novel promoters for carbon dioxide absorption in potassium
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Novel promoters for carbon dioxide absorption in potassium carbonate
solutions
By
Guoping Hu, BEng, MEng
Submitted in total fulfilment of the requirements of the degree of Doctor of Philosophy
January 2018
Department of Chemical Engineering
Melbourne School of Engineering
The University of Melbourne
Australia
I
Abstract
Carbon dioxide is a major driver for climate change and carbon capture and storage (CCS) is
widely recognized as an effective way to reduce CO2 emissions to mitigate climate change.
However, managing the cost of carbon capture to an acceptable level is of vital importance to
deploy it at an industrial scale. Potassium carbonate solvent (K2CO3) shows promise as a solvent
for carbon capture due to its low cost, low corrosivity, low degradation rates and low
environmental impact. However, as the absorption rate of CO2 using K2CO3 is relatively slow,
improving its absorption kinetics via adding rate promoters is crucial for reducing the capital cost
of absorption equipment required to build the carbon dioxide capture plant. In this study, a series
of promoters were investigated to improve the absorption kinetics.
Different promoters including organic promoters, inorganic promoters and enzymatic promoters
have been reported in the literature. From the literature review, a good promoter should be
economically acceptable, stable, non-toxic, non-corrosive, highly efficient, environmentally
benign, recyclable, and have a low vapour pressure. It was recommended that more efforts should
be focused on carbonic anhydrase enzyme and different amino acids, which is the focus of the
present study.
A carbonic anhydrase (NZCA) was first examined as a promoter in potassium carbonate solutions.
The catalysis kinetics of this promoter were tested via the stopped flow technique and a wetted
wall column (WWC). The Michaelis-Menten catalysis parameter (kcat/Km) was determined to be
2.7×107 M-1s-1 at 298 K, resulting an activation energy of 51±1 kJ/mol at 298‒328 K. The
promoting coefficient of the NZCA was determined to be 5.3×107 M-1s-1 using a WWC in 30 wt. %
potassium carbonate solutions (pH ~ 11‒12) at 323 K. Furthermore, the NZCA kept more than 70%
II
of its initial catalysis efficiency after continuously running for 8 hours in 30 wt. % K2CO3 solutions
at pH of 10.6‒10.8 and temperature of 323 K.
Then, histidine was investigated as a promoter for CO2 absorption as this is an important
component in carbonic anhydrase. Results showed that histidine anion ions (His‒) are the main
species reacting with CO2 in basic conditions (pH>9) with a reaction order of 1.18±0.08 across the
temperature range of 298–313 K. The zwitterion mechanism was used to fit the kinetic data and it
showed that both protonation and deprotonation reactions contributed to the overall reaction rate.
Ionic strength was also shown to have a significant influence on the reaction kinetics when the
histidine concentration is high (≥0.2 M). The reaction rate between histidine and CO2 was shown
to be slower than that of glycine and proline and slightly faster than that of taurine at low
concentrations (<0.1 M).
A range of different amino acids were next investigated as promoters. The amino acids
investigated in this study were 2-piperazinecarboxylic acid, asparagine, aspartic acid, glycine,
leucine, lysine, proline, sarcosine, serine and valine. Furthermore, proline, sarcosine, glycine,
leucine and lysine were tested as rate promoters in potassium carbonate solvent for carbon dioxide
absorption using a wetted wall column. Results showed that the anions of the amino acid salts are
the major species reacting with carbon dioxide. Therefore, the promoting effect of amino acid salts
is sensitive to changes in pH due to changes in species distribution. Sarcosine and proline were the
most effective promoters among the amino acid salts tested in this study with comparable
promoting performance at higher pH values (≥12.5) but with sarcosine more effective at lower pH
values (<12.5). Compared to 0.5 M monoethanolamine (MEA), sarcosine and proline showed
faster rate promotion effects for carbon dioxide absorption into 30 wt% potassium carbonate
III
solvents at high pH (>12.0), while the promoting performance of MEA was comparable with that
of proline and slightly poorer than that of sarcosine at low pH (<12.0) conditions.
Lastly, a carbonic anhydrase mimicking polymer was synthesized and characterized as a catalyst
for the CO2 hydration reaction. Results showed that the lower critical solution temperature (LCST)
of PNiPAm-co-CyclenZn is 33.7 oC which is close to the physiological temperature. Above the
LCST, PNiPAm-co-CyclenZn undergoes a phase transition from a swollen hydrated state to a
shrunken dehydrated state. This property can potentially enable easy separation of PNiPAm-co-
CyclenZn from the CO2 loaded solution exiting the absorber column so that it does not enter the
high temperature stripping column. In the reaction between CO2 and H2O, the catalysis coefficient
at 298 K of PNiPAm-co-CyclenZn was determined to be 380±20 M–1s–1 at a pH of 7.36 and
2330±40 M–1s–1 at a pH of 9.06. Arrhenius fitting of the catalysis coefficients showed an activation
energy of 60±2 kJ/mol at pH of 7.36. This study presents the first example of a temperature
responsive polymeric catalyst for carbon dioxide absorption.
Results from this study can guide recommendations for choosing promoters for industrialized
CO2 capture process using a potassium carbonate aqueous solution and will allow for a CO2
capture process with lower costs.
IV
Declaration
This is to certify that:
i) The thesis comprises only my original work except where indicated in the preface;
ii) Due acknowledgement has been made in the text to all other material used;
iii) The thesis is fewer than 100 000 words in length, exclusive of tables, maps,
bibliographies and appendices.
……………………….
Guoping Hu
November 2017
V
Preface
Results in Chapters 2, 4, 5, 6, 7 have been published in peer reviewed journals (listed below) and
the contents have been modified to fit the purpose of this thesis.
Chapter 2
Guoping Hu, Nathan Nicholas, Kathryn Smith, Kathryn Mumford, Sandra Kentish, Geoff
Stevens. Carbon dioxide absorption into promoted potassium carbonate solutions: A review.
International Journal of Grenhouse Gas Control, 2016 (53), 28–40.
Guoping Hu, Kathryn Smith, Yue Wu, Sandra Kentish, Geoff Stevens. Recent progress on the
performance of different rate promoters in potassium carbonate solvents for CO2 capture. Energy
Procedia, 2017 (114), 2279–2286
Chapter 4
Guoping Hu, Kathryn Smith, Nathan Nicholas, Joel Yong, Sandra Kentish, Geoff Stevens.
Enzymatic carbon dioxide capture using a thermally stable carbonic anhydrase as a promoter in
potassium carbonate solvents. Chemical Engineering Journal, 2017 (307), 49–55.
Chapter 5
Guoping Hu, Kathryn Smith, Liang Liu, Sandra Kentish, Geoff Stevens. Reaction kinetics and
mechanism between histidine and carbon dioxide. Chemical Engineering Journal, 2017 (307), 56–
62.
Chapter 6
Guoping Hu, Kathryn Smith, Yue Wu, Sandra Kentish, Geoff Stevens. Screening amino acid
salts as rate promoters in potassium carbonate solvent for carbon dioxide absorption. Energy &
Fuels, 2017 (31), 4280–4286
VI
Chapter 7
Guoping Hu, Zeyun Xiao, Kathryn Smith, Sandra Kentish, Luke Connal, Geoff Stevens. A
carbonic anhydrase inspired temperature responsive polymer based catalyst for accelerating
carbon capture. Chemical Engineering Journal, 2018 (332), 556–562
VII
Acknowledgements
This work presented in the thesis was conducted with the assistance of a number of people and
organizations to whom I would like to express my sincere gratitude:
Professor Geoff Stevens, Professor Sandra Kentish, Dr. Kathryn Smith and Dr. Nathan Nicholas,
for their supervision, advice, encouragement and support throughout the project.
Dr. Gabriel Da Silva, my advisory committee chair, for his suggestions and support.
Department of Chemical Engineering, the CO2 solvent group members Frank Wu, Dr. Nouman
Mirza, Indrawan, Alita Aguiar, Siming Chen, Thomas Moore and Dr. Kathryn Mumford,
engineering workshop member Justin Fox, general office staff Dr. Michelle de Silva, Tabitha
Cesnak, Cara Jordan and Louise Baker, academics and fellow postgraduates Dr. Qi Zheng, Sam
Law, Fan Wu, Hiep Lu, April Li, Hongzhan Di, Dr. Yong Wang, Dr. Jinguk Kim, Dr. Zheng Li,
Dr. Shinji Kanehashi, Dr. Lina Wang, Dr. Liang Liu and Dr. Colin Scholes etc., who provided
assistance and support throughout the project and my study, either spiritual or knowledgable.
My collabrators, Dr. Joel K. Yong, Dr. Zeyun Xiao and Dr. Luke Connol, for providing research
materials and inspiring discussions.
Peers and senior researchers for hosting my visit and inspiring discussions on scientific research
topics.
Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research
Council, for infrastructure and funding support.
Peter Cook Centre for CCS research and CO2CRC, for infrastructure support and funding support
for travelling.
VIII
Melbourne School of Engineering, for financial suport to travelling.
The University of Melbourne, for financial support during my study.
The Gordon Conference Committee, for providing travel funds sponsoring my attendancy to the
conference.
Finally to all my friends and family not mentioned above, who were always there to provide
support and encouragement, through the many chanllenges and triumphs encountered while
completing this project.
IX
Table of Contents
Abstract .......................................................................................................................................... I
Declaration .................................................................................................................................. IV
Preface .......................................................................................................................................... V
Acknowledgements .................................................................................................................... VII
Table of Contents ........................................................................................................................ IX
List of Figures ............................................................................................................................ VII
List of Tables .............................................................................................................................. XI
Nomenclature ................................................................................................................................. i
Chapter 1 Background .............................................................................................................. - 1 -
1.1 Reducing carbon dioxide from the atmosphere ............................................................... - 1 -
1.2 Techniques for carbon dioxide reduction ........................................................................ - 2 -
1.2.1 Solvent absorption for capturing carbon dioxide ...................................................... - 3 -
1.2.2 Sorbent adsorption for capturing carbon dioxide ..................................................... - 4 -
1.2.3 Membranes for capturing carbon dioxide ................................................................. - 5 -
1.2.4 Mineral carbonation for capturing carbon dioxide ................................................... - 6 -
1.2.5 Cryogenics distillation .............................................................................................. - 7 -
1.2.6 Others ........................................................................................................................ - 8 -
1.3 Potassium carbonate solvent absorption system for carbon dioxide capture .................. - 8 -
1.4 Current challenges with the potassium carbonate absorption process .......................... - 10 -
1.5 Aim of this study ........................................................................................................... - 11 -
Chapter 2 Literature Review ................................................................................................... - 12 -
2.1 Inorganic Promoters ...................................................................................................... - 12 -
2.1.1 Arsenite ................................................................................................................... - 12 -
2.1.2 Boric acid ................................................................................................................ - 13 -
2.1.3 Vanadate ................................................................................................................. - 16 -
2.1.4 Other inorganic promoters ...................................................................................... - 17 -
2.2 Organic Promoters ......................................................................................................... - 18 -
2.2.1 Amines .................................................................................................................... - 18 -
2.2.1.1 Monoethanolamine ........................................................................................... - 18 -
X
2.2.1.2 Diethanolamine ................................................................................................ - 25 -
2.2.1.3 Piperazine ......................................................................................................... - 25 -
2.2.1.4 Other amines .................................................................................................... - 27 -
2.2.2 Amino acid salts ..................................................................................................... - 27 -
2.3 Enzymatic promoters ..................................................................................................... - 29 -
2.3.1 Carbonic anhydrase ................................................................................................ - 29 -
2.3.2 Metal compounds mimicking carbonic anhydrase ................................................. - 32 -
2.4 K2CO3 pilot plant studies with rate promoters .............................................................. - 34 -
2.5 Lessons learnt from the literature .................................................................................. - 35 -
2.5.1 Promoting mechanisms ........................................................................................... - 35 -
2.5.2 Comparison of different promoters and remarks .................................................... - 37 -
Chapter 3 Experimental .......................................................................................................... - 41 -
3.1 Stopped flow technique ................................................................................................. - 41 -
3.1.1 Stopped flow ........................................................................................................... - 41 -
3.1.2 Stopped flow methods ............................................................................................ - 45 -
3.1.3 Stopped flow validation .......................................................................................... - 46 -
3.2 Wetted wall column technique ...................................................................................... - 47 -
3.2.1 Wetted wall column ................................................................................................ - 47 -
3.2.2 Wetted wall column methods ................................................................................. - 50 -
3.2.2.1 Gas film mass transfer coefficient ................................................................... - 50 -
3.2.2.2 Liquid physical mass transfer coefficient ........................................................ - 51 -
3.2.2.3 Enhancement factor .......................................................................................... - 52 -
3.2.2.4 Surface renewal model ..................................................................................... - 52 -
3.2.2.5 Pseudo first order reaction constant ................................................................. - 53 -
3.3 Polymer Characterization .............................................................................................. - 54 -
3.4 Materials ........................................................................................................................ - 55 -
Chapter 4 A Thermally Stable Carbonic Anhydrase as a Promoter in Potassium Carbonate
Solvents for Carbon Dioxide Capture ....................................................................................... - 58 -
4.1 Introduction ................................................................................................................... - 58 -
4.2 Results and discussion ................................................................................................... - 59 -
4.2.1 Stopped flow experiments ...................................................................................... - 59 -
XI
4.2.2 Wetted wall column experiments ........................................................................... - 65 -
4.2.3 Comparison of results from stopped flow and wetted wall column ....................... - 68 -
4.3 Conclusions ................................................................................................................... - 69 -
Chapter 5 Reaction Kinetics between Histidine and Carbon Dioxide .................................... - 70 -
5.1 Introduction ................................................................................................................... - 70 -
5.2 Results and discussion ................................................................................................... - 73 -
5.2.1 The reaction contribution of different histidine species ......................................... - 73 -
5.2.2 Determination of corrected reaction pseudo-first-order rate constants (kobs’) ........ - 74 -
5.2.3 Zwitterion mechanism fitting with the experimental data ...................................... - 78 -
5.2.4 Influence of ionic strength on the reaction kinetics ................................................ - 80 -
5.2.5 Comparison of histidine with other amino acids .................................................... - 83 -
5.3 Conclusions ................................................................................................................... - 84 -
Chapter 6 Screening of Amino Acids as Promoters for CO2 Absorption ............................... - 86 -
6.1 Introduction ................................................................................................................... - 86 -
6.2 Results and discussion ................................................................................................... - 87 -
6.2.1 Speciation and reaction kinetics of amino acid salts with CO2 .............................. - 87 -
6.2.2 Promotion performance of amino acid salts in potassium carbonate solvent ......... - 90 -
6.2.3 Effect of pH on the absorption kinetics .................................................................. - 92 -
6.2.4 Comparison of amino acids and monoethanolamine (MEA) as rate promoters for CO2
absorption in potassium carbonate solvent ........................................................................ - 94 -
6.3 Conclusions ................................................................................................................... - 95 -
Chapter 7 A Carbonic Anhydrase Mimicking Polymer for Accelerating Carbon Capture .... - 96 -
7.1 Introduction ................................................................................................................... - 96 -
7.2 Results and discussion ................................................................................................... - 98 -
7.2.1 Synthesis and characterization ................................................................................ - 98 -
7.2.1.1 4-Vinylbenzyl Cyclen ...................................................................................... - 99 -
7.2.1.2 PNiPAm-co-Cyclen ....................................................................................... - 104 -
7.2.1.3 PNiPAm-co-CyclenZn ................................................................................... - 104 -
7.2.1.4 LCST Determination ...................................................................................... - 107 -
7.2.2 Carbon dioxide hydration catalysis efficiency of PNiPAm-co-cyclenZn ............ - 110 -
7.3 Conclusions ................................................................................................................. - 113 -
XII
Chapter 8 Conclusions and Recommendations ..................................................................... - 115 -
8.1 Conclusions ................................................................................................................. - 115 -
8.2 Recommendations ....................................................................................................... - 117 -
References ............................................................................................................................. - 119 -
VII
List of Figures
Figure 1.1. Technologies for carbon dioxide capture ............................................................... - 3 -
Figure 1.2. A solvent absorption process for CO2 capture (picture sourced from CO2CRC) .. - 4 -
Figure 1.3. A typical sorbent adsorption process for capturing CO2 (picture sourced from
CO2CRC) .................................................................................................................................... - 5 -
Figure 1.4. Membrane separation mechanisms (picture sourced from CO2CRC) ................... - 6 -
Figure 1.5. A carbonation process reported by Wang et al. [55] .............................................. - 7 -
Figure 1.6. A simplified diagram of cryogenics distillation ..................................................... - 8 -
Figure 1.7. A flow diagram of a traditional absorption process ............................................... - 9 -
Figure 2.1. Structural formula of arsenite ions ....................................................................... - 13 -
Figure 2.2. Simplified equilibrium diagram for borate speciation [18, 112, 113] .................. - 14 -
Figure 2.3. Mechanism for borate-catalysed hydration of CO2 proposed by Guo et al. [101]- 15 -
Figure 2.4. Simplified equilibrium diagram for vanadium (V) speciation in basic to pH neutral
water [96] .................................................................................................................................. - 17 -
Figure 2.5. Kinetics research results for the reaction CO2-MEA ............................................ - 20 -
Figure 2.6. Promoting mechanism of MEA for CO2 absorption in potassium carbonate solutions
................................................................................................................................................... - 22 -
Figure 2.7. Structures of piperazine in aqueous solutions ...................................................... - 26 -
Figure 2.8. Simplified equilibrium diagram for PZ speciation ............................................... - 26 -
