Abstract
Methanol catalytic oxidation over VO x /Al 2O 3, VO x /ZrO 2 and VO x /MgO catalysts has been studied by solid-state nuclear magnetic resonance (NMR) spectroscopy. It was found that stronger acid sites in VO x /Al 2O 3 result in almost the same selectivities for dimethoxymethane, paraformaldehyde and formic acid, and weaker acid sites in VO x /ZrO 2 favor the formation of paraformaldehyde, while the VO x /MgO catalyst with the base support shows high selectivity for formate. Supporting VO x species on γ-Al 2O 3 and ZrO 2 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 VO x /Al 2O 3 catalyst is found to be stronger than that of the VO x /ZrO 2 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.
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