Guylène Costentin

On the partial oxidation of propane and propylene on mixed metal oxide catalysts

Abstract The present review analyses the literature data reported on the partial oxidation of propane to organic compounds (acrolein, acrylic acid and acrylonitrile) over mixed metal oxides, mainly magnesium vanadates, vanadia bismuth molybdates and vanadia antimony. The data were compared to those reported on the partial oxidation of propylene over bismuth molybdate and antimony—tin multicomponent oxides and over cuprous simple oxide. For both reactions, we analyzed the involved reaction mechanisms, intermediate species, active phases and active sites. The role of water produced during the reaction and that of the Bronsted acid sites were shown to be important in the determination of the selectivity of the expected products. The main conclusion of our study is that a good mix of acid-base and redox properties of the oxide surface should permit a controlled orientation of the reaction towards selective products.

Comparative study of CS2 hydrolysis catalyzed by alumina and titania

The study is devoted to the comparison between the activity of γ-alumina and titania (anatase) toward CS2 hydrolysis. It is shown that, without any other sulfur compound, like H2S and SO2, and without O2 traces in the feed, alumina is more active than titania at 320°C. The IR study of CS2 adsorption on alumina evidences the participation of the most basic OH groups and the formation of hydrogen thiocarbonate and hydrogen carbonate species, providing some information on the nature of the active sites and the reaction mechanism. Chemisorbed COS appears as a reaction intermediate. Realistic industrial Claus conditions imply, in addition to CS2 and H2O, the presence of H2S, SO2 and O2 traces in the feed. It appears that the presence of O2 traces in the CS2–H2O mixture brings about a decrease in activity of the alumina and titania. This is due to sulfate formation as shown by the IR analysis of the catalysts after the reaction. Moreover, IR studies evidence that sulfate species are reduced by H2S at 320°C on TiO2, contrary to results obtained on Al2O3, explaining why TiO2 is much more effective than Al2O3 when the CS2+H2O feed also contains H2S and O2 traces.

Propane oxydehydrogenation reaction on a VPO/TiO2 catalyst. Role of the nature of acid sites determined by dynamic in-situ IR studies

Abstract A VPO/TiO 2 catalyst tested in the oxydehydrogenation reaction (ODH) of propane between 300 and 400°C shows satisfactory performances (up to 80% of propene selectivity at 2% of propane conversion at 300°C or 56% of propene selectivity at 9% of propane conversion at 400°C). Addition of water or pyridine in the feed gas tends to decrease the propane conversion and enhances the propene selectivity. It is shown that water increases the number of Bronsted surface acid sites by dissociative adsorption which, in turn, enhances propene selectivity at the expense of the CO x selectivity. These results are in good agreement with spectroscopic IR observations performed under catalytic conditions showing that the Lewis acid sites are linked to CO x formation, whereas it seems that Bronsted sites would rather be linked to propene formation.

Study of H2S selective oxidation on new model catalysts: Influence of composition

Phosphate of transition elements A 3 M 4 (PO 4 ) 6 (A=Fe, Ni, Zn, Mg, Cu, Cr and M=Fe, V, Cr) are prepared by solid methods, at 1223 K. Their activity in H 2 S selective oxidation are compared. In spite of their low specific area, the catalysts develop a good activity (up to 17% of H 2 S conversion for surface area lower than 1 m 2 g -1 ) and an excellent sulphur selectivity (always higher than 95%). Screening experiments show that the best systems always contain Fe as element and, in these cases, Mossbauer characterisations evidence the establishment of Fe 2+ /Fe 3+ mixed valency during the reaction. As shown by XANES and XRD results, the ability of the element in the divalent A site to promote the redox mechanism between M 2+ /M 3+ and H 2 S/S 0 and to prevent sulphidation determines the catalytic activity.

Evidence of the reverse Claus reaction on metal oxides: Influence of their acid–base properties

Abstract Different metal oxides have been impregnated by sulfur S 8 and treated under H 2 O at increasing temperature up to 330°C. In case of TiO 2 and Al 2 O 3 , H 2 S and SO 2 emissions have been detected by gas chromatography, showing that the reverse Claus reaction between sulfur and water occurs. Addition of sulfate ions to TiO 2 and Al 2 O 3 prevents the reaction, whereas addition of sodium to SiO 2 promotes H 2 S and SO 2 formations. Our results evidence that the reverse Claus reaction is sensitive to the acid–base properties of the metal oxides used, as the direct reaction does. However, H 2 S is always evolved at lower temperature than SO 2 , which is discussed in relation with the chemisorption of each product on the metal oxides used, and their possible reactivity. In particular, thiosulfate species are evidenced by infrared spectroscopy on Na/SiO 2 after reaction.

