Quantum chemical analysis of carbon dioxide absorption into aqueous solutions of moderately hindered amines

Aqueous solutions of moderately hindered amines are promising absorbents for the capture of CO2 from a gas stream because of their relatively low heats of reaction and high rates of absorption. To determine the reaction mechanisms underlying the absorption process, we performed a quantum chemical analysis of the absorption of CO2 into aqueous amine solutions and carried out an experimental comparison of representative amines. We correlated properties such as absorption capacity and product ratio with results from free energy calculations using the conductor-like screening model for real solvents (COSMO-RS) coupled with density functional theory (DFT). Additionally, the reaction mechanisms were investigated by transition state optimizations and intrinsic reaction coordinate (IRC) calculations in an aqueous solution phase by DFT calculations using the latest continuum solvation model (SMD/IEF-PCM). The results indicated that a bicarbonate anion likely forms directly by the reaction of CO2, H2O and an amine rather than by the hydrolysis of a carbamate anion.