Metal skeletal isomerization

Bifunctional (metallic and acidic functions) catalysts are applied to a variety of oil-refining and petrochemical processes. For example, paraffin hydroisomerization involves n-paraffins dehydrogenation to a-olefins over metal, skeletal isomerization of n-olefins to isoolefins over acidic sites, followed by hydrogenation of isoolefins to isoparaffins over metal. Zeohte-supposted noble metal catalysts are often used for these types of reactions, which... 

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. 

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. 

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. 

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). 

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. 

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

Mechanisms of Skeletal Isomerization of Hydrocarbons on Metals F. G. Gault... 

All these results were interpreted by a free-radical mechanism with the involvement of alkenes, and smaller (C3,C4) and larger (C7,C8) ring intermediates in aromatization. Skeletal isomerization was found to occur through vinyl shift and C3,C4 cyclic intermediates.202 Transition metals with the exception of Fe and Os, as well as Re, Co, and Cu, are active in aromatization of alkanes. Platinum,... 

Olefins are formed by dehydrogenation of the n-paraffin feed over the metallic hydrogenation-dehydrogenation function and are adsorbed on the acidic surface of the catalyst as carbonium ions by proton addition. After skeletal isomerization they are desorbed as isoolefins and subsequently hydrogenated to the corresponding isoparaffins. The net result (i.e., the formation of carbonium ions) of the action of metal and acid in dual function catalysis is, on pure Friedel-Crafts type catalysts, described by the scheme ... 

Activity and Selectivity for Skeletal Isomerization of n-Butane in the Presence of H2 and Metal-Promoted Catalysts at 573 K (381. 382)... 

Fe(lll)> Fe(100)> Fe(110), with relative activities of 430 32 1 and for Re the order is Re(1120)> Re (1010 > Re (0001), with Re (1120) more than 1000 times more active than Re (0001). For both metals it is proposed that the active site is a metal atom in the second layer of an open surface structure, i.e., an atom in the bulk (having a high density of electron holes near the Fermi level) which is accessible to a gaseous molecule because of the open structure of the surface. This model emphasizes the unique electronic rather than structural sensitivity of this reaction. It is possible that similar electronic effects may contribute to structure sensitivity for other reactions (c.f. skeletal isomerization reactions, see later). 

The product distributions obtained over the catalysts are summarized in Table II for the reaction of butane (29, 31, 32). The activities varied with the kinds of metal oxides that were treated with SbF,. SbF,/ Si02-Ti02 showed the highest activity, and SbF,/Ti02 was highly selective for the skeletal isomerization of butane, the selectivity being 72%. On... 

Hafnium is the third element in the same group with Ti and Zr, the transition metals of the Periodic Table it is predicted that Hf02 is enhanced in acidity by sulfate addition up to superacidity. The catalyst, which was obtained by exposing Hf(OH)4, prepared by the hydrolysis of HfCI4, to I M H2S04 and then calcining, was active for the skeletal isomerization of... 

Another important and well studied paramagnetic ion in the lattice of oxide semiconductors is Zr3+ in Zr02. Zirconia dioxide is widely used both as a catalyst of different chemical processes, and as a carrier for constructing supported metal-complex catalysts. In the last years, sulfated zirconia attracted significant interest as an active and selective catalyst in skeletal isomerization of normal alkanes at low temperatures, cracking of paraffins, alkylation and acylation of aromatics [42, 53 and Refs therein]. The appropriate experimental data are collected in the following Table 8.2. 

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