(472c) Microkinetic Modeling of CO2 Hydrolysis Over Zn-(1,4,7,10-tetrazacyclododecane) Catalyst Based On First Principles: Revelation of Rate-Determining Step

Authors: 
Ma, R., Northwestern University
Broadbelt, L. J., Northwestern University



The emission of greenhouse gases, including carbon dioxide, has been put forth as the main cause of global warming.1,2 Carbon capture and sequestration are regarded as a potential means of mitigating this warming effect.2 One possible remedy is to convert carbon dioxide catalytically to HCO3-. In nature, HCA II, a member of the human carbonic anhydrase (CA) family of enzymes, is one of the fastest known enzymes with a kcat for CO2 hydrolysis at a value of 106 s-1, which approaches the diffusion limit.3 However, due to the high cost of isolation and purification, as well as the unstable nature of enzymes when removed from their natural environment, biomimetic catalysts are a more desirable choice whenever they are available. 1,4,7,10-tetraazacylododecane ([12]aneN4 or cyclen) complex of zinc, to the best of our knowledge, has the highest activities among all biomimetic catalysts to date, with a rate constant of 3300 M-1s-1.4 In this study, density functional theory (DFT) calculations were used to unravel the reaction mechanism and evaluate thermodynamic parameters. We mapped the entire catalytic cycle for CO2 hydration by the zinc-cyclen complex and built a microkinetic model based on first principles kinetics and thermodynamics. The dependence of the observed reaction rate constant on the pH has a sigmoidal shape, which is similar to HCA II.5 The inflection point is identical to the pKa value of the zinc-cyclen complex, indicating that the rate-limiting step involves Zn-cyclen-OH+. The reaction rate constant is calculated to be 3650.5 M-1s-1, while the experimental value has been reported to be 3300 M-1s-1 or 2691.5 M-1s-1.4,6 Since our microkinetic model captured the experimental data well, we then analyzed each elementary step to identify the rate-determining step of the catalytic cycle through calculating the ratio of the net rate to the forward rate of reaction. Our model indicated that adsorption of CO2 is the rate-determining step. In addition, we analyzed the initial CO2 adsorption rate, which corresponds to the reaction: Zn-cyclen-OH- + CO2 → Zn-cyclen-HCO3-. The rate constant for this reaction is calculated to be 1.82×105 M-1s-1, which is on the same order of magnitude as the CO2 adsorption rate constant of a novel biomimetic catalyst reported by Huang and coworkers.7 Ultimately, these results provide a basis for the design of new biomimetic catalysts for CO2 hydrolysis.

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