Steering C–C Bond Cleavage and Hydrogenation in Polystyrene Hydrocracking through External Brønsted Acid Sites over Ni/Zeolites

Abstract

Unlike polyolefin hydrocracking, aromatic substituents in polystyrene (PS) stabilize benzylic species and redirect degradation pathways, making selective catalytic upcycling more difficult. Consistent with this challenge, recycling rates for polyolefins reach only 5.3–10.3%, whereas PS remains below 1%. Here, PS hydrocracking over bifunctional Ni/zeolite catalysts was examined to determine how hydrogen utilization and C–C bond cleavage depend on Brønsted acid sites (BASs) and the accessibility of active sites to polymer-derived intermediates. Non-acidic Ni catalysts mainly consumed hydrogen via aromatic-ring hydrogenation with minimal PS conversion, whereas incorporating BASs enabled depolymerization under mild conditions (300 °C). Across HY zeolites and additional frameworks with similar SiO₂/Al₂O₃ ratios, conversion exhibited non-monotonic behavior: excessive acidity and/or restricted polymer accessibility reduced activity, yielding volcano-like trends and framework-dependent deviations. Poisoning tests indicated that external BASs are required to initiate backbone cleavage. Product distributions further reflected the interplay between acid-site accessibility and hydrogenation capacity, and hydrogen consumption correlated linearly with PS conversion within the Ni/zeolite series. Overall, these results highlight externally accessible Brønsted acidity as a practical handle for tuning Ni/zeolite catalysts toward LOHC-relevant products.