Mechanistic Pathways for Methylcyclohexane Hydrogenolysis over Supported Ir Catalysts

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 (r(H)/r(D) > 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.