Mechanistic Studies on the Photooxidation of 5-Hydroxymethylfurfural by Polyoxometalate Catalysts and Atmospheric Oxygen

Efficient oxidation of 5-hydroxymethylfurfural (HMF) to corresponding furanic products represents an important research focus of biomass valorization, recent research on polyoxometalates (POMs)-catalyzed aerobic oxidation of HMF usually requires high temperature and sometimes high O₂/air pressure. In this work, we report a mild and green approach to photocatalytically transform HMF into various furanic products using atmospheric oxygen as oxidant and POMs as photocatalysts. The influence of different POMs, light sources, and additives were systematically investigated by various experimental and spectroscopic results. Under minimally optimized conditions, 88.0 % HMF can be efficiently photooxidized with as high as 70.2 % furanic yield by TBA-W₁₀ catalyst after 2 h irradiation of 365 nm UV light when coupling with TEMPO and Na₂CO₃ as additives. Finally, detailed mechanistic pathways of HMF photooxidation have been proposed to illustrate the transformation of HMF to various furanic products. This work provides some insightful guidelines for photooxidation of biomass-derived platform chemicals to value-added products under efficient, mild, and green conditions, exhibiting potential practical applications in the future.