Figure 2.9. Different ligands used for mimicking carbonic anhydrase ................................... - 33 -
Figure 2.10. Pilot plant results with different promoters compared with unpromoted solvent[85]
(A: 35 wt. % K2CO3 with L/G of 4; B: 35 wt. % K2CO3 with 3% boric acid; C: 36 wt. % K2CO3
with 9% glycine with L/G of 3; D: 40 wt. % K2CO3 with 10% glycine with L/G of 5; E: 41 wt. %
K2CO3 with 9.1% glycine with L/G of 4) ................................................................................. - 35 -
Figure 2.11. Intermediate formulas of different promoters [101, 152, 159, 175, 180] ........... - 37 -
Figure 3.1. Schematic of stopped-flow technique[194] .......................................................... - 42 -
Figure 3.2. SX.17 MV flow diagrams ..................................................................................... - 43 -
Figure 3.3. Experimental absorbance versus time for CO2 hydration at different CO2
concentrations (wavelength of 400 nm, temperature of 303 K and pH of 7.5) ........................ - 46 -
VIII
Figure 3.4. The structure diagram of the wetted wall column used in the study .................... - 48 -
Figure 4.1. Turnover numbers (a) and Michaelis-Menten constants (b) for NZCA (298‒328 K) -
61 -
Figure 4.2. Catalysis scheme of CO2 hydration by carbonic anhydrase[99, 217] .................. - 62 -
Figure 4.3. Effect of pH on the NZCA activity at 298 K ........................................................ - 64 -
Figure 4.4. Effect of CO2 loading and ionic strength on the catalysis efficiency of the NZCA at
298 K ......................................................................................................................................... - 65 -
Figure 4.5. Promotion effect of the NZCA for CO2 absorption with 30 wt. % K2CO3 solvents
(0.04‒0.32 loading) at 323 K using WWC ............................................................................... - 66 -
Figure 4.6. Thermal stability of the NZCA in 30 wt. % K2CO3 (0.1 loading) at 323 K ......... - 68 -
Figure 5.1. Transformation among different forms of histidine ............................................. - 71 -
Figure 5.2. Distribution of Histidine species under different acidity at 298 K[230] .............. - 71 -
Figure 5.3 Distribution of histidine formations at different temperatures (a: 298 K, b: 303 K, c:
308 K, d: 313 K) ....................................................................................................................... - 73 -
Figure 5.4. Corrected pseudo first order reaction rate constants at different pH and temperatures
................................................................................................................................................... - 74 -
Figure 5.5. Corrected pseudo first order reaction rate constant between CO2 and His‒ at the
temperatures of 298‒313 K ....................................................................................................... - 75 -
Figure 5.6. Double log coordinate plot of observed pseudo-first-order rate constants versus the
concentration of His- ................................................................................................................. - 76 -
Figure 5.7. Determination of reaction constant to His‒ at different temperatures .................. - 77 -
Figure 5.8. Arrhenius plot of the reaction of His‒ with CO2 ................................................... - 78 -
Figure 5.9. Zwitterion mechanism fitting of the reaction between CO2 and His- ................... - 79 -
Figure 5.10. Comparison of extrapolating results in this study with experimental results by Shen
et al.[229] at high histidine concentrations ............................................................................... - 81 -
Figure 5.11. Comparison of extrapolating results from this study with WWC results from
literature[229] using a b value of 0.44 representing the ionic strength impact ......................... - 82 -
Figure 5.12. Comparison of results extrapolated from stopped flow experiments using b=0.67 at
298 K, b=0.65 at 303 K, b=0.46 at 313 K to correct for ionic strength with experimental WWC
results by Shen et al.[229] at high histidine concentrations...................................................... - 83 -
IX
Figure 5.13. Comparison of kinetics results between amino acids and CO2 at low ionic strength
(<0.05 M) .................................................................................................................................. - 84 -
Figure 6.1. Transformation of different species of amino acid salts with pH ......................... - 88 -
Figure 6.2. Distribution of valine ionic species at various pH values .................................... - 88 -
Figure 6.3. Reaction rate between CO2 and amino acid salt solutions (~5 mM) at neutral (7.3±0.2)
and basic pH values (around pKa values, lysine: pH~pKa1, lysine*: pH~pKa2) at 298 K ........ - 89 -
Figure 6.4. Pseudo first order reaction constants between different amino acid anions and CO2:
Lysine# is the lysine species with negative two valency while all other amino acids have negative
one valency (the results for glycine agree with previous research[193], while the data for histidine
was extracted from our previous research[242]) ...................................................................... - 90 -
Figure 6.5. Enhancement factors using 30 wt. % potassium carbonate solvents with and without
amino acid salts (0.5 M) in a WWC at pH of 12.5 and temperature of 323 K ......................... - 91 -
Figure 6.6. Enhancement factors using 30 wt. % potassium carbonate solvents with and without
amino acid salts (0.5 M) in a WWC over a range of pH values at 323 K ................................ - 93 -
Figure 6.7. Enhancement factors using proline, sarcosine and MEA (0.5 M) as promoters in 30
wt% potassium carbonate solvent. Results were obtained using a WWC over a range of pH values
at 323 K ..................................................................................................................................... - 94 -
Figure 7.1. Proposed mechanism for the hydration of CO2 by carbonic anhydrase.[243] ..... - 97 -
Figure 7.2. Synthesis of the cyclenZn pendant PNiPAm and the small molecule of cyclenZn- 99
-
Figure 7.3. 1H NMR of 4-vinylbenzyl cyclen ....................................................................... - 101 -
Figure 7.4. 13C NMR of 4-vinylbenzyl cyclen ...................................................................... - 102 -
Figure 7.5. Electrospray ionization-mass spectrometry (ESI-MS) of 4-vinylbenzyl cyclen - 103 -
Figure 7.6. 1H NMR spectra of PNiPAm-co-cyclen and PNiPAm-co-cyclenZn. The proton
signals from the cyclen moieties are enlarged ........................................................................ - 105 -
Figure 7.7. SEC diagram of PNiPAm-co-Cyclen ................................................................. - 106 -
Figure 7.8. ICP-OES measurement at the wavelengths of 202.548 and 206.200 nm with four
standard solutions of 0, 4, 10, 20 ppm (9.34 mg of PNiPAm-co-CyclenZn dissolved in 10 ml
solution). ................................................................................................................................. - 107 -
X
Figure 7.9. (a) LCST study of the PNiPAm-co-cyclenZn in water (10 mg/ml). (b) Variable
temperature 1H NMR of PNiPAm-co-cyclenZn in D2O. As the temperature increases, the polymer
separated from the solution as evidenced by the loss of signal. ............................................. - 109 -
Figure 7.10. Arrhenius fitting of Michaelis-Menten catalysis coefficients of the PNiPAm-co-
CyclenZn ................................................................................................................................. - 111 -
Figure 7.11. Activity of the PNiPAm-co-CyclenZn catalyst for CO2 hydration reaction showing
the thermal stability and recyclability. Each cycle represents a catalytic assay after heating the
polymer catalyst to 328 K and then cooling and repeating the kinetic assay at 298 K. No measurable
decrease in activity of the polymer catalyst was observed. .................................................... - 113 -
XI
List of Tables
Table 2.1. Some amine promoters in potassium carbonate solutions ..................................... - 18 -
Table 2.2. Kinetic research about CO2 absorption in MEA aqueous solutions ...................... - 21 -
Table 2.3. Promoting performance of amines in potassium carbonate solutions ................... - 24 -
Table 2.4. Promotion performances of different AAS under different conditions ................. - 29 -
Table 2.5. Promotion performances of carbonic anhydrase under different operating conditions -
30 -
Table 2.6. Comparison of different inorganic promoters........................................................ - 38 -
Table 2.7. Kinetics data of different promoters from the literature ........................................ - 39 -
Table 3.1. Buffers and indicators used in this study ............................................................... - 45 -
Table 3.2. Information on reagents used in this work ............................................................. - 56 -
Table 4.1. Catalysis coefficient of the NZCA at different pH values ..................................... - 60 -
Table 4.2. Comparison of carbonic anhydrase catalytic coefficients for CO2 hydration ........ - 63 -
Table 5.1 Thermodynamic properties of histidine .................................................................. - 72 -
Table 5.2. Reaction rate constants with respect to His‒ at different temperatures .................. - 77 -
Table 6.1. pKa values of amino acid salts at 298 K in diluted solutions ................................. - 87 -
Table 7.1. Comparison of catalysis coefficients for PNiPAm-co-CyclenZn and other carbonic
anhydrase mimics.................................................................................................................... - 112 -
i
Nomenclature
General notations
a
Regression constant
A
Central column part of the WWC
Aa m2 Contact area
b mm Optical pathlength
B
Absorption chamber of the WWC
c
Regression constant
C
Bathing chamber of the WWC
d m Diameter
DCO2 m2 s–1 Diffusivity of CO2
E
Enhancement factor
Ea kJ mol‒1 Activation energy
f
Regression constant
g m s‒2 Gravity constant
G m3 s‒1 Gas volumetric flow rate
h m Height of WWC
HCO2 Pa m3 mol‒1 Henry constant
k
Reaction constant
kB
Rate constant of deprotonation reaction
kB-x
Rate constant for deprotonation by x
kcat s‒1 Turn over number
kcat/Km M‒1 s‒1 Catalysis efficiency
kg
mol Pa‒1 m‒2 s‒
1 gas mass transfer coefficient
KG
mol Pa‒1 m‒2 s‒
1 Overall mass transfer coefficient
klo m s‒1 Liquid phase physical mass transfer coefficient
ii
Km M Michaelis-Menten constant
kobs s‒1 Observed first order reaction rate constant
kobs' s‒1 Corrected observed first order reaction rate constant
L m Length of wetted wall column
NCO2 mol m‒2 s‒1 Absorption flux
P*CO2, b Pa CO2 equilibrium partial pressure
PCO2, b Pa CO2 partial pressure in the gas phase
Q
Buffer factor
Ql m3 s‒1 Liquid flowrate
R J mol‒1K‒1 Gas constant
r m s‒1 Reaction rate
Re
Reynolds number
Sc
Schmidt number
Sh
Sherwood number
T K Temperature
t s Reaction time
Tr K Reference temperature
v m s‒1 Linear velocity of the gas
V̇ m3 s‒1
Volumetric flowrate of the
liquid
W m Circumference of the column
α
Molar fraction
Γ kg m‒1 s‒1 Mass rate of flow per unit width
δ m Thickness of a layer
ε M‒1 cm‒1 Extinction factor
μg Pa s‒1 Gas viscosity
ρg kg m‒3 Gas density
ρl kg m‒3 Liquid density
τ s Surface contact time
𝛩
A dimensionless driving force
iii
[CO2] M Concentration of CO2
[H2O] M Concentration of H2O
[MEA] M Concentration of MEA
∆rCpo kJ mol‒1 K‒1 Heat capacity changes
∆rGo kJ mol‒1 Standard molar Gibbs energy
∆rHo kJ mol‒1 Standard molar enthalpy
Abbreviations
AAS
Amino acid salt
Abs
Absorbance
CA Carbonic anhydrase
DEA Diethanolamine
His Histidine
LCST Lower critical solution temperature
MEA Monoethanolamine
NG Not given
NZCA Novozymes carbonic anhydrase
PZ Piperazine
WWC Wetted wall column
- 1 -
Chapter 1 Background
The emission of carbon dioxide into the atmosphere is recognized as a significant driver for
climate change. Carbon capture and storage (CCS) techniques are efficient and effective ways to
reduce carbon dioxide emissions to the atmosphere. However, the cost of any carbon capture
technique has to be reduced to manageable levels before it can be deployed at an industrial scale[1].
1.1 Reducing carbon dioxide from the atmosphere
The total amount of carbon on earth is relatively constant and its distribution among the
lithosphere, the atmosphere and the biosphere remained relatively constant until the industrial
era[2]. The subsequent emission of CO2 to the atmosphere from human activities is recognized as
the main reason for climate change including global warming, changes in sea levels, extreme hot
summers and cold winter, and agricultural problems[3-7]. With an exponentially increasing global
population, there are many basic human needs such as food, water and energy to be met, which
may result in higher quantities of carbon dioxide being emitted into the atmosphere[8]. Therefore,
there has been an increasing focus on the development of new energy resources as well as cleaner
and more efficient energy systems to reduce overall carbon dioxide emissions[5], though there is
still a long way to go before emission targets are met. Fossil fuels, coals and natural gas will remain
as the main sources of energy in the near future as they remain cheap and abundant while also
experiencing much security and stability in their utilization systems[9]. It is inevitable that the
emissions of carbon dioxide to the atmosphere will continue, which will lead to even more climate
change effects. Therefore, we must devise economical, stable, environmentally friendly ways to
reduce these effects.
- 2 -
1.2 Techniques for carbon dioxide reduction
Many methods for the reduction of CO2 atmospheric levels or CO2 emissions have been
investigated, such as re-forestation, ocean fertilization and CO2 mineral carbonation[10-14]. These
processes are able to simultaneously capture and sequester CO2 simultaneously at a low energy
cost. However, these processes alone are not efficient enough to significantly reduce the quantity
of CO2 being emitted to limit climate change. Carbon dioxide capture and storage (CCS)[15] is an
efficient way to reduce carbon dioxide emissions into the atmosphere. However, there is a high
capture cost[16, 17] associated with these capture options and appropriate storage is also required
for preventing the captured CO2 from entering the atmosphere[9]. Carbon capture from
combustion processes can be classified into three configurations depending on at which stage the
CO2 is being captured: pre-combustion, oxyfuel combustion and post-combustion. However, the
capture technologies (Figure 1.1) are similar for all these configurations and include absorption,
adsorption, membrane, cryogenic separation (a single process of capture and compression),
mineralization and a combination of these techniques[18-25].
- 3 -
Figure 1.1. Technologies for carbon dioxide capture
1.2.1 Solvent absorption for capturing carbon dioxide
The absorption of carbon dioxide into an aqueous solvent has been investigated for decades, and
was first used for purifying gases, such as hydrogen gas, natural gas and synthesis gas[26], and
more recently for reducing CO2 emissions[19]. A typical solvent absorption process is presented
in Figure 1.2, in which a mixture gas flows through an absorber and CO2 is captured in the solvent,
then the CO2 loaded solvent is heated to regenerate the solvents and obtain pure CO2. Many
solvents have been investigated for their efficiencies in the absorption of CO2, including
monoethanolamine (MEA), diamines and ternary amines[27], piperazine and its derivatives[28-
30], ammonia[31], amino acid salts[32], ionic liquids[33, 34], and their blends[30, 35-37]. MEA
is the most widely used solvent. However, the use of MEA brings about some disadvantages such
as a high energy penalty for solvent regeneration, its high degradation rate and corrosivity [38-40].
Some research has been conducted from the perspective of reducing energy costs, such as utilising
- 4 -
solar energy in power plants to supplement the total energy requirements[41]. However, much
time is needed for scaling up this technique to the industrial level. In addition, the drawbacks of
solvent degradation and corrosion have to be addressed as well.
Figure 1.2. A solvent absorption process for CO2 capture (picture sourced from CO2CRC)
1.2.2 Sorbent adsorption for capturing carbon dioxide
In an sorbent adsorption process, CO2 is adsorbed from a process stream using solid materials
(Figure 1.3) and then released via a thermal swing (TSA) or pressure swing (PSA)[42]. Both
physical adsorption and chemical adsorption can be used for CO2 capture and a range of sorbent
materials have been investigated such as activated carbon[43], amine sorbents[44, 45], metal
oxides[46], metal-organic frameworks (MOFs)[47] and carbonates[48].
- 5 -
Figure 1.3. A typical sorbent adsorption process for capturing CO2 (picture sourced from
CO2CRC)
1.2.3 Membranes for capturing carbon dioxide
Membrane separation is a technology that can selectively sieve different components from a
mixture of gases and liquids via thin film materials. Membrane materials can be organic (ex.
cellulose acetate, polysulfone and polyimide), inorganic (ex. ceramic and metallic membranes) or
a mixture of both (ex. metal-organic framework (MOF) supported polymeric membranes)[49-51].
In a CO2 separation process, the major driving force is usually CO2 partial pressure (i.e. CO2
concentration). Permeability and selectivity are both important parameters in membrane
- 6 -
technology. The separation mechanisms of gas molecules through a membrane can be categorised
into five models: Knudsen diffusion, molecular sieving, solution-diffusion model (Figure 1.4),
surface diffusion and capillary condensation[52].
Figure 1.4. Membrane separation mechanisms (picture sourced from CO2CRC)
1.2.4 Mineral carbonation for capturing carbon dioxide
Mineral carbonation is a method for capturing and storing CO2 at the same time. Figure 1.5
showed a research using calcium oxide for capturing carbon dioxide to form calcium carbonate.
Mineral carbonation can also provide a pathway for capturing CO2 and releasing the captured CO2
with a high temperature regeneration process in a process known as chemical looping[53]. The
mineral materials for capturing CO2 can be mineral wastes (ex. metallurgy wastes) and metal
oxides (ex. CaO, MgO or a mixture of both)[11, 54]. The major barrier for this technology is the
slow reaction kinetics. However, there has been recent research on enhancing its kinetics via a
carbonic anhydrase enzyme in aqueous solution[55].
- 7 -
Figure 1.5. A carbonation process reported by Wang et al. [55]
1.2.5 Cryogenics distillation
Cryogenics (low temperature distillation) is a method to condense CO2 under low temperature to
produce concentrated liquid CO2 for transport and storage, and the other gases (mainly N2) flow
through to the atmosphere[23, 56]. The advantage of cryogenics distillation is that the CO2 can be
captured and compressed in one step.
- 8 -
Figure 1.6. A simplified diagram of cryogenics distillation
1.2.6 Others
There are some other technologies reported for carbon capture including algae cultivation[57] or
a combination of the different technologies mentioned above[58]. However, more efforts are
needed to make these technologies competitive with thoes mentioned above (1.2.1‒1.2.5).
1.3 Potassium carbonate solvent absorption system for carbon dioxide capture
Potassium carbonate (potash solution) is a good solvent for carbon dioxide capture because of its
low regeneration energy, low degradation and low corrosivity. It was first developed to absorb
carbon dioxide as an impurity from synthesis gas, natural gas, hydrogen gas in a process known
as the “Hot Potassium Carbonate (Benfield) Process”[59, 60].
- 9 -
8 7
1
23
4
5
6
Figure 1.7. A flow diagram of a traditional absorption process
(1–Flue gas, 2–Absorber, 3–Reducing valve, 4–Flashing vessel, 5–Desorber, 6–Closed steam coil,
7, 8–Condenser)
The potassium carbonate solution was widely used in later research on CO2 absorption[61] and a
traditional absorption process is shown in Figure 1.7[60]. The main parts are the absorber and
desorber. The flue gas is fed into the absorber counter-currently to the solvent for absorption. The
loaded solvent is then sent into a desorber, where CO2 is stripped from the solvent by increasing
the temperature and/or decreasing the pressure of the desorber. This desorbed CO2 will then be
compressed and liquefied for storage or utilization, and the regenerated solvent can be channelled
back to the absorber for reuse in the absorption process.
- 10 -
1.4 Current challenges with the potassium carbonate absorption process
Corrosion is an important problem caused by the acidic nature of the flue gas and from the
degradation of solvent. This problem can be mitigated to some degree by adding corrosion
inhibitors into solvents. Potassium dichromate[62], vanadium (V)[63], EDTA[64], CuCO3[63] and
so on have been reported to act as corrosion inhibitors in different equipment.
Another challenge is the degradation of solvents by forming heat stable salts such as potassium
sulphate. Due to the low solubility of these heat stable salts, precipitation of the weakly soluble
potassium sulphate can be employed to minimize the effect of these impurities when H2S and SOx
are present in the flue gas[63].
The precipitation of potassium bicarbonate was also considered as a problem for leading to pipe
blockages in the system. However, a novel precipitation technique for the absorption of CO2 with
highly concentrated potash solution was proposed by Mumford et al.[65]. The crystallization of
the solvent should be investigated in detail to manage possible problems caused by solids.
The major shortcoming of the potassium carbonate absorption system is its slow reaction kinetics
in comparison to the commonly used MEA absorption system. Research has indicated that physical
mass transfer can be enhanced by the chemical reactions (Reactions 1.5.1–1.5.4) when the
absorption temperature is higher than 318 K, but that the chemical reactions are not fast enough to
be instantaneous even at a temperature of 378 K[66, 67], indicating that largely improving
absorption kinetics cannot be obtained by solely increasing temperature.
𝐶𝑂2(𝑔) ⇌ 𝐶𝑂2(𝑎𝑞)
𝐶𝑂2(𝑎𝑞) + 𝑂𝐻−(𝑎𝑞) ⇌ 𝐻𝐶𝑂3
− (𝑎𝑞) (𝐹𝑎𝑠𝑡) 1.5.1
𝐻𝐶𝑂3−(𝑎𝑞) + 𝑂𝐻−(𝑎𝑞) ⇌ 𝐶𝑂3
2−(𝑎𝑞) + 𝐻2𝑂(𝑎𝑞) (𝐼𝑛𝑠𝑡𝑎𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠) 1.5.2
𝐶𝑂2(𝑔) + 𝐻2𝑂(𝑎𝑞) ⇌ 𝐻2𝐶𝑂3(𝑎𝑞) (𝑆𝑙𝑜𝑤) 1.5.3
- 11 -
𝐻2𝐶𝑂3(𝑎𝑞) + 𝑂𝐻−(𝑎𝑞) ⇌ 𝐻𝐶𝑂3
−(𝑎𝑞) + 𝐻2𝑂(𝑎𝑞) (𝐼𝑛𝑠𝑡𝑎𝑛𝑡𝑎𝑛𝑒𝑜𝑢𝑠) 1.5.4
As can be seen from the reaction regime, reaction 1.5.1 is fast but not fast enough to be treated
as instantaneous. When the pH of the solvent is greater than 9.0, Reaction 1.5.3 is negligible in
comparison with Reaction 1.5.1[67-69], hence the rate limiting step of the absorption process is
Reaction 1.5.1. Since Reaction 1.5.1 is not fast enough, the absorption kinetics is slow. Therefore,
a tall absorber is needed to get high absorption efficiency, leading to a very high capital investment
and operation penalty. Adding rate promoters and improving the mass transfer efficiency in the
absorption column[70, 71] are recognised as good options for improving the slow kinetics.
