Proton‐Exchanged Montmorillonite With Interlayer Accessibility as a Solid Acid Catalyst for the Upcycling of Polyoxymethylene

Engineering plastics such as polyoxymethylene (POM) are highly resistant to depolymerization due to their robust C─O backbones, posing a major challenge for sustainable recycling. Herein, we report a solid acid catalyst based on proton-exchanged montmorillonite (H-Mont) that efficiently upcycles POM into cyclic acetals under mild conditions. H-Mont exhibits high catalytic performance, converting both pristine and post-consumer POM, outperforming conventional solid and liquid acid catalysts in both catalytic efficiency and reusability. Mechanistic studies reveal that interlayer accessibility generated by solvent-induced swelling, together with strong Brønsted acidity of H-Mont, synergistically overcomes interfacial limitations between the catalyst and semi-crystalline POM, enabling efficient depolymerization. This represents the first demonstration of a streamlined, reusable heterogeneous catalysis for POM upcycling toward cyclic acetals and establishes a strategy for transforming chemically resilient engineering plastics into valuable chemicals.