Hydrocracking demethylation

Hydrocracking, 30 48-52 behavior, thermal, 29 269 catalytic, 26 383 deethylation, 30 50 demethylation, 30 50 metallocarbene formation, 30 51-52 of f -decane, 35 332-333 primary coal liquids, 40 57 procedure, 40 66-67 product distribution, 30 49 reactions, over perovskites, 36 311 suppression by sulfur, 31 229 zeolite-supported catalysts, 39 181-188... 

Main reactions in CR processes are dehydrogenation of cyclohexane and alkylcylohexanes, cyclization of alkanes, isomerization of n-parafines, alkylcyclopentanes and alkylaromatics, and hydrocracking. Secondary reactions are the demethylation and cracking of cyclic compounds. 

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. 

On palladium, the major hydrocracking reaction is demethylation, which is equally effective, in 2-methyl- and 3-methylpentane, for primary-secondary and primary-tertiary C-C bonds, and therefore, since metallocarbenes cannot be formed from tertiary carbon atoms, can in no way be ascribed to a 1,2-dicarbene mechanism. Since demethylation is also the major process in the reaction of neopentane on palladium (34), a possible precursor for demethylation could be a 1,3-diadsorbed species attached to two metal atoms (Scheme 56) instead of one as in the metallocyclobutane mechanism (see Scheme 29). 

This reaction probably involves the participation of more than two carbon atoms and the extension of a delocalized n-electron system. The results are readily explained by the unequal stabilities of the two possible intermediates, involving carbene formation on the methyl group and on the alkyl group, respectively (Scheme 74). In the first case, demethylation easily occurs by metallocarbyne formation, while in the second case, stabilization by carbene-olefin isomerization (very fast compared to hydrocracking) prevents dealkylation. However, when the a-carbon atom in the alkyl group cannot dehydrogenate to give a metallocarbene, as for 1-methyl-1-terf-butylcyclohexane, dealkylation prevails over demethylation. 

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