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_nCOO] (n=1, 2 and 7), and water were investigated. The cation-anion interactions occur via the α-¹H on P_{4 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 P_{4 4 4 4}]C_nCOO] 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 P_{4 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.