Showing papers on "Dehydrogenation published in 2000"

Kinetics and Mechanism of Oxidative Dehydrogenation of Propane on Vanadium, Molybdenum, and Tungsten Oxides

Abstract: The effect of cation identity on oxidative dehydrogenation (ODH) pathways was examined using two-dimensional VOx, MoOx, and WOx structures supported on ZrO2. The similar kinetic rate expressions obtained on MoOx and VOx catalysts confirmed that oxidative dehydrogenation of propane occurs via similar pathways, which involve rate-determining C−H bond activation steps using lattice oxygen atoms. The activation energies for propane dehydrogenation and for propene combustion increase in the sequence VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2; the corresponding reaction rates decrease in this sequence, suggesting that turnover rates reflect C−H bond cleavage activation energies, which are in turn influenced by the reducibility of these metal oxides. Propane ODH activation energies are higher than for propene combustion. This leads to an increase in maximum alkene yields and in the ratio of rate constants for propane ODH and propene combustion as temperature increases. This difference in activation energy (48−61 kJ/mol) be...

Handbook of Commercial Catalysts: Heterogeneous Catalysts

Abstract: ACETOXYLATION Ethylene + Acetic Acid a Vinyl Acetate Butadiene + Acetic Acid a 1,4 Diacetoxy-2-Butene a 1,4 Butanediol a Tetrahydrofuran Propylene + Acetic Acid a Allyl Acetate a Allyl Alcohol ALKYLATION Introduction Alkylation of Benzene and Toluene Alkylation of Phenols Higher Alkylphenols Alkylation of Polynuclear Aromatics Alkylation of Aromatic Amines Transalkylation and Disproportionation Lower Aliphatic Amines by Alkylation of Ammonia (Amination) AMMONOLYSIS Phenol + Ammonia a Aniline Meta-Cresol + Ammonia a Meta-Toludine Diethylene Glycol + Ammonia a Morpholine AMMOXIDATION Propylene a Acrylonitrile Methane a HCN Isobutylene a Methacrylonitrile Aromatic Methyl Compounds a Nitriles CARBONYLATION Carboxylation of Olefins to Carboxylic Acids Carbon Monoxide + Chlorine a Phosgene DEHYDRATION OF ALCOHOLS 1-Phenylethanol a Styrene DEHYDROCHLORINATION C10 - C13 Chloridea Linear Olefins DEHYDROGENATION Dehydrogenation of Ethylbenzene Styrene Derivatives from Other Alkyl Aromatics Dehydrogenation of Lower Alkanes Alcohols to Aldehydes or Ketones Three Other Dehydrogenations EXPOXIDATION Ethylbenzene a Propylene Oxide + Styrene HYDRATION Ethylene a Ethanol Propylene a Isopropyl Alcohol N-ButenesaSec-Butanol Acrylonitrile a Alkylamide HYDROCHLORINATION Alkenes Alcohols HYDROGENATION General Background Hydrogenation of Aromatic Rings Hydrogenation of Heterocyclic Compounds Hydrogenation of Aliphatic Unsaturates Hydrogenation of Nitriles to Amines Hydrogenation of Nitroaromatics Hydrogenation of Haloaromatics Hydrogenation of Carbonyl Compounds Hydrogenation of Resins, Rosins, and Waxes Selective Hydrogenation of Fats and Oils Miscellaneous Hydrogenations HYDROGENOLYSIS Natural Fatty Acids and Fatty Esters a Fatty Alcohols Dimethylterephthalate a 1,4 Dimethylolcyclohexane Toluene a Benzene (Hydroalkylation) Methyl and Dimethyl Napthalene ISOMERIZATION Meta-Xylene a Para and Ortho-Xylene OXIDATION (INORGANIC) Sulfur Dioxide a Sulfur Trioxide a Sulfuric Acid Ammonia a Nitric Oxide a Nitrogen Dioxide a Nitric Acid Hydfrogen Sulfide a Sulfur OXIDATION (ORGANICS) General Ethylene a Ethylene Oxide Propene a Acrolein Propene a Acrolein a Acrylic Acid Butane or Benzene a Maleic Anhydride Ortho-Xylene or Naphthalene a Phthalic Anhydride Anthracene a Anthraquinone Methanol a Formaldehyde Isobutylene or Tert-Butyl Alcohol a Methacrolein a Methacrylic Acid OXYCHLORINATION Ethylene a 1,2 Dichloroethane a Vinyl Chloride PETROLEUM REFINING Catalytic Reforming Hydroprocessing (General) Hydrotreating Hydrocracking Isomerization Oligomerization (Polymer Gasoline Production) Fluid Catalytic Cracking Oxygenates SYNTHESIS GAS AND ITS PRODUCTS Historical Background Modern Synthesis Gas Production High and Low Temperature Shift Conversion (CO + H2O a H2 + CO2) Naphtha Steam Reforming Methanol Synthesis (Carbon Monoxide + Hydrogen a Methanol) Pure Carbon Monoxide from Synthesis Gas and Its Uses Pure Hydrogen from Synthesis Gases and Its Uses Ammonia Synthesis (Nitrogen + Hydrogen a Ammonia) Appendix

