On the importance of combining bulk- and surface-active sites to maximize the catalytic activity of metal-organic frameworks for the oxidative dehydrogenation of alcohols using alkyl hydroperoxides as hydride acceptors

A series of zirconium-terephthalate UiO-66 metal-organic frameworks (MOF) containing partially uncoordinated zirconium sites (ZrUC) was synthesized, finely characterized by complementary physico-chemical techniques, and employed to catalyze the oxidative dehydrogenation of alcohols in the presence of tert-butyl hydroperoxide (TBHP). These sites were twofold: bulk ZrUC sites created by reducing the content of terephthalate linkers within the framework and intrinsic ZrUC sites existing on the external surface of undefective UiO-66 crystals. Activity of both sites was evidenced by the linear dependency taking place between activity and either missing linkers or external surface area. Combining in one UiO-66 crystal a high density of ZrUC sites within the bulk and a high external surface area enabled to maximize the performances of the catalysts in alcohol dehydrogenation, making possible an oxidation of benzyl alcohol over 90% under mild temperature (T = 40°C) after 2 h of reaction. More challenging secondary alcohols were also oxidized. The formation of Lewis acid sites at the inorganic nodes was followed by FTIR spectroscopy of adsorbed CO. The presence of bulk and/or external ZrUC sites was further disclosed by the observation on hydrated samples of solid-state 1H NMR signals corresponding to the proton of terminal µ1-OH moieties capping the ZrUC sites. Interestingly, two distinct NMR signals corresponding to the terminal µ1-OH moieties of bulk and surface ZrUC sites were identified. Finally, a mechanism of alcohol dehydrogenation using TBHP was proposed relying on the concomitant coordination of benzyl alcohol and TBHP onto the two adjacent ZrUC sites at the zirconium cornerstones.