(638h) Understanding the Mechanisms of Aromatic Hydrogenolysis and Hydrogenation on Metal Surfaces for Polymer Upcycling Applications

Polymer upcycling by hydrogenolysis has gained attention for various hydrocarbon polymers resistant to other upcycling and chemical recycling approaches. Gas-phase C–C hydrogenolysis reactions have been extensively studied on H-covered Ir surfaces; however, these conditions are vastly different to polymer melt conditions during polymer upcycling. To determine the catalytic mechanisms related to polymer upcycling, we examined hydrogenations and hydrogenolysis reactions of a range of model compounds: benzene, toluene, butylbenzene, and 2,4-diphenylpentane. C–C hydrogenolysis studies of alkanes have shown that dehydrogenation elementary steps within the reaction are quasi-equilibrated, forming a pool of hydrocarbons with variable levels of saturation. Polyethylene alkanes can form aromatics, releasing H₂ available for hydrogenolysis reactions elsewhere in the backbone, and alternatively, aromatics in polystyrene can be hydrogenated in reactions competing with hydrogenolysis routes. Preliminary data for toluene hydrogenolysis on Ir(111) surfaces show that ring opening (endocyclic) mechanisms generally have lower free energy barriers than demethylating (exocyclic) mechanisms, which would result in branched products from the upcycling of polystyrene. The methyl substituent has a weak impact on the preferred ring opening location, with activation barriers generally favoring the formation of 2-methyl-hexane over heptane or 3-methylhexane. Toluene demethylation occurs through two low-energy barriers, one cleaving C₆H₅–CH* and the other cleaving C₆H₃–CH*. Preliminary data for hydrogenation steps forming cyclohexane from benzene indicate that hydrogenation barriers are lower than C-C cleavage activation barriers, but C-C cleavage activation barriers increase with increasing saturation of cyclic hydrocarbons. These data inform the catalytic mechanisms of ring opening and cyclization, hydrogenation and dehydrogenation and thus provide insights into polyolefin and polystyrene upcycling.