Chemical Recycling of Polyethylene by Tandem Catalytic Conversion to Propylene

Abstract

Although polyethylene (PE) and polypropylene (PP) are by far the world’s largest volume plastics, only a tiny fraction of these energy-rich polyolefins is currently recycled. Depolymerization of PE to its constituent monomer, ethylene, is highly endothermic and conventionally accessible only through unselective, high temperature pyrolysis. Here, we provide experimental demonstrations of our recently proposed tandem catalysis strategy which uses ethylene to convert PE to propylene, the commodity monomer used to make PP. The approach combines rapid olefin metathesis with rate-limiting isomerization. Mono-unsaturated PE is progressively disassembled at modest temperatures via many consecutive ethenolysis events, resulting selectively in propylene. Fully saturated PE can be converted to unsaturated PE starting with a single transfer dehydrogenation to ethylene, which produces a small amount of ethane (one equiv. per dehydrogenation event). These principles are demonstrated using both homogeneous and heterogeneous catalysts. While selectivity under batch conditions is limited at high conversion by the formation of an equilibrium mixture of olefins, high selectivity to propylene (≥ 94 %) is achieved in a semi-continuous process due to continuous removal of propylene from the reaction mixture.

Catalytic Chemical Recycling of Post-Consumer Polyethylene

“…48 Recently, Hartwig and co-workers, and Guironnet, Scott and co-workers catalytically deconstructed polyethylene through transfer dehydrogenation, isomerization, and metathesis into propylene (Figure 2d). 49,50 In this work, dehydrogenated HDPE was used to develop recyclable-bydesign polyethylene-like materials from hard-to-recycle waste HDPE (Figure 1).…”

Understanding the Structure–Activity Relationships in Catalytic Conversion of Polyolefin Plastics by Zeolite-Based Catalysts: A Critical Review

“…207 Excitingly, this simulated path was recently successfully verified by Conk et al using well-designed homogeneous catalytic systems 208 or by Wang et al using a multifunctional heterogeneous catalyst, respectively. 209 MTO catalyst as an olefin metathesis catalyst. By precisely controlling the reaction conditions and catalyst composition, the dehydrogenation and the ethene metathesis reactions could occur simultaneously in this tandem catalysis process and give propene with selectivity as high as 94%.…”

Thiol–ene Click Chemistry Incorporates Hydroxyl Functionality on Polycyclooctene to Tune Properties

“…25−27 Recent work has investigated dehydrogenation as part of a pathway to reduce polyolefins to smaller molecules by creating C�C then conducting cross metathesis with small alkenes. 28,29 We propose using dehydrogenated polyolefins for polymer-topolymer recycling. Here we elaborate upon thiol−ene click chemistry with mercaptoethanol to functionalize polycyclooctene (PCOE), a model polymer for dehydrogenated PE.…”