Binary metal-organic frameworks: Catalysts for the efficient solvent-free CO2 fixation reaction via cyclic carbonates synthesis

Two types of binary metal-organic frameworks (MOFs) using Cu (from Cu-BTC) and Zr (from UiO-66) as the metal centers have been synthesized by the solvothermal method and characterized using different physicochemical techniques. The catalytic potential of binary MOFs was clearly demonstrated for the cycloaddition reaction of carbon dioxide (CO2) with epoxides under solvent-free conditions. The effects of various parameters such as catalyst amount, temperature, reaction time, and CO2 pressure were studied, and a moderate set of reaction conditions (0.16 mol% of the catalyst, 60 degrees C, 8 h and 1.2 MPa CO2 pressure) was selected for detailed analysis. The synthesized Cu/Zr MOFs were used for the CO2-epoxide cycloaddition reaction with a tetrabutylammonium bromide (TSAR) co-catalyst. The UiO-66/Cu-BTC displayed excellent conversion of epichlorohydrin (ECH) with > 99% selectivity. The appreciable conversion of ECH with the UiO-66/Cu-BTC/TBAB system was influenced by the synergistic effect of the Cu and Zr metals and the Br ion from TBAB. The scope of extending this catalysis to various epoxides was established, and a recyclability study was also conducted. Finally, based on our previous DFT (density functional theory) studies and experimental inferences, a plausible reaction mechanism for the binary MOF-catalyzed epoxide-CO2 cycloaddition reaction was proposed.