Relationship between ethanol gas sensitivity and surface catalytic property of tin oxide sensors modified with acidic or basic oxides

Abstract: A semiconductor gas sensor using SnO 2 was loaded with acidic or basic oxides (5 wt.%) to investigate ethanol-gas sensing properties and related catalytic properties. The sensitivity to ethanol gas at 300°C increased tremendously with an addition of a basic oxide (e.g., La 2 O 3 ), while it hardly changed with that of an acidic oxide (WO 3 ). It turned out that the addition of the basic metal oxide to SnO 2 brought about enhancement of catalytic activity not only for the dehydrogenation of ethanol gas to CH 3 CHO but also for the consecutive oxidation of CH 3 CHO to CO 2 . On the other hand, the acidic metal oxide enhanced only the dehydration reaction, showing even an adverse effect on the consecutive oxidation. Based on these results, it was concluded that the enhancement of the catalytic oxidation activity to an appropriate level could be a reason for the high sensitivity to ethanol gas for the sensors loaded with basic oxides, particularly one loaded with La 2 O 3 .

Ethanol steam reforming on Rh/Al2O3 catalysts

Abstract: Ethanol steam reforming on Rh/Al2O3 catalysts has been the object of our research project. The mixture used for the research testing was prepared with a high water content (H2O/C2H5OH = 8.4 mol/mol) in order to simulate the composition of the ecological fuel product from vegetable biomass fermentation. The experimental tests were carried out in a fixed bed reactor at a programmed temperature between 323 and 923 K. The maximum temperature (T = 923 K) is the standard working temperature of a molten carbonate fuel cell able to make direct use of the hydrogen produced for ethanol steam reforming. The reaction mechanism starts with the initial dehydrogenation and/or dehydration of the ethanol, followed by rapid conversion of the products into methane, carbon monoxide and carbon dioxide. The acid support (Al2O3) assists the dehydration of the alcohol, while all the other reactions are catalyzed by the Rh, although in different measures. For this reason, with increase in Rh content, there is also a progressive i...

The x-ray structure of d-amino acid oxidase at very high resolution identifies the chemical mechanism of flavin-dependent substrate dehydrogenation

Abstract: Flavin is one of the most versatile redox cofactors in nature and is used by many enzymes to perform a multitude of chemical reactions. d-Amino acid oxidase (DAAO), a member of the flavoprotein oxidase family, is regarded as a key enzyme for the understanding of the mechanism underlying flavin catalysis. The very high-resolution structures of yeast DAAO complexed with d-alanine, d-trifluoroalanine, and l-lactate (1.20, 1.47, and 1.72 A) provide strong evidence for hydride transfer as the mechanism of dehydrogenation. This is inconsistent with the alternative carbanion mechanism originally favored for this type of enzymatic reaction. The step of hydride transfer can proceed without involvement of amino acid functional groups. These structures, together with results from site-directed mutagenesis, point to orbital orientation/steering as the major factor in catalysis. A diatomic species, proposed to be a peroxide, is found at the active center and on the Re-side of the flavin. These results are of general relevance for the mechanisms of flavoproteins and lead to the proposal of a common dehydrogenation mechanism for oxidases and dehydrogenases.

