Valorization of non-recyclable polypropylene manufacturing waste via catalytic co-hydrothermal liquefaction with biomass using industrial alumina–silica waste as a heterogeneous catalyst

Non-recyclable polypropylene (PP) manufacturing waste and lignocellulosic/algal biomass residues represent two underutilized waste streams that are commonly disposed of by landfill or incineration. This study investigated the co-hydrothermal liquefaction (co-HTL) of sawdust, rice straw, and Chlorella with non-recyclable PP manufacturing waste under subcritical water conditions (350 °C), using industrial alumina–silica waste recovered from liquid nitrogen processing as a low-cost heterogeneous catalyst. The catalyst was characterized by scanning electron microscopy (SEM), FTIR spectroscopy, and N 2 physisorption. Product distribution, elemental composition, higher heating value (HHV), Van Krevelen analysis, and FTIR spectroscopy were evaluated to determine the influence of catalyst addition on product quality. A screening-level environmental assessment was also performed to compare the estimated embodied carbon footprint of the waste-derived catalyst with reported values for synthetic and bio-derived zeolite catalysts. Catalyst addition increased the maximum bio-crude yield to 47.2 wt.% for the Chlorella–PP blend, while reducing the solid residue from 46.7 wt.% to 37.3 wt.%. The catalyst was mesoporous, with a BET surface area of 226.6 m 2 g −1 and an average pore diameter of 5.3 nm. Co-processing with PP substantially improved fuel characteristics, with the highest HHV reaching 32.95 MJ kg⁻ 1 and the O/C atomic ratio decreasing from 0.43 to 0.19, indicating enhanced deoxygenation and improved energy density. FTIR and Van Krevelen analyses were consistent with increased hydrocarbon formation and reduced oxygenated functional groups following catalytic treatment. The observed improvements are proposed to arise from catalyst-assisted cracking of polypropylene together with enhanced interactions between PP-derived hydrocarbons and biomass-derived intermediates during co-liquefaction. The waste-derived catalyst exhibited an estimated embodied carbon footprint of approximately 0.13 kg CO₂-eq kg⁻ 1 , which is substantially lower than values reported for conventional synthetic and bio-derived zeolite catalysts. These results demonstrate that industrial alumina–silica waste can serve as an effective low-cost heterogeneous catalyst for upgrading bio-crude during co-hydrothermal liquefaction, while simultaneously valorizing two problematic industrial waste streams. Further investigation of catalyst stability, aqueous-phase composition, long-term catalyst reusability, and comprehensive life cycle and techno-economic assessment is required before scale-up.