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 kH = 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 (kH/kD) 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.