Bifunctional metal-doped ZIF-8: A highly efficient catalyst for the synthesis of cyclic carbonates from CO2 cycloaddition

The acid-base properties of ZIF-8 are successfully tuned by doping different transition metal ions into the crystalline lattice, for their effects on the catalytic activity of the CO2 cycloaddition reaction. The as-synthesized metal-doped ZIF-8 samples maintain rhombic dodecahedral morphology, high specific surface areas and high thermal stability of pure ZIF-8. Coexistence of acidic and basic sites allowed metal-doped ZIF-8 to be a highly efficient heterogeneous catalyst for the CO2 cycloaddition reaction without co-catalyst and solvent. Both weak and moderate acid/base sites are active sites for cycloaddition reaction, but the weak acid/base sites contribute more to the increase in catalytic activity. Fe/ZIF-8 catalyst shows the best catalytic activity among the metal-doped ZIF-8 samples tested with a 96.6% yield of cyclic carbonate, attributed to its large amount of weak acid/base sites. Correlation between acid-base characteristics and catalytic activities has been established, which is of great significance for the development of catalysts for CO2 cycloaddition.