Dehydrocyclization of n-hexane

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 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). 

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 the reaction scheme in Figure 5.2, the dehydrocyclization of n-hexane proceeds via formation of n -hexene on dehydrogenation centers, followed by cyclization of the n-hexene to methylcyclopentane on acidic centers. The methylcyclopentane then is converted to benzene in the manner already described. Alternatively, it seems possible that a hexadiene may be an intermediate in the reaction sequence. Such a sequence would involve formation of a hexadiene on platinum sites, followed by cyclization on acidic centers to form a cyclic olefin, methylcyclopentene (1). 

However, there is also evidence that dehydrocyclization may proceed by another route involving only the metal component of the catalyst. It has been observed that unsupported platinum powders catalyze the dehydrocyclization of n-heptane (21). Also, Dautzenberg and Platteeuw (25) report that dehydrocyclization of n-hexane to benzene occurs over a catalyst in which platinum is supported on a nonacidic alumina. Since bifunctional catalysis with participation of acidic sites is then presumably eliminated, the activity is attributed to the platinum itself. 

Figure 10 Dehydrocyclization of n-hexane to benzene at 733K on Pt-MLZeolites. Influence of zeolite basicity (45,46).

Similar mechanisms apply to the isomerization and dehydrocyclization of n-hexane to methyl cyclopentane and then benzene, as shown in Table 6.15. 

The dehydrocyclization of n-hexane catalyzed by platinum-loaded alkali L zeolite reported by Bernard [34] became the subject of recent industrial development [35]. In contrast to the acidic commercial reforming catalyst, the new L zeolite catalyst reported by Hughes et al. [35] is nonacidic Pt-BaK-L zeolite. Prestimably, the properties of platinum clusters by themselves account for all the catalytic activity. The catalyst is extremely sensitive to poisoning by sulfur, but with a thoroughly desulfurized feed, a one-year run has been successfully completed with refinery light naphta. 

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. 

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

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... 

The first approach to the cyclic mechanism of isomerization was the finding that the interconversion of n-hexane and methylpentanes takes place under the conditions where the nonselective mechanism of hydrogenolysis (Mechanism A) is the only one operating that is, on 0.2% Pt/AljOj (32). The identical product distributions in isomerization of hexanes and hydrogenolysis of methylcyclopentane suggested that both reactions involve a common intermediate with a methylcyclopentane structure. It was then proposed that the species responsible for dehydrocyclization of hexanes are a,j8, -triadsorbed species involving a single metal atom (55) (Scheme 40). 

This reaction, like dicarbene recombination, also has its analog in coordination chemistry, that is, reductive elimination of tetramethylene and pentamethylene ligands from platinum complexes yields cyclobutane and cyclopentane, respectively (777). According to this direct ring closure mechanism, the observed selectivity for dehydrocyclization of n-alkanes on metals (nonformation of quaternary-secondary and tertiary-secondary C-C bonds in reactions of type A and B) should be interpreted in terms of simple steric effects. However, although, in the case of platinum, the concepts of steric hindrance could account for the change of selectivity that occurs with decreasing metal particle size (i.e., cyclization of n-hexane takes place on... 

Figure 2. Dehydrocyclization/Cracking Selectivity Ratio plotted as a function of n-hexane conversion A SAP0-11,...

As it is mentioned earlier, there is a significant change in selectivities of toluene and Cg+ aromatics with the addition of zinc. The preferential increase of toluene indicates the possibility of toluene formation from the direet dChydrocyclization/direct aromatization of n-heptane over the Zn/HZSM-5 (Kms), in addition to the cracking-and-oligomerization route (Kai). The direct dehydrocyclization of hydrocarbons (hexane and above) was also reported by Giannetto et al [49] from their studies over Ga-HZSM-5 catalyst. 

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. 

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... 

On the other hand, the selective dehydrocyclization, which does not allow the formation of secondary-primary C-C bonds, must involve only two methylic carbon atoms in the 1 and 5 positions. Although the reverse reaction (selective hydrogenolysis of methylcyclopentane) could be observed on platinum catalysts of low dispersion at 220°C (86), the selective dehydrocyclization of methylpentanes on these catalysts is detectable only at higher temperatures (280°-300°C), where it competes with another process, ascribed to Mechanism C (33). Fortunately, it was found recently that iridium supported on AI2O3 or SiOj selectively catalyzes at 150°C the cyclic type interconversion of 2-methyl- and 3-methylpentanes (88). n-Hexane under the same conditions yields only cracked products (702) (Scheme 52). Similarly,... 

Finally, the partially selective Mechanism C in hydrogenolysis of cyclopentanes has a counterpart in dehydrocyclization of methylpentanes and n-hexane. The intervention of this mechanism, involving metallocyclobutane intermediates, is strongly supported by studies of aromatization (see Section V). 

The kinetics of 1-5 ring closure were investigated in parallel for aliphatic and aromatic hydrocarbons on Pt-C (755-757). The apparent activation energy for dehydrocyclization is always higher (by 7-15 kcal/mol) in the case of monosubstituted benzenes (n-propyl-, sec-butyl-, and isobutyl-benzenes) than in the case of paraffins (ethylpentane, isooctane, n-hexane). The same is not true, however, for dehydrocyclization of o-ethyltoluene and isooctane, which occur with similar activation energies (757). This result is quite understandable if one considers that the first elementary step in the dehydrocyclization of monosubstituted benzenes but not of disubstituted benzenes results in a loss of aromaticity. 

Ad (a), as has been found by Bernard [39], platinum on the nonacidic zeolite KL is superior to conventional reforming catalysts in the dehydrocyclization of hexane to benzene (460 °C, molar ratio Ih hexane 6). It is proposed that zeolite KL is unique in its ability to prevent agglomeration of the small Pt particles required for the reaction. Chevron workers partially exchanged KL towards a Pt-BaKL catalyst [40]. The aromatization selectivity over Pt-BaKL was high and nearly constant (between 90 and 82 %) for n-alkanes having six to nine carbons. A process was developed by Chevron under the name AROMAX. 

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