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, H₂ O localises around the carboxylate headgroups, inducing an electron inductive effect towards the oxygens, leading to ion-pair separation. Studies with D₂ 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} ][C₇ COO] 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 H₂ O. The absorption of CO₂ in equimolar IL-H₂ O mixtures forms a hydrogen carbonate, through a proposed reaction of the CO₂ with an intermediate hydroxide, and carboxylic acid.