CO₂ capture in ionic liquid 1-alkyl-3-methylimidazolium acetate: a concerted mechanism without carbene

Abstract

Ionic liquids (ILs) provide a promising medium for CO₂ capture. Recently, the family of ILs comprising imidazolium-based cations and acetate anions, such as 1-ethyl-3-methylimidazolium acetate (EMI⁺OAc^-), has been found to react with CO₂ and form carboxylate compounds. N-Heterocyclic carbene (NHC) is widely assumed to be responsible by directly reacting with CO₂ though NHC has not been detected in these ILs. Herein, a computational analysis of CO₂ capture in EMI⁺OAc^- is presented. Quantum chemistry calculations predict that NHC is unstable in a polar environment, suggesting that NHC is not formed in EMI⁺OAc^-. Ab initio molecular dynamics simulations indicate that an EMI⁺ ion "activated" by the approach of a CO₂ molecule can donate its acidic proton to a neighboring OAc^- anion and form a carboxylate compound with the CO₂ molecule. Analysis of this termolecular process indicates that the EMI⁺-to-OAc^- proton transfer and the formation of 1-ethyl-3-methylimidazolium-2-carboxylate occur essentially concurrently. Based on these findings, a novel concerted mechanism that does not involve NHC is proposed for CO₂ capture.