular basket" concept.
For the first time, they reported the synergetic effect with respect to capture capacity and
adsorption/desorption kinetics between polyethylenimine (PEI) and ordered mesoporous
silica material MCM-41.
2-4
Separately, MCM-41 adsorbed 8.6 mg CO2/g; PEI adsorbed
109 mg CO2/g. However, when 50 wt% PEI was supported on the MCM-41, the
adsorbent captured 112 mg CO2/g, almost twice higher than the combined capacity of
separated MCM-41 and PEI (8.6/2+109/2=59 mg CO2/g). The percentage desorbed for
the PEI was also increased from 56.4% to 99.8%. The enhanced adsorption/desorption
characteristics are due to the porous support providing an open and accessible backbone
for the stabilization of a large number of amino adsorption sites.
3
In the following work,
Belmabkhout et al. were able to engineer a MCM-41 material with expanded pore size
(PE-MCM-41). After grafting triamine on both supports, improved CO2 adsorption
capacity on PE-MCM-41 was observed, especially at low and medium CO2
concentrations.
5
Inspired by a series of promising results, there have been tremendous

59

efforts on developing better porous inorganic oxide materials suitable for supporting
amines.
As in the process of studying silica supported organoamine adsorbents, it becomes
clear that the morphology of the porous support material plays a key role in determining
the overall performance. Such mesoporous materials are often characterized by their
surface area, pore diameter, pore volume and pore window diameter. There have been a
few groups studying the effect of pore structure and pore size of the support on the CO2
capture performance. Zelenak et al. prepared sorbents by grafting 3-
aminopropyltriethoxysilane (APTS) to MCM-41, SBA-12 and SBA-15.
6
For the
preparation of efficient amine-based mesoporous silica, they found the lower limit of the
pore size to be about 35 Å. Below this size, the adsorption of CO2 on the amine sites
inside the pores was very limited due to limited diffusion into the pores. Son et al.
performed a study on a series of mesoporous material, MCM-41, MCM-48, SBA-15,
SBA-16, and KIT-16 impregnated with PEI with a molecular weight (M w) of 800 in a
ratio of 1 to 1.
7
The adsorption capacity and adsorption kinetics increased primarily as a
function of the pore size of the supports. They concluded that the average pore diameter
was the most important variable dictating the adsorption capacity and kinetics. MCM-41
with the lowest pore diameter (2.8 nm) had the lowest CO2 adsorption capacity. The use
of KIT-6, with the largest pores (6.0 nm) in a 3D arrangement, resulted in the highest
CO2 adsorption capacity (135 mg CO2/g adsorbent at 75 °C) that was believed to be due
to an easier access to the active amino sites of PEI impregnated inside the pores. Yan et
al. prepared a series of SBA-15 based mesoporous materials with various average pore
diameter and pore volume, and loaded them with 50% PEI (linear, Mw of 423).
8
They

60

found that the adsorption capacity was dependent on the total pore volume of the support
rather than their pore diameter. In addition, the relationship between adsorption capacity
and pore volume was linear, supported by a strong correlation coefficient.
Although the reports of both Son and Yan allowed to gain some insight into the
factors governing the preparation of efficient amine based mesoporous adsorbents, it
seems that neither of their work was conclusive. First, their experimental protocols were
very different. Son et al based their study on supports synthesized from different
procedures. For example, MCM-41 used cetyltrimethylammonium bromide (CTMABr)
as a templating agent, whereas SBA-15 and SBA-16 were prepared with triblock
copolymers as surfactants. Although the resulting mesoporous materials were
characterized by a set of values, namely surface area, average pore diameter and total
pore volume, their properties and morphology could be different accordingly leading to
different adsorbents. Furthermore, Yan et al. synthesized most of their supports based on
the same procedure and only varied the aging time in the process.
8
However, the variety
of the supports was probably too limited to draw a definitive conclusion. Yan et al. listed
only four supports, with a relatively narrow total pore volume distribution ranging from
0.7 cm
3
/g to 1.2 cm
3
/g, to determine whether the adsorption capacity and pore volume are
linearly related. Similarly, Son’s supports only ranged from 2.8 nm to 6.0 nm in average
pore diameter.
In regards of these limitations, a study with supports exhibiting a wider selection
of average pore size and total pore volume but based on a similar synthesis method is
clearly warranted. Recent development in the preparation of mesocellular foams (MCF)
in which pore volume and size can be relatively easily tuned, make this type of support a

61

good candidate for such a study.
9-10
A series of MCFs were therefore prepared and tested
as supports for PEI and their effect on CO2 adsorption capacity and kinetics determined.
Reaction conditions for the synthesis of the support have been screened to maximize its
pore volume, pore diameter, and surface area. The resulting material features pore
volumes of up to 4.17 cm
3
/g, significantly surpassing similar materials reported in the
literature. Combined with the high surface area, large pore diameter and interconnected
wall structures, this material is suitable for loading PEI for the purpose of CO2 separation.
The obtained sorbents were tested for CO2 adsorption capacity, kinetics, stability and
regenerability.
3.2 Results and Discussion
3.2.1 Preparation of MCF supports: screening of synthesis conditions
Part of the procedural work was based on Genggeng Qi et al.
11
and
Schimidt et al.’s publicaitons.
9
Reaction conditions have been screened to improve
the structure and morphology of the support to suit our needs. Specifically, the
effects of ammonium fluoride concentration, rate of heating during the calcination
process and equilibration time of trimethylbenzene (TMB) were studied.
3.2.1.1 Effect of ammonium fluoride concentration during MCF preparation.
Ammonium fluoride plays several roles in the synthesis of mesoporous materials.
First, it is a mineralizer that increases the solubility of silicate. Secondly, it catalyzes the
oxolation reaction and enables the building of the oxide framework.
12
In addition, the
NH4F/Si molar ratio promotes the enlarging of the window size and may possibly
produce mesostructures that suit our needs.
9

62

The effect of the ammonium fluoride concentration is shown in Table 3-1 and
Figure 3-1. By altering the [NH4F]/[Si] molar ratio in the system during the preparation,
the morphology of the supports is varied (see 3.4 Experimental for detail about the
preparation). The highest surface area was obtained when no NH4F was used. However,
the average pore size and pore volume were the lowest in the absence of NH 4F. The
initial addition of NH4F significantly decreased the surface area, but the trend plateaued
for [NH4F]/[Si] ratios higher than 0.86 staying at around 600 m
2
/g. Both average pore
diameter and pore volume increased until the [NH4F]/[Si] ratio reached a value of about
0.5, after which they increased only moderately or plateaued. From Table 3-1, it can be
seen that under these conditions, ammonium fluoride helps to expand the pore diameter
and pore volume but has a somewhat negative effect on the surface area.
Using tetraethylorthosilicate (TEOS) as a silica source, Schmidt-Winkel et al.
9

achieved excellent mesostructures when the [NH4F]/[Si] molar ratio was 0.03, which was
significantly lower than the ratio we used with sodium silicate as the silica source. In the
present study, we selected sodium silicate because the formed foam would have more
hydroxyl groups on its surface, helping to bind more strongly the impregnated PEI, while
at the same time also potentially increasing the hydrothermal stability of the adsorbent.

63

Table 3-1 Physical data for supports prepared with various NH4F concentrations
Experiment
#
[NH 4F]/[Si] Surface area (m
2
/g) APS
(A)
PV
(cm
3
/g)
S-[NH 4F]-0 0.00 942 46 1.09
S-[NH 4F]-0.09 0.09 788 75.8 1.49
S-[NH 4F]-0.42 0.42 694 101 1.77
S-[NH 4F]-0.86 0.86 622 104 1.61
S-[NH 4F]-1.28 1.28 724 119 2.15
S-[NH 4F]-1.71 1.71 607 110 1.68
S-[NH 4F]-2.57 2.57 600 127 1.90

3.2.1.2 Effect of the heating rate during the calcination process
Calcination is the final step in the preparation of mesocellular silica foam. It
thermally condenses the silica framework as well as removes the organic template and
water. Typical calcination temperatures to remove the surfactant P123 and organic
components are generally between 500 and 700 °C. Early in the development of
mesoporous materials, a slow heating rate was recommended to avoid a possible collapse
of the porous network. Therefore, 1 or 2 °C/min has been the standard heating rate for
preparing mesoporous materials of the M41S family, HMS, MSU-X, SBA-15, etc. This
protocol was challenged after Bagshaw et al. experimented with fast heating rate of up to
100 °C/min. They observed that due to the low combustion temperature of PEO based
templates, low heating rates resulted in the sample being without pore filling for a longer
time, affecting the structure of the mesoporous solid. In the case of Si-MSU-X in

64

particular, they noticed that its structural integrity is best preserved when exposed to the
calcinations for the shortest period of time, i.e. at the fastest rate of heating.
13

Here, we have investigated the effect of heating rate on the mesostructure of the
supports. As shown in Table 3-2, among the heating rates we tested (1 °C/min, 2 °C/min,
5 °C/min, and 10 °C/min), an intermediate rate of 5 °C/min maximized at the same time
the surface area, average pore diameter and pore volume. Although different systems may
be optimized at different heating rates, in the case of the present support, the heating rate
of 5 °C/min was found to be optimal to obtain the largest pore volume with
correspondingly large surface area and average pore diameter.

0
100
200
300
400
500
600
700
800
900
1000
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Surface area (m
2
/g)
[NH
4
F]/[Si]
(a)
0
20
40
60
80
100
120
140
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Average pore diameter (Å)
[NH
4
F]/[Si]
(b)
0
0.5
1
1.5
2
2.5
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Pore volume (cc/g)
[NH
4
F]/[Si]
(c)

0
100
200
300
400
500
600
700
800
900
1000
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Surface area (m
2
/g)
[NH
4
F]/[Si]
(a)
0
20
40
60
80
100
120
140
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Average pore diameter (Å)
[NH
4
F]/[Si]
(b)
0
0.5
1
1.5
2
2.5
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Pore volume (cc/g)
[NH
4
F]/[Si]
(c)

65

Figure 3-1 Influence of the concentration of ammonium fluoride on (a) the surface area
of MCF (BET), (b) the average pore diameter of MCF, (c) the total pore volume of MCF.
Table 3-2 Effect of heating rate on the physical characteristics of MCFs
Experiment # Heating rate
(°C/min)
Surface area
(m
2
/g)
APD (Å) PV (cm
3
/g)
S-1C/min 1 492 150 1.85
S-2C/min 2 519 129 1.67
S-5C/min 5 543 154 2.10
S-10C/min 10 606 122 1.85

3.2.1.3 Effect of the addition of swelling agent TMB
For the purpose of loading PEI to absorb CO2, mesoporous cellular foam with
interconnected voids is preferred to allow easy diffusion of CO2 through the material.
With this in mind, the swelling agents trimethylbenzene (TMB), was added to the system,
not only serving the role of expanding pore volume and pore size, but also potentially to
create more channels by breaking down the ordered mesoporous structure.
10,14

We monitored the addition of TMB and the effect of equilibration time on the
pore structure. As shown in Table 3-3, when no TMB is added, surface area, average pore

0
100
200
300
400
500
600
700
800
900
1000
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Surface area (m
2
/g)
[NH
4
F]/[Si]
(a)
0
20
40
60
80
100
120
140
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Average pore diameter (Å)
[NH
4
F]/[Si]
(b)
0
0.5
1
1.5
2
2.5
0.00 0.50 1.00 1.50 2.00 2.50 3.00
Pore volume (cc/g)
[NH
4
F]/[Si]
(c)

66

diameter and pore volume are the lowest in the series at 543 m
2
/g, 154 Å and 2.1 cm
3
/g,
respectively. Initial addition of TMB, even without any equilibration time improved all
these parameters. An equilibration time of 2 h allowed a further significant increase for
all these parameters. Surface area increased to 688 m
2
/g whereas the pore volume
doubled compared to the support prepared without TMB (4.17 cm
3
/g versus 2.1 cm
3
/g).
Longer equilibration time (24 h) did not result in further expansion of the pore diameter
and volume. On the contrary, average pore diameter decreased to 203 Å, and total pore
volume decreases to 3.34 cm
3
/g.
Table 3-3 Effect of the addition of trimethylbenzene (TMB) and equilibration time on the
structure of MCFs
Experiment # TMB/P123
(w/w)
a
Equilibration
time (h)
Surface area
(m
2
/g)
APS (Å) PV (cm
3
/g)
S-noTMB 0 -- 543 154 2.1
S-TMB-eq.0h 1.75 0 618 191 2.96
S-TMB-eq.2h 1.75 2 688 243 4.17
S-TMB-eq.4h 1.75 4 796 152 3.03
S-TMB-eq.24h 1.75 24 658 203 3.34
a
Weight ratio using 42g TMB.
9

Meanwhile, all the samples were analyzed under TEM (Figure 3-2). As shown I
in Figure 3-2, shorter equilibration time in the presence of TMB led to a structure
resembling “tree branches”, while 24 h of equilibration time resulted in a closed spherical
structure. Further evidence (vide infra) suggests that adsorption is more effective when
PEI was loaded on the “tree branched” support. Indeed the tree branched material seems
to have better diffusive characteristics.

67

From the perspective of accommodating more PEI for CO2 adsorption, pore
volume is of primary importance, because it is the pore volume that determines how
much PEI can be loaded inside the pores. PEI has a density of 1.03 g/cm
3
. One gram of
MCF with for example a PV of 1.5 cm
3
/g can thus theoretically load a maximum of 1.6 g
of PEI inside its pores. According to the theory of synergistic enhancement, the CO 2
adsorption performance is closely related to the portion of PEI loaded inside the pore.
15

Therefore, pore volume is the major factor. In addition, pore diameter and window
opening are also important factors, considering the process of CO2 adsorption by PEI.
Large pore diameters and windows openings facilitate the accessibility of the PEI’s active
amino groups by CO2.

68

Figure 3-2 TEM pictures of supports prepared with and without TMB and after various
equilibration times
S-no TMB
S-TMB-eq.0h
S-TMB-eq.2h
S-TMB-eq.4h
S-TMB-eq.24h
200 nm 100 nm
100 nm
100 nm
S-no TMB
S-TMB-eq.0h
S-TMB-eq.2h
S-TMB-eq.4h
S-TMB-eq.24h
200 nm 100 nm
100 nm
100 nm

69

3.2.2 Preparation of CO2 adsorbents based on MCF and PEI
3.2.2.1 Characteristics of the supports tested for the preparation of CO2
adsorbents with PEI
The CO2 adsorbents were prepared from PEI and MCF by simple mixing of the
components in methanol followed by evaporation of the solvent under vacuum (see
Experimental). In a first series of experiments, the effect of the morphology and
structural characteristics (surface area, pore volume, pore diameter) of various MCF
supports on the CO2 adsorption capacity was studied. Five supports described in Table
3-4 with a wide range of pore volume and diameter were chosen and classified following
their total pore volume from S1 to S5. The supports all followed a type IV adsorption
isotherm characteristic of mesopores (20 to 500 Å) within the solid (Figure 3-3). This
can be clearly seen in the pore size distribution measured following the BHJ method
(Figure 3-4). The surface areas of these supports were comparable, in a range from 500 –
700 m
2
/g, sufficient for loading PEI through silanol group hydrogen bonding and
physical interaction with the support. The average pore diameter increased from 65.7 Å
to 243 Å going from S1 to S5. Total pore volume varied from 0.98 cm
3
/g to 4.17 cm
3
/g
from S1 to S5. This allowed the study of the effect of pore volume and pore diameter on
the PEI impregnation and CO2 adsorption. S1 to S3 were prepared in the absence of TMB
and had a very similar maximum at around 85Å in the pore diameter distribution (Figure
3-4). The cumulative pore volume increased from S1 to S3 (Figure 3-5). S4 and S5
prepared in the presence of TMB had a wider pore diameter distribution which was also
shifted toward larger pore diameter. S5, the MCF with the largest total pore volume had
also the largest average pore diameter. From Figure 3-5, it can also be observed that in

70

the case of S5 most of the porosity is due to pores with a diameter larger than ~120Å,
which is fairly large. All the supports had, however, a quite broad distribution in pore
diameter.
Table 3-4 Characteristics of the supports used in the study of the effect of pore volume
and pore diameter on the preparation of PEI-based adsorbents and their CO2 adsorption
capacity
Support

Surface
area (m
2
/g)
Average pore
diameter (Å)
Total pore volume
(cm
3
/g)
Pore diameter
distribution
Reference
S1 596 65.7 0.98
Small peak at 35 Å, big
one at 85 Å
---
S2 519 129 1.67
Broad peak from ~60 Å
to ~200 Å
S-2C/min
S3 543 154 2.10
broad peak from ~60 Å
to ~200 Å
S-no TMB
S-5C/min
S4 618 191 2.96
Very broad peak from
~40 Å to ~200 Å
S-TMB-
eq.0h
S5 688 243 4.17
Very broad peak from
~60 Å to ~250 Å
S-TMB-
eq.2h

71

Figure 3-3 N2 adsorption/desorption isotherm for MCF S1 to S5.

