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

Manganese oxides (MnO x ) 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 MnO x oxides with surface abundant unsaturated Mn and O species. The defective MnO x (MnO x –D) catalyst exhibits 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 MnO 2 . Moreover, MnO x –D catalyst demonstrates broad substrate scopes for various biomass–derived alcohols and displays 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 is established that the presence of surface–adsorbed H 2 O species on MnO x –D catalyst significantly influences its catalytic performance. Mechanism studies reveal that the aerobic oxidation of biomass–derived R–CH 2 –OH compounds into the corresponding aldehydes can proceed via radical and non–radical pathways. Notably, the catalytic activity of MnO x –D catalyst in both pathways is mediated by the surface adsorbed H 2 O species. In the radical route, surface adsorbed H 2 O and O 2 species are converted into active •OH radical species, while in the non–radical route, surface adsorbed H 2 O can form a complex of HMF––H 2 O species. Both processes enhance the abstraction of H from the O–H and C–H bonds in the R–CH 2 –OH compounds on MnO x –D catalyst. This discovery could potentially guide the development of efficient metal oxidic catalysts for the oxidation of biomass–derived R–CH 2 –OH compounds.