Bifunctional tandem catalytic upcycling of polyethylene to surfactant-range alkylaromatics

Abstract

•H2 redistribution converts polyethylene to surfactant-range alkylaromatics •Metal/acid bifunctional catalysts are more effective than either component separately •Strong Brønsted acid sites control rates of C–C bond scission, isomerization, and cyclization •Metal and acid sites are not required to be in close proximity 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. 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.