Isomerization mechanism, bifunctional alkane

In this section we present experimental evidence for a bifunctional alkane isomerization mechanism obtained by selective poisoning of the acidic sites of Pd-NiSMM with pyridine, which was pulse-injected into the liquid hydrocarbon feed stream. The possibility of additional poisoning of the metallic sites was checked by studying the hydrogenation of benzene and the isomerization and ring opening of methylcyclopentane (MCP). 

The proposed mechanism for the isomerization of n-alkanes on bifunctional catalysts (60,61) is presented in Figure 14. From this mechanism an equilibrium between paraffins and olefins is established on the metal function. Then the olefins diffuse towards the Bronsted sites, where they become protonated and rearranged to give the branched carbenium ions. This, which is the rate controlling step, is followed by the desorption and hydrogenation, to yield the branched paraffins. 

Hence, the rate depends only on the ratio of the partial pressures of hydrogen and n-pentane. Support for the mechanism is provided by the fact that the rate of n-pentene isomerization on a platinum-free catalyst is very similar to that of the above reaction. The essence of the bifunctional mechanism is that the metal converts alkanes into alkenes and vice versa, enabling isomerization via the carbenium ion mechanism which allows a lower temperature than reactions involving a carbo-nium-ion formation step from an alkane. 

The hydroconversion of -alkanes over noble metal-loaded zeolites follows a bifunctional mechanism, in which the reactant is dehydrogenated on the metal particles and the formed alkene is isomerized and/or cracked over the acid sites of the zeolite, followed by a subsequent hydrogenation of all unsaturated products on the metallic sites (see reaction Scheme 7). 

Molecular heats of adsorption play a role in many catalytic reactions. Figure 6.23 illustrates this for an isomerization reaction catalyzed by a solid acid. As explained in Chapter 3, the hydroisomerization of alkanes on a zeolite-supported metal proceeds through a bifunctional reaction mechanism, in which the metal has the function of activating C-H bonds and H2 at a low reaction temperature. The alkane-alkene equilibrium is established by metal catalysis, and the alkene is protonated and isomerized by the acidic protons of the zeolite... 

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