Showing papers on "Dehydrogenation published in 2004"

Altering Hydrogen Storage Properties by Hydride Destabilization through Alloy Formation: LiH and MgH2 Destabilized with Si

Abstract: Alloying with Si is shown to destabilize the strongly bound hydrides LiH and MgH2. For the LiH/Si system, a Li2.35Si alloy forms upon dehydrogenation, causing the equilibrium hydrogen pressure at 490 °C to increase from approximately 5 × 10-5 to 1 bar. For the MgH2/Si system, Mg2Si forms upon dehydrogenation, causing the equilibrium pressure at 300 °C to increase from 1.8 to >7.5 bar. Thermodynamic calculations indicate equilibrium pressures of 1 bar at approximately 20 °C and 100 bar at approximately 150 °C. These conditions indicate that the MgH2/Si system, which has a hydrogen capacity of 5.0 wt %, could be practical for hydrogen storage at reduced temperatures. The LiH/Si system is reversible and can be cycled without degradation. Absorption/desorption isotherms, obtained at 400−500 °C, exhibited two distinct flat plateaus with little hysteresis. The plateaus correspond to formation and decomposition of various Li silicides. The MgH2/Si system was not readily reversible. Hydrogenation of Mg2Si appears...

Steam reforming of bio-ethanol on alkali-doped Ni/MgO catalysts: hydrogen production for MC fuel cell

Abstract: The effects of alkali addition (e.g. Li, Na, K) on the behavior of Ni/MgO catalyst in the bio-ethanol steam reforming have been investigated. Li and Na promote the NiO reduction but negatively affect the dispersion of the Ni/MgO catalyst, whereas K does not significantly affect either morphology or dispersion. Li and K enhance the stability of Ni/MgO mainly by depressing Ni sintering. Coke formation on bare and doped catalysts occurs but with orders of magnitude lower rates than those claimed for Ni supported on an acidic carrier. The peculiar influence of the mean Ni particle size on the turnover frequency (TOF s −1 ) has been explained by invoking a structure-sensitive character of the ethanol dehydrogenation reaction considered as being the first step of the reaction which evolves according to the following mechanism: ethanol hydrogenation → acetaldehyde decomposition → steam reforming of methane and water gas shift reactions.

Highly effective pincer-ligated iridium catalysts for alkane dehydrogenation. DFT calculations of relevant thermodynamic, kinetic, and spectroscopic properties.

Abstract: The p-methoxy-substituted pincer-ligated iridium complexes, (MeO−tBuPCP)IrH4 (RPCP = κ3-C6H3-2,6-(CH2PR2)2) and (MeO−iPrPCP)IrH4, are found to be highly effective catalysts for the dehydrogenation of alkanes (both with and without the use of sacrificial hydrogen acceptors). These complexes offer an interesting comparison with the recently reported bis-phosphinite “POCOP” (RPOCOP = κ3-C6H3-2,6-(OPR2)2) pincer-ligated catalysts, which also show catalytic activity higher than unsubstituted PCP analogues (Gottker-Schnetmann, I.; White, P.; Brookhart, M. J. Am. Chem. Soc. 2004, 126, 1804). On the basis of νCO values of the respective CO adducts, the MeO−PCP complexes appear to be more electron-rich than the parent PCP complexes, whereas the POCOP complexes appear to be more electron-poor. However, the MeO−PCP and POCOP ligands are calculated (DFT) to show effects in the same directions, relative to the parent PCP ligand, for the kinetics and thermodynamics of a broad range of reactions including the addition o...

Hydrogen storage by reversible hydrogenation of pi-conjugated substrates

Abstract: Processes are provided for the storage and release of hydrogen by means of a substantially reversible catalytic hydrogenation of extended pi-conjugated substrates which include large polycyclic aromatic hydrocarbons, polycyclic aromatic hydrocarbons with nitrogen heteroatoms, polycyclic aromatic hydrocarbons with oxygen heteroatoms, polycyclic aromatic hydrocarbons with alkyl, alkoxy, nitrile, ketone, ether or polyether substituents, pi-conjugated molecules comprising 5 membered rings, pi-conjugated molecules comprising six and five membered rings with nitrogen or oxygen hetero atoms, and extended pi-conjugated organic polymers. The hydrogen, contained in the at least partially hydrogenated form of the extended pi-conjugated system, can be facilely released for use by a catalytic dehydrogenation of the latter in the presence of a dehydrogenation catalyst which can be effected by lowering the hydrogen gas pressure, generally to pressures greater than 0.1 bar or raising the temperature to less than 250° C. or less, or by a combination of these two process parameters.

