A hierarchical ZSM-5 zeolite catalyst crystallized at 120 °C held its gasoline-range fraction above 70% for 6.83 hours of continuous microwave-assisted pyrolysis of high-density polyethylene and still retained 63.55% after 11 hours — performance figures that put catalyst deactivation, the chronic cost driver in thermochemical plastic valorization, at the center of a new peer-reviewed study.

Huiyan Zhang's team at Southeast University published the work in Sustainable Carbon Materials on 8 April 2026 (DOI: 10.48130/scm-0026-0013). The study identifies crystallization temperature as a single, readily controllable synthesis parameter that governs the balance between porosity, acidity and lifetime in ZSM-5 used for catalytic plastic pyrolysis.

Why catalyst lifetime matters for the stream. Catalytic pyrolysis converts energy-rich plastic waste into olefins, aromatics and liquid fuels, and ZSM-5 is the workhorse zeolite for these reactions because its microporous framework and strong Brønsted acidity drive selective cracking and aromatization. The problem is runtime. Polymer decomposition generates bulky, reactive intermediates that diffuse slowly through the zeolite's narrow channels. Extended residence promotes coke formation, blocks pores, cuts off acid sites and deactivates the catalyst within hours. Secondary mesopores can improve molecular transport, but researchers have lacked a consistent account of how one controllable synthesis variable could balance porosity and acidity while holding performance during continuous conversion.

The experiment. The researchers synthesized five hierarchical ZSM-5 catalysts in a one-pot hydrothermal process at 120, 140, 180, 200 and 220 °C, each held for 24 hours. After calcination and ammonium ion exchange, the zeolites were converted to protonic form and labelled T-120 through T-220. X-ray diffraction confirmed all five retained the phase-pure MFI framework, so performance differences trace to morphology and acidity rather than structural change. Scanning electron microscopy showed the distinction: T-120 consisted of open, nanocrystal-assembled aggregates with abundant intercrystalline spaces, while higher crystallization temperatures produced progressively coarser, more densely intergrown crystals.

Nitrogen adsorption put Brunauer-Emmett-Teller surface areas at 434–467 m² g⁻¹ across all samples. Mesopore volume diverged sharply: T-180 held the largest at 0.157 cm³ g⁻¹; T-220 the smallest at 0.075 cm³ g⁻¹. Ammonia temperature-programmed desorption showed crystallization temperature shifted both total acidity and acid-strength distribution.

For testing, the team deposited the zeolites on silicon carbide foam in a continuous microwave-assisted fixed-bed reactor, feeding HDPE at 60 g h⁻¹ and pyrolyzing at 500 °C under nitrogen. They defined catalyst lifetime by the proportion of liquid products volatilizing at or below 200 °C — the gasoline-range fraction — analyzed by thermogravimetric analysis and gas chromatography-mass spectrometry at defined operating intervals.

The numbers. Every catalyst decayed. T-120 decayed slowest, losing 2.42 percentage points of gasoline-range fraction per hour. T-200 fell below the 70% threshold after 2.36 hours; T-220 crossed it after 3.16 hours. T-120 stayed above 70% for 6.83 hours and retained 63.55% at 11 hours.

Product chemistry tracked the same pattern. For T-140, the combined benzene, toluene and xylene fraction collapsed from 38.3 wt% at 0.5 hours to 4.0 wt% at 6.5 hours, while olefins and paraffins rose — a signature of fading catalytic upgrading. T-120 held aromatics far longer, declining more gradually from 57.6 wt% to 30.8 wt% over 11 hours.

The interpretation. Durability, the authors conclude, does not hinge on maximizing acidity or mesopore volume in isolation. T-180 had the largest mesopore volume but not the longest life. The winning combination was T-120's open nanograin architecture plus sufficient acidity, which preserved accessible reaction pathways — secondary cracking and aromatization — as carbon deposits accumulated. Crystallization temperature, applied as a single control knob, could simplify industrial production of durable zeolites for converting discarded plastics into fuels and chemicals.

The research was funded by the National Natural Science Fund for Distinguished Young Scholars of China (Grant No. 52425607) and the Natural Science Foundation of Jiangsu Province (BK20240010).

For now, these results sit at bench scale — 60 g h⁻¹ of HDPE in a fixed bed, not a tonne-per-hour line. The milestone to watch is whether the 120 °C one-pot synthesis survives scale-up testing with mixed, contaminated plastic feeds, where coke loads and feed impurities will test the 11-hour lifetime claim against real waste streams.