The Pennsylvania State UniversityThe Graduate School
John and Willie Leone Family Department of Energy and Mineral Engineering
UTILIZING IMMOBILIZED BIOCATALYSTS ON ACTIVATED CARBON BLACK
ADSORBENTS FOR CARBON DIOXIDE CAPTURE FROM SIMULATED AMBIENT
AIR UNDER A pH SWING ADSORPTION PROCESS
A Thesis in
for the Degree of
ii
The thesis of Antonio Rafael Cuesta was reviewed and approved* by the following:
Chunshan Song Distinguished Professor of Fuel Science and Professor of Chemical Engineering; Director of EMS Energy Institute Thesis Advisor
Randy L. Vander Wal Professor of Energy and Mineral Engineering and Materials Science and Engineering
Jonathan P. Mathews Associate Professor of Energy and Mineral Engineering
Sanjay Srinivasan Professor of Petroleum and Natural Gas Engineering; John and Willie Leone Family Chair in Energy and Mineral Engineering Interim Associate Department Head for Graduate Education of Energy and Mineral Engineering
*Signatures are on file in the Graduate School
iii
ABSTRACT
are both fully regenerating and economical. A potentially energy-saving and
regenerating pH swing system is currently utilized by biocatalysts. The main objective is
to develop, test and analyze a synthetic pH Swing Adsorption (pHSA) system as well as
a pHSA compatible solid adsorbent to capture CO2 from a simulated ambient air gas
stream. Furthermore, comparing its performance to other CO2 sorbent systems is
necessary to determine the feasibility of pHSA implementation. The lead developed
adsorbent is a carbon black co-activated with potassium carbonate and nitrogenous
copolymer that is impregnated with immobilized bovine carbonic anhydrase and thereby
deemed “BCA/KN-CB”. BCA/KN-CB has preliminarily demonstrated both a competitive
CO2 adsorption capacity and a limited regenerative ability under experimental pHSA
conditions. In addition, BCA-based adsorbents achieved higher adsorption capacities
than non-BCA adsorbent counterparts. BCA-based adsorbents displayed better
regenerative stability when the adsorbent was chemically activated with K2CO3. The
point of zero charge of adsorbents BCA/KN-CB and KN-CB showed change between
pHSA steps and came close to returning to the initial point of zero charge after one
pHSA cycle. While the cost of the biocatalyst easily makes its use for CO2 capture
impractical, the operational and remaining material costs are competitive to MSA and
TSA solid adsorbent systems. The scientific contribution of the thesis is the concept of a
pH swing adsorption/biocatalytic adsorbent system that can effectively operate under
ambient conditions and has competitive CO2 adsorption capacities compared against
other swing adsorption CO2 capture systems.
iv
1.2 CARBON CAPTURE ABSORBENTS ................................................................. 3
1.3 RECENT SWING ADSORPTION PROCESSES FOR CARBON CAPTURE ................ 4
1.4 SURFACE MODIFIED CARBON ADSORBENTS .................................................10
1.5 CARBONIC ANHYDRASE FOR CARBON CAPTURE ...........................................12
CHAPTER 2 THEORY .......................................................................................15
CHAPTER 3 OBJECTIVES ...............................................................................26
3.1 PROBLEM STATEMENT ................................................................................26
3.2 MAIN OBJECTIVES ......................................................................................27
3.3 RESESARCH GOALS ...................................................................................27
CHAPTER 4 METHODOLOGY .........................................................................28
CHAPTER 5 RESULTS AND DISCUSSION .....................................................46
5.1 MATERIAL SYNTHESIS DISCUSSION .............................................................46
5.2 EXPERIMENTAL SETUP DISCUSSION ............................................................47
5.4 PHSA SORBENT PERFORMANCE .................................................................51
5.5 PHSA ADSORBENT PORE PROPERTIES........................................................60
CHAPTER 6 FUTURE WORK ...........................................................................71
6.1 PHSA SORBENT DESIGN ............................................................................71
6.2 PHSA PROCESS DESIGN ............................................................................71
CHAPTER 7 CONCLUSION ..............................................................................75
vi
Figure 1-2. Mechanisms of CO2 capture into MEA solution2……….………………………4
Figure 1-3. Mechanism for the reaction of CO2 with (a) primary, (b) secondary and (c)
tertiary amines3……………………………...…………………………………………..6
Figure 1-4. CO2 capture mechanism for MSA sorbents4…………..…………….………….8
Figure 1-5. Heat treating of biomass to produce ordered biochar material5……..…..….10
Figure 1-6. SEM images of the activated carbon precursor (A), then nitrogenized (B),
then carbonized (C), and under TEM (D)6………………………..…………………12
Figure 1-7. Carbonic anhydrase suggested biological mechanism7……….….…………13
Figure 2-1. The theoretical sorption binding energy approaching the free energy of
mixing with increasing pressure8…..…………………………………………………16
Figure 2-2. Free energy of reaction diagram56..........................……………………….….17
Figure 2-3. Effect of oxidation time on pore volume distribution. Additionally, effect of
pore classification presence based volume distribution on N2 and CO2 adsorption
isotherms9..............................................................................................……………….…19
Figure 4-2. The pHSA system lab scale prototype…………………………………………33
Figure 4-3. The comprehensive pHSA cycle illustration.................................................36
Figure 4-4. Proposed surface configurations through pHSA (Phase 1): Initial step........37
Figure 4-5. Proposed surface configurations through pHSA (Phase 1): Capture step…38
Figure 4-6. Proposed surface configurations through pHSA (Phase 2): Regeneration
step…………………………………………………….……………………………..…40
vii
Figure 4-7. Proposed surface configurations through pHSA (Phase 3): Initial step of the
following cycle…………………………………………………………………………………..41
Figure 5-1. CO2 gas blank curves for the adsorption experiment (~90s data
frequency)………………………………………………….………………………………....…52
Figure 5-2. Experimental CO2 adsorption curves (5x, red) compared to the gas blank
(~90s data frequency) curves (blue)…………………….…………………..………….……53
Figure 5-3. CO2 gas blank curves for the adsorption experiment (~63s data
frequency)……………………………………………………………….………………………53
Figure 5-4. Experimental CO2 adsorption curves (5x, red) compared to the gas blank
(~90s data frequency) curves (blue)…………………….…………………..………….……53
Figure 5-5. N2 isotherm plot of pHSA adsorbents; Po ~ 1 atm……………….……..……61
Figure A-1. Atmospheric CO2 concentration at Mauna Loa, HI. Courtesy of NOAA…...92
Figure A-2. Ice core data showing over 400 thousands years of relative temperature to
present climate and its correlation to CO2/CH4 atmospheric concentrations.
Courtesy of Marian Koshland Science Museum……………………………………93
Figure A-3. KN-CB PZC surface charge density vs. pH plot: Initial step………..……….93
Figure A-4. KN-CB PZC surface charge density vs. pH plot: Capture step……..………94
Figure A-5. KN-CB PZC surface charge density vs. pH plot: Regeneration step……...94
Figure A-6. KN-CB PZC surface charge density vs. pH plot: Initial* step……………….95
Figure A-7. KN-CB PZC surface charge density vs. pH plot: Initial step………………...95
Figure A-8. KN-CB PZC surface charge density vs. pH plot: Capture step……………..96
Figure A-9. KN-CB PZC surface charge density vs. pH plot: Regeneration step………96
viii
Figure A-10. KN-CB PZC surface charge density vs. pH plot: Initial* step……………...97
Figure A-12. A front-facing view of the pHSA system lab scale prototype with the
furnace closed……………………………………………………………………………99
Figure A-13. Inficon 3000 Micro-GC (portable series) used for gas analysis...............100
Figure A-14. The GC inlet flow controller and gas filter system. The inlet filter removes
moisture from the gas stream and reduces oxygen in the inlet gas stream……………101
Figure A-15. Multi-Flow Controllers and Master Display for the three gases outlined in
Figure 4-1......................................................................................................................102
Figure A-16. A display of all the adsorbents used for benchmark TSA and pHSA
adsorbents from left to right: N-CB, KN-CB, KN-CB (not activated), BCA/N-CB, BCA/CB,
BCA/KN-CB (Batch 2), BCA/KN-CB (Batch 1), PEI-30-FS, PEI-30-C4.……………......103
ix
Table 5-2. pHSA: BCA/KN-CB performance based on regeneration pH………….……..55
Table 5-3. pHSA: biocatalyst effect on CO2 adsorption and regeneration…………........57
Table 5-4. pHSA: adsorbent modification effect on CO2 adsorption and regeneration…58
Table 5-5. pHSA adsorbent pore properties from N2 adsorption tests performed by a
Micromeritics Tristar II 3020 surface area and porosity analyzer…………………………60
Table 5-6. Adsorbents’ point of zero charges through one pHSA cycle…….……………62
Table 5-7. Integral analysis results comparing operations of swing adsorption
systems………………………………………………………………………………………….69
Table A-11. Cost model for CO2 capture plants using MEA scrubbing, in 2000 and
201210………...………………………………………………………………………….………98
CAH – Carbonic Anhydrase
MSA – Moisture Swing Adsorption
specific activity ≥3,500 W-A units/mg protein, lyophilized powder, Item Number
C2624 from Sigma Aldrich
SBA-15 – Amine-functionalized mesoporous silica-based molecular sieve
MBS – Molecular basket sorbent, a term coined by the Song group
CB – Carbon black from Cabot Corporation, model PBX-51.
xi
ACKNOWLEDGEMENTS
I thank Dr. Chunshan Song for teaching me the importance of fundamental
understanding in the work at hand. His guidance for learning as a part of this work is
much appreciated. Thank you, Drs. Xiaoxing Wang, Dongxiang Wang, and Xiao Jiang,
for having provided me with relatable wisdom that has helped me face intimidating goals
in academia. I would like to acknowledge the entire Clean Fuels and Catalysis Program
under direction of Dr. Chunshan Song for providing a space where I felt comfortable to
follow my passions and dreams. In addition, I would like to thank my committee
members for being teachers through the course of my graduate studies, as they have
influenced me in the classroom and on the job and not just assisting me with my thesis.
