| The emission of greenhouse gases, including carbon dioxide, has been put forth as the main cause of global warming. Carbon capture and sequestration are regarded as a potential means of mitigating this warming effect. One possible remedy is to convert carbon dioxide catalytically to bicarbonate, but new and robust catalysts are needed to effect this transformation in the harsh environment of absorption processes. 1,4,7,10-tetraazacylododecane ([12]aneN4 or cyclen) complex of zinc has the highest activity among all biomimetic catalysts to date, with a rate constant of 3300 M -1s-1. In this work, density functional theory (DFT) calculations were used to unravel the reaction mechanism and evaluate thermodynamic parameters. The entire catalytic cycle for CO2 hydration by the zinc-cyclen complex was mapped and a microkinetic model was built based on first principles kinetics and thermodynamics. The dependence of the observed reaction rate constant on the pH was simulated and compared with experimental data. To aid in the interpretation of the complex microkinetic model, an analytical rate expression was derived on the basis of a simplified potential energy surface.;Similar analysis was applied to a series of other carbonic anhydrase mimics, e.g., zinc-cyclam, zinc-(1,5,9-triazacyclododecane), and cobalt-cyclen. Importantly, the microkinetic model quantifies how the reaction rate and the associated overall rate constant varies as a function of time, underscoring that observed overall rate constants are a function of the species' concentrations used to extract them, which may not be the initial conditions. The decrease of initial reaction rate over 0 to 12 ms was ascribed to the decrease of the concentration of the catalytic form, Zn-OH-, which was primarily converted to Zn-HCO3-. Finally, the conversion of CO2 at 1000 ms as a function of pH was calculated to compare the relative activity of these catalysts. |
