Showing papers on "Dehydrogenation published in 1987"

A membrane reactor using palladium

Abstract: La deshydrogenation du cyclohexane en benzene et hydrogene est une reaction reversible; pour obtenir une reaction complete, on utilise une membrane de palladium pour separer l'hydrogene au fur et a mesure de sa formation

Selective catalytic dehydrogenation of alkanes to alkenes

Abstract: Linear and cyclic alkanes can be selectively dehydrogenated to the corresponding alkenes both thermally and photochemically (254 nm) with iridium complexes as catalysts. In the photochemical case, a sacrificial hydrogen acceptor is not required and H/sub 2/ is evolved directly. Preferential initial formation of the least stable alkene (e.g., methylenecyclohexane from methylcyclohexane) is explained by attack at unhindered C-H bonds by an oxidative addition to the metal. A subsequent ..beta..-elimination gives the alkene. A key feature of the catalyst, (IrH/sub 2/(eta/sup 2/-O/sub 2/CCF/sub 3/)(PR/sub 3/)/sub 2/) (4), is that the chelating acetate group can open up to allow ..beta..-elimination to take place in the alkyl hydride intermediate (IrH(R)(eta/sup 2/-O/sub 2/CCF/sub 3/)(PR/sub 3/)/sub 2/). Prolonged reaction times lead to progressive isomerization of the alkene to give the thermodynamic product (e.g., 1-methylcyclohexene from methylcyclohexane). Up to 32 turnovers of dehydrogenation are seen. Deactivation of the catalyst takes place by P-C hydrogenolysis of the PAr/sub 2/ ligand in the thermal case; the rise in the amount of ArH formed parallels the fall-off in activity of the catalyst. P-C cleavage does not take place in the photochemical system (R = cyclohexyl).

The catalytic activation and functionalization of carbon-hydrogen bonds. Aldimine formation by the insertion of isonitriles into aromatic carbon-hydrogen bonds

Abstract: Many examples of the activation of aromatic and aliphatic carbon-hydrogen bonds by homogeneous transition-metal complexes have appeared over the past few years, offering attractive new routes to organometallic species. While several of these reports involve oxidative addition of low valent metal complexes to alkanes or arenes, these new adducts have not proven to be useful for the generation of functionalized hydrocarbon products. Reports of successful metal-based alkane functionalization include free radical oxidations, intramolecular cyclizations of alkyl carbenoid species to given cyclopentanones, aromatic isonitrile insertion to give indoles, and alkane dehydrogenation to produce olefins. Arene functionalization reactions commonly depend upon the presence of tethering groups, although the production of benzaldehyde, benzoic acid, styrene, and phenylsiloxane insertion products have been recently reported. They report here a new type of iron catalyzed insertion of isonitrile into the C-H bond of arenes to produce aldimines (eq 1). RNC + C/sub 6/H/sub 6/ (Fe) ..-->.. ..delta..G/sup 0/ approx. = -11 Kcal/mol C/sub 6/H/sub 5/ - CH = NR (1).

Catalytic oxidative steam dehydrogenation process

Abstract: A novel process is disclosed for the steam dehydrogenation of dehydrogenatable hydrocarbons in the vapor phase in conjunction with oxidative reheating of the intermediate products The process utilizes a single catalyst to perform both the selective oxidation and steam dehydrogenation functions The particular catalyst employed comprises a Group VIII noble metal component, a Group IA and/or a Group IIA component and may contain among other modifiers a Group IIIA or IVA metal, and a halogen component The catalytic components are supported on an inorganic substrate such as alumina

Dehydrogenation catalyst compositon

Abstract: A novel catalytic composite comprising a platinum group metal component; a modifier metal component selected from the group consisting of a tin component, germanium component, rhenium component and mixtures thereof; an optional alkali or alkaline earth metal component or mixtures thereof, an optional halogen component, and an optional catalytic modifier component on a refractory oxide support having a nominal diameter of at least about 850 microns. The distribution of the platinum group metal component is such that the platinum group component is surface-impregnated where substantially all of the platinum group metal component is located at most within a 400 micron exterior layer of the support. The effective amount of the modifier metal component is uniformly dispersed throughout the refractory oxide support. The distribution of the surface-impregnated platinum metal component is such that the novel catalyst has particular utility as a hydrocarbon dehydrogenation catalyst in a hydrocarbon dehydrogenation process.

