Michel Forissier

Correlations between X-ray photoelectron spectroscopy data and catalytic properties in selective oxidation on SbSnO catalysts

Abstract A series of SbSnO catalysts has been studied by X-ray photoelectron spectroscopy (XPS) as a function of Sb content (from 1.7 to 39.7 atom%) and activation temperature. At low activation temperature (500 °C) surface and bulk compositions are comparable, while at higher activation temperatures strong surface enrichment in Sb occurs. Detailed analysis of the XPS line shapes shows that two phases may exist at the surface of the catalyst. The first one, which is observed at low Sb content and low activation temperature (500 °C), is assigned to Sb(V) in solid solution in the SnO 2 lattice. The second one, which is observed for Sb contents larger than 5% or for high-temperature activations, is assigned to an Sb 2 O 4 phase. Catalytic properties for selective oxidation of propylene were studied in the 350 to 400 °C range. The selectivity for acrolein increases when surface Sb content is increased either by enhancement of the Sb concentration or by higher activation temperature. It is then postulated that the catalytic phase, which gives selective oxidation, consists of an Sb 2 O 4 phase lying at the surface of a solid solution of Sb (V) in SnO 2 .

Kinetic model based on a molecular description for catalytic cracking of vacuum gas oil

Abstract A kinetic model based on a molecular approach is proposed to represent the cracking reactions of industrial feedstocks on a commercial equilibrium catalyst. Special attention is given to the definition of lumps to represent correctly the effect of the feedstock composition on the gasoline composition through the most important reactions occurring between lumps. The latter are defined not only by their boiling range but also by their chemical composition. The kinetic model is built from experiments with a small fixed bed reactor (the microactivity test). The analysis of cracked products is done by chromatography simulated distillation and a detailed gas chromatographic analysis of gases and gasoline. The study is done at 803 K with various reaction conditions (catalyst hold-up, initial catalyst coke content) to observe primary and successive products. Three typical commercial feedstocks (aromatic, naphthenic or paraffinic) are used to distinguish the effects of feedstock components. The selected reactions are grouped as: (i) cracking reactions by β scission of paraffin, alkyl-aromatic or naphthenic chains, (ii) condensation reactions of olefins possibly with aromatic hydrocarbons, (iii) cyclization of olefins, (iv) hydrogen transfer reactions. The attention is focused on the deactivation function of the catalyst. This function is experimentally determined by testing previously coked catalysts. It takes into account chemical deactivation by coke fouling and diffusional limitation due to pore plugging by coke. The results show that this function depends on the feedstock composition. The reactor is modelled as isothermal, plug flow and unsteady because of the deactivation by coke. The kinetic expressions suppose generally an order one versus each reactant. The kinetic constants are determined by non-linear adjustment with experimental data. The obtained set of kinetic constants describes satisfactorily the experimental results. The kinetic model shows the importance of the condensation and hydrogen transfer reactions for coke formation and gasoline quality. It can be used for modelling a commercial FCC riser.

Hydrogen solubility in straight run gasoil

AbstractThe solubility of hydrogen in a straight run gasoil is measured using a chromatographic method, which is rst validated with aknown organic liquid (cyclohexane). Under the experimental conditions (up to 4MPa, temperatures from 298 to 675K), these resultsarebestdescribedbyChao–SeaderandimprovedZudkevitch–Joe(withadjustmentparameter b 0 =0:6)models.Henry’slawconstantisdeterminedatfourtemperatures.? 2002ElsevierScienceLtd.Allrightsreserved. Keywords: Hydrogen;Cyclohexane;Solubility;Gasoil;Vaporpressure; Equation of state; Multiphase reactor 1. IntroductionIn many industrial processes of oil rening (hydro-genation, hydrotreatments such as hydrodesulfurization,hydrodenitrogenation, ...), kinetics is often related to thehydrogenpressure.However,intheseprocessesthecatalystisincontactonlywiththeliquidphasecontainingthedis-solvedhydrogen;therefore,thekineticratewouldinvolvethe hydrogen concentration in the liquid phase. If masstransfers are not limiting (as expectedin the Mahoney–Robinson reactor), hydrogen solubility in the conditionsof reaction is a way to access a goodestimation of thehydrogen concentration in the liquid phase; on the otherhand,ifamasstransferlimitationoccurs,adiusionmodelmay be used, implying the knowledge of the liquid-phasehydrogen concentration. In both cases, the knowledge ofhydrogen solubility is of interest for studying the kineticsofhydrotreatments.Whereas many data are available for hydrogen solu-bility in pure components (Young, 1981), only few re-searchers have workedwith complex mixtures, especiallywith petroleum products (Harrison, Scheppele, Sturm, G the methodis validatedby measure-mentsoncyclohexane(inwhichthehydrogensolubilityhas

Catalytic cracking in riser reactors: core-annulus and elbow effects.

Abstract The results of a simple model of vacuum gasoil cracking are presented:the radial profiles of superficial gas velocity, solid concentrations and axial profiles of conversion and yields are predicted by a plug flow model with a gas velocity profile and a radial dispersion coefficient. The catalyst to gas slip coefficient is fitted to industrial results. The core annulus structure of the bed results in a substantial reduction of gasoline yield.