1.5 Aim of this study
The aim of this project is to accelerate the carbon dioxide absorption rate using potassium
carbonate solvent via exploring different rate promoters. By doing this, the scale of carbon capture
absorption columns can be potentially decreased and the overall costs for carbon capture can be
minimized.
- 12 -
Chapter 2 Literature Review
A promoter can be classified into one of the three classes: inorganic, organic or enzymatic
promoters. Much research has been conducted on the addition of promoters into potassium
carbonate solutions for carbon dioxide absorption, such as hypochlorite[72], [73], bromine and
hypobromite[74], sulphite, selenite and tellurate[75]. However, the most widely studied promoters
in potassium carbonate solutions have been amines such as monoethanolamine (MEA),
diethanolamine (DEA) and piperazine (PZ) [26, 76-81], amino acids [82-86], arsenious acid [87-
90], boric acid [91-93], vanadates [94-96] and carbonic anhydrase [97-101].
2.1 Inorganic Promoters
2.1.1 Arsenite
Arsenious acid (H3AsO3) is one of the best promoters for hydration of CO2 and the promotion of
CO2 absorption in potassium carbonate solutions. Arsenious acid has been researched widely
because of its excellent promoting performance, high stability, favourable ionization constant, high
solubility, availability and low cost [102-107]. It was used in industrial absorption of CO2 as a
promoter in potassium carbonate solutions more than 50 years ago [88] and was also used as a
promoter in amine and sodium carbonate-bicarbonate solutions [87-89]. A packed column for CO2
absorption using arsenite promoted potassium carbonate solution was designed by Kumar in 1989
[90]. The arsenite ion (Figure 2.1) is recognized as the promoting species as it has a single lone
pair of electrons, which can serve to neutralize the Lewis acidity of CO2, and it also has a pyramidal
structure similar to NH3, which allows for direct and facile interaction between CO2 and the base
to form a CO2·base complex [67]. Almost all research performed has shown that arsenite is a good
catalyst for CO2 hydration. However, as it is toxic and carcinogenic [83, 108, 109], it is no longer
used as a promoter in commercial applications.
- 13 -
Figure 2.1. Structural formula of arsenite ions
2.1.2 Boric acid
Boric acid has been studied extensively in the laboratory as a promoter for CO2 absorption in
potassium carbonate solutions as it is environmentally benign, economically affordable and
tolerant to oxidative and thermal conditions. It also has no significant influence on the vapour
liquid equilibria (VLE) of CO2 at low concentrations and no interaction with other minor
components such as sulphur oxide in the flue gas [18, 69, 91-93, 109, 110].
In aqueous solutions, the speciation of boric acid is influenced by pH. At low concentrations,
B(OH)4‒ is dominant in basic solutions (pH˃9.3) [18, 93, 101, 111]. However, when the
concentration of boric acid is higher than 0.025 M, polyborates will form (Figure 2.2) and the
concentration of B(OH)4‒ may be restricted [18]. This is also shown in the study by Ahmadi et al.
[91], in which the CO2 absorption did not change significantly beyond a certain boric acid
concentration.
- 14 -
Figure 2.2. Simplified equilibrium diagram for borate speciation [18, 112, 113]
The promoting mechanism of boric acid was first proposed by Guo et al. (Figure 2.3) [101], in
which the boric acid-water complexes deprotonate to form the active species B(OH)4‒ (Equation
2.1.1, Step ①, Figure 2.3), then B(OH)4‒ reacts with CO2 to form an intermediate B(OH)3·HCO3
‒
(Equation 2.1.2, Step ②, Figure 2.3), and the HCO3‒ in the intermediate is replaced with water
forming HCO3‒ and regenerating the promoter (Equation 2.1.3, Step ③, Figure 2.3).
𝐵(𝑂𝐻)3 ∙ 𝐻2𝑂 → 𝐵(𝑂𝐻)4− + 𝐻+ 2.1.1
𝐵(𝑂𝐻)4− + 𝐶𝑂2 → 𝐵(𝑂𝐻)4𝐶𝑂2
− 2.1.2
𝐵(𝑂𝐻)4𝐶𝑂2− +𝐻2𝑂 → 𝐵(𝑂𝐻)3 ∙ 𝐻2𝑂 + 𝐻𝐶𝑂3
− 2.1.3
- 15 -
Figure 2.3. Mechanism for borate-catalysed hydration of CO2 proposed by Guo et al. [101]
However, when boric acid was used in a pre-combustion pilot plant demonstration in 2012 by
Smith et al. [114], the promoting performance was not evident. This was attributed to the decrease
of pH when adding boric acid to the potassium carbonate solution. It was reported [114] that the
pH value of potassium carbonate solution with a loading of 0 decreased from 12.3 to 10.9 upon an
addition of 3 wt.% boric acid. Therefore, the rate of Reaction 1.5.2 could counteract the
improvements provided by boric acid when the loading is high. However, the reduction in pH due
to the addition of boric acid can be overcome by the changing the addition of boric acid to its salts.
Another possible reason is the CO2 solubility decrease in carbonate solutions due to the addition
of boric acid[93], which will reduce the driving force for CO2 absorption, and therefore, influence
the promoting performance of boric acid.
- 16 -
2.1.3 Vanadate
Vanadium (V) compounds were initially used as additives in amine systems to solve corrosion
problems [115-117]. It was later found that vanadate also has a promoting effect on CO2 hydration
[94] and was used as a promoter together with sodium or potassium borate in potassium carbonate
solutions [110]. However, the addition of vanadate may reduce the CO2 solubility in potassium
carbonate solution [95]. Recently, it was found that the promoting species of vanadium were
HVO42‒
and HV2O73‒
(Figure 2.4) and the catalysing performance of both active species was
comparable to MEA at low concentration [96]. The catalysis performance of HVO42‒
was more
efficient than arsenites including HAsO32‒
and H3AsO3 [108]. However, as vanadium species are
comparatively sensitive to pH values and vanadium concentrations. The concentrations of HVO42‒
and HV2O73‒
decrease with a decrease in pH (due to CO2 being absorbed). Further, polyvanadates
species forming as the total concentration of vanadium increases. This limits the effectiveness of
vanadium (V) as it can only be used in small concentration, thus vanadium is not recognised as a
suitable promoter in industrialized CO2 absorption process. Therefore, vanadium (V) is more
suitable as a corrosion inhibitor rather than a rate promoter.
- 17 -
Figure 2.4. Simplified equilibrium diagram for vanadium (V) speciation in basic to pH neutral
water [96]
2.1.4 Other inorganic promoters
Other inorganic promoters have been investigated for improving CO2 absorption rates in K2CO3
including phosphate and silicate [102], hypochlorite [88, 102], selenite and tellurate [102].
However, they all have various drawbacks including poor promoting performance, instability,
corrosiveness and toxicity.
Phan [108, 109] concluded that all species that feature O‒ or OH groups, or that act as Lewis
bases with CO2 as a Lewis acid, or that have a pyramidal or tetrahedral structure to facilitate the
CO2 molecule approaching the base site, could potentially act as a catalyst. This is a similar
conclusion to that of Dennard and Williams [72] who stated that the oxyanions of promoters should
have a lone pair of electrons and have the ability to act as acceptors for promoting CO2 absorption.
Most inorganic promoters are thermally stable and resistant to degradation, but they may switch
between different speciation at different concentrations, temperatures or pH values. This is the
- 18 -
main reason for some promoters such as boric acid and vanadate not demonstrating good
performance over a range of industrial operating conditions.
2.2 Organic Promoters
2.2.1 Amines
Research on the absorption of CO2 into amine promoted potassium carbonate mixtures was first
conducted in 1967 by Danckwerts and McNeil [105]. Amines can be classified as primary,
secondary and tertiary depending on the number and orientation of carbon atoms bonded to the
amine functional group. Primary and secondary amines are usually used as promoters in potassium
carbonate solutions, while tertiary functional group amines are seldom found as promoters as they
do not have a significant promoting effect [69, 76, 118]. The most widely used amines in the
literature are monoethanolamine (MEA), diethanolamine (DEA) and piperazine (PZ) (Table 2.1).
Table 2.1. Some amine promoters in potassium carbonate solutions
Amine Abbreviation Formula
2-aminoethanol
(monoethanolamine)
MEA
2,2'-iminodiethanol
(diethanolamine)
DEA
Piperazine
PZ
2.2.1.1 Monoethanolamine
Monoethanolamine (MEA) has been the main solvent used for CO2 absorption for many years as
it has fast absorption rate, high absorption capacity and good selectivity for CO2 [118, 119].
- 19 -
Research investigating the reaction kinetics between CO2 and MEA is summarized in Table 2.2.
In the present case, recent kinetic data has been included and the techniques used to measure
reaction kinetics have been provided. As shown in Figure 2.5 and Table 2.2, the kinetic results [76,
118-127] vary between different investigations. This discrepancy may result from the use of
different physical properties, measurement techniques and reaction regime assumptions.
However, the limitations associated with pure MEA as a solvent has led to the use of mixtures of
potassium carbonate and MEA as an alternative solvent which can simultaneously, overcome the
slow CO2 absorption kinetics of potassium carbonate solutions while minimising the drawbacks
of MEA including evaporation and degradation[26, 120, 128]. MEA promotes the absorption of
CO2 via two pathways. The first pathway is the increase in OH- concentration in the solvent when
MEA is added and the carbamate forms. The second pathway is the zwitterion mechanism pathway
(Figure 2.6), which is the dominant promoting mechanism for MEA.
- 20 -
Figure 2.5. Kinetics research results for the reaction CO2-MEA
According to published data [22, 129-131], MEA has been shown to have good absorption and
promoting performance (Table 2.3). MEA reacts with CO2 in aqueous solution via the zwitterion
mechanism (Figure 2.6) [76, 77], in which MEA first reacts with CO2 to form a zwitterion
intermediate, and then the intermediate reacts with a base to form bicarbonate ions and regenerate
MEA. In this reaction, the zwitterion formation (Step ①, Figure 2.6) is the rate-determining step
as the zwitterion (HO(CH2)2HNH+COO‒, Figure 2.6) is not stable.
- 21 -
Table 2.2. Kinetic research about CO2 absorption in MEA aqueous solutions
CAmine /mol/L Amine Additives Apparatus Reaction order with
respect to amine and CO2
T/K kMEA/ L/(mol·s) Reference
0.0091- 0.06 MEA - Stopped flow method 1 278-303 log𝑒k𝑀𝐸𝐴 = 25.53 −5076 ± 180
𝑇
[127]
- MEA - Stirred cell 1 298-353 log𝑒k𝑀𝐸𝐴 = 24.98 −4775
𝑇
[120]
- MEA - Review 1 ≤ 313K log𝑒𝑘𝑀𝐸𝐴 = 26.81 −5400
𝑇
[118]
0.013 -1.5 MEA - Stopped flow method 1 278-298 log𝑒𝑘𝑀𝐸𝐴 = 27.47 −5617
𝑇
[126]
0 - 2.2 MEA 30 wt.% K2CO3 Wetted wall column 1 316-356 log𝑒𝑘𝑀𝐸𝐴 = 22.17 −3825
𝑇
[76]
0 - 0.4 MEA 1.5, 1.7 M AMP Wetted wall column 1 303-313 log𝑒𝑘𝑀𝐸𝐴 = 36.63 −8534
𝑇
[125]
0.1 - 0.5 MEA 0.5, 1.0 M TEA Wetted wall column 1 303-313 log𝑒𝑘𝑀𝐸𝐴 = 26.43 −5376
𝑇
[124]
- MEA - Stirred cell 1 318-333 log𝑒𝑘𝑀𝐸𝐴 = 29.23 −6354
𝑇
[123]
- MEA - Stirred cell 1 278-353 log𝑒𝑘𝑀𝐸𝐴 = 25.30 −4955
𝑇
[119]
3.0 - 9.0 Loaded
MEA
- Laminar jet 1 293-333 log𝑒𝑘𝑀𝐸𝐴 = 22.25 −4412
𝑇
[122]
1.0 - 5.0 Loaded
MEA
- Wetted wall column 1 298-343 *{log𝑒𝑘𝑀𝐸𝐴
𝑇 = 23.64 −4112
𝑇
log𝑒𝑘𝑀𝐸𝐴𝛾
= 23.86 −4742
𝑇
[121]
*Superscript T based on concentration-based model; superscript γ based on activity based model
- 22 -
Figure 2.6. Promoting mechanism of MEA for CO2 absorption in potassium carbonate solutions
The promoting reaction rate can be determined by Equation 2.2.1, where r is the reaction rate,
[CO2], [MEA] and [B] are the concentrations of CO2, MEA and basic species, respectively, kMEA
and k-MEA are the rate constants of the reverse zwitterion formation reaction (Step ①, Figure 2.6),
and kB is the rate constant of deprotonation reaction (Step②, Figure 2.6).
r =kMEA[CO2][𝑀𝐸𝐴]
1+k−MEA∑kB[B]
2.2.1
B can be hydroxyl ions (OH−), MEA itself, and other basic species (such as H2O, HCO3−, and
CO32− in MEA promoted potassium carbonate solutions), by which the reactions are as follows
(Equation 2.2.2‒2.2.6), where 𝑘𝐵−𝑥 is the rate constant for deprotonation by x (H2O, MEA, HCO3−,
OH− and CO32−) and is a characteristic constant of solvent.
HO(CH2)2HNH+COO− + H2O
kB−H2O→ HO(CH2)2HNCOO
− + H3+O 2.2.2
HO(CH2)2HNH+COO− + CO3
2−kB−CO3
2−
→ HO(CH2)2HNCOO− + HCO3
− 2.2.3
HO(CH2)2HNH+COO− + HCO3
−kB−HCO3
−
→ HO(CH2)2HNCOO− + H2CO3 2.2.4
HO(CH2)2HNH+COO− + OH−
kB−OH−→ HO(CH2)2HNCOO
− + H2O 2.2.5
- 23 -
AmH+COO− + AmHkB−MEA→ HO(CH2)2HNCOO
− + HO(CH2)2HNH2− 2.2.6
Therefore, the reaction rate can be written as Equation 2.2.7.
r1 =kMEA[CO2][MEA]
1+k−Am1
kB−H2O[H2O]+kB−CO3
2−[CO32−]+kB−HCO3
−[HCO3−]+kB−OH−[OH
−]+kB−MEA[𝑀𝐸𝐴]
2.2.7
As the zwitterion (HO(CH2)2HNH+COO−, Figure 2.6) is not stable, the zwitterionic formation is
the rate limiting step, 1 ≫𝑘−𝑀𝐸𝐴
𝑘𝐵[𝐵], thus, the overall reaction order dependency on amine and CO2
is unity (Equation 2.2.8, n=1)[118]. However, Dugas[132] has reported a second order
dependency on MEA (Equation 2.2.8, n=2) at greater amine concentration (7 M), in which 1 ≪
𝑘−𝑀𝐸𝐴
𝑘𝐵[𝐵] and the deprotonation step can be the rate limiting step.
r1 = kMEA[CO2][MEA]𝑛 2.2.8
- 24 -
Table 2.3. Promoting performance of amines in potassium carbonate solutions
Amines K2CO3
con. wt.%
Amine
con. wt.%
Temperature
K
Acceleration* Reference
MEA 1.8 0.5 291 0.2 [26]
MEA 30 5 336 15 [76]
MEA 30 10 336 45 [76]
MEA 25 5 294 6 [78, 131]
DEA 25 5 294 2.6 [78, 131]
DEA NG NG 363 4-5 [133]
DEA 20 2 353 1.6 [134]
DEA 25 2 323-363 ~3 [135]
DEA 25 5 323-363 ~6 [135]
PZ 20 5 333 10 [64, 136]
MDEA 25 5 294 1 [78, 131]
*𝐴𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 =𝐶𝑂2 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 𝑖𝑛 𝑝𝑟𝑜𝑚𝑜𝑡𝑒𝑑 𝐾2𝐶𝑂3 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑠
𝐶𝑂2 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 𝑖𝑛 𝑢𝑛𝑝𝑟𝑜𝑚𝑜𝑡𝑒𝑑 𝐾2𝐶𝑂3 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑠 𝑎𝑡 𝑠𝑎𝑚𝑒 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑠− 1
NG–Not given
Amine promoted potassium carbonate solutions are the most mature promoted systems for CO2
absorption. They have been used in industrial plants for CO2 scrubbing of synthesis gas and pilot
plants for CO2 capture from power plant flue gases [78, 137]. However, the addition of MEA
increases the regeneration energy requirement for solvent regeneration and the degradation and
corrosion problems still exist, thus, a search for alternative promoters is still required.
- 25 -
2.2.1.2 Diethanolamine
Diethanolamine (DEA) has also been widely studied as a promoter for CO2 absorption in
potassium carbonate solutions [62, 77, 115, 120, 135, 138].
The kinetic data was well reviewed by Blauwhoff et al. [119] and Versteeg et al. [118], and is
summarized in Table 2.3. As shown in Table 2.3, the CO2 absorption rate into potassium carbonate
solutions can be enhanced by adding a small amount of DEA (2 wt.% to 5wt.%). Recently, a DEA
promoted potassium carbonate solution was used for post-combustion CO2 capture in a tray
column and showed good absorption performance [139].
The promoting pathways of DEA are often assumed to be similar to MEA with a reaction order
of one with respect to CO2. However, the reaction order in terms of DEA is not always the same
and research results showed that the reaction order can range between one and two under different
conditions (OH− and DEA concentrations) [118, 119]. The zwitterion mechanism is also widely
used to interpret the kinetics data [140]. However, some researchers have proposed a termolecular
mechanism for the interpretation of the kinetic data [141], in which the intermediate is not a
zwitterion, but a loosely-bound complex [142].
2.2.1.3 Piperazine
Piperazine (PZ) has recently become very popular as a promoter due to its low vapour pressure,
good promoting performance, low degradation and low corrosivity [136, 143-150]. It is reported
to have better promoting performance than DEA [151] and be comparable to or even better than
that of MEA [151].
Piperazine in aqueous solutions can have several forms (Figure 2.7 and Figure 2.8) depending on
the pH value and CO2 concentration. These forms are protonated PZ (PZH+), diprotonated PZ
(H+PZH+), PZ carbamate (PZCOO–), protonated PZ carbamate (H+PZCOO–) and PZ dicarbamate
- 26 -
(PZ(COO–)2) [152], of which the deprotonated PZ exists when pH value is below 5.5 and should
not exist in basic potassium carbonate solutions, and the protonated PZ is the most active species
for reacting CO2 and forming PZ carbamate. When the loading is high, the protonated PZ
carbamate will absorb CO2 and form PZ dicarbamate, so that every mole of solvent can react with
two moles of CO2.