A special thank you to my family, which has and will always believe in me and my
pursuit for making a difference in the world using the gifts I am meant to share. To my
wife, thank you, I love you, and I hope this exploration of the new and unchartered
inspires you to push yourself likewise in your own career pursuits.
1
1.1 Carbon Dioxide and Climate Change
Since the start of civilization, human activities have contributed to disrupting the
natural negative trend of atmospheric concentration of CO2. The concentration began to
stabilize until the Industrial Revolution, when mass consumption of fossil fuels and other
carbon-based fuels skyrocketed ever since. Because CO2 and other byproducts of fossil
fuel consumption are greenhouse gases, they hinder the loss of heat on Earth into space
and change regional climates. Climate change can lead to hardships in the biosphere
that would threaten the current way of life. Greenhouse gas emission control over recent
decades has successfully prevented certain anthropogenic byproducts from
accumulating in the atmosphere and contributing to the warming trend. Due to the lack
of regulation and sheer volume of emission, carbon dioxide is the most prevalent threat
of the greenhouse gases. Currently, the global yearly per capita rate of anthropogenic
CO2 is approximately five tonnes per person11. The concentration of CO2 in the
atmosphere has broken through 400 ppm (see Figure A-1) in a consistently increasing
trend for over fifty years. However, this milestone is largely unprecedented, as this
concentration has not happened on Earth for hundreds of thousands of years (see
Figure A-2). Figure 1-1 illustrates the untapped potential in ambient carbon capture:
2
Figure 1-1. Comprehensive carbon cycle1
The large reservoir of CO2 in the atmosphere and ocean surface is at least two
orders of magnitude1 more than the total amount of released anthropogenic CO2. The
flux of carbon released into the atmosphere by fossil fuel consumption is less than a
twentieth1 of the total carbon flux globally. Whether naturally occurring or anthropogenic
in origin, the atmospheric and oceanic carbon dioxide could be captured to prevent
climate change. The required effectiveness of a large scale ambient capture operation
would then be much lower than flue gas or similar capture schemes in order to
significantly reduce the atmospheric CO2 concentration. This is a needed advantage,
since in the pursuit to return CO2 concentrations to pre-21st century levels, a total stop to
fossil fuel consumption would be insufficient12. Therefore, future research and
investment foci should center on a comprehensive solution involving ambient air capture.
CO2 concentrations are 250 times more dilute in air than in combustion gas streams,
which directly affect reaction kinetics for the worse. In terms of thermodynamics, the
3
required sorbent strength is lower for higher concentration streams8, however, the
ambient condition reactions are still favorable and shown to occur over different CO2
capture techniques4,13.
1.2 Carbon Capture Absorbents
The main application of early carbon capture sorbents were life support systems
for small-environments such as submarines and spacecraft14,15. The main motivations of
this research in the 1980’s were to find sorbents that can be regenerative while still cost
and space efficient. An ion-exchange solid amine system14 was developed, and
demonstrates how early solid amines have been of interest. The field’s abilities peaked
with developments, enabling spacecraft with carbon capture systems for life support to
replace LiOH systems, including a liquid amine/polymer membrane system (HSC+)15
with an CO2 absorption capacity of 8 wt.%15 using a pressure swing absorption (PSA)
process. Under PSA, absorption or adsorption occurs at high pressures and desorption
occurs when the saturated sorbent is exposed to a low-pressure or vacuum system to
shift the equilibrium between adsorbed and desorbed gas.
Current industrial carbon capture revolves around CO2 from flue gas streams in
coal or natural gas powered power plants. A common industrial liquid amine,
monoethanolamine (MEA), is a primary amine-containing alcohol that uses a gas
absorption process via acid-base reactions. The amine scrubber can be regenerated
with heating the saturated liquid, where the captured CO2 forms carbamate and the
carbamate degrades back to carbonate species and CO2. The mechanisms of this
approach have recently been understood as complex12, with many intermediate
equilibrium reactions happening simultaneously. Carbon dioxide concentrations in the
amine solutions < 33 mol% form carbamate from CO2, while at higher concentrations
4
carbonate species in solution are the main reactant to form carbamate2. Figure 1-
2illustrates the mechanisms of the chemical absorption process.
Figure 1-2. Mechanisms of CO2 capture into MEA solution2
The temperature swing process used by amine scrubbing starts CO2 absorption
by cooling flue gas from more than 100oC to about 50-55oC. Once loaded with CO2, the
amine solution is heated to about 120oC to trigger the CO2 desorption mechanisms in
Figure 1-2. The theoretical 1:1 molar ratio between amine and CO2 is a very attractive
property of amine scrubbing that has drawn decades of engineering and upscaling.
1.3 Recent Swing Adsorption Processes for Carbon Capture
At the turn of the millennium, environmentalism became a bigger part of scientific
research and the carbon capture field1,16–20. The connection between emission control of
power plants and polymeric amines began as an attempt to solve the problems of amine
scrubbing. The development took the form of poly(ethylenimine), or PEI, impregnated on
a mesoporous silica material, named PEI-MCM-4121. The base precursor to the material
5
polymeric amine has initially been proposed as a single-step mechanism22,23:
a) R2NH···B + CO2 (g) R2NCO2 —:BH+
With B as another base amine or water molecule, the proton shuffle moves to the
base. Further research has proposed transition states depending on the presence of
water3,23:
b) R2NH+CO2 - -R2NH R2NCO2
- + R2NH2 +
b) R2NH+CO2 - - H2O R2NH+CO2
- -H2O
The polymeric amine contains primary, secondary and tertiary amine functional
groups that react with CO2 through different mechanisms. While primary and secondary
amines favorably react with CO2, H2O presence can activate tertiary amine mechanisms.
6
Figure 1-3. Mechanism for the reaction of CO2 with (a) primary, (b) secondary and (c)
tertiary amines3
The sorbent utilized temperature swing adsorption (TSA) as the regeneration
driver. Specifically for PEI adsorbents under TSA21,24, CO2 adsorbs onto the amine sites
at an optimal temperature of 75oC and desorbs when the sorbent is heated to 120oC.
Using a high surface area material, PEI impregnation successfully increased adsorption
7
capacity twice above pure PEI, to 215 mg CO2/g PEI21. Activated carbon impregnated
with PEI is shown to rival the adsorption capacity of PEI impregnated into MCM-41 and
other ordered sorbents and be hundreds of times less expensive25.
In carbon dioxide capture research, the most recent evolution in the field is
ambient and direct air capture, even though its potential has been discussed as early as
199917. Through critical thinking on the feasibility and relative ease to develop materials
compared to flue gas carbon dioxide capture17,26, the main advantage of the ambient air
capture versus flue gas capture is the potential ability to counteract the anthropogenic
carbon emissions. While flue gas capture would require all CO2 emitted to be captured in
the sorbents, ambient air capture requires only a modest capture efficiency of CO2out of
the total available CO2 in the atmosphere to neutralize current emissions. The new
strategy would require a new sorbent and regeneration system. The developed sorbent
is a polymeric amine anionic exchange resin, polystyrene backbone chains with
quaternary ammonium26 incorporated into the resin structure. The resin is loaded at 60%
wt.%26 onto its poly (propylene) sheet support. The resin is regenerated by a moisture
swing adsorption (MSA) process, where CO2 is adsorbed under dry conditions and is
released when the resin is exposed to moisture. Figure 1-4 shows the proposed
mechanism of the ion exchange resin for CO2 capture.
8
Figure 1-4. Carbon dioxide capture mechanism for MSA sorbents4
Under constant ambient conditions, the sole switch between humid and dry
conditions trigger the MSA process. As shown, the theoretical molar ratio is 2:1 between
quaternary ammonium sites and CO2. The adsorption capacity of the ion exchange resin
(IER) sorbent is theoretically estimated to be 3.74 wt.%, yet the experimentally
determined adsorption capacity was determined to be about 1.1 wt% 6 even when
allowed excess time to reach saturation. However, regeneration cycles of TSA sorbents
take less time for saturation, while more work is needed to make the IER sorbent
economical. The regeneration moisture substance was shown to be more effective as a
brine4, enhancing aqueous CO2 interactions.
