Dehydrocyclization Of Alkanes Research Articles

Fullerene black: Structure, properties and possible applications

This review concerns the fullerene black, a poorly known nano-sized carbon material, the insoluble residue after extraction of fullerenes from fullerene soot obtained by arc vaporization of carbon material (usually graphite) in a helium atmosphere. This by-product of the production of fullerenes, whose yield reaches 80 wt %, is a finely dispersed material with a particle size of 40–50 nm. It includes amorphous carbon, graphitized particles, and graphite. Test reactions showed the presence in the structure of fullerene black of curved surfaces, and, like fullerenes, of alternating nonconjugated ordinary and double bonds. In addition to the double bonds, its structure includes dangling bonds in the concentration not higher than one per 1200 carbon atoms. Fullerene black absorbs oxygen from the atmosphere and water, and enters into the reactions of nucleophilic addition. The fullerene black cannot be graphitized, and its application is extremely important. The fullerene black was shown to activate hydrogen and thus to undergo a hydrogenolysis in the absence of a catalyst and to catalyze the dehydrogenation and dehydrocyclization of alkanes. This carbon nanomaterial can be used as a sorbent and a catalyst carrier, a tribotechnical additive; it can interact with carbide-forming metals and harden their surface.

Structure and Properties of Cobalt-Exchanged H-ZSM5 Catalysts for Dehydrogenation and Dehydrocyclization of Alkanes

Co/H-ZSM5 catalysts with Co/Al ratios of 0.09−0.22 were prepared by aqueous exchange. Turnover rates for propane conversion to propene and to C6−C8 aromatics on these catalysts are about 10-fold higher than on H-ZSM5. The selectivities to propene, aromatics, and H2 are also higher on Co/H-ZSM5 than on H-ZSM5. The rate of D2 exchange with OH groups increases with increasing Co/Al ratio, suggesting that Co cations catalyze D2 dissociative chemisorption steps that limit the rate of isotopic exchange. Co cations also catalyze hydrogen recombinative desorption steps, which limit the rate of propane dehydrogenation and aromatization reactions. The density of residual zeolitic hydroxyls was measured by D2−OH isotopic exchange and by changes in the intensity of OH infrared bands as a function of Co content. D2−OH and infrared measurements showed that Co2+ cations replace 1.1−1.3 zeolitic protons, suggesting the predominant presence of Co2+−O−Co2+ dimers, with some Co2+ monomers, each bridging two next-nearest nei...

Alkane dehydrocyclization mechanism

A review of results impacting the dehydrocyclization mechanisms for monofunctional catalysts proposed to date has been made. The data indicate that, while alkane conversions at low temperatures (about 300°C or less) may involve reversible adsorption, irreversible adsorption of the alkane is the rate determining step at higher temperatures representative of naphtha reforming. The extent of irreversible adsorption depends upon hydrogen partial pressure, with alkane adsorption being irreversible near atmospheric pressure and gradually changing to reversible adsorption at naphtha reforming pressure. At all pressures, primary aromatic products are formed by a mechanism involving direct six-carbon ring formation. At low pressures, the cyclopentane cyclization product yields carbon on catalyst and cracked products. At higher pressures, the cyclopentane structures undergo nonselective hydrogenolysis to produce isomers of the alkane feed by a monofunctional metal catalyzed pathway. A common cyclization pathway is proposed for the formation of C 5- and C 6-ring structures. With bifunctional catalysts, the acid catalyzed cyclization is more rapid than the monofunctional metal cyclization.

Dehydrocyclization ofn-Octane: Role of Alkene Intermediates in the Reaction Mechanism

The objective of the present study is to define whether alkenes, formed during alkane dehydrocyclization, are reconverted to the alkane and to elucidate the role of alkanes in the dehydrocyclization mechanism. In a competitive conversion, an alkene is converted much more rapidly thann-octane (C8D18). The lack of H/D exchange indicates that the adsorption of then-octane is essentially irreversible, whereas some H/D exchange in the added alkene suggests that alkene desorption does occur; however, the added alkene does not desorb as the alkane, in conformity with irreversible adsorption of the alkane. The results demonstrate that the addition of a labeled alkene cannot provide data to establish the validity of the sequential dehydrogenation reaction pathway.

Dehydrocyclization of Alkanes Over Zeolite-Supported Metal Catalysts: Monofunctional or Bifunctional Route

