Tandem Heterogeneous Catalysis for Polyethylene Depolymerization via an Olefin-Intermediate Process

Abstract

The accumulation of plastic waste in the environment has prompted the development of new chemical recycling technologies. A recently reported approach employed homogeneous organometallic catalysts for tandem dehydrogenation and olefin cross metathesis to depolymerize polyethylene (PE) feedstocks to a mixture of alkane products. Here, we build on that prior work by developing a fully heterogeneous catalyst system using a physical mixture of SnPt/γ-Al₂O₃ and Re₂O₇/γ-Al₂O₃. This heterogeneous catalyst system produces a distribution of linear alkane products from a model, linear C₂₀ alkane, <i>n</i>-eicosane, and from a linear PE substrate (which is representative of high-density polyethylene), both in an <i>n</i>-pentane solvent. For the PE substrate, a molecular weight decrease of 73% was observed at 200 °C in 15 h. This type of tandem chemistry is an example of an olefin-intermediate process, in which poorly reactive aliphatic substrates are first activated through dehydrogenation and then functionalized or cleaved by a highly-active olefin catalyst. Olefin-intermediate processes like that examined here offer both a selective and versatile means to depolymerize polyolefins at lower severity than traditional pyrolysis or cracking conditions.