Polyethylene upcycling to selective monoaromatics: Revisiting porosity-acidity roles in ZSM-5 frameworks

Plastic waste demands catalytic routes beyond mechanical recycling. Under fast LDPE cracking (500 °C, 1 min) within a helium atmosphere (1.0 mL/min), we quantified how ZSM-5 porosity and acidity jointly control monoaromatic formation by comparing two parent (microporous) ZSM-5 zeolites (different Si/Al ratios) with NaOH-desilicated hierarchical analogues (0.3–0.5 M), with and without 2.5 wt% Ni. Parent ZSM-5 delivered 36.1–39.1 wt% monoaromatics, whereas desilication increased light olefins and lowered monoaromatics to 26.6–35.2 wt%. These results revise the assumption that hierarchical ZSM-5 universally enhances aromatics, revealing external and mesopore Brønsted acidity drives C–C scission, while aromatization requires confinement and longer residence time at stronger acid sites within micropores, with Ni accelerating the rate-limiting dehydrogenation of cyclic carbocations. Silanol formation improved Ni dispersion but promoted the formation of hardly reducible Ni species and higher solid residue. We introduced “monoaromatic-selective acidity (MSAcid),” integrating reactive acidity and microporosity to unify monoaromatic yield trends across metal-free and Ni-modified zeolites. Monoaromatic yields showed exponential-asymptote patterns with MSAcid, predicting ultimate limits of 39.4 wt% for individual MFI and 52.9 wt% for Ni-MFI. Heart-cut analysis over Ni-MFI identified 320–350 °C and 350–400 °C as the dominant formation windows for olefins and monoaromatics, respectively, providing a practical design metric for polyolefin-to-aromatic catalysis. • Mesopores promoted β-scission but reduced monoaromatic formation from LDPE. • Both in-micropore Brønsted and Lewis acidity contributed to PE-to-aromatics. • Desilication lowered monoaromatics from 36.1–39.1 wt% to 26.6–35.2 wt%. • Ni-MFI synergy enhanced monoaromatics via Brønsted acidity and dehydrogenation. • Loading 2.5 wt% Ni enabled up to 34.3% monoaromatic promotion on MFI.