Showing papers on "Dehydrogenation published in 1989"

Photochemical dehydrogenation of alkanes catalyzed by trans-carbonylchlorobis(trimethylphosphine)rhodium: Aspects of selectivity and mechanism

Abstract: Deshydrogenation photochimique des methyl- et isopropyl cyclohexanes. Obtention d'alkyl cyclohexenes, de methylidene et d'isopropenyl cyclohexanes

Hydrogen production from ethanol catalysed by Group 8 metal complexes

Abstract: Different strategies for the catalytic thermal production of hydrogen from ethanol are discussed and demonstrated using various Group 8 metal catalysts, in the presence of added base. Where the metal has a low affinity for carbon monoxide, e.g. in [PtH(PEt3)3]+, simple dehydrogenation of ethanol to ethanal and its aldol condensation products is observed. When the metal has a high affinity for CO, CO abstraction from the formed ethanol occurs and, as in reactions catalysed by [RhCl(PPh3)3] or [RhH(PPri3)3], can poison the reaction. In some cases, the CO abstraction reaction can be used to promote the thermodynamically favourable reaction of formation of hydrogen, methane, and carbon monoxide; although irradiation with visible light is often required to release the carbon monoxide from the metal centre {e.g. [RhH(CO)(PPri3)2] in the absence of base}. Finally, in catalytic reactions carried out in the presence of base, water–gas shift type chemistry is observed in reactions catalysed by [Rh(bipy)2]Cl, so that ethanol can be converted into 2H2, CH4, and CO2. In the cases of [Rh(bipy)2]Cl and [RuH2(N2)(PPh3)3], rates of hydrogen production of > 100 catalyst turnovers h–1, corresponding to > 1 l per litre of catalyst solution per hour can readily be sustained over long periods. The role of base in, and the mechanisms of, these interesting reactions are discussed; as are synergistic effects and reasons for the success of [Rh( bipy)2]Cl and [RuH2(N2)(PPh3)3] as catalysts for hydrogen production.

Aromatization of hydrocarbons by oxidative dehydrogenation catalyzed by the mixed addenda heteropoly acid H5PMo10V2O40

Abstract: The mixed addenda heteropoly acid H{sub 5}PMo{sub 10}V{sub 2}O{sub 40} dissolved in 1,2-dichloroethane with tetraglyme, forming the (tetraglyme){sub 3}-H{sub 5}PMo{sub 10}V{sub 2}O{sub 40} complex, catalyzes the aromatization of cyclic dienes at moderate temperatures in the presence of molecular oxygen. Dehydrogenations of exocyclic dienes such as limonene show that dehydrogenation is preceded by isomerization to their endocyclic isomers. Aromatization takes place by successive one-electron transfers and proton abstractions from the organic substrate to the heteropoly acid the latter being reoxidized by dioxygen coupled with the formation of water.

Probing ensemble effects in surface reactions. 1. Site-size requirements for the dehydrogenation of cyclic hydrocarbons on platinum(111) revealed by bismuth site blocking

Abstract: Un modele cinetique est propose pour expliquer la dissociation des especes adsorbees sur les catalyseurs ou les sites acides ont ete partiellement dilues

Oxygen-hydrogen and carbon-hydrogen bond activation in ethylene glycol by atomic oxygen on silver(110): heterometallacycle formation and selective dehydrogenation to glyoxal

Abstract: The absorption and reaction of ethylene glycol [(CH 2 OH) 2 ] on clean and oxygen-covered Ag(110) surfaces have been studied with temperature-programmed reaction spectroscopy (TPRS) and high resolution electron energy loss vibrational spectroscopy (EELS). On the clean surface (CH 2 OH) 2 was reversibly adsorbed, desorbing with an activation energy of 15 kcal/mol at 250 K from a monolayer state and a 225 K from a multilayer state with a heat of sublimation of 16 kcal/mol. Vibrational spectra of both the multilayer and monolayer were in good agreement with that of liquid (CH 2 OH) 2 , indicating that (CH 2 OH) 2 is only weakly bonded to the Ag(110)

The Role of Metal Cations in the Transformation of Lower Alkanes into Aromatic Hydrocarbons

Abstract: The reactions of ethane, propane, pentane, and hexane over H–ZSM-5, Ga–ZSM–5, and Zn-ZSM-5 zeolite were carried out to find the role of cationic species for the formation of aromatic hydrocarbons. In the ethane conversion, zinc cations act as dehydrogenation center for ethane. For other hydrocarbons, the cracking of the starting hydrocarbons on acid centers is the first step of the conversion, and metal cations act as catalyst for the dehydrogenation of the olefinic intermediates. The high activity of the zinc or gallium cations for the olefin dehydrogenation is demonstrated in the conversion of lower olefins and methanol.

A process for the production of mono-olefins by the catalytic oxidative dehydrogenation of gaseous paraffinic hydrocarbons having two or more carbon atoms

Abstract: A process for preparing mono-olefins by catalytic oxidative dehydrogenation of e.g. ethane, propane or butanes is provided. The process employs a catalyst which is capable of supporting combustion beyond the normal fuel rich limit of flammability. Preferred catalysts are platinum group metals supported on a monolith e.g. platinum.

