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 CC 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 CC rupture form by sequential quasi-equilibrated dehydrogenation steps that break CH bonds, form Cmetal bonds, and desorb chemisorbed H atoms (H*) from H*-covered surfaces. Activation enthalpies (Delta H-), entropies (Delta S-), and the number of H-2(g) formed with transition states are larger for 3CxC rupture than for 2C2C or 2C1C cleavage for all cycloalkane reactants and Ir cluster sizes. 3CxC rupture transition states bind to surfaces through three or more C atoms, whereas those for less-substituted 2C2C bonds cleave via alpha,beta species bound by two C atoms. 3CxC rupture involves larger Delta H- than 2C2C and 2C1C because the former requires that more CH bonds cleave and H* desorb than for the latter two. These endothermic steps are partially compensated by Cmetal bond formation, whereas the formation of additional H-2(g) gives larger Delta S-. CC rupture transition states for cycloalkanes have less entropy than those for CC bonds in acyclic alkanes of similar size because C6 rings decrease the rotational and conformational freedom. Delta H- values for all CC 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. Delta H- for 3CxC cleavage is more sensitive to cluster size because their transition states displace more H* than those for 2C2C or 2C1C 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 CX bond cleavage (where X = O, S, and N atoms) reactions during hydrotreating processes.