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

The interactions between aprotonic tetrabutylphosphonium carboxylate ionic liquids (ILs), [P 4 4 4 4 ][C n COO] ( n =1, 2 and 7), and water were investigated. The cation‐anion interactions occur via the α ‐ 1 H on [P 4 4 4 4 ] + and the carboxylate headgroup of the anion. Upon addition, H 2 O localises around the carboxylate headgroups, inducing an electron inductive effect towards the oxygens, leading to ion‐pair separation. Studies with D 2 O and [P 4 4 4 4 ][C n COO] revealed protic behaviour of the systems, with proton/deuterium exchange occurring at the α‐ 1 H of the cation, promoted by the basicity of the anion, forming an intermediate ylide. The greater influence of van der Waals forces of the [P 4 4 4 4 ][C 7 COO] system allows for re‐orientation of the ions through larger interdigitation. The protic behaviour of the neat ILs allows for CO 2 to be chemically absorbed on the ylide intermediate, forming a phosphonium‐carboxylate zwitterion, signifying proton exchange occurs even in the absence of H 2 O. The absorption of CO 2 in equimolar IL‐H 2 O mixtures forms a hydrogen carbonate, through a proposed reaction of the CO 2 with an intermediate hydroxide, and carboxylic acid.