Mechanisms of CO₂ capture in ionic liquids: a computational perspective

We present computational studies of CO₂ sorption in two different classes of ionic liquid. The addition of carbon dioxide to four superbase ionic liquids, [P₃₃₃₃][Benzim], [P₃₃₃₃][124Triz], [P₃₃₃₃][123Triz] and [P₃₃₃₃][Bentriz], was studied using the DFT approach and considering anions alone and individual ion pairs. The addition of CO₂ to the anion alone clearly resulted in the formation of a covalently-bound carbamate function with the strength of binding correlated to experimental capacity. In the ion pair however the cation significantly alters the nature of the bonding such that the overall cohesive energy is reduced. Formation of a strong carbamate function occurs at the expense of weakening the interaction between anion and cation. In [N₁₁₁₁][l-ALA], a representative amino acid ionic liquid, evidence was found for a low-energy monomolecular mechanism for carbamate formation, explaining the 1 : 1 molar uptake ratio observed in some amino acid ionic liquids. The mechanism involves proton transfer to the carboxylate group of the aminate anion.