The Table 1-1 explains the different CO2 capture techniques discussed in terms
of strengths and weaknesses, resulting in a summarized list of what is desired in a
hypothetical CO2 capture system.
CO2 Capture Method Advantages Disadvantages
Pressure Swing
1.4 Surface Modified Carbon Adsorbents
Carbonaceous materials used as adsorbents all seem to go through a common
process by which the carbon content rises and a more ordered system presents itself as
porous carbons. Pyrogenic carbon adsorbents are products of pyrolysis or other anoxic
carbonization processes between 500 and 1000oC where the C:O and C:H ratio
increases, the pH of the carbon surface increases and the surface area of the material
increases27,34. These products result in chars of biomass or coal tars or other fossil fuel
fluids, making for different shapes and sizes of the particulates. Under comprehensive
review, one of the strongest physisorption performances of the post-combustion
adsorbent field27 show a CO2 saturation capacity of 7.3 wt.% under ambient conditions in
a pure CO2 stream34. The adsorbent is a biochar from sugarcane bagasse shown in the
following production SEM images5,34 that met the pyrogenic carbon criteria for an
adsorbent.
Figure 1-5. Heat treating of biomass to produce ordered biochar material5
The sugarcane bagasse CO2 isotherm showed a Langmuir style curve suggesting
monolayer adsorption34 as well as suggesting the higher temperature-treated biochar
11
adsorbed more CO2 34. No result suggests that CO2 at a low partial pressure can be
effectively adsorbed is given. The physisorption multilayers are formed in given enough
time, which in this case the adsorption step is 75 minutes27, could be the main cause of
the high adsorption capacity. In addition, it is conceivable that if applied to ambient
moisture conditions, H2O may compete for physisorption among other gases, as
physisorption is not selective. The kinetics showed slow saturation in pure CO2 < 2
hours34 compared to industrial sorbents and TSA adsorbents, and any amount of low
concentration CO2 adsorbed would be slower.
Activation of carbon material is a physicochemical modification technique.
Activation refers to increasing pore volume (preferably micro/mesopores), surface area
and surface functional groups. Physical activation refers to both carbonization of the
material and oxidation of the surface for functional group formation. Therefore, stronger
binding energies of CO2 are expected as a result. Steam or CO2 activation with heat
treatment are common processes.27,35,36 Steam activation has shown to produce
exceptional biochar adsorbents, where under ambient conditions in pure CO2 yield an
8.8 wt.%35 adsorption capacity, while achieving 4.4 wt.%35 under a 0.1 atm pure CO2
stream.
Chemical activation can be performed with nitrogenous materials, hydroxides or
carbonate salts27,37. Carbonization by heat treatment may be complemented by carbon
surface/activation agent reactions that create surface functional groups. Nitrogenous
functional groups may be either an amine, pyrrolic, or pyridinic. By activating with both
urea and KOH, a biochar from biomaterial can achieve adsorption capacities of 21
%wt.38 under ambient conditions in a pure CO2 gas stream. Under the same system
conditions, a KOH activated char can reach the similar capacities39. This difference
12
shows that the species of biomass material and activation agent concentration is crucial
to the maximum adsorption potential. The adsorption isotherms show high adsorption in
very low pressures6,36,39, which gives promise to explore adsorption of CO2 from ambient
air.
Figure 1-6. SEM images of the activated carbon precursor (A), then nitrogenized (B),
then carbonized (C), and under TEM (D)6
In comparing abilities of the physically activated carbon and chemically activated
carbon, the extra step significantly increases the specific adsorption of CO2.
Nitrogenized surfaces are credited with charging the surface and providing more
adsorption pathways6,40–43. However, basic activation agents are just as strong
adsorption enhancers as nitrogenous precursors. The strength of the binding energy,
site selectivity, faster adsorption saturation times, and greater adsorption capacities all
work in concert to infer that CO2 would better benefit from chemically activated carbon
adsorbents than their physically activated counterparts.
1.5 Carbonic Anhydrase for Carbon Capture
A potentially energy-saving, fully regenerating mechanism currently exists being
utilized by biocatalysts. These carbonic anhydrase metallo-enzymes rely on pH swings
13
as their driver for readying their adsorption or desorption reactions. Initial studies have
tested these biocatalysts, determining their strengths and weaknesses for the specific
task of CO2 capture44–47. The enzymes are inefficiently dense at 30,000 g/mol, and are
water soluble in the bodies of animals, plants, and bacteria. In the lungs, they contribute
to the release of CO2 when exhaling. The reaction rate of bicarbonate formation is one of
the fastest for an enzyme, as it is over 4,000 times faster than MEA44 in CO2 conversion.
Figure 1-7. Carbonic anhydrase suggested biological mechanism7
The natural enzyme has a water ligand that can deprotonate and serves as a
Lewis base, reacting with carbon dioxide to make a bicarbonate ligand. This ligand
rearranges and enters the solution, returning the ligand back to a water ligand. The
deprotonation is credited to the enzyme’s ability to easily shift protons44. The enzyme
has a hydrophilic and hydrophobic side, due to the three tetrahedral-oriented nitrogen-
containing histidine ligands. Furthermore, the ability of surrounding amino acids to
pKa = 6.8
14
donate electrons to the metallic center enhances the attack of electrophilic carbon in
CO2 , and these multiple properties are behind the belief why the reaction rates are of
the fastest among enzymes44,48. The pKa of the complex is 6.8, so the equilibrium point
of a carbonic species system with the biocatalyst should be ~ 6.8. This means that when
the pH is > 6.8 the hydroxylated form is dominant which attacks CO2, and when the pH
is < 6.8 the hydrated enzyme will dehydrate the bicarbonate and release CO2. The
catch-and-release cycle of CAH indeed can be observed as dependent on the pH swing,
and manipulating the environment pH would force the biocatalyst to convert or release
CO2. In aqueous solutions, carbonic anhydrase in humans can convert over 1,000,000
CO2 molecules per second44 and can convert over 200,000 bicarbonate molecules per
second44. Therefore, biocatalyst enhancement in amine scrubbing or in a hydrous
solution where CO2 was bubbled has been the focus of research. These values
decrease when immobilized on a support while immobilization also protects against
biocatalyst degradation and activity loss, showing CO2 conversion amount
enhancements to carbonate, ammonia or MEA solution-based scrubbing47,49–53.
However, non-aqueous CO2 conversion with carbonic anhydrase have not been reported
for CO2 conversion. Carbon dioxide/bicarbonate conversion in a moist gas stream would
be the operating requirement of carbonic anhydrase if used for swing adsorption CO2
capture.
15
2.1 Thermodynamics of Carbon Dioxide Sorption
Adsorption of CO2 is generally known to be an exothermic and
thermodynamically favorable phenomenon (ΔG < 0 kJ/mol), where a gas adheres to a
surface, behaving like a condensate. Therefore, at high temperatures adsorption’s
thermodynamic favorability diminishes, while low temperatures reduce favorable reaction
kinetics. Absorption of CO2 is not gas deposition on a surface, but a phenomenon of a
physical or chemical interaction between CO2 and the bulk material’s surface used to
make CO2 a component of the material. Because most sorption processes are done in
an open system and subject to temperature and pressure changes, the Hemholtz free
energy cannot be used to quantify the sorption binding energy54, so Gibbs free energy is
used to make this thermodynamic analysis instead. The Gibbs free energy is generally
given by:
ΔG = ΔH – TΔS
The free energy of reaction systems can be estimated by combining Gibbs
equation and the ideal gas law to give:
ΔG = -nRTln(Keq)
Gmix = - PdV = -nRTln(V2/V1) = -nRTln(P1/P2)
In a gas stream, the energy requirement of gas separation (ex. CO2) from an air
stream is determined by the free energy of mixing. And when used to describe the free
energy of mixing of ideal gases the Gibbs free energy equation becomes:8,55
16
Gmix(P0, PCO2) = RT((PCO2/P0)ln(PCO2/P0) + ((P0 - PCO2)/P0)ln((P0 – PCO2)/P0))
At modest concentrations of CO2, the free energy of mixing of CO2 gas in the
stream can be used to estimate the free energy of CO2 sorption if the sorbent efficiency
remains low. Figure 2-1 represents this estimation, where the energy requirements of a
sorbent increases as the target partial pressure of CO2 at the system exit decreases8.
Therefore, atmospheric pressure and temperature sorbents at are not required to be
much stronger, if at all, than the mixing free energy of the system.
Figure 2-1. The theoretical sorption binding energy approaching the free energy of
mixing with increasing pressure8
The free energy of separation for CO2 from air as function of the partial pressure
at the exit of the system. The temperature T = 300 K, the total pressure P0 = 100 kPa.
