MCM-41 supported quaternary ammonium ionic liquids as an effective heterogeneous catalyst for CO2 cycloaddition reaction

Ionic liquid immobilization is an effective means for preparing metal-free heterogeneous catalysts for the CO2 conversion. Herein, a series of quaternary ammonium ionic liquids functionalized MCM-41 heterogeneous catalysts, integrating hydrogen bond donors and nucleophilic ion sites, were prepared by a post-synthetic modification method for the CO2 cycloaddition reaction. The physicochemical properties of the catalysts were analyzed by XRD, EA, SEM, TEM, BET, FT-IR, TGA, NMR and XPS. The results show that the quaternary ammonium ionic liquids are successfully immobilized onto the MCM-41 molecular sieves. The obtained catalysts maintain an excellent pore structure and have the good thermal stability. The effects of catalytic conditions on the catalytic performance, the recyclability of the catalysts and their generalizability to epoxide substrates were systematically investigated. In addition, the catalytic performance was evaluated for low concentrations of CO2 (20% CO2, 80% N2). The synergy of multifunctional active sites makes the obtained materials exhibit the high catalytic activity without metals, co-catalysts and solvents. Furthermore, a possible mechanism for the conversion of CO2 to cyclic carbonate catalyzed by the MCM-41-N/CH3(1:3) was proposed.