Showing papers on "Dehydrogenation published in 1998"

New catalytic functions of Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys in the conversions of methanol

Abstract: Pd and Pt supported on ZnO, Ga2O3 and In2O3 exhibit high catalytic performance for the steam reforming of methanol, CH3OH+H2O→CO2+3HH2, and the dehydrogenation of methanol to HCOOCH3, 2CH3OH→HCOOCH3+2HH2. Combined results with temperature-programmed reduction (TPR) and XRD method revealed that Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys were produced upon reduction. Over the catalysts having the alloy phase, the reactions proceeded selectively, whereas the catalysts having metallic phase exhibited poor selectivities.

Bond-selective dissociation of alkanethiol based self-assembled monolayers adsorbed on gold substrates, using low-energy electron beams

Abstract: We have conducted a study of electron-stimulated reactions in butanethiol, octanethiol, dodecanethiol, and hexadecanethiol monolayers adsorbed onto Au/mica substrates, using in situ infrared spectroscopy to quantify the processes; the electron dose dependence of the depletion of various C–H stretching modes has permitted the determination of the first dissociation cross sections for electron stimulated reactions in self-assembled organic monolayers. Electron-induced dehydrogenation of alkanethiol/Au/mica films in the 0–15 eV regime is shown to proceed principally via dissociative electron attachment, thus confirming previous work that directly measured H2 desorption yields during irradiation. The dissociation probabilities exhibit a well-resolved maximum at 10 eV, with a full-width at half-maximum of ∼4 eV. Unlike previous studies, our spectroscopic investigation shows that the dehydrogenation is not uniformly distributed throughout the organic film, but is strongly localized near the methyl terminations ...

Reactivities and Collision-Induced Dissociation of Vanadium Oxide Cluster Cations

Abstract: Reactivities and collision-induced dissociation of vanadium oxide cluster cations are investigated using a triple quadrupole mass spectrometer coupled with a laser vaporization source. The dominant peaks in the mass distribution correspond to cluster ions with stoichiometries of (VO2)n(V2O5)m(O2)q+. Collision-induced dissociation studies of the vanadium oxide species V2O4-6+, V3O6-9+, V4O8-10+, V5O11-13+, V6O13-15+, and V7O16-18+ show that VO2, VO3, and V2O5 units are the main building blocks for most of these clusters. The reaction pathways observed for these vanadium oxide clusters include molecular association, cracking, dehydration, and oxygenation of the neutral hydrocarbons with the reactivities of specific clusters differing from species to species. For example, V3O7+ is very efficient in the dehydrogenation of 1,3-butadiene and in the cracking of 1-butene. On the other hand, V3O6+ produces only molecular association products with these same reactants. To help explain these differences in reactivit...

Copper-catalyzed homolytic and heterolytic benzylic and allylic oxidation using tert-butyl hydroperoxide

Abstract: Allylic and benzylic alcohols were oxidized in good yields to the respective ketones by tert-butyl hydroperoxide (TBHP) in the presence of copper salts under phase-transfer catalysis conditions. This dehydrogenation was found to proceed via a heterolytic mechanism. CuCl2, CuCl, and even copper powder were equally facile as catalysts, as they were all transformed in situ to Cu(OH)Cl which was extracted into the organic phase by the phase-transfer catalyst (PTC). Deuterium labeling experiments evidenced the scission of the benzylic C–H bond in the rate-determining step. Nonproductive TBHP decomposition was not observed in the presence of the alcohol substrates. Conversely, the oxygenation of π-activated methylene groups in the same medium was found to be a free radical process, and the major products were the appropriate tert-butyl peroxides. Catalyst deactivation, solvent effects, and extraction effects are discussed. By applying Minisci’s postulations concerning the relative reactivity of TBHP molecules towards tert-butoxyl radicals in protic and nonprotic environments, the coexistence of the homolytic and the heterolytic pathways can be explained. A complete reaction mechanism is proposed, wherein the free-radical oxidation obeys Kochi’s mechanism, and the heterolytic dehydrogenation is based on either a high-valent CuIVO species or a [Cu(OH)Cl]2 species.

