M.M. Bettahar

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.

Gas phase hydrogenation of benzaldehyde over supported copper catalysts

Abstract Benzaldehyde hydrogenation over copper catalysts supported on Al2O3, SiO2, TiO2, CeO2 and ZrO2 has been studied at atmospheric pressure and 100–350°C. The reaction competitively produced benzylalcohol, toluene and benzene with yields depending on the nature of the support and reaction temperature. The obtained order of activity was attributed to metal and acid–base surface properties. The important gap in activity (multiplied by 2 to 23) between 300°C and 350°C, observed for all catalysts except Cu/SiO2, was attributed to an adsorption phenomenon, i.e. base sites or low reaction temperature inhibited the reaction course, whereas acidic sites or high reaction temperature enhanced the catalyst activity. Benzylalcohol was obtained with high selectivity (83% at 68% of conversion) from only 100°C with the SiO2 acidic support, whereas selective formation of toluene or benzene was observed at higher reaction temperature and depended on the nature of the support. The relative selectivity of toluene and benzylalcohol was governed by the adsorption strength the alcohol OH function on the catalyst surface. High selectivity to benzene was attributed to the easiness of the catalyst to break the CH aldehydic bond and stability of the surface organic entity formed. The parallel study of the reduction of benzaldehyde under N2 flow threw some light on the relation between metal and acid–base surface properties of the catalysts and reaction paths in benzaldehyde hydrogenation.

Nickel Nanoparticles Supported on Silica of Low Surface Area. Hydrogen Chemisorption and TPD and Catalytic Properties

Nickel metal nanoparticles supported on silica of low surface area (15 m2 g-1) were prepared by reduction of nickel acetate by hydrazine in aqueous medium. Their gas-phase stability and surface properties depended on thermal pre-treatment under H2 or air atmosphere. Small nickel particles (<10 nm), in oxidized or reduced state, are strongly resistant to reductive or oxidative treatment respectively. For H2-treated catalysts, H2 chemisorption and TPD results suggested the occurrence of spillover hydrogen between the metal nickel phase and silica. For air then H2 treated catalysts, hydrogen spillover seemed to involve the NiO phase. The activity of the catalysts in gas-phase benzene hydrogenation also depended on the thermal pre-treatment. Pre-calcined then reduced catalysts exhibited higher TOFs than non pre-calcined catalysts, suggesting that the presence of NiO phase may have influenced the hydrogenation process.

Reaction mechanisms and kinetics in the n-hexane cracking over zeolites

Abstract The reaction mechanisms of the cracking of n-hexane over MFI zeolites ( Si Al =10–75 ) have been studied at 400°C at low conversions in a microflow reactor. The reaction was found of first order in the partial pressure of n-hexane and second order in the aluminium content of the zeolites. The analysis of the kinetic parameters and selectivities showed that the reaction predominantly proceeded through the protolytic monomolecular mechanism involving the formation then the decomposition of an adsorbed carbonium ion-like species. This decomposition leads to n-alkanes and adsorbed carbenium ion-like species or, most probably, to n-olefins. The linear olefins (mainly butenes) should be the primary products of the cracking of n-hexane and, as such, should be considered as the main source of the subsequent reactions, notably the hydride transfer reactions. The latter reaction was favoured at high conversions, long contact times and high aluminium contents. It is suggested that, in the n-hexane cracking, the acid as well as the conjugated base sites of the zeolite play an important role in the protolysis process and explain the kinetic orders reported here and in the literature data. Two reaction paths have been envisaged: (i) the second order/high temperature (>450°C) mechanism which involved one n-hexane molecule and one zeolite acid site in the rate determining step and; (ii) the third order/low temperature mechanism which involved one n-hexane molecule and two zeolite acid sites in the rate determining step. In the latter case the protolysis step needed the anchimeric assistance of a second active site. In the proposed mechanistic schemes, the gaseous n-alkane molecule was adsorbed on Bronsted acid sites then decomposed with the help of the conjugated base sites.

In situ FT-IR and kinetic study of methanol synthesis from CO2/H2 over ZnAl2O4 and Cu–ZnAl2O4 catalysts

Abstract The kinetics of the CO2/H2 reaction over ZnAl2O4 and Cu–ZnAl2O4 catalysts at 250°C up to 0.3 MPa have been followed by in situ FT-IR spectroscopy. Both methanol and carbon monoxide formation were enhanced in presence of copper. They were also produced by independent routes through different adsorbed species. Formate (type I, I′ and II), methoxy and carbonate species were identified on the support and, in addition, copper formate and copper carbonyl species when copper was present. The hydrogenation of carbonate species to copper formate species was found rate determining in methanol synthesis over the Cu–ZnAl2O4 catalyst whereas type I formate species were shown to be the active intermediate for this reaction over the ZnAl2O4 support. Carbon monoxide resulted from the water gas shift reaction probably through the same species as methanol formed over the Cu–ZnAl2O4 catalyst whereas it seemed to stem from formate species of type II in the case of the ZnAl2O4 support. Type II formate species were shown inactive in presence of copper whereas the methoxy species adsorbed on the support were found inactive in presence and in absence of copper. The comparison of these results with those previously obtained with the CO/H2 mixture showed that the nature and the role of the detected species strongly depended on the reactive atmosphere and on the presence or not of copper in the catalyst composition.

