Dehydrocyclization reaction mechanism

The mechanism of the dehydrocyclization reaction is not completely understood. Two alternatives were proposed (11), one which proceeds via a silaimine mechanism, exemplified by reaction (25), and one wherein ring closure occurs by nucleophilic displacement of hydrogen, reaction (26). 

The focus of the next four chapters (Chapters 14-17) is mainly on the theoretical/computational aspects. Chapter 14 by T. S. Sorensen and E. C. F. Yang examines the involvement of p-hydrido cation intermediates in the context of the industrially important heptane to toluene dehydrocyclization process. Chapter 15 by P. M. Esteves et al. is devoted to theoretical studies of carbonium ions. Chapter 16 by G. L. Borosky and K. K. Laali presents a computational study on aza-PAH carbocations as models for the oxidized metabolites of Aza-PAHs. Chapter 17 by S. C. Ammal and H. Yamataka examines the borderline Beckmann rearrangement-fragmentation mechanism and explores the influence of carbocation stability on the reaction mechanism. 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

In contrast to this mechanism, the one proposed in our work operates direct from the oxidation state of the alkane feedstock. The same alkyl cation intermediate can lead to both alkane isomerization (an alkyl cation is widely accepted as the reactive intermediate in these reactions) and we have shown in this paper that a mechanistically viable dehydrocyclization route is feasible starting with the identical cation. Furthermore, the relative calculated barrier for each of the above processes is in accord with the experimental finding of Davis, i.e. that isomerization of a pure alkane feedstock, n-octane, with a dual function catalyst (carbocation intermediate) leads to an equilibration with isooctanes at a faster rate than the dehydrocyclization reaction of these octane isomers (8). 

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. 

As a result of the studies discussed above, a reasonably consistent picture of the kinetics and mechanism of the dehydrocyclization reaction over oxide catalysts has evolved. However, as pointed out earlier in this section, relatively few kinetic data have been reported for dehydrocyclization over platinum-alumina reforming-type catalysts. The data which have been reported include those of Hettinger and co-workers (H7), who studied the dehydrocyclization of re-heptane over platinum catalysts. These investigators found that the rate of dehydrocyclization decreased with increasing total pressure at a constant hydrogen-to-hydrocarbon ratio (Fig. 9). They also reported that the extent of dehydrocyclization was substantially greater for re-nonane than for re-heptane, which is consistent with the results obtained on oxide catalysts. In a later study of the kinetics... 

The situation is much different when an acidic support is used. First, the C8 aromatic products have a distribution that approaches an equilibrium composition. The Pt catalyst on an acidic support is both more active and produces aromatics more selectively than Pt on a nonacidic support (84). It is concluded that the bifunctional mechanism involving cyclization by the acid site followed by a bifunctional ring expansion/dehydrogenation reactions is much more rapid than the monofunctional metal catalyzed dehydrocyclization reaction. For the catalyst based on an acidic support, the tin added initially acts as a catalyst poison (Figure 5), at least during the initial 1-2 weeks of usage. 

Kinetics and Mechanism. The dehydrocyclization of alkanes may occur by two different mechanism involving only the metallic function, or by a bifunctional mechanism where the dehydrogenation-hydrogenation steps occurs on the metal, and the cyclization occurs on the acid sites. The metallic mechanism is very sensitive to poisons such as sulfur and coke (18), and because of this in the commercial process the paraffins dehydrocyclization occurs mainly by the bifunctional mechanism (8). Details about the paraffin dehydrocyclization reaction on metals can be found elsewhere (9,32-34). 

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. 

Muller (ISO) has recently shown that the dehydrocyclization of 2,2,4,4-tetramethylpentane to 1,1,3,3-tetramethylcyclopentane occurs on thick polycrystalline platinum film catalysts with a rate that is comparable to the formation of 1,1,3-trimethylcyclopentane from 1,2,2-trimethyl-pentane. As Muller points out, reactions (10)—(12) cannot occur from 2,2,4,4-tetramethypentane, and it is clear that either these mechanisms are inadequate, or at least there must be an alternative mechanism available. Muller suggests mechanism (14) which requires two adjacent platinum sites. 

