Alkenes dehydrocyclization

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. 

In the dehydrocyclization of alkanes it is clear that ring closure can take place both in a metal-catalyzed reaction and as a carbocationic process. The interpretation of the reforming process proposed by Heinemann and coworkers,123 therefore, is not a complete picture of the chemistry taking place. The scheme they presented (Fig. 2.1) attributes cyclization activity solely to acidic sites. The ample evidence available since requires that metal-catalyzed C5 and C6 ring-closure possibilities be included in a comprehensive interpretation. Additionally, the metal component plays and important role in carbocationic reactions in that it generates carbocations through the formation of alkenes. 

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. 

Dehydrocyclizatlon Pt(0) can be used for dehydrogenation of alkanes to alkenes. Ti(0) is known to absorb H2 to form a dihydride. Paquette et al.1 reasoned that a combination of the two metals could in principal effect dehydrocyclization. Indeed, cyclooctane when heated with Pt(0) and Ti(0) (1 1) adsorbed in A1203 is converted into bicyclo[3.3.0]octane (equation I). 

The activity of OH groups on the alumina surface can be markedly enhanced by the proximity of Cl ions. In the commercial catalysts, 7-alumina is treated with HCl to make it a highly active catalyst. 7-Alumina cannot readily catalyze the skeletal isomerization of alkenes because of its weak acidity. On the other hand, chlorinated alumina is highly active for skeletal isomerization and other strong-acid catalyzed reactions which are desirable in reforming. The strength of acid sites can be controlled by the extent of chlorination. If the Cl content is too low, the reactions which occur on the acid centers slow down and the octane number of reformate drops. If excess Cl ion is present, the extent of hydrocracking increases relative to dehydrocyclization. 

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