(638g) Reactivity Investigation and Mechanistic Insights for the Hydrogenolysis of Polyethylene over Silica-Supported Earth-Abundant Cobalt Catalysts

Chemical repurposing of plastics, especially polyolefins, has emerged as a promising route to valorize the “end-of-use” plastic waste and mitigate its environmental release. Hydrogenolysis with rare-earth metal catalysts, particularly Ru, has been shown to effectively depolymerize polyethylene (PE) waste. However, the selectivity towards valuable liquid-range products remains a challenge. In addition, utilizing earth-abundant metal catalysts is essential to ensure the sustainability of plastics valorization. Here we show the ability of silica-supported, earth-abundant cobalt (5 wt % Co/SiO₂) catalysts to produce liquid-range products (C₅-C₃₀) with high selectivity, having tested from 200-300°C, 20-40 bar H₂, and 2-32 h. At an optimized reaction condition of 275°C, 30 bar H₂, and 8 h reaction time, 55% yield towards liquid products on a carbon-mole basis was achieved, which comprised 75% of non-solid products, with gas yields limited to ~19%. Furthermore, by tracking the product evolution, we infer a multi-pathway mechanism, including a dominant, non-terminal cleavage mechanism over the Co catalyst on the polymer chain, which drives the high liquid product selectivity. Lastly, we demonstrate the real-world viability of the catalyst, successfully applying it to various post-consumer polyethylene samples (HDPE jug, LDPE bag, and LDPE bottle) and demonstrating its effective recyclability and regenerability post-reaction. The catalyst remained active to polyethylene over 4 reactions, but the selectivity of reaction products shifted heavily towards liquids after the first reaction. After regenerating the catalyst in air at 450°C, selectivity shifted back towards high gas yields. We combined powder X-ray diffraction (PXRD) of the post-reaction recycled and regenerated catalysts, finding that the cobalt phase shifted from fully oxidized Co₃O₄ to CoO under reactions conditions, but that the shift in selectivity is likely due to coke formation. These results and insights move the field toward more sustainable and economically viable catalysts for chemical upcycling of waste plastics.