Oxidehydrogenation of isobutyric acid to methacrylic acid on iron phosphate catalysts: an experimental and theoretical study

Summary.

Iron phosphates and hydroxyphosphates of different compositions and therefore of different structural organizations have been used for isobutyric acid oxidehydrogenation to methacrylic acid in the presence of excess water. XRD and Mossbauer studies of the solids allowed us to characterize the crystalline structure of the samples before and after catalytic reaction. The best catalysts were shown to exhibit trimers of FeO₆ octahedra sharing oxygen ion faces and isolated one from the other by a cationic vacancy. Theoretical calculation using a simple extended Hückel theory allowed us to show that for such iron oxide trimers the excess electron of Fe²⁺ with respect to the two Fe³⁺ cations was localized over the central and a lateral Fe cation. For dimers or larger clusters than trimers either the excess electron was shown to be localized on one Fe cation or completely delocalized along the Fe cations, respectively. The optimum catalytic property was assigned to the limited delocalization of the excess electron to a nearby cation while no electron delocalization or a too large electron delocalization led to a less performing catalyst.