Ionic liquid functionalized bimetallic UiO-66-Br with multiple active sites for efficient catalytic oxidative carboxylation of olefins and CO2

The direct oxidative carboxylation of olefins and CO2 to synthesize cyclic carbonates involves a tandem process of epoxidation and CO2 cycloaddition reactions. However, designing a multifunctional catalyst for this reaction remains challenging. Herein, a composite catalyst ImBr@UiO-66-Br(Co) was synthesized by post-metallization of Co2+ and subsequent functionalization with an imidazole ionic liquid (IL), which was used effectively for this reaction. The effects of temperature, CO2 pressure, catalyst dosage, and reaction time on the direct oxidative carboxylation of olefins to cyclic carbonates were investigated. The results showed that ImBr@UiO-66-Br(Co) exhibited optimal performance in the solvent-free reaction system with styrene (3 mmol), tert-butyl hydroperoxide (6 mmol), and tetrabutylammonium bromide (TBAB, 50 mg) at 80 degrees C under 1 MPa CO2 for 24 h, achieving a styrene conversion of 90.3 % and styrene carbonate selectivity of 80.7 %. The excellent catalytic activity of ImBr@UiO-66-Br(Co) was attributed to the synergistic interactions of the Lewis acidity Zr4+ sites, the strong nucleophilicity provided by Br-, the weak base sites from tertiary amine and quaternary ammonium groups in imidazole IL, and the redox-active Co2+ sites. Additionally, the catalyst was easy to separate and maintained stable activity and selectivity over at least five cycles. Finally, the potential catalytic mechanism for the direct oxidative carboxylation of olefins to cyclic carbonates was proposed.