Bifunctional Y/g-C3N4 promoted cycloaddition of CO2 to epoxide: Halogen-free and acid-base synergistic catalysis

Cycloaddition reaction of CO2 to epoxides is an important procedure for both CO2 conversion and cyclic carbonate synthesis. Homogeneous alkali halide and quaternary ammonium salt are the general catalysts, while they have the problems of halogen corrosion and residue, intricate separation, and catalyst recycling. Here, a series of heterogenous and halogen-free bifunctional yttrium-coordinated g-C3N4 were synthesized and employed to catalyze the cycloaddition of CO2 to butylene oxide. Excellent butylene oxide conversion (94.5%) and butene carbonate selectivity (95.0%) were achieved over the typical Y/g-C3N4-0.5 catalyst without any halogen addition. The characterizations demonstrate that the coordinated Y species function well as the preferred Lewis acid sites for epoxide ring opening and butene carbonate desorption, and the uncondensed amino groups are supplied as Lewis base sites to activate CO2. It is the synergistic effect based on acid-base duality that enhances the catalytic performance of Y/g-C3N4 to cycloaddition of CO2 with epoxide.