Gas-phase hydrogenation/hydrogenolysis of phenol over supported nickel catalysts

Abstract: The gas-phase hydrogenation/hydrogenolysis of alcoholic solutions of phenol between 423 and 573 K has been studied using a Y zeolite-supported nickel catalyst (2.2% w/w Ni) and Ni/SiO2 catalysts (1.5−20.3% w/w Ni). This is a viable means of treating concentrated phenol streams to generate recyclable raw material. Phenol hydrogenation proceeded in a stepwise fashion with cyclohexanone as a reactive intermediate while a combination of hydrogenolysis and hydrogenation yielded cyclohexane. Hydrogenolysis to benzene is favored by high nickel loadings and elevated temperatures. A catalytic hydrogen treatment of cyclohexanone and cyclohexanol helped to establish the overall reaction network/mechanism. The possible role of thermodynamic limitations is considered and structure sensitivity is addressed; reaction data are subjected to a pseudo-first-order kinetic treatment. Hydrogen temperature-programmed desorption (H2-TPD) has revealed the existence of different forms of surface hydrogen. Selectivity is interprete...

Dehydrogenation of ethane with carbon dioxide over supported chromium oxide catalysts

Abstract: The oxidative dehydrogenation of ethane into ethylene by carbon dioxide over an unsupported Cr2O3 and several supported Cr2O3 catalysts on metal oxides such as Al2O3, SiO2, TiO2, and ZrO2 was investigated and the effect of support on the catalytic activity was studied. The unsupported Cr2O3 shows medium catalytic activity in this reaction; the support will exert a quite different effect on catalytic behavior. The catalytic activity varies with the nature of supports. Cr2O3/SiO2 catalysts exhibit an excellent performance in this reaction. Cr2O3 loading also affects the catalytic activity; 8 wt.% Cr2O3/SiO2 catalysts can produce 55.5% ethylene yield at 61% ethane conversion at 650°C. Characterization indicates that the distribution of chromium oxide on supports and surface chromium species structure are influenced by the nature of supports. The acidity/basicity and redox property of catalysts determines the catalytic activity in the dehydrogenation of ethane by carbon dioxide.

Selective oxidation of light alkanes over hydrothermally synthesized Mo-V-M-O (M=Al, Ga, Bi, Sb, and Te) oxide catalysts

Abstract: Selective oxidations of ethane to ethene and acetic acid and of propane to acrylic acid were carried out over hydrothermally synthesized Mo-V-M-O (M=Al, Ga, Bi, Sb, and Te) complex metal oxide catalysts. All the synthesized solids were rod-shaped crystallites and gave a common XRD peak corresponding to 4.0 A d -spacing. From the different XRD patterns at low angle region below 10° and from the different shape of the cross-section of the rod crystal obtained by SEM, the solids were classified into two groups: Mo-V-M-O (M=Al, possibly Ga and Bi) and Mo-V-M-O (M=Sb, and Te). The former catalyst was moderately active for the ethane oxidation to ethene and to acetic acid. On the other hand the latter was found to be extremely active for the oxidative dehydrogenation. The Mo-V-M-O (M=Sb, and Te) catalysts were also active for the propane oxidation to acrylic acid. It was found that the grinding of the catalysts after heat-treatment at 600°C in N 2 increased the conversions of propane and enhanced the selectivity to acrylic acid. Structural arrangement of the catalytic functional components on the surface of the cross-section of the rod-shaped catalysts seems to be important for the oxidation activity and selectivity.

