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

Abstract

A series of zirconium-terephthalate UiO-66 metal–organic frameworks (MOFs) containing partially uncoordinated zirconium sites (Zr_UC) were 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 two-fold: bulk Zr_UC sites created by reducing the content of terephthalate linkers within the framework and intrinsic Zr_UC sites existing on the external surface of undefective UiO-66 crystals. The activity of both sites was evidenced by the linear dependency taking place between the activity and either the missing linkers or external surface area. Combining in one UiO-66 crystal a high density of Zr_UC sites within the bulk and a high external surface area enabled the performances of the catalysts in alcohol dehydrogenation to be maximized, making the oxidation of benzyl alcohol over 90% at mild temperature (T = 40 °C) after 2 h of reaction possible. More challenging secondary alcohols were also oxidized. The formation of Lewis acid sites at the inorganic nodes was monitored by FTIR spectroscopy of adsorbed CO. The presence of bulk and/or external Zr_UC sites was further disclosed by the observation on hydrated samples of solid-state ¹H NMR signals, corresponding to the protons of terminal μ₁-OH moieties capping the Zr_UC sites. Interestingly, two distinct NMR signals corresponding to the terminal μ₁-OH moieties of bulk and surface Zr_UC 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 Zr_UC sites at the zirconium cornerstones.