In-situ mechanistic elucidation of 1-hexadecene pyrolysis as a polyethylene model compound

The pyrolysis mechanism of polyethylene (PE) remains elusive owing to its inherent complexity and the difficulty in identifying reactive intermediates. In this work, 1-hexadecene was employed as a model compound and pyrolyzed in a custom low-pressure flow tube reactor. The pyrolysis intermediates and products were detected via in-situ electron ionization molecular beam mass spectrometry (EI-MBMS) across 300–980 °C. A total of up to 59 species were identified, including primary pyrolysis intermediates (PPIs), primary pyrolysis products (PPPs), secondary reaction intermediates (SRIs), and secondary reaction products (SRPs), with 16 key species quantified. Three distinct pyrolysis stages were revealed: at the initial stage (320–680°C), 1-hexadecene underwent limited decomposition, dominated by β -scission near the double bond, producing mainly unsaturated hydrocarbons, while hydrogen transfer reactions exhibited pronounced olefin selectivity. At the stage with increasing temperature (680–880°C), the radical concentration increased sharply, promoting extensive β -scission and the onset of secondary reactions. Aromatic precursors such as cyclopentadiene emerged, accompanied by intensified dehydrogenation. At the final stage (880–980°C), the reactant was nearly depleted, and olefins underwent deep cracking. Both Diels–Alder reactions and the chain reaction hydrocarbon aggregation (CHRCR) mechanism jointly drove aromatization, with the propargyl radical (C 3 H 3 ) serving as a key intermediate for the first aromatic ring formation. This study systematically elucidates the pyrolysis pathway of 1-hexadecene from radical initiation to aromatic and small-molecule formation, highlighting the temperature-dependent transition from primary bond scission to extensive dehydrogenation and aromatization. The findings offer fundamental insights for accurate modeling and optimization of polyethylene pyrolysis in chemical recycling.