Figure 2.7. Structures of piperazine in aqueous solutions
Figure 2.8. Simplified equilibrium diagram for PZ speciation
According to pilot plant test results reported in the literature [80, 153], 3.6M K+/1.8M PZ and 5M
K+/2.5M PZ are thought to be good solvents for CO2 absorption as their absorption rates are
comparable to that of 5M and 7 M MEA, respectively. In addition, the combined utilization of
potassium carbonate and PZ can reduce the enthalpy by 20% compared with PZ solutions at 40–
70 oC [145]. However, as the solubility of PZ is limited, problems associated with precipitation of
- 27 -
PZ and other salts have been observed [146]. In addition, economic estimates [148, 149] are needed
for future scale up of the PZ promoted potassium carbonate solution.
2.2.1.4 Other amines
Many other amine promoters have also been studied such as methyl diethanolamine (MDEA)
[131], diglycolamine (DGA), diisopropanolamine (DIPA) [78], methyl-amino ethanol (MAE) [77],
2-ethylaminoethanol (EAE) [26] as well as some sterically hindered amines [116, 133].
While amines have been studied extensively and many of them show potential as good promoters,
the corrosion, volitivity, toxicity and degradation problems of pure organic amine solutions still
exist, although are somewhat reduced in a promoter situation. Therefore, the search for new
promoters continues.
2.2.2 Amino acid salts
Amino acid salt (AAS) solutions are another class of solvents for CO2 capture. Some such as
glycine, L-alanine and L-proline, have very good properties such as a fast reaction rate, large CO2
solubility, commercial availability, high surface tension, low toxicity and low volatility [154-157].
However, they also have some limitations such as a high regeneration energy penalty due to the
formation of stable carbamates. Therefore, the use of these salts as a promoter in potassium
carbonate solutions is an area of active research [82, 84].
Primary and secondary AAS may have three forms (Equations 2.2.9–2.2.11) in aqueous solutions,
i.e. acidic, zwitterionic and basic [158, 159] . The acidic state does not take part in the reaction
with CO2 [155].
Acid state: R1NHCHR2COOH + H3O+ ⇌ R1NH2
+CHR2COOH + H2O 2.2.9
Zwitterion state: R1NH2+CHR2COOH + H2O ⇌ R1NH2
+CHR2COO− + H3O
+ 2.2.10
Deprotonated zwitterion state: R1NH2+CHR2COO
− + H2O ⇌ R1NHCHR2COO− + H3O
+ 2.2.11
- 28 -
The reaction between CO2 and AAS can be explained by the zwitterion mechanism [154, 158]
similar with MEA (2.2.12–2.2.13).
Zwitterion carbamate formation: R1NHCHR2COO− + CO2 ⇌ O− OCN+(R1)HCHR2COO
−
2.2.12
Deprotonation: O− OCN+(R1)HCHR2COO− + B ⇌ O− OCNR1CHR2COO
− + BH+
2.2.13
Published research on AAS as a promoter is still limited. Results for arginine [82, 83], histidine
[82], glycine [84, 85], sarcosine [84] and proline [84] have been published as promoters for CO2
absorption into potassium carbonate solutions (Table 2.4). Arginine and histidine have been found
to be effective promoters in potassium carbonate solution as they have been reported to increase
both the absorption and desorption rates [82, 83]. In addition, they also decrease CO2 partial
pressure, which means the driving force for CO2 absorption and CO2 solubility would increase
[82]. However, the low solubility of histidine may limit its utilization. Glycine, sarcosine and
proline have also been studied as promoters [84], and shown to have very good promoting
performance, in which glycine is the cheapest. Glycine was used as a promoter in a pilot plant
demonstration and a 6 fold improvement of the CO2 recovery rate was obtained by adding 10 wt. %
glycine into 35 wt. % K2CO3 aqueous solution [85].
- 29 -
Table 2.4. Promotion performances of different AAS under different conditions
AAS K2CO3 con.
wt.%
AAS con.
M
Temperature
K
Acceleration* Reference
Arginine 35 0.077 322 0.44 [82]
35 0.387 322 1.35 [82]
Histidine 35 0.104 322 1.54 [82]
Glycine 30 1.0 333 22 [84]
Sarcosine 30 1.0 333 45 [84]
Proline 30 1.0 333 14 [84]
*𝐴𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 =𝐶𝑂2 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 𝑖𝑛 𝑝𝑟𝑜𝑚𝑜𝑡𝑒𝑑 𝐾2𝐶𝑂3 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑠
𝐶𝑂2 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 𝑖𝑛 𝑢𝑛𝑝𝑟𝑜𝑚𝑜𝑡𝑒𝑑 𝐾2𝐶𝑂3 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑠 𝑎𝑡 𝑠𝑎𝑚𝑒 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑠− 1
Based on the above overview, more research is needed to test the performance of different AAS
such as alanine [156] glutamic acid [160] and taurine [86], especially a comparison of various
amino acids and better understanding their reaction/promotion mechanisms.
2.3 Enzymatic promoters
2.3.1 Carbonic anhydrase
Carbonic anhydrase (CA) has been investigated intensely since it was found to be a catalyst for
the hydration of CO2 in the human body in 1933 [161] . This enzyme can be classified as α, β, γ, δ
or ζ based on its structure and source [99, 162]. Carbonic anhydrase is environmentally friendly,
an efficient catalyst for CO2 hydration and has little influence on the CO2 vapour liquid equilibrium
(VLE) and heat of absorption. Thus, it has attracted researchers to study its promoting performance
in potassium carbonate solutions [97], amine solutions [163] and attached to membranes [20].
- 30 -
As shown in Table 2.5, carbonic anhydrases increase CO2 absorption in potassium carbonate
solution by 6–20 times by adding only 300 mg/L at low temperature (298 K). However, when the
temperature increases to 313–338 K, most forms of carbonic anhydrase will lose their activity [164,
165]. Indeed, the harsh conditions of industrial absorption operation such as high temperatures and
high pH can lead to enzyme denaturation which will negatively influence its promoting
performance [166]. Special measures such as immobilization [98] or the development of thermally
stable enzyme variants [167] need to be undertaken before it can be used as a rate promoter on an
industrial scale.
Table 2.5. Promotion performances of carbonic anhydrase under different operating conditions
CA# con.
mg/L
K2CO3 con.
wt.%
Temperature
K
Acceleration* Reference
300 20 298 8.8–11.3 [168]
300 20 313 5.2–6.4 [168]
300 20 323 3.4–4.0 [168]
300 20 313–333 2–6 [169]
55 30 313 0.3 [166]
300 20-30 298 6–20 [170]
300 20-30 323 2–8 [170]
# The forms of carbonic anhydrases used were different among different researchers
*𝐴𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 =𝐶𝑂2 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 𝑖𝑛 𝑝𝑟𝑜𝑚𝑜𝑡𝑒𝑑 𝐾2𝐶𝑂3 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑠
𝐶𝑂2 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 𝑖𝑛 𝑢𝑛𝑝𝑟𝑜𝑚𝑜𝑡𝑒𝑑 𝐾2𝐶𝑂3 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑠 𝑎𝑡 𝑠𝑎𝑚𝑒 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑠− 1
- 31 -
A number of investigations have been conducted on the immobilization of carbonic anhydrases,
showing that immobilization would increase the stability of carbonic anhydrases but decrease its
activity [98, 99, 171]. Therefore, more work is required to improve the immobilization technology
to maintain its performance.
Recently, a directed evolution method was used to develop high temperature resistant carbonic
anhydrases and results showed that these carbonic anhydrases could endure 360 K in 2.1 M MDEA
solutions at pH˃10 for 6 days without a measurable decrease in activity [167]. However, more
tests are required before it can be deployed in commercialized systems.
The active site of most carbonic anhydrase enzymes consists of a Zn2+ ion coordinated with three
imidazole groups from histidine molecules and a water molecule or hydroxyl group depending on
the pH value [172]. While many different catalysing mechanisms have been proposed [162, 173]
for promoting CO2 hydration in potassium carbonate solutions, it has been widely accepted that
there are three promotion steps: [162, 173, 174] deprotonation of the water molecule to form the
active complex (Equation 2.3.1), CO2 reaction with hydroxide to form an intermediate complex
(Equation 2.3.2) and exchange of HCO3– with H2O to regenerate the enzyme (Equation 2.3.3).
Enzyme·H2O ⇌ H+ + Enzyme·OH– 2.3.1
Enzyme·OH– + CO2 ⇌ Enzyme·HCO3– 2.3.2
Enzyme·HCO3– + H2O ⇌ HCO3
– + Enzyme·H2O 2.3.3
Carbonic anhydrase could be an excellent promoter if ways to overcome the shortcomings can be
found such as improving stability under industrial conditions, limiting decreased activity after
immobilization and considering the problem of reuse.
- 32 -
2.3.2 Metal compounds mimicking carbonic anhydrase
As the structures of carbonic anhydrases are very complex, understanding the structure-reactivity
relationship of carbonic anhydrases can be achieved by mimicking the enzyme using a metal
complex such as Zn (II) [175-178], Co (II) [175, 176, 179], Cu (II) [176, 177], Ni (II) [180], Hg
(II) [175] and Cd (II) [181]. Of these, Zn (II) is the most promising metal according to research
results [175-177]. Recently, zinc complex mimics of carbonic anhydrases have been used as
promoters in solvents to improve the absorption kinetics and overcome the drawbacks of carbonic
anhydrases under typical industrial conditions [165, 172, 182].
Many ligands (Figure 2.9) have been used for synthesising metal compounds that mimic carbonic
anhydrases such as 1,4,7-triazacyclononane [183], 1,5,9-triazacyclododecane [176, 183-185],
1,4,7,10-tetraazacyclododecane [165, 172, 176, 177, 183], 1,4,7,10-tetraazacyclotridecane [183],
Nitrilotris (2-benzimidazolylmethyl-6-sulfonic acid) [186] and Tris (2-
benzimidazolylmethyl)amine [185-187], (Figure 2.9). Of these, 1,4,7,10-tetraazacyclododecane
and Nitrilotris (2-benzimidazolylmethyl-6-sulfonic acid have been shown to have the best
performance for catalysing the hydration of CO2 [172, 185, 186].
- 33 -
Figure 2.9. Different ligands used for mimicking carbonic anhydrase
Research results by Kim et al. [182] showed that different metal salts also had an influence on
the promotion effects. Different zinc salts have been used including Zn(OH)2 [182, 186], ZnCl2
[182, 186], Zn(NO3)2 [175, 182], Zn(OAc)2 [171, 182] and Zn(ClO4)2 [172, 177]. Zn(OH)2 was
found to have the highest promotion effect at the conditions investigated[182].
The promoting mechanism of metal compounds is very similar to that of carbonic anhydrases as
they have similar structures, which also includes deprotonation, CO2 addition and HCO3–
substitution [173].
Even though the performance of carbonic anhydrases mimics are poorer than the native
enzyme[165] and their promoting performances are influenced by a number of factors including
pH [165] and anions [185, 186], the metal compounds provide a direction for promoter
development. Therefore, more research into the synthesis, incorporation of second sphere
- 34 -
interactions [188], promoting performance and immobilization to improve its stability [171] is
expected.
2.4 K2CO3 pilot plant studies with rate promoters
Rate promoted potassium carbonate solvent has been tested by our research group using an
industry based (1 tonne per day CO2) solvent absorption pilot plant and in a laboratory based (0.1
tonne per day CO2) pilot plant. Boric acid (~3 wt. %) in potassium carbonate was tested under pre-
combustion capture conditions using the 1 tonne per day solvent absorption pilot plant[114]. As
shown in Figure 2.10, it was found that the addition of boric acid showed no enhancement on the
CO2 absorption rate. This is consistent with previous kinetics research that shows that boric acid
has a very limited promotion impact[69]. Additionally, the addition of boric acid can potentially
decrease the pH value of the solvent which will decrease the reaction rate between CO2 and
hydroxide ions. Potassium glycinate (~9.5 wt. %) has been investigated as a rate promoter using a
laboratory scale pilot plant[85] and in a 1 tonne per day pilot plant under post-combustion capture
conditions at a brown coal fired power station[189]. The absorption rate of CO2 in glycinate (~9.5
wt. %) promoted potassium carbonate (30 wt. %) solvent increased to 5‒6 times compared with
unpromoted solvents, indicating that glycinate is a promising promoter for the system. However,
it should also be noted that the addition of glycinate also slightly increased the pressure drop and
holdup measured in the packed absorption column which is likely due to a reduction in the surface
tension of the solvent. However, there is still a lack of data for the heat requirement in regeneration
of glycinate promoted potassium carbonate solvents which is very important for further estimation
of costs.
- 35 -
Figure 2.10. Pilot plant results with different promoters compared with unpromoted solvent[85]
(A: 35 wt. % K2CO3 with L/G of 4; B: 35 wt. % K2CO3 with 3% boric acid; C: 36 wt. % K2CO3
with 9% glycine with L/G of 3; D: 40 wt. % K2CO3 with 10% glycine with L/G of 5; E: 41 wt. %
K2CO3 with 9.1% glycine with L/G of 4)
2.5 Lessons learnt from the literature
2.5.1 Promoting mechanisms
Although the physical and chemical properties between different promoters are quite different,
their structures have some similarities. The similarity between all promoters is that they have OH
or O– groups, or can act as Lewis bases with CO2 as a Lewis acid, which is consistent with the
conclusion drawn by Phan [109] and Dennard and Williams [72].
- 36 -
Previous research has suggested a range of promotion mechanisms [101, 152] and the overall
outcomes are consistent. All promoters catalyse the reaction between CO2 and H2O (CO2 hydration)
and the promotion mechanism can be summarized by the following three steps (a−c).
a. Deprotonation of promoter to form activated species
[Promoter] → [Activated Promoter] + H+
b. CO2 addition forming intermediate
[Activated Promoter] + CO2 → [Intermediate]
c. HCO3- substitution to regenerate promoters
[Intermediate] + H2O → [Promoter] + HCO3-
CO2 + H2O → H+ + HCO3–
As shown in Figures 2.3, 2.6, 2.8 and Equations 2.1.1−2.1.3, 2.2.9−2.2.13 and 2.3.1−2.3.3, the
activated species of the promoters are the deprotonated species, indicating that promoters dissolved
in potassium carbonate solutions deprotonate first to form activated species. Then the deprotonated
species react with dissolved CO2 to form an intermediate (Figure 2.11). After that, the
intermediates react with base species substituting HCO3- and regenerating promoters. Specifically,
for amines following the zwitterion mechanism, the deprotonation (Step a) follows intermediate
formation (Step b).
- 37 -
Figure 2.11. Intermediate formulas of different promoters [101, 152, 159, 175, 180]
2.5.2 Comparison of different promoters and remarks
The promoting performance of a range of promoters is summarized in Table 2.6. Ideal promoters
should be economically acceptable, stable, non-toxic, non-corrosive, highly efficient,
environmentally benign, recyclable, and have a low vapour pressure.
- 38 -
Table 2.6. Comparison of different inorganic promoters
Promoter Promoting performance Toxicity Stability Volatility Corrosivity Pilot plant performance
Arsenic acid
Boric acid
Vanadate ?
MEA
DEA
PZ
Glycine
Sarcosine ? ?
CA ? ?
CA mimicking
compounds
? ? ?
“”-very good, “”-good, “”-poor, “”-very poor, “?”-currently unclear
- 39 -
As concluded in Table 2.6, arsenic acid is toxic and unlikely to be used as a promoter; boric acid
provides very minor promoting performance; vanadate is more appropriate as a corrosion inhibitor
rather than a promoter due to its instability. A comparison of promotion kinetics is shown in Table
2.7 to quantify the promotion performance of the remaining promoters.
Table 2.7. Kinetics data of different promoters from the literature
Promoter Temperature oC K2CO3 % kobs 104 M–1s–1 Reaction order Reference
MEA 63 30 6.1 1 [76]
DEA 60 25 10.8 1 [135]
PZ 25 0 6.7 1 [190]
Glycine 60 30 10.0 1 [84]
Sarcosine 60 30 38.0 1.4 [84]
CA 50 20 9×103 1 [191]
CA
mimicking
compounds
100 13 0.2 1 [165]
Most inorganic promoters are thermally stable and resistant to degradation, but they may switch
between different speciation forms at different concentrations, under different temperatures or pH
values, which is an important reason for the poor performance of boric acid and vanadate.
Although amine promoters are widely used in industrial absorption plants such as MEA and DEA,
they have shortcomings of degradation, evaporation and corrosion to be overcome. Amino acid
salts overcome some of the shortcomings of amines, however, more testing needs to be conducted
on their activity in industrial processes over time, energy costs and promoting performances. For
- 40 -
enzymatic promoters, more development of stable carbonic anhydrases and metal compounds for
mimicking carbonic anhydrases is needed over a range of industrial conditions.
2.6 Scope of this study
As shown in this chapter, different promoters from previous publications were reviewed and
analysed. It was concluded that ideal promoters should be economically acceptable, stable, non-
toxic, non-corrosive, highly efficient, environmentally benign, recyclable, and have a low vapour
pressure. Potential promoters such as thermally stable carbonic anhydrase, further study on amino
acids and carbonic anhydrase mimics were also recommended. Therefore, in this study, a thermally
stable carbonic anhydrase was first investigated as a promoter and its stability was tested (Chapter
3). Then (Chapter 4 & 5), amino acids were investigated as promoters and their promotion
mechanism was revealed. Finally (Chapter 6), a temperature responsive pocketed Zinc compound
inspired from carbonic anhydrase was synthesized (collaboration with Dr. Zeyun Xiao and Dr.
Luke Connal) and its catalysis efficiency was studied.
- 41 -
Chapter 3 Experimental
A range of promoters were screened and tested for improving the reaction kinetics of potassium
carbonate using the stopped flow technique and a wetted wall column. Both technologies have
been widely used in the previous literature[192, 193]. The details of both technologies are
introduced in this chapter for better understanding the methodologies of this study.
3.1 Stopped flow technique
3.1.1 Stopped flow
Experiments conducted using apparatus such as the wetted wall column, laminar jet setup and
stirred cell batch are all heterogeneous techniques (i.e. gas-liquid techniques). The stopped flow
technique is a homogeneous (liquid-liquid) method specially used for fast reaction kinetics
research. A schematic diagram of a stopped-flow apparatus is depicted in detail by Kierzkowska-
Pawlak (Figure 3.1)[194]. As this method is homogeneous, no conversion is needed accounting
for the mass transfer between liquid phase and gas phase, so it is more accurate for measuring the
reaction kinetics between two reactants. However, the stopped flow technique has a limit for
extremely fast reactions related to its dead time (the time required for the reactants to flow from
the final point of mixing to the observation cell) is of the order of milliseconds [194]. Additionally,
as the stopped-flow technique is based on ultraviolet absorption or fluorescence in the mixer (some
machines can also measure the conductivity), highly concentrated or very viscous solutions cannot
be used as a result of the interference introduced by the mixture process. A further constraint is
that it operates only with liquids, and the low solubility of CO2 in water. This limits the range of
concentrations that can be studied.
- 42 -
Figure 3.1. Schematic of stopped-flow technique[194]
In this study, a SX.20 Stopped-flow spectrometer (upgraded) from Applied Photophysics Ltd.
(United Kingdom) was used to conduct the investigation of the reaction kinetics between CO2 and
solvents. A simple introduction of the stopped flow system is given (Figure 3.2).
- 43 -
Controller
Computer
Computer moniter
Sample handling system Monochrometer Lamp
Lamp power
Lamp earth
Lamp positive
Lamp negative
II C bus cables
Analogue control cables
Light guide
Figure 3.2. SX.17 MV flow diagrams
- 44 -
There are several fundamental elements in this stopped-flow spectrometer including a light source,
monochromators, sample handling unit, detection system, temperature controlled water circulator,
computer and software (Figure 3.2). Desirable features are: a high level of automation, high
sensitivity, short dead-time and low volume usage, and a wide range of accessories and alternative
detection options.