Palladium Catalyzed Aerobic Dehydrogenation: From Alcohols to Indoles and Asymmetric Catalysis

Abstract: Catalytic aerobic dehydrogenation of organic substrates is going through a Renaissance. Recent advances in this area have led to the discovery of palladium-catalyzed alcohol dehydrogenations and oxidative hetero- and carbocyclizations. The development of asymmetric catalytic dehydrogenations is the latest advance in a long line of catalytic asymmetric oxidation reactions.

Catalytic Conversion of Alkanes to Olefins by Carbon Dioxide Oxidative Dehydrogenation-A Review

Abstract: The utilization of carbon dioxide (CO2) which is one of the main contributors to the greenhouse effect, has been a topic of global interest, from both fundamental and practical viewpoints. In addition to be the sources of carbon, CO2 might also be utilized as an oxygen source or oxidant, because it can be considered to be a nontraditional (mild) oxidant and oxygen transfer agent. In this paper, CO2 as an oxidant for the selective oxidative conversions of alkanes to alkenes has been reviewed, including methane coupling to ethylene, C-2-C-4 alkanes dehydrogenation to their corresponding olefins, dehydroaromerization of lower hydrocarbons to benzene, and dehydrogenation of ethylbenzene to styrene. It has been shown that CO2 has the potential to offer a promising alternative to oxygen for selective oxidation, whereas the catalyst development is the key to the applications.

CH3O decomposition on PdZn(111), Pd(111), and Cu(111). A theoretical study.

Abstract: Methanol steam re-forming, catalyzed by Pd/ZnO, is a potential hydrogen source for fuel cells, in particular in pollution-free vehicles. To contribute to the understanding of pertinent reaction mechanisms, density functional slab model studies on two competing decomposition pathways of adsorbed methoxide (CH3O) have been carried out, namely, dehydrogenation to formaldehyde and C−O bond breaking to methyl. For the (111) surfaces of Pd, Cu, and 1:1 Pd−Zn alloy, adsorption complexes of various reactants, intermediates, transition states, and products relevant for the decomposition processes were computationally characterized. On the surface of Pd−Zn alloy, H and all studied C-bound species were found to prefer sites with a majority of Pd atoms, whereas O-bound congeners tend to be located on sites with a majority of Zn atoms. Compared to Pd(111), the adsorption energy of O-bound species was calculated to be larger on PdZn(111), whereas C-bound moieties were less strongly adsorbed. C−H scission of CH3O on var...

Kinetics of aqueous-phase reforming of oxygenated hydrocarbons: Pt/Al2O3 and Sn-modified Ni catalysts

Abstract: Reaction kinetics studies were conducted on the aqueous-phase reforming of ethylene glycol to produce hydrogen at temperatures near 500 K over Pt/Al2O3 and Raney NiSn catalysts. Ethylene glycol reforming proceeds through similar mechanisms over Pt and NiSn catalysts, involving initial dehydrogenation of ethylene glycol, followed by C−C bond cleavage and water−gas shift. The initial dehydrogenation of ethylene glycol appears to be kinetically significant over Pt/Al2O3, whereas the subsequent rate of C−C cleavage appears to be kinetically significant over R−Ni14Sn. The reforming reaction is fractional order in the ethylene glycol concentration, because of the strong adsorption of the oxygenated reactant, and negative order in the system pressure, through product inhibition by adsorption of H2 and/or CO at high pressures. High selectivity for hydrogen production is achieved for gas-phase products over Pt/Al2O3, whereas the addition of Sn is necessary to avoid alkane formation by methanation over Ni-based cat...

