Thermochemical conversion of metallized plastic packaging waste: Aluminum in waste enhances polymer pre-cracking and steam-cracker precursor formation over zeolite catalysts
The rapid accumulation of metallized plastic packaging waste (MPPW), consisting of multilayer polymer–aluminum (Al) structures, represents a major environmental challenge. Herein, we present a sustainable valorization route combining catalytic pyrolysis and hydrofinishing, supported by kinetic, thermodynamic, and energy analyses, to produce steam cracker-compatible naphtha while enabling efficient Al recovery. To isolate the role of inherent Al from other additives and impurities, model feedstocks; including virgin LDPE, LDPE–Al, and PolyAl composites, were studied alongside real MPPW. Al-enhanced pre-cracking, driven by improved thermal conductivity, shifted pyrolysis products toward lighter hydrocarbons, particularly over ultrastable Y zeolite CBV-901, whereas a weaker effect was observed over SIRAL 20 HPV, demonstrating the strong dependence of Al's influence on catalyst structure. Combined model-free and model-fitting kinetic analyses were integrated with evaluation of the Thiele modulus (ϕ) and effectiveness factor (η) to quantify intraparticle diffusion and derive intrinsic (i.e., diffusion-corrected) kinetics. The results revealed that Al indirectly enhances diffusion, especially in microporous catalysts relative to amorphous counterparts, by thermally promoting polymer pre-cracking during pyrolysis. Post-pyrolysis wet gravity separation achieved >92% Al recovery at >96% purity. Hydrofinishing of the olefin-rich pyrolysis oil over commercial Ni/SiO2–Al2O3 produced steam cracker-compatible naphtha with H/C ratio and composition closely matching reference cuts. Energy balance analysis showed that the integrated process retains most of the polymer energy while generating substantial recoverable energy. Overall, this work establishes a scalable circular route for converting MPPW into naphtha with efficient Al recovery, identifying inherent Al as a polymer pre-cracking promoter rather than a contaminant.