Jacques C. Védrine

The role of redox, acid-base and collective properties and of cristalline state of heterogeneous catalysts in the selective oxidation of hydrocarbons

The main aspects of heterogeneous selective oxidation on metal oxides are considered, in particular the importance of acid--base and redox properties. Particular emphasis is placed on the dynamics of the surface under catalytic conditions which involves a redox process, dehydroxylation of the surface, reversible formation of point defects and shear plane transformations, changes in collective properties due to the adsorption of reactants/products and reduction of the solid (electrical conductivity variations), amorphous state of a few surface top layers, etc. All these aspects affect the reaction mechanism and, subsequently, the activity and selectivity of the reactions and result in a very complex system, particularly difficult to characterise. However, despite this great complexity some general conclusions can be drawn.

H3PW12O40 supported on Cs modified mesoporous silica: catalytic activity in n-butane isomerisation and in situ FTIR study. Comparison with microporous CsxH3-xPW12O40

Abstract 12-Tungstophosphoric acid, H3PW12O40 (HPW), was supported over conventional mesoporous silica and MCM-41. The acidity of these supported materials and their catalytic activity in n-butane isomerisation were investigated. The differences in activity between these two series of supported HPW were ascribed to different HPW dispersion. Modification of the mesoporous silica and MCM-41 by treatment with cesium carbonate increased their catalytic activity. It was shown that the subsequent impregnation of the HPW resulted in the formation of the cesium salt (Cs3PW12O40) supporting the HPW. An optimum of Cs:HPW ratio was observed, which was interpreted as due to a competition between dispersing the HPW on Cs salt formed and the acid site neutralisation by Cs+ cation. The optimised catalyst, obtained by supporting HPW over Cs modified MCM-41, was found to be more active in the reaction of n-C4 isomerisation than the conventional acidic Cs salt, Cs2HPW12O40. Pyridine adsorption over supported HPW and microporous Cs2HPW12O40 was investigated by in situ FTIR spectroscopy and compared to bulk HPW. Pyridine adsorption over bulk HPW lead to a peculiar spectrum ascribed to the formation of the bis-pyridinium species. Over supported HPW and microporous Cs salt, the classical spectra of the pyridinium species were observed.

Effect of Water on the Partial Oxidation of Propane to Acrylic and Acetic Acids on Mo-V-Sb-Nb Mixed Oxides

It is shown that the major effect of water in the oxidation of propane to acrylic and acetic acids on Mo1V0.3Sb0.25Nb0.08On catalysts is to stabilise the active sites and increase the rates of formation of both acids. The usual effect of favoring desorption of the products is considered to be secondary.

Properties of Cs2.5 salts of transition metal M substituted Keggin-type M1−xPV1MxMo11−xO40 heteropolyoxometallates in propane oxidation

Keggin-type heteropolyoxometallate compounds were prepared with introducing a redox transition metal M (M = Co-II, Fe-III. Ga-III. Ni-II, Sb-III or Zn-II) in the solution before precipitating the Cw(2.5) salts The transition metal M was partly substituting Mo in the Keggin anion and partly replacing the charge balancing protons, as represented by the general formula: Cs2.5H6x-yM1-xPV1MxMo11-xO40. In some experiments, the transition metal cation was simply replacing the charge balancing protons as in Cs2.5H1.5-3xMxPV1Mo11O40 samples with M = Fe3+ or Ga3+. TGA measurements, IR of lattice vibration bands and P-31 MAS-NMR data allowed us to estimate the relative amount of exchanged and substituted transition metal cations and the Keggin anion structure stability. It appeared that introducing a transition metal M in the Keggin anion led in propane oxidation reaction near 340degreesC to much more propylene formation (60-80% selectivity) and lower acetic acid and COx formation than for Cs2.5H1.5PV1Mo11O40 Sample, while Keggin anion was observed not being destroyed during the reaction. When the transition metal cation was exchanged for protons, it was observed that the effect of redox transition metal was much less pronounced while the number of acid sites was decreased. At last, water introduced in the reaction feed led for all samples to a sharp increase in acetic acid and decrease in COx and propylene. This allowed us to propose a two-pathway reaction mechanism for the reaction with one pathway via isopropyl alcohol and then acetone being favoured by water and the strong Bronsted acidity of the heteropolyoxometallate compounds.

Influence of the nature and porosity of different supports on the acidic and catalytic properties of H3PW12O40

In order to obtain highly dispersed heteropolyacid (HPA) species, H3PW12O40 was supported on various supports exhibiting different porosities and surface chemical properties. Amorphous and monoclinic amphoteric zirconias, activated montmorillonite (AC) and hexagonal silica (HMS) were chosen as supports. It was observed that the zirconia support partly decomposed HPA at low coverage, giving the lacunary anion PW11O397−, due to the reaction with basic hydroxyl groups, whereas montmorillonite and HMS did not. Catalytic properties for n-butane to isobutane isomerisation at 473 K and propan-2-ol decomposition at 353 K were compared for all samples as a function of HPA loadings and compared to data already published on bulk H3PW12O40 and Cs1.9H1.1PW12O40 samples. It was found for HPA/AC samples that, although their activity for propan-2-ol decomposition varied linearly with HPA loading, their activity for n-butane isomerisation was very weak, which indicates a weaker acid strength by supporting the HPA. This is probably due to the exchange of protons from the HPA by the exchangeable cations of the montmorillonite, the new protons associated with the clay being much less acidic. It also appeared, when comparing with catalytic data already published on for HPA/HMS, that HMS was the best support for HPA without modifying appreciably its catalytic and thus their acid properties, the HPA being certainly bonded to the HMS walls by hydrogen bonding. Assuming a diameter of 1.2 nm for each Keggin anion, the turnover frequency (TOF) values for n-butane isomerisation at 473 K were calculated per surface Keggin species for HPA/HMS, bulk H3PW12O40 and Cs1.9H1.1PW12O40, and were found to be equal to 155, 113 and 100×10−5 s−1, respectively, and thus to be very close, showing that the acid strength of the three samples was comparable. It was found to equal only 3.4×10−5 s−1 for the HPA/AC sample, in agreement with a weaker acid strength. At last, mesoporosity of the support was found to favour n-butane isomerisation reaction.

