Showing papers on "Hydrocarbon published in 1993"

Dehydrogenation and aromatization of methane under non-oxidizing conditions

Abstract: The dehydrogenation and aromatization of methane on modified ZSM-5 zeolite catalysts has been studied under non-oxidizing conditions with a fixed bed continuous-flow reactor and with a temperature programmed reactor. The results show that benzene is the only hydrocarbon product of the catalytic conversion of methane at high temperature (973 K). The catalytic activity of ZSM-5 is greatly improved by incorporating a metal cation (Mo or Zn). H2 and ethene have been directly detected in the products with a mass spectrometer during TPAR. A carbenium ion mechanism for the activation of methane is suggested.

A study of carbon deposition on catalysts during the partial oxidation of methane to synthesis gas

Abstract: The deposition of carbon on catalysts during the partial oxidation of methane to synthesis gas has been investigated and it has been found that the relative rate of carbon deposition follows the order Ni>Pd>Rh>Ir. Methane decomposition was found to be the principal route for carbon formation over a supported nickel catalyst, and electron micrographs showed that both “whisker” and “encapsulate” forms of carbon are present on the catalyst. Negligible carbon deposition occurred on iridium catalysts, even after 200 h.

Effect of micellar solubilization on biodegradation rates of hydrocarbons

Abstract: Batch experiments were conducted with a strain of Pseudomonas aeuroginsa and a strain of Ochrobactrum anthropi, both Gram-negative bacteria, growing on aqueous solutions containing straight-chain hydrocarbons solubilized in small micelles (204 nm) of nonionic surfactants. Measurements of optical density, a quantity proportional to bacterial cell concentration, and hydrocarbon content were made as a function of time. Since no macroscopic hydrocarbon drops were present and therefore there was no opportunity for the bacteria to attach themselves to oil-water interfaces, the results provided unambiguous confirmation that solubilization greatly enhances rates of hydrocarbon degradation in these systems compared to rates observed with bulk liquid hydrocarbon in the absence of surfactants. Solubilization of n-decane and n-tetradecane in micelles reduced the times required for cell density to double during exponential growth by a factor of {approximately}5 for one bacterial strain compared to results obtained for surfactant-free experiments. The improvement was even greater for the other strain. Except for very low hydrocarbon concentrations, the Monod model for a single substrate was able to describe reasonably well the time dependence of both cell growth and hydrocarbon consumption for experiments with n-decane. 28 refs., 4 figs.

Collisions of protic and aprotic gases with hydrogen bonding and hydrocarbon liquids

Abstract: We explore collisions of Ne, CH4, NH3, and D2O with glycerol, a hydrogen bonding liquid, and with squalane, a liquid hydrocarbon. The experiments are carried out by directing a molecular beam at a continuously renewed liquid surface in vacuum and monitoring the identity and velocity of the scattered products with mass spectroscopy. We observe both direct inelastic scattering and trapping desorption when the gases strike the liquids. The polyatomic gases thermalize readily at low collision energies but rebound more frequently as the incident energy increases. We find that impulsive energy transfer is extensive and depends only weakly on the type of gas or liquid; for encounters leading to direct scattering, the gases appear to undergo hard spherelike collisions with the CH2 and CH3 groups of squalane and the OH and CH2 groups of glycerol. The gases accommodate differently on the two liquids, however: Neon and methane equilibrate more efficiently on squalane, ammonia thermalizes equally well on each liquid, and water is trapped more frequently by glycerol. The differences in trapping probabilities are smaller than expected from their solubilities, but they roughly follow trends in the free energies and enthalpies of solvation. Our results suggest that thermal accommodation in gas–liquid collisions reflects both the mechanical roughness and softness of hydrocarbons and the strong attractive forces between protic gases and the OH groups of glycerol.

Role of propene in the selective reduction of nitrogen monoxide in copper-exchanged zeolites

Abstract: Gas switching experiments, examining the first few minutes of reaction, have been used to study the selective reduction of nitrogen monoxide by propene on a Cu-ZSM-5 catalyst in an oxygen-rich gas mixture. It has been found that the conversion of nitrogen monoxide to nitrogen reaches a steady-state activity in a very short period of time. It is concluded that carbon deposition is not responsible for the conversion of nitrogen monoxide into nitrogen. It is proposed instead that the hydrocarbon and oxygen act to maintain the active copper sites in an oxidation state suitable for direct nitrogen monoxide decomposition.

Performance, selectivity, and mechanism in Cu-ZSM-5 lean-burn catalysts

Abstract: Performance of Cu-ZSM-5 materials as three-way control catalysts of emissions from advanced lean-burn automotive engines has been studied by laboratory methods. Control of NO x under thexe oxidizing conditions is poorly precedented and particularly difficult. Fresh Cu-ZSM-5 catalysts exhibit moderate NO x reduction performance over a broad temperature range. The nitric oxide is reduced by hydrocarbon species present in the simulated exhaust, not by carbon monoxide or hydrogen. Propene is more effective than propane. Catalyst compositions of maximum effectiveness for NO x reduction are different when propene or propane is used as the reducing agent. We suggest copper-alkyl species are formed by an oxidative activation and that these intermediates add nitric oxide to form N -nitroso- N -alkylhydroxylamate species bound to cupric centers. Such a mechanism would account for the known requirements of: (1) variability in catalytic effectiveness as the reducing agent varies; (2) the inability of carbon monoxide or hydrogen to engage in selective NO x reduction; and (3) the requirement of oxygen for selective NO x reduction.

