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 (ΔH⧧), entropies (ΔS⧧), and the number of H2(g) formed with transition states are larger for C−C rupture than for C−C or C−C cleavage for all cycloalkane reactants and Ir cluster sizes. C−C rupture transition states bind to surfaces through three or more C atoms, whereas those for less-substituted C−C bonds cleave via α,β species bound by two C atoms. C−C rupture involves larger ΔH⧧ than C−C and C−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 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 C−C cleavage is more sensitive to cluster size because their transition states displace more H* than those for C−C or C−C 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.