Biocatalytic Carbon Capture And Conversionbiocatalytic Carbon Capture And Conversion

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 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. 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.