Methyl substituents at C-C bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic C2-C8 alkanes on Ir, Rh, Ru, and Pt catalysts. C-C cleavage transition states form via equilibrated dehydrogenation steps that replace several C-H bonds with C-metal bonds, desorb H atoms (H*) from saturated surfaces, and form λ H2(g) molecules. Activation enthalpies (δH‡) and entropies (δS‡) and λ values for 3C- xC cleavage are larger than for 2C-2C or 2C-1C bonds, irrespective of the composition of metal clusters or the cyclic/acyclic structure of the reactants. 3C- xC bonds cleave through ∝,ß,γ- or ∝,ß,γ,δ-bound transition states, as indicated by the agreement between measured activation entropies and those estimated for such structures using statistical mechanics. In contrast, less substituted C-C bonds involve ∝,ß-bound species with each C atom bound to several surface atoms. These ∝,ß configurations weaken C-C bonds through back-donation to antibonding orbitals, but such configurations cannot form with 3C atoms, which have one C-H bond and thus can form only one C-M bond. 3C-xC cleavage involves attachment of other C atoms, which requires endothermic C-H activation and H* desorption steps that lead to larger δH‡ values but also larger δS ‡ values (by forming more H2(g)) than for 2C-2C and 2C-1C bonds, irrespective of alkane size (C2-C8) or cyclic/acyclic structure. These data and their mechanistic interpretation indicate that low temperatures and high H2 pressures favor cleavage of less substituted C-C bonds and form more highly branched products from cyclic and acyclic alkanes. Such interpretations and catalytic consequences of substitution seem also relevant to C-X cleavage (X = S, N, O) in desulfurization, denitrogenation, and deoxygenation reactions.
ASJC Scopus subject areas
- Colloid and Surface Chemistry