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

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 FeO/C and CoO/C were designed as carriers for the vanadium-substituted Keggin-type polyoxomolybdate HPMoVO·35HO (PMoV). In this complex system, spinel oxides can effectively adsorb HMF molecules and cooperate with PMoV to catalyze the aerobic oxidation of HMF. As a result, the as-prepared PMoV@FeO/C and PMoV@CoO/C catalysts can achieve efficient conversion of HMF into DFF with almost 100% selectivity. Among them, PMoV@FeO/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 diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy revealed that the V and Mo in PMoV, together with the metal ions in the spinel oxides, act as active centers for the catalytic conversion of HMF. Therefore, it is proposed that PMoV and MO/C (M = Fe, Co) cooperatively catalyze the transformation of HMF into DFF a proton-coupled electron transfer mechanism. This study offers an innovative approach for designing highly selective and recyclable biomass oxidation catalysts.