Lewis Acid-Base Bifunctional Crystals with a Three-Dimensional Framework for Selective Coupling of CO2 and Epoxides under Mild and Solvent-Free Conditions.

Two Lewis acid-base bifunctional, isostructural metal-organic frameworks (MOFs), [Zn3(Hbtc)2(atz)2]·CH3CN·2CH3OH (MOF-1) and [Co3(Hbtc)2(atz)2]·H2O·2DMF (MOF-2), containing NH2 groups and four-coordinated metal sites were fabricated successfully, and the structures were thoroughly characterized by X-ray crystallography, powder X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and N2 adsorption/desorption techniques. Satisfactorily, the novel MOFs with a three-dimensional framework were highly efficient and stable heterogeneous catalysts for selective fixation of CO2 to cyclic carbonates under mild conditions in the absence of solvent. The influences of various reaction parameters on catalytic performance were systematically investigated. Also the catalysts showed excellent versatility to epoxides with different substituents and could be easily recyclable with high activity. Additionally, the self-catalyzed mechanism as well as synergistic mechanism with tetrabutylammonium bromide (Bu4NBr) for CO2 conversion was proposed. Novel Lewis acid-base bifunctional crystals with three-dimensional frameworks were successfully developed and showed exceptionally efficient activity for selective CO2 cycloaddition to epoxides under mild and solvent-free conditions. [ABSTRACT FROM AUTHOR]