Hierarchically nanoporous copolymer with built-in carbene-CO2 adducts as halogen-free heterogeneous organocatalyst towards cycloaddition of carbon dioxide into carbonates

Abstract

A serial of porous polymers incorporating N‑heterocyclic carbene-CO2 adducts (designated as NHC-CO2 adducts) were facilely synthesized via radical polymerization of N-vinylimidazolate and ethyleneglycol dimethacrylate plus post-treatment with dimethyl carbonate (DMC). The polymers possessed tunable chemical structure, large surface area along with abundant meso-macropores and enhanced CO2 uptake which functioned as metal-halogen-free heterogeneous organocatalysts for solvent-free cycloaddition of CO2 to styrene oxide under 10 bar CO2 pressure, exhibiting highly catalytic activity and stable recyclability. Further, turnover frequency (TOF) of 7.1 h−1 (94.3% yield, 12 h) was obtainable under ambient condition, much higher than those of reported metal-halogen-free heterogeneous catalysts, even comparable with existing halogen-containing ones. Furthermore, the catalyst could efficiently convert various epoxides including internal epoxides into corresponding cyclic carbonates under mild condition (1 bar, 70–120 °C). The excellent catalytic performance was ascribed to the appropriate proportion of NHC-CO2 adducts in porous skeleton structure triggered by DMC. A plausible reaction mechanism was proposed to understand NHC-catalyzed fixation of CO2. This study highlights porous organic polymer is a versatile platform of NHC-CO2 adducts to construct eco-friendly solid catalyst towards efficient CO2 conversion.