J.C. Volta

Oxidative dehydrogenation of propane over VMgO catalysts

Abstract Oxidative dehydrogenation of propane was studied at 500-550°C over VMg-oxide catalysts and over the reference phases orthovanadate, pyrovanadate, and metavanadate of magnesium. Characterization of the reference phases was performed by XRD, IR spectroscopy, TEM, STEM, and 51 V NMR and compared with the VMgO catalysts. In contrast with previous results by Kung, it was observed that the actual catalyst for dehydrogenation of propane to propene is pyrovanadate of magnesium. This specificity is explained on the basis of a dynamic model of the working catalyst favored by the corner-sharing VO 4 tetrahedra of the V 2 O 7 4− units.

A study by in situ laser Raman spectroscopy of VPO catalysts for n-butane oxidation to maleic anhydride I. Preparation and characterization of pure reference phases

An in situ laser Raman spectroscopy (LRS) cell has been constructed in order to study the evolution of the local structure of the vanadium phosphate catalysts for n-butane oxidation to maleic anhydride in catalytic conditions. The LRS cell is described with the on line disposal for detection of the evolving gases. The first issue concerns the preparation and the physicochemical characterization of the reference phases of the VPO system: (VO)2P2O7, αII, β, γ, and δ VOPO4. Their purity was controlled by X-ray diffraction and 31P and 51V solid NMR and their LRS spectra were studied in the 800–1200 cm−1 range characteristic of the PO and VO bonds. Among these phases only δ VOPO4 is partly transformed (into αII VOPO4) in the catalytic conditions. From the evolution of their respective Raman spectra with temperature and with conditions of hydration, new proposals for the structure of γ VOPO4 are given. They are in agreement with the solid state NMR data. Raman spectra of the phases exhibit features specific enough to allow identification of the different VPO structures.

Molecular description of active sites in oxidation reactions: Acid-base and redox properties, and role of water

Abstract Oxidation reactions in heterogeneous catalysis usually involve a Mars and van Krevelen mechanism which includes activation of the substrate on a metallic cation, insertion of oxygen from lattice oxygen ions, a redox mechanism on the catalyst surface, and the transfer of several electrons. It follows that such a reaction necessitates both acid-base and redox properties of a catalyst the acid site being of Lewis type (cations) and the basic sites being the surface O 2- or OH - species which could exhibit electrophilic or nucleophilic properties. The active site should be able to fulfil the following requirements: H abstraction from the substrate, oxygen insertion, and electron transfer. It has been shown to correspond to an ensemble of atoms of limited size in an inorganic molecular complex. It could correspond to local structural defects including steps, kinks, coordinatively unsaturated cations or to clusters of atoms on the surface. Some examples are described namely: 1. (i) n-butane oxidation to maleic anhydride on (VO) 2 P 2 O 7 catalyst where four dimers of vanadyl cations on the (100) face were suggested to form the active site; 2. (ii) isobutyric acid oxidative dehydrogenation to methacrylic acid on iron hydroxy phosphates where trimers of iron oxide octahedra were shown to constitute the most efficient and selective catalytic site while water was observed to be absolutely necessary to facilitate the reaction which corresponds to hydroxylated surface sites ensuring the redox mechanism; 3. (iii) propane oxidative dehydrogenation to propene on VMgO samples which was shown to depend both on VO x arrangements with respect to MgO and on the basicity of the material induced by MgO while vanadium cations induced acidic features.

Vanadium phosphorus oxides for n-butane oxidation to maleic anhydride

Abstract This publication presents recent developments of the knowledge of the vanadium phosphorus oxides used as catalysts for n -butane oxidation to maleic anhydride. We emphasize particularly the nature of the active phase, the active centres and the role of the redox and acido-basic properties contrasting informations between catalysts presenting different degree of disorder. The nature of the active oxygen species and the mechanistic aspects of the reaction are discussed from studies conducted in our laboratory and by comparison with the present knowledge.

Dynamic processes on vanadium phosphorous oxides for selective alkane oxidation

Abstract The use of complementary physicochemical tools (XRD, Raman spectroscopy, XPS, 31 P NMR, and electron microscopy techniques), sometimes used in in situ conditions has allowed to evidence the dynamic processes occurring during the oxidation of light alkanes on the vanadium phosphorus oxide (VPO) system. The transformations of the VPO system in the course of the oxidation of n-butane to maleic anhydride and of the oxidation of propane to acrylic acid are contrasted in connection with the evolution of the catalytic performances.

Structural transformation sequences occurring during the activation of vanadium phosphorus oxide catalysts

The structural transformations which occur when a VOHPO4·0.5H2O precursor is activated in a mixture of n-butane–air at 400 °C have been studied using a combination of XRD, TEM and 31P NMR spin–echo mapping. The VOHPO4·0.5H2O precursor was prepared by reducing V2O5 with isobutyl alcohol and had a plate-like morphology with a [001] normal. A systematic series of ‘activated’ catalysts were then prepared in which the activation time was varied between 0.1 and 132 h. Structural characterisation studies on these samples show the structural evolution of the catalyst during the activation procedure. At the periphery of the platelet a direct topotactic transformation from [001] VOHPO4·0.5H2O to [100](VO)2P2O7 occurs. In the interior of the platelet a more complex indirect transformation sequence occurs. Regions are shown to exist where the VOHPO4·0.5H2O precursor initially transforms epitaxially into δ-VOPO4. As the activation time increases the domains of δ-VOPO4, which are embedded in a disordered matrix, shrink and further transform to the final (VO)2P2O7 phase. An attempt has also been made to correlate measured catalytic performance data with the catalyst microstructure at various stages of the transformation process. It is found that there is a distinct parallel between improving catalytic performance and a decrease in the amount of V5+ phases present. A further sample of the same hemihydrate precursor was activated in a pure N2 atmosphere for comparison. In this case, the whole VOHPO4·0.5H2O platelet was observed to undergo a direct topotactic transformation to the (VO)2P2O7 phase. The transformation mechanism, however, involved the homogeneous nucleation of discrete epitaxial patches of crystalline (VO)2P2O7 over the surface of the platelet. This is followed by the growth and eventual coalescence of the patches upon extended activation.

Correlation with the redox V5+/V4+ ratio in vanadium phosphorus oxide catalysts for mild oxidation of n-butane to maleic anhydride

Abstract The redox properties of vanadium phosphorus oxide (VPO) catalysts have been compared to their catalytic performance in fuel-lean conditions for two sets of (VO) 2 P 2 O 7 catalysts obtained after calcination under nitrogen at different temperatures or after different postoxidation periods under dioxygen. It is shown that doping the VPO catalysts with Co and Mo makes possible for the catalyst to produce maleic anhydride in fuel-rich conditions. In both situations, catalytic performances are dependent on the vanadium oxidation state of the catalysts.

Membrane reactor for selective oxidation of butane to maleic anhydride

A simulation of a packed-bed membrane reactor acting as an oxygen distributor for the selective oxidation of n-butane to maleic anhydride (MA) has been performed by recreating specific reactive atmospheres in a microreactor. In the membrane reactor, the oxidation state of the catalyst depends on its position in the bed, leading to an important change in the MA yield. However, this heterogeneity can be turned to an advantage using a reverse of n-butane flow. Co-promoted catalysts have also been developed to enhance the global performance of the membrane reactor.

Vanadium-molybdenum phosphates supported by TiO2-anatase as new catalysts for selective oxidation of ethane to acetic acid

Dispersion of vanadium and molybdenum phosphates on titanium oxide (anatase) below the monolayer gives good catalysts for direct oxidation of ethane to acetic acid. By comparison with the dispersion of only vanadium phosphate, the higher selectivity to acetic acid for vanadium and molybdenum phosphates has been explained by an interaction between molybdenum and vanadium as it can be deduced from electron spin resonance and laser Raman spectroscopy studies.

Niobium oxide based materials as catalysts for acidic and partial oxidation type reactions

Abstract Niobic acid, H 8 Nb 6 O 19 · x H 2 O, was synthesized and studied for its acidic features as a function of its dehydroxylation extent. It was observed to be strongly acidic, using NH 3 adsorption calorimetry and isopropanol conversion reaction as probe techniques, and to be weakly acidic on its dehydrated form, Nb 2 O 5 . The mixed oxide Al 2 o 3 :Nb 2 o 5 in 1:1 molar ratio prepared from aluminum and niobium oxalates was shown to be amorphous up to 1023 K where it crystallized in the form of AINbO 4 and to exhibit higher acidity than Nb 2 O 5 dehydrated, as expected from Tanabes model but much less acidic than the niobic acid form. The single Nb 2 o 5 and mixed A1 2 O 3 :Nb 2 o 5 oxides were used as supports for grafting MoO x and VO x species, respectively. It was observed that for MoO x /Nb 2 o 5 samples new strong acid sites were created while the redox properties were not as satisfactory as expected. For VO x /Al 2 O 3 :Nb 2 O 5 samples, at relatively low coverage (ca. 15°10), acidic features were not appreciably modified (only slightly enhanced) while new redox features were generated resulting in a rather satisfactory mild oxidation catalyst. The degree of condensation of VO x species on the mixed oxide surface, including some possible epitaxial grafting, even if not clearly characterized, appeared to play a determining role in oxidative dehydrogenation properties. This constitutes a new example of structure sensitivity in partial oxidation reactions on oxides.