Selective electrophilic activation of alkanes

Abstract: One major drawback for the direct use of alkanes as chemicals is their chemical inertness toward most of the usual reagents. For this reason, costly, large-scale refinery operations such as catalytic reforming and vapocracking are needed, which necessitate the use of noble metal catalysts and/or high temperature to activate the strong C-H and C-C bonds and yield the primary chemical building blocks such as ethylene, propylene, butadiene, benzene, toluene, and xylene. In response to the challenge to use more economically a larger share of alkanes, and increasing number of research groups are working on the activation functionalization processes of the saturated hydrocarbon C-H bond. The goal is to overcome smoothly and selectively the chemical inertness of the starting (paraffinic) material. A book and special issue have been published recently on this subject. Protonated alkanes are reaction intermediates in superacid-catalyzed protolysis. Protonation occurs on all C-C and C-H [sigma]-bonds independently of the further reactivity of the protonated alkane as confirmed by using [sup 2]H labeled superacids. Direct and selective C-H bond protolysis is limited to tertiary alkanes whereas linear alkanes undergo less selective C-C bond cleavage. 31 refs., 3 figs.

Process for the preparation of hydrocarbon fuels

Abstract: A process for the preparation of hydrocarbon fuels comprises the steps of: a) contacting a mixture of carbon monoxide and hydrogen with a hydrocarbon synthesis catalyst at elevated temperature and pressure to prepare a substantially paraffinic hydrocarbon product; b) contacting the hydrocarbon product so-obtained with hydrogen in the presence of a hydroconversion catalyst under conditions such that substantially no isomerisation or hydrocracking of the hydrocarbon product occurs; and c) contacting at least part of the hydrocarbon product of step (b) with hydrogen in the presence of a hydroconversion catalyst under conditions such that hydrocracking and isomerisation of the hydrocarbon feed occurs to yield a substantially paraffinic hydrocarbon fuel.

Modelling the chemistry of ozone, halogen compounds, and hydrocarbons in the arctic troposphere during spring

Abstract: The box model MoccaIce has been developed to study the chemistry of the arctic boundary layer. It treats chemical reactions in the gas phase and in the aerosol, as well as exchange between the 2 phases. Photolysis rates vary according to the solar declination during polar sunrise. Apart from the standard tropospheric chemistry of ozone, hydrocarbons, and nitrogen species, the reaction mechanism includes sulfur and the halogens Cl, Br, and I. Modeling an ozone depletion event, we found that iodine species contribute to the chemical destruction of ozone significantly if IO mixing ratios are about 1 pmol/mol. The reactions of BrO with BrO and IO are the main pathways of the ozone destruction cycle. Hydrocarbon concentrations decrease during ozone depletion events due to reaction with halogen atoms. The rate of ozone destruction depends on whether the addition of Br to C 2 H 4 and C 2 H 2 yields inert products or intermediates from which Br can be regenerated. Bromine and HCHO are positively correlated. The model produces HCHO during ozone depletion events, though not as much as reported from field observations. After the destruction of ozone has been competed, the halogen species are converted to halides and subsequently scavenged by aerosol particles. DOI: 10.1034/j.1600-0889.49.issue5.8.x

Catalytic Reduction of Nitrogen Monoxide by Methane over Palladium-Loaded Zeolites in the Presence of Oxygen

Abstract: Palladium ion-exchanged H-ZSM-5 showed a high catalytic activity for the removal of dilute nitrogen monoxide in the presence of methane and oxygen at 350–550 °C. The presence of palladium and (protonic) acidity is essential for the high activity.

The oxidative dehydrogenation of ethane over molybdenum-vanadium-niobium oxide catalysts: the role of catalyst composition

Abstract: The oxidative dehydrogenation of ethane has been studied at atmospheric pressure using molybdenum-vanadium-niobium oxide catalysts in the temperature range of 350–450 °C. The presence of all three oxides together is necessary in order to have active and selective catalysts. The best results have been obtained using a mixture having a Mo ∶ V ∶ Nb ratio of 19 ∶ 5 ∶ 1. Our studies of the variation of oxide composition suggest that the active phase is based on molybdenum and vanadium. Niobium enhances the intrinsic activity of the molybdenum-vanadium combination and improves the selectivity by inhibiting the total oxidation of ethane to carbon dioxide. The apparent activation energies for the conversion of ethane to ethylene, carbon monoxide and carbon dioxide were 18, 27 and 17 kcal/mol, respectively. The addition of water vapor to the gas stream does not affect the product distribution on this catalyst.

