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

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 k_cat 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 pK_a 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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            (4)        Zhang, X. P.; Vaneldik, R. Inorganic Chemistry 1995, 34, 5606.

            (5)        Silverman, D. N.; Lindskog, S. Accounts Chem Res 1988, 21, 30.

            (6)        Koziol, L.; Valdez, C. A.; Baker, S. E.; Lau, E. Y.; Floyd, W. C.; Wong, S. E.; Satcher, J. H.; Lightstone, F. C.; Aines, R. D. Inorganic Chemistry 2012, 51, 6803.

            (7)       Huang, D. G.; Makhlynets, O. V.; Tan, L. L.; Lee, S. C.; Rybak-Akimova, E. V.; Holm, R. H. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 1222.