Bifunctional tandem catalytic upcycling of polyethylene to surfactant-range alkylaromatics

Summary

Catalytic conversion of waste polyolefins to value-added alkylaromatics could contribute to carbon recycling. Compared with tandem hydrogenolysis/aromatization of polyethylene (PE) catalyzed by Pt/γ-Al₂O₃ at 280°C, both a 5-fold enhancement in the rate of C–C bond scission and a doubling of the molar yield of alkylaromatics were achieved using a more acidic Pt/F-Al₂O₃ catalyst instead. Bifunctional (metal/acid) catalysts also generate alkylaromatic products with lower average carbon numbers (ca. C₂₀), similar to conventional anionic surfactants. Because physical mixtures of weakly acidic Pt/γ-Al₂O₃ or non-acidic Pt/SiO₂ with strongly Brønsted acidic Cl-Al₂O₃ or F-Al₂O₃ are also effective, the tandem reaction does not require nanoscale intimacy between metal and acid active sites. Kinetic studies using triacontane (norm-C₃₀H₆₂) as a model for PE show that the Pt-catalyzed dehydrogenation/hydrogenation reactions are quasi-equilibrated, while the acid-catalyzed C–C bond scission and skeletal transformations (isomerization and cyclization) determine the overall rates of depolymerization and aromatic formation.