Showing papers on "Dehydrogenation published in 1992"

Aromatization of short chain alkanes on zeolite catalysts

Abstract: On MFI catalysts, low cost liquefied petroleum gas (LPG ) can be transformed into valuable aromatics (mainly C 6 -C 8 benzenics) and into hydrogen. Unfortunately methane and ethane are also produced in significant amounts. A reduction of the production of these unwanted compounds would render the aromatization process economically more attractive. The reaction pathways of propane aromatization were established on H-ZSM-5 pure or loaded with platinum or with gallium. On H-ZSM-5 the first step is the dehydrogenation and the cracking of the reactant through carbonium ion intermediates. The resulting alkenes (propene and ethylene) undergo rapid successive reactions via carbenium ion intermediates: oligomerization, cyclization, hydrogen transfer. The selectivity to aromatics is limited because of the formation of methane by propane cracking and of alkanes by hydrogen transfer. Platinum increases the rate of propane transformation significantly but a higher production of methane and ethane is found on PtH-ZSM-5 catalysts, owing to the hydrogenolysis of alkanes and of alkylaromatics and to the hydrogenation of ethylene on the platinum sites. Gallium improves both the rate and the selectivity of propane aromatization. The aromatization occurs, like on PtH-ZSM-5, through a bifunctional pathway, gallium catalyzing the dehydrogenation of the alkane reactant to alkenes and of naphthenic intermediates to aromatics, and the acid sites catalyzing the oligomerization of light alkenes and the cyclization of C 6 -C 8 alkenes. The better selectivity to aromatics is obtained for GaMFI catalysts with gallium species well dispersed within the zeolite and being very active for dehydrogenation (e.g. with gallosilicates or galloaluminosilicates steamed under mild conditions). At high temperature, in presence of hydrogen (hence during aromatization) GaMFI catalysts would undergo various modifications: reduction of gallium species, migration within the zeolite, reaction of these species with the protonic sites. The nature of the gallium active species and their role in the various steps are discussed.

Direct observation of surface reactions by scanning tunneling microscopy: Ethylene→ethylidyne→carbon particles→graphite on Pt(111)

Abstract: We have used variable temperature, ultrahigh vacuum scanning tunneling microscopy (STM), in both static and time‐dependent experiments, to study the chemistry of the ethylene/Pt(111) system. Images of ethylene which exhibit long‐range order have been obtained at a sample temperature of 160 K. The conversion of ethylene to ethylidyne has been observed directly in STM images. This conversion reaction is observed to occur in a ‘‘patchy’’ manner across the surface at saturation coverage. As the reaction proceeds, well‐defined islands of unreacted ethylene continue to be clearly observed. Further dehydrogenation of the ethylidyne formed from ethylene leads to carbon containing particles dispersed randomly across the sample. After annealing the ethylidyne covered sample to 500 K, the surface is uniformly covered with carbon containing particles which exhibit a bimodal distribution of heights (one and two atomic layers) consisting of an average of ten and twenty carbon atoms, respectively. Further annealing to 7...

Conversion of light alkanes into aromatic hydrocarbons: VI. Aromatization of C2-C4 alkanes on H-ZSM-5 —reaction mechanisms

Abstract: The conversion of ethane, propane, n-butane, isobutane and of the corresponding alkenes was studied at 530°C on a H-ZSM-5 zeolite (Si/Al=40). Butanes transform about 4 times more rapidly than propane and 100 times more than ethane. From propane and butanes the primary products result from dehydrogenation and cracking, cracking being more rapid than dehydrogenation. Ethane undergoes mainly dehydrogenation and leads slowly to methane. It is suggested than these reactions occur through scission of carbonium ions formed by alkane protonation. The more stable the carbonium ion the higher the reaction rate. The olefinic primary products undergo very rapid reactions through carbenium ion intermediates: oligomerization, cyclization of the oligomers followed by aromatic formation through hydrogen transfer from naphthenes to light alkenes. Whatever the reactant, the limiting step of aromatization is the initial formation of alkenes.

Role of surface acidity on vanadia/silica catalysts used in the oxidative dehydrogenation of ethane

Abstract: The oxidative dehydrogenation of ethane in ethene has been investigated in the 753–863 K temperature range over V 2 O 5 /SiO 2 catalysts prepared by grafting or wet impregnation methods. The acidic features of fresh and used catalysts were studied by ammonia and pyridine adsorption using microcalorimetry and DRIFT spectroscopy. V 2 O 5 /SiO 2 catalysts can be regarded as medium acidic materials. At the same conversion level of ethane, selectivity to ethene was favored by low V 2 O 5 loadings. Ethane conversion increased with the vanadium content and the ammonia uptake of the V 2 O 5 /SiO 2 catalysts. Some Bronsted-type sites disappeared in the course of the catalytic reaction, leading to minor changes in the acidic character of the catalysts.

