Showing papers on "Dehydrogenation published in 1999"

Methanol–steam reforming on Cu/ZnO/Al2O3 catalysts. Part 2. A comprehensive kinetic model

Abstract: Surface mechanisms for methanol–steam reforming on Cu/ZnO/Al 2 O 3 catalysts are developed which account for all three of the possible overall reactions: methanol and steam reacting directly to form H 2 and CO 2 , methanol decomposition to H 2 and CO and the water-gas shift reaction. The elementary surface reactions used in developing the mechanisms were chosen based on a review of the extensive literature concerning methanol synthesis on Cu/ZnO/Al 2 O 3 catalysts and the more limited literature specifically dealing with methanol–steam reforming. The key features of the mechanism are: (i) that hydrogen adsorption does not compete for the active sites which the oxygen-containing species adsorb on, (ii) there are separate active sites for the decomposition reaction distinct from the active sites for the methanol–steam reaction and the water-gas shift reaction, (iii) the rate-determining step (RDS) for both the methanol–steam reaction and the methanol decomposition reaction is the dehydrogenation of adsorbed methoxy groups and (iv) the RDS for the water-gas shift reaction is the formation of an intermediate formate species. A kinetic model was developed based on an analysis of the surface mechanism. Rate data were collected for a large range of conditions using a fixed-bed differential reactor. Parameter estimates for the kinetic model were obtained using multi-response least squares non-linear regression. The resultant model was able to accurately predict both the rates of production of hydrogen, carbon dioxide and of carbon monoxide for a wide range of operating conditions including pressures as high as 33 bar.

Oxidation of Methanol on 2nd and 3rd Row Group VIII Transition Metals (Pt, Ir, Os, Pd, Rh, and Ru): Application to Direct Methanol Fuel Cells

Abstract: Using first principles quantum mechanics [nonlocal density functional theory (B3LYP)], we calculated the 13 most likely intermediate species for methanol oxidation on clusters of all 2nd and 3rd row Group VIII transition metals for all three likely binding sites (top, bridge, and cap). This comprehensive set of binding energies and structures allows a detailed analysis of possible reaction mechanisms and how they change for different metals. This illustrates the role in which modern quantum chemical methods can be used to provide data for combinatorial strategies for discovering and designing new catalysts. We find that methanol dehydrogenation is most facile on Pt, with the hydrogens preferentially stripped off the carbon end. However, water dehydrogenation is most facile on Ru. These results support the bifunctional mechanism for methanol oxidation on Pt−Ru alloys in direct methanol fuel cells (DMFCs). We find that pure Os is capable of performing both functionalities without cocatalyst. We suggest that...

Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts. I. Influence of surface chemical groups

Abstract: Activated carbons were used as catalysts for the oxidative dehydrogenation of ethylbenzene. The type and amount of functional groups on the surface of the carbon catalysts was modified by oxidative treatments in the gas or in the liquid phase, while heat treatments at different temperatures were used to selectively remove some of the functional groups. The performance of these catalysts was evaluated in terms of conversion, styrene yield and selectivity. The results show that the gas phase treatments lead to improved performance associated with an increase in the amount of carbonyl/quinone groups on the surface, which were identified as the active sites for the reaction. A good correlation between catalytic activity and the concentration of these surface groups was obtained.

Molecular Studies of Catalytic Reactions on Crystal Surfaces at High Pressures and High Temperatures by Infrared−Visible Sum Frequency Generation (SFG) Surface Vibrational Spectroscopy

Abstract: Infrared−visible sum frequency generation (SFG) is a surface-specific vibrational spectroscopy that can operate in a pressure range from ultrahigh vacuum (uhv) to atmospheric pressures. SFG is therefore one of the few surface science techniques that permits atomic scale monitoring of surface species during catalytic reactions at high pressures (around 1 atm) and high temperatures. Using single-crystal surfaces of transition metals, platinum and rhodium, reaction rates can be simultaneously determined by gas chromatography, and correlations between the concentration of adsorbates under reaction conditions and the observed turnover numbers can help to elucidate the reaction mechanism. To bridge the gap to traditional surface science experiments, SFG is also employed under uhv or low pressures. The technique has been successfully applied to the adsorption and oxidation of CO, hydrocarbon conversion such as ethylene hydrogenation and cyclohexene hydrogenation and dehydrogenation on Pt(111). The experiments de...

