Isomerization hydrogenolysis and

Many other authors studied the catalytic activity of palladium in more complicated hydrogenation reactions because of being coupled with isomerization, hydrogenolysis, and dehydrogenation. In some cases the temperatures at which such reactions were investigated exceeded the critical temperature for coexistence of the (a + /3)-phases in the other case the hydrogen pressure was too low. Thus no hydride formation was possible and consequently no loss of catalytic activity due to this effect was observed. 

Most multipromoted catalysts have been described for the catalytic reforming of petroleum. For this process it is typical, that several reactions take place simultaneously dehydrogenation of cyclohexanes, dehydroisomerization of alkylcyclopentanes and dehydrocyclization of alkanes. Isomerization, hydrogenolysis, and hydrocracking are also involved in the process. 

Figure 4. Selectivity in isomerization, dehydrocycllzation and hydrogenolysis (cracking) of Pt/Cu alloys (on Si02). Bulk composition of alloys (% Pt) indicated. (Reproduced with permission from Ref.30. Chem.Soc.London)...

Gault and coworkers [ 149] have observed that the distribution of products obtained by hydrogenolysis and isomerization of methylcyclopentane was the same as those obtained with hexane. They proposed two competing mechanisms a selective mechanism implying an a, a, p, j0-tetra-adsorbed species and a non-selective mechanism implying coordinated olefin and bis-carbene intermediates (Scheme 38). 

The metallic component of HCK catalysts provides hydrogenation, dehydrogenation, hydrogenolysis, and isomerization. The number and nature of reactive hydrogen species created by the interaction of a bifunctional catalyst with hydrogen is not well understood [103], on the other hand, neither the action of those species on the catalytic sites is understood. The main limitation in this understanding is the dynamic character of the interaction however, now that in situ characterization techniques are becoming available, research would soon defeat the limitations. 

From the chemical point of view, in diasteieoselective syntheses, several kinds of reactions like hydrogenation [273,277-286], hydrogenolysis [287-293], isomerization [294], and epoxidation [295-300] are involved. Hydrogenation is the most important application of heterogeneous catalysis because of its potential to produce a wide variety of chiral functional groups. 

Transition metals are used as catalysts for a variety of reactions hydrogenation, hydrogenolysis and isomerization of hydrocarbons (group VIII metals), oxidation of... 

Fig. 4. 3C complexes, the existence of which can be seen in experimental evidence from exchange [neopentane Rh (Pt)] and from hydrogenolysis and isomerization (neopentane, neohexane). As in Fig. 3, the known ( ) and the speculative aspects ( ) of the 3C complex formation are indicated. 

Fig. 10. Selectivities in hexane conversions versus temperature for benzene formation (Be), hydrogenolysis (Hy), methylcyclopentane formation (MCP), isomerization (ISOM), and dehydrocyclization (Dehy) (9 wt. % Pt on inert Si02).

The approach of finding correlation between adsorption properties and hydroge-nolysis led to the interpretation of cracking patterns. Later, the realization of relationships between hydrogenolysis and other metal-catalyzed reactions (isomerization) resulted in a much better understanding of the characteristics of hydrogenolysis reactions. 

THE ilECHANISM OF HYDROGENOLYSIS AND ISOMERIZATION OF OXACYCLOALKANES ON METALS, PART X ... 

The hydrogenolysis and isomerization of methyloxirane were studied over various Pt catalysts in order to determine the number and nature of the active sites. The steps were found to be the probable active sites and the transformation is structure-sensitive. The regioselectivity is not affected by variation in the catalyst structure, so it is determined by the nature of the metal. 

During studies of the hydrogenolysis and isomerization of the 2-Me-oxa-cycloalkanes on transition metal catalysts, it was found that different metals have different regioselectivities (refs 1,2). On Cu and Ni catalysts, primarily the C-0 bond adjacent to the substituent is split, leading to the formation of a primary alcohol or aldehyde (ref. 3), while on Pt and Pd catalysts mainly the more distant C-0 bond undergoes cleavage (ref. 4) yielding a secondary alcohol or ketone (Scheme 1). 

The hydrogenolysis and isomerization of methyloxirane take place via a mechanism in which hydrogen is involved in the rate-determining step. This reaction occurs on the step sites. 

The cylization of alkylaromatics over platinum catalysts is usually accompanied by isomerization, hydrogenation and dehydrogenation, fragmentation (i.e., hydrogenolysis and cracking), and other reactions. 

Formation of 1,1,4-trimethylcycloheptane from car-3-ene depends on two properties of palladium, the strong tendency of palladium to promote isomerization and to hydrogenolyze vinylcyclopropane systems. In the bicyclic unsaturated acid, IV, prior isomerization is not necessary to move the double bond into conjugation, yet here, too, platinum and palladium give quite different results. The saturated bicyclic acid is formed over platinum, whereas over palladium a mixture of cyclohexane-carboxylic acid and benzoic acid results through hydrogenolysis and disproportionation (20). 

The per cent of dicyclohexylamine formed in hydrogenation of aniline increases with catalyst in the order ruthenium < rhodium platinum, an order anticipated from the relative tendency of these metals to promote double bond migration and hydrogenolysis (30). Small amounts of alkali in unsupported rhodium and ruthenium catalysts completely eliminate coupling reactions, presumably through inhibition of hydrogenolysis and/or isomerization. Alkali was without effect on ruthenium or rhodium catalysts supported on carbon, possibly because the alkali is adsorbed on carbon rather than metal (22). 

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