Probing strong steric hindrance effects in aqueous alkanolamines for CO2 capture from first principles

Abstract

Carbon dioxide (CO2) absorption into aqueous amine solvents is known to be governed by two competing reactions leading to carbamate or bicarbonate formation. Sterically hindered alkanolamines with different degrees of methylation are experimentally known to cause different favorability in forming carbamate or bicarbonate. However, the fundamental mechanisms responsible for the preferential formation of carbamate or bicarbonate still remain unclear. Herein, we present CO2 absorption behavior with several different sterically hindered amines, especially the effects of methylation and temperature. Our ab initio metadynamics simulations clearly demonstrate that free-energy barriers for the carbamate and bicarbonate formation reactions strongly depend on the degree of steric hindrance of hydrophobic methyl groups. This in turn directly impacts on the kinetic favorability of the two competing reaction pathways. Our analysis shows that various degrees of steric effects cause considerably different temperature-dependent degrees of solvation of the nitrogen atom of hindered amines, which may directly affect CO2 accessibility to form carbamate. Moreover, the enhanced solvation as a result of increased steric effects particularly at low absorption temperatures lead to kinetically facile protonation of the nitrogen atom of amines, initiating and promoting bicarbonate formation. Our work sheds light on the effects of various degrees of steric hindrance, with different temperature-dependent solvation degrees governing reaction mechanisms and rates during CO2 capture.