Low-Cost Polymer-Supported Quaternary Ammonium Salts as High-Efficiency Catalysts for Cycloaddition of CO2 to Epoxides

Abstract

Polystyrene grafted with quaternary ammonium salts were obtained by direct quaternarization of chloromethylated polystyrene with N,N-dimethylethylamine, N,N-dimethylglycine (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. Polystyrene bearing quaternary ammonium salt ionic liquids (denoted as PS-QNS and PS-CQNS) were fabricated and evaluated for solventless synthesis of cyclic carbonates from CO2 and epoxides. The catalysts are low in cost and simple to prepare, and show good activity, selectivity, stability and reusability towards the cycloaddition reaction.