Figure 3-4 Pore size distribution of MCF S1 to S5.
0
500
1000
1500
2000
2500
3000
0 0.2 0.4 0.6 0.8 1
Volume at STP (cm
3
/g)
Relative pressure (P/P
0
)
S1
S2
S3
S4
S5
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
20 200 2000
dV(d) (cm
3
/Å/g)
Pore Diameter (Å)
S1
S2
S3
S4
S5

72

Figure 3-5 Cumulative pore volume as a function of pore diameter of MCF S1 to S5
3.2.2.2 Preparation and characteristics of the adsorbents prepared with PEI
Polyamines impregnated MCF have been studied by a few research groups in
recent years.
11,16-17
Zhao et al. reported the use of PEI-MCF for the adsorption of CO2.
18

However, this study included only one MCF with a pore volume of 3.14 cm
3
/g and a pore
diameter of 113 Å. Chaffee et al. also tested PEI/MCF
19
to selectively adsorb CO2
whereas Jones et al. used poly(allylamine)/MCF.
20
Here again only one or two MCF
supports with a limited range in pore volume and surface area were employed. Amine
and polyamines chemically bound on the surface MCF have also been considered by
Jones et al.,
21
Chaffee et al.
22
and others.
16

In the present studies, branched PEI with a high molecular weight of 25,000
g/mol (PEI25k) was selected for the experiments in large part because of its low volatility
and high CO2 adsorption capacity.
23-24
On the five selected supports (S1 to S5), PEI25k
was loaded through a typical impregnation method (see Experimental). Weight ratios of
support vs. PEI25k were 1:1, 1:2, 1:3, 1:4, and 1:5, corresponding to a PEI loading of
50%, 67%, 75%, 80% and 83%, respectively. All the selected supports formed free
flowing white powders when impregnated with PEI at contents of 50% and 67% (Table
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
20 200 2000
cm
3
/g
Pore Diameter (Å)
S1
S2
S3
S4
S5

73

3-5). However, when the PEI content was further increased to 75%, using the support
with the lowest pore volume resulted in the formation of a sticky solid that was not
suitable for CO2 adsorption applications. The maximum amount of PEI which could be
loaded on this support had been exceeded. Proceeding to higher PEI loadings a similar
phenomenon was observed for supports with higher pore volumes and pore diameters. S2
and S3 with a pore volume of 1.67 and 2.10 cm
3
/g, respectively, did not form a free
flowing solid at a loading of 80% PEI. With a PEI content of 83% only S5, the support
with the highest pore volume (4.17 cm
3
/g), was able to form a suitable solid adsorbent.
The fact that larger amounts of PEI could be loaded on the supports with a higher pore
volume seems logical.
Table 3-5 Aspect of the adsorbents prepared from MCF and PEI25k
Support 50% PEI 67% PEI 75% PEI 80% PEI 83% PEI
S1 solid solid Sticky solid NP NP
S2 solid solid solid Sticky solid NP
S3 solid solid solid solid NP
S4 solid solid solid solid Sticky solid
S5 solid solid solid solid solid
NP: not prepared

74

Table 3-6 Surface area (BET, m
2
/g) of the adsorbents prepared from MCF and PEI25k
Support No PEI 50% PEI 67% PEI 75% PEI 80% PEI 83% PEI
S1 596 TS TS Sticky solid NP NP
S2 519 59.8 11.1 TS Sticky solid NP
S3 543 57.8 13.3 3.84 TS NP
S4 618 82 40 10.5 0.73 Sticky solid
S5 688 132 51.5 15.5 4.1 TS
TS: too small; NP: not prepared
Table 3-7 Total pore volume (cm
3
/g) of the adsorbents prepared from MCF and PEI25k
Support No PEI 50% PEI 67% PEI 75% PEI 80% PEI 83% PEI
S1 0.98 TS TS
Sticky
solid
NP NP
S2 1.67 0.43 0.050 TS
Sticky
solid
NP
S3 2.10 0.50 0.085 0.025 TS NP
S4 2.96 0.70 0.32 0.082 0.0022
Sticky
solid
S5 4.17 1.12 0.45 0.10 0.0175 TS
TS: too small; NP: not prepared

75

Figure 3-6 Total pore volume of adsorbents based on MCFs as a function of PEI25k
loading in the adsorbent

Figure 3-7 Pore Distribution of S5 MCF impregnated with PEI in various concentration
The surface area and total pore volume of all the adsorbents (i.e. non-sticky solids)
were also measured and the results are presented in Table 3-6 and Table 3-7, respectively.
As a general trend, the surface area decreased with increasing PEI loading. The total pore
volume pattern followed the same pattern as can be seen in Table 3-7 as well as Figure
3-6. However, for a similar PEI loading the supports with the highest initial total pore
0.001
0.01
0.1
1
10
0 50 67 75 80 83
Total pore volume (cm
3
/g)
PEI concentration (%)
S1-PEI25k
S2-PEI25k
S3-PEI25k
S4-PEI25k
S5-PEI25k
-0.0020
0.0000
0.0020
0.0040
0.0060
0.0080
0.0100
0.0120
20 200 2000
dV(d) (cc/Å/g)
Pore diameter (Å)
S5-PEI25k-50
S5-PEI25k-67
S5-PEI25k-75
S5-PEI25k-80

76

volume were able to retain a higher surface area than supports with a similar surface area
but lower total pore volume. A higher pore volume as well as average pore diameter of
the support (Table 3-4) is therefore highly important for the preparation of adsorbents
able to accommodate high PEI loading while retaining reasonable surface area and pore
volume to allow an easier access for CO2 molecules. Figure 3-7 shows the pore size
distribution of the support with the highest pore volume (S5) impregnated with various
amounts of PEI25k. It can be noticed that despite an obvious decline in pore volume, the
pore size distribution does not change dramatically going from a concentration of 50 to
75% with a peak at around 140 Å and 160 Å, respectively. At a PEI concentration of 80%,
no more peak is discernible in the pore size distribution as most of the porosity
disappears. This indicates that even at loadings as high as 75%, some large pores are still
present which probably allow an easier access to the active amino sites for CO 2. The
adsorbents with favorable physical properties, i.e. free flowing solids, were tested for
CO2 adsorption.
3.2.3 CO2 adsorption capacity measurements
3.2.3.1 Effect of pore volume on CO2 adsorption
The CO2 adsorption capacity of each sorbent was measured by TGA. The sorbent
was first heated to 110 °C under nitrogen (60 mL/min) to desorb CO2 and water present
on the surface. The flow was then switched to 60 mL/min pure CO2 for 180 min for
adsorption followed by 60 mL/min pure nitrogen at 85 °C for 90 min in the desorption
step. This procedure was repeated for 25 °C, 55 °C, and 85 °C to determine the influence
of temperature on the CO2 adsorption. The results are presented in Table 3-8 as well as in
Figure 3-8 and Figure 3-9.

77

It can be observed that, as a general trend, the adsorption capacity at a given
temperature and concentration of PEI25k increased with increasing pore volume and pore
diameter of the support. For example at 25 °C and a concentration of PEI25k of 50%, the
CO2 adsorption capacity increased from 0.53 mmol CO2/g for S1 to 1.93 mmol/g for S5.
A similar effect was observed at higher PEI25k loadings and temperatures. This agrees
with previous studies conducted by other groups, suggesting that adsorption capacity
benefits from larger pore size and/or larger pore volume (see Introduction).
6-8

Table 3-8 CO2 adsorption capacity (mmol/g) on MCF/PEI25k with various PEI loading
50% PEI 67% PEI 75% PEI 80% PEI 83% PEI
(°C) 25 55 85 25 55 85 25 55 85 25 55 85 25 55 85
S1
0.53 1.36 2.55 0.43 1.24 2.86
S2
1.35 2.47 3.22 1.22 2.69 4.36 1.03 2.34 4.36
S3
1.21 2.31 2.69 1.16 2.68 4.45 1.06 2.49 4.39 0.74 1.65 3.31
S4
1.64 2.64 3.13 1.26 2.92 4.57 1.06 2.38 4.58 0.87 2.23 4.55
S5
1.93 3.08 3.68 1.81 3.54 4.33 1.54 3.40 5.08 1.54 3.23 5.22 1.45 3.06 5.25

At 25 °C, the highest adsorption was obtained at the lowest loading of 50% PEI
on all the supports (Figure 3-8 (a) and Figure 3-9 (a)). At a temperature of 55 °C, the
adsorption capacity displayed a maximum at 67% PEI for all supports except S1 with the
lowest pore volume which had the highest adsorption at a PEI loading of 50% (Figure
3-8(b) and Figure 3-9 (b)). At 85 °C, the increase in PEI concentration resulted generally
in an increase in the CO2 adsorption capacity. This increase was, however, most
pronounced at lower loadings, i.e. between 50 and 67% (Figure 3-8 (c)). At higher
loadings, the adsorption reached a plateau and the addition of larger amounts of PEI did

78

not improve further the adsorption capacity. In the case of S3, the adsorption even
decreased when the PEI loading was increased from 75 to 80% probably due to the filling
of the pores by PEI.

Figure 3-8 CO2 adsorption capacity on Sx-PEI25k as a function of PEI loading. (a) 25 °C,
(b) 55 °C, and (c) 85 °C.
0
0.5
1
1.5
2
2.5
40 50 60 70 80 90
Adsorption capacity (mmol CO
2
/ g
adsorbent)
25 C
0
0.5
1
1.5
2
2.5
3
3.5
4
40 50 60 70 80 90
Adsorption capacity (mmol CO
2
/ g
adsorbent)
55 C
0
1
2
3
4
5
6
40 50 60 70 80 90
Adsorption capacity (mmol CO
2
/ g
adsorbent)
PEI concentration (%)
85 C
S1-PEI25k
S2-PEI25k
S3-PEI25k
S4-PEI25k
S5-PEI25k
(a)
(b)
(c)

79

Figure 3-9 Effect of the pore volume of the support on CO2 adsorption capacity of
adsorbents prepared with various concentrations of PEI25k. (a) 25 °C, (b) 55 °C, and (c)
85 °C.
0.00
0.50
1.00
1.50
2.00
2.50
0 1 2 3 4 5
Adsorption Capacity (mmol CO 2 /g sorbent)
25 °C
50% 67% 75% 80%
0
0.5
1
1.5
2
2.5
3
3.5
4
0 1 2 3 4 5
Adsorption Capacity (mmol CO
2
/g sorbent)
55 °C
0
1
2
3
4
5
6
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Adsorption Capacity (mmol CO 2 /g sorbent)
Pore Volume (cm
3
/g)
85 °C
(a)
(b)
(c)

80

Raising the temperatures had a positive effect on the adsorption capacity of pure
CO2 which increased accordingly on all supports and PEI loadings. This pattern is the
opposite of the one usually observed with commonly used liquid amines for CO 2 capture
such as MEA and DEA. A similar trend had already been observed by us and others
when using PEI as the active component in supported amines.
3,23
It is probably due to the
nature of PEI25k, which is a very viscous gel. Lower viscosity at higher temperature is
believed to result in a better accessibility of the amino groups of the adsorbent for CO2
molecules and improved reaction kinetics, leading to higher CO2 adsorption.
3
This effect
is more pronounced in adsorbents with higher concentrations of PEI, where PEI is less
uniformly dispersed on the solid support. With increasing PEI loadings, the surface of the
support is increasingly saturated with PEI and access to some amino groups is made more
difficult. This is further supported by the fact that the best utilization of the amino groups
in PEI was obtained at lower PEI loadings as can be seen in Figure 3-10 representing the
CO2 adsorption capacity per g PEI on S5-PEI25k. A maximum adsorption of 325 mg
CO2 per gram PEI was obtained at 85 °C on the adsorbent containing 50% PEI. This
value is almost identical to the one we obtained previously on PEI/fumed silica
adsorbents with the same PEI loading.
23
Increasing the PEI concentration resulted only in
a decrease of the CO2 adsorption capacity per g of PEI. This decrease was however more
pronounced at lower temperatures (55 °C and 25 °C). Therefore, although a higher PEI
loading might increase the overall CO2 adsorption capacity of the adsorbent, the most
effective use of the polyamine is achieved at lower loadings. A better dispersion of PEI
also induced a faster desorption of CO2 during the regeneration step. Desorption was
therefore faster in adsorbents with lower PEI loadings as can be seen in Figure 3-11

81

showing the percentage of desorption as a function of time on S5-PEI25k. The desorption
time gradually increased going from a PEI25k content of 50% to 83% on S5-PEI25k.
Whereas on S5-PEI25k-50 the desorption was nearly complete after only 3 min, almost
15 min were necessary to achieve the same with S5-PEI25k-83.

Figure 3-10 Effectiveness of PEI utilization. CO2 Adsorption capacity in mg CO2/g PEI
in the adsorbent measured at various temperatures on S5-PEI25k as a function of PEI
loading.
On the other hand, the bulk density of S5-PEI25k increased significantly from
0.11 g/mL for the adsorbent containing 50% PEI to 0.40 g/mL for the one containing 83%
PEI. At 85 °C, this corresponded to an adsorption capacity of 0.41 and 2.1 mmol CO2/mL
adsorbent for S5-PEI25k-50 and S5-PEI25k-83, respectively. This five-fold increase in
CO2 adsorption capacity per mL is much higher than what would be expected from a
simple increase in PEI concentration from 50 to 83 %. On a volume basis, the use of
adsorbents with a higher PEI content is therefore advantageous as it would reduce the
overall size and capital cost of the adsorption unit.
0
50
100
150
200
250
300
350
40 50 60 70 80 90
mg CO 2 /g PEI
PEI in sorbent (%)
25 °C 55 °C 85 °C

82

Figure 3-11 Desorption completion at 85 °C as a function of time of adsorbents based on
S5 containing PEI25k loadings from 50% to 83%
3.2.3.2 Effect of the molecular weight of PEI on CO2 adsorption
It has previously been shown that the lower the molecular weight of PEI, the
higher the CO2 adsorption capacity of the adsorbent.
23
When impregnated on fumed silica
in a ratio of 1:1, linear PEI with a Mw of 423 adsorbed 173 mg CO2/g adsorbent. Similar
adsorbents prepared with branched PEI with a Mw of 800 and 25000 adsorbed 147 mg
CO2/g and 130 mg CO2/g, respectively. Shorter oligomers such as
pentaethylenehexamine (PEHA) and tetraethylenepentamine (TEPA) adsorbed the most
at 192 mg CO2/g and 200 mg CO2/g, respectively. The higher adsorption capacity of
ethylenimine oligomers and linear PEI can most probably be attributed to the fact that
these contain only primary and secondary amines which are both active for CO 2 capture.
The higher molecular weight PEI used were branched and therefore contained beside
primary and secondary amines also tertiary amines which under dry conditions do not
adsorb CO2. In addition to this, the viscosity of branched PEI is higher, which also
hinders the access to the active amino sites on the adsorbent.
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20
Desorption (%)
Time (min)
S5-PEI25k-50
S5-PEI25k-67
S5-PEI25k-75
S5-PEI25k-80
S5-PEI25k-83

83

The problem with ethylenimine oligomers and to some extent the low molecular
weight PEIs is their stability over time. Because of their lower boiling point and therefore
increased volatility, they have a tendency to leach out of the adsorbents, resulting in a
loss of adsorption capacity over numerous adsorption/desorption cycles as well as
possible contamination of the system downstream of the adsorbent, narrowing the
adsorbents’ practical application. In order to avoid these potential problems as much as
possible we decided to limit our present study of the effect of molecular weight on the
adsorption capacity of MCF-PEI to branched PEIs with Mw of 800, 1800 and 25000.
These PEIs were impregnated on the support with the highest pore volume (S5) to allow
loadings of up to 83% PEI. To determine the effect of temperature the CO2 adsorption
capacity was measure at 25 °C, 55 °C and 85 °C. The results can be seen in Figure 3-12.

