Cation Vacant Fe_3−x−y V _x □ _y O₄ Spinel-Type Catalysts for the Oxidation of Methanol to Formaldehyde

The potential of Fe–V-oxide catalysts for use in methanol oxidation is explored. Our results show that although FeVO₄ is active and selective for formaldehyde (FA) formation, it is not completely stable towards volatilization under reaction conditions. Attempts to stabilize Fe–V-oxide were made using titania, alumina and silica supports. However, we observe that although some stabilization is achieved using titania and alumina, the supported catalysts are sensitive to volatilization considering the relatively low content of active oxide. Compared with supported V-oxide, the results show that iron causes stabilization of vanadium decreasing its volatility. Considering the observation that the neat FeVO₄ restructures to form a spinel-type phase under influence of the catalysis, we prepared a series of cation vacant spinel-type Fe_3−x−y V _x □ _y O₄ catalysts with various V/Fe ratio and consequent number of cation vacancies □. Opposed to the activity, which is rather constant irrespectively of the vanadium content, the selectivity to FA passes through a maximum of about 90% for Fe/V = 14. A spinel-type phase with the composition Fe_2.62V_0.19□_0.20O₄ was prepared and subsequently preoxidized to different degree. It is observed that the spinel-type structure is stable and that the oxidation of vanadium and iron is balanced by an increasing number of cation vacancies. Moreover, irrespectively of the original degree of preoxidation, it is found that in methanol oxidation a steady state is reached where all samples are equally active and selective and have the same composition both in the bulk and at the surface. The results clearly demonstrate that the spinel-type catalysts are phase-stable, nonvolatile and flexible in that the cations can change oxidation state retaining the same basic structure type and Fe/V ratio.