Catalytic properties of early transition metal nitrides and carbides: n-butane hydrogenolysis, dehydrogenation and isomerization

Abstract: Phase-pure early transition metal nitrides and carbides were prepared via the temperature programmed reaction of metal oxides with NH3 or a CH4/H2 mixture. The nitrides and carbides were mostly mesoporous with surface areas up to 81 m2/g. Their gravimetric butane conversion rates were generally higher than those for a Pt–Sn/Al2O3 catalyst. Activities for the nitrides and carbides ranged from 0.4×1012 to 10×1012 molecules/cm2 s at 723 K and decreased as follows: γ-Mo2N>W2C≈WC>β-W2N≈WC1−x>β-Mo2C>VN≈V8C7≫NbC≈Nb4N3.92. The metal atom type had the most significant effect on the activity and selectivity. The Group VI metal nitrides and carbides were much more active than the Group V metal compounds. In general, the Group VI metal compounds catalyzed butane hydrogenolysis and dehydrogenation with similar selectivities while the vanadium compounds had dehydrogenation selectivities in excess of 98%. The β-W2N catalyst also catalyzed butane isomerization possibly as a consequence of the presence of oxygen on the surface. The effect of lattice structure was significant and obvious for the tungsten carbides where WC (hex) was almost twice as active as WC1−x (fcc) despite having similar C/W ratios. Nitrides and carbides of the same metal and lattice structure had similar activities suggesting that the effect of the non-metal atom type was small. We believe variations in the catalytic properties of the nitrides and carbides were the result of differences between their electronic structures.

Supported Pt–Sn catalysts highly selective for isobutane dehydrogenation: preparation, characterization and catalytic behavior

Abstract: Magnesia-supported platinum–tin catalysts with a Pt content 0.25–3% and atomic ratio Pt/Sn = 1 were prepared from a Pt–Sn bimetallic complex. Characterization of catalysts by XRD, TEM, EDX, XPS and CO singleton frequency indicated the formation of well-dispersed bimetallic particles, the size of which decreased with the metal content. Both dilution and electronic effects on Pt by Sn were evidenced by IR experiments of CO adsorption. The behavior of catalysts in isobutane dehydrogenation at 753 K and atmospheric pressure was related to their structural characteristics and compared to silica- and alumina-supported catalysts prepared by a similar method, and to the respective monometallic catalysts. Magnesia-supported bimetallic catalysts were the most selective to isobutene production. Selectivities higher than 99% were achieved for conversion values of 11%.

HCN Synthesis from Methane and Ammonia: Mechanisms of Pt+-Mediated C−N Coupling

Abstract: The Pt+-mediated coupling of methane and ammonia has been studied both experimentally and computationally. This system serves as a model for the Degussa process for the industrial production of the valuable feedstock hydrogen cyanide. Mass spectrometric studies demonstrate that C−N bond formation is catalyzed efficiently by Pt+. Details of the experimentally observed reaction channels have been explored computationally using the B3LYP hybrid DFT/HF functional. In the first reaction step, Pt+ dehydrogenates CH4 to yield PtCH2+; in contrast, dehydrogenation of ammonia by Pt+ is endothermic and does not occur experimentally. Starting from PtCH2+ and NH3, C−N bond formation, which constitutes the crucial step in making HCN from CH4 and NH3, is achieved via two independent pathways. The major pathway is found to be exothermic by 23 kcal mol-1 and yields neutral PtH and CH2NH2+. The second pathway involves a dehydrogenation to yield the aminocarbene complex PtC(H)NH2+ (ΔrH = −36 kcal mol-1); dehydrogenation of ...

Specific Dehydrogenation of 3-Methylindole and Epoxidation of Naphthalene by Recombinant Human CYP2F1 Expressed in Lymphoblastoid Cells

