Showing papers on "Dehydrogenation published in 2006"

Amineborane-based chemical hydrogen storage: enhanced ammonia borane dehydrogenation in ionic liquids.

Abstract: Ionic liquids are shown to provide advantageous media for amineborane-based chemical hydrogen storage systems. Both the extent and rate of hydrogen release from ammonia borane dehydrogenation are significantly increased at 85, 90, and 95 degrees C when the reactions are carried out in 1-butyl-3-methylimidazolium chloride compared to analogous solid-state reactions. NMR studies in conjunction with DFT/GIAO chemical shift calculations indicate that both polyaminoborane and the diammoniate of diborane, [(NH3)2BH2+]BH4-, are initial products in the reactions.

Efficient catalysis of ammonia borane dehydrogenation.

Abstract: In the presence of an iridium pincer complex, dehydrogenation of ammonia borane (H3NBH3) occurs rapidly at room temperature in tetrahydrofuran to generate 1.0 equivalent of H2 and [NH2BH2]5. A metal borohydride complex has been isolated as a dormant form of the catalyst which can be reactivated by reaction with H2.

Catalytic Alkane Metathesis by Tandem Alkane Dehydrogenation-Olefin Metathesis

Abstract: With petroleum supplies dwindling, there is increasing interest in selective methods for transforming other carbon feedstocks into hydrocarbons suitable for transportation fuel. We report the development of highly productive, well-defined, tandem catalytic systems for the metathesis of n-alkanes. Each system comprises one molecular catalyst (a "pincer"-ligated iridium complex) that effects alkane dehydrogenation and olefin hydrogenation, plus a second catalyst (molecular or solid-phase) for olefin metathesis. The systems all show complete selectivity for linear (n-alkane) product. We report one example that achieves selectivity with respect to the distribution of product molecular weights, in which n-decane is the predominant high-molecular-weight product of the metathesis of two moles of n-hexane.

Modified lithium borohydrides for reversible hydrogen storage (2).

Abstract: In an attempt to develop lithium borohydrides as reversible hydrogen storage materials with high hydrogen storage capacities, the feasibility of reducing the dehydrogenation temperature of the lithium borohydride and moderating rehydrogenation conditions was explored The lithium borohydride was modified by ball milling with metal oxides and metal chlorides as additives The modified lithium borohydrides released 9 wt % hydrogen starting from 473 K The dehydrided modified lithium borohydrides absorbed 7-9 wt % hydrogen at 873 K and 7 MPa The modification with additives reduced the dehydriding starting temperature from 673 to 473 K and moderated the rehydrogenation conditions from 923 K/15 MPa to 873 K/7 MPa XRD and SEM analysis revealed the formation of an intermediate compound that might play a key role in changing the reaction path, resulting in the lower dehydriding temperature and reversibility The reversible hydrogen storage capacity of the oxide-modified lithium borohydrides decreased gradually during hydriding/dehydriding cycling One of the possible reasons for this effect might be the loss of boron during dehydrogenation, but this can be prevented by changing the dehydriding path using appropriate additives The additives reduced the dehydriding temperature and improved the reversibility, but they also reduced the hydrogen storage capacity The best compromise can be reached by selecting appropriate additives, optimizing the additive loading, and using new synthesis processes other than ball milling

Kinetics of Oxidative Dehydrogenation of C2‐C3 Alkanes on Oxide Catalysts

Abstract: This review is a survey of the main facts and concepts appeared in the last years on kinetics of oxidative dehydrogenation (ODH) of ethane, propane, butane and isobutane. The accent is put on the mechanisms and the kinetic models used for the description of this reaction. The main catalysts for ODH of light alkanes and the specific features of each type of alkane are discussed. Influence of the type of supports, promoters, acid–base properties, type of oxygen on the performance of ODH reaction is also discussed. No clear interpretation regarding the mode of alkane activation of each stage of this reaction can be derived from the survey of the literature and the problems which are still waiting for the solution or need further studies are described.

