Protection Against Deactivation of Selective Oxidation Catalysts Due To Spillover Oxygen

We review recent results obtained in our laboratory, which show that certain oxides, called Donors (e.g. alpha-Sb2O4, BiPO4, etc.) effectively protect phases active in selective oxidation against various deactivation processes. The results were obtained with simple mechanical mixtures of oxides, using the oxidation of isobutene to methacrolein and the oxidative dehydrogenation of n-butene to butadiene as a test reaction and several characterization techniques (surface area measurement, x-ray diffraction [XRD], x-ray photoelectron spectroscopy [XPS] and Fourier transform infrared [FTIR]). The presence of a Donor phase exerts several effects, often in conjunction: prevention of the reduction of the surface of the active phase, inhibition of coke formation, stabilization against segregation in certain cases, or promotion of favorable solid-state reactions. This results in improved activity and selectivity and slower deactivation. These effects are attributed to spillover oxygen, emitted by the Donor phases. This constitutes one facet of the role of the remote control mechanism in multiphase, multicomponent oxidation catalysts

Bismuth carboxylates as precursors for the incorporation of bismuth in oxide-based materials

The use of bismuth(III) carboxylates (acetate, oxoacetate, lactate, oxalate) as precursors for the incorporation of this element on oxide supports is evaluated as a new tool to generate bismuth-based oxide phases at the surface of MoO3 or WO3 supports. These insoluble precursors are deposited as small particles from a slurry in a liquid hydrocarbon under appropriate experimental conditions, Bismuth molybdate and tungstate phases are produced by solid-state reactions between in situ generated Bi2O3 and the supporting oxide at 673 K. The samples are characterized by specific surface area measurements, X-ray diffractometry, and X-ray photoelectron spectroscopy, The selective oxidation of isobutene to methacrolein is employed as a reaction test to demonstrate the applicability of this approach, The use of precursors containing lactate or acetate-type ligands is found to generate materials displaying enhanced specific surface area and high bismuth-to-molybdenum surface atomic ratios. This bismuth enrichment of the surface is indicative of the presence of dispersed crystallites of either Bi2O3 itself or ternary Bi-Mo-O or Bi-W-O phases, which are known to promote partial oxidation of alkenes, The performances of these materials in the isobutene to methacrolein conversion are indeed in most cases definitely better than those of the so-called reference materials obtained from the simple mixtures of the same oxides prepared separately. (C) 1996 Academic Press, Inc

Efecto en las propiedades térmicas, magnéticas y catalíticas de la cobaltita dopada con 10 % Mn

Proceed. XXI SICAT, 2008. Proceedings de la conférenc

Synergetic effects in multiphase catalysts: The role of FeSbO4 as donor-acceptor of spillover oxygen

The present work reports experimental data to further investigate the role of FeSbO4 in multiphase catalysts used in the selective oxidation of isobutene to methacrolein. Its oxygen acceptor and donor properties have been evaluated by showing the effect of admired alpha-Sb2O4, which is a good oxygen donor able to increase the selectivity of an acceptor phase, and of MoO3 which generally acts as a good oxygen acceptor, namely carries all the functions necessary for oxidation, but has only a weak ability to dissociate oxygen. Catalysts containing FeSbO4 and alpha-Sb2O4 or MoO3 were prepared: (1) by mechanical mixture; and (2) by artificial deposition of Sb and Mo ions on FeSbO4. The catalysts were characterized, before and after the test, by S-BET, XRD and XPS. The mechanical mixtures and impregnated catalysts gave a synergy in the yield and in the selectivity to methacrolein. Characterization shows that the catalysts are formed by two separate non-contaminated phases in contact. Synergy is explained by the remote control mechanism. It is proposed that FeSbO4 act as a donor of oxygen spillover when it is mixed with MoO3, and as an acceptor in presence of alpha-Sb2O4

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

Influence of the preparation conditions on the physico-chemical and catalytic properties of a vanadium-aluminum oxynitride propane ammoxidation catalyst

The influence of the precipitation pH and of the vanadium concentration in solution on the physico-chemical properties and catalytic activity of the V-Al oxynitride catalyst system was investigated. The V-Al precursor oxide samples were submitted to a nitridation treatment to form the V-Al oxynitride (VAlON), which is the active and stable phase during the catalytic reaction. The VAlON catalyst was subsequently tested in the propane ammoxidation reaction and characterized by BET-surface area, X-ray diffraction, temperature programmed reduction (TPR), nitrogen and metal content analyses. Optimal acrylonitrile (ACN) selectivity was observed for the sample prepared at pH=5.5 and vanadium concentration in solution=30.10(-3) M. This catalyst also showed the highest specific surface area, reduction temperature and nitrogen content. The existence of an optimum balance between the basic properties provided by surface and bulk nitrogen species (N-3 and NH(x)rectangle) and the redox capacity of the vanadium ions (Vrectangle+) would explain the catalytic behavior of the VAlON system in propane ammoxidation reaction