A rational design of a Pd-based catalyst with a metal-metal oxide interface influencing molecular oxygen in the aerobic oxidation of alcohols

摘要

In a green process for selective oxidation of alcohols, the utilization of molecular oxygen as a primary oxidant is the most critical step. Although many palladium (Pd)-based catalysts have shown potential, the role of different Pd-species in the aerobic oxidation reaction is still a matter of discussion. There have been diverse reports, which describe either Pd-0 or Pd2+ as the individual species responsible for the aerobic oxidation of alcohols. Herein we demonstrate that the presence of both Pd0 and Pd2+ species with a Pd-PdO interface stabilized on the surface of reduced graphene oxide (rGO) is important for the oxidation of alcohols. With an optimum Pd2+/Pd-0 ratio, the catalyst catalyzes the oxidation of benzyl alcohol in water with oxygen, resulting in a turnover frequency (TOF) of up to 18 000 h(-1) with 98% selectivity towards the aldehyde. It is proposed that both metallic Pd and its oxide domains, when coexisting with a phase boundary between them, promote the activation of oxygen. On the other hand, rGO provides surface functionalities for the formation and stabilization of Pd-PdO nanoclusters enabling the catalyst to be both stable and reusable. Using histidine as a scavenger for singlet oxygen, we have also determined the importance of oxygen-activation in the reaction. Furthermore, the catalyst is capable of converting various other alcohols into the corresponding carbonyl compounds. Comparison of various catalysts shows that the Pd-PdO@rGO catalyst is the most efficient in terms of TOF, conversion and selectivity for the oxidation of benzyl alcohol using oxygen compared to the reported Pd-based catalysts, particularly when performed under milder reaction conditions. Therefore, the result on Pd-catalyst designing is believed to be of significance for the further developments in the environmentally benign oxidation processes involving molecular oxygen as the oxidant.