84

Figure 3-12 Effect of the Mw of PEI on the adsorption capacity at various temperatures
Not unexpectedly, at 25 °C, the highest adsorption was observed for the adsorbent
containing PEI with the lowest Mw (S5-PEI800), and decreased with increasing Mw of
PEI, regardless of the PEI concentration. The adsorption capacity for S5-PEI800
displayed a maximum at a PEI concentration of 75%. Above this concentration, the
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
50 67 75 80 83
Adsorption Capacity (CO 2 mmol/g sorbent)
PEI loading (%)
25 °C
PEI800 PEI1800 PEI25000
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
50 67 75 80 83
Adsorption Capacity (CO 2 mmol/g sorbent)
PEI loading (%)
55 °C
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
50 67 75 80 83
Adsorption Capacity (CO 2 mmol/g sorbent)
PEI loading (%)
85 °C

85

addition of more PEI resulted in a decrease of the adsorption capacity, probably due to
more PEI present in the pores of the support resulting in lower pore size and limiting the
access of CO2 to the active amino sites. On S5-PEI1800, the adsorption was similar with
PEI concentrations between 50% and 75% and decreased at higher PEI contents. S5-
PEI25k, prepared with the highest MW PEI, had the highest CO2 adsorption capacity at
the lowest PEI concentration (50%). Addition of more PEI had only a detrimental effect
on the adsorption capacity at 25 °C.
With all three PEIs and all the PEI concentrations tested, the corresponding
adsorbents adsorbed increasing amounts of CO2 with increasing temperature. At 55 °C
and 85 °C, the CO2 adsorption followed the same general pattern observed at 25 °C, i.e. a
decrease in adsorption with increasing PEI Mw. However, the difference in adsorption
capacity between the different PEIs decreased with increasing temperature. At 85 °C, S5-
PEI800 and S5-PEI1800 had almost similar adsorption capacity. With increasing
temperatures, the highest adsorption capacity also shifted to higher PEI concentrations.
This shift was most pronounced for S5-PEI25k which adsorbed the most at a PEI
concentration of 80 to 83% at 85 °C compared to 50% at 25 °C. Chemically the three
branched PEIs used are very similar and differ mostly by their M w. Therefore, the
observed patterns seem to confirm the predominant role of physical properties, such as
already mentioned viscosity, on the adsorption capabilities (vide supra). At higher
temperature, the viscosity of the PEI on the surface of the support diminishes allowing
better access to the amino sites and improving the reaction kinetics.
3,23
A CO2 adsorption
of up to 6 mmol CO2/g adsorbent was obtained with S5-PEI800-75 at 85 °C. The highest

86

adsorption per g PEI was obtained with S5-PEI800-50 at 85 °C with 8.16 mmol CO2/g
PEI or 359 mg CO2/g PEI.
3.2.3.3 Adsorbents stability over multi-adsorption/desorption cycles
Besides being able to adsorb large amounts of CO2, an effective adsorbent should
also be easily regenerated and stable over numerous adsorption/desorption cycles. In
order to test the chemical and thermal stability of our sorbents, they were submitted to a
total of 100 short desorption/adsorption cycles (over 40 h of cycling). Each cycle
included a 10 min adsorption step under 95% CO2 and a 15 min desorption step under
nitrogen as a stripping gas. Both steps were run isothermally at 75 °C. For each cycle the
CO2 adsorption capacity was determined by the difference in weight between the lowest
point during each desorption and the highest point during the next adsorption phase. S5-
PEIx-80 with x=800, 1800 and 25k were tested for their stability. The TGA plots for
these measurements are presented in Figure 3-13. Figure 3-14 shows that regardless of
the Mw of PEI, none of the sorbents underwent degradation within 100 cycles under these
conditions. One reason could be that a large pore volume can accommodate more PEI
inside the pores and be less affected by the formation of urea as proposed by Sayari et
al.
25
In this work, we chose PEI with relatively high molecular weights. Although, PEIs
with lower molecular weight, such as PEI 423 and TEPA, are generally reported to have
higher adsorption capacity, they suffer from significant amine loss and therefore
adsorption capacity loss as described in a number of previous multicycles studies.
11,16-
17,26-30

87

Figure 3-13 TGA plot of 100 Adsorption/desorption cycles performed at 75 °C (10 min
adsorption under CO2, 15 min desorption under N2) on (a) S5-PEI800-80, (b) S5-
PEI1800-80, (c) S5-PEI25k-80
98
100
102
104
106
108
110
112
114
116
118
120
0 500 1000 1500 2000 2500
Weight (%)
Time (min)
98
100
102
104
106
108
110
112
114
116
118
120
0 500 1000 1500 2000 2500
Weight (%)
Time (min)
98
100
102
104
106
108
110
112
114
116
118
120
0 500 1000 1500 2000 2500
Weight (%)
Time (min)
(c) S5-PEI25k-80
(b) S5-PEI1800-80
(a) S5-PEI800-80

88

Figure 3-14 Stability of S5-PEIx sorbents over 100 adsorption/desorption cycles at 75 °C
(10 min adsorption under CO2, 15 min desorption under N2)
Interestingly in these short cycles, the adsorbent containing PEI with the highest
Mw (25k) showed the largest cyclic CO2 uptake (about 157 mg CO2/g for S5-PEI25k-80),
followed by S5-PEI1800-80 and S5-PEI800-80 (with the lowest Mw PEI). This had,
however, more to do with the desorption step than the adsorption step as can be seen in
Figure 3-15 showing the first 5 adsorption/desorption cycles of the three S5-PEI-80 tested.
After desorption at 110 °C for 30 min the adsorbent with the lowest M w PEI adsorbed the
most CO2 (almost 200 mg/g) during the first adsorption. The 15 min desorption at 75 °C
were, however, insufficient and only 75% of the CO2 could be desorbed, resulting in an
apparent loss of adsorption capacity (Figure 3-16). The adsorbents prepared with
PEI1800 and PEI25k had a lower initial adsorption capacity (close to 160 mg/g). Their
desorption capacity was, however, higher with 95% for S5-PEI1800-80 and an essentially
complete desorption for S5-PEI25k-80 (Figure 3-16 (a)). Given enough time, all
adsorbents were able to release all the CO2 initially adsorbed. This indicates that
adsorbents based on PEI with higher Mw have faster apparent desorption kinetics. This
could be due to the fact that PEI with a higher M w because of its size, does probably not
110
115
120
125
130
135
140
145
150
155
160
165
170
0 20 40 60 80 100
CO
2
absorption capacity (mg/g)
Number of cylces
S5-PEI800-80
S5-PEI1800-80
S5-PEI25k-80

89

penetrate as deep into the pores as lower molecular PEI and a larger part of the PEI
remains outside the pores where access for CO2 and its desorption are easier.

Figure 3-15 First five adsorption/desorption cycles on S5-PEI-80 containing PEI with
various Mw.
At lower PEI loadings this effect was less pronounced (Figure 16b-d) due to a
better dispersion of the PEI on the surface of the support. However, even at the lowest
PEI concentration (50%), the desorption on the adsorbent containing the PEI with the
highest Mw was still the fastest (Figure 3-16). On the other hand, the adsorption kinetics
were very fast and similar regardless of the Mw of the PEI (Figure 3-17).
98
100
102
104
106
108
110
112
114
116
118
120
0 100 200
Weight (%)
Time (min)
(c) S5-PEI25k-80
98
100
102
104
106
108
110
112
114
116
118
120
0 100 200
Weight (%)
Time (min)
(a) S5-PEI800-80
98
100
102
104
106
108
110
112
114
116
118
120
0 100 200
Weight (%)
Time (min)
(b) S5-PEI1800-80

90

Figure 3-16 Desorption capacity as a function of time for adsorbents containing PEI with
various Mw and concentrations
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35 40 45 50
Desorption (%)
Time (min)
S5-PEI800-80
S5-PEI1800-80
S5-PEI25k-80
80% PEI
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35 40 45 50
Desorption (%)
Time (min)
S5-PEI800-75
S5-PEI1800-75
S5-PEI25k-75
75% PEI
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35 40 45 50
Desorption (%)
Time (min)
S5-PEI800-67
S5-PEI1800-67
S5-PEI25k-67
67% PEI
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35 40 45 50
Desorption (%)
Time (min)
S5-PEI800-50
S5-PEI1800-50
S5-PEI25k-50
50% PEI
Desorption time used
in cycling experiments

91

Figure 3-17 Adsorption capacity as a function of time for the adsorbents containing PEI
with various Mw and PEI concentrations of 50% and 80%
Overall, from these short adsorption/desorption cycle measurements it can be seen
that up to a PEI concentration of 67%, the cyclic CO2 adsorption capacity decreased with
increasing PEI Mw, i.e. S5-PEI800 > S5-PEI1800 > S5-PEI25k (Figure 3-18). At a PEI
concentration of 75%, however, the highest cyclic capacity was obtained with PEI1800
and at a PEI concentration of 80% with PEI25k. The overall highest cyclic adsorption
capacity under these conditions was obtained with S5-PEI800-67 and S5-PEI-1800-75.
Although the desorption kinetics were faster on PEI25k, it might therefore be more
advantageous to use PEI with lower molecular weight to achieve high cycling capacity,
but making sure that they do not suffer from leaching problems.
0
10
20
30
40
50
60
70
80
90
100
0 5 10
Adsorption (%)
Time (min)
S5-PEI800-50
S5-PEI1800-50
S5-PEI25k-50
0
10
20
30
40
50
60
70
80
90
100
0 5 10
Adsorption (%)
Time (min)
S5-PEI800-80
S5-PEI1800-80
S5-PEI25k-80
50% PEI
80% PEI

92

Figure 3-18 Adsorption capacity in short adsorption/desorption cycles at 75 °C as a
function of PEI Mw and concentration (average of 9 measurements, 10 min adsorption
under CO2, 15 min desorption under N2)
3.3 Conclusion
Mesocellular foams (MCF) with a broad range of pore volume and size were
prepared by varying a number of synthesis parameters such as ammonium fluoride
concentration, effect of swelling agent trimethylbenzene (TMB), equilibration time and
calcination heating rates. The resulting materials featured pore volumes of up to 4.17
cm
3
/g, significantly surpassing similar materials reported in the literature. Combined with
a high surface area of 500 to 700 m
2
/g, large pore diameter and interconnected wall
structures, these materials were suitable for loading PEI. The obtained sorbents were
tested for CO2 adsorption capacity, kinetics, stability and regenerability.
When using PEI25k it was observed that the adsorption capacity at a given
temperature and concentration of PEI increased with increasing pore volume and pore
size of the support. This agrees with the suggestion that PEI loading and distribution on
100
110
120
130
140
150
160
170
180
190
40 50 60 70 80 90
CO
2
adsroption capacity (mg/g)
PEI concentration (%)
S5-PEI800
S5-PEI1800
S5-PEI25k

93

the support and consequently adsorption capacity benefits from larger pore size and/or
larger pore volume. Adsorption up to 5.25 mmol CO2/g adsorbent (230 mg/g) were
obtained at 85 °C with the adsorbent based on the support with the highest pore volume.
Decreasing PEI molecular weight resulted in increasing adsorption capacity. At 85 °C,
the adsorbent based on PEI800 and the support with the highest pore volume resulted in
CO2 adsorption of up to 6 mmol/g (265 mg/g). At room temperature (25 °C) the highest
adsorption was obtained with the adsorbent containing the least PEI (50%). However, as
the temperature was increased to 55 °C followed by 85 °C, the maximum in adsorption
shifted to adsorbents with higher PEI loadings.
The adsorption capacity of the adsorbents based on MCF and PEI did not
decrease over 100 adsorption/desorption cycles at 75 °C. Interestingly, whereas the
adsorption of CO2 was similarly fast with all the PEIs, the desorption step increased with
increasing PEI molecular weight. PEI25k had the fastest desorption kinetics. Overall the
highest adsorption capacity (about 180 mg/g, 4.1 mmol/g) using short
adsorption/desorption cycles was obtained with S5-PEI800-67 and S5-PEI-1800-75.
The CO2 adsorption results obtained here were in the top tier compared to the
ones reported in the literature. Preparation of PEI based adsorbents clearly benefited from
the utilization of support with larger pore volume and diameter, which in turn led to
improved CO2 adsorption capabilities.
3.4 Experimental
3.4.1 Chemicals
Triblock copolymer surfactant poly(ethylene oxide)-block-poly(propylene oxide)-
block-poly(ethylene oxide) (P123, EO20PO70EO20, Mw = 5800 g/mol, Aldrich), sodium

94

silicate solution (26.5% SiO2, Aldrich), ammonium fluoride (Aldrich), glacial acetic acid
(EMD Chemicals), hydrochloric acid (EMD Chemicals), and 1,3,5-trimethylbenzene
(TMB, Alfa Aesar) were used to prepared the adsorbent supports. Deionized water (DI)
was generated with a Milli-Q integral pure and ultrapure water purification system from
Millipore. Two branched polyethylenimines (PEIs) with molecular weights average (Mw)
of ~25,000 g/mol and ~800 g/mol were purchased from Aldrich and denoted as PEI25k
and PEI800, respectively. Branched PEI1800 (Mw ~ 1800g/mol) was purchased from
Alfa Aesar. All chemicals were directly used as received unless otherwise stated.
3.4.2 Preparation of adsorbent supports
The mesoporous silica support was prepared by a “sol-gel approach”. In a typical
preparation of S5 (vide infra), 24.2 g P123 was added to 375 mL of DI water and 23 mL
of glacial acetic acid. This mixture was kept under stirring at 40 °C for 18 h to obtain a
homogenous solution. The pore swelling agent, 42 mL of 1,3,5-trimethylbenzene (TMB)
was added and the solution stirred for an additional 2 h. After that, 2.52 g of ammonium
fluoride was added. Within a minute, a solution of 36 mL sodium silicate and 250 mL DI
water was slowly poured into the prepared solution. The combined reaction mixture was
vigorously stirred for 10 min before letting it sit under static conditions at 40 °C for 24 h.
The temperature was increased to 70 °C and the solution aged for another 24 h. The
resulting white suspension was filtered on a Buchner funnel and washed copiously with
DI water. Any organic components present were removed by calcination at 560 °C for 6 h
with a temperature ramp of 5 °C/min from room temperature to 560 °C to afford a light
and fluffy white solid.

95

To study the effects of support composition and morphology on the CO2
absorption capacity of the prepared adsorbents, a series of mesoporous silica support
were synthesized by modifying various variables as well as based on previous reports.
More details are given in the SI.
3.4.3 Preparation of adsorbents
PEI was coated on the supports by a wet impregnation method. Desired amounts
of PEI and support were mixed in methanol solution. After mixing for 24 h, the methanol
was evaporated on a rotary evaporator. The prepared adsorbent was further evacuated
under high vacuum at r.t. overnight. Samples were labeled as MCF-x, where x represents
the PEI weight percentage. All adsorbents were stored in closed vials until further
investigation.
3.4.4 Measurement of CO2 adsorption capacity
The CO2 adsorption and desorption measurements were performed on Shimadzu
TGA-50 thermogravimetric analyzer. Usually the adsorbents were tested for CO2
adsorption at 25, 55, and 85 °C using a 95% CO2/5% N2 gas mixture.
Typically 5 mg of solid adsorbent was loaded in a platinum pan and placed into
the TGA instrument. The sample was initially heated to 110 °C under a pure N2
atmosphere (flow = 60 mL/min) and this temperature was maintained for 30 min to
desorb water and CO2 from the surface. The temperature was then lowered to 25 °C and
the adsorbent exposed to 95% CO2 (flow = 60 mL/min) for 3 h. After that the gas flow
was switched back to N2 and the temperature increased to 85 °C for 90 min desorption.
The second adsorption cycle was carried out under 95% CO2 at 55 °C for 3 h followed by
desorption at 85 °C under N2 for 90 min. The third adsorption cycle was carried out at

96

85 °C for 3 h. Finally 10 adsorption/desorption cycles were carried out isothermally at
85 °C. Fifteen minutes adsorption under 95% CO2 was followed by 25 min desorption
under N2 for each cycle.
For regenerability studies, under isothermal conditions with 100
adsorption/desorption cycles or more, the adsorbent sample was pre-treated as described
above (110 °C). The temperature was then lowered to 75 °C. For each cycle, an
adsorption step at 75 °C (10 min) under 95% CO2 (60 mL/min) was followed by a
desorption step at 75 °C (15 min) under pure N2 (60 mL/min).
3.4.5 Characterization
3.4.5.1 Surface area and pore analysis
Nitrogen adsorption/desorption isotherms were measured at -196 °C with a
Quantachrome NOVA 2200e surface area and pore volume analyzer. The samples were
pre-treated at 250 °C under vacuum for at least 3 h. The specific surface area was
calculated by the multipoint Brunauer-Emmett-Teller (BET) method. The total pore
volume was evaluated at a P/P0 close to 0.995. The Barrett-Joyner-Halenda (BJH)
method was used to calculate the pore volume and pore size distribution using the
desorption branch of the isotherm. A transmission electron microscope (TEM, JEOL
JEM-2100F) was used to observe the morphologies of the supports and adsorbents.
3.4.5.2 Thermogravimetric analysis to determine the organic content of the
prepared adsorbents.
The organic content of the adsorbents was determined by weight loss using
thermogravimetric measurements on a Shimadzu TGA-50 thermogravimetric analyzer

97

under an air flow of 30 mL/min in a temperature range increasing from 25 to 800 °C with
a heating rate of 10 °C/min.
3.4.5.3 Density measurements
The tapped density of the adsorbent was measured by placing a known amount of
the adsorbent into a graduated cylinder, which was tapped continuously for 2 min. The
volume occupied by the adsorbent was then recorded and the density of the solid in g/mL
determined.

