Mechanistic understanding of metal–acid synergetic hydroconversion of polyethylene under mild conditions over a Ru/MOR catalyst

Abstract

Hydroconversion has been demonstrated as a viable method to efficiently deconstruct polyolefins under relatively mild conditions. The hydrogenolysis of C–C bonds over Ru-based catalysts can convert polyolefins into liquid alkanes and methane. To suppress methane generation and obtain high-value isomer, further exploration of the mechanism for the cooperative deconstruction of polyolefins by bifunctional catalysts with metal–acid sites is necessary. In this study, we utilized ball milling to enhance the accessibility of acid sites within MOR zeolite, while the fragmented pore structure ensured that the loaded Ru metal was sufficiently accessible for long-chain alkanes. The sequential accessibility of metal–acid sites boosted the hydroconversion efficiency of low-density polyethylene (LDPE). Through systematic investigations involving catalysts with various Ru loadings and Si/Al ratio of zeolites, it is discovered that the smaller-sized Ru particles not only facilitate the generation and desorption of olefinic intermediates in hydrocracking but also promote the hydrogenation and desorption of alkyls and suppress the cascade of terminal C–C bond cleavage in hydrogenolysis. Liquid fuel (C5-C21) yield was increased by 27% for the catalyst loaded with 0.7 wt% Ru on mMOR20 compared to 1.7 wt% loadings, while methane yield was only one-third of the latter. Concurrently, the distribution of acid sites on the zeolite surface affected the aggregation process of Ru atoms. This study deeply unveils the synergistic interaction of metal–acid sites, facilitating internal C–C bond cleavage to obtain liquid fuel and sheds light on catalyst design for waste polyolefins upcycling.