Upcycling plastics to cut greenhouse gas emissions

The team, from University of Illinois Urbana-Champaign and University of California, Santa Barbara, worked with materials science company Dow on the process, described by researchers as a breakthrough.

“The world needs more and better options for extracting the energy and molecular value from its waste plastics,” said co-lead author Susannah Scott, distinguished professor and Mellichamp chair of Sustainable Catalytic Processing at UC Santa Barbara.

The researchers explained that conventional plastic recycling methods result in low-value plastic molecules, offering little incentive to recycle the ‘mountains’ of plastic waste accumulated over the past several decades.

“Turning polyethylene [PE] into propylene, which can then be used to make a new polymer, is how we start to build a circular economy for plastics,” Scott said in a statement.

Damien Guironnet, co-author and professor of chemical and biomolecular engineering at Illinois, described how the team began by conceptualising the approach and demonstrating its promise first through theoretical modelling. 

Guironnet, who published the first study outlining the necessary catalytic reactions in 2020, said that the team has now proved the process can be done experimentally in a way that is scalable and potentially applicable to current industry demands.

The new study, published in the Journal of the American Chemical Society, announces a series of coupled catalytic reactions that transform PE into the building block propylene, the key ingredient to produce PP (polypropylene).

This establishes a proof-of-concept for upcycling PE plastic with more than 95 per cent selectivity into propylene. According to researchers, they have built a reactor that creates a continuous flow of propylene that can be converted into PP easily using current technology.

“Our preliminary analysis suggests that if just 20 per cent of the world’s PE could be recovered and converted via this route, it could represent a potential savings of GHG emissions comparable to taking three million cars off the road,” said Garrett Strong, a graduate student associated with the project.

The goal is to cut each very long PE molecule many times to obtain many small pieces, which are the propylene molecules, the team said. First, a catalyst removes hydrogen from the PE, creating a reactive location on the chain. Next, the chain is split in two at this location using a second catalyst, which caps the ends using ethylene. Finally, a third catalyst moves the reactive site along the PE chain so the process can be repeated. Eventually, all that is left are a large number of propylene molecules.

“Now that we have established the proof of concept, we can start to improve the efficiency of the process by designing catalysts that are faster and more productive, making it possible to scale up,” said Scott.

“Since our end-product is already compatible with current industry separation processes, better catalysts will make it possible to implement this breakthrough rapidly.” 

Dow senior scientist and co-author Ivan Konstantinov added: “Dow is taking a leading role in driving a more circular economy by designing for circularity, building new business models for circular materials, and partnering to end plastic waste.

“As a funder of this project, we are committed to finding new ways to eliminate plastic waste and are encouraged by this approach."