Computational Insights into Biomimetic CO2 Hydration Activities of (Poly)borate Ions

摘要

Borate-based compounds have been observed to be CO2 capture agents in natural and synthetic setups. Carbon capture in such systems has been proposed to take place via hydration of CO2 resulting in bicarbonate ion formation. Several experimental studies have reported borate-based catalysts for CO2 hydration reaction, and the mechanism of the reaction has been proposed to follow the enzymatic carbonic anhydrase action. In view of the absence of detailed physical insights into the mechanistic aspects of borate-catalyzed CO2 hydration, computational investigations were carried out in this study under the density functional theory framework to explain the mechanism of the reaction over borate-based systems. Three borate-based systems, namely, borate ions, triborate ions, and tetraborate ions, were tested as the catalysts for the aqueous phase carbon capture reaction. Free energy landscapes provided the details of the elementary steps of the reaction, concluding the mechanism of the reaction to be indeed biomimetic consisting of parallel and bent CO2 complexation, intramolecular proton transfer, bicarbonate ion complex formation and displacement of bicarbonate ion by water molecule to form (poly)borate-water complexes. All three ions were found to be active for catalyzing the reaction. NMR and FTIR spectra of all the intermediates proposed during the biomimetic mechanism were computed and compared against the experimentally reported spectra. The comparative analyses proved the identities of the intermediates, thus further confirming the mechanism of the reaction.