Study of a new iron phosphate catalyst for oxidative dehydrogenation of isobutyric acid

A new method of preparation of iron phosphate-based catalysts has been developed. The catalysts obtained are characterized by a single phase which is very active and selective in methacrylic acid (MAA) formation. This phase, identified as Fe-2(PO3OH)P2O7, is an selective but much more active than industrial iron phosphate-based catalysts prepared by another method. The similarities between the catalytic properties of this phase and the active phase of the industrial catalyst (alpha Fe-3(P2O7)(2) which is superficially oxidized and hydrated to the form Fe3-x3+Fex2+(PO3OH)(3+x)(PO4)(1-x)) support the role of the different species previously proposed especially the important role of the PO3OH groups and of the clusters of face-sharing Fe-O octahedra (7, 25). These clusters correspond to dimers (Fe2O9)(13-) in Fe-2(PO3OH)P2O7, whereas they correspond to trimers (Fe3O12)(16-) in the active phase of industrial catalysts. However, a reduced amorphous phase has been shown to form at the surface of the Fe-2(PO3OH)P2O7 phase which may contain the same species as the active phase of the industrial catalyst, explaining in this way the similarities observed in the catalytic properties. The greater activity of the new catalysts can be explained by the fact that the industrial catalysts contained not only the alpha Fe-3(P2O7)(2) phase but other inactive phases. The role attributed to water in shifting the hydration equilibrium and stabilizing superficially the Fe,(PO,OH),PO, phase is confirmed. Finally, it appears that the new catalysts are hydrated in the bulk rather than only on the surface as was the case for alpha Fe-3(P2O7)(2) and this is likely to have a direct effect on the ability of the solid to hydrate more easily, allowing a stabilization of the catalytic properties at lower partial pressures of water or even in the absence of added water. (C) 1996 Academic Press, Inc.