Green conversion of CO2 by N-heterocyclic carbene-CO2 adducts grafted hierarchical porous silica microspheres

The multiphase catalysts DMSNs-Cb-X containing NHC-CO₂ adducts were synthesized by soft template method that is, first preparing dendritic multi-model pore silica nanoparticles (DMSNs) with central-radial structure, and then grafting imidazoline functional groups, and finally allowing to react with dimethyl carbonate to graft NHC-CO₂ adducts. The investigations had the scope to improve the performance and to elucidate the effect of NHC-CO₂ adducts on the CO₂ cycloaddition reaction. The optimum catalyst DMSNs-Cb-1 and the optimum reaction conditions (200 mg, 100 °C, 10 h) were obtained via thermogravimetric analysis. CO₂ cycloaddition reaction with epoxide DMSNs-Cb-1 can convert CO₂ and epoxide to cyclic carbonate at ambient pressure without solvent and co-catalysts (1 bar, 100 °C). The reaction proceeds with high yield and high conversion frequency (TOF = 27.4 h⁻¹). The prepared catalyst is easily recovered and still has high activity after five cycles of experiments. The excellent catalytic performance of the obtained catalyst can be attributed to the synergy of hierarchical porous structure and NHC-CO₂ adducts acting as CO₂ adsorption sites as well as zwitterionic catalytic sites. This work demonstrates an efficient strategy for green conversion of CO₂ into cyclic carbonates.