Dehydrocyclization pathways

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. 

It will be clear from the results so far presented that both C5 and C dehydrocyclization products can be formed, with aromatization proceeding (one would expect) by further dehydrogenation of the initially formed C6 ring-closure species. There is another pathway for the production of aromatics based upon cyclization of a linear triene (133), but this is of relatively small importance, and is only significant at all at quite high temperatures and low hydrogen partial pressures. 

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... 

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. 

Commercial reforming catalysts have both metal and acid sites. Both could contribute to cyclization. If there are four or more carbon atoms in the side chain of a mono-alkylaromatic or ortho-substituted dialkylaromatic hydrocarbon, cyclization can yield either five- or six-membered rings. This multiplicity of reaction pathways helps to clarify the roles of the metal and acid components in dehydrocyclization and other reactions. 

The rate of dehydrocyclization increase with the number of aromatic rings in the molecule (19). The dehydrocyclization of alkylnaphthalenes can follow the same pathways as the cyclization of alkylbenzenes C5-dehydro-cyclization gives benzindans and benzindenes, while C6-dehydrocyclization yields anthracenes and phenanthrenes. In addition to these two pathways, a-substituted alkylnaphthalenes can cyclize to acenaphthenes and acenaphthylenes ... 

There are at least two C6-dehydrocyclization mechanisms one of these proceeds through arylalkene intermediates and corresponds to the hexatriene-type C6-dehydrocyclization of paraffins. The other pathway is direct ring closure. It is probably related to C5-dehydrocyclization. 2-Butylnaphthalene may differentiate between the two mechanisms phenanthrene is probably formed by the first reaction, anthracene by the second. 

In some cases, a complete estimation of the relative contributions of the various pathways of cyclic and bond shift types requires the simultaneous use of a number of C-labeled molecules. Thus far the most complicated example is the isomerization of 3-methylhexane. This molecule, which dehydrocyclizes in three different ways, to 1,2-dimethylcyclopentane, 1,3-dimethyIcyclo-pentane, and ethylcyclopentane (Scheme 17), may isomerize by 23 different pathways, consisting of both cyclic and bond shift types. In particular, four parallel pathways account for n-heptane, and five for self-isomerized 3-methylhexane (4J). Therefore, even when using all the possible labeled molecules, one cannot distinguish between all the isomerization pathways, since the complete location of C in 3-methylhexane cannot be completely achieved. 

Indeed the carbene-alkyl insertion mechanism in Scheme 45 neatly explains why the rates of dehydrocyclization of 1, 2, and 3 are so similar. However, since 2-methylhexane also undergoes 1-5 dehydrocyclization, involvement of methylenic carbon atoms and not simply terminal carbon atoms must also be possible. The pathway for the C7-alkanes must be the reverse of nonselective hydrogenolysis of methylcyclopentane (Mechanism A), since it also results in isomerization to 2,4-dimethylpentane and 3-methylhexane, most likely via adsorbed 1,3-dimethylcyclopentane (scheme 46). It is... 

Metallocarbene formation by hydrogen shift explains the observed selectivity in the 1,5-dehydrocyclization of 3-methylhexane on Pt/AljOj (41). Three cyclic intermediates may be formed from this molecule, 1,2-dimethylcyclopentane (4), 1,3-dimethylcylopentane (5), and ethylcyclopentane (6). By using several selectively C-labeled 3-methylhexanes, the contribution of each parallel pathway both in cyclic type isomerization and in dehydro-cylization to gaseous cyclic molecules was determined. Relative rates of 3 2 1 were observed for 1-5, 2-6, and 6-7 ring closure (giving 5, 4, and 6, respectively) (Scheme 49 and Table VII), whatever the dispersion of the platinum (2-10%) and the temperature (32O°-38O°C). 

---