Properties of molybdate species supported on silica

Molybdenum species have been deposited on a high surface area silica by the impregnation method and further calcination at 500 °C in air. Mo loading was varied from 0–10 wt % Mo. I.r., u.v.–visible, XPS and EDX-STEM techniques have been used to characterize the nature of the molybdenum species as a function of Mo loading. At low Mo content ( < 3 wt % Mo) interaction between silanol groups and molybdate ions occurred resulting in the formation of monomeric tetrahedral MoO2–4 species with a u.v. band near 245 nm. For intermediate loadings (3–8 wt %) polymeric octahedral molybdate species were identified with a u.v. band near 340 nm. At high Mo content crystallites of MoO3 were also observed. Catalytic properties for both isopropyl alcohol conversion in air at 100 °C and propene oxidation at 400 °C were studied as a function of Mo loading. It was observed that the catalysts are acidic as evidenced by isopropyl alcohol dehydration into propene at low Mo content and their activity increased with Mo content following the amount of MoO2–4 species. Polymeric molybdate species were observed to exhibit mainly redox-type properties as evidenced by isopropyl alcohol oxidative dehydrogenation into acetone and by propene oxidation into propanal (electrophilic attack) and into acrolein (allylic-type reaction). MoO3 crystallites were observed to exhibit usual properties of unsupported MoO3 catalysts.

Synergy effects in the catalytic properties of bismuth molybdates

Different phases (α, β and γ) and intimate equimolar mixtures of the α and γ phases of bismuth molybdate have been investigated for the mild oxidation of propene into acrolein. The mixtures were prepared either as a coprecipitate or as intimately compressed and ground mixtures. A synergy effect between the α and γ phases was observed with an enhancement in activity and selectivity for an intimate equimolar mixture. A study by X-ray diffraction, X-ray photoelectron spectroscopy and infrared spectroscopy allowed us to follow the effect of mixing on the different phases in the bulk or at the outer layers. Changes in elemental composition in either Mo or Bi because of mixing or preparation conditions were not detected.

Dependence of selectivity on surface structure of MoO3 catalysts

The mild oxidation of propylene has been studied on oriented and non-oriented supported MoO3 catalysts. In the case of the oriented MoO3–graphite catalysts, the orientation of the MoO3 crystallites and catalytic properties are strongly dependent on preparation conditions.Reaction specificity of the MoO3 crystalline faces has been observed. By radiocrystallographic and electron-microscopy studies it has been possible to identify the reactive faces. Catalytic studies have allowed us to suggest that, at low conversion, acrolein and carbon dioxide issue from different catalytic sites; these are situated on the (100) MoO3 faces for acrolein and on the (010) MoO3 faces for carbon dioxide.On the basis of stereochemical and electronic considerations, a mechanism for the oxidation of propylene to acrolein on the (100) MoO3 face has been proposed.

Electrical behaviour of powdered tin–antimony mixed oxide catalysts

A series of tin + antimony mixed oxide powders, calcined at 773 K, has been studied by electrical conductivity. A continuity in the electrical behaviour is found between semiconducting SnO2 and insulating Sb2O4. Pure stannic oxide is an n-type semiconductor and its free electrons come from the first ionization of anionic vacancies whose concentration is ≈ 1018 cm–3 at 608 K under 2.13 × 104 Pa O2. The enthalpy of formation of these vacancies and the ionization energy of their 2nd electrons have been estimated. As the Sb content increases, antimony dissolves into the SnO2 structure in the 5+ state, which increases the conductivity, σ, up to a maximum corresponding to 6.1 Sb atom %. Around this value formation of the Sb2O4 phase begins. The increased conductivity of mixed oxides with high Sb content (>20 atom %), compared with that of Sb2O4, which is an insulator, is attributed to a doping effect by Sn4+ cations in Sb3+ lattice positions of Sb2O4. Comparison of how both σ and catalytic properties vary with Sb content shows that electron transfer between catalyst and adsorbed species is not the rate limiting step in propene oxidation and that the solid solution of Sb5+ in SnO2 cannot constitute the active phase for acrolein formation.

Mechanism of 1,1-d2 propene oxidation over oxide catalysts

Abstract CD 2 CHCH 3 was oxidized over bismuth molybdate, tin-antimony mixed oxides, and supported molybdenum and vanadium oxide catalysts. The deuterium retention is high (>90%) in the recovered propene. Percentage retentions of deuterium in the acrolein agree with literature data when bismuth molybdate is used as catalyst. On SbSnO and supported Mo and V oxides, no isotope effect is noticed for the abstraction of the second hydrogen from the olefin. The slow step of the reaction may therefore be different for the oxidation of propene on BiMoO and SbSnO. The ethanal produced by oxidation of CD 2 CHCH 3 contains only minor amounts of deuterium, whatever the catalyst used. It is suggested that partial oxidation of propene to acrolein and CC bond rupture are parallel reactions which involve different intermediates. Possible mechanisms adapted from organic chemistry are presented to explain these findings.

Structural properties of Sb–Sn—O mixed oxide catalysts

Mixed oxides of tin and antimony were investigated by X-ray diffraction and Mossbauer spectrometry as a function of chemical composition and of firing temperature. At low calcination temperatures, antimony is present as Sb5+ dissolved in the SnO2 lattice at 5 atom % Sb and a mixture Sb5++ Sb3+ at higher concentrations.Upon calcination a demixing of the solid solution occurs and an antimony oxide phase is formed. From X-ray diffraction, this phase is identified as a bidimensional Sb2O4 layer. The balance of electrical charges in the lattice is achieved by two different mechanisms as a function of the Sb content: delocalization of electrons in the conductivity band at low antimony content and reduction of Sb5+ to Sb3+ at higher antimony percentage. Good selectivities are obtained for propene oxidation when the system is biphasic; it is postulated that the actual catalyst consists of an oriented film of Sb2O4 supported by the Sb5+–SnO2 solid solution.