Effects of pretreatments on state of gallium and aromatization activity of gallium/ZSM-5 catalysts

Abstract: Additional evidence is presented for the role of gallium reduction in preparing active Ga/ZSM-5 catalysts for alkane aromatization. The gallium in ion-exchanged ZSM-5, both with and without additional Ga 2 O 3 , can be reduced to Ga + , in the former case up to a limit determined by the number of anionic framework sites. An ion-exchanged, reduced catalyst exhibits similar redox behavior, and is as active for propane aromatization, as catalysts prepared from purely physical mixtures of Ga 2 O 3 and H-ZSM-5. Although reduction generates dispersed, active gallium species in zeolite channels, it is shown that for ethane dehydrogenation to aromatics the reduced (with hydrogen) Ga/ZSM-5 catalysts are not as active or selective as are materials which have been substantially reoxidized either by air or by traces of oxygen in inert gases. A combination of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) studies has been used to confirm that rapid reoxidation takes place at 550°C. However, the reoxidation does not regenerate β-Ga 2 O 3 , but rather a higher energy, dispersed Ga 3+ which appears to be optimal for production of aromatics from ethane. This form of Ga 3+ is stable under reaction conditio

Energetics of surface reactions on (100) diamond plane

Abstract: The energetics of several possible diamond (100) surface growth reactions was investigated by a semiempirical quantum chemical method. These reactions included thermal and radical-driven dehydrogenation, hydrogen atom migration, and diamond growth by the addition of methyl, acetylene, carbon monoxide, and carbon atoms

Oxidative Dehydrogenation Of Propane Over Supported-Vanadium Oxide Catalysts

Abstract: Vanadium supported on SiO2, AI2O3, TiO2, MgO, La2O3, Sm2O3 and Bi2O3 as well as La, Sm and Bi orthovanadates have been studied for the oxidative dehydrogenation of propane. It has been found that V-O species in which the V is in octahedral and tetrahedral positions are active, the tetrahedral being less active and more selective. The higher the concentration of species VO2+, and species V2O5 the lower is the selectivity to propene. The acid-base/redox properties of the support and the metal, in the vanadates, controls the proportion of the different vanadium species formed.

Efficient Transfer-Dehydrogenation of Alkanes Catalyzed by Rhodium Trimethylphosphine Complexes under Dihydrogen Atmosphere

Abstract: RhL 2 Cl(CO) (1; L=PMe 3 ), a known catalyst for the photodehydrogenation of alkanes, is found to catalyze thehighly efficient thermal (nonphotochemical) transfer-dehydrogenation of alkanes under high-pressure hydrogen atmosphere. The proposed mechanism involves addition of H 2 , loss of CO, and transfer of H 1 to a sacrificial acccptor, thereby generating RhL 2 Cl, the same catalytically active fragment formed by photolysis of 1

Isobutane dehydrogenation on chromia/zirconia catalysts

Abstract: Isobutane dehydrogenation was studied on CrOxZrO2 (ZC) catalysts (0.04 to 1.9% chromium by weight) in a flow apparatus at 673–873 K. Up to 823 K, molar selectivity to isobutene was ⩾ 95%, while the only gaseous by-products were propene, methane and traces of propane. No skeletal isomerization occurred up to 873 K. A deactivation was observed due to the formation of coke. The activity decay was considerably delayed by adding a few percent hydrogen to the reactant mixture. In all cases, catalytic activity was completely recovered by treating the catalyst with flowing oxygen at 773 K over 22 reaction regeneration cycles. The catalyst surface was conditioned mainly by the reaction atmosphere, so that sample reduction with carbon monoxide at 623 K before catalysis did not influence the activity above 773 K. The turnover frequency per total surface chromium atom was found to be independent of surface chromium concentration at all given temperatures, which indicated a mononuclear active site. A comparison with chromia/silica and chromia//gg-alumina samples of similar chromium content revealed a higher activity for the zirconia-based catalysts. The overall picture shows close similarities with that of propane dehydrogenation previously studied in our laboratory on the same catalytic system. It is concluded that the site previously proposed for propane dehydrogenation is also responsible for the activity in the dehydrogenation of isobutane; this site involves a mononuclear CrIII species with two coordinative vacancies and, most probably, an adjacent O2− ion of the support.