The chemistry and physics of coking

Abstract: The causes of coke formation during petroleum refining are only now beginning to be understood. They are closely related to the mechanism of the thermal decomposition of the petroleum Constituents and to changes in the character of the liquid medium. It was formerly believed that coke formation was, a polymerization reaction whereupon the chemical precursors to coke immediately formed macromolecules when subject to the processing temperatures. This is not so. And it is the initial stages of the thermal decomposition which determine the ultimate path of the reaction. Coke formation is a complex process involving both chemical reactions and thermodynamic behavior. Reactions that contribute to this process are cracking of side chains from aromatic groups, dehydrogenation of naphthenes to form aromatics, condensation of aliphatic structures to form aromatics, condensation of aromatics to form higher fused-ring aromatics, and dimerization or oligomerization reactions. Loss of side chains always accompanies thermal cracking, and dehydrogenation and condensation reactions are favored by hydrogen deficient conditions.

On the nature of the active site for the ethylbenzene dehydrogenation over iron oxide catalysts

Abstract: The dehydrogenation of ethylbenzene to styrene was studied over single-crystalline iron oxide model catalyst films grown epitaxially onto Pt(111) substrates. The role of the iron oxide stoichiometry and of atomic surface defects for the catalytic activity was investigated by preparing single-phased Fe3O4(111) and α-Fe2O3(0001) films with defined surface structures and varying concentrations of atomic surface defects. The structure and composition of the iron oxide films were controlled by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), the surface defect concentrations were determined from the diffuse background intensities in the LEED patterns. These ultrahigh vacuum experiments were combined with batch reactor experiments performed in water–ethylbenzene mixtures with a total gas pressure of 0.6 mbar. No styrene formation is observed on the Fe3O4 films. The α-Fe2O3 films are catalytically active, and the styrene formation rate increases with increasing surface defect concentration on these films. This reveals atomic surface defects as active sites for the ethylbenzene dehydrogenation over unpromoted α-Fe2O3. After 30 min reaction time, the films were deactivated by hydrocarbon surface deposits. The deactivation process was monitored by imaging the surface deposits with a photoelectron emission microscope (PEEM). It starts at extended defects and exhibits a pattern formation after further growth. This indicates that the deactivation is a site-selective process. Post-reaction LEED and AES analysis reveals partly reduced Fe2O3 films, which shows that a reduction process takes place during the reaction which also deactivates the Fe2O3 films.

States of Pt in Pt/C catalyst precursors after impregnation, drying and reduction steps

Abstract: The characterization of Pt/C catalysts after impregnation and activation steps has been carried out by using different techniques: TPD; TPR; XPS; EXAFS and H 2 chemisorption. Furthermore, the catalytic activity of the samples has been tested in the cyclohexane dehydrogenation. The catalysts were prepared from a purified peach pit derived carbon and two H 2 O 2 -functionalized supports obtained from it. Results showed that the interaction of H 2 PtCl 6 with the carbon implies a redox process in which after impregnation and drying steps, the metal complex is stabilized as Pt 2+ on the carbon surface. The functionalization treatment seems to have no effect on the resulting state of platinum. During the activation step (thermal treatment in H 2 ), apart from the reduction of Pt species to Pt 0 , catalyzed and non-catalyzed reactions involving CO and CO 2 desorbed from the carbon surface, take place. It was also found that the Pt 2+ species are reduced to zerovalent Pt even by thermal treatment of the dried samples with He. The electronic state of reduced platinum is not modified by the differences in the support surface chemistry.

Conversion of ethane into benzene on Mo2C/ZSM-5 catalyst

Abstract: The reaction of ethane on unsupported and supported molybdenum compounds has been investigated at 773–973 K. ZSM-5 was used as support. Reaction products were analyzed using gas chromatography. Changes in the composition of catalyst samples were followed by X-ray photoelectron spectroscopy. Unsupported MoO3 is partially reduced with ethane above 800 K to give H2O and CO2. The formation of H2, C2H4 and CH4 was also observed. Mo-free ZSM-5 exhibited relatively high activity towards dehydrogenation, hydrogenolysis and aromatization of ethane above 773 K. Deposition of MoO3 on ZSM-5 significantly enhanced the conversion of ethane and also the selectivity of benzene production. Alteration of the catalytic behaviour of MoO3/ZSM-5 in time on-stream at 773–973 K was attributed to the reaction of MoO3, to carbon deposition and to the formation of Mo2C. Unsupported Mo2C catalyzed the dehydrogenation of ethane without producing benzene. In contrast, Mo2C/ZSM-5 was found to be an effective catalyst in the aromatization of ethane. At 973 K the conversion of ethane was ∼67% and the selectivity to benzene formation 31%.