In our calculations we will first discuss our results starting with both the 2-and 3- octyl cations (the 4- octyl cation cannot form a 1,6-p-H-structure). The n-octane conversion to aromatics, as described by Davis (8), is a good test of our proposed mechanisms, for several reasons (1) his experimental observation would require the formation of approximately equal amounts of 1,2-dimethylcyclohexane (o-xylene) and ethylcyclohexane (ethylbenzene), even though in our mechanism the structure of the needed 1,6-p-H cation intermediates are quite different, and (2) the formation of to- and p-xylene requires a prior isomerization of n-octane to 2- and 3- methylheptane, and this must be a faster reaction than the dehydrocyclization (or at least competitive with it). If our mechanisms are valid, we should be able to reproduce some aspects of the above results. 

The stepwise dehydrocyclization of hydrocarbons with quaternary carbon atoms over chromia was interpreted by Pines 94). Here a skeletal isomerization step prior to cyclization was assumed. This is not of a cationic type reaction, and the results were explained by a free radical mechanism accompanied by vinyl migration (Scheme IXA). Attention is drawn to the fact that... 

Recent data on other alloys confirm the overall classification presented above, but at the same time lead to some refinement of the picture. For example, the most diluted Pt-Au alloys revealed isomerization, identified as running via 3C intermediates. This evidence was obtained (248) by establishing the fact that pentane isomerizes on most diluted Pt-Au alloys with 100% selectivity, whereas this molecule can only isomerize via the 3C complexes. This conclusion has been confirmed by the isotopic labeling method (269). It is therefore reasonable to assume that this isomerization can also proceed on isolated Pt sites, as can a part of dehydrocyclization and the dehydrogenation. We must conclude on the basis of this information that on metals like Pt, the fast multisite and the slow one-site mechanisms of hydrocarbon reactions may operate in parallel with each other. 

Since dehydrocyclization and ring cleavage are reversible processes, the same mechanisms may be operative in both transformations. Two basic cleavage patterns—a selective and a nonselective ring opening—are known. In the selective process only secondary-secondary carbon-carbon bonds are broken. Conversely, only the formation of carbon-carbon bonds between methyl groups is allowed in ring formation. In contrast, all bonds are cleaved equally in the nonselective reaction. Bonds adjacent to quaternary carbon atoms are exceptions, since they are not cleaved at all. 

The formation of aromatics by the catalytic dehydrocyclization of paraffins with chains of six or more carbon atoms has been known for some time. Certain oxides of the 5th and 6th subgroups of the periodic table, such as chromia and molybdena, were shown early to be particularly effective catalysts for the reaction. Consequently, most of the reported studies of the kinetics and mechanism of the reaction have been carried out using these catalysts (P6, H4, H5). Since the available data on the kinetics of dehydrocyclization over oxide catalysts have been reviewed by Steiner (S9) in 1956, only a brief summary of the work will be made here, primarily for the purpose of orientation. The relatively few kinetic data which have been reported for dehydrocyclization over the bifunctional platinum on acidic oxide catalysts will be discussed subsequent to this. 

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. 

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 his 1940 review Plate subjected the experimental material on dehydrocyclization of paraffins published to that time to a critical analysis (304) and concluded that aromatization of paraffins at the temperatures employed will depend upon the selection of proper catalysts in order to suppress the competing reactions, that the multiplet theory satisfactorily explains the mechanism of cyclization, and that intermediate formation of olefins is conceivable on oxide catalysts but can hardly occur on platinum. 

Having characterized the three hydrogenolysis mechanisms by their precursor species dicarbenes (Scheme 34), 7c-adsorbed olefins (Scheme-36), and metallocyclobutanes (Schemes 38 and 39), the knowledge of the overall mechanism of cyclic type isomerization requires the identification of the precursor species in 1-5 dehydrocyclization, the reverse reaction of hydrogenolysis of cyclopentanes. 

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