The direct catalytic conversion of alkanes into aromatics has found potentially important industrial applications. Initially only alkanes with 6 and more carbon atoms in the chain were concerned. Supported platinum catalysts were found active for the aromatization of alkanes; the drawbacks of these catalysts were their deactivation with time on stream and the existence of simultaneous parallel reactions. Much discussion has been published on the aromatization of C6+ alkanes. A bifunctional mechanism which involves both the metal and the acid sites of the support and a monofunctional mechanism involving only the metallic sites operate over, respectively, Pt supported on acidic support and Pt supported on nonacidic support. In the present review the mechanisms proposed for the aromatization of alkanes are described. Over monofunctional Pt catalysts two possible mechanisms prevail: 1,6 ring closure on the Pt surface involving primary and secondary C-H bond rupture, followed by dehydrogenation of the cycloalkanes into aromatics (1,5 ring closure to a lesser extent also contributes to aromatic production); or dehydrogenation of the alkanes into olefins, dienes, and trienes followed by thermal ring closure. Zeolites were found most suitable as support for preparing catalysts more active and more selective in the alkane aromatization. In addition catalysts based on noble metals supported on zeolite appeared more resistant against deactivation by coke. In this review the aromatization of hexane, heptane, and octane over Pt-zeolite catalysts is discussed in detail. Comparisons between different zeolite structures and different dehydrogenation sites are given. In particular a critical analysis of the results and interpretation concerning Pt-KL catalysts strongly suggests that the exceptional high selectivity towards aromatization of n-hexane exhibited by Pt-KL could not be explained by only the nest or constraint effect exerted by the channel dimension and morphology, not by only the terminal cracking properties, not by only the partial electron transfer from the zeolite support to the Pt particles, and not by only the Pt particle size. Zeolite structure also affects the aromatic product distribution, in particular when the alkane contains more than 7 carbon atoms. It is shown how Pt on medium-pore zeolites such as In-ZSM-5, silicalites will favor the aromatization of C8 alkane isomers into ethylbenzene-styrene with respect to other C8 aromatics. Aromatization of light alkanes, C2-C5, requires the increase of the hydrocarbon chain length up to 6 carbon atoms and higher, followed by cyclization reaction. Recently new processes to convert C2-C5 alkanes into aromatics have been disclosed, M2-forming from Mobil, Cyclar from BP-UOP, and Aroforming from IFP-Saluted. In general these processes use bifunctional catalysts possessing a dehydrogenating and an acid function. The catalysts consist of a metal ion or metal oxide supported on a microporous acid solid. In this review we analyze the results concerning mainly platinum supported on pentasil-type zeolite. It is shown that althoug Pt has better dehydrogenating properties as compared with gallium and zinc, the efficiency of catalysts based on Pt-ZSM-5 for light alkane aromatization is less because undersirable reactions such as hydrogenolysis and ethene (olefins) hydrogenation occur on the platinum surface, resulting in the production of unreactive alkanes, CH2, C2H6. These drawbacks could be partially suppressed by alloying Pt and by increasing the reaction temperature.

Reaction Pathway for Alkane Dehydrocyclization

Naphtha reforming to produce high octane gasoline is an important process. Many reaction mechanisms are involved in this process. For example, the study of the fundamentals of this process led to the concept of bi- or poly-functional catalysis. The results of this study provide additional mechanistic information about the dehydrocyclization of an n-alkane to produce aromatics. The reaction coordinate diagram advanced to account for the observation of irreversible adsorption should be modified to account for the present results. 32 refs., 1 fig.

ChemInform Abstract: Platinum‐Tin‐Alumina Catalysts. Comparison of Alkane Dehydrocyclization Activity and Characterization Data

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Catalyt properties of tungsten oxycarbide and carbide in hydrocarbon conversion

AbstractTungsten oxycarbide with specific surface area ⋍ 24 m2/g was prepared from white tungstic acid by reduction and carburization with CO and H2. XRD and EXAFS measurements confirmed that the WOC obtained has a simple hexagonal structure isotypic with α‐WC and the oxygen is inserted into the trigonal prismatic interstices of the crystal lattice. The reactions of n‐hexane, n‐heptane and cyclohexane on WOC were studied and compared with WC prepared by direct synthesis at high temperature. WOC and WC are active for the hydrogenolysis, isomerization and dehydrocyclization of alkanes and dehydrogenation of cyclohexane and their catalytic properties are in many ways similar to Pt catalysts. Oxygen insertion inhibits the catalytic activities of tungsten carbide for hydrocarbon conversion reactions probably due to electronic effect.

Isomerization and hydrogenolysis of trimethylbenzenes with platinum on acidic and nonacidic supports

For commercial high-pressure naphtha reforming conditions, platinum has a similar demethylation activity when supported on acidic or nonacidic alumina. Platinum on nonacidic alumina exhibits a low trimethylbenzene isomerization activity under conditions where platinum on an acidic support produces an equilibrium mixture. The xylene isomers produced by demethylation are consistent with a demethylation mechanism without isomerization. The relative rates of demethylation and isomerization of 1,3,5-trimethylbenzene should be a method of measuring the hydrogenation and acidic functions of bifunctional catalysts. The results also demonstrate that the aromatics formed by alkane dehydrocyclization under reforming conditions do not extensively isomerize when a nonacidic support is used.

Role of hydrogen in transformations of alkanes and olefins on platinized carbon

1. Hydrogen has to be present for the C5 dehydrocyclization and the skeletal isomerization of alkanes, and also the C5 cyclization of olefins, on Pt/C. 2. An interrelationship was established between the reactions for the formation of a five-membered ring from alkanes and olefins and their skeletal isomerization in the presence of H2; it is postulated that the hydrogen enters into the composition of an associative transition complex. 3. The formation of cyclopentanes on Pt/C proceeds via the direct dehydrocyclization of alkanes; cyclization with the formation of intermediate unsaturated compounts is also possible.

Paraffin dehydrocyclization: VI. The influence of metal and gaseous promoters on the aromatic selectivity

The addition of tin to a PtAl 2O 3 catalyst greatly enhanced the activity and/or decreased the rate of deactivation for the dehydrocyclization of alkanes. There appeared to be an optimum ratio of Pt:Sn = 1:4 for a 0.6% Pt catalyst. The addition of Sn, Ag, or Cu to a RhAl 2O 3 catalyst had a similar effect with Sn showing the greatest improvement. The addition of tin altered the ethylbenzene (EB): ortho-xylene (OX) ratio from 1:1 for pure 0.6% Pt to EB:OX = 1:2 for the PtSn (1:4) catalyst. The addition of thiophene and the dehydrocyclization of the octenes or octynes also changed the EB:OX ratio to favor the OX in a similar manner. The addition of tin or thiophene greatly decreased the second cyclization to the bicyclic aromatic; e.g., the addition of tin produced a tenfold decrease in the indan (bicyclic)/total aromatic ratio. The results are interpreted to be consistent with a selectivity that is determined by an electronic factor.