Dehydrogenation of ethylbenzene with an activated carbon-supported vanadium catalyst

Abstract: Dehydrogenation of ethylbenzene over supported vanadium catalysts was carried out at 723–923 K, W / F of 35–105 g-cat h/mol in a carbon dioxide atmosphere (carbon dioxide/ethylbenzene of 50–70 mmol/mmol) and in an argon atmosphere. The catalyst, 1.0 mmol of vanadium loaded on 1 g of activated carbon, afforded the highest ethylbenzene conversion of 67.1%, a styrene yield of 54.2% and a styrene selectivity of 80.8% at 550 K with a W / F of 70 g-cat h/mol of ethylbenzene in carbon dioxide. Ethylbenzene conversion and styrene yield in the presence of carbon dioxide were 14.0% higher than those in argon. During the course of the reaction, carbon dioxide was reduced to the corresponding amount of carbon monoxide against the styrene yield. As a result, the same amount of water was produced.

Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts: 3. Catalyst deactivation

Abstract: Extended catalytic tests show that coke deposition is responsible for the deactivation of activated carbon catalysts in the oxidative dehydrogenation of ethylbenzene (ODE). Temperature-programmed desorption (TPD), DRIFTS, textural and elemental analyses of used catalysts have shown that not only the great majority of the micropores become blocked, but also that the amounts of oxygen and hydrogen in the catalyst composition increase with time on stream, leading to a material increasingly more reactive towards oxidation. It was observed that after a few days on stream, the rate of carbon gasification became larger than the rate of coke deposition, leading to a decrease in catalyst weight. Working under milder conditions (lower temperature and oxygen partial pressure) delayed this effect. The increase in the concentration of surface groups with time does not result in a proportional increase in the activity of the catalysts, because the majority of the groups created are not active for the ODE reaction.

SFG-surface vibrational spectroscopy studies of structure sensitivity and insensitivity in catalytic reactions: cyclohexene dehydrogenation and ethylene hydrogenation on Pt (1 1 1) and Pt (1 0 0) crystal surfaces

Abstract: The effect of the surface structure of Pt (1 1 1) and Pt (1 0 0) has been investigated for cyclohexene hydrogenation and dehydrogenation, and ethylene hydrogenation by using sum frequency generation. Cyclohexene dehydrogenation is a structure sensitive reaction, and the rate was found to proceed more rapidly on the Pt (1 0 0) crystal surface than on the Pt (1 1 1) crystal surface. On Pt (1 0 0), the major reaction intermediate during cyclohexene dehydrogenation was 1,3-cyclohexadiene, whereas on Pt (1 1 1), both 1,3- and 1,4-cyclohexadiene were present. Both 1,3- and 1,4-cyclohexadiene can dehydrogenate to form benzene, although the reaction proceeds more rapidly through the 1,3-cyclohexadiene intermediate. Because of this, the structure sensitivity of cyclohexene dehydrogenation is explained by noting that there are both a fast and slow reaction pathway for Pt (1 1 1), whereas there is only a fast reaction pathway on Pt (1 0 0). Ethylene hydrogenation is a structure insensitive reaction. Both ethylidyne and di-σ-bonded ethylene are present in both Pt (1 1 1) and Pt (1 0 0) under reaction conditions, although the ratio of the concentrations of the two species are different. The rate of the reaction was found to be 11±1 and 12±1 molecules per site per second for Pt (1 1 1) and Pt (1 0 0), respectively. Since the reaction rate is essentially the same on the two surfaces, while the concentration of ethylidyne and di-σ-bonded ethylene are different, these species must not be the active species which turnover under catalytic ethylene hydrogenation. The most likely species which turnover are π-bonded ethylene and ethyl, and their concentrations are near the detection limit of SFG.

