Thermo-Catalytic Pyrolysis of Polypropylene over Co- and Zn-Supported H-Mordenites: Mechanistic Insights into Upcycling

Abstract Polypropylene (PP), one of the most widely used plastics, is a major contributor to global plastic waste. H-mordenite (HM) zeolites impregnated with cobalt (Co) and zinc (Zn) were employed as catalysts for the thermo-catalytic pyrolysis of PP to generate light hydrocarbons and hydrogen. Comprehensive characterization of the catalysts shows that metal incorporation reduced the BET surface areas due to partial pore filling, while TEM and FESEM images confirm unchanged crystallinity with well-dispersed Co and Zn species. Temperature desorption studies with NH3 show abundant Brønsted and Lewis acid sites in HM, with Co and Zn further tuning the acidity of zeolite. The H2-TPR studies indicate that Co formed reducible oxide species capable of generating metallic Co sites under reaction conditions, while Zn remains primarily as framework-stabilized Zn2+ with limited reducibility. The XPS studies confirm the coexistence of metallic Co and cobalt oxides, promoting dehydrogenation and C–C bond scission, whereas Zn remained as Zn2+ and –O–Zn–OH species, enhancing Lewis acidity and limited secondary cracking. Catalytic pyrolysis experiments yielded over >90% PP conversion with 65% selectivity toward light olefins (C2=–C4=) at 450 °C for a 1:1 catalyst-to-polymer ratio with Co-HM, outperforming similar experiments with Zn-HM and pristine HM. The catalyst stability test performed for both Co-HM and Zn-HM catalysts showed propene selectivity to decrease slightly from 53 to 49% after 20 h time-on-stream (TOS) studies, while for the Zn-HM catalyst, a decrease from 45 to 40% was observed over the same period. Density functional theory (DFT) calculations are consistent with these observations, showing that Co sites strongly stabilize propene intermediates via π back-donation and lower dehydrogenation barriers, while Zn exhibits weaker binding and limited stabilization due to filled d-orbitals. These findings suggest that although Co-HM and Zn-HM yield comparable polypropylene conversion with similar activity, each catalyst operates through a different catalytic pathway to yield the final products. While Co-HM provides reducible Co species with Brønsted–Lewis acidic sites that favor olefin formation, Zn-HM primarily modifies Lewis acidity, resulting in different distribution of products.