Dehydrocyclization of alkylaromatics

The dehydrocyclization of alkylaromatics was first described more than four decades ago. In 1936 Moldavskii and Kamusher reported the formation of naphthalene from u-butylbenzene on chromia at 475°C (7, 2). In 1945 Herington and Rideal reported the formation of indene from w-propyl-benzene over chromia-alumina (3). Platinum-containing catalysts were first used for these reactions in 1956 by Kazanskii and co-workers 4-6). 

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... 

Monoalkylaromatics and or/Ao-substituted dialkylaromatics containing at least three carbon atoms in the side chains can easily undergo dehydrocyclization to form an additional ring. One can classify cyclizations of alkylaromatics in many different ways. 

Bragin and co-workers found that over platinum-on-carbon catalysts, both paraffins and alkylaromatics follow zero-order kinetics. Activation energy for C5-dehydrocyclization in which the new bond is formed between two sp3 hybridized atoms is substantially less than the activation energy of cyclization in which the new bond is formed between one sp3 hybridized atom and the sp2 hybridized carbon atom of the aromatic ring. Over one batch of platinum-on-carbon catalyst, Bragin and co-workers obtained 20 kcal/mol and 27.5 kcal/mol activation energies for the dehydrocyclization of paraffins and monoalkylbenzenes, respectively (6). Another batch of platinum on carbon (which differed only in some minor details of preparation from the first batch), gave 14 kcal/mol for the cyclization of l-methyl-2-ethylbenzene and isooctane, and 29 kcal/mol for the cyclization of secondary butylbenzene ( ) (Fig. 1). 

Platinum-catalyzed cyclization of alkylaromatics is faster than the cyclization of paraffins because the presence of the aromatic ring enhances the rate. The rate of dehydrocyclization further increases with the number of aromatic rings in the feed molecule. A comparative study of the de-... 

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. 

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