(388c) Understanding Hydrogenolysis Activity and Binding Strength of Aromatic and Naphthene Model Compounds on Iridium and Ruthenium Surfaces during Polystyrene Upcycling

Polymer upcycling by hydrogenolysis has gained attention for various hydrocarbon polymers resistant to solvolysis and other upcycling and chemical recycling methods targeting heteroatom linkages between monomers. Gas-phase C–C hydrogenolysis reactions of alkanes have been extensively studied on H-covered Ir surfaces; however, these conditions markedly differ from conditions during polymer upcycling. To determine the catalytic mechanisms related to polystyrene upcycling, we examined hydrogenation and hydrogenolysis reactions of 2-butylbenzene (BB) and 2-butylcyclohexane (BCH) model compounds. C–C hydrogenolysis studies of saturated acyclic and cyclic hydrocarbons have shown that dehydrogenation elementary steps within the reaction are quasi-equilibrated, forming a pool of hydrocarbons with variable levels of saturation. Alternatively, feeding polystyrene enables initial hydrogenation of the aromatic rings prior to C–C scission, greatly impacting H₂ pressure dependence of hydrogenolysis rates. Preliminary data for BB and BCH hydrogenolysis on Ir(111) surfaces show that ring opening reactions at the para and meta positions generally have lower free energy barriers than ring opening at the ortho position or cleaving exocyclic C–C bonds, which would result in branched products from the upcycling of polystyrene. Among exocyclic mechanisms, C₄ dealkylation dominates over the C₂ cleavage reaction, indicating that it would be easier to remove aromatic substituents from polystyrene than to break the polymer's C–C backbone. Preliminary data for the adsorption of styrene dimers and trimers on Ru(0001) surfaces indicate a strong impact of phenyl orientation on adsorbates with several aromatic ring substituents, implying a relationship between polystyrene tacticity and binding mode onto Ru catalysts. These data inform the catalytic mechanisms of ring opening, dealkylation, hydrogenation and dehydrogenation of alkylbenzenes along with the thermodynamics of adsorption for styrene dimers and trimers providing insight into polystyrene upcycling.