Efficient homogenous catalysis of CO2 to generate cyclic carbonates by heterogenous and recyclable polypyrazoles

The cycloaddition between CO2 and epoxides to produce cyclic carbonate is an attractive and efficiency pathway for the utilization of CO2 as C1 source. The development of catalyst to mediate cycloaddition between CO2 and epoxides at low temperature and pressure is still a challenge. Herein, a series of polypyrazoles with glass transition temperature (Tg) in the range of 42.3–52.5 °C were synthesized and served as catalyst to mediate the cycloaddition of CO2 and epoxides by the assistant of tetrabutylammonium bromide. The catalytic behaviors of polypyrazole on the model cycloaddition of CO2 to epichlorohydrin, including the reaction parameters optimization and versatility were investigated in detail, and excellent yield (99.9%) and selectivity (99%) were obtained under the optimized reaction conditions of 70 °C and 1.0 MPa for 6.0 h. Noteworthily, the polypyrazole acts as homogeneous catalyst during reaction (higher than Tg). And under room temperature, polypyrazoles can be easily separated and recovered, which is a promising feature of a heterogeneous catalyst. Furthermore, the reaction mechanism was proposed. The DFT calculation suggested that the formation of hydrogen bond between pyrazole and epoxide greatly reduced the energy barrier, which play an important role in promoting CO2 cycloaddition.