Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

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 (Δ H ⧧ ), entropies (Δ S ⧧ ), and the number of H 2 (g) formed with transition states are larger for 3 C– x C rupture than for 2 C– 2 C or 2 C– 1 C cleavage for all cycloalkane reactants and Ir cluster sizes. 3 C– x C rupture transition states bind to surfaces through three or more C atoms, whereas those for less-substituted 2 C– 2 C bonds cleave via α,β species bound by two C atoms. 3 C– x C rupture involves larger Δ H ⧧ than 2 C– 2 C and 2 C– 1 C 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 H 2 (g) gives larger Δ S ⧧ . C–C rupture transition states for cycloalkanes have less entropy than those for C–C bonds in acyclic alkanes of similar size because C 6 rings decrease the rotational and conformational freedom. Δ H ⧧ 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. Δ H ⧧ for 3 C– x C cleavage is more sensitive to cluster size because their transition states displace more H* than those for 2 C– 2 C or 2 C– 1 C bonds. These data and their mechanistic interpretation provide guidance for how surface coordination, reaction temperatures, and H 2 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.