Abstract: 3-Methylindole (3MI) is a naturally occurring pulmonary toxin that requires metabolic activation. Previous studies have shown that 3MI-induced pneumotoxicity resulted from cytochrome P-450-catalyzed dehydrogenation of 3MI to an electrophilic methylene imine (3-methyleneindolenine), which covalently bound to cellular macromolecules. Multiple cytochrome P-450s are capable of metabolizing 3MI to several different metabolites, including oxygenated products. In the present study, the role of human CYP2F1 in the metabolism of 3MI was examined to determine whether it catalyzes dehydrogenation rather than hydroxylation or ring oxidation. Metabolism was examined using microsomal fractions from human lymphoblastoid cells that expressed the recombinant human CYP2F1 P-450 enzyme. Expression of CYP2F1 in the lymphoblastoid cells proved to be an appropriate expression system for this enzyme. Products were analyzed using HPLC and the mercapturate, 3-[(N-acetylcystein-S-yl)methyl]indole, of the reactive intermediate was identified and quantified. Product analysis showed that human CYP2F1 efficiently catalyzed the dehydrogenation of 3MI to the methylene imine without detectable formation of indole-3-carbinol or 3-methyloxindole. High substrate concentrations of 3MI strongly inhibited production of the dehydrogenated product, a result that may indicate the existence of mechanism-based inhibition of CYP2F1 by 3MI. Recombinant CYP2F1 demonstrated remarkable selectivity for the bioactivation of 3MI to the putative dehydrogenated reactive electrophile. Bioactivation of naphthalene to its pneumotoxic epoxide by CYP2F1 was also demonstrated.

Effect of Ga addition to Pt/Al2O3 on the activity, selectivity and deactivation in the propane dehydrogenation

Abstract: In this paper a study about the effect of the Ga addition to Pt/Al 2 O 3 on the performance in propane dehydrogenation is reported. Mono and bimetallic catalysts were characterized by tests reactions, TPR, H 2 chemisorption, FT-IR of preadsorbed CO, and tested in propane dehydrogenation by using a pulse technique. Results showed that the selectivity to propylene is enhanced by the Ga addition, while the catalyst deactivation and the carbon formation are decreased when the Ga content increased. Ga appears to have a very low effect on the acidity function, but it modifies the structure of the metallic phase in a different way according to the previous reduction temperature. Thus, at low reduction temperature (573 K) the action of Ga can be related only to geometric effects, while after reducing at high temperature (773 K), additional effects including Ga blocking, and in a minor extension, an electronic modification of Pt sites, would take place.

Effects of structural defects and acid–basic properties on the activity and selectivity of isopropanol decomposition on nanocrystallite sol–gel alumina catalyst

Abstract: The surface acid-basic properties of sol-gel alumina catalysts were studied by Fourier transform infrared FTIR spectroscopy of pyridine adsorption and temperature-programmed desorption of CO and NH . The number of acid and 23 basic sites on the samples varied with the calcination temperatures of the samples. The populations of the three different aluminum ions—tetrahedral, pentacoordinated and octahedral, which were identified by the 27 Al MAS NMR, were strongly affected by the sample calcination temperature and the crystalline composition. In the reaction temperature range between 100 and 2508C, isopropanol decomposition on sol-gel alumina catalysts was carried out. It was found that isopropanol decomposition on alumina catalyst was a structural-defect sensitive reaction. The dehydrogenation selectivity to acetone depended on the surface basic sites and the concentration of aluminum vacancies in the crystalline structure of g-Al O. 23 Bimolecular reaction to isopropylether was largely governed by the pentacoordinated aluminum ions which were related to the coordinately unsaturated aluminum ions. A mechanism for the formation of isopropylether was proposed: oxygen . q vacancies were suggested to involve the adsorption step of isopropanol, an intermediate species, CH HC , reacted with 32

Catalytic dehydrogenation (DH) of light paraffins combined with selective hydrogen combustion (SHC): I. DH → SHC → DH catalysts in series (co-fed process mode)

Abstract: Sb2O4, In2O3, WO3 and Bi2O3, supported on 50% SiO2, were found to be highly selective hydrogen combustion (SHC) catalysts. Their respective selectivities are 99.8, 99.7, 98.5 and 98.1% at 500°C, atm pressure, WHSV 2 h−1 and C°3/C3=/H2/O2 = 80/20/20/10. Their activities vary greatly, reflected by the first-order hydrogen combustion constants (kH2, s−1) which are: In2O3 1.57, Bi2O3 0.53, WO3 0.36, Sb2O4 0.22. Among the SiO2, Al2O3, TiO2 and ZrO2 supports tested, ZrO2 was found to be the best overall carrier for the highly active In2O3. When a 0.7 wt% Pt-Sn-ZSM-5 dehydrogenation (DH) catalyst and a 10 wt% In2O3/ZrO2 SHC catalyst are used in a sequential microreactor DH → SHC → DH co-fed process mode, higher than equilibrium yields of light olefins are obtained from the corresponding paraffins. At 550°C, atmospheric pressure and WHSV of 2 h−1 propylene yields of 29.7% at 97% selectivity (0.3 air/propane in SHC), and 33% at 89% selectivity (0.6 air/propane in SHC) are realized, compared to the equilibrium yield of 25% at 99% selectivity when only the DH catalyst is used. The yield improvements over equilibrium dehydrogenation are 19 and 32%, respectively. Under the same operating conditions but 0.2 air/isobutane ratio in the SHC stage, the isobutylene yield from isobutane is 47.5% at 99+% selectivity as compared to 40% and 99+% selectivity when only the DH catalyst is used; i.e. an 18% yield improvement over equilibrium. The above metal oxide SHC catalyst systems might find application to improve conventional DH processes such as Oleflex and Catofin.