The lower line is the free energy of mixing; the upper line represents the free energy of a
sorbent reaction capable of driving the partial pressure down to the exit pressure. The
sorbent energy diverges as the exit pressure goes to zero. Related to the above
analyses, thermodynamics of adsorption should be analyzed to produce the equilibrium
between adsorbate and adsorptive that directly determines how much adsorptive can be
17
adsorbed. The point of equilibrium occurs when just as much sorption is occurring as
desorption, and knowing the equilibrium gives insight to sorbent efficiency.
2.2 Kinetics of Carbon Dioxide Sorption
While the free energy of adsorption is a metric of its favorability, another
important quality remains: what timescale does the reaction happen and go to
completion? The timescale is represented by the rate of reaction and is affected by
temperature, component concentrations and the activation energy. The difference
between activation energy and free energy of a reaction is illustrated below:
Figure 2-2. Free energy of reaction diagram56
The activation energy is not related to the Gibbs free energy, but represents the
energy barrier of the high-energy intermediate’s formation before reaching the more
favorable product. The general equation for rate and kinetic analysis is:
r = -k[A]x = -kPx
k = koe(-Ea/RT)
18
Where the rate is dependent (or not) on the reactant concentration or partial pressure.
The magnitude of dependency on concentration is given by x, the order of reaction. The
rate constant k can be expanded into terms of temperature, and if ln(k) vs. 1/T was
plotted, the general Arrhenius equation in point-slope form describes the plot:
ln(k) = (-Ea/R)/T + ln(ko)
While the rate constant is temperature dependent, the activation energy is a
constant whose magnitude affects only how temperature dependent is the rate constant.
Therefore, the order of reaction only affects the rate and timescale of reaction, and in
terms of adsorption the main two orders are x=1 and x=2.
For general first order reactions:
r = d[A]/dt = -k[A]
ln[A] = -kt + ln[Ao]
ln[A/Ao] = -kt
r = d[A]/dt = k[A]2
1/[A] = kt + 1/[Ao]
1/[A] – 1/[Ao] = kt
Time of CO2 adsorption to near completion is important as to cross compare
adsorption techniques, favoring the quickest of reactions. The kinetic favorability,
however, may conflict with the thermodynamic favorability of adsorption, where a lower
temperature would increase the Gibbs free energy magnitude. More variables may be at
play, and more seriously affect the kinetics of applying the adsorbent/gas system.
19
The pore properties of the adsorbents is a key factor in the kinetics CO2
adsorption. Diffusion into the pores can be affected by the pore sizes and the percentage
of the pore sizes. As discussed in the adsorption isotherms section, the adsorption
isotherms of an adsorbent differ and can enhance or hinder CO2 adsorption. The
following plot9,37 shows how micropores significantly enhance pore volume that
translates enhanced favorability of adsorption. It is worth noting that depending on your
adsorptive gas, different volumes will result.
Figure 2-3. Effect of oxidation time on pore volume distribution. Additionally, effect of
pore classification presence based pore volume distribution on N2 and CO2 adsorption
isotherms9
Activated carbon adsorbents and factors that affect the CO2 reaction kinetics
have been studied and it is found that many experimental conditions affect reaction
kinetics. Because of this, kinetic analyses are highly circumstantial on a swing
adsorption design. Since pHSA in a gas stream has not been studied, more work on its
design should be completed before significant kinetic results can be determined. The
following points can be made about CO2 adsorption on activated carbons57:
20
does not change the equilibrium adsorption capacity.
Flow rate – reaction rate increases with increasing volumetric flow rate, but does
not change the equilibrium adsorption capacity.
Temperature – reaction rate increases with increasing temperature, and also
increases the equilibrium adsorption capacity (a thermodynamic property).
Particle size – reaction rate decreases with increasing particle size, but does not
change the equilibrium adsorption capacity.
Static bed height – reaction rate increases with increasing in aspect ratio, but
does not change the equilibrium adsorption capacity.
Reaction order - activated carbon CO2 adsorption follows 2nd order kinetics as
well as carbon anhydrase catalyzing CO2 hydration 44,58–60.
2.3 Synthesis of Biocatalyst Modified Adsorbents
The concept of a pH swing mechanism is not new and even used as close to the
field of ambient carbon capture as ex-situ carbon sequestration through mineralization
processes. Current reviews in mineralization61,62 have determined the high level of
promise of a pH swing mechanism when using ammonium salts for providing the basic
pH needed for carbonate precipitation. In mammalian bodies, carbonic anhydrase (CAH)
is an enzyme biocatalyst that assists in converting carbon dioxide to bicarbonate and the
reverse reaction. The research initiative for CAH immobilization sprouted from the same
desirable characteristics as carbon dioxide adsorbents, active site stability and
regeneration. Immobilization of biocatalysts and cells has been researched for decades
and fundamental understandings lead to its situational prominence over
polymerization63, a recent developing concept. The main limitation of polymerization lies
within the nature of CAH, its large molecular weight and steric hindrances49.
21
Immobilization is not a very strict process, so a key advantage of immobilization
synthesis is that one could find the most efficient, cheapest, or most chemically suitable
synthesis. Immobilization techniques found include enzyme aggregation46,64,
adsorption46,64 and covalent attachment64,65. In that order it seems, the performance of
the biocatalysts seem to diminish with aqueous capture processes of high concentration,
with enzyme aggregation and adsorption being the two more strongly considered
options. The main concern with the various cross-links, polymerizations, or aggregations
of catalysts is that it introduces another diffusion barrier limitation, decreasing adsorbent
efficiency, and adding complexity in the synthesis process.
Our group’s synthesis procedures regarding PEI immobilization on activated
carbon, fumed silica, MCM-41, or SBA-15 do not have any fundamental differences or
additional material to ensure impregnation. Therefore, any processes found in literature
that study immobilization of CAH on one of these supports should be able to apply to all
forms of possible supports used by the Song group. Therefore, the primary focus of
research in a pHSA system should be an adsorption-based synthesis of a supported
biocatalyst. Regarding adsorption onto solid supports, the most appropriate examples of
successful products, not necessarily or telling to performance, include polymeric
solids47,49–51,65–72, silica-based solids46,73,74 and graphitic solids.52,74 Typically, the
adsorptive biocatalyst is impregnated with similarities to the Song group’s impregnation
of PEI onto solid supports, and are outlined through. Forming a simple method will take
considerations the following people have done regarding the differences in the chemistry
of the solid (carbon, silica, etc.), and critiquing the details of synthesis will be not the
priority in my immediate work.
22
Considerations of establishing proper surface environment for CO2 adsorption
and pH swing interactions will be the last piece of the comprehensive process/material
groundwork in discussion. Experimental synthesis of introducing nitrogen in a CO2
adsorbent can be accomplished through low energy nitrogen ion sputtering onto a
carbon surface43, though more commonly by an organic precursor or mix of precursors
that are exposed to many variations of washes, activations and carbonizations, as
opposed to post-activation modification of carbon materials75. Commonly, organic resins
or polymeric carbon materials are synthesized to create adsorbents6,38,40,42,76,77.
Activation and carbonization is required to unlock the CO2 adsorption potential,
compared to the raw nitrogen material42, where proper thermal treatment significantly
increases CO2 adsorption76,77.
Because many synthesis variations of these materials have been demonstrated
to produce successful adsorbents, a determined representation of the nitrogen material
synthesis will be the basis for preferential materials. For instance, using microwave
irradiation and pressurization synthesis for the polymer to be activated and carbonized
as previously discussed42 or requiring high temperature ammonia gas treatments for
nitrogen-containing carbon fibers40 complicates the process, and does not enhance
adsorption performances in low concentration, small pore environments. While the pH of
nitrogen-containing polymeric resins or their activated carbon products can be
speculated as basic, the pH of nitrogen-containing coal-derived activated carbon has
been reported to be 9.441, which tends to be where immobilized carbonic anhydrase
activity is optimized74. The ammonia solution separation and steam activation of
ammonium humates from coal to make nitrogen-containing activated carbon41 is
straightforward, but also exciting as the material is not thought of as a material to be
23
used for CO2 adsorption or carbonic anhydrase immobilization for that matter. The more
commonly synthesized resin-based polymers or activated carbons become chars.
Important factors while considering this synthesis pathway have been researched
including templating76, carbon dioxide carbonization temperatures76–78, and choice
nitrogen precursor76,77. Carbonization temperatures throughout the literature have
remained in the range of 400-800oC, but specifically for these resins, the optimal
temperature for diluted carbon dioxide streams are low around 500oC76,77. While
melamine-based resins can be more complexly synthesized with resorcinol to make
intricate spherical geometry of the activated material, the special char geometry can
increase the CO2 adsorption performance two to four fold78 under thermal activation in
nitrogen gas compared to similar activation of melamine-based resin activation
procedures76,77. Therefore, supported nitrogenous material promoting the material’s CO2
adsorption capacity can show relative performance to synthesis-modified activated
carbon spheres without the expensive, complex, time-consuming adsorbent synthesis.
