Polyethylene (PE) and polypropylene (PP) account for the majority of plastic waste and are among the longest-lived waste plastics in the environment. Due to their structure consisting entirely of C–C and C–H bonds, these polyolefins are particularly intractable to functionalization by traditional organic transformations. Pyrolytic cleavage, hydrogenolysis, and alkane metathesis are existing methods for the valorization of polyolefins which create mixtures of value-added hydrocarbons. However, methods are still needed by which aliphatic polyolefins can be converted to one or two high-value products with high selectivity. We envisioned that the olefins installed in polyolefin chains during catalytic cracking could be intercepted and used to selectively transform polyolefins to light olefins by a tandem process of olefin isomerization and cross metathesis with ethylene (isomerizing ethenolysis). During this process, the long-chain terminal olefins resulting from cracking polyolefins are isomerized to internal olefins. These internal olefins then undergo cross-metathesis with ethylene (ethenolysis) to yield shorter terminal olefins. This tandem process of olefin isomerization and ethenolysis then iteratively cleaves the polyolefin chains until PE chains are converted to propylene and PP chains to mixtures of propylene and isobutylene. By conducting the catalytic cracking of PE and PP in the presence of ethylene over WO₃/SiO₂ as olefin metathesis catalyst and Na/γ-Al₂O₃ as olefin isomerization catalyst we have successfully converted PE to propylene and PP to a mixture of propylene and isobutylene in greater than 95% yield.

Natalie Lefton

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