98

3.5 References
(1) CRC Handbook of Chemistry and Physics; 83rd ed.; Lide, D. R. CRC
Press: Boca Raton, 2002-2003.
(2) Xu, X.; Song, C.; Miller, B. G.; Scaroni, A. W. Fuel Process. Technol.
2005, 86, 1457.
(3) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Energ.
Fuels 2002, 16, 1463.
(4) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Micropor.
Mesopor. Mater. 2003, 62, 29.
(5) Belmabkhout, Y.; Sayari, A. Adsorption 2009, 15, 318.
(6) Zeleňák, V.; Badaničova, M.; Halamová, D.; Čejka, J.; Zukal, A.; Murafa,
N.; Goerigk, G. Chem. Eng. J. 2008, 144, 336.
(7) Son, W.-J.; Choi, J.-S.; Ahn, W.-S. Micropor. Mesopor. Mater. 2008, 113,
31.
(8) Yan, X.; Zhang, L.; Zhang, Y.; Yang, G.; Yan, Z. Ind. Eng. Chem. Res.
2011, 50, 3220.
(9) Schmidt-Winkel, P.; Lukens, W. W.; Yang, P.; Margolese, D. I.; Lettow, J.
S.; Ying, J. Y.; Stucky, G. D. Chem. Mater. 2000, 12, 686.
(10) Schmidt-Winkel, P.; Lukens, W. W. J.; Zhao, D.; Yang, P.; Chmelka, B.
F.; Stucky, G. D. J. Am. Chem. Soc. 1999, 121, 254.
(11) Qi, G.; Fu, L.; Choi, B. H.; Giannelis, E. P. Energ. Environ. Sci. 2012, 5,
7368.

99

(12) Voegtlin, A. C.; Ruch, F.; Guth, J. L.; Patarin, J.; Huve, L. Micro. Mater.
1997, 9, 95.
(13) Bagshaw, S. A.; Bruce, I. J. Micropor. Mesopor. Mater. 2008, 109, 199.
(14) Leventis, N.; Mulik, S.; Wang, X.; Dass, A.; Patil, V. U.; Sotiriou-
Leventis, C.; Lu, H.; Churu, G.; Capecelatro, A. J. Non-Crystalline Solids 2008, 354, 632.
(15) Xu, X.; Song, C.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Energ.
Fuels 2002, 16, 1463.
(16) Liu, S.-H.; Hsiao, W.-C.; Sie, W.-H. Adsorption 2012, 18, 431.
(17) Yan, W.; Tang, J.; Bian, Z.; Liu, H. Ind. Eng. Chem. Res. 2012, 51, 3653.
(18) Zhao, J.; Simeon, F.; Wang, Y.; Luo, G.; Hatton, A. RSC Advances 2012,
2, 6509.
(19) Subagyono, D. J. N.; Liang, Z.; Knowles, G.; Chaffee, A. L. Chem. Eng.
Res. Design 2011, 89, 1647.
(20) Chaikittisilp, W.; Khunsupat, R.; Chen, T. T.; Jones, C. W. Ind. Eng.
Chem. Res. 2011, 50, 14203.
(21) Li, W.; Bollini, P.; Didas, S.; Choi, S.; Drese, J. H.; Jones, C. W. ACS
Appl. Mater. Interfaces 2010, 2, 3363.
(22) Liang, Z.; Fadhel, B.; Schneider, C. J.; Chaffee, A. L. Adsorption 2009, 15,
429.
(23) Goeppert, A.; Meth, S.; Prakash, G. K. S.; Olah, G. A. Energ. Environ. Sci.
2010, 3, 1949.
(24) Meth, S.; Goeppert, A.; Prakash, G. K. S.; Olah, G. A. Energ. Fuels 2012,
26, 3082.

100

(25) Sayari, A.; Belmabkhout, Y. J. Am. Chem. Soc. 2010, 132, 6312.
(26) Chen, C.; Yang, S.-T.; Ahn, W.-S.; Ryoo, R. Chem. Commun. 2009, 3627.
(27) Liu, Y.; Shi, J.; Chen, J.; Ye, Q.; Pan, H.; Shao, Z.; Shi, Y. Micropor.
Mesopor. Mater. 2010, 134, 16.
(28) Wang, X.; Li, H.; Liu, H.; Hou, X. Micropor. Mesopor. Mater. 2011, 142,
564.
(29) Wang, W.; Wang, X.; Song, C.; Wei, X.; Ding, J.; Xiao, J. Energ. Fuels
2013, 27, 1538.
(30) Qi, G. G.; Wang, Y. B.; Estevez, L.; Duan, X. N.; Anako, N.; Park, A. H.
A.; Li, W.; Jones, C. W.; Giannelis, E. P. Energ. Environ. Sci. 2011, 4, 444.

101

Chapter 4. Applicability of fumed silica supported branched polyethylenimine as an
adsorbent for direct air capture
4.1 Introduction
Since the development of the concept of CCS, CO2 capture from relatively
concentrated industrial streams such as flue and exhaust gases of coal burning power
plants, cement or aluminum factories, and fermentation plants has generated a lot of
attention. It has been the subject of a vast number of studies by numerous academic as
well as industrial groups. Advances in this field have been well covered in recent
publications.
1-6
However, much less attention has been given to CO2 capture from the air,
for which the available data is still quite limited and has only recently attracted increasing
interest.
7-8
Whereas practical applications have been developed for the essential removal
of CO2 from submarines and spacecrafts,
9-10
the separation and recovery of CO2 from
ambient air on a larger scale is still in its infancy.
8
Adsorbents based on strong bases such
as Ca(OH)2,
11
NaOH,
12-16
and combination thereof have been proposed but their use is
energy intensive because of the high temperatures required for their regeneration. Solid
sorbents based on amine and polyamine either bound chemically or attached physically
on supports such as silica, have been recognized as potential candidates and are being
tested for the CO2 capture from the air.
10,17-27

Following our earlier studies on branched polyethylenimines (BPEIs)
impregnated on precipitated silica and fumed silica,
28-30
we have performed a study on the
properties of amine-silica hybrid adsorbents for the direct adsorption of CO2 from the air.
Continuing on the theme, characteristics of the amine-silica hybrid material were

102

comprehensively investigated. In this Chapter, the effect of particle size, PEI loading and
molecular weight, adsorption and desorption temperatures, gas stream flow rate, heat of
adsorption and desorption are examined. Since all the materials used in our study are
commercially available and the preparation is straightforward, the developed adsorbents
can find applications for practical direct CO2 capture from the air.
4.2 Results and Discussion
Previous research have shown that adsorbents based on oligomers of ethylenimine
such as tetraethylenepentamine (TEPA) and pentaethylenehexamine (PEHA) impregnated
on precipitated silica suffered from leaching issues, despite the fact that they generally
adsorbed more CO2 than their higher molecular weight counterparts.
29,31-37
Part of the
polyamine was lost overtime resulting in decreasing adsorption capacity and possible
contamination of the system downstream of the adsorption unit. Other amines with
relatively low boiling points including monoethanolamine, diethanolamine,
diisopropanolamine and 2-(2-aminoethylamino)-ethanol had similar issues.
29,37-40

Polyethylenimines, especially the ones with molecular weights higher than ~600-
800 g/mol were found to be more stable with regard to possible leaching problems.
Branched PEI with a high molecular weight (Mw=25000 g/mol) showed no signs of
leaching. Because of its low volatility, it was therefore selected as adsorbent material of
choice. Fumed silica (FS, Aerosil 380) was coated with PEI by simply mixing it with a
solution of PEI in methanol followed by evaporation of the solvent.
The reaction of CO2 with branched PEI is represented in Scheme 4-1. The
repeating unit of the polymer in this scheme is only a simplified model representation
showing the three different types of amines present in PEI. The primary and secondary

103

amino groups in PEI react with CO2 to form carbamates. These carbamates can react
further to form bicarbonates in the presence of water. Under dry conditions, two amino
groups are therefore necessary for every CO2 molecule that is captured. On the other
hand, only one amino group is theoretically needed to capture a molecule of CO 2 under
humid conditions. Thus, the CO2 adsorption capacity of PEI should be higher under
humid conditions and indeed this was observed.
28
The addition of water also greatly
stabilized the adsorbents based on PEI and grafted amines by inhibiting the formation of
urea species.
41-42

Scheme 4-1 Reaction of CO2 with PEI

4.2.1 Effect of the particle size of the adsorbent
In a first series of experiments, the effect of particle size of FS-PEI-50 on the CO2
adsorption capacity from air was determined. Figure 4-1 shows the typical shape of the
CO2 concentration measurements. The first segment of the curve displays the CO 2
concentration in the air when the adsorbent was bypassed (approx. 400-420 ppm). Once
the air flow was directed to the adsorbent, complete adsorption of CO2 was observed
resulting in a CO2 concentration close to 0 ppm. Following this initial period of
adsorption (before breakthrough), the adsorbent began to saturate and a gradual increase
in CO2 concentration in the outlet gas was observed until complete saturation was

104

achieved. The areas of both CO2 adsorption before and after breakthrough were
integrated to calculate the total adsorption capacity.
In Figure 4-1 and Figure 4-2, it can be seen that the particle size had a pronounced
effect on the adsorption behavior. A decrease in particle size resulted in a longer initial
period where all of the CO2 present in the inlet air was adsorbed and the outlet
concentration was essentially free of CO2. For particles smaller than 0.25 mm as well as
particles between 0.25 and 0.50 mm, the time required for complete adsorption was about
10 h. For larger particles with a size of 0.5-1.7 mm and > 1.7 mm, this time was much
shorter at only 8 h and 4.5 h, respectively. The saturation slope also decreased with
increasing particle size. With particles < 0.25 mm, the adsorbent reached saturation
relatively fast after the initial total CO2 adsorption. With particles larger than 1.7 mm,
this saturation occurred much slower over a period of about 30 h. The overall adsorption
capacity increased only slightly going from particles < 0.25 mm to > 1.7 mm (Figure 4-2).
The adsorption during the initial period, however decreased rapidly with increasing
particle size, from about 58 mg CO2/g with particles < 0.25 mm to 24 mg CO2/g with
particles >1.7 mm. CO2 adsorption measurements performed by TGA showed a similar
trend with the fastest adsorption kinetics obtained with particles < 0.25 mm and between
0.25 mm and 0.50 mm and lower adsorption rates as the particle size increased.
The CO2 desorption step exhibited the same pattern. TGA measurements of CO2
desorption showed that the desorption rate decreased substantially when the particle size
increased (Figure 4-3). Whereas particles < 0.25 mm and between 0.25 mm and 0.50 mm
had similar desorption rates at 85 °C, particles larger than 1.7 mm had the slowest
desorption rate.

105

The diffusion of gases (air containing CO2 or pure air) inside the adsorbent
particles might be more difficult in larger particles, explaining the observed decrease in
both the rate of adsorption and desorption with increasing particle size.

Figure 4-1 Effect of the particle size of FS-PEI-50 on the CO2 adsorption from air at
25 °C

0
100
200
300
400
500
0 10 20 30 40 50
CO
2
concentration (ppm)
Time (h)
<0.25 mm
0.25-0.50 mm
0.5-1.7 mm
> 1.7 mm

106

Figure 4-2 Total CO2 adsorption and CO2 adsorption under 10 ppm as a function of FS-
PEI-50 particle size
For most applications, maximum adsorption in a shorter time period is preferred.
Some applications such as alkaline fuel cells also require “CO2 free” air to avoid
deterioration of the electrolyte. In both cases, the utilization of the adsorbent with the
highest adsorption capacity before breakthrough, i.e. with a smaller particle size, would
be preferred. Of course, other parameters have also to be taken into account such as
pressure drop along the adsorbent bed, which would increase as the particle size
decreases. In the case of particles smaller than 0.25 mm, a slight pressure developed
upstream of the adsorbent bed (~0.10 bar). Particles with a size of 0.25-0.50 mm are a
good compromise and was therefore the size of choice of the particles used for the other
studies reported in this chapter.
0
10
20
30
40
50
60
70
80
90
<0.25 0.25-0.50 0.5-1.7 >1.7
Adsorption / mg CO
2
per g adsorbent
Particle size / mm
total CO2 adsorption from air (mg/g)
CO2 adsorption from air under 10 ppm(mg/g)
total CO
2
adsorption from air (mg/g)
CO
2
adsorption from air under 10 ppm (mg/g)

107

Figure 4-3 Effect of the particle size of FS-PEI-50 on CO2 desorption at 85 °C
4.2.2 Influence of PEI loading
The effect of PEI loading on the CO2 adsorption characteristics of FS-PEI was
studied. For further studies, the same particle size fraction of 0.25-0.50 mm was selected
for all PEI loadings. As might be expected, the CO2 adsorption capacity decreased with
decreasing PEI loading, from 73.7 mg/g with FS-PEI-50 to 18.0 mg/g with FS-PEI-20
(Figure 4-4 and Table 4-1). However, the ratio between the initial phase when all the CO2
is adsorbed from the incoming gas and the total adsorption increased from 0.70 to 0.88
for FS-PEI containing 50 and 20% PEI, respectively. For FS-PEI-20, most of the CO2
was captured during the initial phase; the difference between initial CO2 adsorption and
total CO2 adsorption becoming larger with increasing PEI loadings (Figure 4-5). The
slope of saturation, also increased more than 10 fold while going from a PEI loading of
50% to 20%. This slope expressed in ppm/h is the slope of the linear part of the curve in
Figure 4-4 between the initial adsorption (before breakthrough) and the total saturation.

0
20
40
60
80
100
0 10 20 30 40
CO
2
desorption / %
Time / min
<0.25 mm
0.25-0.50 mm
0.5-1.7 mm
> 1.7 mm

108

Table 4-1 Effect of PEI loading in FS-PEI on the CO2 adsorption characteristics at 25 °C
Adsorbent Surface
area
[m
2
/g]
Volume
of
pores
[cm
3
/g]
Total CO 2
adsorption
from air
[mg/g]
CO 2
adsorption
from air
under 10
ppm
[mg/g]
Ratio
adsorption
under 10
ppm/total
adsorption
Slope of
saturation
[ppm/h]
FS-PEI-50 27.2 0.40 73.7 51.8 0.70 109
FS-PEI-33 79.9 1.06 50.0 40.8 0.82 349
FS-PEI-25 108 1.42 34.5 29.4 0.85 518
FS-PEI-20 114 1.49 18.0 15.8 0.88 1476

The faster saturation might in part be due to the fact that less amine sites are
present at lower PEI loadings and thus the required time for saturation may be shorter.
Our observations, however, point to a better dispersion of PEI on the surface of fumed
silica at lower loadings, allowing for an easier access of CO2 to the active amino sites.
This is also supported by the higher surface area and total pore volume of adsorbents
containing lower PEI loadings. Whereas FS-PEI-50 had a surface area of 27.2 m
2
/g and a
pore volume of 0.4 cm
3
/g, FS-PEI-25 had much higher surface area of 108 m
2
/g and a
pore volume of 1.42 cm
3
/g. FS-PEI-20, with the lowest PEI content had the highest
surface area and pore volume with 114 m
2
/g and 1.49 cm
3
/g, respectively. A slight shift to
larger pores was noticed with decreasing PEI content (Figure 4-6). However, the general
distribution of the pore size did not change much with a maximum around 350 Å to 390
Å regardless of the PEI loading. The presence of these relatively large pores in the
adsorbent is beneficial for CO2 to access the active amino sites. However, this contrasts
strongly with the pore distribution in the fumed silica support, which showed essentially
no peaks.

