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

Plastic waste is a mounting problem that lacks globalstrategy,largely due to inadequate recycling capabilities. One alternativegaining traction in the heterogeneous catalysis community is the upcyclingof polyolefins into value-added products, such as hydrogen-free conversionto alkylaromatic compounds over Pt/Al2O3. Here,we examined the activity of nominally metal-free, mesoporous silica-aluminamixed oxide materials (SiO2-Al2O3) for the conversion of polyethylene into aromatic compoundsat temperatures of and below 280 & DEG;C. Yields with the silica-aluminacatalysts are comparable to those obtained over Pt(1 wt %)/Al2O3 under identical conditions, and product selectivitycan be tuned by altering reaction conditions or the acid site densityof the SiO2-Al2O3. Notably,the fraction of polyaromatic products increases with the Bronstedacid site density of the catalyst, as does the degree of polymer deconstruction.These catalysts can be reused without regeneration, and their activityimproves with each recycling event, producing soluble product yieldsup to 83%. Preliminary work on the mechanism of the reaction suggeststhat acid sites are responsible for initiating depolymerization andaromatization reactions, in analogy to previous work in the literature.This work showcases the activity of SiO2-Al2O3 for polyolefin deconstruction/aromatizationat subpyrolytic temperatures and lays the foundation for future studiesinvolving solid acid and bifunctional catalysts. Mesoporous silica-alumina converts polyethyleneplasticinto tunable aromatic compounds at subpyrolytic temperatures withoutthe use of metals or added hydrogen.