Hydrocarbon Deposition during Polyolefin Upcycling: Irreversible Adsorption and Surface Reactions of Polyethylene and Ethylene Oligomers on Silica Supports

Catalytic conversion of polyolefins to value-added products offers an alternative route to capture value from plastic waste. Here we initially examine reactions of a polyethylene model (hexatriacontane, C₃₆H₇₄) on a Pt/SiO₂ catalyst under typical hydrogenolysis and hydrocracking temperatures, which leads to irreversibly adsorbed surface hydrocarbons identified after extraction of hexatriacontane with excess hot toluene. The IR spectra of these catalysts after extraction reveal only aliphatic C-H stretches. SiO₂ alone leads similar hydrocarbon adsorption on the surface where extended extraction fails to fully remove the adsorbed hydrocarbons from neat silica. The amount of hydrocarbon irreversibly adsorbed increases nearly 10-fold when the reactant is changed from hexatriacontane to polyethylene (M_n = 4000 Da), but the adsorbed quantity is insensitive to reaction temperature (200-300 °C). These results demonstrate significant, nonextractable hydrocarbon deposition on catalyst support surfaces without dehydrogenation catalyst present at temperatures typical of catalytic deconstruction of polyolefin waste, which may limit catalyst turnover and impact the product distribution.