The mechanism of the selective oxidation of methanol on two V-Ti oxide catalyst samples, prepared by impregnation and coprecipitation techniques, respectively, is investigated. The interaction of methanol and its oxidation products (i.e., formaldehyde, dimethoxymethane, formic acid, and methyl formate) is studied by FT-IR spectroscopy and compared with the results of flow reactor measurements performed at different temperatures, contact times, and methanol/oxygen molar feed ratios. The data are interpreted on the basis of a reaction mechanism which involves the following steps: (i) condensation of methanol with surface VOH groups; (ii) H abstraction from methoxy groups leading to coordinated formaldehyde; (iii) formation of dioxymethylene species by interaction of adsorbed formaldehyde with nucelophilic sites; (iv) reaction of dioxymethylene species with methanol to give dimethoxymethane; (v) successive oxidation of dioxymethylene groups to formate ions; (vi) reaction of these ions either with methanol to produce methyl formate or with water to give formic acid; (vii) decomposition of formate species to produce carbon monoxide; (viii) parallel oxidation of methanol to carbon dioxide. The behaviors of the surface species are compared with those monitored on other systems and the catalyst requirements for the title reaction are discussed.

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The mechanism of methanol oxidation to methyl formate over coprecipitated vanadium-titanium oxide catalysts previously proposed is confirmed and further clarified, and the nature of active sites in this reaction is assessed by using surface sensitive techniques in the catalyst characterization, by performing a series of flow reactor experiments where intermediates and reaction products are added to the reactor feed, and by comparing the adsorption of methanol and formaldehyde on titanium and vanadium-titanium oxides. The reaction mechanism is proved to consist of successive oxidation steps, and the active sites for each step are identified. An alternative route to methyl formate by a Cannizzaro-type disproportionation reaction of dioxymethylene species is found to be of minor relevance. The capability of coprecipitated vanadium-titanium oxide to catalyze the formation of methyl formate is correlated to the existence of dioxymethylene species with an intermediate stability as compared to pure titania and vanadia.