Unraveling the Role of Mesoporosity and Acidity of Hierarchical Aggregated HZSM-5 in HDPE Vapor Catalytic Cracking for Value-Added Hydrocarbons

The catalytic upcycling of end-of-life polyethylene by using hierarchical zeolites has garnered attention. Fine-tuning these zeolites’ micromesoporous structure and acidity is crucial for process efficiency, which requires a comprehensive understanding of their impact on polyethylene cracking. The impact of the mesoporosity and acidity of aggregated hierarchical HZSM-5 zeolites on high-density polyethylene (HDPE) vapor cracking was studied. The results indicated that the hierarchical structure enhanced accessibility to internal acid sites, facilitating monomolecular cracking and aromatization. Intercrystalline mesopores additionally heightened the alkylated aromatic yields. The Si/Al ratio shows a positive correlation toward the C5+ aliphatic hydrocarbon cracking, which boosts the formation of C2–C4 olefins and BTEX (benzene, toluene, ethylbenzene, and xylene). However, the presence of mesopores also yielded more heavy aromatics, thus increasing the risk of coking. The role of acid sites spatial distribution was investigated by poisoning the acid sites in micropores via pre-coking. The results indicated that the cracking of HDPE vapor occurred on the external acid of the zeolite, followed by the subsequent C5+ aliphatic hydrocarbon cracking on the mesopore and micropore to form aromatics and C1–C4 products. The formation of BTEX mainly occurred in the micropore, while the heavier aromatics and other alkylated benzenes were formed in the mesopore. The results of this study highlighted the importance of a proper hierarchical structure and acid site distribution in zeolites for effective and stable polyolefin cracking.