Testing of ZSM-5/NiO/Al₂O₃ composite catalytic system in a fixed-bed cavity microwave pyrolyzer for the recycling of plastic into structured carbon materials and fuel

ABSTRACT This study investigated the conversion of plastic waste into structured carbonaceous materials via microwave-assisted catalytic pyrolysis using ZSM-5, Al₂O₃, and NiO-based catalysts. With an emphasis on the synergistic effects of multi-component catalysts in enhancing carbon quality and graphitization, binary and ternary catalysts, such as Al₂O₃/NiO, ZSM-5/NiO, ZSM-5/Al₂O₃, and ZSM-5/Al₂O₃/NiO, were used in a fixed-bed cavity microwave pyrolyzer to recycle mixed-density HDPE/LDPE plastics. Carbon nanotubes (CNTs) were the primary product, while liquid and gaseous products were examined as secondary outputs to support the pyrolysis process. Experimental results demonstrated that the ternary ZSM-5/Al₂O₃/NiO system achieved the highest conversion efficiency of 86%, yielding 14% of highly structured CNTs. Similarly, the ZSM-5/Al₂O₃ catalyst produced higher-quality CNTs with an 18% yield. The highest intensity ratio of D and G bands in Raman spectroscopy was 1.14 for carbonaceous material produced with the ZSM-5/NiO and Al₂O₃/NiO catalyst system, followed by ZSM-5/Al₂O₃ (1.03), Al₂O₃/NiO (0.94), and ZSM-5/NiO (0.86). The liquid product was composed of aromatic hydrocarbons and phenolic compounds, while gas analysis revealed hydrogen as a major component of the gaseous product. These findings highlight the synergistic role of acidic ZSM-5, supportive Al₂O₃, and redox-active NiO in enhancing cracking, aromatization, and dehydrogenation reactions.