Structure sensitivity of hydrogenolytic cleavage of endocyclic and exocyclic C-C bonds in methylcyclohexane over supported iridium particles

Structure sensitivities, H-2 pressure effects, and temperature dependencies for rates and selectivities of endo- and exocyclic C-C bond cleavage in methylcyclohexane were studied over supported Ir catalysts. The rate of endocyclic C-C bond cleavage first decreased and then increased with declining Ir dispersion from 0.65 to 0.035. The ring opening (RO) product distribution remained unchanged with varying H-2 pressure on small Ir particles, while further shifting to methylhexanes with increasing H-2 pressure on large particles. In contrast, the rate and selectivity of exocyclic C-C bond cleavage decreased monotonically with increasing H-2 pressure and decreasing Ir particle size. The distinct dependencies of endocyclic and exocyclic C-C bond cleavage pathways on Ir dispersion and H-2 pressure suggest that they are mediated by surface species with different ensemble size requirements. DFT calculations were performed on an Ir-50 cluster and an Ir(111) surface, with or without pre-adsorbed hydrogen atoms, to provide insight into the observed effects of particle size and H-2 pressure on RO pathways. On small Ir particles, the calculated dehydrogenation enthalpies for all endocyclic bonds were similar and affected to similar extents by H-2 pressure; on large particles, the selectivity to n-heptane (via substituted C-C bond cleavage) was even lower than on small particles as a result of the least favorable adsorption and dehydrogenation energetics for hindered bonds. (c) 2012 Elsevier Inc. All rights reserved.