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 qua ntities of antimony butoxide. BET surface area measurement, XRD, Mossbauer spectroscopy, CTEM-AEM, XPS and ISS were used to characterize phase and sur face architectures and their changes after calcination and catalytic reacti on. Before calcination, antimony was present as pure oxide or hydroxide, pa rtly as particles and partly as an incomplete monolayer on the surface of F e-2(MoO4)(3). After calcination at 400 degrees C, antimony got detached fro m the Fe-2(MoO4)(3) surface and aggregated very intensively, partly as Sb2O 4 and partly, through reaction with the iron molybdate, as a mixture of dis torted FeSbO4 and MoO3. After reaction or calcination at 500 degrees C, mor e distorted FeSbO4 and MoO3 are formed, separated from Fe-2(MoO4)(3). Selec tive oxidation of isobutene to methacrolein was carried out on the calcined material. Impregnated catalysts show considerably improved catalytic perfo rmances compared to the pure Fe-2(MoO4)(3) phase or mechanical mixtures of it with alpha-Sb2O4. The catalytic performances are explained by several ca talytic cooperations via spillover oxygen. These cooperative effects involv e all the oxide phases present in the material having worked as catalyst: F e-2(MoO4)(3) (pure or possibly contaminated by small amounts of antimony ox ide), FeSbO4, MoO3 and SbyOx.