On the Key Influence of Amino Acid Ionic Liquid Anions on CO2 Capture.

Abstract

Amino acid ionic liquids (AAILs) are promising green materials for CO2 capture and conversion due to their large chemical structural tunability. However, the structural understanding of the AAILs underlying the CO2 reaction dynamics remains uncertain. Herein, we examine the steric effects of AAIL anions with various chemical structures on CO2 capture behavior. Based on ab initio free-energy sampling, we assess reaction mechanisms for carbamate formation via a two-step reaction pathway with a zwitterion intermediate undergoing dynamic proton transfer. Our results show that free-energy barriers for carbamate formation can be significantly reduced as the degree of steric hindrance of the anions decreases. Further analyses reveal that reduced steric hindrance of anions causes markedly stronger intermolecular interactions between zwitterion and anions, leading to an increased kinetically favorable intermolecular proton transfer for carbamate production. We also describe the correlation strength between intramolecular interactions within the zwitterion and intermolecular interactions between the zwitterion and anions. We conclude that the favored structural flexibility due to the less steric hindrance of the zwitterion leads to enhanced intermolecular interactions, facilitating proton transfer to nearby AAIL anions for carbamate formation. Our study provides invaluable insight into the influence of various degrees of steric hindrance of the AAIL anions governing CO2 chemisorption. These findings may aid in the design of optimal AAIL solvents for the CO2 capture process.