Theoretical investigation of the amine structural properties on the CO2 adsorption mechanism over amine-functionalized mesoporous silica sorbents

A solid understanding of the adsorption mechanisms of carbon dioxide (CO2) over amine-modified sorbents as well as the effect of amine structural properties on these mechanisms are essential for the systematic design of sorbents that would effectively reduce the CO2 emissions. This study aims to examine the interaction of CO2 with various amine-modified silica sorbents thoroughly by using density functional theory (DFT) modeling, with the purpose of clarifying the potential impact of amine functionality, surface amine density, surface silanol density, alkyl linker length and the structure of the amine (cyclic vs. linear) on the adsorption behavior and mechanism. Our calculations performed over APTMS (3-aminopropyl)tri-methoxysilane), MAPS (3-(methylamino)propyl-trimethoxysilane), and PYR (pyrrolidine) based sorbents indicated that CO2 was adsorbed in the forms of carbamic acid, ammonium carbamate and surface-bound carbamate structures. Surface-bound carbamates had extreme stability and were observed in the case of short alkyl linkers. Complex hydrogen bonding networks were indispensable for feasible adsorption of CO2 and hydrogen bonding interactions were the most deterministic factor on the adsorption performance making surface amine density, surface silanol density, amine structure, and alkyl chain length quite significant. Amine functionality, on the other hand, seemed to be less conclusive regarding the adsorption mechanism under the applied conditions.