Damien Guironnet and graduate students Vanessa DaSilva and Nicholas Wang in their laboratory. Photo: Heather Coit/The Grainger College of Engineering, University of Illinois Urbana-Champaign.
Damien Guironnet and graduate students Vanessa DaSilva and Nicholas Wang in their laboratory. Photo: Heather Coit/The Grainger College of Engineering, University of Illinois Urbana-Champaign.

Scientists from the University of Illinois at Urbana-Champaign, the University of California (UC) Santa Barbara, and Dow have developed a breakthrough process to transform the most widely produced plastic – polyethylene (PE) – into the second-most widely produced plastic, polypropylene (PP), which could help reduce greenhouse gas emissions.

“The world needs more and better options for extracting the energy and molecular value from its waste plastics,” said co-lead author Susannah Scott, professor and chair of sustainable catalytic processing at UC Santa Barbara. Conventional plastic recycling methods result in low-value plastic molecules and thus offer little incentive to recycle the mountains of plastic waste that have accumulated over the past several decades. But, Scott added, “turning polyethylene into propylene, which can then be used to make a new polymer, is how we start to build a circular economy for plastics.”

“We started by conceptualizing this approach and demonstrated its promise first through theoretical modeling – now we have proved that it can be done experimentally in a way that is scalable and potentially applicable to current industry demands,” said co-lead author Damien Guironnet, a professor of chemical and biomolecular engineering at the University of Illinois at Urbana-Champaign. Guironnet first outlined the necessary catalytic reactions in a paper in the Journal of Physical Chemistry A in 2020.

The new study, reported in a paper in the Journal of the American Chemical Society, details a series of coupled catalytic reactions that can transform PE, which makes up 29% of the world’s plastic consumption, into the building block propylene that is the key ingredient to produce PP, which accounts for close to 25% of the world’s plastic consumption.

The study establishes a proof-of-concept for upcycling PE plastic into propylene with more than 95% selectivity. The researchers have built a reactor that creates a continuous flow of propylene, which can be converted into PP easily using current technology — making this discovery scalable and rapidly implementable.

“Our preliminary analysis suggests that if just 20% of the world’s PE could be recovered and converted via this route, it could represent a potential savings of greenhouse gas 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 numerous times to obtain many small pieces, which are the propylene molecules. To do this, a catalyst first 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 with 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.

“Think of cutting a baguette in half, and then cutting precisely-sized pieces off the end of each half — where the speed at which you cut controls the size of each slice,” Guironnet said.

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

The work reported in this paper is highly complementary to work recently reported by another group in Science. Both groups used virgin plastics and similar chemistries. However, the other team used a different process in an enclosed batch reactor. This process requires much higher pressure – which is energy intensive – and needs to recycle more ethylene.

“If we are to upcycle a significant fraction of the over 100 million tons of plastic waste we generate each year, we need solutions that are highly scalable,” Guironnet said. “Our team demonstrated the chemistry in a flow reactor we developed to produce propylene highly selectively and continuously. This is a key advance to address the immense volume of the problem that we are facing.”

Dow researchers were also involved in this work. “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," said Dow senior scientist and co-author Ivan Konstantinov. "As a funder of this project, we are committed to finding new ways to eliminate plastic waste and are encouraged by this approach."

This story is adapted from material from the University of Illinois at Urbana-Champaign, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.