Stepwise reaction mechanism of carbon dioxide capture by aqueous glycine: An ab initio free energy study

Direct air capture (DAC) is an emerging technology that aims to alleviate the massive post-combustion CO₂ emissions by removing CO₂ from air. Aqueous amino acid absorbents showing excellent physicochemical properties offer a promising approach to separate CO₂ from gas stream. Despite numerous experimental studies conducted to investigate the thermodynamics and kinetics of CO₂ capture by amino acid salt solutions, the theoretical reaction model has not been well examined at the atomistic scale. In this work we have investigated a two-step mechanism involving CO₂ addition and deprotonation steps. Here, we aim to quantify the stepwise reaction free energy profile as well as the reaction rate constants for CO₂ captured by aqueous glycine anions. Density function theory is used to describe both the nuclear and electron degrees of freedom in condensed phase, and on-the-fly ab initio molecular dynamics (AIMD) simulations are performed to accurately sample the reactive phase space. The stepwise reaction mechanism is firstly explored by metadynamics simulations, and later quantified by free energy calculations using umbrella sampling simulations. The stepwise reaction rate constants and the zwitterion intermediate lifetime are estimated together by transition state theory and unimolecular dissociation trajectory calculations. The predicted reaction rate constant is in excellent agreement with the published experimental value.