ISOBUTYRIC ACID OXIDATIVE DEHYDROGENATION OVER IRON HYDROXYPHOSPHATES.1. CATALYTIC PROPERTIES AND ROLE OF WATER

The catalytic activity and selectivity of several mixed-valence hydrox yphosphates in the oxidative dehydrogenation of isobutyric acid to met hacrylic acid were determined and compared to those of an industrial i ron phosphate-based catalyst. The phases were characterized before and after catalytic reaction; after catalysis they appeared to belong to a solid solution of the type Fe4-x3+Fe3x2+(PO4)(3)(OH)(3-3x)O-3x with less than or equal to x less than or equal to 1. The compounds of this solid solution underwent a reversible reduction through a reduction-h ydroxylation mechanism without structural change. This mechanism is pr oposed to be the basis of the catalytic activity of the compounds, whi ch suggests that one of the roles of water added to the gas feed is to displace the hydration equilibrium.