Synthetic Origin-Dependent catalytic activity of Metal-Organic Frameworks: Unprecedented demonstration with ZIF-8 s on CO₂ cycloaddition reaction

In this study, the catalytic activities of three types of zeolitic imidazolate framework-8 (ZIF-8) (two types were synthesized in aqueous and methanolic media, while the other was commercially available) with respect to CO₂ cycloaddition to epichlorohydrin were evaluated. Surprisingly, only the ZIF-8 that was synthesized in an aqueous medium showed a marked catalytic performance, leading to the generation of the desired product, chloropropene carbonate. On the contrary, the other two types of ZIF-8 showed little to no catalytic activity. It is conjectured that the water molecules occluded inside the as-synthesized ZIF-8 triggered the dissociation of Zn-N bonds, resulting in a concomitant transformation to a new, dense phase. Accordingly, it is plausible that dissociated Zn or N species were formed during the reaction. In particular, we found that the dissociated N species arising from the dissociation of Zn-N bonds were pyrrolic and pyridinic, while the dissociated Zn species were bound to hydroxyl groups. Finally, complementary interpretation of CO₂– and NH₃-based temperature-programmed desorption and NH₃-adsorbed Fourier transform infrared spectroscopy results revealed that the dissociated Zn species connected with hydroxyl groups, observed only in the ZIF-8 synthesized in an aqueous medium, served simultaneously as Lewis acids and bases and, thus, were crucial to the CO₂ cycloaddition reaction.