Hydrogenation and Hydrocarbonation and Etching of Single-Walled Carbon Nanotubes

Abstract: We present a systematic experimental investigation of the reactions between hydrogen plasma and single-walled carbon nanotubes (SWNTs) at various temperatures. Microscopy, infrared (IR) and Raman spectroscopy, and electrical transport measurements are carried out to investigate the properties of SWNTs after hydrogenation. Structural deformations, drastically reduced electrical conductance, and an increased semiconducting nature of SWNTs upon sidewall hydrogenation are observed. These changes are reversible upon thermal annealing at 500 °C via dehydrogenation. Harsh plasma or high temperature reactions lead to etching of nanotubes likely via hydrocarbonation. Smaller SWNTs are markedly less stable against hydrocarbonation than larger tubes. The results are fundamental and may have implications to basic and practical applications including hydrogen storage, sensing, band gap engineering for novel electronics, and new methods of manipulation, functionalization, and etching of nanotubes.

Effects of Ce addition on the Pt-Sn/γ-Al2O3 catalyst for propane dehydrogenation to propylene

Abstract: The effects of Ce addition on the Pt-Sn/gamma-Al2O3 catalysts for propane dehydrogenation to propylene have been investigated by reaction tests and some physicochemical characterizations like XRD, BET, NH3-TPD, H-2-TPD, H-2-TPR, H-2-chemisorption, CO-FTIR and TPO The results show that the Ce addition could greatly improve the catalytic performance and catalytic stability of the Pt-Sn/gamma-Al2O3, which is reported as the optimal catalyst for propane dehydrogenation to propylene We could keep > 38% of propane conversion, > 98% propylene selectivity and > 37% propylene yield over 50 h in the reaction of propane dehydrogenation to propylene over Pt-Sn/Ce-Al2O3 catalysts at 576 degrees C, 3800 h(-1) and H-2/C3H8/Ar = 1/1/5 The presence of Ce in the Pt-Sn/Ce-Al2O3 catalysts could not only stabilize the active states of Pt, Sn and support, but could also suppress the coke accumulation on the catalyst during reaction, and further improve the catalytic performance of Pt-Sn/gamma-Al2O3 (c) 2006 Elsevier BV All fights reserved

Investigation of the Mechanism of the Intramolecular Scholl Reaction of Contiguous Phenylbenzenes

Abstract: Two mechanisms of the Scholl reaction were investigated in the series 1, 2, ..., n-oligophenylbenzenes (n = 2, 3, 4, 6) at the B3LYP/6-31G(d) level of theory. A mechanism involving generation of a radical cation followed by C−C bond formation and dehydrogenation is unlikely on the basis of unfavorable energies of activation. A mechanism involving generation of an arenium cation followed by C−C bond formation and dehydrogenation is energetically feasible. An explanation for the facile polycondensation of hexaphenylbenzene to hexa-peri-hexabenzocoronene, where six new aryl−aryl bonds are formed, is provided. Kinetic simulations based on the calculated activation energies of the arenium cation mechanism predict that intermediates will not accumulate; this is supported by mass balance experiments. Reaction optimization studies suggest that PhI(O2CCF3)2/BF3·OEt2 or MoCl5 are superior to FeCl3 or AlCl3/CuCl2. This is a full account of our work reported partially as a communication previously (Rempala, P.; Kroul...

Ethanol dehydrogenation over copper catalysts on rice husk ash prepared by ion exchange

Abstract: Samples of copper on rice husk ash (Cu/RHA) have been prepared by the ion exchange method, with various copper loadings, and have been calcined at different temperatures. Such samples were tested for dehydrogenation of ethanol to acetaldehyde. The samples were characterized by DSC, XRD, FTIR, TEM, XPS, TPR, BET, and H 2 –N 2 O titration techniques. FTIR spectra illustrate the formation of chrysocolla in uncalcined and calcined Cu/RHA catalyst precursors. DSC, XRD and XPS analyses illustrate that, in Cu/RHA catalyst precursor after drying at 383 K, copper is present as Cu(OH) 2 species. After calcination at 723 K, copper is present as Cu 2+ ions and as CuO species. After reduction of the calcined Cu/RHA sample at 523–573 K, copper is present as Cu + and/or Cu 0 species along with unreduced Cu 2+ species. TEM images show that copper crystallites are spherical in shape and are evenly distributed. TPR results reveal that various copper loadings in calcined Cu/RHA catalyst precursor exhibit similar metal-support interactions (MSI). Ethanol conversion for dehydrogenation of ethanol is found to be independent of calcination temperature and has little effect on Cu loading. Ethanol is selectively converted to acetaldehyde at the reaction temperature of 483–548 K. The Cu/RHA catalysts exhibit higher catalytic activity and lower deactivation rate than Cu/SiO 2 catalysts. The activity of Cu/RHA catalysts is found to depend on Cu surface area.

