Zn-based ionic liquids as highly efficient catalysts for chemical fixation of carbon dioxide to epoxides

Abstract

The novel Zn-based task-specific ionic liquids (Zn-TSILs) catalysts were developed for the coupling of carbon dioxide and epoxides to form cyclic carbonates under mild reaction conditions without using additional organic solvents and cocatalysts. Due to the synergistic effects of the cation and anion in this catalytic system, excellent yields and selectivities to cyclic carbonates were achieved with high TOF values up to 794 h⁻¹. Among the catalysts investigated, OH-containing Zn-TSILs showed better activity than COOH-containing Zn-TSILs, and [(CH₂CH₂OH)Bim]ZnBr₃ was found to be the best. Additionally, the influences of CO₂ pressure and catalyst concentration were also investigated over [(CH₂CH₂OH)Bim]ZnBr₃. In addition, the rate constants as well as the activation energies for the cycloaddition reaction catalyzed by Zn-TSIL and TSIL were comparatively determined. The activation energy was calculated to be 34.1 kJ mol⁻¹ for bare TSIL catalyst, whereas the Zn-TSIL reduced the activation energy value by 14.7 kJ mol⁻¹. Moreover, the Zn-TSIL was easily recyclable without significant loss of activity, representing the exceptionally promising candidate for the effective fixation of CO₂ to epoxides.