Nitrogenous functional groups on carbon adsorbent surfaces by impregnation
has been characterized by XPS38,42. Because at 500oC nitrogenous functional groups
start to decompose, various nitrogenous adsorbents start to display converging nitrogen
contents of about 30 wt.%41,42 after activation. Mainly, pyrrolic, pyridinic and quaternary
nitrogen functional groups were found38,42, which has been attributed to KOH activation
and subsequent decomposition of less stable nitrogenous groups42. Both pyridinic and
quaternary nitrogen has been shown to effectively and preferentially adsorb CO2 from
even low concentrations4,43 In addition, nitrogen sites exhibit a localized adsorption
phenomenon43. The following excerpt79 from IUPAC defines localized (or immobile)
adsorption.
24
“A situation in which the freedom of molecules of adsorbate to move
about the surface is limited. Adsorption is immobile when kT is small
compared to ΔE, the energy barrier separating adjacent sites. The
adsorbate has little chance of migrating to neighboring [sic] sites and
such adsorption is necessarily localized”.
The term “kT” is the mathematic product of Boltzmann’s constant and temperature and is
representative of the minimum amount of energy required to raise the system entropy80.
The energy barrier ΔE is the difference in energy between the preferential high energy
adsorption site and the adjacent lower energy adsorption site80. Because the localized
adsorption site has an energy of adsorption much higher than the kT energy required to
increase the adsorbate’s entropy, the adsorbate is not given enough energy to desorb.
Chemical activation of carbonaceous materials is widely done to produce
commercially available carbons and its effective CO2 adsorption has been previously
discussed in the introduction. Potassium bases like KOH and K2CO3 can be impregnated
into a porous carbon for this kind of activation. Specifically, activation of nitrogenous
compounds occurs after carbonization using KOH38,42,81 and co-activation of nitrogenous
resins on carbons using K2CO3 76,77 have been demonstrated to enhance the resulting
adsorbents’ CO2 performance.
While functional groups on PBX-51 and BP2000 carbon black comprise of
hydroxyl and carboxyl groups according to Cabot82, those groups and others can be
produced by potassium base activation. Ether83, lactonic41,84,85 and quinonic84 groups
have been found from FTIR and XPS analysis besides hydroxyl and carboxyl groups.
Activation temperatures at 500oC or more will start to decompose and reduce the
amount of acidic groups41,75,86,87, raising basicity and increasing π electrons from PAH
formation. Potassium can be utilized as a CO2 adsorption promoter as well, where K+
25
ions interact with oxygenated surface functional groups due to ion exchange during wet
impregnation83. Being that potassium ions are a Lewis acid with more electron affinity
than hydrogen ions, potassium can displace oxygen-bonded hydrogen. Even after
activation, filtrations and washings of the material, XPS and EDX characterizations have
been found to confirm interacted K+ on the surface57,83,88. Adding K2CO3 at 50% the
weight of the activated carbon being impregnated during synthesis can been shown via
EDX to keep enough potassium in the porous carbon to be around 29-30 wt.% of the
resulting adsorbent57. Different methods have been used to confirm carbon anhydrase
on a porous sorbent. Because of the large molar density of the enzyme, impregnation
into porous sorbents may not be simple as eventually a certain pore size will prevent
further diffusion into smaller pores. FT-IR spectra65 has been reported to confirm
carbonic anhydrase immobilized on a silica-based SBA-15 via its histidine groups. FE-
SEM has been also previously reported to suggest surface morphological changes from
carbonic anhydrase on the surface of SBA-15 via adsorption or glutaraldehyde cross-
linking immobilization techniques64. EDX spectra50 has been reported to identify the Zinc
metal center belonging to carbonic anhydrase immobilized on functionalized biochar and
supported by both FTIR and SEM confirmation49,50. Through measuring enzyme activity,
functionalized biochar was shown to immobilize >72%50 of the available carbonic
anhydrase, while other specialized functionalized materials are reported to immobilize up
to 99% of available carbonic anhydrase49,50.
26
3.1 Problem Statement
Most carbon capture methods require swing adsorption systems that are energy-
intensive under ambient air conditions. Biomimetic catalysis for carbon capture has
limitations in an aqueous system and requires immobilization to maintain catalyst
activity. Commercial sorbents lack the selectivity, adsorption capacity and regenerative
performance to be a significant carbon capture material without activation agents and
surface modifications. Though research has been done separately on these issues, a
culmination of these methods could solve their individual limitations. To date, there is no
technique or individual material that boasts the adsorption capacity to modified sorbents,
fast kinetics of carbon-converting biocatalysts, and efficient regeneration schemes in
ambient air conditions. A new carbon capture system with a low-energy regeneration
cycle could help increase the feasibility of ambient air carbon capture on an industrial
scale.
Adsorption and desorption events showing cyclic regeneration using pHSA
drivers need to be observed to validate the proposal. The synergistic effect of
simultaneous biomimetic catalysis and surface adsorption is not well understood. How
immobilized biocatalysts perform in a low concentration of water and respond to pH
change due to water introduction or removal need to be determined for a thorough
assessment of pHSA’s potential. Low cost, efficient carbon capture is even more a
priority in the ambient air field, even more than a very high adsorption capacity or a high
heat of adsorption. The barrier of entry is high for a market not connected to an existing
infrastructure, such as power plants emitting flue gas. Therefore, finding a regenerative
27
sorbent that boasts low cost and low adsorption energy chemistry would be the goal
when pursuing ambient air capture. Faster regeneration in ambient air capture means
faster carbon processing and less important the strength of the sorbent becomes,
meaning the cost will lower for both the material and operation. So far, PSA, TSA and
MSA are insufficient to address regeneration in ambient conditions and requisite capture
performance.
3.2 Main Objectives
1. Demonstrate whether the concept of using biocatalysts on a modified adsorbent
could enhance the adsorbent’s CO2 capture performance under ambient
conditions.
2. Demonstrate whether the concept of utilizing a pH swing adsorption (pHSA)
system could regenerate modified adsorbents for CO2 capture.
3. Perform an integral analysis to compare the CO2 capture performance and
potential application of pHSA with other swing adsorption processes.
3.3 Research Goals
1. Immobilization of bovine carbonic anhydrase (BCA) onto a modified carbon
adsorbent can enhance its adsorption capacity and cyclic regeneration under the
pHSA process.
2. A carbon adsorbent regardless of BCA immobilization or chemical modification
can possess regenerative ability through the pHSA process.
3. BCA-immobilized, modified carbon adsorbents under the pHSA process can
compete in CO2 capture, CO2 regeneration or system costs with both PEI-
modified solid adsorbents under the TSA process and ion exchange resin sheets
under the MSA process.
Development of the sorbent requires multiple synthesis procedures, activation
and drying to produce the material. The material would use the pHSA design in the
following experimental design to demonstrate its potential performance and sorbent
regeneration abilities. The necessary syntheses for the proposed work and preliminary
analyses include benchmark sorbents, the proposed bio-catalytic sorbents, ammonium
bicarbonate regeneration solutions, and potassium chloride ice baths.
In a way to swing the pH on the adsorbent surface, a regeneration solution can
be introduced via spray or vapor to the adsorbent. The ammonium bicarbonate
regeneration solutions were created as both basic and acidic. The acidic solution is
created from CO2 gas bubbling after ammonium bicarbonate (by Sigma Aldrich) addition
into distilled water, inducing a pH of 5.6. The basic solution is created from the
ammonium bicarbonate addition until the pH measures about 7.8. With the goal of a
basic or acidic pH being foremost, there is precedent for ammonium bicarbonate in
aqueous CO2 capture systems89–91.
After the adsorbent bed, a briny ice bath will be used as a condenser to control
the bulk of the moisture in the exit stream before heading through the finer
moisture/oxygen reduction filter equipped onto the micro-GC. The 3.6M potassium
chloride solution ice bath utilizes the freezing point depression property of the brine
(approximately -6 to -9oC) for thoroughly condensing any effluent vapor.
The benchmark sorbents are synthesized using the procedure established by the
Song group for polyethylenimine-based (PEI) sorbents92. Specifically, through works
29
within the Song group, it has been shown that 30 wt.% PEI loaded sorbents (instead of
the optimal 50 wt.% loading) yield higher adsorption capacities if under dilute CO2
conditions13,21,25,93. In addition, the Song group has also made advancements in finding
commercially available adsorbents, such as porous carbon materials to support PEI,
instead of costly synthesized MCM-41 or SBA-15 supports25. Using this previous work,
the commercially available sorbents chosen to synthesize the 30 wt.% PEI-modified
sorbents are BP2000 coal tar carbon black (Cabot Corp.) deemed “C4” and CAB-O-SIL
untreated fumed silica (Cabot Corp.) deemed “M5”. For the creation of the benchmark
sorbents, 20 mL of methanol (Sigma Aldrich) was stirred for 30 minutes at room
temperature. Polyethylenimine (Sigma Aldrich) with a molecular weight of 600 g/mol was
added to the methanol until dissolved. The C4 or M5 was then added to the solution with
more methanol as necessary to thoroughly disperse the supporting material. Stir-drying
overnight and subsequent vacuum drying for at least a day formed the final material. The
material was extracted from the synthesis beaker and crushed by mortar and pestle to
break up visible agglomerates, resulting in particle sizes loosely including 60-75
microns94.