Chemical process and catalyst

Abstract: A catalyst which comprises zinc, together with an effective amount of a platinum group metal on a support which is a material having the silicalite structure, the framework of said material consisting essentially of silicon and oxygen atoms or of silicon, zinc and oxygen atoms. There is also provided a process for the dehydrogenation of a C2 to C10 paraffin to yield an alkene product which process comprises contacting the paraffin under dehydrogenation conditions with a catalyst according to the invention.

The importance of heterogeneous and homogeneous reactions in oxidative coupling of methane over chloride promoted oxide catalysts

Abstract: The formation of ethene from ethane and methane in a silica reactor has been studied both in the presence and in the absence of chloride-containing catalysts. Some homogeneous conversion of ethane to ethene occurs in the gas phase through direct dehydrogenation, oxidative dehydrogenation, and, when HCl is present, chlorine radical induced reactions. Methyl chloride is detected in the gas phase but has no influence on the conversion of ethane to ethene. It is shown that under typical catalytic conditions, when a chloride-modified catalyst is used, ethane is mostly produced in the catalyst bed.

The nature of the active phase of the Fe/K-catalyst for dehydrogenation of ethylbenzene

Abstract: The industrial catalyst for high temperature dehydrogenation of ethylbenzene based on iron and potassium oxides undergoes, under reaction conditions, essentially a transformation into magnetite, Fe3O4, and a mixture of ternary oxides containing trivalent iron, viz. K2Fe22O34 and KFeO2. The latter compound constitutes the outside of the catalyst particles and is indeed the catalytically active phase.

Partial oxidation of methane over oxide catalysts. Comments on the reaction mechanism

Abstract: A detailed investigation of the partial oxidation of methane to ethane and ethene is described. In particular a range of oxidants (O2, N2O, O3) have been reacted with a series of catalysts (MgO, Li/MgO, La2O3, Li/La2O3, Sm2O3 and Li/Sm2O3) and these studies have been utilized to investigate the reaction mechanism. Methane oxidation at conversions of ca. 1 % have indicated that ethene as well as ethane is a primary reaction product, but at higher conversions the major fraction of the ethene formed is via oxidative dehydrogenation of ethane in a secondary reaction. Based on this study a reaction mechanism is proposed and discussed. In line with previous studies the results from used of N2O as oxidant confirm that O–(s) is the oxidising species responsible for CH˙3 formation, from CH4, which on dimerisation yields ethane. In addition it is proposed that primary formation of ethene results from the interaction of methane with a different surface oxygen species (e.g. O2–2(s)). The oxides studied, which can be considered as broadly representative of the wide range of oxides utilised as catalysts, become active only at high temperatures ( 700 °C) and at such temperatures the gas phase nature of the overall reaction is dominant. The consequences of this observation are discussed with respect to the design of improved catalysts.

Properties of molybdate species supported on silica

Abstract: 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.

Mössbauer spectroscopic study of iron phosphate catalysts used in selective oxidation

Abstract: Three iron phosphate compounds FePO4, Fe2P2O7 and Fe7(PO4)6 have been synthesized and studied as catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid. Mossbauer spectroscopic characterization of the solids before and after catalytic reaction has allowed us to show that the starting phases were transformed in the conditions of reaction and that a new phase was formed. This new phase is a mixed Fe3+ and Fe2+ phosphate with a Fe3+/Fe2+ ratio of 1.8±0.2 with Mossbauer parameters: δ1=0.47 ± 0.05, Δ1=0.68± 0.02 mm.s-1 and δ2=1.20 ± 0.05, Δ2=2.73±0.02 mm.s-1. This new phase appears to be elective for methacrylic acid formation.

Combined oxidation and dehydrogenation in a palladium membrane reactor

Abstract: Simulation, dans un reacteur a membrane de palladium, du couplage d'une reaction exothermique d'oxydation et d'une reaction de deshydrogenation du butene-1. Presentation des resultats dans des conditions adiabatiques et isothermes

Conversion of Light Alkanes Into Aromatic Hydrocarbons. 3. Aromatization of Propane and Propene on Mixtures of HZSM5 and of Ga2O3

Abstract: The activities for propane aromatization of physical mixtures of HZSM5 (25 mg) and Ga2O3 (5 to 200 mg) are much greater than the sum of the activities of the pure components. This synergic effect is characteristic of a reaction in which catalytic sites of the two components (bifunctional catalysis) participate. Experiments with model reactants propane, propene, 1-hexene, 1-heptene, methylcyclohexene and methylcyclohexane confirm that gallium oxide catalyzes the dehydrogenation of alkanes into olefins and of naphthenes into aromatics but is not active for oligomerization and cyclization reactions. The aromatization of propane on the mixture occurs in the following way: on gallium oxide propane is dehydrogenated into propene; propene diffuses from gallium oxide to HZSM5, on which it undergoes oligomerization and then cyclization; naphthenic compounds diffuse from HZSM5 to gallium oxide where they are dehydrogenated into C6-C8 aromatics.