Effects of Metal–Acid Proximity on Aromatics Production in CO2-Assisted Catalytic Pyrolysis of Polypropylene over Fe-Modified ZSM-5

CO2-assisted catalytic pyrolysis presents a viable and promising approach to addressing plastic waste pollution and mitigating climate change. However, the effects of the metal–catalyst combination mode and the spatial distance between metal–acid sites on catalytic performance remain unclear. In this study, the reaction behaviors of the configurations, Fe3O4 and ZSM-5 in tandem catalysis (Fe3O4&HZ), their physical mixture (Fe3O4-HZ), and Fe-loaded ZSM-5 (Fe/HZ), were compared in polypropylene pyrolysis under a CO2 atmosphere. The aromatic contents followed this order: Fe/HZ > Fe3O4-HZ > Fe3O4&HZ > ZSM-5 > Fe3O4. Specifically, Fe/HZ with the highest degree of metal–zeolite proximity achieved an aromatic content of 66.1%, significantly higher than the 34.2% obtained with Fe3O4&HZ, demonstrating that closer metal–acid proximity promoted aromatic formation. Moreover, Fe/HZ significantly reduced coke deposition. Based on characterization results from XRD, SEM, TEM, XPS, and NH3-TPD, the enhanced spatial proximity between metal and acid sites strengthened the functional synergy between iron-based redox sites and zeolitic Brønsted acid sites. This synergy facilitated the reverse water–gas shift reaction of CO2, which consumed hydrogen generated during aromatization and shifted the reaction equilibrium toward enhanced aromatic production. These findings would offer theoretical and strategic insights into the optimization of CO2-assisted catalytic pyrolysis systems for the sustainable upcycling of plastic waste.