Hydrogenolysis mechanisms

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

Fig. 20.4 A cyclopropane hydrogenolysis mechanism based on the Horiuti-Polanyi alkene hydrogenation mechanism.

Having characterized the three hydrogenolysis mechanisms by their precursor species dicarbenes (Scheme 34), 7c-adsorbed olefins (Scheme-36), and metallocyclobutanes (Schemes 38 and 39), the knowledge of the overall mechanism of cyclic type isomerization requires the identification of the precursor species in 1-5 dehydrocyclization, the reverse reaction of hydrogenolysis of cyclopentanes. 

The first approach to the cyclic mechanism of isomerization was the finding that the interconversion of n-hexane and methylpentanes takes place under the conditions where the nonselective mechanism of hydrogenolysis (Mechanism A) is the only one operating that is, on 0.2% Pt/AljOj (32). The identical product distributions in isomerization of hexanes and hydrogenolysis of methylcyclopentane suggested that both reactions involve a common intermediate with a methylcyclopentane structure. It was then proposed that the species responsible for dehydrocyclization of hexanes are a,j8, -triadsorbed species involving a single metal atom (55) (Scheme 40). 

Figure 1 Butane hydrogenolysis mechanism. C. denotes a paraffin of carbon number j, C. the corresponding adsorbed species, k. the adsorption rate constant, k. the desorption rate ...

Non Seleclive m ethy Icy clopen lane hydrogenolysis = Mechanism "a"... 

Selective m ethy Icy clopen tane hydrogenolysis = Mechanism "b ... 

A simple mathematical model was formulated for the successive methyl abstraction hydrogenolysis mechanism illustrated in Figure 10. 

Note that the model does not include secondary methanolysis, i.e. readsorption and additional demethylation of the longer fragment once desorbed. This is for reasons such as the relative demethylation reactivities C12 / C13, Cn / C13 etc. being unknown and, in particular, that products from the non-specific hydrogenolysis mechanism were to be considered too. Therefore, the methanolysis model and the derived a-values, only hold for low to medium of conversion. 

However, one can also consider this a hydrogenolysis mechanism as shown on the top pathway in Figure 4 with subsequent trapping of the HCl by base to prevent facile backreactions. In many ways hydrogenolysis can be... 

Sulfur removal is accomplished directly by a hydrogenolysis mechanism or indirectly by prior hydrogenation. Figure 13.6 shows the two possible pathways for dibenzothiophene HDS. In the direct route, the S-atom is eliminated and replaced by hydrogen. The other mechanism requires saturation of one... 

Rednce coke formation in favor of liquid products by means of a hydro-cracking/hydrogenolysis mechanism... 

---