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Mechanistic Pathways for Methylcyclohexane Hydrogenolysis over Supported Ir Catalysts

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journal contribution
posted on 11.09.2014, 00:00 by Hui Shi, Oliver Y. Gutiérrez, Anmin Zheng, Gary L. Haller, Johannes A. Lercher
H/D isotope effects on methylcyclohexane hydrogenolysis over Ir/Al2O3 catalysts were examined by combining measured rates with theoretical estimates provided by partition function based analyses. Normal H/D isotope effects (rH/rD > 1) were observed for endocyclic and exocyclic C–C bond hydrogenolysis. Hydrogenolysis is concluded to occur via stepwise dehydrogenation followed by cleavage of the C–C bond and subsequent hydrogenation of the cleaved entities. The so-called “multiplet” mechanism (i.e., the C–C bond of a flat-lying physisorbed cyclic molecule is cleaved upon the attack of a coadsorbed H atom) is unequivocally excluded. For ring-opening, either C–C bond cleavage or C–H­(D) bond reformation may be rate-determining, due to their indistinguishable isotope effects under the studied conditions. C–H­(D) bond dissociation does not control the rate of C–C bond hydrogenolysis. For the exocyclic cleavage of the methyl group, a higher degree of unsaturation of the surface intermediate and the potential impact of mobile H atoms on large Ir particles are noted.

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