The light source of the stopped flow used in this study is a Xenon arc source as this produces an
intense emission over a large wavelength range (far-UV to the NIR region) and is sufficiently
intense as to be suitable for fluorescence measurements. There are alternative light sources
sometimes used for specific applications; for example, a Xenon-Mercury lamp may be preferred
for fluorescence applications where an intense Mercury emission line can be used to excite the
fluorophore of interest.
The monochromator is designed to select the light wavelength i.e. the excitation wavelength for
fluorescence measurements or the wavelength of interest for absorbance measurements.
The sample handling unit is used to rapidly mix the reagents into an observation cell, and
coordinate the stopping of the flow with the start of detection (Figure 3.1).
The detection system (usually one or more photomultiplier detectors) is used to record changes
in absorbance, fluorescence and light scattering. Photodiode array detectors are also sometimes
used to enable acquisition of time-resolved spectra (multi-wavelength kinetic data).
A computer and software (SX. 20) is used to control most aspects of the instrument. Curve fitting
and, in some cases, global analysis software is used for fitting multi-wavelength kinetic data.
A temperature controlled water circulator (W15, Grant Instrument, Cambridge, UK, ±0.1 °C) is
used to regulate the reagent temperature and the temperature of the observation cell.
- 45 -
3.1.2 Stopped flow methods
A buffer solution method developed by Khalifah[195] and since then been used widely in enzyme
kinetics research was used in this study for investigating the reaction kinetics between CO2 and
solvents. The reaction rate can be calculated using a buffer factor (Equation 3.1.1) and the
absorbance change with respect to time (Equation 3.1.3), where Q is the buffer factor[195], α is
the mole fraction of the base forms of the buffer and indicator, respectively, b is the optical
pathlength (10 mm), ε (extinction factor) of the acid and base forms of the indicators and dA/dt│t=0
is the initial absorbance changing rate. Typical buffers and indicators used in this study is listed in
Table 3.1.
𝑄 =𝛼𝐵(1−𝛼𝐵)𝐶𝐵
𝛼𝐼𝑛(1−𝛼𝐼𝑛)𝑏𝐶𝐼𝑛∆ 3.1.1
𝛼 =10(𝑝𝐻−𝑝𝐾𝑎)
1+10(𝑝𝐻−𝑝𝐾𝑎) 3.1.2
𝑘𝑜𝑏𝑠 = −𝑄
[𝐶𝑂2]0
𝑑𝐴
𝑑𝑡|𝑡=0
3.1.3
Table 3.1. Buffers and indicators used in this study
Buffer pKa Indicator pKa ∆ε M‒1cm‒1 Wavelength nm
Imidazole 7.14 4-Nitrophenol 7.15 17900 400
m-cresol 8.22 Diethylimidazole 8.32 37800 578
Thymol blue 9.0 Ampso 8.9 35600 598
In each experiment, seven repeat runs were performed and the average absorbance was used to
fit an exponential relationship with time (Equation 3.1.4) using the Marquardt algorithm[141] to
calculate the initial reaction rate. The reaction rate constants kOH− and kH2O were obtained from the
validation experiments (3.1.3) and compared with literature results[196].
- 46 -
An experimental example of CO2 hydration reaction with carbonic anhydrase enzyme is given in
Figure 3.3.
𝑦 = 𝑎 + 𝑓𝑒𝑐𝑥 3.1.4
Figure 3.3. Experimental absorbance versus time for CO2 hydration at different CO2
concentrations (wavelength of 400 nm, temperature of 303 K and pH of 7.5)
3.1.3 Stopped flow validation
The reaction rate of CO2 with OH− and CO2 with H2O was measured for validating the stopped
flow equipment. The reaction rate constant of CO2 with OH− was measured at 298‒318 K and the
Arrhenius equation (Equation 3.1.4) was used to determine the activation energy of 56.4 kJ/mol,
which agrees well with the results of 57.8 kJ/mol presented by Guo et al.[101] and 55.4 kJ/mol
reported by Pinsent et al.[197]. The reaction rate of CO2 with OH− at 298 K (7044 M‒1s‒1) also
agrees well with other literature data 6000 M‒1s‒1 [198] and 7900 M‒1s‒1 [102]. The reaction rate
constant of CO2 with H2O was 0.044±0.001 s‒1 at 298 K, which is comparable to the previous
- 47 -
research of 0.037±0.002 s‒1[195] and 0.0386 s‒1[199]. The CO2 hydration rate constant follows the
Arrhenius equation (Equation 3.1.5) with an activation energy of 72.8 kJ/mol, agreeing well with
literature results of 77.9 kJ/mol[200] and 74.1 kJ/mol[201].
𝑘𝑂𝐻−(𝑀−1𝑠−1) = 5.53 × 1013𝑒
−6786𝑇 (𝐾)⁄
3.1.4
𝑘𝐻2𝑂 (𝑠−1) = 2.57 × 1011𝑒−8759/𝑇 (𝐾) 3.1.5
3.2 Wetted wall column technique
3.2.1 Wetted wall column
A wetted wall column (WWC) (Figure 3.4) was used to test the CO2 absorption rate in different
solvents. The flowchart of the WWC is shown in Figure 3.5. In a WWC, solvent is pumped through
the central column (A, Figure 3.4) and an evenly distributed film can be obtained on the outer
surface of the column. The solvent is contacted with gas in the chamber (B, Figure 3.4) and
absorption of CO2 is monitored by measuring the inlet and outlet gas flowrate and CO2
concentration. Also, a liquid sample was obtained at the beginning and end of each experiment to
compare with the gas phase absorption results. The contact area between the gas and liquid film
formed on the surface of the central column is 51.7 cm2. The temperature of the system was
controlled by an external water bath with a thermal controller and both gas and solvent were
preheated in this water bath. The calculation methodology is similar to that given by Thee et al.
[69].
- 48 -
102.0 mm
Gas out
Bathing liquid out
25.0 mm
Gas inLiquid out
Liquid in
12.7 mm
Bathing liquid in
116.9 m
m
139.6 m
m
BoltFixing to
stand
BoltFixing to
stand
Gasket
A
B
C
Figure 3.4. The structure diagram of the wetted wall column used in the study
- 49 -
CO2/N2
Cylinder Saturator
Water Bath
Solvent
Tank
Sample Port
Positive
Displacement
Pump
Drain
CO2 AnalyzerCold Trap Mass Flow
Meter
Vent
Pressure
Regulator
Gas
Solvent
Bathing liquid
Lines
WWC
Mass Flow
Meter
Vent
Vent
Figure 3.5. Flowchart of the WWC used in this research (modified from Thee[192])
- 50 -
3.2.2 Wetted wall column methods
3.2.2.1 Gas film mass transfer coefficient
By introducing the Sherwood number[202], a dimensionless number which is a ratio of the
convective mass transport to diffusive mass transport, the gas phase film mass transfer coefficient
can be found from Equation 3.2.1, where Sh is Sherwood number, R (8.31432 J·mol‒1K‒1) is the
gas constant, T (K) is the temperature, d (m) is the hydraulic diameter of the annulus, DCO2 is the
diffusivity of CO2.
𝑆ℎ =𝑘𝑔𝑑
𝐷𝐶𝑂2 3.2.1
So, the gas mass transfer coefficient is as Equation 3.2.2.
𝑘𝑔 =𝑆ℎ𝐷𝐶𝑂2
𝑑 3.2.2
According to Bishnoi[203] and Pacheco[204], the Sherwood number can be found from Equation
3.2.3 in a similar WWC equipment, where Re is Reynolds number, Sc is Schmidt number, h is the
height of the WWC.
𝑆ℎ = 1.075 (𝑅𝑒𝑆𝑐𝑑
ℎ)0.85
3.2.3
The Schmidt number is defined as Equation 3.2.4, where μg (Pa·s‒1) is the gas viscosity, ρg (kg·m‒
3) is the gas density.
𝑆𝑐 =𝜇𝑔
𝜌𝑔𝐷𝐶𝑂2 3.2.4
The Reynolds number is as Equation 3.2.5, where v (m·s‒1) is the linear velocity of the gas.
𝑅𝑒 =𝜌𝑔𝑣𝑑
𝜇𝑔 3.2.5
This gas film phase mass transfer coefficients can be obtained by Equation 3.2.6.
𝑘𝑔 = 1.075𝑣0.85𝐷𝐶𝑂2
0.15
𝑑0.15 3.2.6
- 51 -
3.2.2.2 Liquid physical mass transfer coefficient
A dimensionless driving force is introduced to calculate the liquid phase physical mass transfer
coefficient (Equation 3.2.7)[202].
𝑘𝑙𝑜 =
V̇
𝑎(1 − 𝛩) 3.2.7
For calculating the dimensionless driving force, a dimensionless penetration distance can be
found from Equation 3.2.8[202], in which τ (s) is the surface contact time.
𝛩 = 1 − 3√𝜂
𝜋 (𝜂 <0.01) 3.2.8
𝛩 =0.7857
𝑒5.121𝜂+0.1101
𝑒39.21𝜂+
0.036
𝑒105.6𝜂+0.0181
𝑒204.7𝜂 (𝜂 >0.01) 3.2.9
𝜂 =𝐷𝐶𝑂2𝜏
𝛿2 3.2.10
In the case of a gas transferring into a falling film, the liquid physical mass transfer coefficient
(klo) can be calculated by Equation 3.2.11[205], where L (m) is the length of wetted wall column,
Γ (kg·m‒1·s‒1) is the mass rate of flow/unit width, δ (m) is the thickness of a layer, �̇� [m3·s‒1] is
the volumetric flowrate of the liquid, ρl [kg·m‒3] is the liquid density, g [m·s‒2] is the gravity
constant and W [m] is the circumference of the column. The approach by Dugas[35] which uses a
simplified equation (Equation 3.2.13) for calculating the liquid phase physical mass transfer
coefficient, in which Ql (m3·s‒1) is the liquid flowrate and Aa (m
2) is the contact area and proved
that the results were comparable to those calculated by Equation 3.2.11.
𝑘𝑙𝑜 = (
6𝐷𝐶𝑂2𝛤
𝜋𝜌𝛿𝐿)1 2⁄ 3.2.11
𝛿 = √3𝜇𝑙�̇�
𝜌𝑙𝑔𝑊
3 3.2.12
𝑘𝑙𝑜 =
31 3⁄ 21/2
𝜋1/2𝑄𝑙1 3⁄ ℎ1 2⁄ 𝑊2 3⁄
𝐴𝑎(𝑔𝜌
𝜇)1/6𝐷𝐶𝑂2
1 2⁄ 3.2.13
- 52 -
3.2.2.3 Enhancement factor
The overall mass transfer flux can be written as Equation 3.2.14, where PCO2, b (Pa) is the CO2
partial pressure in the gas phase, P*CO2, b (Pa) is the CO2 equilibrium partial pressure of the system.
𝑁𝐶𝑂2 = 𝐾𝐺(𝑃𝐶𝑂2,𝑏 − 𝑃𝐶𝑂2,𝑏∗ ) 3.2.14
This increase in the absorption rate due to chemical reaction is often described using an
enhancement factor, E. The value of E is always greater than or equal to one. Levenspiel [13]
formulated a general rate equation with chemical reactions as Equation 3.2.15 and the
enhancement factor can be calculated by Equation 3.2.16.
1
KG=
1
kg+
H
klo𝐸
3.2.15
𝐸 =𝐻𝐶𝑂2𝐾𝐺
𝑘𝑙𝑜 3.2.16
3.2.2.4 Surface renewal model
A range of models including film theory, penetration and surface renewal theories, and eddy
diffusivity theories have been proposed for describing the interfacial mass transfer of gas-liquid
systems[204, 206]. The Danckwerts surface renewal theory[207] was used in this study with the
WWC as it has been widely used in literature and proved having high accuracy[120, 208, 209]. In
this theory, by assuming that the interfacial liquid is replaced by the liquid from the bulk solution
and therefore has the local mean bulk concentration, the chance of an element at the interface is
independent of its length of time being exposed. Assuming that the gas mass transfer resistance to
be very small and the CO2 equilibrium partial pressure in the liquid phase is very small compared
to CO2 partial pressure in the gas phase as experiments are often operated at high CO2 partial
pressure, thus
1
kg≈ 0
- 53 -
PCO2,b − PCO2,b∗ ≈ PCO2,b 3.2.17
The flux of CO2 can be expressed as Equation 3.2.18, where the enhancement factor can be
written as Equation 3.2.19.
𝑁𝐶𝑂2 =𝑘𝑙𝑜𝐸
𝐻𝐶𝑂2𝑃𝐶𝑂2,𝑏 3.2.18
𝐸 = √1 +𝑘𝑜𝑏𝑠𝐷𝐶𝑂2
(𝑘𝑙𝑜)2
3.2.19
3.2.2.5 Pseudo first order reaction constant
For CO2 absorption with potassium carbonate solvents in a WWC, the liquid reactant can be
assumed to be constant across the film and be represented by the bulk solution concentration.
The pseudo first order assumption can be written as follows (Equation 3.2.20).
𝐷𝐶𝑂2𝜕2[𝐶𝑂2]
𝜕2𝑥2= 𝑘𝑂𝐻[𝑂𝐻
−][𝐶𝑂2] + 𝑘𝐻2𝑂[𝐶𝑂2] 3.2.20
Based on mass balance and Henry’s law, when CO2 is absorbed or desorbed from the solvent in
a WWC equipment, the Equation 3.2.21[192] can be used, where a is the surface mass transfer
area.
𝑉𝑙𝑑[𝐶𝑂2]𝑏
𝑙
𝑑𝑡= 𝑘𝑙
𝑜𝑎([𝐶𝑂2]𝑖𝑙 − [𝐶𝑂2]𝑏
𝑙 ) 3.2.21
𝑉𝑙𝑑[𝐶𝑂2]𝑏
𝑙
𝑑𝑡=𝑘𝑙𝑜𝑎
𝐻(𝑃𝐶𝑂2,𝑖 − 𝑃𝐶𝑂2,𝑏
∗ ) 3.2.22
Taking desorption of CO2 from CO2 loaded water as an example and assuming the desorption
from water is completely liquid film controlled (the mass transfer resistance in the gas phase is
negligible compared with that in the liquid phase), the partial pressure of CO2 at the gas phase
would be: PCO2, i=PCO2, b.
𝑉𝑙𝑑[𝐶𝑂2]𝑏
𝑙
𝑑𝑡=𝑘𝑙𝑜𝑎
𝐻(𝑃𝐶𝑂2,𝑏 − 𝑃𝐶𝑂2,𝑏
∗ ) 3.2.23
- 54 -
The molar concentration of CO2 in the gas phase is related to its partial pressure:
[𝐶𝑂2]𝑏𝑔=𝑃𝐶𝑂2,𝑏
𝑅𝑇 3.2.24
Taking Equation 3.2.24 into Equation 3.2.23, Equation 3.2.25 can be obtained.
𝑉𝑙𝑑[𝐶𝑂2]𝑏
𝑙
𝑑𝑡=𝑅𝑇𝑘𝑙
𝑜𝑎
𝐻([𝐶𝑂2]𝑖
𝑙 − [𝐶𝑂2]𝑏𝑙 ) 3.2.25
According to mass balance, the CO2 gas absorbed from the gas phase by the liquid equals the
increase of CO2 in the liquid phase (Equation 3.2.26), where G (m3·s‒1) is the gas volumetric flow
rate in the WWC, the superscript “g, in” and “g, out” indicate the concentrations of the inlet and outlet
gas, respectively.
𝐺([𝐶𝑂2]𝑔,𝑖𝑛 − [𝐶𝑂2]
𝑔,𝑜𝑢𝑡) = 𝑘𝑙𝑜𝑎([𝐶𝑂2]𝑖
𝑙 − [𝐶𝑂2]𝑏𝑙 ) 3.2.26
3.3 Polymer Characterization
1H NMR and 13C NMR spectra were obtained with a Varian 400 MHz spectrometer using the
specified solvent. Chemical shifts (δ) were reported relative to the solvent residue peak and are in
ppm (δCHCl3 = 7.26 ppm, δD2O = 4.79 ppm). Mass spectra were recorded on an Agilent 6520
QTOF MS. Polymer dispersity and molecular weights were determined by Size Exclusion
Chromatography (SEC) using calibration curves obtained from polystyrene standards. To analyse
polymers by SEC, the sample was dissolved in spectrometry grade Dimethylformamide (DMF) at
a concentration of 10 mg/mL and filtered through a 0.45 µm TeflonTM syringe filter. 50 µL was
injected into the instrument at a flow rate of 1 mL/min. The SEC instrument was equipped with a
Shimadzu RID-10 refractometer (λ = 633 nm) and Shimadzu SPD-20A UV-vis detector using two
Phenomenex Phenogel columns (5 µm bead size, 104 and 106 Å porosity) in series, operating at
70 °C. DMF with 0.05 mol·L–1 lithium bromide was used as the mobile phase. Polymer lower
critical solution temperature (LCST) measurements were completed using UV-Vis transmission
measurements using a quartz cuvette with a 10 mm path length on a Cintra 2020 UV-Vis fitted
- 55 -
with a heating cell; any resulting temperature changes were measured with an external
thermocouple. The concentration of Zn in the polymer was determined via an inductively coupled
plasma optical emission spectroscopy (ICP-OES, Varian 720-ES) at a wavelength of 206.200 nm
and 202.548 nm.
3.4 Materials
The information on all reagents used in this work is listed in Table 3.1. Chemicals were used as
received unless otherwise stated. Azobisisobutyronitrile (AIBN) was recrystallized in methanol
prior to use. N-isopropylacrylamide (NiPAm) supplied by Tokyo Chemical Industry Co., Ltd was
recrystallized from a mixture of toluene/hexane (v/v 2:3) prior to use. Purified water (Elix
Millipore, resistivity > 15 MΩ cm‒1) was used to prepare all solutions Compressed air pre-treated
with a gas generator (Parker Filtration & Separation Division, Balston, US) to remove water and
oil residues was used as the driving gas for the stopped flow equipment. The carbonic anhydrase
(NZCA) used in this study was provided by Novozymes A/S (Bagsvaerd, Denmark). The enzyme
was produced by microbial fermentation in a benign host organism, which was removed during
recovery of the enzyme broth and is not present in the sample.
- 56 -
Table 3.2. Information on reagents used in this work
Chemicals Purity Suppliers
1,4,7,10-tetraazacyclododecane >97.0% Sigma-Aldrich
2-piperazinecarboxylic acid ≥98% Sigma
4-nitrophenol ≥99.5% Fluka Analytical
4-vinylbenzyl chloride Sigma-Aldrich
Alizarin yellow GG (5-(3-
Nitrophenylazo) salicylic acid sodium
salt) 50% Sigma
Ampso 99% Chem-supply
Aspartic acid For biochemistry Merck KGaA
Azobisisobutyronitrile (AIBN) Sigma-Aldrich
Carbon dioxide Coregas Australia
Carbon dioxide and nitrogen mixtures
10.1%
BOC Australia
29.9%
55.0%
90.1%
Dichloromethane (DCM) Reagent grade Chem-Supply
Diethyl ether RCI premium grade ACI Labscan
Dimethylimidazole 98% Sigma-Aldrich
Glycine AR Chem-Supply
Imidazole 99% Chem-supply
Leucine Bulk Nutrients
Lysine Bulk Nutrients
m-cresol 90% Sigma-Aldrich
- 57 -
Chemicals Purity Suppliers
Methanol Reagent grade Chem-Supply
MOPS (3-(N-
Morpholino)propanesulfonic acid) >99.5% Sigma-Aldrich
N-isopropylacrylamide (NiPAm) 98% Tokyo Chemical Industry Co., Ltd
Nitrogen Liquid Coregas
Novozymes carbonic anhydrase
Novozymes A/S (Bagsvaerd,
Denmark)
Potassium bicarbonate 99.70% Sigma-Aldrich
Potassium carbonate 99% Fluka Analytical
Potassium hydroxide LR Chem-Supply
Proline For biochemistry Merck KGaA
Sarcosine 98% Sigma
Serine ≥98% Sigma
Sulfuric acid 98% Science Supply
Tetrahydrofuran (THF) GPR
Reactapur VWR Chemicals
Thymol blue 98% Sigma-Aldrich
Toluene (Analar Normapur) VWR Chemicals
Valine ≥99% Sigma
Zinc perchlorate hexahydrate Sigma-Aldrich
- 58 -
Chapter 4 Thermally Stable Carbonic Anhydrase as a Promoter in
Potassium Carbonate Solvents for Carbon Dioxide Capture
4.1 Introduction
As discussed in Chapter 2, carbonic anhydrase is an enzyme that is found in mammals, marine
species and bacteria[99]. It is an efficient enzyme for catalysing the CO2 hydration reaction with
water to form bicarbonate. It has been widely studied due to its important functionality in the
human body and has high catalysis efficiency and selectivity for carbon dioxide hydration[210].
