Catalyst Control of Alkylbenzene Molecular Architecture in Tandem Catalytic Hydrogenolysis-Aromatization of Polyethylene

Abstract

Upcycling of polyethylene (PE) can transform waste material into chemically valuable building blocks such as linear alkylbenzenes, with potential use in anionic surfactants. Alkylbenzenes can be formed by tandem hydrogenolysis-aromatization of PE, catalyzed by Pt nanoparticles supported on an acidic oxide. In the complex mixture of hydrocarbon products, the difficulty in identifying precise structures makes it hard to predict how the choice of catalyst and variations in PE feedstock will influence molecular architectures. Here, we report a detailed characterization of the alkylbenzenes, with insight into the catalytic mechanism that controls their formation. Acid-catalyzed skeletal isomerization is the key factor that determines branching, initiates cyclization, and dictates aromatic substitution patterns. Highly acidic Pt/F-Al2O3 catalyst yields predominantly 3,4-dialkyl-substituted toluenes, in which the lightly branched alkyl substituents have a very small number of (mostly methyl) branches, located predominantly in nonbenzylic positions. In contrast, less acidic Pt/γ-Al2O3 catalyzes the formation of virtually unbranched 2,3-dialkyltoluenes. The unexpected difference in aromatic substitution reflects the dependence of the alkane cyclization mechanism on catalyst acidity. However, the structures of the alkylbenzenes are almost independent of the initial PE microstructure (e.g., LDPE vs HDPE). Furthermore, the type of PE feedstock has little effect on the average alkyl chain length and negligible influence on the branch frequency and distribution in the alkyl side-chains of the alkylbenzene products, resulting in a consistent, low prevalence of benzylic branching compared to conventional alkylbenzenes. These features make the process well-suited to the conversion of mixed postconsumer PE, while the insights into alkylbenzene regioselectivity inform catalyst design to obtain desired linear alkylbenzene structures, and facilitate investigation of structure-property relationships in surfactants sourced from waste PE.