Intrinsic Activity of Silica-Alumina for the Conversion of Polyethylene into Tunable Aromatics Below Pyrolytic Temperatures

Plastic waste is a growing catastrophe that lacks global strategy, largely due to inadequate recycling capabilities. The search for a superior recycling process has spanned decades and disciplines, but recently, low-temperature hydroconversion reactions have gained increasing attention. These reactions typically produce fuel-range alkanes using moderate hydrogen pressures, but questions surround the economics of such conversions given the products are of relatively low monetary value. Other low-temperature reactions that produce value-added products have been proposed. One prominent example is the hydrogen-free tandem hydrogenolysis/aromatization reaction pioneered by Zhang et al., which converts polyethylene into alkylaromatics using a Pt/Al₂O₃ catalyst at 280 °C. These compounds are valuable as lubricants and are currently produced through more energy-intensive routes. A techno-economic assessment for this type of polymer conversion has not been published, but the catalyst will undoubtedly be a cost driver given the low polymer/catalyst ratios and energy-intensive catalyst regeneration procedures detailed in the literature. Lowering catalyst costs and increasing efficiency are paramount to moving the technology forward.<br/><br/>In a preliminary step to address these problems, we examined the intrinsic activity of nominally metal-free, mesoporous silica-alumina mixed oxide materials (SiO₂-Al₂O₃) for the conversion of polyethylene into aromatic compounds. Herein, we demonstrate that SiO₂-Al₂O₃ deconstructs polyethylene into smaller, aromatic molecules under identical conditions as those used by Zhang et al. during reactions with Pt/Al₂O₃. Yields are comparable, the catalyst can be reused without regeneration, and product selectivity can be tuned by altering reaction conditions or the acidity of the SiO₂-Al₂O₃. Notably, the fraction of polyaromatic products increases with the Brønsted acidity of the catalyst, as does the degree of polymer deconstruction. Preliminary work on the mechanism of the reaction suggests that acid sites are responsible for initiating depolymerization and aromatization reactions in analogy to previous work in the literature. This work showcases the intrinsic activity of SiO₂-Al₂O₃for polyolefin deconstruction/aromatization at sub-pyrolytic temperatures and lays the foundation for future studies involving solid acid and bi-functional catalysts.