The principal reactions related to CO2 absorption in potassium carbonate solutions include
Reactions 4.1.1‒4.1.3. Reaction 4.1.1 (CO2 hydration reaction) dominates in acidic to neutral pH
conditions, while Reaction 4.1.2 occurs in alkaline solutions. The catalysis efficiency can be
calculated using Equations 4.1.4–4.1.6, where r is the overall reaction rate for a particular NZCA
concentration, kobs is the observed reaction rate coefficient and [CO2] is the concentration of carbon
dioxide. The observed rate coefficient (Eqaution. 4.1.5) can be expressed as the sum of that relating
to hydration by hydroxide ions (Reaction 4.1.2, kOH[OH–]) and that relating to enzyme promoted
hydration (Reaction 4.1.3, k’obs). Michaelis-Menten kinetics has been used extensively by other
workers to describe the hydration by carbonic anhydrase[195, 211, 212]. In this case (Equation
4.1.6), the turnover number (kcat) represents the maximum number of CO2 molecules converted to
HCO3– per carbonic anhydrase molecule per second, while the Michaelis-Menten constant (Km) is
the concentration of carbon dioxide when the initial rate is equal to half the maximum rate. The
catalysis efficiency of the carbonic anhydrase is represented by kcat/Km.
𝐶𝑂2 + 𝐻2𝑂𝑘𝐻2𝑂→ 𝐻𝐶𝑂3
− + 𝐻+ 4.1.1
𝐶𝑂2 + 𝑂𝐻−𝑘𝑂𝐻→ 𝐻𝐶𝑂3
− 4.1.2
- 59 -
𝐶𝑂2 + 𝐻2𝑂 + 𝑁𝑍𝐶𝐴𝑘𝑁𝑍𝐶𝐴→ 𝑁𝑍𝐶𝐴 + 𝐻𝐶𝑂3
− + 𝐻+ 4.1.3
𝑟 = 𝑘𝑜𝑏𝑠[𝐶𝑂2] 4.1.4
𝑘𝑜𝑏𝑠 = 𝑘𝑜𝑏𝑠′ + 𝑘𝑂𝐻[𝑂𝐻
−] 4.1.5
𝑘𝑜𝑏𝑠′ =
𝑘𝑐𝑎𝑡
𝐾𝑚+[𝐶𝑂2][𝑁𝑍𝐶𝐴] 4.1.6
Enzymatic carbon dioxide capture[162] has been widely tested for facilitating the CO2 capture
process in solvent absorption[31, 68, 99, 163, 166, 168, 169, 191, 213-215], and sorbent adsorption
and sequestration[97, 216]. However, solvent absorption usually operates under harsh conditions
such as high pH, high temperature and high ionic strength where many commercial enzymes are
unstable. Thus, the development of more stable carbonic anhydrases is needed. The aim of this
research is to accelerate the absorption rate of carbon dioxide using a commercially available and
thermally stable carbonic anhydrase as a promoter in potassium carbonate solvents.
4.2 Results and discussion
4.2.1 Stopped flow experiments
The turnover numbers (kcat, s-1) and Michaelis-Menten constants (Km, M) of the NZCA containing
solutions (Table 4.1) determined using the stopped flow technique at different temperatures are
shown in Figure 4.1. Both parameters increase with an increase in temperature and the turnover
numbers increase faster than the Michaelis-Menten constants, indicating that the enzyme catalysis
activity is increasing with the increase of temperature in the range of 298‒328 K.
- 60 -
Table 4.1. Catalysis coefficient of the NZCA at different pH values
pH buffer Indicator kcat/ Km ×10-7 M‒1s‒1
6.90 Imidazole 4-nitrophenol 1.8
7.24 Imidazole 4-nitrophenol 2.3
7.68 Imidazole 4-nitrophenol 2.7
7.90 Imidazole 4-nitrophenol 3.1
8.00 Diethyl imidazole M-cresol purple 3.2
8.27 Diethyl imidazole M-cresol purple 4.9
8.30 Diethyl imidazole M-cresol purple 5.0
- 61 -
Figure 4.1. Turnover numbers and Michaelis-Menten constants for NZCA (298‒328 K)
- 62 -
The turnover number and Michaelis-Menten constants of the carbonic anhydrase at 298 K are
calculated to be 3.4×105 s‒1 and 12.5 mM, respectively. This result is comparable with other
investigations on carbonic anhydrase with the catalysis efficiency (kcat/Km) lying in the range of
7.9×106 to 1.6×108 M‒1s‒1 (Table 4.2). The catalysis efficiency increased with pH, as measured in
the pH range of 6.8‒8.3. According to the CO2 hydration reaction catalysis mechanism (Figure
4.2)[99, 217], the conversion of carbonic anhydrase coordinated with water to hydroxide ions is
critical for the functioning of carbonic anhydrase. Solutions with higher pH can provide more
hydroxide ions for the conversion, and therefore, can improve the catalysis efficiency of the
carbonic anhydrase. It can be concluded that the NZCA is an efficient catalyst for CO2 hydration
at moderate temperatures and pH values.
Figure 4.2. Catalysis scheme of CO2 hydration by carbonic anhydrase[99, 217]
- 63 -
Table 4.2. Comparison of carbonic anhydrase catalytic coefficients for CO2 hydration
Enzyme T (K) pH kcat (×105s-1) Km mM kcat/Km (×10-7 M-1s-1) Reference
HCA C 298 7 10 8.3±1.2 12 [218]
HCA II 298 ‒ 1.1±0.2 9.7±2.3 12±2 [211]
HCA II 293 7.5 9.5 11.4 8.3 [219]
HCA C 298 7.0 5.7 9 6.3 [195]
HCA XIV 293 7.5 3.12 8.0 3.9 [219]
mCA XV 293 7.5 4.7 14.2 3.3 [212]
NZCA 298 7.68 3.4 12.5 2.7 This work
HCA IX 298 8.6 3.8 15.8 2.4 [220]
CA – – 4 26 1.5 [221]
HCA XIII 293 7.5 1.5 13.8 1.1 [222]
HCA B 298 7.0 0.32 4 0.79 [195]
The temperature dependence of turnover number was then determined according to Equation
4.2.1.
𝑘𝑐𝑎𝑡 = 3.11 × 1014𝑒−6190/𝑇 4.2.1
The activation energy of 51±1 kJ/mol is within the normal activation energy range (16.7‒83.7
kJ/mol) for enzymes[223], but is somewhat larger than the activation energy of 45 kJ/mol reported
for a human carbonic anhydrase I [224] and 35.9±0.8 kJ/mol for a human carbonic anhydrase
II[225].
The catalysis efficiency (kcat/Km) across a range of pH values at 298 K is represented in Figure
4.3, with the data presented in Table 4.1.
- 64 -
Figure 4.3. Effect of pH on the NZCA activity at 298 K
As shown in Figure 4.3, the catalysis efficiency of NZCA increases with pH in the pH range of
6.8 to 8.3. This agrees well with the trend of increasing catalysis efficiency with pH for carbonic
anhydrase mimicking compounds[184]. These results indicate that the enzyme favours high pH
conditions and gives confidence for testing under harsher conditions including high pH and
temperatures seen in industrial CO2 capture plants.
The catalysis efficiency of NZCA also increases slightly with ionic strength (addition of KCl) as
can be seen in Figure 4.4. Conversely, the catalysis efficiency of NZCA decreases with increasing
bicarbonate concentration (or CO2 loading).
- 65 -
Figure 4.4. Effect of CO2 loading and ionic strength on the catalysis efficiency of the NZCA at
298 K
4.2.2 Wetted wall column experiments
Figure 4.5 shows the corrected observed first order rate constant (kobs’, s‒1) for CO2 absorption in
30 wt. % K2CO3 at different loadings with NZCA as a promoter at 323 K, determined using the
WWC. A small addition of NZCA enhanced this corrected observed first order rate coefficient
significantly, with an increase of more than 10 fold with the addition of 52 µM of NZCA. Results
from WWC experiments, where the pH and ionic strength are both significantly higher, gave a
catalysis efficiency of 5.3×107 M‒1s‒1 with 0.04 loading in potassium carbonate solutions, which
is comparable with previous research by Zhang and Lu[191] (9×107 M‒1s‒1, 323 K) tested with a
stirred cell reactor in 20 wt. % K2CO3 solution and Ye and Lu[169] (1.2×108 M‒1s‒1, 298‒323 K),
also tested with a stirred cell reactor in 20 wt. % K2CO3 solution.
- 66 -
Figure 4.5. Promotion effect of the NZCA for CO2 absorption with 30 wt. % K2CO3 solvents
(0.04‒0.32 loading) at 323 K using WWC
Figure 4.5 shows that the CO2 loading had a slightly negative influence on activity of NZCA
(5.3×107 M‒1s‒1 at 0.04 loading and 3.4×107 M‒1s‒1 at 0.32 loading), indicating that NZCA is still
a good promoter for CO2 absorption in potassium carbonate solutions even at high loading
conditions. As carbonic anhydrase enzyme is a typical enzymatic catalyst, it catalyses both forward
and backward reactions of CO2 hydration. Therefore, the apparent reaction rate can be influenced
by the bicarbonate concentration (CO2 loading). The loss of efficiency was also observed with
experiments in the stopped flow apparatus (Figure 4.3). In this case, the addition of KHCO3 led to
a loss of catalysis efficiency, but addition of KCl at similar concentrations has no effect. The loss
- 67 -
of efficiency suggests that NZCA might be a good catalyst for CO2 desorption if it can tolerate
high temperature conditions (373‒393 K).
As shown in Figure 4.6, the NZCA catalysis efficiency reduced by less than 30% after being
tested for 8 hours in 30 wt. % K2CO3 solutions with pH of 10.6‒10.8 at 323 K. In the first 1.5
hours, the catalysis efficiency of the NZCA dropped by around 20% most likely due to the
denaturation of some enzyme. The catalysis efficiency continued to decrease afterwards but at a
much slower rate. These results show that the NZCA used in this study is relatively stable
compared with other carbonic anhydrase enzymes including one that had only 22% activity
remaining after 1 hour at a pH of 8.0 and temperature of 323 K[226], a carbonic anhydrase (BhCA)
with 50% activity loss after 65 min at 323 K[227] and a carbonic anhydrase (GS6‒046) that
reported showing no enhancement at temperatures over[228] 320 K. However, the NZCA used in
this study does not appear to be comparable with that reported by Ye and Lu with only 50% activity
loss after 2 months at 323 K in 20 wt. % K2CO3 solutions[169]. Furthermore, the activity loss of
NZCA could drop even faster at higher temperatures, indicating a limitation of this enzyme.
- 68 -
Figure 4.6. Thermal stability of the NZCA in 30 wt. % K2CO3 (0.1 loading) at 323 K
4.2.3 Comparison of results from stopped flow and wetted wall column
While the enzyme is clearly affected by such experimental conditions, catalysis efficiencies at
323 K from the stopped flow (6.0×107 M‒1s‒1) and the WWC (5.3×107 M‒1s‒1) are not dissimilar,
even though the pH (7.5 versus 10.9); ionic strength (≤0.1 versus 7.2M) and loading (0 versus 0.04)
are significantly different. This indicates that the enzyme is relatively resilient to these extreme
conditions, with a net loss in efficiency of around 12 %. However, this observation is likely to be
affected by differences between the two experimental techniques.
Furthermore, according to the thermal stability testing (Figure 4.6), the NZCA maintained more
than 70% of its initial promoting coefficient after exposing it to 30 wt. % K2CO3 solutions with a
pH of 10.6‒10.8 at 323 K for 8 hours, indicating that the enzyme is relatively stable at the
conditions studied. Further studies on enzyme immobilization may be needed for studying an
- 69 -
industrial absorption-desorption process. Immobilization can increase the stability of carbonic
anhydrase[221] under industrial conditions and avoid the carbonic anhydrase running through the
desorption process which operate at much higher temperatures (≥373 K) than the absorption
process, which can make the reuse of carbonic anhydrase possible.
4.3 Conclusions
The catalysis kinetics of a thermally stable carbonic anhydrase (NZCA) promoter were tested via
the stopped flow technique and a wetted wall column (WWC). The Michaelis-Menten catalysis
parameter (kcat/Km) was determined to be 2.7×107 M-1s-1 at 298 K with the promoting catalysis
reaction activation energy of 51±1 kJ/mol at 298‒328 K. The catalysis coefficient of the NZCA
was determined to be 5.3×107 M-1s-1 using a WWC in 30 wt. % potassium carbonate solutions (pH
~ 11‒12) at 323 K. The corrected observed first order rate coefficient increased more than 10 fold
by adding 52 nM of NZCA. The NZCA is comparatively stable with more than 70% of its initial
catalysis efficiency maintained after continuously running for 8 hours in 30 wt. % K2CO3 solutions
at pH of 10.6‒10.8 and temperature of 323 K.
- 70 -
Chapter 5 Reaction Kinetics between Histidine and Carbon Dioxide
5.1 Introduction
Within the carbonic anhydrase enzyme, histidine acts as a ligand for the zinc cation, assisting the
catalysis of hydration reaction between carbon dioxide and water. It has been studied as a solvent
for carbon dioxide capture by Shen et al. [229]. In Shen’s research, the reaction kinetics between
CO2 and histidine was measured using a wetted wall column and a zwitterion mechanism was used
for fitting the experimental results. However, the reaction mechanism is not clear and the influence
of ionic strength was not well represented. Therefore, further investigation into the reaction
mechanism between CO2 and histidine and the influence of ionic strength is performed in this
study.
Histidine (His) is an essential amino acid. It is polybasic, with several pKa values (5.1.1–5.1.3,
Figure 5.1, Figure 5.2) [230].
[𝐻𝑖𝑠]2+ ⇔ 𝐻+ + [𝐻𝑖𝑠]+ pKa1=1.54 (298 K) 5.1.1
[𝐻𝑖𝑠]+ ⇔ 𝐻+ + [𝐻𝑖𝑠]± pKa2=6.07 (298 K) 5.1.2
[𝐻𝑖𝑠]± ⇔ 𝐻+ + [𝐻𝑖𝑠]− pKa3=9.34 (298 K) 5.1.3
- 71 -
Low pH High pH
[His]2+
[His]+
[His]±
[His]-
Figure 5.1. Transformation among different forms of histidine
Figure 5.2. Distribution of Histidine species under different acidity at 298 K[230]
The equation of Clarke and Glew (5.1.4)[231] was used to represent the influence of temperature
on the pKa values, in which Tr is a reference temperature, R is the gas constant, ∆rGo is the standard
- 72 -
molar Gibbs energy, ∆rHo is the standard molar enthalpy and ∆rCp
o is the heat capacity changes.
The thermodynamic properties of different histidine formations are given in Table 5.1[230] and
the calculation results of the distribution of histidine formations at different temperatures are
shown in Figure 5.3 (a: 298 K, b: 303 K, c: 308 K, d: 313 K).
𝑅 ln𝐾 = −∆𝑟𝐺
𝑜
𝑇𝑟+ ∆𝑟𝐻
𝑜 (1
𝑇𝑟−1
𝑇) + ∆𝑟𝐶𝑝
𝑜 (𝑇𝑟
𝑇− 1 + ln
𝑇
𝑇𝑟) +
𝑇𝑟
2(𝜕𝑟∆𝑟𝐶𝑝
𝑜
𝜕𝑇)𝑝(
𝑇
𝑇𝑟−𝑇𝑟
𝑇− 2 ln
𝑇
𝑇𝑟)
5.1.4
Table 5.1 Thermodynamic properties of histidine
Reactions ∆rGo (kJ/mol) ∆rCp
o (kJ/mol/K) ∆rHo (kJ/mol)
5.1.1 8.8 0 3.6
5.1.2 34.65 0.176 29.5
5.1.3 53.31 -0.233 43.8
- 73 -
a b
c d
Figure 5.3 Distribution of histidine formations at different temperatures (a: 298 K, b: 303 K, c:
308 K, d: 313 K)
5.2 Results and discussion
5.2.1 The reaction contribution of different histidine species
The corrected pseudo first order reaction rate constant (kobs’, s‒1) determined using the stopped
flow technique at pH values of 7.2 and 9.5 and temperatures of 298‒313 K are shown in Figure
5.4. Figure 5.4 shows that the reaction rate between CO2 and buffer solutions at pH of 7.2 is
negligible compared with the reaction rate at pH of 9.5. As shown in Figure 5.1, the main histidine
- 74 -
species at a pH of 7.2 is His± (92.6% of His± with 0.6% of His- and 6.8% of His+ at 298 K). At pH
9.5 and 298 K the distribution of histidine is 59.1% His‒ with 40.9% of His±. Therefore, it can be
concluded that the reaction between CO2 and His‒ is the main reaction occurring under basic
conditions, which is similar to the reaction between CO2 and glycine reported by Guo et al.[193].
Figure 5.4. Corrected pseudo first order reaction rate constants at different pH and temperatures
5.2.2 Determination of corrected reaction pseudo-first-order rate constants (kobs’)
The corrected pseudo first order reaction rate constants (kobs’, s‒1) between CO2 and His‒ at
temperatures of 298‒313 K and His‒ concentrations of 0 to 9 mM are shown in Figure 5.5. It shows
that the corrected pseudo first order reaction rate constant increases sharply with increases in His‒
concentration, indicating that the addition of histidine has a major contribution to the reaction
between CO2 and the solution. Also, the corrected pseudo first order reaction rate constant
increased as temperature increased.
- 75 -
Figure 5.5. Corrected pseudo first order reaction rate constant between CO2 and His‒ at the
temperatures of 298‒313 K
As the solution used in the stopped flow equipment is very dilute (ionic strength ≤0.05M in this
study), the influence of ionic strength can be ignored and the solution can be treated as an ideal
solution. Therefore, the reaction order with respect to His– can be obtained by plotting the corrected
pseudo first order reaction rate constant versus the histidine concentration on a log-log plot (Figure
5.6).
- 76 -
Figure 5.6. Double log coordinate plot of observed pseudo-first-order rate constants versus the
concentration of His-
The slopes in Figure 5.6 are identical within experimental error with a value of 1.18±0.08 across
the temperature of 298–313 K with all coefficients of determination (r2) more than 0.99, which is
comparable with the reaction order of between 1.22 and 1.45 for histidine reported by Shen et al.
[229]. The reaction rate constant can be obtained by plotting kobs’ versus [His‒]1.18 (Figure 5.7), and
the reaction rate constants at different temperatures with respect to His‒ are listed in Table 5.2.
