Catalytic fixation of CO₂ to cyclic carbonates over biopolymer chitosan-grafted quarternary phosphonium ionic liquid as a recylable catalyst

We prepared chitosan-grafted quarternary phosphonium ionic liquid (1-butyl-triphenylphosphonium bromide) by a simple method and used it as catalyst for the synthesis of cyclic carbonates through CO₂ cycloaddition to epoxides in the absence of co-catalyst and solvent. High yield of propylene carbonate (96.3%) is obtained at 120 °C and 2.5 MPa in 4 h. We studied the roles of chitosan hydroxyl groups and ionic liquid counter anions on catalytic activity. It is proposed that the facilitation of ring opening of epoxides is a combined result of polarization (by hydrogen bonding due to hydroxyl groups), electronic interaction (by [BuPh₃P]⁺), and nucleophilic attack (by bromide anion). The catalyst can be easily recovered and reused as demonstrated in a test of five runs without showing any significant loss of activity. From the view point of commercial application, the catalyst is attractive because it is low-cost, ecology-safe, stable and efficient. Furthermore, it is potentially applicable for fixed-bed continuous flow reactors.