(638f) Using Branched Model Compounds to Understand the Mechanism of C–C Bond Hydrogenolysis of Polyethylene on Catalytic Metal Surfaces

C–C hydrogenolysis reactions, used in ring opening and chain shortening processes, provide an alternate method to chemically upcycle polyolefins, circumventing the difficulty in processing mixed waste streams and the quality loss associated with traditional mechanical recycling. Previous studies investigating the mechanism of C–C bond hydrogenolysis in linear alkanes have shown that the reaction proceeds through analogous partially dehydrogenated intermediates of the type RCCR’* in which both C atoms have lost two H each, regardless of chain length (J. Phys. Chem. (2019), p. 5421-5432; J. Phys. Chem. (2016), p. 8125-8138). While the rates of such reactions varied dramatically with chain length because of attractive dispersive interactions that favor activations in large alkanes; those activation mechanisms went unchanged. Studies on small, branched alkanes have shown that, counterintuitively, hydrogenolysis at C–C branches (having less H) proceed through reaction intermediates that are more extensively dehydrogenated. This excess dehydrogenation occurs at methyl groups vicinal to the cleaved bond (ACS Catal. (2016), p. 469-482) in isobutane and 2,3-dimethylbutane. To elucidate the mechanism of C–C bond hydrogenolysis at the branch points of polyolefins (i.e., far from chain ends), we study here the hydrogenolysis mechanisms of isobutane, 3-ethylpentane, and 4-propylheptane at low H₂ pressures using density functional theory (DFT) calculations on Ir(111), Pt(111), and Ru(001) surfaces. This study then determined how large a model compound needs to be to reliably predict the mechanism of the analogous reaction in an arbitrarily large molecule (i.e., can isobutane-derived results be extrapolated to 5-pentyl-nonane). In doing so, we have also examined other factors such as what species cover metal surfaces during polyolefin hydrogenolysis, and how ‘large’ transitions states can be penalized relative to ‘small’ transition states on those crowded surfaces. These results give insights into how polyolefin upcycling can be understood through model compound DFT studies.