(391f) Probing the Mechanisms of Hydrogen-Free Polyethylene Depolymerization over Ruthenium-Zeolite Catalysts Using Automated Analytical Platforms

Emerging plastic depolymerization techniques, including hydrogenolysis, offer promising new routes towards achieving a circular plastics economy. However, the use of molecular hydrogen in these techniques raises significant safety, sustainability, and economic concerns. Low-temperature, hydrogen-free processes using bifunctional metal-acid catalysts, such as Pt/γ-Al₂O₃, are effective in producing gaseous and liquid hydrocarbons, especially long-chain alkylaromatics. However, these processes require long reaction times (~24 hr) and produce wide, convoluted product distributions. To address the problem of narrowing these distributions, we must first establish an understanding of the complex kinetics and mechanisms of hydrogen transfer in these environments. In this study, we demonstrate the activity of catalysts comprised of ruthenium and zeolite beta in polyethylene depolymerization reactions (280ºC, 24 hr, 9 bar N₂). These reactions are shown to produce mixtures of gaseous (C₃-C₅) and liquid (C₆-C₂₀) hydrocarbons alongside larger waxy products. Mechanistic probing of these reactions is challenging due to slow analytics, as multiple catalytic pathways – including isomerization, cyclization, and aromatization – lead to hundreds of products. To address this challenge, we developed an automated analytical workflow which: 1) implements retention time-based labelling for rapid gas chromatography (GC) mass spectrometry (MS) and GC-FID peak matching; 2) extrapolates empirical response factors to estimate those for compounds within the same homologous series or chemical lump; and 3) integrates these methods into a platform facilitated by several user interfaces. This work validates the efficiency of this workflow by comparing its results to manual analyses, finding that it achieves a 10x reduction in analysis time. We subsequently apply this workflow to probe the effects of noble metal and Brønsted acid site intimacy and co-fed aromatic intermediates on hydrogen transfer kinetics and mechanism. Overall, our analytical platform enables the efficient probing of hydrogen transfer mechanisms and intermediates in hydrogen-free polyethylene depolymerization.