109

Figure 4-4 Effect of PEI loading in FS-PEI on CO2 adsorption from air at 25 °C

Figure 4-5 Total CO2 adsorption and CO2 adsorption under 10 ppm as a function of PEI
loading in FS-PEI

Figure 4-6 Effect of PEI loading on the pore size distribution of FS-PEI
0
100
200
300
400
500
0 10 20 30 40 50
CO
2
concentration / ppm
Time / h
FS-PEI-20
FS-PEI-25
FS-PEI-33
FS-PEI-50
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60
CO
2
adsorption / mg CO
2
per g
adsorbent
PEI content / %
total CO
2
adsorption from air
CO
2
adsorption from air under 10 ppm
0.000
0.001
0.002
0.003
0.004
0.005
0.006
10 100 1000 10000
dV(d) / cm
3
.Å
-1
.g
-1
Pore Diameter / Å
FS-PEI-50
FS-PEI-33
FS-PEI-25
FS-PEI-20
fumed silica

110

The active ingredient for the adsorption of CO2 in the studied adsorbents was PEI.
Therefore the specific adsorption capacity for PEI contained in the adsorbent at different
PEI concentrations was also determined. The results given in mg of CO 2 adsorbed per
gram PEI are presented in Figure 4-7. The best utilization of PEI in the case of CO2
capture from air was obtained at concentrations of 33 and 50 % PEI with about 150 mg
CO2/g PEI. At lower loadings, the efficiency of PEI utilization diminished strongly. This
is quite different from the behavior we observed with pure CO2, where the best utilization
of PEI was achieved at lower PEI concentrations with values up to 350 mg CO 2/g PEI at
PEI loadings under 30%.
29
A similar trend has been observed by Sanz et al. on SBA-15
impregnated with PEI (Mw of 800 g/mol) while using pure CO2 and 15% CO2 in
nitrogen.
43
While using SBA-15-PEI-50 with a PEI loading of 50%, they observed a
minimal decrease in the efficiency of the amine use (expressed in mol CO2 adsorbed per
mol of N on the adsorbent) going from 0.18 to 0.17 for CO2 concentrations of 100% and
15%, respectively. Under the same conditions, a much more pronounced decrease from
0.25 to 0.145 was obtained on SBA-15-PEI-10 with a PEI loading of 10%. At a higher
CO2 pressure of 4.5 atm the difference in amine efficiency was even larger compared to 1
atm of CO2. Whereas the efficiency increased only slightly to 0.19 for SBA-15-PEI-50, it
more than doubled for SBA-15-PEI-10 to 0.52. As observed in our studies, with
decreasing CO2 concentration, the adsorption capacity decreased proportionally faster in
adsorbents with lower PEI loadings. This effect could be due to the interaction between
PEI and silanol groups on the surface of the support. A similar observation was made by
Wang et al. in their infrared study of CO2 adsorption on PEI impregnated on SBA-15.
44

Coating with PEI is in fact one of the ways to passivate the acidic silanol groups on

111

fused-silica capillaries used for example in the separation of amino acids and peptides.
45

Because of this interaction, part of the amino groups is less available for CO2 adsorption.
However, once the surface is saturated with PEI the excess amino groups would be free
to react with CO2. At higher PEI concentration, this effect should be less pronounced due
to the large excess of PEI compared to the number of silanol groups. Increase in the
concentration or pressure of CO2 could shift the equilibrium reaction with PEI from
silanol groups towards CO2, explaining the higher efficiency of PEI utilization observed.
Interestingly, this is also supported by our recent results reported on organoamines
grafted on silica.
46
A higher graft density was obtained when the organoamine-silane
coupling agent was first saturated with CO2 prior to grafting. The amino groups of the
organoamine reacted with CO2 to form carbamates, which greatly reduced their ability to
interact with the silanol groups present on the surface of the support. More silanol groups
were therefore available to react with the silane resulting in a higher graft density and
subsequently to a substantially higher CO2 adsorption capacity of the synthesized
adsorbent.

Figure 4-7 Efficiency of PEI utilization for CO2 adsorption from air at 25 °C as a
function of PEI loading in FS-PEI
0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60
CO
2
adsorption / mg CO
2
per g PEI
PEI content / %
total CO2 adsoprtion from air total CO
2
adsorption from air
(mg/g PEI)

112

4.2.3 Effect of temperature on the adsorption
Considering the gas separation processes from the atmosphere, temperature plays
a significant role in the adsorption capacity. A series of experiments were therefore
carried out under the same conditions, except for the capture temperature. Figure 4-8
represents the measurements obtained for CO2 adsorption from air at temperatures from
25 ºC to 85 ºC on FS-PEI-33 (0.25-0.50 mm) at a flow rate of 335 mL/min. In Figure 4-9,
the adsorption capacity as a function of temperature is plotted. FS-PEI-33 (0.25-0.50 mm)
had the highest adsorption capacity of 50 mg CO2/g adsorbent at 25 °C. Increasing the
temperature resulted in a decrease in the adsorption capacity. At 85 °C the adsorption
capacity became negligible. On FS-PEI-50, the maximum adsorption capacity occurred
on a slightly higher temperature (Figure 4-10 and Figure 4-11). From 70 mg/g at 25 °C,
the adsorption capacity peaked at 75 mg CO2/g at 35 °C before decreasing at higher
temperatures (Figure 4-11). TGA experiments also confirmed this maximum adsorption
capacity at a temperature of 35 °C for FS-PEI-50. In the case of FS-PEI-50 too, almost no
adsorption was observed at 85 °C. On both FS-PEIs, the desorption of the CO2 adsorbed
close to room temperature could therefore be easily performed at 85 °C either with a
stripping gas or under vacuum. The difference in temperature between maximum
adsorption and the temperature needed for desorption is relatively small (50-60 °C),
especially compared to more traditional direct air adsorbents such as alkaline solution of
NaOH that require very high temperatures for their regeneration.
15,47
The use of these
solid adsorbents has therefore the potential to decrease significantly the amount of energy
required for the separation of CO2 from the air. Because of the relatively low regeneration

113

temperature, low grade heat or waste heat from some other process could be used.
Utilization of solar heat is also a possibility.
The ratio between adsorption before breakthrough and total adsorption was higher
at all temperatures on FS-PEI-33 compared to FS-PEI-50 (Table 4-2 and Table 4-3). As
pointed out, vide supra, this is due to a the better dispersion of the amine on the surface
of the silica support at lower PEI loadings, which allows CO2 to access more easily the
active amino groups as compared to FS-PEI-50.

Figure 4-8 CO2 adsorption from air as a function of time on FS-PEI-33 (0.25-0.50 mm)
at various temperatures

Figure 4-9 Adsorption as a function of temperature on FS-PEI-33 (0.25-0.50 mm)
0
100
200
300
400
500
0 5 10 15 20 25
CO
2
concentration / ppm
Time / h
25 C
35 C
45 C
55 C
65 C
85 C

0
10
20
30
40
50
60
20 40 60 80 100
Adsorption / mg CO
2
per g adsorbent
Temperature / C
Total CO2 adsorption from air
(mg/g)
CO2 adsorption from air
under 10 ppm (mg/g)
total CO
2
adsorption from air
CO
2
adsorption from air under 10
ppm

114

Figure 4-10 CO2 adsorption from air as a function of time on FS-PEI-50 (0.25-0.50 mm)
at various temperatures

Figure 4-11 CO2 adsorption as a function of temperature on FS-PEI-50 (0.25-0.50 mm)
Table 4-2 Temperature dependence of adsorption characteristics of FS-PEI-33 (0.25-0.50
mm)
Temperature
[°C]
Total CO 2
adsorption
from air
[mg/g]
CO 2 adsorption
from air under
10 ppm [mg/g]
Ratio adsorption
under 10
ppm/total
adsorption
Slope of
saturation
[ppm/h]
25 50.0 41.0 0.82 349
35 48.0 42.5 0.89 396
45 39.2 34.5 0.88 256
55 22.5 16.2 0.72 205
65 6.8 5.2 0.76 922
85 1.1 0.3 0.23 4046

0
100
200
300
400
500
0 10 20 30 40
CO
2
concentration / ppm
Time / h
25 C
35 C
45 C
55 C
65 C
85 C
0
10
20
30
40
50
60
70
80
20 40 60 80 100
Adsorption / mg CO
2
per g adsorbent
Temperature / C
T otal CO2 adsorption from air
(mg/g)
CO2 adsorption from air under
10 ppm (mg/g)
total CO
2
adsorption from air
CO
2
adsorption from air under 10
ppm

115

Table 4-3 Temperature dependence of adsorption characteristics of FS-PEI-50 (0.25-0.50
mm)
Temperature
[°C]
Total CO2
adsorption
from air
[mg/g]
CO2
adsorption
from air
under 10
ppm
[mg/g]
Ratio
adsorption
under 10
ppm/total
adsorption
Slope of
saturation
[ppm/h]
25 70.0 50.3 0.72 121
35 75.0 60.0 0.80 119
45 65.3 48.1 0.74 136
55 34.5 13.1 0.38 44
65 8.4 3.9 0.47 292
85 0.8 0.0 0.00 3154

4.2.4 Effect of flow rate on the adsorption
On FS-PEI-33, the adsorption capacity did not depend too much on the flow rate.
When increasing the flow rate from 335 mL/min to 945 mL/min, the total adsorption
capacity decreased only slightly from 50 to 47 mg CO2/g as did the ratio of adsorption
before breakthrough to total adsorption (Figure 4-12 and Table 4-4). On FS-PEI-50, the
adsorption capacity decreased more significantly between a flow rate of 335 mL/min and
945 mL/min from 73.7 to 61.0 mg CO2/g (Figure 4-13 and Table 4-4). The ratio of
adsorption before breakthrough to total adsortption decreased however only slightly. In
TGA experiments with an even higher gas flow compared to the amount of adsorbent,
FS-PEI-50 adsorbed about 62.6 mg CO2/g, which is close to the value obtained here.
Noteworthy is that even at the highest air flow rate of 945 mL/min (31 cm/s), both
adsorbents were able to extract all the CO2 from the incoming air for an extended period
of time as can be seen by the CO2 concentration remaining close to 0 ppm (Figure 4-12

116

and Figure 4-13). This indicates that less adsorbent could be used to extract CO2 from the
air by simply allowing for more frequent regeneration steps.

Figure 4-12 Effect of air flow rate on the CO2 adsorption as a function of time on FS-
PEI-33 (0.25-0.50 mm)

0
100
200
300
400
500
0 5 10 15 20
CO
2
concentration / ppm
Time / h
335 mL/min
667 mL/min
945 mL/min

117

Figure 4-13 Effect of air flow rate on the CO2 adsorption as a function of time on FS-
PEI-50 (0.25-0.50 mm)

Table 4-4 Temperature dependence of adsorption characteristics of FS-PEI-33 and FS-
PEI-50
Adsorbent Flow rate
[mL/min]
total CO2
adsorption from
air [mg/g]
CO2
adsorption
from air
under 10 ppm
[mg/g]
Ratio adsorption
under 10
ppm/total
adsorption
FS-PEI-33 335 50.0 41.0 0.82
FS-PEI-33 667 47.0 38.7 0.82
FS-PEI-33 945 47.0 37.1 0.79
FS-PEI-50 335 73.7 51.8 0.70
FS-PEI-50 667 61.8 41.3 0.67
FS-PEI-50 945 61.0 40.0 0.66

0
100
200
300
400
500
0 10 20 30 40 50
CO
2
concentration / ppm
Time / h
335 mL/min
667 mL/min
945 mL/min

118

4.2.5 Desorption and adsorption cycling
To study its CO2 desorption behavior, FS-PEI-50 was submitted to a regeneration
by temperature swing in which it was exposed to a higher temperature under a flow of
nitrogen (335 mL/min). Temperatures between 70 ºC and 100 ºC were tested for these
regeneration experiments. The CO2 concentrations of the outlet gases as a function of
time are presented in Figure 4-14. At 100 ºC, the CO2 concentration showed a maximum
value of 10%. A fast desorption occurred and in about 20 min all the CO2 was essentially
desorbed. At 85 ºC, total desorption took some 40 min and the CO2 concentration reached
a maximum of 4%. Lowering further the desorption temperature to 70 ºC dramatically
increased the time needed for the desorption to more than 120 min. After each desorption
step, an adsorption step was performed at 25 ºC under air. The adsorption capacity
measured was similar (72 to 75 mg/g) to the one obtained during the first adsorption
using vacuum at 85 ºC for the desorption of CO2. This means that given enough time,
most if not all the CO2 present on the adsorbent can be desorbed under nitrogen even at
70 ºC. Desorption experiments conducted by TGA on FS-PEI-50 showed a similar trend
and full desorption could be obtained even at temperatures as low as 50 °C under air (see
Figure 4-15). However, the reaction rate was about 35 times faster at 85 °C than at 50 °C.
The effect of flow rate of the swipe gas (N2) on the desorption at 85 ºC was also
determined. This can be seen in Figure 4-16 in which the time needed for desorption
decreases when the flow rate is increased. With a flow rate of 1000 mL/min, total
desorption was achieved in less than 30 min. At a rate of 100 mL/min, about 90 min were
needed.

119

In order to show the regenerability of the adsorbents, they were subjected to four
consecutive adsorption/desorption cycles. The adsorption was performed at 25 ºC and the
desorption at 85 ºC under a flow of air (335 mL/min). Over these four cycles both FS-
PEI-33 and FS-PEI-50 did not show a decrease in adsorption capacity (Figure 4-17). This
also indicates that the PEI coated on fumed silica did not degrade significantly at 85 °C in
the presence of oxygen. However, to determine the long-term oxidative stability of PEI
based adsorbents studies with a much larger number of adsorption/desorption cycles
under these conditions are still required.

Figure 4-14 Effect of temperature on the CO2 desorption as a function of time on FS-
PEI-50
0 20 40 60 80 100 120 140
0
2
4
6
8
10
12
CO
2
concentration / %
Time / min
0 20 40 60 80 100 120 140
0
2
4
6
8
10
12
Time / min
0 20 40 60 80 100 120 140
0
2
4
6
8
10
12
Time / min
70 C
85 C
100 C

120

Figure 4-15 CO2 desorption under air on FS-PEI-50 at various temperatures. Desorption
conducted after CO2 adsorption from air (400 ppm) at 25 °C. TGA measurements

Figure 4-16 Effect of flow rate on the CO2 desorption at 85 °C as a function of time on
FS-PEI-50
0
20
40
60
80
100
0 50 100 150 200
Desorption / %
Time / min
85 C
70 C
60 C
50 C
0 20 40 60 80 100 120
3.0
2.5
2.0
1.5
1.0
0.5

Time / min
100 mL/min
335 mL/min
1000 mL/min
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
3.0
CO
2
concentration / %
Time / min
0 20 40 60 80 100 120
0.0
0.5
1.0
1.5
2.0
2.5
3.0

Time / min

121

Figure 4-17 Adsorption/desorption cycles for FS-PEI-33 and FS-PEI-50
4.2.6 Heat (reaction enthalpy) of adsorption and desorption of CO2 on FS-PEI
The heat of adsorption and desorption of CO2 on PEI based adsorbents were
measured using differential scanning calorimetry. Figure 4-18 shows a typical heat of
adsorption graph obtained for the adsorption of pure CO2 at 85 °C. After the adsorption
was completed, heat of the desorption of the same sample was measured by heating the
sample from 0 °C to 140 °C under nitrogen (heating rate of 5 ºC / min).
0
10
20
30
40
50
60
70
80
1 2 3 4
Adsorption / mg CO
2
per g adsorbent
Pass
FS-PEI-33
FS-PEI-50

122

Figure 4-18 Typical heat of adsorption graph obtained on the Perkin Elmer DSC 7 for
FS-PEI-33. Isotherm 85 °C for 20 min
According to these measurements, the heat (enthalpy) of adsorption for FS-PEI-
50 was determined to be -83 kJ/mol CO2, whereas the enthalpy for desorption was 82
kJ/mol CO2. These values are an average of 5 measurements. The enthalpy of adsorption
and desorption measured on FS-PEI-33 were -85 and 87 kJ/mol CO2, respectively. For
both adsorbents the results obtained for the adsorption are in good correlation with the
ones obtained for desorption and are within the experimental error. These values are
consistent with the reaction of an amine with CO2. Monoethanolamine (MEA) has a heat
of adsorption around -85 kJ/mol, diethanolamine (DEA) about -72 kJ/mol. A tertiary
amine such as triethanolamine, which forms much weaker bonds with CO2 has a heat of
adsorption of -48 kJ/mol. These values are also close to the ones obtained by Satyapal et
al. on PEI bonded on a high-surface-area solid poly(methyl methacrylate) solid support.
The measurements of the heat of adsorption of CO2 on this adsorbent using a gas mixture