Conversion of Fischer-Tropsch heavy end products with platinum/boron-zeolite beta catalyst having a low alpha value

Abstract: The use of high silica to alumina ratio large pore zeolites, in combination with a hydrogenation/dehydrogenation component, to convert Fischer-Tropsch heavy end products which may contain paraffins, olefins and oxygenates into low pour point distillates with high cetane number and extra high VI lube is described The lube can be dewaxed by a conventional solvent process, by a catalytic process, or by increasing the severity of the hydroisomerization step

Some fundamental and practical aspects of the ammoxidation of alkylbenzenes

Abstract: Various aspects of the ammoxidation of the alkylbenzenes are discussed, with emphasis being placed on the reaction kinetics and mechanism, dependent on the chemical nature of the catalyst. Over one group of catalysts based on vanadium pentoxide, ammoxidation occurs via the benzoate-like surface intermediate. All of them are active in both ammoxidation and partial oxidation of alkylaromatic hydrocarbons. Over another group of catalysts active in oxidative dehydrogenation as well as in ammoxidation reactions, the latter reaction proceeds via the amine and imine-like intermediates. Irrespective of the reaction mechanism, the rate-limiting step is related to the alkyl C-H bond dissociation, occurring by either the homolytic or the heterolytic process. From this point of view, different possibilities for controlling the ammoxidation process are discussed. The most effective one is a modification of the catalyst with acid-base or redox additives. In conclusion, industrial applications and technologies for these reactions are briefly reviewed.

Catalytic Properties Of Promoted Vanadium Oxide In The Oxidation Of Ethane In Acetic Acid

Abstract: ReO3-like oxides belonging to various systems such as : [V-O], [V-P-O] and [Mo-V-Nb], have been prepared, characterized and studied in the mild oxidation catalysis of ethane, Pure and Pd doped (VO)2P2O7 are found to be very selective catalysts for the direct oxidation of ethane in acetic acid and anhydride as low as 250°C, whereas oxides having the composition [Mo0.73V0.18Nb0.09] catalyze selectively either the oxidative dehydrogenation of ethane in ethylene near 350°C, or its oxidation to acetic acid at lower temperatures, according to the mode of preparation.

Mathematical analysis on catalytic dehydrogenation of ethylbenzene using ceramic membranes

Abstract: A mathematical model is employed to evaluate the performance of a membrane reactor for the catalytic dehydrogenation of ethylbenzene to styrene. The model previously discussed in the literature has been modified to include side reactions for estimating product selectivities. Yields from the side reactions, e.g., benzene and toluene, have been analyzed for a proposed hybrid system, i.e., a fixed bed reactor in conjunction with a membrane reactor

Alkylaromatic process with removal of aromatic by-products

Abstract: An improved process for the production of alkylated aromatic compounds by paraffin dehydrogenation and aromatic alkylation is disclosed. Aromatic by-products normally formed in paraffin dehydrogenation are selectively removed using at least one aromatics removal zone. Removal of these aromatic by-products significantly reduces the deactivation rate of solid alkylation catalysts. The improved process produces a detergent alkylate product that is significantly more linear than that produced by the prior art process.

Calorimetric study of vanadium pentoxide catalysts used in the reaction of ethane oxidative dehydrogenation

Abstract: Vanadium pentoxide catalysts have been studied in the partial oxidation reaction of ethane in the 723-843 K temperature range. The relationship between the acid-base properties and the catalytic behavior was investigated. The number and character of acidic sites of V{sub 2}O{sub 5} catalysts were determined by studying the adsorption of a basic molecule using microcalorimetry. The reducibility level and the evolution of the surface state, as well as the heat evolved, were studied by using a pulse method with pure ethane only. The reaction of ethane oxidative dehydrogenation was studied by a continuous flow method and the activation energies for the formation of C{sub 2}H{sub 4} and CO were calculated. The selectivity of the catalyst was interpreted in connection with the acid-base properties. The strong sites were observed to decrease rapidly with time on stream, although the catalysts were still active. Temperature-programmed reduction of V{sub 2}O{sub 5} using a TG-DSC coupling was also investigated with hydrogen, ethylene, or ethane as reducers. The different heats of reduction are given. It was observed that C{sub 2}H{sub 4} is a much more efficient reducing agent than H{sub 2} and C{sub 2}H{sub 6}. Following each reduction, reoxidation studies by oxygen were performed in the more » same equipment showing clearly different step in the reoxidation process. 20 refs., 8 figs., 1 tab. « less

Kinetic study of the initial stages of dehydrogenation of cyclohexane on the platinum(111) surface

Abstract: The initial dehydrogenation in the conversion of cyclohexane to benzene has been studied by laser-induced thermal desorption (LITD) combined with Fourier transform mass spectrometry (FTMS). Our previous work has shown that the initial dehydrogenation of cyclohexane occurs at ∼180 K while benzene is not formed at a substantial rate until ∼280 K. A stable surface intermediate exists in the temperature range 180-280 K. We have studied the kinetics of the dehydrogenation of cyclohexane to form the intermediate in the temperature range 175-215 K

