C6 dehydrocyclization

Platinum catalyzes at least two types of C6- dehydrocyclization, one of which involves olefinic intermediates (13, 28, 29). In the case of paraffins, this latter reaction involves the ring-closure of hexatrienes (30, 31). In the C6-dehydrocyclization of n-butylbenzene and n-pentylbenzene, phenyl-butadiene and phenylpentadiene could correspond to these triene intermediates (13, 14). The second C6-dehydrocyclization mechanism is similar to C5-dehydrocyclization, and may not involve olefinic intermediates. 

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 ... 

No investigator has observed C6-dehydrocyclization involving the pen-carbon atom of naphthalene. Phenalene and 2,3-dehydrophenalene have not been detected over any of the catalysts investigated. 

The C5-dehydrocyclization of 2-w-butylnaphthalene can give 4,5- or 5,6-methylbenzindans and benzindenes. Similarly, C6-dehydrocyclization can yield either phenanthrene or anthracene. The product distribution depends on reaction temperature and catalyst type. The C5-dehydrocyclization of l-(2-naphthyl)-butene depends on the acidity of the alumina, but... 

C6-dehydrocyclization does not (50). As a consequence, over nonacidic platinum catalysts above 400°C, C6-dehydrocyclization predominates over C5-dehydrocyclization (27). Furthermore, the phenanthrene/anthracene ratio is independent of catalyst acidity (52). The effect of reaction temperature is, however, very interesting. Over platinum-on-carbon catalyst between 350°C and 400°C, more anthracene is formed than phenanthrene. Above 450°C phenanthrene is the main product (55). Phenanthrene is also the main product over chromia-alumina between 360°C and 440°C whereas, as seen above, anthracene is formed in this temperature range over platinum-on-carbon catalyst (54). 

If this is true, the simultaneous formation of anthracene and phenanthrene from 2-n-butylnaphthalene gives us an extraordinary and fortunate opportunity to differentiate between two types of C6-dehydrocyclization (27). Anthracene might be the product of direct cyclization, a mechanism related... 

Erivanskaya and co-workers also studied the dehydrocyclization of 2-n-butylnaphthalene over supported palladium, rhodium, and iridium catalysts (56-55). Palladium-alumina showed the lowest C6-dehydrocyclization activity, but was the most active for the C5-dehydrocyclization of 2-n-butyl-naphthalene. A later study showed, however, that this enhanced activity was due to the high chlorine content of the palladium-alumina catalyst and not to some mysterious inherent catalytic activity of palladium (56). 

Platinum can catalyze C5- and C6-dehydrocyclizations. The two reactions are parallel. Interconversion is very limited between five- and six-membered ring products over nonacidic platinum catalysts. Platinum-catalyzed dehydrocyclization does not involve carbonium ions. 

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. 

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. 

Starting from C5 molecules, dehydrocyclization (into cyclopentane and derivatives of cyclopentane) is also possible. From C6 on up, aromatization also occurs. These two reactions comprising a dehydrogenation step are only observable at temperatures which on most metals are higher than the region where hydrogenolysis (hydrocracking) is first observed. 

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. 

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... 

Data on rates of dehydrocyclization rD and cracking rc of n-heptane at 495°C and 14.6 atm are given in Table 5.2 for platinum-iridium on alumina and platinum-rhenium on alumina catalysts, and also for catalysts containing platinum or iridium alone on alumina (33). The rate rD refers to the rate of production of toluene and C7 cycloalkanes, the latter consisting primarily of methylcyclohexane and dimethylcyclopentanes. The rate of cracking is the rate of conversion of n-heptane to C6 and lower carbon number alkanes. 

The reduction in the formation of C9+ aromatics in presence of Zn/HZSM-5 can be explained by the reaction path way Km2 provided by zinc (Fig. 7). Once C6-C8 oligomers are formed, these are converted to Ce-Cg aromatics by dehydrocyclization reaction (Km2) at a rate faster than cyclization and hydrogen transfer reactions (Ka2), thus suppressing oligomerization to continue beyond Cs level through Kas which, in turn, reduces the formation of C9+ aromatics in the product. 

Catalyst Support Alumina. The support used exclusively in commercial naphtha reforming catalysts is alumina. As previously mentioned, the Pt/KL-zeolite might be used for dehydrocyclization of linear C6-C8 paraffins, but not for straight-run naphthas reforming. 

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