Optical basicity: a scale of acidity/basicity of solids and its application to oxidation catalysis.

This chapter aims to summarize most of the previous findings using the optical basicity scale. First, the concepts of optical basicity and of ionic-covalent parameter are recalled and some examples given. Then, the choice of ionization potential as a means to account for a thermodn. selectivity is justified. The linear correlations obtained when plotting differences in ionization potential (DI) against the optical basicity are then discussed. Finally, the thermodn. nature of optical basicity is discussed and attempts to account for the linearity of correlations between DI and optical basicity are proposed

Preparation and characterization of VOx/TiO2 catalytic coatings on stainless steel plates for structured catalytic reactors.

1 The parameters to be controlled to coat metallic walls by VOx/TiO2 catalysts which are used in the mild oxidation of hydrocarbons and NOx abatement are studied. Stainless steel (316 L) was chosen because of its large application in industrial catalytic reactors. TiO2 films on stainless steel were obtained by dip-coating in two steps. Superficially oxidized plates were first dipped in Ti-alkoxide sol-gel to be coated by a very thin layer of TiO2. On this anchoring layer was then deposited a porous film of titania by dipping plates in an aqueous suspension of TiO2 particles.After calcination, VOx species were grafted to TiO2/SS plates and their loading was determined by means of X-ray Photoelectron Spectroscopy. The chemical and mechanical resistances of films were controlled by several tests. Laser Raman Spectroscopy, X-Ray Diffraction and Scanning Electron Microscopy were used to characterize the samples after each step of preparation. The porous texture was determined using a thermobalance. The dispersion and the nature of VOx species and the value of theoretical monolayer of VOx on TiO2/stainless steel are shown to depend on the surface V/Ti ratio, in the same manner as for VOx/TiO2 coating anodised aluminum plates and as for VOx/TiO2 powders, . Therefore, we have demonstrated that the shaping of TiO2 has no influence on the characteristics of the active phase, which is of prime importance for catalytic applications in structured reactors

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

Transient behaviour of dense membranes of BIMEVOX (Me = Cu, Co) catalysts in the oxidation of propane

ME-doped Bi4V2O11 (BIMEVOX) oxides are highly oxide ion conducting materials and this property may be profitably used in selective oxidation of hydrocarbons. The catalytic properties of BICUVOX and BICOVOX when shaped as dense membranes displayed in catalytic dense membrane reactor are examined in the oxidation of propene and of propane. Mirror-polished BICUVOX and BICOVOX membranes studied previously were poorly active for propene oxidation because of a small number of active sites but showed an excellent stability and reproducibility (lasting more than one month) during which products of mild oxidation (acrolein, hexadiene) and CO were formed. Membranes with depolished surfaces exhibit high conversions of propene (up to 60 mol% ), and also of propane (up to 20 mol%) but – contrary to mirror polished membranes – a complex transient behaviour is observed during which syngas production occurs. The membrane polarization followed by in-situ Solid Electrolyte Potentiometry shows that the oxygen reservoir is far higher than expected on the reaction side which is separated (by the membrane) from the oxidising side where (diluted) oxygen is reduced to O2- specie. The influence of oxygen partial pressure on the catalytic performance suggests that the electronic conductivity of the material is limiting the oxygen flux through the membrane, and thus is determining the catalytic properties and transient behaviours of depolished membranes

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

Selective oxidation of hydrocarbons in a catalytic dense membrane reactor: Catalytic properties of BIMEVOX (Me = Ta)

