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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

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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

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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%

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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

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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.

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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

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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

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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

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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.

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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.

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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 -

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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 -

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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 -

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Chapter 8 Conclusions and Recommendations ..................................................................... - 115 -

8.1 Conclusions ................................................................................................................. - 115 -

8.2 Recommendations ....................................................................................................... - 117 -

References ............................................................................................................................. - 119 -

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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 -

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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 -

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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 -

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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 -

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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 -

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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

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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

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[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

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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.

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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].

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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

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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].

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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

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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].

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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.

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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].

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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.

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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

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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.

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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.

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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.

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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

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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.

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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.

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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

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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].

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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.

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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.

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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

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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

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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

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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.

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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

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(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

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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

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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].

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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].

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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

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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.

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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].

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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

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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.

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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].

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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).

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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.

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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

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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

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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.

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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.

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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).

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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

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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.

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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].

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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

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- 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].

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- 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

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- 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])

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- 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

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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 =

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

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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

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- 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

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- 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

Page 81: Novel promoters for carbon dioxide absorption in potassium ...

- 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.

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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

Page 83: Novel promoters for carbon dioxide absorption in potassium ...

- 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

Page 84: Novel promoters for carbon dioxide absorption in potassium ...

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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

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- 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.

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- 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

Page 87: Novel promoters for carbon dioxide absorption in potassium ...

- 61 -

Figure 4.1. Turnover numbers and Michaelis-Menten constants for NZCA (298‒328 K)

Page 88: Novel promoters for carbon dioxide absorption in potassium ...

- 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]

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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.

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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).

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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.

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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

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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.

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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

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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.

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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

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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

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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

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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

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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.

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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).

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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.

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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].

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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].

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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.

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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).

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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).

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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

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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].

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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

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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.

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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

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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 – – –

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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+

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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

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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

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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).

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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.

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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.

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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

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(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.

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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.

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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

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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.

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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

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= 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]+.

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Figure 7.3. 1H NMR of 4-vinylbenzyl cyclen

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Figure 7.4. 13C NMR of 4-vinylbenzyl cyclen

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Figure 7.5. Electrospray ionization-mass spectrometry (ESI-MS) of 4-vinylbenzyl cyclen

[M+H]+

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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.

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Figure 7.6. 1H NMR spectra of PNiPAm-co-cyclen and PNiPAm-co-cyclenZn. The proton

signals from the cyclen moieties are enlarged

ppm

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0 10 20 30 40

0.082

0.084

0.086

RI

Time (min)

Figure 7.7. SEC diagram of PNiPAm-co-Cyclen

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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

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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.

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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)

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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

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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).

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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.

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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

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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.

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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

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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

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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;

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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.

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