Chemical Recycling of Polyethylene by Tandem Catalytic Conversion to Propylene

TLDR

Polyethylene and polypropylene are the largest-volume plastics, yet only a tiny fraction are recycled, and depolymerizing PE to ethylene is highly endothermic and typically requires high-temperature pyrolysis. The study demonstrates a tandem catalysis strategy that converts polyethylene to propylene by using ethylene as a reactant. The tandem process couples rapid olefin metathesis with rate‑limiting isomerization and transfer dehydrogenation to ethylene, and is implemented with both homogeneous and heterogeneous catalysts. Monounsaturated polyethylene is progressively disassembled at modest temperatures via consecutive ethenolysis events, selectively yielding propylene, and a semicontinuous process achieves ≥94 % propylene selectivity by continuously removing propylene, whereas batch conditions limit selectivity at high conversion.

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 are 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. Monounsaturated 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 (1 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 semicontinuous process due to the continuous removal of propylene from the reaction mixture.