Pure and Applied Chemistry

Abstract: The structure–activity relationship and reaction mechanism for selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) in toluene were studied on vanadium oxide domains on TiO₂, Al₂O₃, Nb₂O₅, ZrO₂, and MgO and with a wide range of VO_x surface densities. The structures of these catalysts were characterized by X-ray diffraction (XRD), diffuse reflectance UV–vis spectroscopy (UV–vis DRS), and Raman spectroscopy, and their reducibility was probed by H₂-temperature programmed reduction. The structures of the VO_x domains evolved from monovanadate to polyvanadate structures with increasing the VO_x surface densities, and finally to crystalline V₂O₅ clusters at surface densities above one-monolayer capacity. Within one-monolayer capacity, higher VO_x surface densities and more reducible supports led to higher reducibility and reactivity of the VO_x domains. The support surfaces covered with polyvanadates and V₂O₅ clusters and the supports with acidity favored the formation of DFF. The correlation between the reducibility and reactivity, together with the kinetic studies, suggests that the HMF oxidation to DFF proceeds via the redox mechanism involving the V⁵⁺/V⁴⁺ redox cycles and the reoxidation of V⁴⁺ to V⁵⁺ by O₂ as the rate-determining step. These results may provide guidance for the design of more efficient catalysts for the HMF oxidation to synthesize DFF.