123

containing 2% CO2 in air was -94 kJ/mol; about 10 kJ/mol higher than the values that we
have obtained by DSC.
10
The higher heat of adsorption observed could be in part due to
the fact that at these lower CO2 concentrations, the amino groups that react with CO2 are
probably the most active ones (e.g. primary amines) binding CO2 more strongly and
requiring therefore more energy during the regeneration step. As the CO2 concentration
increases, slightly weaker amino groups binding somewhat less strongly CO2 can
increasingly contribute to CO2 adsorption reducing the needed energy for desorption per
mol of CO2.
In the desorption of CO2 from aqueous solutions of alkanolamines, heating the
MEA solution as a whole to around 110 C is necessary. A heat of desorption of about 7.7
kJ / g CO2 (330 kJ/mol CO2) is thus required, considering a 20% MEA aqueous solution
and an absorption of 0.4 mol of CO2 for 1 mol of MEA. This value has been estimated
using thermodynamic data from the literature. Less energy is needed for the desorption
step on the fumed silica supported material because it is composed of organics and SiO 2,
both of which have a lower heat capacity compared to water (about 2.1 – 2.5 J.g
-1
.C
-1
for
the organics and 0.96 J.g
-1
.C
-1
for silica compared to 4.19 J.g
-1
.C
-1
for water).
Consequently, to heat FS-PEI-50 from 25 °C to 110 °C, about 146 J/g adsorbent of heat
energy is needed. If an adsorption capacity of 75 mg CO2/g adsorbent is assumed, 13.3 g
of adsorbent would be needed to capture 1 g of CO2. About 1930 J (85 kJ/mol) are
needed to desorb 1 g of CO2. This means that about 146 × 13.3 + 1930 = 3872 J (3.872 kJ)
are required to heat the adsorbent from 25 °C to 110 °C and desorb 1g of CO 2. According
to these calculations, this is only about half what is needed for aqueous MEA-based
systems (7.7 kJ/g CO2). As we have seen, a temperature much lower than 110 °C is

124

sufficient to desorb CO2 from the solid adsorbent, further decreasing the energy needs of
the hybrid type material. It should also be pointed out that aqueous adsorbent solutions
such as MEA solutions might not be the best choice for CO2 adsorption from the air
because of the large volume of gas to be treated and ensuing loss of water and low boiling
MEA from the system.
8
A large pressure drop along the liquid adsorption column might
also be energetically unacceptable.
The apparent activation energy of the CO2 desorption reaction was also
determined using a TGA apparatus. For these measurements, adsorption was first
performed by flowing air containing 400 ppm CO2 over the adsorbent (FS-PEI-50) for 10
h. Desorption was then performed under pure air (0 ppm CO2) at temperatures ranging
from 50 °C to 85 °C in successive adsorption/desorption cycles. The desorption under
these conditions followed first order reaction kinetics with respect to CO2. The observed
reaction rate constants for CO2 desorption obtained at various temperatures were plotted
in an Arrhenius plot (Figure 2-19 and Figure 2-20). The apparent activation energy (Ea)
for the desorption of CO2 on FS-PEI-50 was 97 kJ/mol CO2. This value seems relatively
high considering that the desorption occurs even at temperatures as low as 50 °C.
Interestingly, however, it is close to the absolute value of -94 kJ/mol CO2 obtained for the
heat of adsorption by Satyapal et al.
10
using also a relatively low 2% CO2 concentration
in air. The absolute value of the heat of reaction for the CO2 adsorption and desorption
steps in the system studied based on PEI are generally similar to the ones observed in our
case (-83 and 82 kJ/mol, respectively for FS-PEI-50), indicating that in terms of energy,
the rate limiting transition state in the desorption step is close to the end products, i.e.
amino groups and gaseous CO2 (late transition state).

125

Figure 4-19 Ln of the CO2 concentration as a function of time at various temperatures on
FS-PEI-50

Figure 4-20 Arrhenius plot for the CO2 desorption on FS-PEI-50
4.2.7 Effect of the molecular weight of PEI on the adsorption
PEI with molecular weights from 423 to 25000 impregnated on fumed silica were
tested for their capacity to adsorb CO2 from the air. Adsorbents with PEI loadings of 33
and 50% were investigated. In all the cases adsorbents with a size of 0.25-0.50 mm were

126

used. All the PEIs in this study were branched except for PEI(423), which was linear. The
results are presented in Figure 4-18 and Table 4-5. The adsorption characteristics of
PEI(423) and PEI(800) were very similar at a PEI loading of 33%. Their adsorption
capacities (72-75 mg CO2/g) were, however, significantly higher than the adsorbents
containing PEI(1800) and PEI(25000) with 50 and 56 mg CO2/g, respectively. The initial
adsorption capacity before breakthrough followed the same order as the PEI M w, i.e.
increasing from PEI(800) to PEI (25000). At a higher PEI loading of 50%, the highest
adsorption capacity was also obtained with PEI (800) at 107 mg CO2/g (2.4 mmol CO2/g).
FS-PEI(423)-50 with a lower Mw PEI had a slightly lower adsorption capacity but was
still noticeably higher than on the adsorbents with higher M w PEI of 1800 and 25000
which had a very similar total CO2 adsorption capacity (~74 mg CO2/g). At a PEI loading
of 50%, the initial adsorption capacity before breakthrough did not follow a clear trend.
However, the initial adsorption capacity of FS-PEI(800)-50 with about 76 mg CO2/g was
significantly higher than the one for FS-PEI(25000)-50 and FS-PEI(1800)-50. As a
general trend, the use of PEI with lower molecular weight offers advantages in terms of
both total adsorption capacity and initial adsorption capacity. However, as already
pointed out, vide supra, lower molecular weight PEI can be more prone to leaching
leading to a loss of capacity overtime, negating the initial positive effect gained by using
them in adsorbent formulations.
29

Compared to other solid adsorbents based on PEI and different supports reported
in the literature, FS-PEI adsorbents exhibited some of the highest CO2 adsorption
capacities from air (Table 4-6). With PEI 800, the most commonly used PEI for these
studies, the highest adsorption capacity, 107.3 mg CO2 per g of adsorbent, was obtained

127

with our adsorbent based on fumed silica. The fact that fumed silica is also easily
available in large quantities from a variety of suppliers for a relatively low cost and that
the preparation of the adsorbent is extremely easy and straightforward is also an
advantage.

Figure 4-21 Effect of PEI molecular weight on CO2 adsorption from the air. (a) On FS-
PEI(x)-33 (0.25-0.50 mm). (b) On FS-PEI(x)-50 (0.25-0.50 mm)
0
100
200
300
400
500
0 10 20 30 40 50
CO
2
concentration / ppm
Time / h
FS-PEI(423)-50
FS-PEI(800)-50
FS-PEI(1800)-50
FS-PEI(25000)-50
(b)
0
100
200
300
400
500
0 5 10 15 20 25 30
CO
2
concentration / ppm
Time / h
FS-PEI(423)-33
FS-PEI(800)-33
FS-PEI(1800)-33
FS-PEI(25000)-33
(a)

128

Table 4-5 Effect of PEI molecular weight on the adsorption of CO2 from the air
Adsorbent total CO2 adsorption
from air [mg/g]
CO2 adsorption from air
under 10 ppm [mg/g]
FS-PEI(423)-33 71.9 59.8
FS-PEI(800)-33 74.7 62.7
FS-PEI(1800)-33 56.0 48.0
FS-PEI(25000)-33 50.0 40.8
FS-PEI(423)-50 103.0 64.1
FS-PEI(800)-50 107.3 75.9
FS-PEI(1800)-50 74.6 42.5
FS-PEI(25000)-50 73.7 51.8

Table 4-6 Comparison of FS-PEI with other solid adsorbents based on PEI for CO2
adsorption from the air
[a] linear PEI
4.3 Conclusion
Fumed silica supported PEI adsorbents are promising candidates for capturing
CO2 from dilute sources including the air. They are easily prepared from widely available
Support Branched
PEI
[MW]
PEI
concentration
in the
adsorbent
[%]
Adsorption
temperature
[°C]
CO 2
concentration
in the gas
[ppm]
CO 2
adsorption
capacity
[mg/g
adsorbent]
CO 2
adsorption
capacity
[mg/g
PEI]
Fumed silica
(this work)
423 50 25 400 103.0 206
800 50 25 400 107.3 215
1800 50 25 400 74.6 149
25000 50 25 400 73.7 147
Fumed silica 25000 50 25 400 74.8 150
28

Mesoporous
silica
(CARiACTG10)
800 45 25 400 103.8 231
48

Mesocellular
silica foam
(MCF)
800 46 25 400 76.6 166
23

2500
[a]
49 25 400 46.2 94
SBA-15 800 40 25 400 46.2 116
49

Alumina 800 50 25 400 76.6 153
49

Polymeric resin
HP20
800 50 25 400 99.3 199
27

Zr-SBA-15 800 35 25 400 37.4 107
50-51

129

and industrially produced materials and are able to adsorb CO2 reversibly under mild
conditions in repeated adsorption/desorption cycles. Their CO2 adsorption capacity is the
highest at or close to room temperature (25-35 ºC), which is preferable for capture from
the air. With increasing temperature, the adsorption capacity decreased. At 85 ºC, almost
no CO2 adsorption occurred. As expected, in the range from 20 to 50% PEI loading, the
adsorption capacity increased with increasing PEI content. However, contrary to what
was observed when pure CO2 was used, the best utilization of PEI (highest CO2
adsorption per g of PEI) was obtained with the adsorbent containing the highest
concentration of PEI (33% and 50 %). On the other hand, the ratio between the
adsorption before breakthrough and total adsorption increased when the PEI loading was
lowered. This is probably due to the better dispersion of the amine on the surface of the
silica support at low PEI loadings, which allows CO2 to access more easily the active
amino groups. When the PEI loadings are increased, the surface of the support is
increasingly saturated with PEI and access to some amino groups is made more difficult.
Particle size also played an important role in the adsorption kinetics. Although the total
adsorption increased slightly with increasing particle size, the initial adsorption before
breakthrough decreased considerably. The adsorption capacity diminished slightly with
increasing flow rate. Although desorption occurred at temperatures between 70 and 100
ºC, a higher temperature allowed for a much faster desorption. These relativly low
regeneration temperatures allow the use of “waste heat” available in many industrial
processes or even solar heat. As a general trend, decreasing the PEI molecular weight
resulted in an increase in adsorption capacity with an adsorption of 107 mg CO2/g (2.4
mmol CO2/g) obtained on the adsorbent prepared with PEI(800) on fumed silica.

130

However, lower molecular weight PEI can be more prone to leaching than its higher
homologues which could lead to a loss of capacity overtime and a possible contamination
of downstream equipment. Considering the results obtained in this paper, FS-PEI-50 with
a particle size in the 0.25-0.50 mm range seems to be the most practical system for
adsorption/desorption of CO2 from the air. If leaching is found to be no issue, the
utilization of PEI (800) seems to be preferable as it allows for a higher adsorption
capacity compared to higher PEI homologs.
Owing to their favorable characteristics for direct air capture of CO2, fumed silica
supported PEI adsorbents could be utilized for a variety of application such as
purification of gas streams from CO2 in submarines and other closed-circuit breathing
systems. They could also be used in alkaline fuel cells and batteries for which an air
source free of CO2 is essential to avoid the reaction of the strong electrolyte (typically
NaOH or KOH) with CO2 leading to the formation of carbonates. The development in
our laboratory of robust and inexpensive iron-air batteries intended for large scale energy
storage in grid applications, which likewise need CO2 free air for their basic electrolytes
could also benefit from this adsorbent. While further studies are necessary to better
understand and improve supported amine based adsorbents, they are clearly practical and
economically promising candidates for the CO2 capture from ambient air under relatively
mild conditions.
4.4 Experimental
4.4.1 Chemicals
Branched polyethylenimines (PEI) with molecular weights average (M w) of
~25,000 g/mol and 800 g/mol were purchased from Aldrich. Branched PEI with an

131

average molecular weight (Mw) of 1800 g/mol was obtained from Alfa Aesar. Linear PEI
with an average molecular weight (Mn) of 423 was purchased from Aldrich. They were
labeled as PEI(423), PEI(800), PEI(1800) and PEI(25000). Aerosil® 380 (hydrophilic) is
a fumed silica that was provided by Evonik. All chemicals were used as received unless
otherwise stated.
4.4.2 Preparation of adsorbents
PEI was coated on the supports by a wet impregnation method. Desired amounts
of PEI and support were mixed in methanol solution. After mixing for 24 h, the methanol
was evaporated on a rotary evaporator. The prepared adsorbent was further evacuated
under high vacuum at r.t. overnight. Samples are labeled as FS-PEI-x, where x represents
weight percentage of PEI in the adsorbent. All adsorbents were stored in closed vials
until further investigation. To study the effect of particle size, the solids obtained were
sieved into four ranges of sizes; smaller than 250 μm, 250-500 μm, 500-1700 μm and
larger than 1700 μm.
4.4.3 Characterization
Nitrogen adsorption/desorption isotherms were measured at 77 K with a
Quantachrome NOVA 2200e surface area and pore volume analyzer. The specific surface
area was calculated by the multipoint Brunauer-Emmett-Teller (BET) method. The total
pore volume was evaluated at a P/P0 close to 0.995. The Barrett-Joyner-Halenda (BJH)
method was used to calculate the pore volume and pore size distribution using the
desorption branch of the isotherm.
The heat of adsorption and desorption of CO2 were measured using Differential
Scanning Calorimetry (DSC). The instrument used was a Perkin Elmer DSC 7. The heat

132

of adsorption for the adsorption of CO2 was obtained at 85 °C under pure CO2. After the
adsorption was completed, heat of the desorption of the same sample was measured by
heating the sample from 0 °C to 140 °C under nitrogen.
4.4.4 Measurement of CO2 adsorption and desorption capacity
The CO2 adsorption and desorption measurements were performed in an all-glass,
grease free flow system. The adsorbent, typically 3 g, was packed in a U-shaped glass
tube (inner diameter of 8 mm) placed in a temperature controlled oil bath. Prior to
adsorption measurements, the adsorbent was heated to 85 ºC under vacuum at a pressure
of 65 mTorr for 3 h to remove adsorbed CO2 and water. The weight loss due to desorption
of water and CO2 was generally comprised between 2 and 10%. The weight of the sample
after treatment was used to calculate the CO2 adsorption capacities. For the adsorption
tests, 335 mL/min of air with a concentration of 400-420 ppm was used and obtained by
mixing a 1900 ppm CO2/air gas mixture with CO2 free air in the appropriate ratio with
mass flow controllers. The mixed gas was then passed over the adsorbent at various flow
rates and temperatures. The CO2 concentrations of the air before and after adsorption
were monitored by a Horiba VIA-510 CO2 analyzer equipped with an IR detector
specifically intended for CO2 measurements placed in-line with the adsorption setup. The
concentration range used was 0-2000 ppm CO2. Before each run, the analyzer was
calibrated with reference gases; CO2 1900 ppm and ultra-zero grade air for the zero. The
CO2 concentration was recorded as a function of time via LabView 8.6.
Desorption was performed using two different methods:
a) By applying vacuum (~65 mTorr) at 85 °C for 3 h

133

b) By heating the adsorbent containing U-tube (70-100 °C) and then passing air (335
mL/min) through it. In this case, the outlet gas was analyzed on a Horiba VIA-510 CO2
analyzer with a range of 0-20% CO2. The CO2 concentration was recorded as a function
of time via LabView 8.6. Immediately after opening of the air flow onto the saturated
adsorbent, the concentration in CO2 spiked to 2-10% CO2 and then slowly decreased until
reaching the inlet CO2 concentration (400-420 ppm).
TGA measurements of CO2 adsorption and desorption were carried out on a
Shimadzu TGA-50.

