Bond shift, skeletal isomerization

Benzene formation from all isohexanes had a similar energy of activation value. With platinum this was nearly twice as high as that of n-hexane aromatization (62) with palladium black, however, nearly the same values were found for -hexane and isohexanes (97a). This indicates a common rate-determining step for aromatization with skeletal rearrangement. This is not the formation and/or transformation of the C5 ring. We attribute benzene formation to bond shift type isomerization preceding aromatization. It requires one step for methylpentanes and two steps for dimethyl-butanes this is why the latter react with a lower rate, but with the same energy of activation. 

Banks and Bailey concluded that disproportionation occurs when two molecules are adsorbed with the breakage of two opposite bonds without hydrogen shift. Polymerization occurs when two molecules are adsorbed with the formation of a four-center complex and then desorb with the breakage of one bond and hydrogen shift. Skeletal isomerization occurs when one molecule is adsorbed with the formation of a four-center complex and then desorbed With the breakage of one bond and hydrogen shift. 

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. 

Slow double-bond shifts and little skeletal isomerization H-transfer is minor and nonselective for tertiary olefins only small amounts of aromatics formed from aliphatics at 932°F (500°C)... 

Rapid double-bond shifts, extensive skeletal isomerization, H-transfer is major and selective for tertiary olefins large amounts of aromatics formed from aliphatics at 932°F (50t) O... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Two main pathways of metal-catalyzed skeletal rearrangement have been distinguished bond shift mechanism and C5 cyclic isomerization (7, 8). 

Skeletal ring contraction steps of primary C7 and Cg rings are more probable than bicyclic intermediates (132b). Aromatization of methylcyclo-pentane indicated no carbonium mechanism with a nonacidic catalyst. Instead, Pines and Chen (132b) proposed a mechanism similar to that defined later as bond shift. This is a methyl shift. Two additional isomerization pathways characteristic of chromia have also been demonstrated vinyl shift (94) and isomerization via C3 and C4 cyclic intermediates (90a). These were discussed in Section III. 1,1-Dimethylcyclohexane and 4,4-dimethyl-cyclohexene gave mainly toluene over various chromia catalysts. Thus, both skeletal isomerization and demethylation activities of chromia have been verified. The presence of an acidic almnina support enhances isomerization dual function effects are thus also possible. 

Low-molecular-weight hydrocarbons (C4 and C5 alkanes) usually undergo isomerization through a simple bond shift. The transformation of [l-l3C]-butane, for instance, yields isobutane via skeletal isomerization and the isotopomer [2-13C]-butane 155... 

Much work has also been carried out on the reactions in the presence of H2 of branched alkanes, and on the mechanism of their skeletal isomerization [6, 7], The use of 13C-labeled molecules permits alternative reaction pathways to be distinguished. Thus, for example, most of the 3-methylpentane formed from 2-methylpentane has followed the bond-shift route, but most of the n-hexane has resulted from the cyclic mechanism. Labeled molecules also allow mechanisms of aromatization of C7 and Cg alkanes to be followed... 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

Over the range of conditions, 1-butene decomposes more rapidly than either of the 2-butene isomers. Double-bond shift and geometrical isomerization accompany the decomposition of the n-butenes however, skeletal isomerization does not occur, as isobutene is not found among the products of the pyrolysis. Isomerization reactions apparently are kinetically controlled, as equilibrium distributions are not generally observed. Trans cis ratios in the products do not correspond to equilibrium at either the maximum or the average reactor temperatures, and in some cases the ratio falls below equilibrium values based on American Petroleum Institute (API) data (14). However, none of these data exceed the equilibrium values based on more recent thermodynamic data (15). 

It is found that Mode E behaves similarly to the zeolite free Pt-Re/Al203 Both catalysts have a relatively high proportion of isomer products which could be formed over the metal surface via a bond-shift mechanism [8]. Isomers are formed by doublebond isomerization and skeletal isomerization reactions at both the acid sites of the alumina support and the metal sites. The later provides a dehydrogenation-hydrogenation function and the acid sites an isomeiization function for the olefins to dehydrogenate from paraffins over the metal function, since it is known that olefin isomerization proceeds much quicker than the respective paraffin isomerization [8]. On the other hand, branched paraffins are less easily cracked than linear ones [10]. Therefore, once isomers are formed over conventional reforming catalysts, they are likely to be the final products. Evidently, the isomerization of paraffin requires the metal function in the bimetallic catalyst, and so does the paraffin aromatization. This can also explain the obseiwed decrease in the isomers and aromatics production with time-on-Hne since it is well- known that coke preferentially deposits on a metal surface first [14]. 

Other catalysts containing WO have been shown to have activity in double bond shift and in metathesis (ref. 6,7,8). The conditions for skeletal isomerization developed in the present work do not favour the metathesis reaction and it makes no apparent contribution to the product. However double bond shift does accompany skeletal isomerization. 

The skeletal isomerization was associated with double bond shift 1-ene to 2-ene. The equilibrium favours the 2-ene at these reaction temperatures so that it is not obvious that a shift of the double bond is an essential step in the branching reaction. The 2-enes undergo the same branching reaction. 

Excessive reduction, particularly after oxidation treatments favours hydrogenation probably due to the production of zero valent tungsten. The oxidized catalyst on the other hand lacks skeletal isomerization activity but shifts the double bond. This is a reaction characteristic of acidic catalysts. 

Normal oleims Doable bond shifts dowly litde skeletal isomerization Double bond shifts rapidly extensive skeletal isomerization... 

Cracked within side chain Double bond shifts slowly little skeletal isomerization Hydrogen transfer is a minor reaction and is nonselective for tertiary olefins... 

Cracked next to ring Double bond shifts rapidly extensive skeletal isomerization Hydrogen transfer is an important reaction and is selective for tertiary olefins Crack at much higher rate than corresponding paraifins Crack at about same rate as paraffins with equivalent structural groups (9)... 

The simplest explanation of the observed fact is that a shift of the double bond is possible by a successive 1,3-hydride transfer. Two additional arguments are in favour of this proposal The metastable spectra of 2- and 3-heptene are practically indistinguishable from the label data for 1-heptene, and terminal D atoms have a much smaller tendency (if any) to scramble, contrary to atoms within the inner positions. Since the latter observation is also made for alkyl fragmentations, this is therefore not a strong argument for an absence of a skeletal isomerization. However, finer details, such as the different time dependence for different fragments, are not explained by such a model. 

Investigations which concern the mechanisms of skeletal rearrangements of saturated hydrocarbons induced by heterogeneous transition metal catalysis are of great interest for industrial applications, e.g. for petroleum reforming processes The developments in this field were reviewed recently by Hejtmanek, and by Maire and Garin , who focussed on the probable reaction mechanisms which include bond-shift and cyclic mechanisms for the skeletal isomerization of acyclic alkanes. Scheme 1 summarizes the... 

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