Characterization of carbon supported palladium catalysts: inference of electronic and particle size effects using reaction probes

Abstract: Carbon supported palladium catalysts have been used in a wide range of industrial reactions, especially hydrogenation in the fine chemical industry. However, there are a lot of difficulties to characterize this system, mainly due to intrinsic characteristics of the carbon materials. This work aimed to evaluate the effect of dispersion and electronic density of palladium on the catalytic performance of different reactions, besides characterizing Pd/C system by several techniques. Two samples of activated carbon and graphite were used in this work. Some pretreatments of activated carbon samples with HCl, HNO3 and O2 were performed before the impregnation with palladium. Supports and catalysts were characterized by atomic absorption spectroscopy, N2 adsorption isotherms at 77 K, temperature programmed desorption (TPDHe), temperature programmed reduction (TPR), CO chemisorption and X-ray photoelectron spectroscopy (XPS). Four reaction probes were performed with the catalysts: cyclohexane dehydrogenation, 1,3-butadiene hydrogenation, dichlorodifluoromethane (CFC-12) and trichloromethane hydrodechlorination. Depending on the electronic density, which is affected by the chemical environment of metal, carbene intermediate formation can be favored, yielding a particular selectivity pattern. The dispersion seems to influence the activity of the reactions, but not selectivity patterns.

Oxidative dehydrogenation of propane on V2O5/Al2O3 and V2O5/TiO2 catalysts: understanding the effect of support by parameter estimation

Abstract: In this paper, the effect of the oxide support for supporting the vanadium oxide phase is studied by estimating the reaction parameters for the oxidative dehydrogenation of propane. To achieve this objective, several V 2 O 5 /Al 2 O 3 and V 2 O 5 /TiO 2 catalysts were synthesized by an incipient-wetness-impregnation technique. The supported vanadium oxide catalysts were characterized and the surface area, monolayer coverage and reducibility were determined. The surface area of the catalyst samples was not significantly affected with supported vanadium oxide loading. It was observed that the catalysts contain only molecularly dispersed vanadium oxide species below monolayer coverage, and molecularly dispersed and crystalline V 2 O 5 above monolayer coverage. TPR studies revealed that the V 2 O 5 /Al 2 O 3 samples were more difficult to reduce relative to the V 2 O 5 /TiO 2 samples. The monolayer or near-monolayer catalysts, 10% V 2 O 5 /Al 2 O 3 and 4% V 2 O 5 /TiO 2 , were selected for detailed kinetic analysis. A Mars–van Krevelen (MVK) model containing eight parameters was chosen for this purpose. The parameters were estimated using a genetic algorithm (GA), which optimizes a suitable objective function for a non-linear multi-response system. From the parameters estimated, it was determined that a similar catalytic cycle occurs independent of the oxide support. However, the rate at which the catalytic cycle occurs appears to be much faster on the more reducible titania support compared to the rate on the less reducible alumina support. The degree of reduction varies along the length of the reactor and depends on the support. Thus, the support has a significant effect on the reaction parameters for the oxidative dehydrogenation of propane over supported vanadium oxide catalysts.

Performance of PtSn catalysts supported on MAl2O4 (M: Mg or Zn) in n-butane dehydrogenation: characterization of the metallic phase

Abstract: The Sn(0.3 or 0.5 wt.%) addition to Pt(0.3 wt.%)/ZnAl 2 O 4 and Pt(0.3 wt.%)/MgAl 2 O 4 catalysts leads to an increase of the activity and selectivity to olefins in the n -butane dehydrogenation reaction. Slight differences in the catalytic behaviour were found between 0.3 and 0.5 wt.% of Sn added to Pt in both catalyst series. Besides, the bimetallic catalysts also show a good stability through the five successive reaction-regeneration cycles, mainly the PtSn/MgAl 2 O 4 one. The mono- and bimetallic catalysts were characterized by using different techniques: tests reactions of the metallic phase (cyclohexane dehydrogenation and cyclopentane hydrogenolysis), temperature programmed reduction, X-ray photoelectron spectroscopy, H 2 chemisorption, microcalorimetric measurements of the propylene adsorption and XRD. Results show that the nature of the metallic phase in monometallic samples appears to be different for the two supports. Thus, metallic particles with a very low concentration of hydrogenolytic sites (steps, corners and edges) would exist in the Pt/ZnAl 2 O 4 catalyst in contrast with the structure of the metallic phase of the Pt/MgAl 2 O 4 one, where the existence of an important concentration of hydrogenolytic sites is clearly observed (according to the cyclopentane hydrogenolysis results). The modification of Pt by the Sn addition clearly improves the catalytic behaviour in n -butane dehydrogenation due to important changes in the structure of the metallic phase. Thus, when Sn is added to Pt/ZnAl 2 O 4 and Pt/MgAl 2 O 4 catalysts the metallic surface structure seems to be more complex. In fact, results would indicate not only a partial formation of PtSn alloys or intermetallic compounds between Pt 0 and a fraction of Sn(0), but also a surface enrichment in Sn, dilution effects as well as the presence of tin stabilized on the support, probably as Sn(II/IV) oxides and SnCl 2 species.

