Solvent-free aerobic photocatalytic oxidation of C(sp3)–H and C(sp3)–OH to CO bonds

Abstract

The development of highly active and selective photocatalytic systems for the oxidation of benzylic C(sp³)–H and C(sp³)–OH to CO bonds under solvent-free conditions with molecular oxygen as an oxidant is attractive and challenging. In this study, a carbon- and PdO-doped Nb₂O₅-based catalyst (PdO/C–Nb₂O₅) was designed and synthesized. This photocatalyst showed excellent solvent-free aerobic photocatalytic performances for two typical oxidation reactions of benzylic C(sp³)–H and C(sp³)–OH to CO compounds. It was found that the doping of carbon and PdO in Nb₂O₅ can modify its surface electrons, increase oxygen vacancies, and change the positions of its valence band and conduction band. Besides, the kinetic study showed that the oxidation of C–H in ethylbenzene to C–OH is the rate-limiting step in the two-step oxidation of ethylbenzene to acetophenone. Moreover, a photocatalytic reaction mechanism was proposed based on these experiments. In addition, the solvent-free aerobic photocatalytic oxidation system showed a good substrate tolerance for benzylic C(sp³)–H and C(sp³)–OH compounds with different substituents to the corresponding ketones and aldehydes.