Bifunctional ZIF-78 heterogeneous catalyst with dual Lewis acidic and basic sites for carbon dioxide fixation via cyclic carbonate synthesis

Zeolitic imidazole framework-78 (ZIF-78) hexagonal prisms, a combination of Zn metal and imidazolate linkers, were successfully synthesized using a facile hydrothermal process. The as-prepared ZIF-78 hexagonal prisms possessed a bifunctional property with dual Lewis acidic (Zn atom) and basic sites (-NH groups of the imidazolate linkers). This bifunctional property allowed the ZIF-78 crystals to be a potential heterogeneous catalyst in the CO2/propylene oxide (PO) cycloaddition reaction for the synthesis of propylene carbonate (PC). To the best of our knowledge, this is the first work to use microporous ZIF-78 crystals as a catalyst for the CO2/PO cycloaddition reaction without the addition of any co-catalyst or solvent. The effects of reaction temperature, pressure and reaction time on the PO conversion, PC selectivity and yield were also investigated in this work. The highest PC yield of 54% was obtained at a reaction temperature of 150 degrees C, pressure of 10 kg/cm(2) and reaction time of 15 h. Moreover, a possible mechanism of the cycloaddition reaction was also proposed in this study.