Dehydrogenation of ethylbenzene to styrene over Fe2O3/Al2O3 catalysts in the presence of carbon dioxide

Abstract: An Fe2O3 (10 wt%)/Al2O3 (90 wt%) catalyst prepared by a coprecipitation method was found to be effective for dehydrogenation of ethylbenzene to produce styrene in the presence of CO2 instead of steam used in commercial processes. The dehydrogenation of ethylbenzene over the catalyst in the presence of CO2 was considered to proceed both via a one-step pathway and via a two-step pathway. CO2 was found to suppress the deactivation of the catalyst during the dehydrogenation of ethylbenzene.

Hydrogen-absorbing alloys

Abstract: Improvement of hydrogen capacity in hydrogen-absorbing alloys has been achieved in recent years. Mg-based alloys which were synthesized by ball milling showed lower dehydrogenation temperatures than intermetallic Mg-based alloys. This technique is also effective for preparing a novel Mg-based amorphous alloy, MgNi, and its hydride. Besides conventional intermetallic compounds such as LaNi5, solid solution alloy, ‘Laves phase related BCC solid solution’ with body-centered-cubic structure showed a hydrogen capacity of 2.2 mass% at room temperature. Alanate, which is not an interstitial hydride, was found to react with gaseous hydrogen reversibly with a catalyst, and its hydrogen capacity was more than 3 mass%.

Chromium Oxide Supported on Different Al2O3 Supports: Catalytic Propane Dehydrogenation

Abstract: Chromium oxide was supported on different commercial aluminas. Their characteristics were assessed by different techniques (X-ray diffraction, X-ray photoelectron, laser Raman, and electron paramagnetic resonance spectroscopies, and Brunauer−Emmett−Teller surface, temperature-programmed reduction, and temperature-programmed desorption measurements). The variation of the Cr3+/Cr6+ ratio was accounted for by the interaction between the chromium phase and the support. The tendency of Cr3+ to form aggregates such as Cr2O3 was induced by the support. The features of the support exert an influence upon chromium stabilization both in its oxidation states 6+ and 3+ and in the coordination of surface chromia species. The total acid sites and the oxidized chromium have a marked effect on activity and selectivity at initial operation times.

The oxidative dehydrogenation of ethane with CO2 over Mo2C/SiO2 catalyst

Abstract: Mo2C prepared on SiO2 was found to be an effective catalyst for the dehydrogenation of ethane to produce ethylene in the presence of CO2. The selectivity to ethylene at 850–923 K was 90–95% at an ethane conversion of 8–30%. With the increase of the temperature the dry reforming of ethane became also a significant process. It is assumed that the Mo oxycarbide formed in the reaction between CO2 and Mo2C plays an important role in the activation of ethane.

Characterization of copper and vanadium containing heteropolyacid catalysts for oxidative dehydrogenation of propane

Abstract: Multicomponent Keggin-type heteropolyacid catalysts were prepared by refluxing MoO3, V2O5 and CuO in aqueous solutions of phosphoric acid. Characterization of the recovered solids by elemental analysis, thermal gravimetric analysis, infra-red spectroscopy, UV–Vis reflectance spectroscopy, and X-ray absorption spectroscopy at the K edges of Mo and Cu, indicated V ions substituted for Mo ions in phosphomolybdic acid, whereas Cu ions did not substitute into the Keggin units. The Cu ions apparently functioned as charge-balancing counter ions in the neutral solid since formation of a Cs salt lowered the Cu content. The activity of Cu,V-containing heteropolyacid for oxidative dehydrogenation of propane to propene at 573 K was greater than that of H3PMo12O40 and H4PMo11VO40, but was similar to that of a physical mixture of CuO and H4PMo11VO40. Substitution of Cu into a Keggin unit does not appear to be necessary for promotion of the oxidative dehydrogenation reaction.