Catalytic deconstruction of waste polyethylene with ethylene to form propylene

TLDR

Converting polyolefins to monomers could turn the largest waste plastic fraction into valuable carbon feedstock, but breaking their carbon–carbon bonds is challenging due to their resistance to selective transformations. The study aims to produce propylene by partially dehydrogenating polyethylene followed by tandem isomerizing ethenolysis of the resulting unsaturated chain. High‑density polyethylene is first dehydrogenated with an iridium‑pincer or Pt/Zn‑silica catalyst to generate up to 3.2 % internal olefins, then a second‑generation Hoveyda‑Grubbs metathesis catalyst together with PdP(tBu)3(μ‑Br)2 selectively converts the unsaturated polymer to propylene in yields exceeding 80 %. These results demonstrate that mild catalysis can effectively deconstruct otherwise stable polyolefins into propylene.

Abstract

The conversion of polyolefins to monomers would create a valuable carbon feedstock from the largest fraction of waste plastic. However, breakdown of the main chains in these polymers requires the cleavage of carbon–carbon bonds that tend to resist selective chemical transformations. Here, we report the production of propylene by partial dehydrogenation of polyethylene and tandem isomerizing ethenolysis of the desaturated chain. Dehydrogenation of high-density polyethylene with either an iridium-pincer complex or platinum/zinc supported on silica as catalysts yielded dehydrogenated material containing up to 3.2% internal olefins; the combination of a second-generation Hoveyda-Grubbs metathesis catalyst and [PdP( t Bu) 3 (μ-Br)] 2 as an isomerization catalyst selectively degraded this unsaturated polymer to propylene in yields exceeding 80%. These results show promise for the application of mild catalysis to deconstruct otherwise stable polyolefins.