Zeolites are widely studied as promising catalysts for the chemical recycling of plastics due to their inherent microporosity and shape‐selective properties. The accessibility of polymer chains to internal acid sites is limited, and initial reactions are supposed to occur near the external surface. A deeper understanding of the role of external acid sites is required to optimize the structure and morphology of the zeolites. In this study, a novel ZSM‐5 zeolite catalyst with Brønsted acid sites selectively localized only on the external surface (ZSM‐5‐SA) is synthesized via selective ion exchange using bulky tetrapropylammonium ions, followed by calcination. Catalytic testing of low‐ and high‐density polyethylene revealed that the initial degradation is mainly triggered at Brønsted acid sites located on the external surface and near‐surface internal regions. Once protonated, the polymer chains undergo β‐scission, leading to similar product distributions regardless of the acid site density. These findings highlight that a small number of spatially accessible acid sites can effectively initiate and propagate the cracking reaction. These findings establish a direct link between the acid site location and reaction pathway and offer a rational design principle for advanced zeolite catalysts tailored for polymer cracking and chemical upcycling.
Nakai et al. (Thu,) studied this question.