Heterogeneous catalyst alkane isomerization

It is necessary to note that many hydrocarbon transformations on heterogeneous catalysts proceed with intermediate formation of the same species although, they give rise to different products depending on the natnre of the catalyst or conditions of the process. Eor example, dehydrogenation and isomerization of alkanes involve stages of the formation of coordinated to the metal surfaces species in the case of catalysis by metals (see below. Figure III.3). Carbe-nium ions are formed if a metal oxide is used as a catalyst. 

The mechanism of dehydrogenation and dehydrocyclyzation of alkanes on heterogeneous catalysts is connected with that of isomerization. Thus a study of the initial stages of propane activation over H-ZSM-5 zeolite by NMR, demon-... 

Hydride complexes of platinum have received considerable study since the preparation of PtHCl(PEt3)2- Spectroscopic studies by NMR techniques have been widely used because of the structural information which can be obtained from coupling constant data to Pt and other nuclei. Platinum is widely used as a heterogeneous catalyst, and vibrational studies on platinum hydride complexes have been useful for comparison of a hydrogen atom bonded to a single platinum with that bonded to a surface. Complexes of platinum have been used to catalyze hydrogenation, hydrosilylation and isomerization reactions with alkenes and alkynes, as well as H/D exchange reactions on alkanes. Hydride complexes are frequently proposed as intermediates in these reactions, and the pathways related to the known chemistry of hydride complexes. 

The diene cyclization shown in Eqn. 2, has been reported to take place only over RhCl3 and Wilkinson s catalyst (ref. 6). We have found that it also occurs when run over supported Rh catalysts. The heterogeneously catalyzed reaction is particularly sensitive to the nature of the solvent used. With alcohols or other solvents which can adsorb on the catalyst, there is an apparent competition with the adsorption of the double bonds and the cyclization does not take place. In alkane solvents, which do not interact with the catalyst, the reaction occurs with reasonable facility. This cyclization is run routinely at 145°C in a flow system with a decane solution of 5 passing through a small column containing a Rh/Al2Oj catalyst. The product composition was related to the time 5 was in contact with the catalyst. With fast flow rates (short contact times) 6 was the primary product of the reaction but the isomerized species, 7 and 8, were produced when slower flow rates were used. This indicates that 6 was the primary product of the reaction but that it was isomerized over the catalyst to 7 and 8. 

The interaction of saturated C—H and C—C bonds with heterogeneous metal catalysts forms the basis of widely applied reactions such as isomerization, cracking, and re-forming of alkanes. In recent years, much attention has been devoted to the selective activation of C—H bonds by transition metal complexes in homogeneous solution under mild conditions.601 604 In principle, an alkane can undergo oxidative addition to a noble metal complex according to... 

Further, the sites for set B must be heterogeneous because Dt for isomeric olefin falls off much more rapidly than T>i for alkane, as shown in Tables VII and VIII. That is, on catalysts activated at lower temperatures, hydrogenation activity by process (3) seems to be larger at those sites which give extensive exchange. 

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