Selective oxidation of methanol over supported vanadium oxide catalysts as studied by solid-state NMR spectroscopy

Methanol catalytic oxidation over VOx/Al2O3, VOx/ZrO2 and VOx/MgO catalysts has been studied by solid-state nuclear magnetic resonance (NMR) spectroscopy. It was found that stronger acid sites in VOx/Al2O3 result in almost the same selectivities for dimethoxymethane, paraformaldehyde and formic acid, and weaker acid sites in VOx/ZrO2 favor the formation of paraformaldehyde, while the VOx/MgO catalyst with the base support shows high selectivity for formate. Supporting VOx species on γ-Al2O3 and ZrO2 leads to the formation of Brønsted acid sites as revealed by the adsorption of probe molecules. The acid strength of Brønsted acid sites on the VOx/Al2O3 catalyst is found to be stronger than that of the VOx/ZrO2 catalyst which has the acid strength similar to zeolite HZSM-5's. The proposed bridging hydroxyl models accounting for the Brønsted acid sites formation were also confirmed by quantum chemical calculation.

Graphical abstractSolid-state NMR spectroscopy results show that stronger acid sites in VOx/Al2O3 lead to almost the same selectivities for dimethoxymethane (DMM), paraformaldehyde and formic acid, and weaker acid sites in VOx/ZrO2 favor paraformaldehyde synthesis, while the VOx/MgO catalyst with base support shows high selectivity for formate. 13C single-pulse MAS NMR spectra (with proton decoupling) of methanol and oxygen reaction on VOx/Al2O3 at various temperatures.Figure optionsDownload full-size imageDownload as PowerPoint slide