Reactions skeletal isomerization

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. 

Hosoi et al. reported that Pt/S042--Zr02 persists a high activity for a long period in alkane skeletal isomerization when the reaction is carried out in the... 

In addition to this skeletal isomerization reaction, Anderson and Avery (24) showed that in a suitable isotopically labeled hydrocarbon, a reaction leading to positional isomerization occurred. Thus, with n-butane-l-13C as the reactant, the isomerization products were 2-(methyl-13C) propane, and 7i-butane-2-13C ... 

The isomerization reactions in the skeleton of the reactant prior to aromatization clearly involve the basic processes which we have already discussed in some detail. In passing we may note that conversion to aromatic is so favorable at any temperature (say) >350°C that this would be, of itself, sufficient reason for an adsorbed cyclo-Ce intermediate to be of negligible importance compared to cyclo-C5 as a pathway for skeletal isomerization at these temperatures. 

A wide range of nonacidic metal oxides have been examined as catalysts for aromatization and skeletal isomerization. From a mechanistic point of view, chromium oxide catalysts have been, by far, the most thoroughly studied. Reactions over chromium oxide have been carried out either over the pure oxide, or over a catalyst consisting of chromium oxide supported on a carrier, usually alumina. Depending on its history, the alumina can have an acidic function, so that the catalyst as a whole then has a duel function character. However, in this section, we propose only briefly to outline, for comparison with the metal catalyzed reactions described in previous sections, those reactions where the acidic catalyst function is negligible. 

Reactions over chromium oxide catalysts are often carried out without the addition of hydrogen to the reaction mixture, since this addition tends to reduce the catalytic activity. Thus, since chromium oxide is highly active for dehydrogenation, under the usual reaction conditions (temperature >500°C) extensive olefin formation occurs. In the following discussion we shall, in the main, be concerned only with skeletally distinguished products. Information about reaction pathways has been obtained by a study of the reaction product distribution from unlabeled (e.g. 89, 3, 118, 184-186, 38, 187) as well as from 14C-labeled reactants (89, 87, 88, 91-95, 98, 188, 189). The main mechanistic conclusions may be summarized. Although some skeletal isomerization occurs, chromium oxide catalysts are, on the whole, less efficient for skeletal isomerization than are platinum catalysts. Cyclic C5 products are of never more than very minor impor-... 

Skeletal Isomerization, -Elimination, and Ring Expansion Reactions... 

For (XX), L py, it is likely that the major reaction path involves initial skeletal isomerization to give (XXI) followed by rapid solvolysis of this isomer. The solvolysis of this isomer is strongly metal-assisted since the intermediate carbonium ion is stabilised by the metal-alkene resonance form as shown in the Scheme. The product is the 1-D2 isomer. Now, the skeletal isomerization of (XX) is expected to be retarded by free pyridine and cannot occur when L2 = 2,2 -bipyridyl C7). Hence under these conditions the reaction must occur by solvolysis of (XX) giving largely the 3-D2 isomer. However, the product formed under these conditions is still about 30% of the 1-D2 isomer (Table I). 

The reversal of the insertion reaction [Eq. (10)] is not normally observed [in contrast to nickel hydride addition to olefins, Eq. (9)]. An exception is the skeletal isomerization of 1,4-dienes (88, 89). A side reaction—the allylhydrogen transfer reaction [Eq. (5)]—which results in the formation of allylnickel species such as 19 as well as alkanes should also be mentioned. This reaction accounts for the formation of small amounts of alkanes and dienes during the olefin oligomerization reactions (51). 

Anderson that at this level the mechanistic details are a matter of opinion (7). There is, however, a difference as far as the number of surface atoms participating in the reaction is concerned. Mechanism A requires more than one Mechanism C, however, requires only one metal atom. Van Schaik et al. 89) reported skeletal isomerization according to Mechanism A over platinum-rich platinum-gold alloys, whereas over gold-rich catalysts, isolated platinum atoms could promote Mechanism C only. Garin and Gault (82) assumed the formation of a C4 cyclic intermediate with the insertion of a platinum atom as the fourth member of the ring. This concept of Mechanism B would also involve one metal atom. 

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... 

The isomerization of light olefins is usually carried out to convert -butenes to isobutylene [12] with the most frequently studied zeolite for this operation being PER [30]. Lyondell s IsomPlus process uses a PER catalyst to convert -butenes to isobutylene or n-pentenes to isopentene [31]. Processes such as this were in larger demand to generate isobutene before the phaseout of MTBE as a gasoline additive. Since the phaseout, these processes often perform the reverse reaction to convert isobutene to n-butenes which are then used as a metathesis feed [32]. As doublebond isomerization is much easier than skeletal isomerization, most of the catalysts below are at equilibrium ratios of the n-olefins as the skeletal isomerization begins (Table 12.5). 

Byggningsbacka, R., Lindfors, L.-E., and Kumar, N. (1997) Catalytic activity of ZSM-22 zeolites in the skeletal isomerization reaction of 1-butene. 

In the case of alkenes, 1-pentene reactions were studied over a catalyst with FAU framework (Si/Al2 = 5, ultrastable Y zeoHte in H-form USHY) in order to establish the relation between acid strength and selectivity [25]. Both fresh and selectively poisoned catalysts were used for the reactivity studies and later characterized by ammonia temperature programmed desorption (TPD). It was determined that for alkene reactions, cracking and hydride transfer required the strongest acidity. Skeletal isomerization required moderate acidity, whereas double-bond isomerization required weak acidity. Also an apparent correlation was established between the molecular weight of the hard coke and the strength of the acid sites that led to coking. 

This mechanism is supported by reports of comparable reaction rates obtained during skeletal isomerization with the corresponding alkenes over metal-free zeolites [52]. 

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