Used as dense membranes in a catalytic dense membrane reactor, Bi2V0.8Ta0.2O5.5 (BITAVOX) is more or less active in the oxidation of propane, ethane and propene according to its surface roughness. When the surface roughness is low, membranes are poorly active in the oxidation of propane (Conversion < 2%). CO and propene are the main products and the stability is high over long experimental periods. BITAVOX membranes with increased surface roughness exhibit high conversions in the oxidation of propane, ethane and propene. Activities and selectivities evolve with time. In a first short period (<250s), products are similar to those of polished samples, then mainly H2 and CO are produced and subsequently H2 and coke. These evolutions are probably related to surface and sub-surface restructuring due to an insufficient oxygen supply through the membrane to face the high catalytic activity induced by the increase in surface area. The results suggest that the O2- diffusion is limited by the low electronic conductivity of BIMEVOX materials. Nevertheless the membrane is not reduced irreversibly, even in coking conditions. This is confirmed by ex-situ characterisations carried out on the materials (XRD, SEM, XPS). An improvement of the electronic conductivity of this material is necessary in order to stabilize the intermediate catalytic behaviour which is interesting for syngas or H2 production from light hydrocarbons

Bi2V1-xMexO5.5-y (Me = Ta, Ni) Membranes for Selective Oxidation of C1-C3 Alkanes in a Catalytic Dense Membrane Reactor

Pure and Me-doped Bi2V1-xMexO5.5-y (BIMEVOX) (Me = Ta, Ni) materials exhibit a transient behaviour during the oxidation of C1-C3 alkanes when shaped as dense membranes separating two compartments fed with diluted hydrocarbon and with air, respectively (catalytic dense membrane reactor). Alkenes are first produced at low conversion and CO and H2 follow at higher conversion. The H2/CO ratio increases up to values close to the stoichiometric ones and continues to increase after a peak of activity during which coke builds up. The same transient phenomena as initially observed happen after burning the coke by replacing diluted hydrocarbon by air. This in situ regeneration of the membrane means that the structure and integrity of the membranes have not been modified, as indeed shown by physicochemical analyses (XRD, SEM, XPS). The catalytic properties are mainly related to the presence of ME which affects the electronic conductivity of the materials. At variance with quite neutral BITAVOX, the behaviour of BINIVOX is close to that of BICOVOX which is also a p-type semiconductor. Only BINIVOX is able to oxidise methane (conversion ca. 22-25 mol%) to syngas at low temperature (650°C) in a quite steady manner

Catalytic dense membranes of doped Bi4V2O11 (BIMEVOX) for selective partial oxidation: chemistry of defects versus catalysis

A catalytic dense membrane reactor (CDMR) is used to physically separate the reaction step from the reoxidation of the catalyst. By decoupling the redox mechanism prevailing in mild oxidation of hydrocarbons, the operating conditions may be optimized resulting in an increase of selectivity. The membranes are made up of BIMEVOX oxides, obtained by partial substitution of V in Γ-Bi4V2O11 by ME (Co, Cu, Ta). Experiments performed on BIMEVOX dense membranes using propene and propane are described in terms of, (i) active sites on polished or unpolished surfaces, (ii) operating conditions (T, pO2 in the high oxygen partial pressure compartment), which determine the selectivity, either to mild oxidation products (acrolein, hexadiene, CO), or to partial oxidation products (CO, H2), and, (iii) nature of ME cations and relative properties. The discussion deals with the respective role of electronic vs. oxide ion conductivities which depend on defects in the structure as well as on the redox properties of cations

Oxygen permeation in bismuth-based materials part I: Sintering and oxygen permeation fluxes

Oxygen permeation measurements were performed on two layered bismuth based oxide ceramics: a rhombohedral phase belonging to the Bi2O3-CaO system, (Bi2O3)0.73- (CaO)0.27 (BICAO) and a BICOVOX phase. Oxygen permeability for these systems was compared to permeability of the cubic fluorite type structure with composition (Bi2O3)0.75(Er2O3)0.25 (BE25). Low oxygen permeability was observed for the pure ceramic. As for BE25, permeability was considerably increased if 40 vol% of silver was added to BICAO. In contrast, permeability was not improved by addition of gold to BICOVOX. For this latter phase, the oxygen molecular exchange at the surface is clearly the limiting step in the oxygen transfer