Surface adsorbed–H2O promoted aerobic oxidation of biomass–derived alcohols on MnOx catalysts

Manganese oxides (MnOx) have been emerged as highly active catalysts for the aerobic oxidation of biomass-derived compounds. Herein, a facile urea–redox hydrothermal method was employed to scalable synthesis of MnOx oxides with surface abundant unsaturated Mn and O species. The defective MnOx (MnOx–D) catalyst exhibited remarkable catalytic performance, enabling the room-temperature aerobic oxidation of biomass–derived alcohols with yields exceeding 99%. This represents a substantial 90.8-fold increase in activity compared to commercial MnO2. Moreover, MnOx–D demonstrated broad substrate scopes for various biomass–derived alcohols and displayed recyclability for the aerobic oxidative conversion of 5-hydroxymethylfurfural (HMF) over five cycles. Through a comprehensive analysis involving X–ray photoelectron spectroscopy (XPS), in–situ infrared (IR) spectroscopy, and controlled experiments, it was established that the presence of surface–adsorbed H2O species on MnOx–D catalyst significantly influences its catalytic performance. Mechanism studies revealed that the aerobic oxidation of biomass–derived R–CH2–OH compounds into the corresponding aldehydes can proceed via radical and non–radical pathways. Notably, the catalytic activity of MnOx–D catalyst in both pathways is mediated by the surface adsorbed H2O species. In the radical route, surface adsorbed H2O and O2 species are converted into active •OH radical species, while in the non–radical route, surface adsorbed H2O can form a complex of HMF––H2O species. Both processes enhance the abstraction of H from the O–H and C–H bonds in the R–CH2–OH compounds on MnOx–D catalyst. This discovery could potentially guide the development of efficient metal oxidic catalysts for the oxidation of biomass–derived R–CH2–OH compounds