Synthesis of the proposed catalytic adsorbent reflects the multiplicity of the
techniques required to promote CO2 adsorption and utilize the proposed pHSA
regeneration system. To mimic the benchmark TSA sorbents, fumed silica and carbon
black are also used as a commercially available sorbent material. While both materials
have shown to be successful sorbents, they have very different surface chemistries so
the preliminary use of both will provide a broad basis for determining a proper material to
further analyze. Specifically, CT-1221 treated fumed silica (Cabot Corp.) deemed “FS”
and PBX-51 lead acid battery carbon black additive (Cabot Corp.) deemed “CB” were
chosen. PBX-51 is a carbon black created as a close substitute for “C4” that is now
30
discontinued as advertised by the Cabot Corporation, with similar surface area and
particle size properties that also boasts dispersion and electrical conductivity82.
First, CB is co-wet impregnated with potassium carbonate and melamine-
formaldehyde copolymer as follows. A 0.1M K2CO3 solution is prepared so that the
amount of K2CO3 is 50% of the weight of the to-be-added CB. The 100 mL aqueous
solution is stirred at room temperature until complete dissolution, and stirring (medium
high setting) is continued through the entire wet impregnation. Poly(melamine-co-
formaldehyde) copolymer is a nitrogenous polymeric substance of average Mn ~ 432 that
will provide the nitrogen enrichment on the adsorbent. The copolymer is in a butanol
solution of 84 wt%, as the copolymer does not readily dissolve in water. Therefore, a
weighed-out amount of copolymer in butanol solution that would result in a 1:1
(copolymer: K2CO3) weight ratio was further dissolved in 10-15 mL of high purity butanol
(Sigma Aldrich). The thoroughly dissolved copolymer solution was added to the K2CO3
solution that was heated to about 95oC to further enhance the solubility and dispersion of
the copolymer and ion exchange of potassium onto the to-be-added CB. After observing
the miscibility of the two solutions, the prescribed CB amount was added (twice the
weight of the added precursors) to the hot solution at 95oC. After CB addition the mixture
was stirred for 5 hours and subsequently filtered using a 1 micron filter paper under
vacuum suction to dryness .
After retrieval from the wet phase, the co-activation process step of K2CO3 and
copolymer begins. Using the same PHSA system design described under Experimental
Design, the retrieved material was placed in the tube reactor between quartz wool as a
fixed bed. Under a 10 mL/min flow of inert nitrogen gas, 1 hour ramping from room
temperature to 500oC was performed and kept at 500oC for 1 hour. This co-activation will
simultaneously provide conditions for chemical activation and copolymer carbonization.
31
After the co-activation is completed and back to room temperature, the adsorbent is
washed with distilled water to remove any loose byproducts or materials trapped on, but
not interacting with, the adsorbent surface. The resulting product is now deemed “KN-
CB”.
As discussed previously, the method for the most effective and stable carbonic
anhydrase impregnation is by adsorption and subsequent cross-linking51,73. While in wet
phase 0.1M phosphate buffer solution of pH ~ 7.4 (Sigma Aldrich), wet-impregnation of
carbonic anhydrase was performed in 250mL at room temperature under medium high
stirring. After stirring, lyophilized carbonic anhydrase from bovine erythrocytes (Item
Number C2624, Sigma Aldrich), or BCA, is weighed-out to an amount that would be 50
wt.% of the to-be-added activated adsorbent and added to the buffer solution.
Immediately after observing even and thorough dissolution, the adsorbent to be stirred
for 1 hour. Using the same vacuum filtration design, the solid phase is retrieved and
readied for the next step.
A second, separate 250mL 0.1M phosphate buffer solution under medium high
stirring and room temperature is required for the crosslinking of the carbonic anhydrase
via the glutaraldehyde cross-linking agent. The 50% by volume glutaraldehyde aqueous
solution (Sigma Aldrich) is added to the buffer solution to result in a miniscule 0.1% by
volume glutaraldehyde concentration. Immediately after, the previously retrieved solid
phase is added to the stirring solution for up to 1 hour. Recovery from the second wet
phase mimics the previous vacuum filtration methods, except the washing solution is
now 0.05M Trizma buffer solution (Sigma Aldrich) for removal of loose material,
contaminants and preservation of the cross-linked biocatalysts. Upon drying on the filter,
the adsorbent goes through overnight vacuum drying, finalizing the adsorbent deemed
“BCA/KN-CB”. For short term storage, refrigeration between 2-5oC is required, and deep
32
freezing < -15oC is required for long term storage. Loss of biocatalyst activity will happen
depending on temperature exposure and increasing age, where on the worst case
scenario, thawing carbonic anhydrase can reduce activity up to 50% in 6 months53,95.
4.2 Experimental Design
Figure 4-1. The proposed pHSA system
The main experiment is performed using a tabletop model of a pHSA-based CO2
capture system. This setup will give data related to the stipulated research goals 1 and 2
to determine the success of meeting main objectives 1 and 2 from Chapter 3.
Determining how much carbon dioxide is captured and released is the key elements of
the work that can demonstrate the concept of the swing adsorption process. Comparing
how gas concentration curves are measured in the Inficon 3000 micro-GC TCD without
the sorbent in the bed to GC readings with the sorbent will be the basis for measuring
CO2 capture and release. Helium carrier gas is used for increased sensitivity to nitrogen
and less hazardous GC effluent given the laboratory’s design.
33
Figure 4-2. The pHSA system lab scale prototype
The gases used for the system include 1% CO2 balance air gas, N2 regeneration
gas and He GC-carrier gas. The CO2-containing gas is split into two streams so that one
of the streams will be sent through a bubbler to be saturated with moisture. After the
bubbler, the streams will be mixed to form a moist simulated air stream. Because the two
streams use different multi-flow controllers, various mixing ratios can be achieved and
therefore different moisture contents as well., The mixing in this case is to specifically
34
achieve a moisture content of 0.5% by volume concentration in the gas stream, which
simulates ambient air conditions26. The resulting volumetric concentration of CO2 is
about 25 times more in the simulated air gas than atmospheric air. While 1% by volume
concentrations can still be considered a dilute component, the concentration increase is
for more easily demonstrating the proposed concepts. A higher concentration of gas will
result in more pronounced detection via micro-GC and more quantitative results. The
same concept of gas mixing to achieve moisture content is used on the nitrogen gas
stream for regeneration, where heating the solution between 45-50oC26 yields a
volumetric moisture content ~10%. The reactor is a steel column housed in an insulated
heating jacket at 35oC for both adsorption and desorption, representing a typical warm
Arizona day26. Sorbent samples are placed in a 3.17mm inner diameter steel column to
perform like a packed-bed reactor with a target aspect ratio of 10-11, where the samples
are held within the column by surrounding quartz wool filaments. The aspect ratio target
does cause variation in the mass of adsorbent in the packed bed between sample trials,
which could range from 50-150 mg. The particle size range also may affect the weight
differences between samples, where the adsorbent agglomerates are larger than 1
micron (filter size) and smaller than 105 microns (140 mesh US Standard Series).
The effluent gases will be condensed of any water before being filtered for the
micro-GC by the KCl-saturated ice bath, which ensures the lifespan of the micro-GC inlet
filter and the micro-GC. Following freezing point depression curves of KCl-salted water,
the freezing point should hover between -9 and -7oC. The solution bubbler for the
simulated air contains distilled water, however, the nitrogen gas bubbler contains a0.1M
ammonium bicarbonate (Sigma Aldrich) brine solution, made acidic via CO2 injection or
basic. The nominal pH by Sigma Aldrich of a 0.1N NH4HCO3 solution is ~ 7.8, higher
than that of carbonic anhydrase, which has a pKa ~ 6.8-7.0. Therefore, the solution in
35
theory would swing the pH of the sorbent back to being basic to catalyze CO2 production
via BCA and effectively regenerate the basic surface pH of the adsorbent.
After loading the adsorbent, nitrogen gas is flown through the sample at 75.8
mL/min to create an inert baseline of inlet gas into the GC. Adsorption starts upon the
switch to the CO2-containing gas, which is also flown at 75.8 mL/min, and the micro-GC
will take gas concentration measure over time. The desired result is a CO2 concentration
curve over time. When creating a fixed bed of all quartz wool and applying the same
adsorption step, a curve is also created due to the delay it takes for the gases to travel
through the reactor to the GC. This “empty” curve will be used as a “gas blank” that will
represent no CO2 adsorbed. Any loss in curve area when using a sorbent-loaded fixed
bed can be calculated as CO2 lost in the stream by CO2 adsorption onto the adsorbent.
All the bed and flow parameters were purposefully kept consistent to ensure flow
behaviors were also consistent between each pHSA trial but also between the
benchmark TSA sorbents. TGA analysis is a common method for adsorption capacity
measurements as well, but if moisture is present in the stream, TGA cannot determine
whether CO2 or H2O was adsorbed.
