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C–C cleavage in C2–C10 n-alkanes involves quasi-equilibrated C–H activation steps to form dehydrogenated intermediates on surfaces saturated with H atoms. These reactions are inhibited by H2 to similar extents for C–C bonds of similar substitution in all acyclic and cyclic alkanes and, thus, show similar kinetic dependences on H2 pressure. Yet, turnover rates depend sensitively on chain length because of differences in activation enthalpies (ΔH⧧) and entropies (ΔS⧧) whose mechanistic origins remain unclear. Density functional theory (DFT) estimates of ΔH⧧ and ΔG⧧ for C–C cleavage via >150 plausible elementary steps for propane and n-butane reactants on Ir show that hydrogenolysis occurs via α,β-bound RC*–C*R′⧧ transition states (R = H, CxH2x+1) in which two H atoms are removed from each C*. Calculated ΔH⧧ values decrease with increasing alkane chain length (C2–C8), consistent with experiment, because attractive van der Waals interactions with surfaces preferentially stabilize larger transition states. A concomitant increase in ΔS⧧, evident from experiments, is not captured by periodic DFT methods, which treat low-frequency vibrational modes inaccurately, but statistical mechanics treatments describe such effects well for RC*–C*R⧧ species, as previously reported. These findings, together with parallel studies of the cleavage of more substituted C–C bonds in branched and cyclic alkanes, account for the reasons that chain length and substitution influence ΔH⧧ and ΔS⧧ values and the dependence of rates on H2 pressure and consequently explain differences in hydrogenolysis reactivities and selectivities across all alkanes.
Hibbitts et al. (Tue,) studied this question.