Bifunctional tandem catalytic upcycling of polyethylene to surfactant-range alkylaromatics

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/ g-Al2O3 at 280 degrees 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/ g-Al2O3 or non-acidic Pt/SiO2 with strongly Bronsted 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.