A correlation between crystal structure and catalytic activity in the solid solutions CdMoxW1−xO4

Abstract Two CdMoxW1−xO4 solid solutions with the scheelite and the wolframite structures were synthesised via different techniques and their structures were determined from powder refinements. The limit of the reciprocal solubility of Mo in CdWO4 has been checked. Several compositions of the solid solutions have been tested in the catalytic mild oxidation of propane or propene and relations between the structures and the catalytic activity are proposed.

Control of calcium accessibility over hydroxyapatite by post-precipitation steps: influence on the catalytic reactivity toward alcohols

Hydroxyapatites are increasingly used as heterogeneous catalysts since they present atypical behaviours for many acid base reactions. The aim of this study was to discuss the possible involvement of Ca2+ Lewis and/or PO-H Brønsted acid sites belonging to the hydroxyapatite system in the conversion of 2-methylbut-3-yn-1-ol, a model molecule that is known to account for the acid base properties, and of ethanol into n-butanol. A series of hydroxyapatite samples with similar bulk properties was prepared from a lone precipitation batch, but by varying the conditions of the washing and drying steps. Although the surface depth probed by XPS exhibited similar average composition, ISS analysis revealed a gradient of calcium concentration in the first surface layers. In fact, the different conditions of drying and washing resulted in a modulation of the relative amount of Ca2+ and PO-H accessible on the top surface, as revealed by the adsorption of the CO molecule monitored by FTIR. The conversion in the two alcohol molecules is linearly dependent on the nature of the acid base pairs involved: when accessible on the top surfaces, due to their stronger acidity, the Ca2+ Lewis acid sites are preferentially involved, but they are less efficient than PO-H, as illustrated by the linear decrease of the conversion levels with the increasing relative amount of accessible Ca2+ cations. It is thus concluded that PO-H sites enhance the performances of the catalysts for the two reactions, and that washing and drying conditions allowing us to decrease the calcium accessibility at the benefit of PO-H should be favoured.

Structure-sensitivity study of partial propene oxidation over AV2P2O10 vanadium phosphate compounds

The activity and selectivity of five AV2P2O10 compounds have been examined with respect to the mild oxidation of propene at different temperatures. These materials with orthorhombic symmetry (ACa, Cd) or monoclinic symmetry (ABa, Pb, Cd) have been prepared as well crystallized samples in an evacuated silica tube. Their crystalline structure, followed by X-ray diffraction (XRD), was unchanged after the catalytic reaction and their structural properties made them ideal model catalysts to study structure-sensitivity effects. Orthorhombic compounds were more active than monoclinic ones. Catalytic results were discussed, taking into account the nature of the inserted cations, the distortion of the framework and the organization of the lattice polyhedra.

Effects of the structural and cationic properties of AV2P2O10 solids on propane selective oxidation

Abstract The activity performances of five AV 2 P 2 O 10 compounds of either orthorhombic (ACd, Ca), or monoclinic (ACd, Ba, Pb) symmetry types, have been compared for propane partial oxidation. They present a rather good activity (9% of propane conversion at 460°C) and a high selectivity (60% of propene selectivity). Results are discussed as a function of the differences in the structural properties. Kinetic studies were also performed for propane and propene oxidation reaction.

Probing the strength, concentration and environment of basic sites in zeolites by IR spectroscopy

Abstract Three protic probe molecules were used to characterize by FTIR spectroscopy the strength, concentration and environment of basic sites in series of faujasites (FAU structure) in which either the Al content (Y, X and LSX) or the nature of the framework charge compensating alkali cation (Na, K, Cs) was varied. The frequency shifts observed for adsorbed MBOH and adsorbed methylacetylene well account for variations in the basic strength of the zeolitic framework oxygen atoms. Interestingly, methylacetylene also informs on the environment of the basic sites. Lastly H 2 S dissociation brings quantitative information on the amount of basic sites. The results show the high potential and the complementarity of these protic probes to describe the strength, concentration and environment of basic sites.