Rates and locations of C-C cleavage during the hydrogenolysis of alkyl-cyclohexanes determine the isomeric products of ring opening and the yield losses from dealkylation. Kinetically relevant transition states for C-C rupture form by sequential quasi-equilibrated dehydrogenation steps that break C-H bonds, form C-metal bonds, and desorb chemisorbed H atoms (H) from H∗-covered surfaces. Activation enthalpies (ΔH±), entropies (ΔS±), and the number of H2(g) formed with transition states are larger for 3C-xC rupture than for 2C-2C or 2C-1C cleavage for all cycloalkane reactants and Ir cluster sizes. 3C-xC rupture transition states bind to surfaces through three or more C atoms, whereas those for less-substituted 2C-2C bonds cleave via α,β species bound by two C atoms. 3C-xC rupture involves larger ΔH± than 2C-2C and 2C-1C because the former requires that more C-H bonds cleave and H∗ desorb than for the latter two. These endothermic steps are partially compensated by C-metal bond formation, whereas the formation of additional H2(g) gives larger ΔS±. C-C rupture transition states for cycloalkanes have less entropy than those for C-C bonds in acyclic alkanes of similar size because C6 rings decrease the rotational and conformational freedom. ΔH± values for all C-C bonds in a given reactant decrease with increasing Ir cluster size because the coordination of exposed metal atoms influences the stabilities of the H∗ atoms that desorb more than those of the transition states. ΔH± for 3C-xC cleavage is more sensitive to cluster size because their transition states displace more H∗ than those for 2C-2C or 2C-1C bonds. These data and their mechanistic interpretation provide guidance for how surface coordination, reaction temperatures, and H2 pressures can be used to control ring-opening selectivities toward desirable products while minimizing yield losses. These findings are consistent with trends for the hydrogenolysis of acyclic isoalkanes and seem likely to extend to C-X bond cleavage (where X = O, S, and N atoms) reactions during hydrotreating processes.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films