Theoretical study of the catalytic oxidation mechanism of 5-hydroxymethylfurfural to 2,5-diformylfuran by PMo-containing Keggin heteropolyacid

Abstract

The mechanism for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) catalysed by PMo-containing Keggin heteropolyacid (H₃PMo₁₂O₄₀) has been systematically investigated at the M06/6-31++G(d,p), Lanl2dz level in dimethylsulfoxide. For H₃PMo₁₂O₄₀ in dimethylsulfoxide, the most stable species was [PMo₁₂O₄₀]³⁻, which was the catalytic active species for the aerobic oxidation of HMF to DFF. Over the [PMo₁₂O₄₀]³⁻ active species, the reaction of 2HMF + O₂ → 2DFF + 2H₂O was associated with three successive reaction stages: the oxidation of the first HMF to DFF by [PMo₁₂O₄₀]³⁻, the import of O₂ to form the peroxide [PMo₁₂O₄₁]³⁻ and the oxidation of the second HMF to DFF by [PMo₁₂O₄₁]³⁻, regenerating [PMo₁₂O₄₀]³⁻. The oxidation of each HMF involves two main reaction steps: the cleavage of the O–H bond in the hydroxyl group and cleavage of the C–H bond in the methylene group of HMF. The turnover frequency determining the transition state was the first-step C–H bond cleavage in the methylene group of HMF with a rate constant of k_H = 1.345 × 10⁸ exp(−153 476/RT), while the turnover frequency determining the intermediate was representative of the HMF-containing molecular complex on [PMo₁₂O₄₀]³⁻. The value of the kinetic isotope effects (k_H/k_D) is predicted to be about 4.2–5.9 over the temperature range 373–433 K. This study provides an insight into the catalytically crucial step in the oxidation of HMF to DFF.