Cyclopentanes, dehydrocyclizations

Dehydrogenation of cyclohexanes Isomerization/dehydrogenation of cyclopentanes Dehydrocyclization of paraffins... 

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... 

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. 

Dehydrocyclization of straight-chain or branched acyclic alkanes—the reverse of the hydrogenolysis of cyclopentanes—takes place on the same platinum-charcoal catalyst under similar conditions. This reaction is often accompanied by aromatization (20-27) (Scheme 4). 

The cyclic mechanism (Scheme 7), which involves dehydrocyclization to an adsorbed cyclopentane intermediate C, followed by ring cleavage and desorption of the products, and is responsible for the isomerization of larger molecules on dispersed platinum-alumina catalysts (52, 55). 

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. 

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 sextet-doublet model was adapted for 1-5 dehydrocyclization, the reverse of cyclopentane hydrogenolysis, and it was proposed that physically adsorbed alkane reacts with chemisorbed hydrogen according to a push-pull mechanism (772) (Scheme 55). 

As in the case of hydrogenolysis of cyclopentane, the change in selectivity observed in pulse and flow systems for the 1-5 dehydrocyclization of n-heptane to ethylcyclopentane and 1,2-dimethylcyclopentane, for instance, was interpreted by two modes of adsorption involving five or seven carbon atoms in contact with the surface (775) (Fig. 3). 

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... 

The paraffin dehydrocyclization reaction is slower than the dehydrogenation of cyclohexanes, dehydroisomerization of cyclopentane, and isomerization of paraffins. Because the rate is not so high, it is not possible to reach the equilibrium conversion. The larger fraction of the transformation of the paraffins into aromatics occurs in the last reactor of the reforming unit, in which the largest fraction of catalyst is loaded, and the higher operation temperature is used. 

Fig. 5.2 Mechanism of coke formation from n-paraflins. DHC dehydrocyclization (on metal and acid sites) DH dehydrogenation (on metal sites) or hydrogen transfer (on acid sites) C condensation on acid sites, CP cyclopentane, CPde cyclopenta-diene., MCP methylcyclopentane, MCPde methylcyclopentadene.

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

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