(219a) Engineering Aqueous Amine Solvents Based on Steric Effects for CO2 Capture from First Principles Simulations

Carbon dioxide (CO₂) emissions from fossil fuel consumption to the atmosphere remain a grand challenge and a major threat to humanity. Currently, the amine scrubbing method based on chemical absorption with aqueous amine solvents is the most widely implemented technology for the post-combustion CO₂ capture process, which is governed by two competing reactions leading to carbamate or bicarbonate formation. Sterically hindered alkanolamines with various degrees of hydrophobicity are experimentally found to influence the formation of these products, but the underlying mechanisms responsible for such preferential product formations are not well understood. In this work, we will present the CO₂ absorption behavior of several sterically hindered amines, focusing on the effects of different degrees of methylation and temperature. Our ab initio molecular dynamics with enhanced sampling simulations show that the degree of steric hindrance strongly influences the free-energy barriers for both carbamate and bicarbonate reaction pathways and, thus, impacts their kinetic favorability. Our analyses further demonstrate that various degrees of steric effects cause considerably different temperature-dependent solvation of the nitrogen atom of hindered amines, which may directly affect CO₂ accessibility to form carbamate. At low absorption temperatures, the enhanced solvation resulting from increased steric hindrance leads to kinetically facile amine protonation, promoting the formation of bicarbonate. Our study provides insights into the effects of various degrees of steric hindrance, with different temperature-dependent solvation natures governing reaction mechanisms and rates during CO₂ capture, thereby aiding in designing optimal solvents for the CO₂ capture process.