Study of cumene oxidation over zirconia-, titania- and alumina-based complex oxides obtained by sol-gel methods: Activity structure relationships

Activity-structure relationships have been studied in the low temperature liquid phase cumene oxidation over Fe-O/ZrO2, Fe-O/TiO2 and Fe-O/Al2O3 complex oxides obtained by sol-gel methods. The study has been carried out by the measurements of overall kinetics, product distribution and by X-ray diffraction and Mossbauer spectroscopic methods. Kinetic study in the presence of homogeneous initiator (azo-bis-diazobutyronitrile) has led to the conclusion that the active surface of complex oxides participates in chain initiation most probably via R-H bond rupture. Three groups of complex oxides are described: (1) two-phase polycrystalline zirconia-based catalysts showing high activity at low iron loading; (2) one-phase polycrystalline zirconia- and titania-based catalysts exhibiting either activity drop at some critical iron content (zirconia) or monotonic activity dependence (titania) on iron loading; (3) amorphous alumina-based catalysts containing gamma-ferric oxide clusters and showing monotone activity dependence. Electronically excited terminal Fe=O groups related to the surface FeO6 polyhedra and iron-containing species located at the interface have been suggested as active centers acquiring the anion-radical behavior. When comparing thermodynamically stable titania (anatase) with metastable one-phase zirconia (cubic or tetragonal) as host matrices, the suggestion was introduced that greater activity of zirconia-based catalysts is due to more energy transfer from the host matrix to the particular active center.