Chemical Open-Loop Recycling of Polyethylene

The scientific challenge currently receiving much attention is the catalytic conversion of non-biodegradable polymers into versatile chemical platform molecules. As a model of a chemical upcycling process, we have developed a homogeneous catalytic system to break down persistent polyethylene waste into valuable chemical intermediates that could ultimately be used to produce important chemical products, including environmentally friendly, biodegradable plastics. In the first step, a smart pyrolysis of polyolefin waste yields oils, containing long-chain olefins as the major components. Then, for the next transformation step, tailored BICAAC-Ru olefin metathesis catalysts were used in combination with an alkene isomerization catalyst (RuHCl(CO)(PPh3)3) for the transformation of the pyrolysis oil to propylene via isomerization metathesis (ISOMET) reaction in ethylene atmosphere. Eventually, translation of the highly efficient single-metal catalyst system enabled ISOMET reaction to a 900 mL reactor setup and repetitive batch experiments could prove the long-term stability of the catalyst system and the highest turn over number (TON = 3800) reported so far for propylene using polyethylene municipal waste feedstock. Propylene content in the gas phase achieved the 20 vol%. Ultimately, these results pave the way for the large-scale applicability of this process as a relevant demonstration of the combined application of adapted catalyst design and chemical engineering optimization with the aim of establishing a multi-dimensional circular economy concept in the chemical industry.