Thermally activated construction of open metal sites on a Zn-organic framework: An effective strategy to enhance Lewis acid properties and catalytic performance for CO2 cycloaddition reactions

Abstract

For the synthesis of cyclic carbonates from CO2 and epoxides, coordinatively unsaturated metal sites of metal–organic frameworks (MOFs) could play a vital role as active Lewis acid centers for facilitating the ring-opening of epoxides. Hence, constructing open metal sites (OMSs) on MOFs will be an effective strategy to improve their catalytic activity for the CO2 cycloaddition reaction. Herein, a simple thermal treatment method is performed on [Zn2(iso)2(bpy)2]n to regulate its OMS content and corresponding Lewis acid properties. When the thermal treatment is performed at 300 °C, the obtained [Zn2(iso)2(bpy)2]n-300 retains the MOF structure well but possesses abundant OMSs (Zn2+ sites) due to the complete removal of the guest solvent. The exposed Zn2+ sites as Lewis acid centers display good affinity for CO2 adsorption during the cycloaddition reaction. Among the as-obtained catalysts, [Zn2(iso)2(bpy)2]n-300 exhibits the best catalytic activity for the synthesis of propylene carbonates from CO2 and propylene oxide. By quantitatively determining the acid strength and amount of the samples, it is verified that the moderate Lewis acid centers exhibit a more important role than the weak ones, and the catalytic activity is in accordance with their acid contents.