Low-cost polymer-supported quaternary ammonium salts as high-efficiency catalysts for cycloaddition of CO₂ to epoxides

Polystyrene grafted with quaternary ammonium salts were obtained by direct quaternarization of chloromethylated polystyrene with N,N-dimethylethylamine, N,Ndimethylglycine (denoted as PS-QNS and PS-CQNS, respectively). These two polymer-supported ionic liquids were found to show high catalytic efficiency towards the cycloaddition of CO₂ to epoxides. The effects of catalyst amount, reaction temperature, initial CO₂ pressure, and reaction time on the yield of cyclic carbonates were investigated systematically. Under the optimal reaction conditions (150 °C, initial CO₂ pressure 2.0 MPa and 5 h), propylene carbonate yield and selectivity in the cycloaddition of CO₂ to propylene oxide over PS-CQNS were 96.2 and 99.3 % whereas those over PS-QNS were 97.9 and 99.5 %, respectively. It was observed that the reusability of PS-QNS was excellent. From the viewpoint of industrial application, PS-QNS is attractive in terms of catalytic efficiency, cost and simplicity in preparation.