Biocatalytic Carbon Capture and Conversion
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BIOCATALYTIC CARBON CAPTURE AND CONVERSION A DISSERTATION SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNNESOTA BY SHUNXIANG XIA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY PING WANG, ADVISOR MAY 2018 © SHUNXIANG XIA 2018 Acknowledgements First and fore most I want to thank my advisor, Dr. Ping Wang in the Department of Bioproducts and Biosystems Engineering at the University of Minnesota. It has been an honor to be his Ph.D. student. I appreciate all his contributions of time, ideas, and patience to make my Ph.D. experience productive and stimulating. I am grateful to all my Dissertation Committee: Prof. Bo Hu, Prof. Kechun Zhang and Prof. William Tai Yin Tze. Each of the members has provided me extensive personal and professional guidance and taught me a great deal about both scientific research and life in general. The lab group has been a source of friendships as well as good advice and collaboration. I especially thank to my group members: Dr. Xueyan Zhao, Dr. Bo Tang, Dr. Guoying Dai, Mr. Ben Frigo-Vaz and Mr. Zhen Qiao. I would like to acknowledge honorary Prof. Jungbae Kim group in Korean. We worked together on carbon capture and release experiments, and I very much appreciated his enthusiasm, intensity, willingness to share his knowledge. Lastly, I would like to thank my family for all their love and encouragement. For my parents who raised me concerning science and supported me in all my pursuits. And most of all for my loving, supportive, and patient wife Fang Xu whose dedicated support during the final stages of this Ph.D. is so appreciated. Thank you. i Abstract As the most significant anthropogenic greenhouse gas, carbon dioxide emitted as an industrial pollution waste can be probably most effectively handled by the Carbon Capture and Storage (CCS) strategy. The energy efficiency of currently available CCS technologies has been, however, quite deterring for large scale practice of CCS. Searching energy efficient CCS strategies via biological means, this research examines factors limiting reaction equilibrium conversion and reaction kinetics for biocatalytic carbon conversion reactions. It is assumed that, biocatalytic carbon conversion can be realized at energy cost close to theoretical efficiency, thanks to the unique energy transfer features of biological reactions; however, equilibrium thermodynamics, as well as kinetics, have to be improved to realize intensified reactions to match the scale and rate of industrial carbon emission. Among the numerous CO2-conversion and CO2-elution reaction pathways found in biology, the isocitrate dehydrogenase (ICDH) reaction in reductive tricarboxylic acid (RTCA) cycle was taken as our study model system due to its reversibility and again high reaction intensity. The overall objective of this research is to establish an ICDH based carbon capture and release strategy with high energy efficiency and reaction intensity, improved equilibrium conversion yield and enhanced catalyst stability. For reaction equilibrium, the pH sensitivity of ICDH was first examined by studying the effect of system pH on reaction equilibrium both theoretically and experimentally. As the theoretical prediction was relying on the Debye–Hückel theory, experimentally ii determined reaction equilibrium was obtained by finding the equilibrium point with zero net reaction rate. The results showed that ICDH reaction exhibited a high pH sensitivity and pH was proven as an effective factor to manipulation the reaction direction. With the pH changed between 5 and 9, the reaction equilibrium constant can be shifted by a factor of ~500 fold. To enhance reaction kinetics, screening ideal enzyme and optimizing reaction condition were conducted. ICDH from Chlorobium limicola revealed the highest specific activity: 15.3 U/mg at pH 6 for carboxylation and 90.2 U/mg at pH 9 for decarboxylation. The optimum temperature and ionic strength for ICDH was 45oC and 200 mM respectively. With higher reaction rate, the gas phase carbon dioxide instead of bicarbonate was considered as the primary substrate for carboxylation. For carbon capture capacity, additional thermodynamic driving force can be created via designing of cascade reaction scheme. In this study, aconitase was introduced and isocitrate was further converted to citrate, lowing the overall Gibbs free energy of reaction. The cascade reactions had the potential to capture 0.48 mol of carbon for each mole of the substrate, approaching the intensities realized with chemical absorbents such as MEA. Enzyme immobilization was conducted to improve system stability. During the process, 20 ~ 30 mg enzyme can be absorbed on 1 gram of mesoporous silicon foam (MSF) within 5 mins with the specific activity of decarboxylation in the order of 6 U per gram MSF. After immobilization, enzyme stability against harsh environmental factors such as high temperature, extreme pH and high shear stress was improved significantly. iii With economic analysis, the ICDH strategy can bring a 66% improvement in energy efficiency. The cost and stability of cofactor (NADPH) were the most significant barriers to the large-scale application of ICDH, mainly due to the poor stability of cofactor under the acid condition. This work demonstrates the feasibility of a novel carbon capture strategy applying reversible enzyme reactions. It reveals that reaction equilibrium can be effectively by manipulating pH and reaction pathways. The reaction intensity and stability could also be improved through reaction condition optimization and enzyme immobilization. With the discovery of new source enzymes and molecular modification of existing enzymes, the intensity and stable of biocatalytic CCS are expected to be proficient in comparison to traditional chemical carbon capture strategies, benefiting from the overall energy efficiency of biological reactions. iv Table of Contents Acknowledgements ......................................................................................................... i Abstract .......................................................................................................................... ii Table of Contents ........................................................................................................... v List of Tables ................................................................................................................. x List of Figures .............................................................................................................. xii Chapter 1. General Introduction .................................................................................... 1 1.1 Carbon dioxide emission and environment concerns ........................................... 1 1.2 Carbon capture and storage .................................................................................. 4 1.3 Biological CCS ..................................................................................................... 9 1.4 Biochemical CCS ............................................................................................... 10 1.5 Objectives ........................................................................................................... 12 Chapter 2. Literature Review ....................................................................................... 14 2.1 Biocatalysts based carbon capture...................................................................... 14 2.2 pH sensitivity of biocatalytic carbon capture ..................................................... 20 2.3 Reaction intensity of biocatalytic CCS .............................................................. 21 2.4 Reaction stability of biocatalytic CCS ............................................................... 26 2.5 Economy feasibility of CCS ............................................................................... 32 Chapter 3. pH Sensitivity of Biocatalytic Carboxylation Reaction ............................. 37 3.1 Introduction ........................................................................................................ 37 3.2 Materials and methods ....................................................................................... 38 3.2.1 Materials ...................................................................................................... 38 3.2.2 Experimentally determination of Gibbs free energy of reaction ................. 38 3.2.3 Theoretical calculation of Gibbs free energy of reaction ............................ 39 3.3 Results and discussion ........................................................................................ 39 3.3.1 Experimental determination of equilibrium constant .................................. 39 v 3.3.2 The effect of pH on equilibrium constant .................................................... 42 3.3.3 Debye–Hückel model of ICDH reaction ..................................................... 43 3.3.4 Prediction of carbon dioxide fixation with ICDH ....................................... 47 3.3.5 pH sensitivity of other biochemical reactions for carbon fixing ................. 48 3.4 Conclusions ........................................................................................................ 50 Chapter 4. Characteristics of ICDH for Carbon Capture ............................................. 51 4.1 Introduction ........................................................................................................ 51 4.2 Material