134

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Chapter 5. Criteria of selecting an application-specific supported amine adsorbent
based on CO2 capture conditions – a literature review
5.1 Introduction
What is the best CO2 adsorbent? – This is the question often asked, when people
want to select an adsorbent for their CO2 capture application. Unfortunately, this is not a
case of one size fitting all. From years of experience studying the silica supported
organoamine adsorbents, we found that selecting an adsorbent is all about trade-offs.
Although the development of new materials has driven the field to a higher level, and the
performance of solid adsorbents have improved in many aspects. Nevertheless, the trade-
offs exist – a gain in adsorption capacity can result in a decrease in stability, or faster
reaction kinetics may be associated with an increase in energy input during the
regeneration step. With respect to selecting the best, rather efficient adsorbent, there are
many aspects that need to be considered. Cost, including initial capital cost and future
operation cost, is the most important factor. There are a number of economic studies on
the process of scaling up carbon capture and sequestration (CCS) and direct air capture
(DAC)
1-3
, which is beyond the scope of this dissertation. As chemists, what we can
contribute to the scientific community is our understanding about selecting adsorbents
based on the specific application, especially based on the characteristics of the gas stream.
For each application, the CO2 concentration, adsorption (and desorption) temperature,
humidity level, and other involved gaseous components can differ, and selecting an
application-specific adsorbent could maximize efficiency and reduce overall cost.
As discussed in Chapter 1, there are three approaches in the proposed CCS: pre-
combustion capture, post-combustion capture, and oxyfuel combustion capture. Physical

139

conditions in the feed gas stream for pre-combustion capture is different from post-
combustion and oxyfuel combustion. In coal pre-combustion capture, syngas (contains
primarily H2 and CO) is converted by a water gas shift reactor to H2 and CO2. CO2 is
separated from the H2 before combustion. The shifted syngas contains about 50% H2, 40%
CO2, 2% CO, and the remaining 8% is sulfur oxides and other gases. The pressure of the
inlet gas stream is around 400 psi. The temperature of the gas is cooled to 40 °C for the
removal of acidic gases (CO2 and SOx). In post-combustion capture, CO2 content in the
gas stream is lower, which is in the range of 15-20% for coal-fired flue gas. The
adsorption pressure is close to 1 atm, and humidity is 5-7% by volume. Capturing CO2
directly from ambient air or from other dilute sources for various subsidies and incentives
add variety to this problem. Table 5-1 provides the gas stream conditions for some
representative applications.
Table 5-1 Gas stream conditions of representative applications
4

Application CO2
concentration
Temperature Relative
Humidity
Other components
Pre-combustion
capture
35.5% 40 °C 0.2% H2, CO, H2S, SOx
Post-combustion
capture
15-20% 50-75 °C 5-7% N2, O2
Oxyfuel capture >80% O2
Direct air
capture
400 ppm 25 °C 20-80% N2, O2
Gas sweetening
processing
40% 35-50 °C -- CH4, H2S
5

Cement
production sites
25% -- -- SOx, NOx
Contained
environment
1000-5000
ppm
25 °C -- N2, O2

Despite the complex situations involved in evaluating an application-specific
solid adsorbent, the generally acknowledged considerations are:

140

a. Adsorption capacity
b. Adsorption/desorption kinetics
c. Selectivity towards CO2
d. Stability
e. Regenerability
f. Tolerance for other gaseous components
In this Chapter, criteria are provided on selecting a solid supported organoamine
adsorbent based on the CO2 concentration, adsorption/desorption temperature, relative
humidity and other issues concerning the nature of mixed gaseous components. The list is
provided based on our experience and results reported in the literature. Recent advances
and progress in the field are also covered.
5.2 Selecting a solid support
Solid supports provide surface area and hollow volume for the loading of amines
and polyamines resulting in hybrid amine/support adsorbents. These supports generally
adsorb CO2 only by physiosorption. Physiosorption of CO2 on the supports is very
limited, especially compared to the chemisorption. Since the initial introduction of this
type of hybrid adsorbent with silica as a support, various materials have been proposed
and studied as supports for amines. Based on their chemical composition, the popular
supports are silica, alumina, carbon, and metal organic framework (MOF).
Mesoporous silica is a type of the most studied amine support. In early studies,
the expansion of pore diameter was found to enable more effective loading of amine and
provide wide diffusion channels, therefore increasing the CO2 adsorption capacity. In
recent years, studies on the effect of pore structure have been reported by many research

141

groups.
6-8
The advancements in controlled synthesis of mesoporous silica materials have
enabled the study of the effect of pore structure on the adsorption of CO2 in hybrid
adsorbents. In Chapter 3, we were able to synthesize a hierarchal of mesocellular silica
foam (MCF) with pore volume ranging from 0.98 to 4.17 cm
3
/g. Exhaustive studies on
this series of material showed that the adsorption of CO2 on supported PEI definitely
benefited from a larger pore volume. Thus, our understanding of the effect of
mesostructure of the silica supports on the CO2 capture characteristics has been
significantly boosted. Overall, higher surface area and large pore size (volume and
diameter) improve the amine loading and decrease the resistance in CO2 mass transfer,
and consequently enhance the adsorption capacity and kinetics.
Similar to silica, alumina is a naturally occurring metal oxide that has been
frequently employed in industrial catalysis and adsorption. Capturing CO2 using bare
porous alumina has been studied by a few groups.
9-12
Impregnating mesoporous alumina
with organoamine to increase adsorption efficiency is also being explored. Mesoporous
alumina has analogous pore morphology to mesoporous silica as they both have large
surface area, big pore volume and diameter that could efficiently support organoamines.
Plaza et al. studied a series of amine (DETA, DIPA, AMPD, TEA, PEHA and PEI)
modified alumina adsorbents and evaluated their performance by thermogravimetric
analysis.
9
The support they chose was a commercially available activated alumina,
containing mostly meso- and macropores and had a BET surface area of 271 m
2
/g.
Hybrid material achieved adsorption capacities as high as 8 w.t.% at 373 K. Chen et al.
prepared PEI functionalized mesoporous alumina (MA: surface area 812 m
2
/g and pore
volume 0.83 cm
3
/g). The PEI/MA hybrid material captured 120 mg CO2/g at 75 °C,

142

which was comparable with the amine functionalized silica adsorbents.
11
Further studies
by Chaikittisilp et al.
10
revealed that alumina is more resistant to structural deformation
under steam treatment compared to silica. Amine modified mesoporous alumina and
silica were treated under steam at 105 °C for 24 h. The structural information obtained by
nitrogen adsorption-desorption isotherms were compared before and after the treatment.
While both materials experienced some structural alteration under the investigated
conditions, alumina supported organoamine adsorbents were far more stable than their
silica counterparts.
Porous carbon, including activated carbon, synthesized mesoporous carbon, and
carbon nanotubes (CNTs), is another popular type of support material. Like silica and
alumina, adsorbents based on porous carbon can be prepared through both physical
impregnation and chemical grafting. For activated carbon, the diverse raw materials such
as coconut shell, pig bones, fire wood, and corncob, are used to obtain a great variety of
porous moieties. Their large surface area exceeding 500 m
2
/g provides great support for
organoamines. Due to the amorphous nature of the material, pore sizes vary from one to
the other, with some of the pores in the micropore regions. These are not very useful for
our applications. Synthesized mesoporous carbons generally have more defined structures.
For example, Wang et al. synthesized mesoporous carbon with a surface area of 999 m
2
/g
and total pore volume of 3.10 cm
3
/g.
13
After physically impregnated with branched PEI
(Mw 600) and a diffusion additive (Span 80), the adsorbents could efficiently adsorb CO2
from dilute sources as low as 5000 ppm and 400 ppm for 3.34 and 2.25 mmol/g,
respectively. Carbon nanotubes (CNTs) are also interesting support materials. Ye et al.
used commercially available CNTs and impregnated them with TEPA.
14
Despite the

143

relatively low surface area of only 87.03 m
2
/g and pore volume of 0.4887 mL/g, the
CNTs-TEPA reported an impressively high adsorption capacity of 2.97 mmol/g at 298 K
using a gas feed containing 2% CO2. Graphene and graphene oxide, the newly appeared
functional materials, have many traits of a good support such as large surface area, high
porosity, and interconnected pores. Liu et al. prepared a solid CO2 adsorbent through in
situ polymerization of aziridine on hydroxylated graphene (HG).
15
The HG-PEI exhibited
an adsorption capacity of 4.13 mmol/g at 25 °C from pure CO2, which is comparable to
the best performing ones based on silica support. Authors attributed the good
performance to the large accessible pores of the support and the uniform distribution of
the grafted amine. Another interesting observation from carbon based adsorbents,
especially CNTs and graphene, is the lower regeneration temperature. Liu et al. believed
it was due to the high thermal conductivity that enabled faster heat transfer in the material,
thus CO2 could be desorbed very rapidly.
15
Dillon et al. suggested that the hydrophobic
nature of the CNTs caused the polyamine to spread better on the surface and
consequently react fast in both CO2 adsorption and desorption stages.
16
Carbonaceous
supports offer unique properties in terms of reduced mass and volume, better thermal
conductivity, and chemical stability.
14
In addition, supports based on more than one
component sometimes give attractive functions, for example silica-alumina or silica-
carbon
17
are covered in a number of studies.
5.3 Selecting the amine based on CO2 concentration
5.3.1 CO2 capture from concentrated sources
As shown in Table 1, the CO2 concentrations in applications of interest range
from 0.04% to 50% and higher. So far, the majority of adsorbents developed have been

144

tested in relatively concentrated sources, ranging from typical flue gas concentration of
10-15% to pure CO2. Recent advances in the field are summarized in several reviews.
18-20

5.3.2 CO2 capture from near ambient air concentration
In 1999, Lackner et al. proposed the concept of direct air capture (DAC) to
manage the atmospheric CO2 level and eventually achieve the negative carbon emission.
Since then, there has been a growing interest in applying adsorbents to capture CO 2 from
the atmosphere. Compared to CO2 capture from concentrated sources, direct air capture is
indeed more challenging because the CO2 concentration in the air is only about 1/250 as
that in a typical flue gas containing 10% CO2. Thus, it requires the amine to effectively
bind with the scarce CO2 molecules in the gas stream. At the same time, diffusion
resistance in the porous channel should be minimal. Supported organoamine hybrid
adsorbents were found to be effective in capturing CO2 from very low concentrations. A
comprehensive review on direct air capture can be found in the literature.
21

With respect to selecting a hybrid adsorbent for capturing low concentration CO2,
the structure of the amine plays an important role. Many research groups have studied
this topic based on Class 2 adsorbents amine grafted on the support. Didas et al.
22
studied
the role of amine structure on the CO2 adsorption from ultradilute sources. In their
publication, they compared adsorbents containing only primary, secondary, or tertiary
amines and found that primary amine showed the strongest CO2 affinity. It agreed with
the fact that primary amines have the highest heat of CO2 adsorption. Thus, the Jones
group suggested that for the separation of CO2 concentration from ultradilute sources,
adsorbents rich in primary amines should be considered. As an endeavor, they prepared a
polymeric amine, called poly(allylamine) (PAA), containing only primary amino groups

145

on the carbon backbone.
23
PAA, as well as branched PEI and linear PEI, were loaded on
a mesoporous silica support. The adsorption capacities of the three adsorbents were
compared at 400 ppm and 10% CO2. As expected, at 400 ppm, PAA adsorbed more CO2
than linear PEI did. However, surprisingly, the PAA was not better than branched PEI in
any of the cases. The authors believed the orientation of the individual primary amino
groups in the polymers contributed to this result.
The materials for direct air capture are also interesting for applications in CO 2
separation from near ambient air concentrations in contained environments, such as in
airplanes and submarines. Studies have shown that within low-to-moderate CO2
concentrations, the increasing level has direct adverse effects on human decision-making
performance.
24
Human exposure to high concentrations of CO2 has adverse health effects
from loss of attention, increased heart rate, to serious coma and even death (Table 5-2). In
these cases, capturing CO2 at concentrations up to 2% is also considered near ambient
concentration capture, and the adsorbents should have similar traits to the ones used in
DAC.
Table 5-2 CO2 levels and the effects of exposure on humans
CO2 Levels Chart
CO2 Levels Exposure Effects
400 ppm Background (normal) outdoor air level.
400 to 1,000 ppm Typical level found in occupied spaces with good air exchange.
1,000 to 2,000 ppm Level associated with complaints of drowsiness and poor air.
2,000 to 5,000 ppm
Level associated with headaches, sleepiness, and stagnant, stale,
stuffy air. Poor concentration, loss of attention, increased heart rate
and slight nausea may also be present.
> 5,000 ppm
Exposure may lead to serious oxygen deprivation resulting in
permanent brain damage, coma and even death.

146

The development of adsorbents for direct air capture is not limited to the purpose
of reducing atmospheric CO2 concentrations. Materials of this category can find various
applications such as in reducing CO2 level in contained environment and in creating CO2
free air for iron-oxygen batteries and alkaline fuel cells. Both primary and secondary
amines are capable of capturing CO2 from ultradilute sources. Due to the higher heat of
CO2 adsorption of primary amines, theoretically, they should be better at adsorbing CO2
from ultradilute sources. This has been shown in the case of Class 2 adsorbents, where 3-
aminopropyl-trimethoxysilane (APS) has higher adsorption capacity than (N-
methylaminopropyl)-trimethoxysilane (MAPS) and (N,N-dimethyllaminopropyl)-
trimethoxysilane (DMAPS) functionalized silica. However, it has not yet been observed
in Class 1 supported polymeric amine systems, which primary amines have higher
reported adsorption capacity under similar conditions. Also known from the liquid amine
systems, primary amines bind more strongly with CO2, and is theoretically and
experimentally proven to be more efficient in scrubbing CO2 from ultradilute sources.
Therefore, it is necessary to explore more polymers rich in primary amines.
In addition, for applications in direct air capture, supported organoamine
exhibited the highest adsorption capacity in the temperature range of 25 to 35 °C. This
close to ambient optimal temperature adds another important characteristics in realizing
direct air capture.
5.4 Selecting the amine based on capture temperature
Most simple amines have boiling points in the range of 100-200 °C. Studies have
shown that exposing amines to elevated temperatures might result in their leaching.
Therefore, the operating temperature in both adsorption and desorption stages of this type

147

of the material is recommended to be lower than 100 °C. As is well acknowledged, the
adsorption reaction between CO2 and amine is exothermic, and the desorption
(regeneration) reaction is endothermic. Ideally, the adsorption at lower temperatures and
desorption at higher temperatures are thermodynamically favored. However, conflicts lie
at both stages and will be explained in the following sections.
5.4.1 Adsorption Stage: thermodynamics vs diffusion control
Thermodynamically, the adsorption of CO2 is favored by lower reaction
temperature. However, in hybrid adsorbents, the adsorption is also largely controlled by
CO2 diffusion, and kinetically, higher temperature allows CO2 to flow to the reaction
sites faster. This conflict is unfortunately a reality in most of the silica supported
organoamine materials. The inversely related properties result in a so-called optimal
adsorption temperature in this type of the material. The effect of temperature on the
adsorption capacity over a certain period of time, also known as working capacity, has
been studied by many groups, and is always measured for almost all the newly developed
amine-silica based adsorbents. When amine-silica based adsorbents are subjected to
different adsorption temperatures, the optimal temperature for each adsorbent is
represented by the maximum adsorption capacity. It is noteworthy that the optimal
temperature for the adsorbent is also CO2 concentration-dependent.
If the temperature of the gas stream is in the ambient region, for applications like
CO2 capture from ambient air, or in the gas sweetening, there exists a larger barrier for
CO2 mass transfer in the hybrid adsorbents. In order to have the highest possible
adsorption capacity, the amine chosen need to be able to spread on the surface of the

148

pores. Also as discussed vida supra, it is helpful to have supports with large porosity and
interconnected channels.
5.4.2 Desorption Stage: energy vs kinetics
In the amine-silica solid adsorbents, temperature or pressure swing are used
during the regeneration step. In the temperature swing, adsorbents are exposed to higher
temperature in the regeneration step. Information on desorption efficiency, temperature
requirement and kinetics are less adequate than it is for adsorption. Nevertheless,
desorption is a crucial factor in calculating the overall energy penalty for the CO 2 capture
technology. Lower regeneration temperature directly translates into energy savings. Thus,
desorption performance, including desorption rate at various temperatures should be
carefully examined with all promising adsorbents.
Supported amine adsorbent reacts with CO2 reversibly. For the same material,
CO2 desorption is faster and more complete at higher temperature, but it is not
economically favorable. Many groups are interested in the role of the amine structure on
the interaction with CO2. A series of Class 2 adsorbents were prepared for improving the
fundamental understanding of this topic, and frequently used aminosilanes are listed in
Figure 1-8. Initially intended to find the better performing amine for CO2 capture for
ultradilute sources, Didas et al. studied the role of the amine structure.
22
They found the
CO2 affinity among the primary, secondary and tertiary amines are in the order of 1°> 2°>
3°. On the other hand, the easiness for desorption order reverses. In the past, chemists
also realized that the use of adsorbents containing mostly secondary amine groups lead to
decreased regeneration temperatures.
25-27
Ko et al. demonstrated that the tertiary amine
(DMAPS) desorbed four times faster than its primary amine counterpart (APS).
28
Our