Dissociation of methane on different transition metals

Abstract: Dissociation of methane on different transition metals M (M=Ru, Ir, Rh, Ni, Pd, Pt, Cu, Ag, Au) has been investigated using a quasi-relativistic density-functional method. Reaction enthalpies for the steps involved are determined. The activation energies have been estimated using the analytic BOC-MP formula. The transition metals, Ru, Rh, …, Pt are shown to exhibit high activity in the dissociation of methane, whereas the coinage metals (Cu, Ag, Au) are very inactive. The conclusion is in agreement with experimental observations. The total dissociation enthalpy ΔH for the complete dissociation of CH 4 to give surface C and H (CH 4,s →C s +4H s ) can be regarded as a measure for the activity of the metal in methane dissociation. The order of the calculated ΔH's is consistent with the order of methane conversions over the metals. The dissociation of methane is also examined in the presence of adsorbed oxygen. Oxygen at on-top site promotes methane dehydrogenation. Oxygen at hollow site promotes methane dehydrogenation on Pt and the coinage metals, but is not beneficial to that on the other transition metals.

Carbon formation on nickel and nickel-copper alloy catalysts

Abstract: Equilibrium, kinetic and morphological studies of carbon formation in CH 4 + H 2 , CO, and CO + H 2 gases on silica supported nickel and nickel-copper catalysts are reviewed. The equilibrium deviates in all cases from graphite equilibrium and more so in CO + CO 2 than in CH 4 + H 2 A kinetic model based on information from surface science results with chemisorption of CH 4 and possibly also the first dehydrogenation step as rate controlling describes carbon formation on nickel catalyst in CH 4 + H 2 well. The kinetics of carbon formation in CO and CO + H 2 gases are in agreement with CO disproportionation as rate determining step. The presence of hydrogen influences strongly the chemisorption of CO. Carbon filaments are formed when hydrogen is present in the gas while encapsulating carbon dominates in pure CO. Small amounts of Cu alloying promotes while larger amounts (Cu : Ni > 0.1) inhibits carbon formation and changes the morphology of the filaments (octopus carbon formation). Adsorption induced nickel segregation changes the kinetics of the alloy catalysts at high carbon activities. Modifications suggested in some very recent papers on the basis of new results are also briefly discussed.

Oxidative dehydrogenation of iso-butane to iso-butene. II. Rare earth phosphate catalysts

Abstract: Among various rare earth phosphates, CePO4 and LaPO4 were found to catalyze the oxidative dehydrogenation of iso-butane to iso-butene at 450–550°C using a feed gas of 75 mol% iso-butane and 5 mol% O2. As for the phosphates of Y, Pr, Nd, Gd, Er, and Yb, iso-butene was formed with a limited selectivity of 20–40% selectivity, and C3H6 formation and deep oxidation into CO and CO2 were promoted over the same temperature range. Temperature-programmed-desorption spectra of NH3 for CePO4 and LaPO4 consisted of two large desorption peaks with maxima at 100°C and 270°C suggesting that these phosphates have strong acid sites. But the rest of the phosphates have only weak acid sites. Rare earth phosphates have no adsorbed oxygen which can be desorbed up to 550°C. The lattice oxygen of CePO4 and PrPO4 can be reduced at 200°C and that of the rest of the phosphates at about 400°C. The amount of oxygen removed up to 550°C in TPR was smaller than the amount corresponding to a monolayer for all phosphates. Oxygen isotope equilibration experiments over CePO4 and LaPO4 revealed that only surface lattice oxygen can participate in the equilibration at 500°C and 550°C. It can be concluded that acidic character is a key factor in the selective oxidative dehydrogenation of iso-butane over the rare earth phosphate catalysts.

