OXIDEHYDROGENATION OF ISOBUTYRIC ACID TO METHACRYLIC-ACID ON IRON PHOSPHATE CATALYSTS - AN EXPERIMENTAL AND THEORETICAL-STUDY

Iron phosphates and hydroxyphosphates of different compositions and th erefore of different structural organizations have been used for isobu tyric acid oxidehydrogenation to methacrylic acid in the presence of e xcess water. XRD and Mossbauer studies of the solids allowed us to cha racterize the crystalline structure of the samples before and after ca talytic reaction. The best catalysts were shown to exhibit trimers of FeO6 octahedra shaping oxygen ion faces and isolated one from the othe r by a cationic vacancy. Theoretical calculation using a simple extend ed Huckel theory allowed us to show that for such iron oxide trimers t he excess electron of Fe2+ with respect to the two Fe3+ cations was lo calized over the central and a lateral Fe cation. For dimers or larger clusters than trimers either the excess electron was shown to be loca lized on one Fe cation or completely delocalized along the Fe cations, respectively. The optimum catalytic property was assigned to the limi ted 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.