Ru nanoparticles supported on N-doped reduced graphene oxide as valuable catalyst for the selective aerobic oxidation of benzyl alcohol

Abstract

The catalytic performance of a series of Ru-based catalysts was evaluated for the selective aerobic oxidation of benzyl alcohol to benzaldehyde under base-free mild conditions. The effect of metal precursor (RuCl3, RuNO(NO3)3 and Ru3(CO)12) and support on catalyst performance was investigated by comparing undoped (rGO) and N-doped (NrGO) reduced graphene oxide with commercial activated carbon and high surface area graphite supports. The surface chemistry and structure of materials were characterized by nitrogen physisorption (BET), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The average Ru nanoparticle sizes were in the range from 1.4 to 2.4 nm, with the smallest particle sizes obtained on rGO support owing to its highest surface area.Catalysts prepared from RuNO(NO3)3 and Ru3(CO)12 precursors exhibit the highest benzyl alcohol conversion to the corresponding aldehyde, with highest conversions observed when NrGO support is employed. Catalysts prepared from Ru3(CO)12 on NrGO support exhibit the highest activity for benzaldehyde formation, which is over three times that of commercial activated carbon supported Ru catalysts.The differences in catalytic performance are attributed to interactions between the acidic product of the reaction and the basic surface sites of the NrGO support, and modification of the surface hydrophobicity. These factors confer a significant rate enhancement in the selective oxidation of benzyl alcohol over Ru/NrGO compared to Ru/rGO. Ru/NrGO is stable under reaction conditions, however progressive deactivation is observed owing to water accumulation at the active site. Catalysts are easily reactivated via heating, with >90% of the original activity recovered on reuse.