Oxidation of methanol to formaldehyde on cation vacant Fe–V–Mo-oxide Original Research Article

A spinel-type catalyst of the composition Fe2.50(1−z/3)V0.20(1−z/3)Mo0.30(1−z/3)□zO4, where z is the number of cation vacancies (□), was prepared under reducing atmosphere. From the reduced sample a number of preoxidized samples were made by TPO treatment of the reduced material to different end temperatures ranging from 150 to 450 °C. XRD shows that the spinel-type structure is maintained irrespective of the degree of preoxidation of the sample and the related increasing number of cation vacancies. The number of cation vacancies z increases from 0.12 for the most reduced sample up to 0.62 for the most oxidized sample. XANES data reveals that in the most reduced sample the cations are essentially Fe2+, Fe3+, V3+, Mo4+ and Mo5+ in the bulk, whereas the most oxidized sample has Fe3+, V4+ and Mo6+. XPS analysis shows that at the surface the metals are mainly in their highest oxidation state, i.e. Fe3+, V5+ and Mo6+. Irrespective of the degree of preoxidation of the catalyst, the same catalytic performance and composition of both the surface and the bulk are approached when the samples are used in methanol oxidation. Both the activity and the selectivity to formaldehyde increase initially with time-on-stream, and in parallel there is some enrichment of vanadium and molybdenum taking place at the surface. At steady state conditions the spinel-type catalyst gives a selectivity to formaldehyde of about 86% at high methanol conversion. Our results demonstrate that the spinel-type structure is flexible and stable at both reducing and oxidizing conditions, allowing the cations to change valence while maintaining the same basic structure type. Moreover, the spinel-type catalyst is stable towards volatilization.