Catalytic cycloaddition of CO2 to epoxides by the synergistic effect of acidity and alkalinity in a functionalized biochar

Cycloaddition of CO2 to epoxides is an efficient way for CO2 fixation and recycling. Currently, the mainly used metal-containing homogeneous catalysts and halide co-catalysts for cycloaddition of CO2 could bring out metal leaching problem, which is harmful to the environment. Herein, we propose a facile way to prepare nitrogen and oxygen-rich porous biochar by one-pot pyrolysis of chitosan under CO2 atmosphere, and demonstrate the high performance of the obtained metal-free carbon material catalyst with plenty of Lewis base sites and Lewis acid sites on cycloaddition of CO2 to epoxides. The doped nitrogen and oxygen show a synergistic effect for this addition reaction, and the catalytic efficiency can be enhanced through tuning the content of nitrogen and oxygen. The yield and selectivity of epichlorohydrin (ECH) can reach to 90% and 97% with no solvents or co-catalysts under relatively mild reaction conditions, respectively. Materials characterizations and theory calculation results suggested that the high catalytic performance of the N,O-codoped porous biochar was ascribed to its excellent adsorption of CO2 and epoxide molecules due to its high specific surface area and consequent activation by its integrated acid–base active sites. This work illustrates a new strategy for recycling CO2 by using it as a pyrolytic agent and a feedstock for the synthesis of cyclic carbonates.