Theoretical insight into the chemo-absorption mechanism of amino acid ionic liquids on CO2

Abstract

Amino acid ionic liquids (AAILs) are acknowledged as potential solvents for CO2 adsorption. However, the mechanism of AAILs adsorbing CO2 has not reached a consensus, which limited the development of high adsorption-performance AAILs. In this work, the mechanism of selected AAILs consisting of 1-Ethyl-3-methylimidazolium ([EMIM]+) cation and amino acids anions ([AA]−) adsorbing CO2 were investigated using density functional theory (DFT) calculations. The nature of the weak interactions between the ionic pairs was found to be hydrogen bonds. The amine group of [AA]− is highly likely to serve as the reactive site to adsorb CO2 with formation of zwitterions. The AAILs-CO2 were discovered to generate carbamic acid through proton transfer in zwitterions. In comparison, the generation of carbamic acid in [EMIM][Gly]-CO2 is a kinetically favorable reaction while in [EMIM][His]-CO2 it is the most thermodynamically favorable, which indicates faster adsorption of CO2 by [EMIM][Gly] and easier adsorption by [EMIM][His]. Furthermore, it was found that the presence of the cation [EMIM]+ weakens the interaction between the amino acid anion and CO2 at different reaction steps. This work revealed the fundamental chemo-absorption mechanism details of AAILs towards CO2, which contributes to rational development of AAILs with optimal CO2 absorption properties.