An ab initio free energy study of the reaction mechanism and rate-limiting steps of CO2 capture by aqueous glycine

Aqueous amino acids are promising absorbents for direct air capture (DAC) of CO2. Herein, we investigate the possibility of kinetic control of CO2 absorption with aqueous anionic glycine (GLY−) by employing extensive ab initio molecular dynamics simulations, free energy analysis, and reaction rate theory. We find that first GLY− binds to CO2 by overcoming a barrier (7.4 kcal/mol) to form a zwitterion intermediate, which then releases a proton by overcoming a similar barrier. Despite the similarity in the barrier, zwitterion formation appears to be the rate-limiting step because it is two orders of magnitude slower (microseconds) than the proton release step. This is predominantly due to stronger nonequilibrium solvent effects for the former that cause many barrier-recrossing events and effectively slow down the reaction rate. Such a detailed fundamental understanding of the amino acid-based CO2-absorption mechanism and rates is key to improving the kinetic efficiency of DAC technology.