STUDY OF CUMENE OXIDATION OVER ZIRCONIA-BASED, TITANIA-BASED AND ALUMINA-BASED COMPLEX OXIDES OBTAINED BY SOL-GEL METHODS - ACTIVITY STRUCTURE RELATIONSHIPS

Activity-structure relationships have been studied in the low temperat ure liquid phase cumene oxidation over Fe-O/ZrO2, Fe-O/TiO2 and Fe-O/A l2O3 complex oxides obtained by sol-gel methods. The study has been ca rried out by the measurements of overall kinetics, product distributio n and by X-ray diffraction and Mossbauer spectroscopic methods. Kineti c study in the presence of homogeneous initiator (azo-bis-diazobutyron itrile) has led to the conclusion that the active surface of complex o xides participates in chain initiation most probably via R-H bond rupt ure. Three groups of complex oxides are described: (1) two-phase polyc rystalline zirconia-based catalysts showing high activity at low iron loading; (2) one-phase polycrystalline zirconia- and titania-based cat alysts 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 cl usters and showing monotone activity dependence. Electronically excite d terminal Fe=O groups related to the surface FeO6 polyhedra and iron- containing species located at the interface have been suggested as act ive centers acquiring the anion-radical behavior. When comparing therm odynamically stable titania (anatase) with metastable one-phase zircon ia (cubic or tetragonal) as host matrices, the suggestion was introduc ed that greater activity of zirconia-based catalysts is due to more en ergy transfer from the host matrix to the particular active center.