Aminoethylimidazole ionic liquid-grafted MIL-101-NH2 heterogeneous catalyst for the conversion of CO2 and epoxide without solvent and cocatalyst

Abstract

Besides the activation of CO₂ by Lewis basic groups, the synergistic effects of Lewis acid sites (or hydrogen bond donor) together with halogen nucleophilic anions to activate epoxide ring opening were also important steps to develop an efficient catalyst for the cycloaddition reaction of CO₂ and epoxides. Herein, an aminoethylimidazole IL-modified MOF catalyst with multiple catalytic active sites (denoted as MIL-101-NHIM-NH₂) was synthesized through an aldimine condensation post-synthesis method. Through thorough characterizations, such as FT-IR, XRD, SEM-EDS mappings, N₂ adsorption and desorption, XPS, elemental analysis and TG measurement, the results showed that the surface of MIL-101-NHIM-NH₂ was successfully grafted with the ionic liquid. Owing to the synergistic effects of multiple active sites containing Cr, –NH₂ and Br⁻ in the MIL-101-NHIM-NH₂ catalyst, MIL-101-NHIM-NH₂ could efficiently catalyze the cycloaddition reaction of CO₂ and epoxide without additional solvent and cocatalyst; 93.5% of propylene carbonate was obtained at 120 °C, 2 MPa and 4 h. Moreover, MIL-101-NHIM-NH₂ also showed better recycling stability within five catalytic runs and easy separation in the CO₂ cycloaddition reaction.