Enhanced catalysis of a vanadium-substituted Keggin-type polyoxomolybdate supported on the M3O4/C (M = Fe or Co) surface enables efficient and recyclable oxidation of HMF to DFF

Abstract

In the reaction of oxidizing 5-hydroxymethylfurfural (HMF), attaining high efficiency and selectivity in the conversion of HMF into DFF presents a challenge due to the possibility of forming multiple products. Polyoxometalates are considered highly active catalysts for HMF oxidation. However, the over-oxidation of products poses a challenge, leading to decreased purity and yield. In this work, metal–organic framework-derived Fe₃O₄/C and Co₃O₄/C were designed as carriers for the vanadium-substituted Keggin-type polyoxomolybdate H₅PMo₁₀V₂O₄₀·35H₂O (PMo₁₀V₂). In this complex system, spinel oxides can effectively adsorb HMF molecules and cooperate with PMo₁₀V₂ to catalyze the aerobic oxidation of HMF. As a result, the as-prepared PMo₁₀V₂@Fe₃O₄/C and PMo₁₀V₂@Co₃O₄/C catalysts can achieve efficient conversion of HMF into DFF with almost 100% selectivity. Among them, PMo₁₀V₂@Fe₃O₄/C exhibits a higher conversion rate (99.1%) under milder reaction conditions (oxygen pressure of 0.8 MPa). Both catalysts exhibited exceptional stability and retained their activity and selectivity even after undergoing multiple cycles. Studies on mechanisms by in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy revealed that the V⁵⁺ and Mo⁶⁺ in PMo₁₀V₂, together with the metal ions in the spinel oxides, act as active centers for the catalytic conversion of HMF. Therefore, it is proposed that PMo₁₀V₂ and M₃O₄/C (M = Fe, Co) cooperatively catalyze the transformation of HMF into DFF via a proton-coupled electron transfer mechanism. This study offers an innovative approach for designing highly selective and recyclable biomass oxidation catalysts.