Periodic density functional theory study of propane oxidative dehydrogenation over V2O5(001) surface.

Abstract: The oxidative dehydrogenation (ODH) of propane on single-crystal V2O5(001) is studied by periodic density functional theory (DFT) calculations. The energetics and pathways for the propane to propene conversion are determined. We show that (i) the C−H bond of propane can be activated by both the terminal and the bridging lattice O atoms on the surface with similar activation energies. At the terminal O site both the radical and the oxo-insertion pathways are likely for the C−H bond activation, while only the oxo-insertion mechanism is feasible at the bridging O site. (ii) Compared to that at the terminal O site, the propene production from the propoxide at the bridging O site is much easier due to the weaker binding of propoxide at the bridging O. It is concluded that single-crystal V2O5(001) is not a good catalyst due to the terminal O being too active to release propene. It is expected that an efficient catalyst for the ODH reaction has to make a compromise between the ability to activate the C−H bond an...

Hydrogenation and Hydro-Carbonation and Etching of Single-Walled Carbon Nanotubes

Abstract: We present a systematic experimental investigation of the reactions between hydrogen plasma and single-walled carbon nanotubes (SWNTs) at various temperatures. Microscopy, infrared (IR) and Raman spectroscopy and electrical transport measurements are carried out to investigate the properties of SWNTs after hydrogenation. Structural deformations, drastically reduced electrical conductance and increased semiconducting nature of SWNTs upon sidewall hydrogenation are observed. These changes are reversible upon thermal annealing at 500C via dehydrogenation. Harsh plasma or high temperature reactions lead to etching of nanotube likely via hydro-carbonation. Smaller SWNTs are markedly less stable against hydro-carbonation than larger tubes. The results are fundamental and may have implications to basic and practical applications including hydrogen storage, sensing, band-gap engineering for novel electronics and new methods of manipulation, functionalization and etching of nanotubes.

Nanoparticle Size Effects on Methanol Electrochemical Oxidation on Carbon Supported Platinum Catalysts

Abstract: The particle size effect observed on the performance of Pt/C electrocatalysts toward the methanol oxidation reaction (MOR) has been investigated with differential electrochemical mass spectrometry (DEMS). The investigation has been conducted under both potentiodynamic and potentiostatic conditions as research on methanol electrochemical oxidation is closely related to interest in direct methanol fuel cells. The particle size effect observed on the MOR is commonly regarded as a reflection of different Pt-CO and Pt-OH bond strengths for different particle sizes. This work focuses mainly on the mechanism of methanol dehydrogenation on platinum which is central to the problem of the optimization of the efficiency of methanol electro-oxidation by favoring the CO(2) formation pathway. It was found that the partitioning of the methanol precursor among the end products on supported platinum nanoparticles is strongly dependent on particle size distribution. Also, it is postulated that the coupling among particles of different sizes via soluble products must be considered in order to understand the particle size effects on the observed trends of product formation. An optimum particle size range for efficiently electro-oxidizing methanol to CO(2) was found between 3 and 10 nm, and loss in efficiency is mostly related to the partial oxidation of methanol to formaldehyde on either too small or too large particles. The possible reasons for these observations are also discussed.

Reforming of oxygenates for H2 production: correlating reactivity of ethylene glycol and ethanol on Pt(111) and Ni/Pt(111) with surface d-band center.

Abstract: The dehydrogenation and decarbonylation of ethylene glycol and ethanol were studied using temperature programmed desorption (TPD) on Pt(111) and Ni/Pt(111) bimetallic surfaces, as probe reactions for the reforming of oxygenates for the production of H2 for fuel cells. Ethylene glycol reacted via dehydrogenation to form CO and H2, corresponding to the desired reforming reaction, and via total decomposition to produce C(ad), O(ad), and H2. Ethanol reacted by three reaction pathways, dehydrogenation, decarbonylation, and total decomposition, producing CO, H2, CH4, C(ad), and O(ad). Surfaces prepared by deposition of a monolayer of Ni on Pt(111) at 300 K, designated Ni−Pt−Pt(111), displayed increased reforming activity compared to Pt(111), subsurface monolayer Pt−Ni−Pt(111), and thick Ni/Pt(111). Reforming activity was correlated with the d-band center of the surfaces and displayed a linear trend for both ethylene glycol and ethanol, with activity increasing as the surface d-band center moved closer to the Fe...