High-resolution electron energy loss spectroscopy and thermal programmed desorption studies of the chemisorption and thermal decomposition of ethylene and acetylene on nickel(100) single-crystal surfaces

Abstract: The chemisorption and thermal decomposition of acetylene and ethylene on Ni(100) surfaces were studied by using thermal programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Acetylene chemisorbs molecularly at 90 K forming a rehybridized (sp/sup 3/) acetylenic moiety on the surface. This fragment is stable up to 270 K, where dehydrogenation takes place to form CCH residues. Heating further, above 400 K, leaves only carbon on the nickel surface. Ethylene also adsorbs molecularly but with little rehybridization, H bonding to the metal. Stepwise dehydrogenation takes place as the crystal temperature is increased. Vinyl is formed as a product of a unimolecular reaction at 170 K for low coverages of ethylene (theta less than or equal to 0.4), but this process is inhibited at higher coverages and only takes place around 200 K. A strong isotope effect was also observed, and C/sub 2/D/sub 4/ decomposition only occurred at temperatures about 40 K higher. Vinyl decomposes further to form an acetylenic moiety at about 230 K. Further heating is followed by stepwise dehydrogenation similar to that observed when acetylene is adsorbed. No H-D scrambling is observed during the thermal treatment for partially or fully deuterated molecules.

Transformation of but-1-ene into aromatic hydrocarbons over ZSM-5 zeolites

Abstract: The yield of aromatic hydrocarbons from but-1-ene increases greatly when zinc cations are incorporated into ZSM-5 zeolites. Thus, the selectivities to aromatics were 38.6% and 77.8% over H-ZSM-5 and Zn-ZSM-5, respectively, at 773 K. For both zeolites, the main aromatic products are benzene, toluene and xylenes. A variety of aromatic products implies that the interconversion among alkenes precedes their aromatization over ZSM-5 zeolites. The ratio of alkanes to aromatics was close to 3 over H-ZSM-5, indicating that aromatics are formed by the successive deprotonation and hydride transfer to carbenium ions from oligomerized products. On the other hand, the ratio was 0.9 over Zn-ZSM-5, indicating that the direct dehydrogenation of intermediate alkenes is involved in aromatization. Over H-borosilicate, the formation of alkanes and aromatics is negligible. The aromatization activity developed on incorporating zinc cations into the borosilicate, indicating the essential role of zinc species in aromatization. The dehydrogenation activity of zinc species is further confirmed by the high selectivity to butadiene from but-1-ene at low contact time over Zn-borosilicate.

Surface reaction pathways of methylamine on the Ni(111) surface

Abstract: The interaction and bond scission sequence of methylamine, CH3NH2, on Ni(111) have been investigated by means of Auger electron spectroscopy and temperature programmed desorption under UHV conditions in the temperature range 87–800 K. Comparisons have been made to the NH3/Ni(111) and N/Ni(111) systems. Methylamine is found to absorb molecularly through its lone pair up to ∼330 K after which a dehydrogenation channel opens which competes with the desorption channel. The sequence of the initial bond breaking was investigated by measuring the initial reactive sticking coefficient of deuterium labeled molecules at 363 K. Methylamine decomposition was found to take place through both ends of the molecule, initially with a slight rate preference at the C end. The CN residue left on the surface by the dehydrogenation process resulted in self‐poisoning of the active sites. This residual species was found to decompose at ∼530 K, leading to N2 desorption and the diffusion of carbon into the crystal at 700–800 K.

Transformation of lower alkanes into aromatic hydrocarbons over ZSM-5 zeolites

Abstract: The yields of aromatic hydrocarbons in the transformation of lower alkanes over ZSM-5 zeolites are greatly enhanced by incorporation of gallium or zinc cations into the zeolites. Effects of contact time on the product distribution in the conversion of hexane over H-ZSM-5 and Ga-ZSM-5 were examined in detail in order to obtain information on the reaction pathway. When contact time is short, only cracking of hexane prevails and when the time is extended, the olefins formed by cracking are converted into aromatics. It is concluded that gallium cations do not affect the capacity for hexane cracking, but they enhance the activity and efficacy for aromatization of olefins. Aromatization of propylene proceeds much more effectively over Zn-ZSM-5 than over H-ZSM-5. The lower yield of propane over Zn-ZSM-5 indicates that zinc cations serve as a catalyst for dehydrogenation of olefinic species. While Zn-borosilicate is not effective for promoting propane activation, it is effective for promoting propylene aromatization. It gives high benzene selectivity at low conversion level, indicating the intermediary of allylic species in the aromatization reaction.