4.3 Proposed pHSA Process
The proposed pHSA process cycle consists of three phases that connect four
sorbent mechanism steps. Figure 4-2 summarizes the pHSA process with three main
phases (capture, regeneration and reset) cycling through three surface configuration
steps (initial, capture and release). The adsorbent in the initial step is exposed to the
simulated air stream in phase 1 and turns into the capture step as CO2 is adsorbed on
the surface. For phase 2, the simulated air is replaced with the regeneration stream to
trigger CO2 desorption, being the regeneration step. Phase 3 is completes the pHSA
36
cycle, where the regeneration bubbler is cut from the inert gas stream and dries the
sorbent back to its initial step properties.
Figure 4-3. The comprehensive pHSA cycle illustration
Figures 4-3 through 4-7 illustrate the process on the adsorbent surface.
1) Phase 1 – Capture
a. Initial Step. If the adsorbent is beginning the first pHSA cycle, the sorbent is
pretreated in a vacuum oven at room temperature for 4-6 hours. Once
loaded in the reactor heated to 35oC, the sorbent is flooded with dry nitrogen
gas for about 8-10 minutes while the GC maintains a consistent data output
showing a nitrogen atmosphere. No significant reactions are expected to
occur in the inert environment.
37
Figure 4-4. Proposed Surface Configurations through pHSA (Phase 1): Initial Step
b. Capture Step. The gas stream is switched from nitrogen to 1% CO2 balance
air. As previously discussed, use of the H2O bubbler yields the desired
moisture content of 0.5% by volume. The gas stream floods the reactor
heated to 35oC for about 8-10 minutes under atmospheric pressure.
During this step, proposed adsorption mechanisms are related to the
discussed CO2 capture enhancements in the theory chapter. The
chemisorption of CO2 on basic surface sites via Lewis acid-base reactions
takes place on the oxygenated and nitrogenous surface functional groups.
The oxygenated functional groups interacting with potassium acts as a weak
cationic surface site and interacts with oxygenated and hydroxyl groups,
thusly enhancing surface basicity. As more H2O and CO2 interact with the
adsorbent, the mechanism halts as it reaches equilibrium.
38
Figure 4-5. Proposed Surface Configurations through pHSA (Phase 1): Capture Step
2) Phase 2 – Release
a. Capture Step. The preamble of phase 2 is the end of the capture step. With
the operational parameters of the capture step consistent with phase 1, CO2
adsorption continues to acidify the surface and the moisture content slightly wets
the surface. As the sorbent surface reduces basicity theoretically below 7 as the
pKa of acarbonic acid system is 6.35, the impregnated carbonic anhydrase
deprotonates. Deprotonation, as part of the proposed mechanism, catalyzes CO2
hydration to bicarbonate ions in the presence of water. This may allow further
CO2 capture before the regeneration step, where either dissociated bicarbonate
adheres to an adjacent surface site or the newly formed bicarbonate stays a
ligand to the BCA. Dissociation of bicarbonate is the preferred option, since BCA
would be free to hydrate more CO2.The hydration mechanism slows when the pH
lowers below 6.8 or lack of water.
39
b. Regeneration Step. The regeneration step starts by switching the 1% by
volumetric CO2 bal. Air gas with regeneration solution-bubbled nitrogen gas for 8-
10 minutes through the reactor at 35oC under atmospheric pressure. The
nitrogen gas is bubbled through the proposed regeneration solution, a ~7.8 pH
ammonium bicarbonate solution heated to approximately 45-50oC with the
intention of making the vapor pressure 0.1 atm or 10% by volume moisture. The
gas stream has a negligible CO2 concentration, causing a vapor pressure
equilibrium shift and desorption of CO2. In addition, the large concentration of
H2O will result in preferential adsorption over CO2 with a low vapor pressure from
the equilibrium shift. Ammonium bicarbonate on the surface exists in solution
with condensed water, and the basic solution interacts with the CO2 acid,
becoming aqueous and eventually gaseous. The regeneration solution, in
following the carbonic anhydrase mechanism, should basify the surface and
induce protonation of the carbonic anhydrase, catalyzing the bicarbonate
dehydration reaction to release CO2. The dehydration mechanism slows when
the pH goes above 6.8.
40
Regeneration Step.
3) Phase 3 – Reset
a. Regeneration Step. As CO2 is released, the surface loses its acidity and
return to the pH of the beginning of the cycle. However, the solution vapor
added to the sorbent/reactor would remain and must be returned to a dried
sample. For phase 3, the gas stream switches to a dry nitrogen gas stream
for 8-10 minutes keeping the reactor at 35oC under atmospheric pressure.
The incoming dry stream shifts the water equilibrium vapor pressure and the
water wet surface vaporates.
b. Initial Step. The dry gas stream will shift the vapor equilibrium, causing more
vapor pressure to leave the sorbent surface with the goal to return the
sorbent to its initial physical properties. While this objective is foreseeable,
41
the chemical properties of the sorbent foreseeably will change and would
easily be suggested through differences in CO2 capture through multiple
cycles.
Figure 4-7. Proposed surface configurations through pHSA (Phase 3): Initial step in
the following cycle
4.4 Adsorbent Pore Characterizations
BET isotherms and surface area estimations were created to better estimate the
adsorption of gases onto a solid surface. With the thought being that adsorption of gases
resembles their condensation behaviors, the BET method is based off of the assumption
that the adsorptive can adsorb in a randomized indefinite amount of layers onto the
porous media surface96. To understand specific adsorptive-adsorbate interactions, a
Langmuir isotherm can be created. The Langmuir isotherm is a model describing the
quantitative ability a gaseous adsorptive adsorbs onto a solid adsorbent with changes of
partial pressure of the adsorbate at a specific temperature97. The model is essentially an
ideal case, where a continuous monolayer of adsorbate along a perfect surface is
assumed. To determine the overall surface area, nitrogen gas is condensed onto the
porous media at 77K to line the surface. Because of the adsorption process is physical,
multiple layers are expected to accumulate on top of the first adsorbed layer and BET
isotherm analysis will be performed. The testing linear equation for the general BET
adsorption isotherm is the following96:
P/v(P0 – P) = 1/(vmc) + (c – 1/vmc)*(P/P0)
This slope-intercept form allows a plot that generally describes the amount
adsorbed versus partial pressure of adsorbate. Vm is a constant describing the volume of
gas required for monolayer adsorption and would be used for surface area
determination. To determine the pore volume or surface area distribution and mean pore
size of the adsorbent, the BJH method will be used to further define the adsorbents.
Using the BET method and the Kelvin equation, the BJH method aims to correct inflation
of multilayers not experimentally founded when pore size become mesopore and
micropore size98. The equation is as follows98:
Vpn = RnΔVn - Rntn∑n-1 j=1 ((p - t)/ p)jApj
43
The equation provided serves as a practical version for determining the pore
volume distributions with mean pore radii to account for adsorbate loss during
desorption98. The instrument of choice, Micromeritics TriStar II 3020 will analyze all of
the above for the adsorbent at each synthesis step (raw material, activated/carbonized,
and biocatalyst immobilized). The main goal of the analyses is to provide a surface area
to be used for point of zero charge measurements to meet research goals 2 and 3.
The point of zero charge (PZC) of a solid surface has been defined in multiple
ways for digestion. The PZC of an adsorbent surface obtains an electrical charge density
of zero, being neutrally charged99 The electrical charge density is dependent on pH, and
the certain pH at which the electrical charge is zero is the PZC. The PZC could be
described as the point when the surface anionic site concentration is the same as the
surface cationic site concentration99.
The common technique to determine PZC is the potentiometric or “fast titration”
method. The method is used to determine the surface charge density of the material
suspended in solution through a range of pH, and the PZC is the pH at which the surface
charge density is zero.
The following surface charge density is as follows99–101:
Traditional procedures show that about 200 mg of samples is mixed into a
solution that is acidified and titrated slowly with the conjugate base and the pH is
measured for every base addition yielding a surface charge density100–102. The equation
for surface charge used in the potentiometric method is consistent throughout the
method’s history. However, the actual procedural specifics does not have such support
or background. The earliest referred paper101 lays out no such procedural specifics, and
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its references do not either. Therefore, a discrepancy exists of the most recent uses
referenced between 40 mL and 30 mL of solution while using the same 200 mg sample
of material. More discrepancies exist, where the same mass of samples is used, but
materials are different, alumina and nickel oxide. These materials will have different
surface areas, surface sites, densities, etc. However, if this observation tells us anything,
it is that there no ratio being adhered to that would require density, surface area or site
density be related volume of solution. In addition, these studies show the difference of
the surface charge density vs. pH plots when the solution has different ionic
concentrations100,101. Even though they have general deviations between models and the
experimental data, being different ionic concentrations of NaNO3 from 0.1 to 0.001M, it is
observable that almost all plots consistently converge at the PZC. Therefore, the specific
concentration doesn’t carry influence, the volume difference between 30 and 40 mL
doesn’t carry influence, and the possible ratios of material characteristics in relation to
solution parameters do not change the PZC values by large enough amounts to notice
by plot observations.