NOx emission control in oxygen-rich exhaust through selective catalytic reduction by hydrocarbon

Abstract: Selective catalytic reduction of nitrogen monoxide by hydrocarbon in the presence of oxygen and sulphur dioxide has very recently been found on copper ion-exchanged ZSM-5 zeolite (Cu-ZSM-5). The reaction proceeds even in the oxidizing atmosphere at temperatures as low as 573 K. Addition of oxygen to the reactant stream is necessary to achieve the selective reduction of nitrogen monoxide. The increment of concentration of hydrocarbon increased the conversion into nitrogen and expanded the active temperature region. After summarizing the new findings on Cu-ZSM-5, the present review introduces catalytic activities of various metal-ion exchanged zeolites and metal-loaded aluminas. Based on the temperature dependence of each catalyst and the change of conversion of nitrogen monoxide into nitrogen with space velocity, Cu-ZSM-5 is suggested as one of the strong candidates for catalysts applicable to lean-burn gasoline and diesel engines, though several problems such as durability to water and overheating remain ...

Structure H hydrate phase equilibria of methane + liquid hydrocarbon mixtures

Abstract: The first phase equilibrium data are reported for mixtures of heavier liquid hydrocarbons+methane forming structure H hydrates. Four-phase equilibrium conditions (water, liquid hydrocarbon, vapor, and structure H hydrate) were measured for three systems: methane+2,2-dimethylbutane, methane+2-methylbutane, and methane+methylcyclohexane. These hydrocarbons constitute a small fraction of crude of reservoirs where they coexist with various gases, including methane. Consequently this work suggests the possibility of the natural occurrence of structure H hydrates in such natural and artificial environments, and these initial phase equilibrium data have the potential to impact industrial perspectives about hydrates

Relative rates of coke formation from hydrocarbons in steam cracking of naphtha .3. aromatic-hydrocarbons.

Abstract: Relative rate constants of coke formation (k) from 18 aromatic hydrocarbons during steam cracking of naphtha at 810 C were determined by application of [sup 14]C-labeled compounds. Benzene is a poor coke precursor (k = 0.3), whereas polycyclic structures like acenaphthylene, anthracene, and chrysene have a high coking potential in the pyrolysis reactor (k = 4.5--6) as well as in the TLE section (k = 12--30). The relation between structure and coke formation rate of aromatic hydrocarbons can be interpreted on the basis of their reactivity in radical reactions. Constituents of the fuel fraction ([ge] C[sub 9]) derived from nonaromatic feed components are more efficient in the TLE fouling than those stemming from benzene derivatives.

Hydrogen transfer and activation of propane and methane on ZSM5-based catalysts

Abstract: Hydrogen exchange between undeuterated and perdeuterated light alkanes (CD4-C3H8, C3D8-C3H8) occurs on H-ZSM5 and on Ga- and Zn-exchanged H-ZSM5 at 773 K. Alkane conversion to aromatics occurs much more slowly because it is limited by rate of disposal of H-atoms formed in C-H scission steps and not by C-H bond activation. Kinetic coupling of these C-H activation steps with hydrogen transfer to acceptor sites (Ga n+, Zn m+) and ultimately to stoichiometric hydrogen acceptors (H+, CO2,O2, CO) often increases alkane activation rates and the selectivity to unsaturated products. Reactions of13 CH4/C3H8 mixtures at 773 K lead only to unlabelled alkane, alkene, and aromatic products, even though exchange between CD4 and C3H8 occurs at these reaction conditions. This suggests that the non-oxidative conversion of CH4 to higher hydrocarbons on solid acids is limited by elementary steps that occur after the initial activation of C-H bonds.

Catalyst components for polymerization of olefins and use thereof

Abstract: Disclosed is a catalyst component for the polymerization of olefins, which is a compound having the following structure: ##STR1## wherein R 1 is a hydrocarbon radical having 1 to 10 carbon atoms or a halogen-containing hydrocarbon radical having 1 to 10 carbon atoms, and R 2 s are each a hydrogen atom, a halogen atom, a siloxy group, a lower-alkyl-substituted siloxy group or a hydrocarbon radical having 1 to 10 carbon atoms, or which is a reaction product of the following sub-components (i) and (ii): sub-component (i) which is a compound having the formula R.sup.1 --B--(OH).sub.2 in which R 1 is a hydrocarbon radical having 1 to 10 carbon atoms or a halogen-containing hydrocarbon radical having 1 to 10 carbon atoms; and sub-component (ii) which is an organoaluminum compound.

Aerobic oxidations of alkanes and alkenes in the presence of aldehydes catalysed by copper salts

Abstract: Oxidation of alkanes to the corresponding alcohols and ketones and epoxidation of alkenes can be performed efficiently at room temperature with molecular oxygen (1 atm) in the presence of an aldehyde and a copper salt catalyst such as Cu(OH)2.