149

group conducted studies on the desorption step for both branched and linear PEI
supported on fumed silica (Class 1 adsorbents). The adsorbent based on linear PEI
containing only secondary amines, desorbed at lower temperatures and faster rates than
its branched counterparts. A detailed explanation is provided in Chapter 2.
Also illustrated in Chapter 2, aqueous tertiary amine systems have the lowest heat
of CO2 absorption, and in theory adsorbents composed predominately tertiary amines
should have the fastest desorption rate and the least energy requirement, which should be
very interesting to compare with its primary and secondary counterparts. However, the
adsorption on tertiary amines is usually slow and if only tertiary amines are present,
water is needed for the adsorption.
In Chapter 4, the effect of branched PEI molecular weights on the desorption
kinetics were also examined. Unexpectedly, PEI with the largest molecular weight of
25000 g/mol desorbed the fastest in the short cycles experiments, followed by PEI1800
and PEI800.
29
The trend however did not apply to the linear PEI adsorbents.
30

5.4.3 The problem with amine leaching and thermal degradation
Another aspect to consider in selecting an amine is to limit its leaching. The
escape of amine molecules can be largely avoided by using lower adsorption/desorption
temperatures and amines with low vapor pressure.
Earlier, as commonly used in liquid amine systems, MEA and DEA were
suggested to be deposited on various supports. However, due to their low molecular
weight and relatively high volatility, the solid adsorbents based on these amines had a
tendency to leach the amine out, progressively reducing the adsorbents’ capacity.
31-32

Another example is the low molecular weight oligomers of ethylenimine such as TEPA

150

and PEHA. They were frequently studied in the amine-silica adsorbents. Although, in an
individual adsorption-desorption cycle, TEPA and PEHA tended to adsorb more CO2
than their higher molecular weights counterparts, they experienced a significant loss of
activity over numerous cycles.
In an application that requires high adsorption or desorption temperatures, it is
important to select a polymeric amine with sufficiently low vapor pressure. In this
context, due to the similar composition in the amines and polymeric amines, the ones
with higher molecular weights, in general, would have higher boiling points and lower
vapor pressures. Except for controlling the operation temperatures, it is also important to
select an amine with sufficiently low vapor pressure, preferably polyamines with higher
molecular weights. To avoid contamination of the gas stream and loss of activity over
numerous adsorption/desorption cycles.
Therefore, the solid supported organoamine adsorbents are recommended to be
used below 130 °C. Above 130 °C, the amine reacts with CO2 to form urea compounds
and the adsorbent cannot be easily regenerated.
33
Although high temperatures are good
for achieving fast adsorption/desorption kinetics, they should be avoided as much as
possible for many reasons, such as 1. reducing amine leaching; 2. inhibiting irreversible
urea formation (see section 5.6.2); 3. preventing other possible degradation pathways
involved with high temperatures, such as oxidative degradation (see 5.6.1); 4. reducing
energy needs. In the cases where high operating temperatures have to be used, the
presence of moisture is reported to reduce the urea formation, and even reverse it.
5.5 Selecting the amine based on humidity level

151

In many applications, moisture is a ubiquitous constituent in the gas stream.
Adsorbents with good stability in the presence of moisture are of great interest.
Fortunately, so far, studies have shown that supported organoamine adsorbents
not only handles the moisture well, but also could benefit from a certain level of humidity
in the gas stream. Due to the reaction mechanism, under dry conditions, primary and
secondary amines react with CO2 in 2 to 1 ratio, whereas tertiary amine does not react
with CO2. As water can act as a free base; in the presence of moisture, primary,
secondary and tertiary amines can react with CO2 in 1 to 1 ratio. Moisture activates the
functionality of tertiary amine and increases the overall amine efficiency (Figure 5-1).
Although in reality, due to the inaccessibility of some amino groups, the amine efficiency
is always lower than theoretical values, there are quite a few studies supporting the fact
that moisture increases the adsorption capacity of the supported organoamine adsorbents.
Other studies also suggest that moisture improves the stability of the adsorbents by
preventing urea formation and even regenerating the degraded amines.
34

Figure 5-1 Reaction of grafted DMAPS with CO2 in presence of water
It is worth mentioning that steam is also known to degrade the porous support.
35

Drage et al.
27
have pointed out that many laboratory studies focus on regenerating the
adsorbents using nitrogen as a sweep gas at elevated temperatures, but this does not
mimic the practical methods in large scale applications where steam or CO2 are more

152

likely to be used. Thus, the effect of moisture (steam) on the stability of the adsorbents,
including both supports and amines, should be examined in detail.
5.6 Selecting the amine based the type of gas components
Depending on the type of feed gas, the gas components of interest are the ones
that might affect adsorbent performance. Nitrogen, although being ubiquitous in all kinds
of feed gases, usually does not affect adsorption capacities, kinetics and stability for
amine functionalized solid adsorbents, and therefore is not of much concern. Oxygen,
moisture and CO2 itself are major components in feed gases. Studies have shown that
they can have impacts on the adsorbents, and will be discussed in detail here. Other
contaminants in flue gas such as SO2, NOx have also known influences, but have not been
extensively studied, and will only be briefly discussed.
5.6.1 Oxidative degradation
To study the amine stability in oxygen-rich environment, Class 2 adsorbents with
amines with only primary, secondary or tertiary amine are grafted on silica support. A
few groups have been involved in identifying the oxidatively degraded species of
supported amines.
36-38
It is becoming apparent that the degradation happens at the
methylene carbon next to the nitrogen atom. Due to irreversible oxidation, the amine is
no longer capable of binding CO2, decreasing its adsorption capacity (Figure 5-2 and
Scheme 2-2).

153

Figure 5-2 Degradation species for branched and linear PEIs

We have also studied the oxidative degradation, but using a Class 1 adsorbent.
Linear polyethylenimine (LPEI), a polyamine containing only secondary amino groups,
was loaded on fumed silica (FS).
30
The adsorbent was subjected to air at 70 and 100 °C.
LPEI-FS retained most of the adsorption capacity (93.6%) after being treated at 70 °C for
20 h. However, when temperature increased to 100 °C, the capture capacity was reduced
drastically to only 7.2% of the original value. This observation agrees with the previous
general findings that oxidative degradation of amines is less pronounced at low
temperatures.
According to the mechanisms described above for the oxidation of amines, the
weight of the organoamine should have increased. It is in contradiction with our
experimental results where TGA recorded almost 4% weight loss in the oxidative

154

environment at 100 °C.
30
And this is not due to of leaching of the PEI, because at the
same temperature, there was only about 1% weight loss under nitrogen. We, therefore,
suspect that there might be some polymer chain cleavage involved with the oxidative
degradation as well.
To recap this subsection, previous studies showed that secondary amines are more
susceptible than primary amines to degradation in oxygen containing atmosphere,
especially at high oxygen concentration and elevated temperatures. If the adsorbent is
applied to oxygen rich atmosphere and at the same time requires high adsorption or
desorption temperatures, secondary amines should be avoided.
5.6.2 CO2-induced degradation
Drage et al. have pointed out that under high regeneration temperature (above
135 °C), CO2 could react with amines to form urea linkage.
33
Sayari et al. have
systematically studied the adsorbents deactivation resulting from CO2-induced
degradation.
39-41
Class 2 adsorbents were examined with degradation intermediates and
pathways suggested for primary and secondary amines (Figure 5-3).

155

Figure 5-3 Sayari et al. proposed CO2-induced degradation mechanisms
34
To summarize, the primary amine is more sensitive to CO2 induced degradation.
Secondary amines experience strong steric hindrance in forming the isocyante
degradation intermediate, and therefore do not readily degrade in CO2atmosphere.
Results are consistent with our studies on FS-LPEI adsorbents.
5.6.3 Other gases of concern (H2S, SOx, and NOx)
The National Energy Technology Laboratory (NETL) suggests a list of properties
that post-combustion CO2 adsorbent should have. Among them, material durability in the
presence of acidic flue gases is an important one.
27,42
For instance, hydrogen sulfide
(H2S), sulfur dioxide (SO2) and nitrogen oxides (NOx) have raised concerns. The effect of
their presence and co-adsorption with CO2 on the supported organoamine adsorbents was
tested by some research groups.

156

First, as shown in Table 5-1, H2S is present in significant portions in natural gas
and pre-combustion stream. To meet the requirement for pipeline transportation, both
CO2 and H2S need to be removed to acceptable levels.
43
Several publications have
reported using silica supported organoamines for simultaneous removal of CO2 and H2S.
Inspired by our previous work on fumed silica support organoamines, Yoosuk et al.
found that this material could be used as an inexpensive adsorbent for both CO2 and
H2S.
44
A co-adsorption experiment with simulated 20% CO2 and 0.36% H2S was used as
the feed gas. CO2 was observed to be adsorbed first and hindered the H2S adsorption.
CO2 adsorption was also favored at higher temperature (80 °C). Concerning the stability,
the adsorption capacities for both gases decreased over five successive adsorption-
desorption cycles. Degradation might have occurred, but authors did not investigate
further. The effect of H2S on the stability of the material was observed by a few research
groups. In these cases, H2S was either used as the feed gas alone or together with CO2. It
is not until recently Miller et al. studied the adsorption of H2S on TEPA and found that
H2S could poison the amino sites through the formation of a strongly adsorbed species of
(HS
-
) NH
3+
-TEPA. Upon exposing to oxygen, the active -NH2 was irreversibly oxidized
to -NO2 species. To mitigate the effect, they suggested adding PEG to reduce the binding
energy of the HS
-,
and further improve the regenerability of the material.
45
To tackle the
problem, a two-stage process was proposed for the removal of CO2 and H2S.
46
Because
CO2 adsorption was favored at higher temperature and H2S was favored at lower
temperature, two adsorbent beds (supported amine) operated at different temperatures can
be operated next to each other. In addition, the one intended for H2S capture could also
incorporate PEG to mitigate the degradation over time.

157

Sulfur dioxide (SO2) is a commonly occurring trace component in flue gases. SO2
is of particular concern for post-combustion capture from coal flue gases and has been
described as decreasing the performance of a range of immobilized amine. Again with
Miller et al., SO2 was captured on the silica supported 1,3-phenylenediamine adsorbent.
Although the material was able to capture 2.8 mmol SO2/mol, they identified strongly
adsorbed SO3
2-
(sulfite) and SO4
2-
(sulfate) species at the amine sites.
47
Further heating
resulted in the irreversible oxidation of amine to -NO2, which led to the rapid decrease of
the adsorbent’s capacity in adsorption/desorption cycles. To lessen the effect caused by
SO2, flue gas desulphurization (FGD) could substantially decrease the concentration in
the flue gas from 1800 ppm to about 50 ppm.
27

To sum up, H2S, SO2 and NOx are gases of concern for co-adsorption of CO2 on
supported organoamines. Studies have confirmed the formation of strongly adsorbed
species, such as -HS, SO3
2-
and SO4
2-
. The subsequent heating often leads to the
irreversible oxidation of amines to -NO2. With regard to H2S, it has shown that adding
PEG could largely reduce the binding energy and help the release of H 2S in the
desorption step. It also minimizes the oxidation of amines. In addition, lowering
operating temperature also alleviate the effect from the presence of these gases and
improve the durability of the amine.
48
In conclusion, in order to increase the practicality
of supported organoamine adsorbents, especially in the CCS applications, the problems
of CO2 associated with the acidic gases need to be fully explored. Studies in the area are
still very limited and should be addressed in future studies.

158

5.7 Conclusion
Currently, the fast developing field for solid CO2 adsorbents has not reached the
stage of being application-specific. Although theoretically, any silica supported
organoamine adsorbent should be able to be applied to many types of gas streams, the
efficiency of the adsorbent varies from one application to the other. As the understanding
of solid CO2 adsorbents continues to grow, scientists and engineers will have better
grasps on what kind of adsorbents are suited best for which specific application. In this
Chapter, previous efforts from our group and other research teams were summarized to
give some criteria on selecting suitable supports or organoamine structures for specific
application conditions.
Silica, alumina and carbon are the three most popular amine supports studied in
the literature. All of them are widely available and have been used in the industry for
many years. They share some similar traits making them suitable for loading amines and
polyamines - to list a few: they all have large surface area, high porosity, and
interconnected network. Meanwhile, they also have unique and attractive properties of
their own. Silica is covered by silanol groups on the surface, which is helpful in
stabilizing the amine through hydrogen bonding. Alumina is steam stable, which can be
useful in the steam stripping regeneration process. Carbonaceous supports offer
advantageous in terms of reduced mass and volume and better thermal conductivity.
For capturing CO2 from dilute sources, including the air, amine and polyamine
containing a higher percentage of primary amines are better suited. Primary amines bind
CO2 stronger than secondary and tertiary amines do, and are therefore the most capable
of grabbing the scarce CO2 molecules resulting from low CO2 concentrations. Although

159

secondary amino groups are also capable of adsorbing CO2 from dilute sources, pros and
cons of using secondary amino groups should be properly investigated in individual cases.
CO2 capture from more concentrated sources is less problematic in this aspect, and
therefore, primary, secondary and tertiary amino groups can all be used.
Supported organoamine adsorbents are suitable of adsorbing CO2 from ambient
temperature up to about 100 °C. However, CO2 concentration-dependent optimal
temperature exists for each adsorbent, where the highest working CO2 adsorption
capacity is observed. In general, the higher the desorption temperature, the faster and
more complete the regeneration step. However, due to the endothermic nature of the CO2
desorption reaction, the cost of energy needs to be considered, and thus, the lowest
possible regeneration temperature with acceptable desorption rate should be selected.
Adsorbents rich in secondary and tertiary amines are capable of desorbing at lower
temperatures than primary amines. Also at higher operating temperatures, polyamines
with low vapor pressure should be considered first to avoid possible amine leaching
problems.
So far, supported organoamine adsorbents are known not to be harmed by the
presence of moisture in the gas stream. In several studies, moisture was actually found to
improve the adsorption capacity of the hybrid adsorbents. This observation is consistent
with the theoretical stoichiometric relationship that amine efficiency doubles with water
being a free base.
Lastly, other gaseous components, which might only represent minor
concentrations, can have major effect on the performance of adsorbents, mainly in terms
of sorbents stability and regenerability. The degradation mechanism and species from

160

oxygen and CO2 are identified. Primary amines are more resistant to oxidative
degradation, but susceptible to CO2-induced degradation compared to secondary and
tertiary amines. Secondary amines are more prone to oxidative degradation but much less
susceptible to CO2 degradation. Studies on other gases such as H2S, SOx, and NOx are
still limited, but should be explored further if the adsorbents have to be applied in any
CCS approaches.

161

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Abstract (if available)

Abstract The rapid increase in the atmospheric carbon dioxide (CO₂) concentration is broadly acknowledged as the main cause for the currently observed global warming and climate change. Concepts such as carbon capture and sequestration (CCS) and direct air capture (DAC) are proposed to mitigate the problems associated with excessive anthropogenic carbon emissions. Interest in capturing CO₂ from large point sources as well as from the air is rising in scientific and engineering communities. Among many options studied, silica supported organoamine materials have emerged as a promising type of CO₂ adsorbent. Due to the complex nature of the reaction between CO₂ and the silica supported organoamine, as it often involves several elementary steps, more fundamental research is needed to understand the reaction process and optimize the adsorbents' structures for capturing CO₂ from various point sources and for diverse purposes. ❧ This dissertation describes the development of silica supported organoamine materials and their practical use as carbon dioxide (CO₂) adsorbents. A particular focus of the dissertation is to understand the structural influence of the organoamine and the silica support on the CO₂ capture characteristics, including CO₂ uptake, adsorption and desorption kinetics, material stability, and its response towards various CO₂ concentration, temperature and relative humidity.

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Asset Metadata

Creator Zhang, Hang (author)

Core Title Carbon dioxide capture using silica supported organoamine adsorbents

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 04/26/2016

Defense Date 03/22/2016

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

Tag amine-functionalized,CCS,CO₂ capture,OAI-PMH Harvest,organoamine,silica,solid CO₂ adsorbent

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Prakash, G. K. Surya (committee chair), Narayan, Sri R. (committee member), Shing, Katherine (committee member)

Creator Email hangz@usc.edu,hzhang626@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-244251

Unique identifier UC11278907

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Legacy Identifier etd-ZhangHang-4380.pdf

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Document Type Dissertation

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Rights Zhang, Hang

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Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

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