Recently published work on EUROPT-1, a 6% Pt/SiO2 reference catalyst

Abstract: EUROPT-1 is a 6% Pt/SiO 2 catalyst which has been thoroughly characterised with respect to its composition, structure, chemisorption behaviour and catalytic properties Extensive further work on this catalyst has been published and this is now reviewed Debye function analysis of X-ray diffraction spectra suggest platinum particles to be cubooctahedral, although preliminary extended X-ray absorption fine structure results do not accord with this model Further isotherms for carbon monoxide, hydrogen and oxygen chemisorption have been reported and enthalpies of adsorption for the last two gases measured Limited results on structure-insensitive reactions such as exchange of methane with deuterium and alkene hydrogenation are available Methane homologation as well as skeletal reactions of alkanes in the presence of hydrogen represent structure-sensitive reactions Some further work on ethane, propane and n-butane is reported; detailed results on n-hexane and methylcyclopentane include changes in rates and selectivities as a function of hydrogen and hydrocarbon pressure They are interpreted in terms of availability of surface hydrogen and hydrogen effects on the formation of carbonaceous deposits of various degree of dehydrogenation Effects of sintering have also been studied, and active centres for various reactions have been tentatively suggested On the whole, EUROPT-1 shows behaviour which is characteristic of platinum catalysts containing small platinum particles but in some limited respects it shows unusual and unexpected properties Reactions of some oxygenated compounds are described briefly Catalytic properties of EUROPT-1 can be modified by metals, oxides, sulphur and nitrogen; treatment with cinchonidine converts it into an enantioselective catalyst for the hydrogenation of methyl pyruvate Finally the importance of selecting appropriate reference supported metal catalyst (s) is stressed and the future role for EUROPT-1 as a possible candidate is pointed out; results must however be compared under carefully defined conditions if they are to have real value

Membrane catalytic reactor comprising heteropolyacid catalyst and polysulfone membrane

Abstract: A membrane catalytic reactor which comprises a heteropolyacid selected from the group consisting of 12-tungstophosphoric acid, 12-molybdophosphoric acid, 12-molybdotungstophosphoric acid, and 12-tungstosilicic acid, and polysulfone membrane is provided. This membrane catalytic reactor is applicable to vapor-phase dehydration, dehydrogenation, oxidation, and simultaneous separation of organic or inorganic materials, particularly vapor-phase dehydration of ethanol.

Propane dehydrogenation and coke formation on chromia-alumina catalysts: effect of reductive pretreatments

Abstract: Propane dehydrogenation and coke formation over a series of alumina-supported chromia catalysts with different chromia contents were examined. X-ray photoelectron spectroscopy (XPS) showed that the calcined catalysts displayed Cr(VI) and Cr(III) in proportions which depended on the overall chromia content. Cr(VI) appeared to be reduced in the initial stages of the dehydrogenation reaction, for which important changes in the selectivity to propene and in the coking rate were noted. These changes were mainly related to the hydrocarbon combustion by the oxygen released from Cr 2 O 3 and to the suppression of the strong acid sites by coking. Coke formation on CO-prereduced catalysts was fast and could be related to the level and stability of the conversion to propene. Both activity and selectivity of the catalysts were determined by the dispersion of Cr and by the effect of Cr content on the porous structure of the catalysts

Catalytic properties of bismuth vanadates based catalysts in oxidative coupling of methane and oxidative dehydrogenation of propane

Abstract: The oxidative coupling of methane (OCM) and the oxidative dehydrogenation of propane (ODHP) have been performed on new mixed Bi-V oxides having a -γ-Bi2MoO6-like structure. The results yield a correlation between the methane conversion and the oxygen diffusivity. The introduction of metals (Fe, Cu, Sr with 10 mol%) improves the C2 selectivity. The use of an oxide with a scheelite structure (BiVO4) shows that, under catalytic conditions, this oxide has the same behaviour as the above solids.

The effect of oxide structure and cation reduction potential of vanadates on the selective oxidative dehydrogenation of butane and propane

Abstract: The oxidative dehydrogenation of butane over a series of orthovanadates of cations of different reduction potentials showed a correlation that the selectivity for dehydrogenation decreased with increasing ease of reduction of the cation. The highest selectivity was observed on Mg orthovanadate. Mg pyrovanadate was nonselective for butane oxidation, but was as selective as Mg orthovanadate in propane oxidation. These results were interpreted by the different structures of the two Mg vanadates.