Iridium hydrocracking

On iridium, hydrocracking involves mainly the rupture of primary-secondary and secondary-secondary C-C bonds, resulting in deethylation. Hydrocracking on this metal is best accounted for by the rupture of a 1,2-... 

On palladium, demethylation (primary-secondary or primary-tertiary C-C bond rupture) is the major hydrocracking reaction. On platinum, the demethylation is still favored, but the other cleavage modes (secondary-secondary and secondary-tertiary C-C bond rupture) become appreciable. On iridium, deethylation predominates, while on nickel, the initial hydrocracking distribution includes a large excess of methane relative to the simple demethylation and deethylation. Finally, on cobalt, extensive cracking to methane accounts for 100% of the overall reaction. These results may be rationalized in terms of metallocarbene chemistry and of the capacities of the different metals to form metallocarbenes. 

While the first process is likely in the case of iridium, nickel, and cobalt, it should not be so easy on platinum, because of its competition with carbene-olefin isomerization (see Section III, Scheme 29). We believe that the only way of explaining why 1,2-dicarbenes may account for the hydrogenolysis of cyclic hydrocarbons (Scheme 34), but only for a minor part for the hydrocracking of acyclic hydrocarbons, is the competition, for the latter, between carbene-dicarbene formation and carbene-olefin isomerization. Carbene-olefin interconversions are unlikely in the case of cyclic hydrocarbons, since a dicarbene species cannot transform into a 1,1,2,3-tetraadsorbed species (l-carbene-2,3-olefin) and further into a 1,1,3-triadsorbed species without C-C rupturing. 

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