Functionalized phosphonium-based ionic liquids as efficient catalysts for the synthesis of cyclic carbonate from expoxides and carbon dioxide

A series of novel functionalized phosphonium-based ionic liquids (FPBILs) were synthesized by a simple method, and first evaluated as catalysts for the synthesis of cyclic carbonates through the cycloaddition of CO₂ to epoxides in the absence of co-catalyst and solvent. The FPBILs perform well in the cycloaddition reaction, especially the carboxyl-functionalized one. Over [Ph₃PC₂H₄COOH]Br, the yield of propylene carbonate is 97.3% (TOF = 64.9 h^-1) at 130 C and 2.5 MPa in 3 h. The synergistic effects of polarization induced by hydrogen bonding and nucleophilic attack of Br^-anion account for the excellent performance. Furthermore, the FPBILs with moderate methylene chain length show superior catalytic activity. It is because they have both strong acidity and weak electrostatic interaction between phosphonium cation and halide anion. The strong acidity facilitates the ring-opening of epoxyl, and the weak electrostatic interaction enhances the nucleophilic attack capability of Br ^-. It is envisaged that the metal- and solvent-free process has high potential for the catalytic conversion of CO₂ into value-added chemicals.