Isomerization bifunctional mechanism

As already mentioned, with time the mid-molecule cleavage typical of a bifunctional catalyst decreases over the molybdenum based catalyst and the demethylation reaction becomes dominant. Demethylation also increases with increasing pressure. Amir-Ebrahimi and Rooney proposed that the metallacyclobutane isomerization mechanism should have a significant methanation and homologation contribution.34 Homologation products were not analysed in this study but have been observed in studies of the C4 and C5 reactions 35 however, methane was an important component of the cracking products over the molybdenum catalysts. 

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). 

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... 

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 classical HCK mechanism on bifunctional catalysts separates the metallic action from that of the acid by assigning the metallic function to the creation of an olefin from paraffin and the isomerization and cracking of the olefins to the acid function. Both reactions are occurring through carbenium ions [102],... 

Dual-function catalysts possessing both metallic and acidic sites bring about more complex transformations. Carbocationic cyclization and isomerization as well as reactions characteristic of metals occurring in parallel or in subsequent steps offer new reaction pathways. Alternative reactions may result in the formation of the same products in various multistep pathways. Mechanical mixtures of acidic supports (silica-alumina) and platinum gave results similar to those of platinum supported on acidic alumina.214,215 This indicates that proximity of the active sites is not a requirement for bifunctional catalysis, that is, that the two different functions seem to operate independently. 

Burch and coworkers (50-53) have reported on the use of Pt-Sn catalysts for hydrocarbon conversions. It was concluded that no proper alloys of Pt and Sn were formed so that this could not account for the changes in the catalytic properties imparted by the presence of Sn (50). Burch and Garla concluded for their catalysts that (i) n-hexane is isomerized by a bifunctional mechanism, (ii) benzene and methylcyclopentane are formed directly from n-hexane at metal sites, and (iii) the conversion of methylcyclopentane requires acidic sites (51). It was also concluded that the Sn (II) ions modified the Pt electronically with the result that self-poisoning by hydrocarbon residues is reduced. However, these later observations were based upon conversions at one bar. 

We examined the ability of our bis-imidazole cyclodextrin artificial enzymes to perform other bifunctionally-catalyzed reactions, where again the availability of the A,B and A,C and A.D isomers let us learn mechanistic details. As an important example, we examined three isomeric catalysts ability to promote the enolization of substrate 48, which binds into the cyclodextrin cavity in water [138]. Here there was again a strong preference among the isomers, but it was the A,D isomer 49 that was the effective catalyst It was also more effective than a cyclodextrin mono-imidazole that cannot use the bifunctional mechanism. 

Fig. 1.4 Bifunctional mechanism of n-hexane isomerization over Pt H zeolites...

With bifunctional Pt acid catalysts, a third mechanism can participate in xylene isomerization, involving the same intermediates as ethylbenzene isomerization (2,11). 

Fig. 9.6 Mechanism of ethylbenzene isomerization over bifunctional catalysts...

Thus, study of the kinetics of n-pentane isomerization on H-mordenite leads to the conclusion that the mechanism of the reaction in question is different from that of isomerization on bifunctional and metal-zeolite catalysts. This difference lies in the manner of carbonium ion formation. With bifunctional catalysts, carbonium ion originates with the attachment of a proton to the olefin molecule, while with H-mordenite it originates as a result of splitting off hydride ion from the saturated molecule of the starting hydrocarbon by mordenite proton, as has been suggested by the above reaction scheme. 

Although the constant metallic activity is a small fraction of the initial activity, does not control the bifunctional mechanisms of the main reforming reactions [paraffin dehydrocyclizatlon and isomerization). The acid function controls these reactions and is the function whose deactivation causes the end of the operation cycle. 

The current theory of bifunctional catalysis assumes that paraffin isomerization is induced by olefin formation at the metal surface, followed by a typical acid-catalyzed reaction of the olefin at the active centers of the acidic component. Consequently, similar skeletal conversions must be found with olefins and an acid catalyst, and paraffins and a bifunctional catalyst. Our findings substantiate this theory. If these results (Figs. 2 and 3) are put together and compared to the predictions of the carbonium mechanism (Fig. 4), one can see that all the expected structures have been obtained in our experiments. 

The bifunctional catalytic n-hexane isomerization was performed in a microflow reactor under atmospheric pressure, using a mechanical mixture (Iw/w) of catalyst and a standard Pt/A Os reforming catalyst (0.35 wt% Pt). The latter was previously reduced at 723 K for 4 h. The catalytic test was performed as follows The mixture Pt/A Os + catalyst was pretreated first under He flow at 723K for 2 hours and then under H2 for 30 min at 493K. 

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