Promoting effect of ceria on the physicochemical and catalytic properties of CeO2–ZnO composite oxide catalysts

Abstract: CeO2–ZnO composite catalysts prepared by amorphous citrate method have been investigated for cyclohexanol dehydrogenation and hydrogen transfer reactions. The precursors and catalysts have been characterized by TGA, CHN analysis, XRD, UV–vis–NIR diffuse reflectance, SEM and acid–base measurements. The amorphous precursors in citrate process contain one molecule of citric acid per Ce4+ or Zn2+ ions. Structural studies of composite oxides indicate the presence of individual oxide phases along with non-equilibrium solid solutions in a limited composition range. The composite oxides contain low coordination Ce3+ and Ce4+ sites. Cyclohexanone was obtained as main product for cyclohexanol transformation reaction carried out over these mixed oxide catalysts due to dehydrogenation on basic sites. The presence of ceria in the composite oxide enhances the surface area and acid–base properties facilitating the dehydrogenation process. At low ceria content, the CeO2–ZnO composite oxide catalysts show higher catalytic activity for both cyclohexanol dehydrogenation and hydrogen transfer reactions due to higher basicity, surface area and smaller crystallite sizes. Hydrogen transfer activity is found to be higher on CeO2(10%)–ZnO catalyst prepared by citrate method compared to the catalyst prepared by decomposition from acetate precursor. This study demonstrates the promoting effect of ceria in CeO2–ZnO catalysts for reactions involving acid–base sites.

Oxidative Dehydrogenation of Ethane over Cr/ZSM-5 Catalysts Using CO2 as an Oxidant

Abstract: Highly active catalysts for oxidative dehydrogenation of ethane with CO2 were characterized by temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), Fourier transform infr...

Novel zeolite composite, method for making and catalytic application thereof

Abstract: A catalytic material includes microporous zeolites supported on a mesoporous inorganic oxide support. The microporous zeolite can include zeolite Beta, zeolite Y (including “ultra stable Y” - USY), mordenite, Zeolite L, ZSM.5, ZSM-11, ZSM-12, ZSM-20, Theta-1, ZSM-23, ZSM-34, ZSM-35, ZSM-48, SSZ-32, PSH-3, MCM-22, MCM-49, MCM-56, ITQ-1, ITQ-2, ITQ-4, ITQ-21, SAPO-5, SAPO-11, SAPO-37, Breck-6, ALPO4-5, etc. The mesoporous inorganic oxide can be e.g., silica or silicate. The catalytic material can be further modified by introducing some metals e.g. aluminum, titanium, molybdenum, nickel, cobalt, iron, tungsten, palladium and platinum. It can be used as catalysts for acylation, alkylation, dimerization, oligomerization, polymerization, hydrogenation, dehydrogenation, aromatization, isomerization, hydrotreating, catalytic cracking and hydrocracking reactions.

Catalysed low temperature H2 release from nitrogen heterocycles

Abstract: In an experimental and computational study, nitrogen- and oxygen-containing heterocycles were compared with carbocycles as liquid substrates for hydrogen release with heterogeneous catalysts. Heteroatom substitution, particularly by nitrogen, favours low temperature H2 release; indoline was fully dehydrogenated in less than 30 min with Pd/C at 110 °C.

Oxidic or Metallic Palladium: Which Is the Active Phase in Pd-Catalyzed Aerobic Alcohol Oxidation?

Abstract: In situ X-ray absorption spectroscopy combined with on-line catalytic measurements using FT−IR spectroscopy unequivocally identified that metallic palladium is the more active phase in the aerobic oxidation of benzyl alcohol than palladium oxide. The aerobic oxidation of benzyl alcohol in cyclohexane at 50 °C was low over oxidized 0.5%Pd/Al2O3 and 5%Pd/Al2O3 catalysts. XANES and EXAFS showed that the catalysts in the as-received state were almost fully oxidized and no reduction of the palladium constituent was observed during time-on-stream. After in situ reduction by hydrogen-saturated cyclohexane, the catalysts were much more active (over 50 times) than before reduction. Both XANES and EXAFS uncovered that the palladium constituent was mainly in a reduced state under these conditions of high catalytic activity. This demonstrates that metallic palladium is the active phase for alcohol dehydrogenation.