Production of aromatics from high density polyethylene over a hierarchical zeolite synthesised from steam-assisted crystallisation

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Abstract
The conversion of high-density polyethylene wastes to aromatic feedstocks in noble-metal-free systems poses a significant challenge. Here, we report a steam-assisted synthesised zeolite (SA-HZSM-5) that can promote the conversion of high-density polyethylene to aromatics, achieving simultaneously high conversion and selectivity (85.4% and 49.2%, respectively) at 260 °C, outperforming noble metal-based catalysts, such as Ru-HZSM-5, under the same conditions. The produced aromatics predominantly consist of valuable monocyclic aromatics (circa 85%). The steam-assisted synthetic method endows a specific distribution of Lewis and Brønsted acid sites in SA-HZSM-5. In situ neutron diffraction and infrared spectroscopy, coupled with modelling, reveal a dual-site adsorption mechanism of mono-olefins intermediates, facilitating their transformation into dienes with suitable spaced double bonds, a feature that is crucial for the sequential cyclisation and dehydroaromatisation into aromatic compounds. This study underscores the potential of hierarchical zeolite materials for efficient upcycling of plastic wastes into chemical feedstocks.

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