Surface species and gas phase products in steam reforming of ethanol on TiO2 and Rh/TiO2

Abstract: The effect of water on the formation and stability of surface species in ethanol adsorption on TiO 2 and Rh/TiO 2 was studied by FTIR. The changes of the gas phase during the adsorption processes and different treatments were monitored and TPD spectra were registered by mass spectrometer. It was found that water enhanced the stability of strongly held C 2 H 5 OH, while the stability of monodentate ethoxide was less due to water. Dehydrogenation of molecularly adsorbed ethanol resulting in the formation of adsorbed acetaldehyde and gas phase H 2 have been proposed as key reaction step. Surface acetaldehyde and gas phase H 2 appeared at lower temperature and in a greater amount in the presence of Rh and water. A possible mechanism of the reaction was discussed.

Theoretical Study of Two-State Reactivity of Transition Metal Cations: The ``Difficult'' Case of Iron Ion Interacting with Water, Ammonia, and Methane

Abstract: The potential energy surfaces corresponding to the dehydrogenation reaction of H 2 O, NH 3 , and CH 4 molecules by Fe + ( 6 D, 4 F) cation have been investigated in the framework of the density functional theory in its B3LYP formulation and employing a new optimized basis set for iron. In all cases, the low-spin ion-dipole complex, which is the most stable species on the respective potential energy hypersurfaces, is initially formed. In the second step, a hydrogen shift process leads to the formation of the insertion products, which are more stable in a low-spin state. From these intermediates, three dissociation channels have been considered. All of the results have been compared with existing experimental and theoretical data. Results show that the three insertion pathways are significantly different, although spin crossings between high- and low-spin surfaces are observed in all cases. The topological analysis of the electron localization function has been used to characterize the nature of the bonds for all of the minima and transition states along the paths.

Methanol dehydrogenation and formation of carbonaceous overlayers on Pd(111) studied by high-pressure SFG and XPS spectroscopy

Abstract: Methanol decomposition on Pd(111) at 300 and 400 K was studied in situ from 5 × 10-7 to 0.1 mbar by combining vibrational sum frequency generation (SFG) and X-ray photoelectron spectroscopy (XPS). Two competing decomposition pathways, i.e., dehydrogenation of CH3OH to CO and H2 and methanolic C−O bond scission, were observed by monitoring the time-dependent evolution of CO/CHxO and of carbonaceous deposits CHx (x = 0−3) via their vibrational and photoemission characteristics. Quantification of carbon-containing species was performed by XPS, while the preferred binding site of CHx was determined by SFG using CO as probe molecule for postreaction adsorption. In contrast to previous reports, Pd(111) was found to be quite active for methanolic C−O bond scission. The CHx formation rate strongly increased with pressure and temperature, leading to immediate catalyst deactivation at 0.1 mbar and 400 K. The combined SFG/XPS data suggest that the carbonaceous residues are highly dehydrogenated, such as CH or carbon...

Effect of ZSM-5 on the aromatization performance in cracking catalyst

Abstract: Cracking of FCC naphtha has been investigated on the model catalysts containing ZSM-5 (Si/Al2=50, 500), Zn-ZSM-5 (Si/Al2=50) and USY. The presence of ZSM-5 and USY causes a tremendous decrease in olefins of gasoline fraction, leading to an enrichment of the aromatics in liquid product. It is evidently that aromatics are generated from lower olefins. The property of lower silica-to-alumina ratio of ZSM-5 and Zn modification enhance the aromatization process for higher cracking reactivity and dehydrogenation. The result of evaluation in confined fluid bed apparatus (CFBA) with the mixture of VGO and VTB as feed further confirms the aromatics generation in cracking reaction stimulated by ZSM-5 additive over base catalysts with different hydrogen transfer activity. Based on these results, relations between scission and aromatization in FCC catalyst containing ZSM-5 zeolite could be proposed. The suggested reaction path could better explain the reasons that effectiveness of ZSM-5 additive to lighter olefins is more profound in base catalyst of low hydrogen transfer activity and why so many researchers had not observed the aromatics generation motivated by ZSM-5 additive in catalytic cracking reaction.