- 77 -
Figure 5.7. Determination of reaction constant to His‒ at different temperatures
Table 5.2. Reaction rate constants with respect to His‒ at different temperatures
Temperature (K) k’AmH (M‒1s‒1)
298 3001.1
303 4229.2
308 6032.8
313 8240.5
As shown in Table 5.2 and Figure 5.7, the reaction rate constant increases with increase in
temperature. Figure 5.8 shows the Arrhenius plot of the reaction rate constants between CO2 and
His‒ (5.2.1) with an activation energy of 52.3 kJ/mol, which is comparable with the values of
glycine (48.2 kJ/mol)[154], proline (43.3 kJ/mol)[232] and lysine (51.0 kJ/mol)[233].
- 78 -
Figure 5.8. Arrhenius plot of the reaction of His‒ with CO2
𝑘𝐴𝑚𝐻(𝑀−1𝑠−1) = 4.43 × 1012𝑒
−6292.9
𝑇 (𝐾) 5.2.1
5.2.3 Zwitterion mechanism fitting with the experimental data
The complete zwitterion mechanism (Equation 2.2.1) was then used to fit the experimental results
in this study, with the three bases (B) OH–, H2O and histidine (Figure 5.9). The protonation reaction
constants and zwitterion deprotonation rate constants at temperatures of 298‒303 K were obtained
by fitting the experimental reaction rate constants using a Levenberg-Marquardt algorithm[141].
- 79 -
Figure 5.9. Zwitterion mechanism fitting of the reaction between CO2 and His-
The forward reaction rate constants of CO2 with histidine (k1) and the ratio of the deprotonation
constant with the reverse reaction rate constant (𝑘𝐴𝑚𝐻
𝑘−1) were obtained and found to follow an
Arrhenius relationship with temperature (5.2.2‒5.2.3). The contribution of other bases (including
water and hydroxide) to the deprotonation step was negligible (with a magnitude of 10‒14),
indicating that histidine is the dominant base for the deprotonation step in this system.
𝑘1(𝑀−1𝑠−1) = 1.15 × 1013𝑒
−6446.8
𝑇 (𝐾) Ea = 53.6 kJ/mol 5.2.2
𝑘𝐴𝑚𝐻
𝑘−1 (𝑀−1) = 1.82𝑒
2156.6
𝑇 (𝐾) Ea = –17.93 kJ/mol 5.2.3
The ratio of k‒1/(∑kB[B]) was within the range of 0.06‒0.30 under the conditions calculated here,
indicating that both the rate of protonation and deprotonation reactions are important for the overall
reaction rate. This is consistent with the results shown in Figure 5.6 which indicated the overall
reaction order of 1.18 standing between unity and two.
- 80 -
5.2.4 Influence of ionic strength on the reaction kinetics
The influence of ionic strength on reaction rates can be complicated as it can affect both reaction
kinetics and reaction equilibria[234]. An exponential relationship (5.2.4) has been used extensively
in the literature for representing the influence of ionic strength[80, 235, 236], where the value of
b is a function of the solution properties and temperature. The value of b was determined to be 0.1
at 283 K and 0.2 at 303 K for CO2 in 0–3 M sodium hydroxide solutions[237] while values of b
= 0.38, 0.44, 0.57 and 0.90 have been given for sarcosine[159], glycine[154], alanine[238] and
threonine[239], respectively. For some reactions such as the hydrogen peroxide decomposition
reaction, b values can also be negative[240].
𝑘 = 𝑘𝑜𝑒𝑏𝐼 5.2.4
The stopped flow technique is usually performed under very low ionic strength conditions so that
the effect of ionic strength is negligible. However, in carbon dioxide capture operations, histidine
is likely to be used in highly concentrated solutions. Therefore, the results obtained in this study
were extrapolated to high histidine concentrations (298–313 K) without considering the effect of
ionic strength and compared with the experimental results by Shen et al.[229] (Figure 5.10).
- 81 -
Figure 5.10. Comparison of extrapolating results in this study with experimental results by Shen
et al.[229] at high histidine concentrations
The extrapolated results (shown by the dotted lines in Figure 5.10) fit well with the experimental
results by Shen et al.[229] at low histidine concentrations (<0.2 M). However, as the histidine
concentration increases, the extrapolated results do not agree with the experimental data, indicating
that ionic strength has a significant influence on the reaction kinetics. In Shen’s research[229], an
estimated b value of 0.44 was used to account for the influence of ionic strength (Equation 5.2.4).
In the present case, the use of b=0.44 was effective at 313K but did not provide a good fit to the
data at 298‒303 K (Figure 5.11). Therefore, a fitting was performed with MATLAB and a better
fit to the experimental data by Shen et al. was obtained using b values of 0.67, 0.65 and 0.46 at
298, 303 K and 313 K respectively (Figure 5.12).
- 82 -
Figure 5.11. Comparison of extrapolating results from this study with WWC results from
literature[229] using a b value of 0.44 representing the ionic strength impact
- 83 -
Figure 5.12. Comparison of results extrapolated from stopped flow experiments using b=0.67 at
298 K, b=0.65 at 303 K, b=0.46 at 313 K to correct for ionic strength with experimental WWC
results by Shen et al.[229] at high histidine concentrations
As shown in Figure 5.12 the extrapolated results fit well with the experimental data (r2 > 0.99)
with these b values. The values fit within the range of other reported b values for amino acids
(0.38‒0.90).
5.2.5 Comparison of histidine with other amino acids
A comparison of the reaction constants for glycine[193], proline[241], taurine[241] and histidine
at low ionic strength is shown as an Arrhenius plot in Figure 5.13. Results showed that the reaction
rate between CO2 and histidine is much slower than that of glycine and proline, but faster than that
of taurine, indicating that the histidine is not a fast reacting amino acid with CO2. This is consistent
with the results obtained at higher concentrations[229].
- 84 -
Figure 5.13. Comparison of kinetics results between amino acids and CO2 at low ionic strength
(<0.05 M)
Histidine is also known to play an important role in carbonic anhydrase enzymes which catalyse
the CO2 hydration reaction with H2O with a catalysis coefficient of ~107 M‒1s‒1. However, in this
study we find that the reaction constant of histidine with CO2 is only ~104 M‒1s‒1. This confirms
that the role of histidine in carbonic anhydrase catalysis is only that of a ligand in accordance with
previous research. This prior work shows that the metal ions (such as Zn, Cd and Co) within the
carbonic anhydrase enzyme plays a more significant role in catalysing the CO2 hydration reaction
with H2O[99].
5.3 Conclusions
The reaction kinetics and reaction mechanism between histidine and CO2 was investigated
experimentally via the stopped flow technique. The reaction order between His‒ and CO2 was
- 85 -
1.180.08 under the conditions studied (pH~9.5, concentration of 0–0.010M and temperature of
298‒313 K). The reaction rate constant between His‒ and CO2 was found to be 𝑘𝑜𝑏𝑠′ (𝑠−1) =
4.78 × 1012𝑒−6319.8
𝑇 (𝐾) [𝐻𝑖𝑠]1.18𝑒𝑏[𝐻𝑖𝑠] with an activation energy of 52.5 kJ/mol, and with b values of
0.67, 0.65, 0.46 at 298, 303 and 313 K, respectively. Use of the zwitterion mechanism indicated
that both the protonation and deprotonation steps are limiting steps. However, the impact of ionic
strength on the reaction kinetics becomes significant when the histidine concentration increases to
higher levels (≥0.2 M). Overall, from the perspective of kinetics, histidine is not considered to be
a good solvent for CO2 absorption. This also further confirmed that metal ions are important parts
for the carbonic anhydrase enzyme to function as a rate promoter for carbon dioxide absorption.
- 86 -
Chapter 6 Screening of Amino Acids as Promoters for CO2 Absorption
6.1 Introduction
As discussed in Chapter 2 and Chapter 5, many amino acids have been considered as fast reactants
with carbon dioxide and thus have been tested as solvents[229] or promoters in different solvent
systems[83]. In this chapter, a range of different amino acids were investigated as reactants and
promoters in potassium carbonate solutions, aiming at identifying the fastest amino acid promoter
and other related important parameters.
The reactions related to the absorption of CO2 when reacting with amino acid salt solutions and/or
amino acid salt promoted carbonate solvents are listed as 4.1.1–4.1.2 and 6.1.1–6.1.3.
𝐻2𝑂 ↔ 𝐻+ +𝑂𝐻− 6.1.1
𝐻𝐶𝑂3− +𝑂𝐻− ↔ 𝐻2𝑂 + 𝐶𝑂3
2− 6.1.2
𝑅2𝐻𝐶(𝑅1𝑁𝐻)𝐶𝑂𝑂− + 𝐶𝑂2 + 𝑂𝐻
−𝑘𝑜𝑏𝑠′
⇔ 𝑅2𝐻𝐶(𝑅1𝑁𝐶𝑂𝑂−)𝐶𝑂𝑂− + 𝐻2𝑂
( ) 6.1.3
- 87 -
6.2 Results and discussion
6.2.1 Speciation and reaction kinetics of amino acid salts with CO2
The amino acids can transform from cations to zwitterions to anions as the pH increases from
low to neutral to high values, respectively (Figure 6.1 and 6.2). Species distribution was calculated
based on the titrated pKa values shown in Table 6.1. It can be seen that the percentage of anions is
negligible at neutral pH conditions (6.0–8.0), while the anions become the principal species present
at high pH (>10.0).
Table 6.1. pKa values of amino acid salts at 298 K in diluted solutions
Amino acid
298 K 323 K
pKa1 pKa2 pKa1 pKa2
2-piperazinecarboxylic acid 9.61 – – –
asparagine 8.88 – – –
aspartic acid 9.96 – – –
glycine 9.81 – 9.31 –
leucine 9.73 – 9.26 –
proline 10.80 – 10.36 –
lysine 9.46 10.74 8.95 10.10
sarcosine 10.27 – 9.73 –
serine 9.24 – – –
valine 9.77 – – –
- 88 -
Figure 6.1. Transformation of different species of amino acid salts with pH
Figure 6.2. Distribution of valine ionic species at various pH values
The effect of amino acid speciation on the reaction kinetics with CO2 were investigated over a
range of pH values at similar amino acid salt concentrations (5 mM) with the stopped flow
technique. As shown in Figure 6.3, the observed pseudo first order reaction rate constant (k’obs)
between the amino acid salts and CO2 at neutral pH (7.3±0.2) was small compared with that at
high pH values, indicating that the anionic species of the amino acid salts is the major species
reacting with CO2, which agrees with previous research for glycine[193] and histidine[242].
H+ H+
- 89 -
Figure 6.3. Reaction rate between CO2 and amino acid salt solutions (~5 mM) at neutral (7.3±0.2)
and basic pH values (around pKa values, lysine: pH~pKa1, lysine*: pH~pKa2) at 298 K
The reaction kinetics between the anions of the amino acid salts and CO2 was then investigated
at pH conditions near the pKa of each amino acid (7.9–10.7) and the pseudo first order reaction
constants are shown in Figure 6.4 as a function of the anion concentration. In this Figure, the
concentration of only the active reacting species (i.e. the anion concentration) is used to compare
the kinetic performance of the different amino acids. The reaction rate constants follow the order
of proline > glycine > sarcosine ≈ valine ≈ aspartic acid >leucine ≈ 2-piperazinecarboxylic acid >
lysine# >histidine ≈ serine > lysine > asparagine. The fast reaction rate between proline and CO2
agrees with the work of van Holst[160], in which the CO2 absorption kinetics using potassium
- 90 -
salts of amino acids were investigated. It can be concluded that proline shows the fastest reaction
rate among the amino acids under the conditions studied here.
Figure 6.4. Pseudo first order reaction constants between different amino acid anions and CO2:
Lysine# is the lysine species with negative two valency while all other amino acids have negative
one valency (the results for glycine agree with previous research[193], while the data for histidine
was extracted from our previous research[242])
6.2.2 Promotion performance of amino acid salts in potassium carbonate solvent
As the stopped flow experiments described above were performed at low temperature (298K) and
low concentration (5 mM), further investigation of absorption kinetics is needed to determine the
performance in industrial potassium carbonate solvents at higher temperature. The preliminary
screening results showed that proline had the fastest reaction rate with CO2. Additionally, sarcosine
has been reported[84, 160] to have fast reaction kinetics under higher concentrations and
temperatures, while glycine, leucine and lysine are all economically favourable as promoters as
- 91 -
they are produced as common nutrients. Therefore, the promoting performance of these five amino
acids (proline, sarcosine, glycine, leucine and lysine) were further tested in the wetted wall column
(WWC) using 30 wt. % potassium carbonate solvents at 323 K, pH of 12.5 and amino acid salt
concentrations of 0.5 M. The carbon dioxide absorption enhancement factors for each amino acid
promoted potassium carbonate sample is shown in Figure 6.5 along with unpromoted potassium
carbonate solvent.
Figure 6.5. Enhancement factors using 30 wt. % potassium carbonate solvents with and without
amino acid salts (0.5 M) in a WWC at pH of 12.5 and temperature of 323 K
As shown in Figure 6.5, the enhancement factors under the same pH conditions were increased
by adding amino acid salts into 30 wt. % potassium carbonate solvents. Among the amino acid
salts examined, glycine had the least effect with an increase in absorption rate of ~3 times, leucine
and lysine showed moderate promoting effect with ~4 times increase in absorption rate, while
proline and sarcosine showed the fastest promoting effect on the absorption rate (~6 times increase).
- 92 -
These results differ from the stopped flow experiments in which glycine was faster than sarcosine.
This could be attributed to different reaction orders between CO2 and the amino acids. The reaction
order between glycine and carbon dioxide has been reported to be unity[193], but that for the
reaction between sarcosine and CO2[84] as between 1.3 and 1.6. This means that as the
concentration of amino acids increases, the reaction rate between glycinate and CO2 increases
linearly regardless of the effect of ionic strength while that of sarcosinate increases with
concentration to the power of 1.3–1.6. These results are consistent with the previous literature[84].
Thus, it can be concluded that proline and sarcosine are the most efficient rate promoters among
the investigated amino acid salts under industrial CO2 capture conditions (pH of 12.5 and
temperature of 323 K) using 30 wt. % K2CO3 solvent.
6.2.3 Effect of pH on the absorption kinetics
According to the experimental results shown in section 3.1, the distribution of amino acid salt
ions is sensitive to pH conditions and the amino acid anions are the main species contributing to
the CO2 absorption reaction. Therefore, the measurement of absorption rate of CO2 in amino acid
promoted potassium carbonate solvents over a range of pH conditions is necessary. The promoting
effects of amino acid salts (0.5 M) have therefore been investigated at different pH values in 30
wt.% potassium carbonate solvents at 323 K (Figure 6.6) using the WWC.
- 93 -
Figure 6.6. Enhancement factors using 30 wt. % potassium carbonate solvents with and without
amino acid salts (0.5 M) in a WWC over a range of pH values at 323 K
As shown in Figure 6.6, the addition of amino acid salts increased the enhancement factor
dramatically at high pH (≥12) conditions, particularly in comparison with the unpromoted
potassium carbonate solvents. The promoting effects of amino acid salts are clearly sensitive to
pH values, which is attributed to the reaction mechanism between CO2 and amino acid salts (only
the anion species of the amino acid salt reacts with CO2) as discussed in Section 6.1. Additionally,
sarcosine showed higher promoting performance at lower pH (<12.5) than proline, while both
amino acids showed comparable performance at higher pH (>12.5). This could be due to the lower
pKa value of sarcosine (refer to Table 6.1). These results are important for interpreting the
promotion effects of amino acids for industrial CO2 capture conditions as promoted potassium
carbonate solutions generally operate at pH values above 12.
- 94 -
6.2.4 Comparison of amino acids and monoethanolamine (MEA) as rate promoters for CO2
absorption in potassium carbonate solvent
Monoethanolamine (MEA) is a widely used solvent for CO2 absorption and it has also been used
as a rate promoter in potassium carbonate solvents[1]. Therefore, the promoting effects of MEA
at 0.5 M (Figure 6.7) in 30 wt% K2CO3 was also measured using the WWC and compared with
the promoting performance of proline and sarcosine under similar experimental conditions.
Figure 6.7. Enhancement factors using proline, sarcosine and MEA (0.5 M) as promoters in 30
wt% potassium carbonate solvent. Results were obtained using a WWC over a range of pH values
at 323 K
As can be seen from Figure 6.7, proline and sarcosine showed better promoting effects when
compared to MEA at high pH (>12.0) conditions in 30 wt. % potassium carbonate solvents at 323
K. This agrees with the previous literature that reports the first order reaction rate constants at 323
K between MEA and CO2 (2.7×104 M–1s–1)[120] are much lower than that of sarcosine with CO2
- 95 -
(3.1×105 M–1s–1)[159] and proline with CO2 (3.2×105 M–1s–1)[158]. However, at lower pH values,
the promoting effects of proline, sarcosine and MEA became similar with the sarcosine promoted
potassium carbonate solvent having slightly faster CO2 absorption due to the changing speciation
of the amino acids. This again highlights the importance of solution pH when determining the
promoting performance of rate promoters for both laboratory research and industrial CO2 capture
applications.
6.3 Conclusions
The anion species of a range of amino acid salts were determined to be the major species reacting
with carbon dioxide via stopped flow experiments. The promoting effects of these salts in 30 wt. %
potassium carbonate solvents were found to be sensitive to pH due to the variation in pKa values
and the corresponding anion reaction rate with carbon dioxide. Sarcosine and proline were found
to be the most effective rate promoters among the amino acid salts tested in this study. Comparable
performance was observed for these two promoters at high pH (>12.5) while at lower pH (<12.5)
sarcosine provided slightly better performance. As promoters, sarcosine and proline both showed
faster enhanced performance than MEA for CO2 absorption in potassium carbonate solvents at
high pH (>12.0), while the promoting performance of MEA was comparable to that of proline and
slightly poorer than that of sarcosine at low pH (<12.0) conditions. The results from this study
highlight the importance of solvent pH when determining the rate promotion performance of
promoters in potassium carbonate solvents for industrial CO2 capture.
- 96 -
Chapter 7 A Carbonic Anhydrase Mimicking Polymer for Accelerating
Carbon Capture
7.1 Introduction
As shown in Chapter 4, carbonic anhydrase has shown highly effective promotion for CO2
absorption in carbonate solvents with only trace amount of addition. However, the thermal stability
is a big barrier hindering its deployment in industry. From the results in Chapter 5 & 6, histidine
is not a quite efficient promoter for CO2 absorption. Therefore, more research is needed for
understanding the nature of carbonic anhydrase and take the advantage of it. In this study, a
temperature responsive polymer was synthesized to mimic the natural carbonic anhydrase. Despite
the complicated protein structure and catalysis mechanism of natural carbonic anhydrase (Figure
7.1), the catalytic site is relatively simple. Researchers have been studying compounds that mimic
carbonic anhydrase to understand the structure-reactivity relationship of the active site for decades.
Metal centres including Zn (II)[175-177], Co (II)[175, 176], Cu (II)[176, 177], Ni (II)[180], Hg
(II)[175] and Cd (II)[181] have been studied, among which Zn (II) is the most promising
metal[175-177]. For Zn compounds, a range of ligands[172, 176, 183-187] have been investigated.
However, research shows that the catalysis efficiency of these carbonic anhydrase mimicking
compounds is still comparatively low compared with natural carbonic anhydrase and the ligands
continue to face degradation or decomposition problems as operating temperatures increase.
Despite understanding the structure of this enzyme, development of an effective polymer based
carbonic anhydrase mimicking catalyst has not yet been realized.