Cu/SiO2 catalysts for methanol to methyl formate dehydrogenation: A comparative study using different preparation techniques

Abstract: A comparative study of the different preparation conditions required to produce supported copper catalysts by the wet impregnation (WI) and the ion-exchange (IE) methods has been carried out. Two silicas with different textural characteristics, and one form of naturally occurring pumice were used as supports. In order to obtain specific chemical species of copper in solution as well as different support reactivities, three different pH values (1, 4.5 and 11.5) for the impregnating solution were tested. The catalysts were characterized by means of different techniques, such as XFS, BET, TPR, dissociative chemisorption of N 2 O, XRD, TEM, EXAFS and XANES and methanol (MeOH) dehydrogenation. The use of copper solutions with high pH, especially after several hours of contact with the chosen carrier, with high solution volume to pore volume ratios, modifies strongly the support texture decreasing the specific surface areas (SSA) of the final catalyst. Copper particle sizes lower than a critical limit, or that did not fulfill a certain ensemble requirement, were not active for methanol dehydrogenation indicating that below such limit, this might be a structure-sensitive reaction. A catalyst (WI-11A) prepared by a simple incipient wetness impregnation method at an alkaline pH similar to that used for the IE technique, showed high dispersion and it was simply activated under reaction conditions (MeOH/N 2 , 230°C) giving conversions around 50% and selectivities to methyl formate (MF) in the order of 75–80% with no deactivation observed after 3 h of time on stream.

A New and One-Pot Synthesis of α,β-Unsaturated Ketones by Dehydrogenation of Various Ketones with N-tert-Butyl Phenylsulfinimidoyl Chloride

Abstract: α,β-Unsaturated ketones were synthesized by one-pot procedure from various ketones in good to excellent yields on treatment of their lithium enolates with N-tert-butyl phenylsulfinimidoyl chloride (1) under mild conditions.

Dehydrogenation of i-butane on CrOx/Al2O3 catalysts prepared by ALE and impregnation techniques

Abstract: Atomic layer epitaxy (ALE), a technique relying on saturating gas–solid reactions, was applied in the preparation of CrOx/Al2O3 catalysts using Cr(acac)3 vapor and air as source materials for CrOx. Vaporized Cr(acac)3 was reacted with preheated Al2O3, and the surface complex formed was treated with air to remove the ligand residues. The Cr loading increased from 1.3 to 12.5 wt.% as the number of saturating Cr(acac)3 and air reactions was increased from one to 10. CrOx/Al2O3 catalysts were also prepared from solution by incipient wetness impregnation (0.3–21 wt.%). XPS and UV–VIS measurements of the catalysts revealed the presence of both Cr6+ and Cr3+. Although the oxidation state distribution was similar, H2-temperature programmed reduction (TPR) and solubility measurements indicated that Cr6+ surface sites were in stronger interaction with Al2O3 and more uniformly distributed in the catalysts prepared by ALE than by impregnation. On the basis of the activity of the catalysts in the dehydrogenation of i-butane, we propose that the dehydrogenation reaction uses both reduced Cr6+, i.e. redox Cr3+, and exposed non-redox Cr3+ sites. Furthermore, the dehydrogenation reaction must be insensitive to the size of the CrOx ensembles since activities were similar for the catalysts prepared by ALE and impregnation. The decay of the dehydrogenation activity in successive prereduction–reaction–regeneration cycles was attributed to a decrease in the number of redox Cr3+ sites.

Catalytic Methanol Decomposition Pathways on a Platinum Electrode

Abstract: Through a combination of potential steps (chronoamperometry) and fast potential sweeps (cyclic voltammetry), temporal advancements involved in methanol decompositionindicated by the decomposition chargeswere referred to the extent of CO poisoning. In a broad potential range, especially at short times, there is an excess of charge above that needed for simple methanol dehydrogenation to surface CO. This indicates that there exist some efficient methanol decomposition pathways that lead to the formation of products other than CO, namely carbon dioxide, but also formic acid and/or formaldehyde. Because the formation of CO2 was not observed by others below the electrode potential of 0.35 V vs RHE (cf. infrared studies reported in J. Phys. Chem. B 1997, 101, 7542), we believe that, at low potentials, methanol dehydrogenation to formic acid and/or formaldehyde accounts for the excess oxidation charge. The formation of the dissolved products along with surface CO confirms the applicability of a dual-path mechani...