Reaction of niobium clusters with benzene-h6 and -d6: evidence for cluster-induced dehydrogenation

Abstract: The gas phase reaction of niobium clusters with benzene-h/sub 6/ and -d/sub 6/ has been studied by using a fast-flow reactor. The products obtained not only depend dramatically on both cluster size and benzene isotope but also depend on the photon energy and fluence used for photoionization detection. With three notable exceptions, the major reaction channel observed with low-fluence (< 80..mu..J/cm/sup 2/), single photon 6.42-eV ionization is molecular addition to the metal clusters. Clusters containing 5, 6, and 11 niobium atoms, however, also give rise to a significant yield of products exhibiting loss of hydrogen for the reaction with benzene-h/sub 6/, but not for that with benzene-d/sub 6/. This kinetic isotope effect is primarily attributed to C-H(D) bond activation but may reflect some aspects of hydrogen desorption from the cluster. These experiments suggest that substantial dehydrogenation of benzene occurs for specific size clusters in the fast-flow reactor. Depending on experimental conditions, the hydrogen (deuterium) may or may not remain bound to the clusters. At high ionizing laser intensities, further desorption of hydrogen is induced by the ionization process, except for complexes containing less than three niobium atoms.

Boron-promoted reducible metal oxides and methods of their use

Abstract: Compositions comprising boron-promoted reducible metal oxides (especially reducible oxides of Mn) and optionally containing alkali and alkaline earth metal components are disclosed, as well as use thereof for hydrocarbon conversions characterized by formation of by-­product water. Particular processes comprise the conversion of methane to higher hydrocarbons and the dehydrogenation of dehydrogenatable hydrocarbons, e.g., dehydrogenation of C₂-C₅ alkanes to form the correspond­ing olefins.

Investigation of the mechanism of photocatalytic alcohol dehydrogenation over Pt/TiO2 using poisons and labelled ethanol

Abstract: Poisoning experiments using pyridine, piperidine, aqueous ammonia, phenol and 2-nitrophenol show that both acid and base sites are involved in the room-temperature photocatalytic dehydrogenation of alcohols (liquid propan-1-ol being chosen for the tests) over a Pt/TiO2 catalyst. The dominant role is played by the base sites, since an acid of the strength of 2-nitrophenol suffices to cancel dehydrogenation, whereas the basic substances only decrease its rate. An additional piece of information on the dehydrogenation mechanism is obtained from the use of gaseous 2,2,2-[2H3] ethanol at 263 K which provides evidence that the β-deuterium atoms are not abstracted, whereas at 295 K an isotopic exchange between the products, H2and 2,2,2-[2H3]ethanal, subsequently occurs.

Photocatalytic dehydrogenation of liquid alcohols by platinized anatase

Abstract: The photocatalytic dehydrogenation of liquid methanol, ethanol, propan-1-ol and propan-2-ol has been investigated using platinized anatase and 366 nm u.v. radiation over the range 278–303 K. Activities and activation energies for carbonyl-compound formation were effectively identical for the four alcohols on Pt(0.5)/TiO2 prepared by photodeposition. The activation energy of 20 kJ mol–1 is associated with photoelectron transport through the anatase to the Pt particles. With Pt(0.5)/TiO2 prepared by impregnation and H2reduction, identical activities for the four alcohols were achieved after O2 treatment. From the effect of u.v. intensity on the rate of propanone formation at 293 K, the limiting quantum yield was found to be 0.45 for photodeposited Pt(0.5)/TiO2 under N2 and 0.82 for support anatase under O2. Arrhenius plots for reaction at reduced u.v. intensities showed that the activation energy fell to zero when these quantum yields were achieved. Different mechanisms follow from the manner in which photoholes are surface trapped; two mechanisms for dehydrogenation with a limiting quantum yield of 0.5 are discussed.

Electrocatalytic oxidative dehydrogenation of saturated hydrocarbons to unsaturated hydrocarbons

Abstract: An electrocatalytic process is described for producing unsaturated hydrocarbon compounds from saturated hydrocarbon compounds The process is conducted in an electrogenerative cell and generally comprises the steps of (A) providing a fuel cell comprising a solid electrolyte having a first surface coated with conductive metal, metal oxide or mixtures thereof capable of catalyzing the reduction of oxygen to oxygen ions, and a second surface coated with metal, metal oxide or mixtures thereof, the two conductive coatings being connected by an external circuit, (B) passing an oxygen-containing gas in contact with the first conductive coating while, (C) passing the saturated hydrocarbon compound-containing feed gas in contact with the second conductive coating, and (D) recovering unsaturated hydrocarbons In a preferred embodiment, the conductive metal coating on the first and second surfaces comprises a silver-containing metal composition In another preferred embodiment, the invention comprises a continuous process for the oxidative dehydrogenation of ethane to ethylene and acetylene while generating electricity in an electrogenerative reactor