Silica- and L-zeolite-supported Pt, Pt/Sn and Pt/Sn/K catalysts for isobutane dehydrogenation

Abstract: Reaction kinetics measurements, temperature programmed oxidation (TPO), transmission electron microscopy, and chemisorption measurements were used to study silica- and L-zeolite-supported Pt catalysts for isobutane dehydrogenation at 798 K in pure isobutane and at 873 K with a 2 : 1 hydrogen : isobutane feed. Highly selective dehydrogenation catalysts were prepared by depositing Pt and Sn on K-L-zeolite, as well as by depositing Pt, Sn, and K on silica. At 873 K and in the presence of hydrogen, the Pt/Sn/K-L catalysts exhibited significantly higher rates of isobutene production compared to the Pt/Sn/K/silica catalyst. Addition of Sn and K to Pt decreased the extent of carbon deposition on the catalysts by reducing the size of the Pt surface ensembles, thereby inhibiting the formation of highly dehydrogenated surface species that lead to coke and other undesirable products (e.g. hydrogenolysis). The presence of Sn may also facilitate the transport of the carbon deposits from the active sites to the support. In the absence of hydrogen, the pores of Pt/K-L-zeolite become blocked by carbonaceous deposits during isobutane dehydrogenation at 798 K. At 873 K and in the presence of hydrogen, the Pt/Sn/K-L catalysts exhibit deactivation due to coking, enrichment of the surface with tin, and/or sintering of the metal particles.

Oxidative dehydrogenation of ethylbenzene with carbon dioxide on alkali‐promoted Fe/active carbon catalysts

Abstract: The catalytic behavior of iron supported on activated carbon was investigated in the oxidative dehydrogenation of ethylbenzene. CO2 was applied as oxidant. High ethylbenzene conversion (>70%) and selectivity towards styrene (>90%) were observed at 550 °C. Apart from styrene, benzene, toluene, carbon monoxide and water were formed as products.

Photocatalytic Dehydrogenation of 2-Propanol on TiO2(110)

Abstract: The photocatalytic production of acetone from 2-propanol and O2 has been investigated on the TiO2(110) surface using molecular beam techniques. We have also studied the thermal reaction of 2-propan...

Dehydrogenation of isobutane to isobutene with iron-loaded activated carbon catalyst

Abstract: Dehydrogenation of isobutane to isobutene was successufully carried out on iron loaded activated carbon catalyst at 873 K. Isobutane conversion of 48%, the yield of 40% and selctivity 80% were obtained with 0.3 mmol iron loaded on 1 g of activated carbon, at 873 K with W/F of 25 g-cat-h/mol of isobutane. Cofeeding of carbon dioxide promoted dehydrogenation as compared to feeding bulk isobutane or isobutane and argon mixture. Hydrogen produced by dehydrogenation was transformed into water in the presence of carbon dioxide, and carbon dioxide was reduced to carbon monoxide. A redox cycle of Fe 3 O 4 and metallic iron is suggested for the catalytic cycle in carbon dioxide by analyzing XRD and XPS. Carbon deposition was reduced under a carbon dioxide stream as compared with the reaction under argon atmosphere.

Surface-Treated Activated Carbons as Catalysts for the Dehydration and Dehydrogenation Reactions of Ethanol

Abstract: Two activated carbons were prepared from olive stones with different degrees of activation and oxidized with a (NH4)2S2O8 solution for variable periods of time to introduce different amounts of oxygen surface complexes. Samples so prepared were characterized to know their surface area, porosity, and surface chemistry and then were used as catalysts in the conversion reactions of ethanol. The dehydration reactions to obtain ethene and ether only occurred on the oxidized samples, where carboxyl groups placed on the external surface of the particles were responsible for these reactions. The dehydrogenation reaction to yield acetaldehyde took place on either acid or basic surface sites placed not only on the external surface, but also on some part of the internal surface. During the reaction some surface active sites for dehydrogenation were lost because some hydrogen remained bound to them. The presence of air in the reactant mixture cleaned some of these sites, increasing the dehydrogenation activity and de...

Rare earth oxide sensors for ethanol analysis

Abstract: Rare earth oxides and tin oxide for ethanol analysis in air are investigated using AC impedance and DC resistance methods. It is found that the unpromoted rare earth oxide sensors give a significantly better sensitivity to ethanol than Pd/SnO 2 sensors prepared by an incipient wetness method. This is attributed to their adsorbed oxygen species of strong basicity whereupon oxidative dehydrogenation of ethanol is effectively carried out. It is interesting to note that the praseodymium oxide shows an entirely different sensing mechanism amongst all of the rare earth oxides tested. Lattice oxygen of the non-stoichiometric PrOx is involved in oxidising ethanol in air at 150–400°C. This results in the destruction of their electron hopping sites in the mixed Pr 4+ and Pr 3+ lattice leading to an increase in the electrical resistance of the material. The resistivity of this new type of oxide sensor increases in proportion to the ethanol concentration in air and recovers to the initial value upon removal of ethanol.