- 97 -
Figure 7.1. Proposed mechanism for the hydration of CO2 by carbonic anhydrase.[243]
Poly(N-isopropyl acrylamide) (PNiPAm) is a temperature responsive polymer that has found
applications in macroscopic gels, microgels, membranes, sensors, biosensors, thin films, tissue
engineering, and in drug delivery[244]. It changes hydrophilicity and hydrophobicity abruptly at
the lower critical solution temperature (LCST). At temperatures lower than the LCST, PNiPAm
orders itself in aqueous solution and the amide group is hydrogen bonded with the water molecules.
At higher temperatures, PNiPAm releases water, becomes hydrophobic and precipitates out from
aqueous solution. Here, we have developed a carbonic anhydrase mimicking material using
PNiPAm as the polymer support. The choice of PNiPAm is threefold; firstly, the interaction of
water with the polymer provides the water necessary for the carbonic anhydrase mechanism;
secondly, the polyamide in the polymer structure mimics the amides of the protein backbone which
is also crucial in the catalytic mechanism; and thirdly, the temperature responsiveness of PNiPAm
- 98 -
enables further understanding of structure function relationships in regard to hydrophobicity of the
nanoenvironment.
1,4,7,10-Tetraazacyclododecane (cyclen) is one of most efficient carbonic anhydrase mimicking
compounds reported to date[183]. In this study, we synthesized a cyclen functionalized PNiPAm
as a temperature responsive polymer mimicking carbonic anhydrase which will precipitate as
temperature increases (>308 K). This will allow the mimicking compound to be readily separated
from the loaded solution of a traditional CO2 solvent absorption process, and thus avoid carryover
into the higher temperature desorption process (typically of the order of 393 K) where it would be
destabilised.
7.2 Results and discussion
7.2.1 Synthesis and characterization
Cyclen functionalized monomer (4–vinylbenzyl cyclen) was synthesized by reacting cyclen with
4-vinylbenzyl chloride in the presence of potassium carbonate (Figure 7.2) and the following steps
were given in details from Sections 7.3.1.1–7.3.1.3.
- 99 -
Figure 7.2. Synthesis of the cyclenZn pendant PNiPAm and the small molecule of cyclenZn
7.2.1.1 4-Vinylbenzyl Cyclen
A mixture of cyclen (344 mg, 2.0 mmol), 4-vinylbenzyl chloride (304 mg, 2.0 mmol), K2CO3 (400
mg) in 50 mL CH3CN was stirred at room temperature for 2 days and then at 60℃ for 1 day. After
removal of solvent, the residue was washed with hexane. The solid was then purified with a reverse
phase chromatography column (C18, methanol/H2O 1:1 as eluent) to afford 113 mg product (yield
39%). 1H NMR (CDCl3, Figure 7.3) δ 7.34 (d, J = 8 Hz, 2 H), 7.25 (d, J = 8 Hz, 2 H), 6.67 (dd, J1
- 100 -
= 17 Hz, J2 = 12 Hz, 1 H), 5.70 (d, J = 17 Hz, 1 H), 5.20 (d, J = 12 Hz, 1 H), 3.64 (s, 2 H), 2.60-
2.90 (m, 16 H). 13C NMR (CDCl3, Figure 7.4) 136.6, 136.5, 129.2, 128.9, 126.4, 113.7, 59.6, 51.9,
51.1, 47.5, 45.7. HRMS (ESI, Figure 7.5), calculated for C17H29N4 289.2392, found 289.2389 [M
+ H]+.
- 101 -
Figure 7.3. 1H NMR of 4-vinylbenzyl cyclen
- 102 -
Figure 7.4. 13C NMR of 4-vinylbenzyl cyclen
- 103 -
Figure 7.5. Electrospray ionization-mass spectrometry (ESI-MS) of 4-vinylbenzyl cyclen
[M+H]+
- 104 -
7.2.1.2 PNiPAm-co-Cyclen
A solution of NiPAm (1.13 g, 10 mmol), 4-vinylbenzyl cyclen (58 mg, 0.20 mmol) and AIBN (3
mg) in THF (3 mL) was bubbled with nitrogen for 5 minutes and then heated at 65 oC for 20 hrs.
After cooling to room temperature, the reaction mixture was precipitated into diethyl ether. The
white solid was collected and dried under vacuum (1.09 g, 92%). The 1H NMR data is shown in
Figure 7.6, 1H NMR (D2O) δ 7.30 (broad, 2 H), 7.15 (broad, 2 H), 3.87 (broad, 44 H), 3.61-3.78
(2 H), 2.61-3.01 (16 H), 1.81-2.33 (45 H), 1.28-1.81 (90 H), 1.12 (broad, 264 H). SEC (Figure 7.7)
Mn 9.5 kDa, Ð 1.93.
7.2.1.3 PNiPAm-co-CyclenZn
A solution of PNiPAm-co-Cyclen (600 mg) in 100 mL of methanol was heated to 60 oC. To this
solution was added zinc perchlorate hexahydrate (56 mg, 0.15 mmol) in 20 mL methanol
dropwisely. The reaction mixture was stirred for another 3 hours at 60 oC. After cooling to room
temperature, most of the solvent was removed and the polymer was precipitated into diethyl ether
and dialyzed in water to remove unreacted zinc salt. The zinc coordinated polymer was obtained
as a white solid (603 mg). The 1H NMR data is shown in Figure 7.6, 1H NMR (D2O) δ 7.29 (broad,
2 H), 7.13 (broad, 2 H), 3.87 (broad, 44 H), 3.61-3.78 (2 H), 2.58-3.30 (16 H), 1.81-2.33 (45 H),
1.28-1.81 (90 H), 1.12 (broad, 264 H). The zinc ion concentration in the copolymer was measured
with ICP–OES (Figure 7.8) which showed 1 gram of the polymer contained 1.8×10-4 mol of zinc
ion. This is in good accordance with the zinc concentration calculated by 1H NMR integration. The
NiPAm to cyclen ratio is 44:1 by 1H NMR integration which corresponds to 1 g polymer
containing 1.9×10-4 mol of cyclen and thus 1.9×10-4 mol of zinc if the cyclen is fully coordinated
to zinc ions.
- 105 -
Figure 7.6. 1H NMR spectra of PNiPAm-co-cyclen and PNiPAm-co-cyclenZn. The proton
signals from the cyclen moieties are enlarged
ppm
- 106 -
0 10 20 30 40
0.082
0.084
0.086
RI
Time (min)
Figure 7.7. SEC diagram of PNiPAm-co-Cyclen
- 107 -
Figure 7.8. ICP-OES measurement at the wavelengths of 202.548 and 206.200 nm with four
standard solutions of 0, 4, 10, 20 ppm (9.34 mg of PNiPAm-co-CyclenZn dissolved in 10 ml
solution).
7.2.1.4 LCST Determination
To examine the LCST of the synthesized polymer, the polymer solution was prepared in distilled
water at a concentration of 10 mg/mL with no pH modification. The sample was then passed
through a 0.45 μm nylon membrane filter, before being added to a 4 mL quartz cuvette with a 1cm
path length. The transmittance of light through the sample was measured by UV−Vis at 540 nm,
with reference to a sample of distilled water (Figure 7.9). A ThermoCell temperature controller
was used to heat the samples at an approximate heating rate of 1°C/min, while the temperature
inside the cuvette was continuously monitored with a thermo probe. The LCST was approximated
- 108 -
as the temperature whereby the transmittance had decreased by 50% from the initial baseline
reading at 30 °C.
Using the 50% transmission temperature as the LCST, the polymer solution displayed a LCST
of 33.5 oC (Figure 7.9a). Above the LCST, PNiPAm-co-CyclenZn undergoes a phase transition
from a swollen hydrated state to a shrunken dehydrated and hydrophobic state. To further confirm
the phase transition and thus the separation of PNiPAm-co-CyclenZn above the LCST, variable
temperature 1H NMR experiments were conducted (Figure 7.9b). Below 30 oC, the polymer was
fully soluble due to its hydrophilic properties. There is no decrease in proton signals as the
temperature changes, with the chemical shifts of the polymer only shifting slightly to higher
frequencies. However, at 35 oC, the proton signals decreased significantly indicating the phase
transition and thus the polymer separation from solution. Upon further increasing the temperature
to 40 oC, the polymer is readily removed from the solution as the proton signals disappeared. The
temperature responsiveness enables ready filtration of the polymer catalyst from the solvent
solution before it is sent to the high temperature solvent regeneration process.
- 109 -
Figure 7.9. (a) LCST study of the PNiPAm-co-cyclenZn in water (10 mg/ml). (b) Variable
temperature 1H NMR of PNiPAm-co-cyclenZn in D2O. As the temperature increases, the polymer
separated from the solution as evidenced by the loss of signal.
(a)
(b)
- 110 -
7.2.2 Carbon dioxide hydration catalysis efficiency of PNiPAm-co-cyclenZn
The Michaelis-Menten catalysis coefficients of the PNiPAm-co-cyclenZn and CyclenZn were
both measured via the stopped flow technique (pH of 7.33, temperature of 293–303 K) as shown
in Figure 7.10. The catalysis coefficient of cyclenZn at 298 K and pH of 7.33 was measured to be
523±7 M–1s–1 (Table 1), which is consistent with previous literature[185] for this compound which
gives a value of 564 M–1s–1 at 298 K and pH of 7.48. In comparison, the PNiPAm-co-CyclenZn
CA mimic also performed well at neutral pH with kcat/Km= 380±20 M–1s–1. However, these values
are still well below those of natural carbonic anhydrase enzymes (Table 7.1). The Michaelis-
Menten catalysis coefficients of PNiPAm-co-cyclenZn was fitted with an Arrhenius expression
(7.3.1) with an activation energy of 60.0±2 kJ/mol, which is slightly lower than the previous
literature results of 68.1 kJ/mol for cyclenZn[165].
𝑘𝑐𝑎𝑡
𝐾𝑀= (1.22 ± 0.03) × 1013𝑒−
(7200±200)
𝑇 Ea=60.0±2 kJ/mol 7.3.1
- 111 -
Figure 7.10. Arrhenius fitting of Michaelis-Menten catalysis coefficients of the PNiPAm-co-
CyclenZn
In the presence of the zinc salt (zinc perchlorate hexahydrate, 2.46 µM) and PNiPAm (7.76 µM)
separately, results showed negligible effect on the CO2 hydration reaction constant (0.047±0.007
s–1, 298 K compared with that of 0.044±0.001 s–1, 298 K[196]) indicating that neither Zn nor
PNiPAm individually speeds up the carbon dioxide consumption rate.
A significant rate enhancement was observed for the polymeric catalyst at basic pH (kcat/Km=
2330±40 M–1s–1), which is also in agreement with previous cyclen literature[184, 196].
Furthermore, the increased performance at higher pH indicates that it can be potentially used in
industrial CO2 solvent absorption processes which are usually operated at high pH conditions
(10.0–11.5).
- 112 -
Table 7.1. Comparison of catalysis coefficients for PNiPAm-co-CyclenZn and other carbonic
anhydrase mimics
Metal Ligand Concentration (mM) kcat/Km (M–1s–1) pH Temp (K) Ref
Zn Cyclen 0.52 523±7 7.33 298 This work
Zn PNiPAm-co-Cyclen 0.26 380±20 7.36 298 This work
Zn PNiPAm-co-Cyclen 0.26 2330±40 9.06 298 This work
Zn Natural carbonic anhydrase – 105–108 8–11 298 [196]
In order to demonstrate the recyclability and thermal stability of the polymer catalyst, a series
of stopped flow kinetic assays were performed. The PNiPAm-co-CyclenZn solution was heated
up to 328 K and then cooled down to 298 K for the kinetic measurement. As shown in Figure 7.11,
no loss in catalysis activity was observed after 6 cycles, indicating that the PNiPAm-co-CyclenZn
is thermally stable at 328 K.
- 113 -
Figure 7.11. Activity of the PNiPAm-co-CyclenZn catalyst for CO2 hydration reaction showing
the thermal stability and recyclability. Each cycle represents a catalytic assay after heating the
polymer catalyst to 328 K and then cooling and repeating the kinetic assay at 298 K. No measurable
decrease in activity of the polymer catalyst was observed.
7.3 Conclusions
A temperature responsive carbonic anhydrase mimicking polymer (PNiPAm-co-CyclenZn) has
been synthesised and characterized. The LCST of PNiPAm-co-CyclenZn is 33.5 oC. Above the
LCST, PNiPAm-co-CyclenZn undergoes a phase transition from a swollen hydrated state to a
shrunken dehydrated state and precipitates from solution. This property can potentially be used to
enable separation of PNiPAm-co-CyclenZn from the CO2 loaded solvent, allowing the polymer
catalyst to be retained in the lower temperature absorption process. The Michaelis-Menten
catalysis coefficient of PNiPAm-co-CyclenZn was determined to be 380±20 M–1s–1 at pH of 7.36
- 114 -
and temperature of 298 K, with an activation energy of 60±2 kJ/mol at 293–303 K. Results also
showed a higher catalysis coefficient (kcat/Km 2330±40 M–1s–1) at higher pH (9.06) and the
synthesized polymer exhibits a high recyclability, which indicates it favours higher pH and can
potentially be used in industrial CO2 solvent absorption processes.
- 115 -
Chapter 8 Conclusions and Recommendations
8.1 Conclusions
Organic promoters, inorganic promoters and enzymatic promoters can be added to potassium
carbonate solvents for enhancing the CO2 absorption rate in a carbon capture process. An ideal
promoter will be economically acceptable, stable, non-toxic, non-corrosive, highly efficient,
environmentally benign, recyclable, and have a low vapour pressure.
The carbonic anhydrase (NZCA) obtained from Novozymes was investigated as a promoter in
potassium carbonate solutions. The Michaelis-Menten catalysis parameter (kcat/Km) was
determined to be 2.7×107 M-1s-1 at 298 K via a stopped flow equipment, with a promoting catalysis
reaction activation energy of 51±1 kJ/mol at 298‒328 K. The catalysis coefficient of the NZCA
was determined to be 5.3×107 M-1s-1 using a WWC in 30 wt. % potassium carbonate solutions (pH
~ 11.0‒12.0) at 323 K. Furthermore, the NZCA kept more than 70% of its initial catalysis
efficiency after continuously running for 8 hours in 30 wt. % K2CO3 solutions at pH of 10.6‒10.8
and temperature of 323 K.
Histidine was also investigated as a reactant with CO2. Results showed that the histidine anion
(amine group) is the main species reacting with CO2, which means in that histidine is only reactive
under high pH conditions (>9.0). The reaction order between histidine anions and CO2 was
determined to be 1.18±0.08 across the temperature range of 298–313 K. The zwitterion mechanism
was used to fit the kinetic data and it showed that both protonation and deprotonation reactions
contributed to the overall reaction rate. Ionic strength was also shown to have a significant
influence on the reaction kinetics when the histidine concentration is high (≥0.2 M). The reaction
kinetic expression was determined to be 𝑘𝑜𝑏𝑠′ (𝑠−1) = 4.78 × 1012𝑒
−6319.8
𝑇 (𝐾) [𝐻𝑖𝑠]1.18𝑒𝑏[𝐻𝑖𝑠] with an
- 116 -
activation energy of 52.5 kJ/mol, and with b values of 0.67, 0.65, 0.46 at 298, 303 and 313 K,
respectively. The reaction rate between histidine and CO2 is slower than that of glycine and proline
and slightly faster than that of taurine at low concentrations (<0.1 M). The low reaction rate
indicates that the role of histidine in carbonic anhydrase catalysis is that of a ligand with the
predominant catalytic activity coming from the metal ion.
A range of different amino acids (2-piperazinecarboxylic acid, asparagine, aspartic acid, glycine,
leucine, lysine, proline, sarcosine, serine and valine) were also investigated as reactants with CO2
or promoters in potassium carbonate solvents. Results showed that the amino anion groups of the
amino acids are the major species reacting with CO2. Therefore, the promoting effect of amino
acid salts is sensitive to changes in pH values due to changes in species distribution of the amino
acids. Sarcosine and proline are the most effective promoters among the amino acid salts tested in
this study with comparable promoting performance at higher pH values (≥12.5) but with sarcosine
more effective at lower pH values (<12.5). Compared to 0.5 M monoethanolamine (MEA) as a
promoter, 0.5 M sarcosine and proline showed faster rate promotion effects for carbon dioxide
absorption into 30 wt% potassium carbonate solvents at high pH (>12.0), while the promoting
performance of MEA was comparable with that of proline and slightly poorer than that of sarcosine
at low pH (<12.0) conditions.
Zinc Cyclen were successfully synthesised to perform as a carbonic anhydrase mimic and its
promoting efficiency was comparable with literature results, which is still lower than that of natural
carbonic anhydrase. A carbonic anhydrase inspired temperature responsive polymer was
synthesized and characterized as a catalyst for catalysing the CO2 hydration reaction. Results
showed that the lower critical solution temperature (LCST) of PNiPAm-co-CyclenZn is 33.7 oC
which is close to the physiological temperature. Above the LCST, PNiPAm-co-CyclenZn
- 117 -
undergoes a phase transition from a swollen hydrated state to a shrunken dehydrated state. This
property can potentially enable easy separation of PNiPAm-co-CyclenZn from the CO2 loaded
solution exiting the absorber column so that it does not enter the high temperature stripping column.
In the reaction between CO2 and H2O, the catalysis coefficient at 298 K of PNiPAm-co-CyclenZn
was determined to be 380±20 M–1s–1 at a pH of 7.36 and 2330±40 M–1s–1 at a pH of 9.06. Arrhenius
fitting of the catalysis coefficients showed an activation energy of 60±2 kJ/mol at pH of 7.36. This
study presents the first example of a temperature responsive polymeric catalyst for carbon dioxide
absorption.
By Comparison, it can be concluded the carbonic anhydrase is an efficient promoter under
appropriate temperature control, while proline and sarcosine are efficient promoters with good pH
control. More work is needed to further develop carbonic anhydrase mimics. However, it is
comparatively difficult to control operation temperatures precisely at large scale especially
industrial scale plants, which can be deadly for carbonic anhydrase. Thus, amino acids such as
proline and sarcosine studied in this work have more potential for further large scales
demonstration without further process modification.
8.2 Recommendations
According to the conclusions drawn above, four further directions of research are suggested:
(1) Development of more stable carbonic anhydrase enzymes;
In this study, the NZCA is thermally stable at 50 oC. However, this temperature is not high
enough due to operational temperature fluctuation, desorption etc. Therefore, developing more
stable carbonic anhydrase is a potential research area to overcome these shortcomings.
(2) Investigation into highly reactive amino acids with low pKa values;
- 118 -
It was concluded that the performance of amino acids is depending on the concentration of anions.
The operational pH of are limited to 9.5-11.5. Therefore, investigation into amino acids with low
pKa values is beneficial to gain higher anion concentrations at certain pH, and thus faster kinetics.
(3) Design and development of more effective carbonic anhydrase mimics;
The carbonic anhydrase mimic work in this study was quite preliminary. More side chains
(thermal responsive at higher temperatures, pH responsive etc.) and structures may be investigated
to obtain faster kinetics and other related properties.
(4) Modelling of absorption kinetics using the experimental data.
This thesis was focusing on experimental work of a range of promoters. The data provides
fundamental results for modelling work such as developing rate-based Aspen custom modeller or
economic modelling work. These modelling work can potentially build confidence for further pilot
plant demonstration or industrial plant operation.
- 119 -
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Minerva Access is the Institutional Repository of The University of Melbourne
Author/s:
Hu, Guoping
Title:
Novel promoters for carbon dioxide absorption in potassium carbonate solutions
Date:
2018
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Novel promoters for carbon dioxide absorption in potassium carbonate solutions
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