Induced Protic Behaviour in Aprotonic Ionic Liquids by Anion Basicity for Efficient Carbon Dioxide Capture

The interactions between aprotonic tetrabutylphosphonium carboxylate ionic liquids (ILs), [P4 4 4 4][CnCOO] (n=1, 2 and 7), and water were investigated. The cation‐anion interactions occur via the α‐1H on [P4 4 4 4]+ and the carboxylate headgroup of the anion. Upon addition, H2O localises around the carboxylate headgroups, inducing an electron inductive effect towards the oxygens, leading to ion‐pair separation. Studies with D2O and [P4 4 4 4][CnCOO] revealed protic behaviour of the systems, with proton/deuterium exchange occurring at the α‐1H of the cation, promoted by the basicity of the anion, forming an intermediate ylide. The greater influence of van der Waals forces of the [P4 4 4 4][C7COO] system allows for re‐orientation of the ions through larger interdigitation. The protic behaviour of the neat ILs allows for CO2 to be chemically absorbed on the ylide intermediate, forming a phosphonium‐carboxylate zwitterion, signifying proton exchange occurs even in the absence of H2O. The absorption of CO2 in equimolar IL‐H2O mixtures forms a hydrogen carbonate, through a proposed reaction of the CO2 with an intermediate hydroxide, and carboxylic acid. An in‐depth insight into the interactions between “aprotonic” ionic liquids with water mixtures, which questions our fundamental understanding of what is considered a protic and aprotonic ionic liquid depends on anion selection. The presence of strong hydrogen bond accepting (basic) anions allows for induced protic behaviour in a typically considered aprotonic cation. This in turn promotes significantly greater CO2 uptake by chemisorption.