Catalytic solvothermal upcycling of mixed polyolefin waste into gasoline-range liquid fuels: Solvent-assisted selectivity and coke kinetics over zeolite-based catalysts

Polyolefins upcycling into high-quality oil is constrained by mass-transfer limitations, excessive temperatures, high hydrogen pressures, and rapid catalyst deactivation in conventional technologies. Without relying on extreme conditions (≥450 °C, long residence times, high H 2 pressures), here we depolymerized mixed polyolefin waste (MPW: PP/LDPE/LLDPE/HDPE) at 400 °C in non-oxygenated solvents (n-heptane and toluene) in presence of large-pore (USY, Beta) and medium-pore (ZSM-5) zeolites, including Ni- and Pt-impregnated USY. Under solvothermal liquefaction (STL) in n-heptane, we achieved 93.6 % solid conversion and 85.9 wt% oil yield from MPW with USY zeolite directing selectivity toward light fraction aromatics (58.8 wt% monoaromatics with a C 7 -C 9 selectivity of 48.2 %) outperforming Beta and ZSM-5, and far exceeding non-catalytic STL (trace monoaromatics and 43.6 wt% olefins). USY retained limited catalytic stability for two consecutive cycles with oil yields near 85 wt% and monoaromatic selectivity above 52 wt%, before gradual dealumination and the loss of strong Brønsted acid sites in the third cycle; it shifted the reaction pathway from deep β-scission toward isomerization, even though the deactivated USY effectively suppresses olefins and yields 30 times more monoaromatics than the non-catalytic test. Notably, coke quantity and regeneration severity are decoupled: USY accumulates the highest coke loading (34.3 wt%) yet exhibits the lowest combustion enthalpy (9.4 kJ/g-coke) among the three zeolites, whereas ZSM-5 deposits the least coke (11.1 wt%) but requires the highest regeneration energy (26.5 kJ/g-coke) due to dense graphitic deposits. The non-oxygenated solvent further modulates this behavior; on USY, switching from n-heptane to toluene reduces the soft-coke fraction from 34 % to 16 % and raises ΔH by 49 %, indicating that the solvent participates in shaping coke chemistry.