Solid-state reactivity of iron molybdate artificially contaminated by antimony ions and its relation with catalytic activity in the selective oxidation of isobutene to methacrolein

Catalysts were prepared by impregnation of Fe-2(MoO4)(3) with different quantities of antimony butoxide. BET surface area measurement, XRD, Mossbauer spectroscopy, CTEM-AEM, XPS and ISS were used to characterize phase and surface architectures and their changes after calcination and catalytic reaction. Before calcination, antimony was present as pure oxide or hydroxide, partly as particles and partly as an incomplete monolayer on the surface of Fe-2(MoO4)(3). After calcination at 400 degrees C, antimony got detached from the Fe-2(MoO4)(3) surface and aggregated very intensively, partly as Sb2O4 and partly, through reaction with the iron molybdate, as a mixture of distorted FeSbO4 and MoO3. After reaction or calcination at 500 degrees C, more distorted FeSbO4 and MoO3 are formed, separated from Fe-2(MoO4)(3). Selective oxidation of isobutene to methacrolein was carried out on the calcined material. Impregnated catalysts show considerably improved catalytic performances compared to the pure Fe-2(MoO4)(3) phase or mechanical mixtures of it with alpha-Sb2O4. The catalytic performances are explained by several catalytic cooperations via spillover oxygen. These cooperative effects involve all the oxide phases present in the material having worked as catalyst: Fe-2(MoO4)(3) (pure or possibly contaminated by small amounts of antimony oxide), FeSbO4, MoO3 and SbyOx.