Effect of Natural Zeolite Modification Route on the Catalytic Pyrolysis of Post-Consumer Polystyrene Toward Styrene-Rich Liquid Products

The catalytic pyrolysis of post-consumer polystyrene (PS) offers a potential route to obtain styrene-rich liquid fractions from plastic waste. In this study, natural zeolites were modified by thermal activation (AT-ZN), acid treatment (AA-ZN), and protonic ion exchange (H-ZN), and their performance was evaluated under different pyrolysis temperatures (400–500 °C), heating rates (10–20 °C min−1), and catalyst loadings (5–10 wt.%). Thermogravimetric analysis indicated that zeolite incorporation shifted the apparent PS degradation profile toward lower temperatures, suggesting that the modified solids altered the polymer’s thermal conversion behavior. Product-yield analysis showed that H-ZN provided the most favorable phase distribution, producing high liquid fractions while limiting solid-residue formation. AT-ZN exhibited an intermediate, comparatively stable response. In contrast, AA-ZN promoted greater solid formation and lower liquid recovery, suggesting that more severe catalytic conditions may favor secondary reactions and the accumulation of carbonaceous residues. Targeted GC–MS analysis revealed that styrene was the dominant aromatic compound among the quantified products, with H-ZN consistently showing the highest styrene proportion in the analyzed liquid fraction. Correlation analysis and ANOVA further indicated that the influence of temperature, catalyst loading, and their interactions depended strongly on the zeolite modification route. Overall, the results demonstrate that the route of modification of the natural zeolite strongly affected its composition, textural properties, acidity distribution, thermal behavior, and catalytic performance during PS pyrolysis. XRF, N2 adsorption–desorption, NH3-TPD, TGA/DTG, and FTIR characterization showed that AA-ZN exhibited the highest Si/Al ratio and BET surface area, whereas H-ZN presented the highest total acidity and the largest contribution of medium- and strong-acid sites. The combined characterization and pyrolysis results indicate that the preservation of styrene-rich liquid products was governed by the balance between acid-site distribution and pore accessibility, rather than by surface area or total acidity considered in isolation.