Physicochemical and catalytic properties of iron-doped silica—the effect of preparation and pretreatment methods

Chemical vapor deposition (1wt% of iron) and wet and wetness impregnations (both giving Fe loading of 1.6 and 2.9 wt%) have been applied for the formation of iron-doped silica. The obtained materials were characterized by means of low-temperature N2 adsorption, XRD, TEM, H2-TPR, ESR, and chemisorption of CO and O2. Their catalytic activity was tested in isopropanol decomposition and oxidation of methanol to formaldehyde. Depending on the preparation procedure various levels and strengths of Fe–silica interactions were observed. The activation conditions determine the nature of the Fe species formed. Fe3+ isolated species are active in isopropanol dehydration whereas Fe–oxide centers exhibit very high selectivity in methanol oxidation to methylformate and formaldehyde.

In-situ FT-IR spectroscopy and kinetic study of methanol synthesis from CO/H2 over ZnAl2O4 and CuZnAl2O4 catalysts

Abstract The CO/H 2 reaction over ZnAl 2 O 4 (support) and CuZnAl 2 O 4 (catalyst) performed at 250°C up to 0.3 MPa, was studied by combining in-situ FT-IR experiments and kinetic investigations. The obtained results showed the existence of two reaction mechanisms for methanol formation involving formate species (mainly on ZnAl 2 O 4 ) and carbonyl species (on CuZnAl 2 O 4 ), as key reaction intermediates. Inactive formate species were also evidenced. Carbon dioxide, which is the initial major product formed on the ZnAl 2 O 4 support, is produced through the partial reduction of the latter.

Higher alcohol synthesis on modified iron based catalysts: copper and molybdenum addition

Abstract The activity of Fe-Cu-Mo-U catalysts in the synthesis of higher (C 1 to C 6 ) alcohols from synthesis gas has been studied at 8.5 MPa. The catalysts have been prepared by coprecipitation and characterized by BET, X-ray diffraction and scanning electron microscopy. The acyl species is the key intermediate in the linear alcohol chain growth. The quantitative estimates of the concentration of the surface species by temperature-programmed desorption experiments allowed the prediction of the ability of the catalyst to induce alcohol chain growth. The study of the carbon monoxide dissociation gives a good estimate of the overall catalytic activity.

Acetylene cyclotrimerization over Ni/SiO2 catalysts in hydrogen atmosphere

Abstract Non-conventional Ni catalysts supported on low surface area SiO 2 were prepared by reduction of nickel acetate by aqueous hydrazine, characterized by XRD and H 2 chemisorption properties or TPSR and tested in acetylene cyclotrimerization in the presence of hydrogen. Their performances were compared with those of classical catalysts supported on silica of low or high surface area. Hydrogen TPD profiles exhibited the presence of active sites involving nickel or nickel in interaction with the support. Hydrogen storage increased for non-classical catalysts or low surface area support. Under the reaction conditions used, no cyclotrimerization of acetylene was observed in the absence of hydrogen, probably as a result of strong adsorption of benzene precursor species. In the presence of hydrogen, benzene and ethylene were the main products in the low reaction temperature regime ( 2 and C 4 hydrogenation products at the expense of cyclotrimerization to benzene. Increasing the partial pressure of acetylene strongly deactivated the catalyst but favoured selectivity of both benzene and ethylene. The presence of both water vapour and hydrogen improved cylotrimerization to benzene at the expense of hydrogenation products, notably at low reaction temperatures. The effect of the different parameters on the sorptive and catalytic properties of the nickel catalysts prepared are discussed, notably the role of hydrogen and water in the determination of activity and reaction paths.

Colloidal nanometric particles of nickel deposited on γ-alumina: characteristics and catalytic properties

The present paper deals with supported nanoparticles of metallic nickel prepared by impregnation of an organometallic colloidal precursor (NiRC complex) on γ-alumina. The obtained supported materials (NiRCS complex) were characterized by STEM, EELS, XRD and TGA and tested in the hydrogenation reaction of benzene and styrene both in the gas and liquid phases. EELS and STEM studies showed that the metallic nickel nanoparticles of the NiRC precursor were well dispersed (1–3 nm) on the support, without size variation during the impregnation step. However, particle growth in NiRCS was observed in samples used after reaction tests. On the other hand, TGA experiments showed that the nickel phase was inserted in an organic matrix which remained in the supported material after impregnation. This organic matrix should be partly removed or totally decomposed by washing with an appropriate solvent or thermal treatment above about 250°C under N2, H2 or air atmosphere respectively. The supported nickel particles were active in the gas phase hydrogenation of benzene in the temperature range of 150–200°C and their activity was enhanced by partial removal of the organic matrix with an organic solvent (e.g. ethanol) or by its total thermal decomposition. They were also found very active in the gas phase hydrogenation of styrene at room temperature. Finally, the supported nickel particles exhibited good hydrogen reservoir properties in organic solvent media.