Hexanes dehydrocyclization

Pt-Sn Surface reactions of SnR4 with Pt/Al203, and by Sn halide reduction by Li/H2 on A1203 / - / - Hexane dehydrocycl- ization... 

In a final paper (737) in this series the novel TeNaX catalyst was compared with Cr203/Al203 catalyst for -hexane dehydrocyclization activity. Both... 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

TABLE 7.39 Kinetic Parameters for n-Hexane Dehydrocyclization Over Metal Catalysts... 

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

When the reactions of alkane molecules larger than the butanes or neopentane are studied, and in particular when the molecule is large enough to form a Cs or a Ce ring, the complexity of the reaction pathway is considerably increased and an important feature is the occurrence, in addition to isomerization product, of important amounts of cyclic reaction products, particularly methylcyclopentane, formed by dehydrocycliza-tion this suggests the existence of adsorbed cyclic species. The question is whether the reaction paths for dehydrocyclization and isomerization are related. There is convincing evidence that they are. Skeletal interconversions involving n-hexane, 2- and 3-methylpentane may be represented. 

Fig. 12. Variation with average platinum particle diameter of the initial rate of reaction (isomerization plus dehydrocyclization) of n-hexane (- -) and 2-methylpentane (-O-) over ultrathin film catalysts at 275°C. Hydrogen/reactant hydrocarbon, 10/1 total reactant pressure 100 Torr.

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

Fig. 4 (a) Stepwise dehydrocyclization of -hexane (21, 62). (b) Temperature programmed desorption of benzene originating from various adsorbates over Pt-AljOs. Temperature of adsorption 25°C. Rate of heating 23°C per minute. Detector monopolar mass spectrometer, the ordinate corresponds to the I intensity of mass number 78, in arbitrary units. For clarity, the thermodesorption curves for other compounds (starting hydrocarbon) hexene from hexa-dienes and hydrogen have not been shown (62c). 

A comparison of the cyclization rates of alkanes and alkenes may help to distinguish between associative and dissociative ring closure mechanisms, just as in the case of Cg dehydrocyclization of hexane and hexenes. 

Fig. 10. Selectivities in hexane conversions versus temperature for benzene formation (Be), hydrogenolysis (Hy), methylcyclopentane formation (MCP), isomerization (ISOM), and dehydrocyclization (Dehy) (9 wt. % Pt on inert Si02).

Additional evidence to this scheme was reported applying temporal analysis of products. This technique allows the direct determination of the reaction mechanism over each catalyst. Aromatization of n-hexane was studied on Pt, Pt—Re, and Pd catalysts on various nonacidic supports, and a monofunctional aromatization pathway was established.312 Specifically, linear hydrocarbons undergo rapid dehydrogenation to unsaturated species, that is, alkenes and dienes, which is then followed by a slow 1,6-cyclization step. Cyclohexane was excluded as possible intermediate in the dehydrocyclization network. 

Dehydrocyclization of n-hexane to form benzene has been a subject of considerable academic and industrial interest since Bernard first reported that platinum clusters supported inside the channels of zeolite L catalyze the reaction with exceptional activity and selectivity (7). The nonacidic nature of the Pt-zeolite L catalyst and correlation of reaction rate with Pt content are consistent with the accepted view that the catalyst is monofunctional, depending solely on Pt metal for catalytic activity (7). However, comparison of aromatization reactivity over nonacidic Pt-zeolites to conventional non-zeolitic catalysts revealed that additional factors contribute to the unusual performance of Pt-zeolites (2). 

FIG. 13. Common intermediate for dehydrocyclization and isomerization of n-hexane and hydrogenolysis of methylcyclopentane (61). 

VI. Lebedeva and V.M. Gryaznov, Effect of hydrogen removing through the membrane catalyst on dehydrocyclization of n-hexane, Izv. Akad. Nauk SSSR, Ser. khim, No. 3, 611 (1981). 

In this same series of reactions it was also found that C-C bond cleavage took place most readily over the 111 face catalyst (A in Fig. 3.2) and to a considerably less extent on the others. This reaction, which is stmcture sensitive, obviously teikes place over ensembles of face atoms. It was also found > 8 that the dehydrocyclization of hexane to benzene, another structure sensitive reaction, took place four times faster on the 111 face of a platinum crystal than on one cleaved to expose the 100 face. The hydrogenolysis of ethane, on the other hand, was found to be significantly faster on Ni (100) than on Ni (111).22 Thus, it is not sufficient merely to define a site as an ensemble of face atoms the orientation of the atoms must also be specified. 

Fig. 3 shows a simplified scheme of the bifiinctional paths proposed by several authors [4,5] for interpreting the n-hexane reforming reaction. The reaction network includes (a) the isomerization and the dehydrocyclization of n-C6 to i-C6 and MCP, respectively, through a... 

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