Therefore, the study’s method to reduce the mass used for the experiment by a
factor of 10 or 20 less without significant PZC deviations seems plausible and open to
testing. Due to the independence of PZC based on ranging concentrations, surface
areas, volume to mass ratio and more, 10 to 20 mg can be used to determine PZC
without consequence even when using 30 or 40 mL of solution. Because the
mass/surface area/number of sites will decrease by an order of magnitude, the
concentration of the solution must be lowered as well. If the surface charge density
changes with pH due to ion exchange, lower concentrations of the solution would more
easily allow observable changes in ion exchange. A very concentrated solution with
small amounts of material would not be as influenced by the material. Instead of 30 mL
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of 0.01 M NaNO3, 30 mL of 0.001 M NaNO3 will be used with 10-20 mg of tested
adsorbents. Another consideration needs to be thought of as experiments commence:
whether BCA can influence pH measurements under the potentiometric method.
4.5 Analysis on Swing Adsorption Carbon Capture Systems
Even though pHSA data is restricted to this study, TSA has been tried on an
industrial scale and many studies on the process exist. TSA (temperature swing
absorption in this case) in MEA scrubbing of CO2 not only has been extensively
researched and engineered, many industrial scale cost analyses have been completed
as well. To better compare the methods, the system/adsorbent properties in terms of
CO2 capture and CO2 regeneration with respect to adsorbent cost, adsorption capacity
and energy consumption. Through these lenses a relative comparison can be made to
determine how far away the pHSA system is from industrial desirability.
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Results and Discussion
The basis of the proposed research relies on a complete, working system
described and successful synthesis of the catalytic sorbent. Through some preliminary
work relating to the laboratory-scale adsorption swing experiment, a confident assertion
can be made that the objectives of the proposed research were attained. Specifically,
discussion on a successful synthesis of the material and results from
adsorption/desorption experiments demonstrate the concept’s ability to adsorb carbon
dioxide and regenerate while giving off carbon dioxide.
5.1 Material Synthesis Discussion
The wet impregnation method was used to adsorb all modifications including the
biocatalyst to the carbon black PBX-51. Even though the wet impregnation technique
involves a water solvent to modify carbons with carbonates or BCA, the nitrogenous
copolymer is not as soluble. The copolymer is packaged with butanol and is soluble in
organic polar solvents due to its methylation. The chemical modifiers (K2CO3 and
poly(melamine-co-formaldehyde) were added to the synthesis wet mixture at 50 wt.% of
the added amount of carbon black, of which 200 mg was added.After collection and co-
activation heat treatment, the mass product retrieved would be approximately 300 mg.
Taking 100-150 mg of the new treated material, another slurry would be created with 50
wt.% of BCA added via wet impregnation. After the glutaraldehyde cross-linking process,
retrieval and drying, the final product mass can be between 150-200 mg. This adsorbent
is BCA/KN-CB, where BCA, K, N and CB stands for adding BCA, K2CO3,
poly(melamine-co-formaldehyde) and PBX-51 respectively to the adsorbent synthesis
procedure. Two batches were synthesized of adsorbents, and other adsorbents
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synthesized with different compositions than BCA/KN-CB are named appropriately. The
BCA used in the syntheses were from sealed containers except for the second batch of
BCA/KN-CB, where the BCA used was form the same bottled used in Batch 1 BCA/KN-
CB stored in a deep freezer for ~1 year, to reduce any differences of the enzyme
product. In summary, the synthesized adsorbents and the weight fraction of material
added during synthesis is:
Batch 1, BCA/KN-CB: BCA0.33/K0.17N.017-CB0.33
Batch 1, KN-CB: K0.25N0.25-CB0.50
Batch 2, BCA/KN-CB: BCA0.33/K0.17N.017-CB0.33
Batch 2, BCA/N-CB: BCA0.33/N.225-CB0.445
Batch 2, N-CB: N0.33-CB0.67
Batch 2, BCA/CB: BCA0.33/CB0.67
Even though the average primary particle of PBX-51 is nanoscale, the average
aggregate size of the carbon black is 4 µm and can be as large as 160 µm103. When
using a 1 µm filter used to separate the material out of the slurry, no observable amount
of carbon black passed through the filter, suggesting that the smaller particulate carbon
black are aggregating on bigger particles. Using a U.S. Standard No. 70 sieve, the
particles were found to be smaller than 210 µm. This range is mostly occurring before
handling, as the carbon black out of the container has a noticeably ranging aggregate
size.
5.2 Experimental Setup Discussion
The fixed bed reactor encased in a tubular oven is positioned vertical with the
flow directional against gravity to reduce particle contamination downstream. The flow
rate of the different gases are kept to 75.8 ml/min by mass flow controllers. The flow
rate, while specifically is not attributable to past work, it is on the same order of
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magnitude of TSA-adsorbent system studies by the Song research group and does not
cause flow impeding pressure drops. The fixed bed is contained between quartz wool,
and depending on the material’s loading it may present flow blockage. The blockage is
best observed from pressure drop measurements across the reactor, where the inlet gas
flow is 1 atm and the outlet is vented to the atmosphere. Depending on the number of
small particles or the ability of the particles to aggregate, carbon black can penetrate the
quartz wool and line the wool until a partial or complete obstruction is achieved.
Referencing the Ergun equation describing fixed bed reactors, a pressure drop from low
void space will reduce linear velocity, which will cause delayed travel of gases to the GC
for analysis. Since the pressure drop of a solely quartz wool bed is negligible, the
pressure drop of adsorbent beds must also negligible, or at least unobserved by the
pressure gauge. Analyzing the amount of CO2 adsorption for each cycle would then be
skewed if compared to a system with different pressure drops and therefore flow
behaviors. To combat pressure drop, samples were pressed slightly with a utensil, which
would make loosely compacted flakes about 1 mm in diameter that would be placed into
the fixed bed. This method was shown to drastically decrease the chance of a small void
space within the bed and reduce the amount of carbon black lining the quartz wool
fillers. Negligible pressure drops were observed and the pHSA experiment would
commence.
5.3 Benchmark TSA Sorbent Performance
The synthesis of the materials were adapted from loosely related previous works
regarding both nitrogen-doped sorbents and immobilized bio/biomimetic catalysts. In
addition, the Song group has recently pursued flue gas sorbents made of commercially
available sorbents, and these works have influenced the choice of material for catalytic
sorbent, being carbon black and treated fumed silica courtesy of Cabot Corporation.
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The first synthesis trial yielded BCA/KN-CB. The carbon black material, even when co-
activated, was relatively hydrophilic. However, the bovine carbonic anhydrase addition
seemed to cause the surface to become slightly hydrophobic. The shift in hydrophilicity
on the synthesis-based perspective gave indication of successful adsorption of BCA
onto the activated carbon black.
For the preliminary work, 1.03% by volume CO2 balance air gas was used as the
adsorption gas for the first stage of the CO2 capture/release mechanisms, being 25
times more concentrated than atmospheric air and 10 times less concentrated in percent
of volume of CO2 than flue gas effluent. The concentration increase will increase the
adsorption uptake by decreasing the thermodynamic barriers of gas separation, if
diluted, for adsorption, but will enhance the amount of adsorption or CO2 hydration
events within a small amount of time. The first experiments done were to validate the
system’s design. Newly made benchmark sorbents referred to in the methodology were
created to test the ability of the system to determine adsorption and desorption of any
tested sorbent. Because MBS-style sorbents utilize temperature swing adsorption
processes, desorption was carried out at 120oC, instead of the adsorption temperature.
The benchmark sorbents can be compared to previous work by the Song group for
consistency of results procured by the new designed system. Table 5-1 shows the
adsorption capacity results of the benchmark sorbents (30PEI/FS and 30PEI/C4) and
the comparison performances of sorbents analyzed in previous works.
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PEI-based TSA
Desorption
PEI Loading (wt.%) 30 30 50 30 50 50
CO2 Concentration
Adsorption Capacity
Reference Present Work
Wang et. al. 2012
Xu et. al. 2002
Wang et. al. 2011
Wang et. al. 2012
From Table 5-1, 30 wt.% PEI loading on sorbents 30PEI/M5 and 30PEI/C4
shows higher adsorption capacities compared to 50 wt.% loading on a related sorbent,
PEI(50)/SBA-15, at similar adsorption temperatures and CO2 concentrations, and vice
versa13. At lower temperatures, adsorption is kinetically controlled and requires smaller
diffusion barriers and less PEI in the pores. This could describe the difference between
the synthesized PEI-30/M5 sorbent vs. the original MCM-41-PEI-30 sorbent adsorption
capacities, where 30 wt.% loadings give less than optimal results under optimal
conditions (75oC, 100 vol.% CO2). The micro-GC gathers enough data to create
51
reasonable capacity data, and with validation from the other works, testing of the
proposed sorbents can commence.
Synthesis of BCA-TFS and BCA-N-CB were prepared as described in the
methodology. Nitrogen-doping was completed with both potassium car