CS2 capture in the ionic liquid 1-alkyl-3-methylimidazolium acetate: reaction mechanism and free energetics

Abstract

Reaction pathways for CS₂ and COS in the ionic liquid, 1-ethyl-3-methylimidazolium (EMI⁺) acetate (OAc⁻), are studied using the ab initio self-consistent reaction field theory (SCRF) and molecular dynamics (MD) computer simulations. It is found that while CS₂ converts to COS nearly at the 100% level through S/O exchange with acetate, both conversion and capture processes are kinetically possible for COS, yielding CO₂/thioacetate and 1-ethyl-3-methylimidazole-2-thiocarboxylate (EMI-COS)/acetic acid as reaction products, respectively. These findings are in excellent agreement with recent experimental observations in the closely related 1-butyl-3-methylimidazolium acetate (BMI⁺OAc⁻) ionic liquid system. Constrained ab initio MD indicates that the capture reaction of COS (and CS₂ if allowed) proceeds in a concerted fashion; viz., proton transfer from EMI⁺ to OAc⁻ and carboxylation of EMI⁺ by COS (and CS₂) occur concurrently, analogous to the concerted pathway proposed recently for CO₂ capture in the imidazolium acetate ionic liquid family. As N-heterocyclic carbene (NHC) is not required, the concerted mechanism is fully consistent with the experimental fact that NHC has not been detected directly in this ionic liquid family. Computational analysis further predicts that if NHC would be present in the ionic liquid, it would react with CS₂ and produce 1-ethyl-3-imidazole-2-dithiocarboxylate, prior to the conversion of CS₂ to COS. Since such a dithiocarboxylate compound was not detected experimentally, the present analysis lends support to the view that NHC is not formed in the pure imidazolium acetate ionic liquid family.