Coadsorption of CO2 and O2 on MgO: An experimental and theoretical ESR study

A paramagnetic species attributed to an anionic coadsorbate of CO2 and O2 has been identified on an uv irradiated MgO surface. Its ESR parameters are as follows: gx=g1=2.040±0.001, gy=g2=2.0072±0.0005, and gz=2.0015±0.0005; Cx ?4 Oe, Cy=3.5±0.5 Oe, and Cz=2.8±0.3 Oe for 13C hyperfine tensor, and Caz=100 Oe and Cbz=50 Oe for 17O hyperfine splittings from molecular oxygen; the corresponding Cy and Cx components were too small to be determined. A theoretical approach to such a species has been made using the CNDO II/SP method. We have studied first the direct interaction of CO2 with O−2 in the gas phase, and then various models for the adsorbed species. The one that best fitted the experimental data corresponded to a species parallel to the surface with a CO2–O2 distance equal to 1.5 A. We suggest that interaction of the O−2 unit with both CO2 and surface ions is necessary in order to obtain theoretical results in qualitative agreement with experimental data.

Role of acid and redox properties on propane oxidative dehydrogenation over polyoxometallates

Cs2.5H1.5PV1Mo11O40 heteropolyoxometallate compounds have been studied for propane oxidative dehydrogenation (ODH) in the 340–400 °C temperature range. Their redox and Bronsted acid properties have been tuned by introducing a redox metal element M such as CoII, FeIII, GaIII, NiII, SbIII or ZnII in a V:M atom ratio equal to 1:1. This introduction was carried out either directly in the synthesis solution or by usual aqueous cationic exchange of protons of the solid Cs salt. TGA and FT-IR analyses allowed us to determine the extent of metal M substitution for MoVI in the Keggin anion and proton replacement by the M cation. It was observed that, under catalytic conditions (C3:O2:He=2:1:2, flow rate 15 cm3 min−1, 12 h on stream), the catalysts were stable, with only a small part of the substituted elements (V and/or M) being extracted from the Keggin anion during the reaction. The presence of these metal M cations enabled us to tune the redox and acid properties of the material and to get high selectivity for propene (60–80% at 5 and 10% propane conversion) at a relatively low temperature (300–400 °C). The direct synthesis method was found more efficient than the classical cationic exchange technique for propane ODH.

EPR investigation of the structure and reactivity of Pd(I) species generated in synthetic mordenite-type zeolite

Abstract A mordenite-type zeolite exchanged with Pd(NH 3 ) 4 2+ ions has been studied by EPR spectroscopy after heat treatment at 500°C in flowing oxygen and then in vacuo . The signal with g ∥ = 2.97 and g ⊥ = 2.146 was unambiguously assigned to Pd(I) ions bonded to four lattice oxygen ions in a pseudo-square-planar symmetry at the center of an eight- or a twelve-membered ring of the zeolite. Pd(I)CO or Pd(I)OH (or H 2 O) adducts were formed and characterized by g ∥ = 2.41 and g ⊥ = 2.105 or g ∥ = 2.35 and g ⊥ = 2.105 , respectively The decrease in the g values with respect to initial Pd(I) ions was interpreted in terms of the ligand effect, with the CO or the OH adduct in an axial position of a pyramidal square-planar complex, and the equatorial positions occupied by the four lattice oxygen ions. The adsorption of molecular oxygen led to a new EPR signal with g ⊥ = 2.050 and g ∥ 1.99 and a hyperfine structure of a sextet and an eleven-line pattern of 77-gauss splitting due to singly or doubly 17 O-labeled diatomic oxygen species. The spectrum was attributed either to an O 2 − ion freely rotating about the internuclear axis or to an O 2 3− ion of V-center type.

Partial oxidation reactions on phosphate-based catalysts

In this work emphasis has been placed on phosphate-based catalysts used for partial oxidation reactions, such as vanadyl pyrophosphate, iron phosphates and hydrogen/hydroxy-phosphates, zirconium hydrogen phosphates as layered compounds used to stabilise/entrap Cr and V oxyhydroxy-macrocations. It is shown that, in partial oxidation reactions, the catalyst surface behave in a rather dynamic and labile way, reconstructing under activation and/or catalytic reaction conditions and adapting itself to the stereochemistry of the reactants. The active sites are shown to have a molecular size, to be isolated and to present several catalytic functions, as hydrocarbon activation, H atom abstraction, lattice oxygen incorporation and electron transfer through the solid material to allow the redox process to occur. The metal cations are the active species and the role of the phosphate tetrahedra is not only to bind the MO6 octahedra together to constitute a dense or layered compound but also to bring some specific redox and acid–base properties.

Importance of site isolation in the oxidation of isobutyric acid to methacrylic acid on iron phosphate catalysts

AbstractIsobutyric acid (IBA) oxidative dehydrogenation to methacrylic acid (MAA) reaction has been studied for a wide variety of iron (hydrogen) phosphates catalysts and mechanical mixtures of several of these phosphates. It was observed that the reaction needs high amounts of water, typically 10–12 mol water per mol IBA, to get stable, active and selective catalysts and follows a Mars and van Krevelen mechanism, as schematised below: