Oxidation of ethane to ethylene and acetic acid by MoVNbO catalysts

The influence of niobium on the physicochemical properties of the Mo-V-O system and on its catalytic properties in the oxidation of ethane to ethylene and acetic acid is examined. Solids based on MoV0.4Ox and MoV0.4Nb0.12Oy composition and calcined at 350 or 400°C were studied by X-ray diffraction, and by laser Raman and X-ray photoelectron spectroscopies. Their reactivity during reduction and reoxidation was examined by in situ XRD and by XPS after pre-treatment. Their stability in air was evaluated by means of Raman spectroscopy during laser heating of particles. Niobium is responsible for both stabilization and nanosize of MoO3 and (VNbMo)5O14 crystals. The high global selectivity to ethylene and acetic acid (90-96 mol%) is related to the presence of both phases while higher activity is owed to nanoparticles. The model already proposed by Merzouki et al. (Stud. Surf. Sci. Catal., 72 (1992) 81) suggesting that MoVNbO catalysts could be made up from (VNbMo)5O14-type microdomains embedded in MoO3 matrix seems still topica

MoVO-based catalysts for the oxidation of ethane to ethylene and acetic acid. Influence of niobium and/or palladium on physicochemical and catalytic properties.

The influence of niobium and/or palladium in MoV0.4Ox on both solid state chemistry and catalytic properties in the oxidation of ethane to acetic acid and ethylene is examined. Catalysts without molybdenum (VNb031Pd3e-4Ox) are also studied for comparison. The structural properties of the precursors and of the catalysts obtained by calcination of precursors at 350 and 400°C are studied by X-ray diffraction, and by laser Raman and X-ray photoelectron spectroscopies. These properties depend on the presence or absence of niobium, and to a lesser extent, of palladium. Nb-free precursors and catalysts are heterogeneous mixtures of crystalline oxides, among which hexagonal and orthorhombic MoO3. The presence of Pd favors the instability of both precursors and catalysts. The catalysts are poorly active (conversion < 4%), but they are mainly selective to acetic acid (SAA max = 61-73 mol%) and to COx (SCOx max = 30-72 mol%). The Nb-containing precursors without or with Pd are more stable, and the catalysts are made up of nanocrystalline particles of V,Nb-doped Mo5O14 and of VxMo1 xO3 x/2. They are active (conversion < 15%) and very selective to ethylene and acetic acid (Stot = 90-96 mol%). The surface being enriched with vanadium in most cases, the discussion deals with the relative role of Nb and Pd and their possible location in the identified oxides. Because no M1 and/or M2 oxides could be identified, synergistic effects between nanocrystals of (VMoNb)5O14 and VxMo1-xO3-0.5x are proposed to account for the high catalytic performance of the multicomponent MoVNb(Pd)oxides

Hydrocraquage d'alcanes sur des systèmes catalytiques amorphes (étude des paramètres régissant l'activité et la sélectivité des catalyseurs)

Notre objectif était de mettre au point des catalyseurs d'hydrocraquage permettant de transformer de façon sélective les distillats sous vide en gazole. Nous nous sommes intéressés aux catalyseurs sulfures de Ni et de W déposés sur silice-alumine, au moyen d'une réaction modèlemodèle : l'hydrocraquage du n-décane. La fonction acide, faible, intervient très peu dans le schéma réactionnel. La réaction majoritaire observée est l'isomérisation du n-décane en isomères monobranchés. La faible importance du craquage bifonctionnel a permis de mettre en évidence l'existence d'une réaction de craquage direct du n-décane. Cette réaction se produit sur la phase sulfure, par un mécanisme faisant intervenir l'arrachement d'un proton par un soufre basique. Nous avons observé que l'utilisation d'une teneur en NiW moitié de la teneur habituelle ne modifiait ni l'activité ni la sélectivité du catalyseur. Le catalyseur non promu W/silice-alumine est aussi actif que le catalyseur NiW/silice-alumine.The aim of this work was to develop catalysts allowing to transform selectively the vacuum distillates into gasoil. The catalysts used in this work were Ni and W sulfides supported on silica-alumina. Hydrocracking of vacuum distillates was modelized by the hydrocracking of n-decan. The acidic function played a very small role in the reaction scheme due to the weak acidity of the support. The main reaction observed was the isomerization of n-decan into monobranched isomers. Owing to the small importance of the bifunctional cracking reaction, a direct cracking reaction of n-decan was evidenced. This reaction took place on the hydro-dehydrogenating function, through abstraction of a proton by a basic sulfur atom. A NiW content half of the habitual one did not modify the activity and the selectivity of the catalyst. The unpromoted W/silica-alumina catalyst was as active as the promoted NiW/silica-alumina catalyst, produced more multibranched isomers and less cracking products.POITIERS-BU Sciences (861942102) / SudocSudocFranceF