Investigation into the Reaction Pathways and Catalyst Deactivation for Polyethylene Hydrogenolysis over Silica-Supported Cobalt Catalysts

Chemical repurposing has emerged as a promising route to valorize “end-of-use” plastic waste and mitigate its release to the environment. In this work, we applied silica-supported cobalt (5 wt % Co/SiO2) catalysts to produce liquid-range hydrocarbons (C5–C30) in the batch phase at 200–300 °C, 20–40 bar H2, and 2–36 h with high selectivity and investigated the reaction pathways, the influence of catalyst phase on the product yields and selectivity, and the catalyst deactivation mechanisms. Reaction conditions were optimized for improving liquid product yields at 275 °C, 30 bar H2, and 8 h reaction time, giving a 55% liquid product yield (C-mole basis), comprising 75% of nonsolid products, with gas yields limited to ∼19%. By tracking product evolution over time and with varying cobalt surface density, we propose a multipathway mechanism, including a dominant, nonterminal C–C cleavage route on the polymer chain over the catalyst, which drives the high liquid product selectivity. The catalyst also showed recyclability over four reactions with reduced activity and a shift in yield toward liquid products after the first reaction. It was effectively regenerated by calcination under air at 450 °C. We combined the reactivity data with powder X-ray diffraction (PXRD), thermogravimetric analysis coupled with mass spectrometry (TGA-MS), and catalyst surface areas via N2 physisorption of various fresh, spent, recycled, and regenerated catalysts to attribute the reduced activity and selectivity shift mainly to the presence of a recalcitrant polymer species embedded on the catalyst, comprising 10.5–18.5 wt % of the spent catalyst, which obstructs access to active sites and increases liquid selectivity and overshadows the influence of carbonaceous coke or catalyst phase reduction to Co. Moreover, we successfully applied the catalyst to various postconsumer polyethylene (HDPE and LDPE) samples. These results move the field toward more sustainable and economically viable catalysts for the chemical upcycling of waste plastics.