Catalytic co-pyrolysis of polylactic acid and high-density polyethylene over hierarchical zeolite catalysts

Catalytic fast co-pyrolysis of polylactic acid (PLA) and high-density polyethylene (HDPE) was investigated using a tandem micro-reactor/gas chromatogram spectrometry (TMR-GC/MS) to selectively produce BTEX (benzene, toluene, ethylbenzene, and xylene). Non-catalytic co-pyrolysis revealed strong synergistic suppression of oxygenated products, which was most pronounced at a PLA/HDPE mass ratio of 0.75/0.25, originating from efficient hydrogen transfer between oxygen-rich PLA-derived intermediates and hydrogen-rich HDPE-derived species. To enhance deoxygenation and aromatization, hierarchical HZSM-5 catalysts were prepared via alkaline desilication using NaOH alone (N-HZ) or in combination with organic bases (TEAOH or TPAOH). Compared with parent HZSM-5, hierarchical catalysts exhibited improved mass transport and increased accessibility of Brønsted acid sites, enabling more efficient conversion of bulky PLA-derived oxygenates. Among the catalysts tested, TPAOH-assisted hierarchical HZSM-5 (TP-HZ) showed the highest BTEX yield of 13.8 wt.%, together with the lowest oxygenate retention and enhanced CO formation, indicating intensified decarbonylation pathways. This superior performance is attributed to the well-balanced micro-mesoporous structure of TP-HZ, which preserves MFI shape selectivity while facilitating diffusion of large intermediates, combined with its higher density of accessible strong acid sites. Multi-cycle experiments further demonstrated that hierarchical catalysts exhibit improved resistance to deactivation compared with parent HZSM-5, highlighting the importance of pore architecture in maintaining BTEX productivity under continuous operation. Overall, this study demonstrates that coupling feedstock-level hydrogen-transfer synergy with hierarchically engineered zeolite catalysts enables efficient upgrading of PLA-containing mixed plastics into high-value aromatics under realistic processing conditions.