Molecular insights into the enhanced rate of CO2 absorption to produce bicarbonate in aqueous 2-amino-2-methyl-1-propanol

Abstract

2-Amino-2-methyl-1-propanol (AMP), a sterically hindered amine, exhibits a much higher CO₂ absorption rate relative to tertiary amine diethylethanolamine (DEEA), while both yield bicarbonate as a major product in aqueous solution, despite their similar basicity. We present molecular mechanisms underlying the significant difference of CO₂ absorption rate based on ab initio molecular dynamics simulations combined with metadynamics. Our calculations predict the free energy barrier for base-catalyzed CO₂ hydration to be lower in aqueous AMP compared to DEEA. Further molecular analysis suggests that the difference in free energy barrier is largely attributed to entropic effects associated with reorganization of H₂O molecules adjacent to the basic N site. Stronger hydrogen bonding of H₂O with N of DEEA than AMP, in addition to the presence of bulky ethyl groups, suppresses the thermal rearrangement of adjacent H₂O molecules, thereby leading to lower stability of the transition state involving OH⁻ creation and CO₂ polarization. Moreover, the hindered reorganization of adjacent H₂O molecules is found to facilitate migration of OH⁻ (created via proton abstraction by DEEA) away from the N site while suppressing CO₂ approach. This leads us to speculate that catalyzed CO₂ hydration in aqueous DEEA may involve OH⁻ migration through multiple hydrogen-bonded H₂O molecules prior to reaction with CO₂, whereas in aqueous AMP it seems to preferentially follow the one H₂O-mediated mechanism. This study highlights the importance of entropic effects in determining both mechanisms and rates of CO₂ absorption into aqueous sterically hindered amines.