Selective upcycling of virgin and post-consumer polypropylene waste into aromatic-rich oil via solvothermal liquefaction in tetralin

Solvothermal liquefaction (STL) of polypropylene (PP) offers a fundamentally different reaction environment from conventional dry pyrolysis, yet its potential for controlled depolymerization of real plastic waste remains underexplored. In this study, virgin and post-consumer polypropylene were converted in a hydrogen-donor solvent (tetralin) at 400-450 °C and residence times of 0-5 h, without the use of catalysts. Near-complete conversion (99.9 wt%) was achieved at 450 °C within 1 h, yielding up to 99.8 wt% liquid product with negligible solid residue (<0.05 wt%) and minimal gas formation (<0.2 wt%). Product oils were dominated by monoaromatic hydrocarbons (up to 75 wt% under inert conditions and >90 wt% with added hydrogen), with a narrow carbon number distribution centered in the C 10 -C 12 range (68-70 wt% under inert conditions, increasing to roughly 80 wt% with added hydrogen). Systematic variation of plastic-to-solvent ratio revealed an optimal ratio of 1:5, balancing hydrogen availability and reactive fragment interactions to maximize aromatic stabilization. Post-consumer PP exhibited conversion behavior and product distributions comparable to virgin polymer, demonstrating strong tolerance to additives and pigments. Blank-run recovery and compositional trends indicate that tetralin is thermally stable in the absence of PP and functions as a hydrogen-transfer medium during PP conversion, suppressing olefin accumulation, repolymerization, and solid formation. Compared with conventional pyrolysis, this solvothermal route enables rapid, high-yield conversion of PP into a stable aromatic-rich liquid with minimal gas and solid formation, without requiring a catalyst. The resulting monoaromatic rich-oil is heavily dominated by a narrow carbon number distribution centered in the C 10 -C 12 range, demonstrating high compatibility with commercial petroleum refinery streams. This highlights a potential scalable, non-catalytic chemical upcycling pathway for upgrading post-consumer polyolefin waste into a refinery-compatible hydrocarbon feedstock or high-octane fuel blending component.