Scientists transform end-of-life PE into PP using catalytic reactions

A group of scientists has developed a process that uses a series of coupled catalytic reactions to transform end-of-life polyethylene into new polypropylene.

The scientists, who are from the University of Illinois Urbana-Champaign, the University of California, Santa Barbara, and Dow, say the process could reduce greenhouse gas emissions (GHG). 

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

Co-lead author Damien Guironnet, a professor of chemical and biomolecular engineering at the University of Illinois, says, “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.”

Guironnet published the first study outlining the necessary catalytic reactions to achieve the transformation in 2020. In that paper, he writes of a possible approach that uses “selective depolymerization to propylene with tandem olefin metathesis and double bond isomerization catalysts.”

The new study published in the Journal of the American Chemical Society, announces a series of coupled catalytic reactions that transform polyethylene, or PE, in the form of high-density polyethylene and low-density polyethylene, which the researchers say make up 29 percent of the plastics used globally, into the building block propylene that is the key ingredient to produce polypropylene, or PP, which the researchers say accounts nearly 25 percent of the world’s plastic consumption. 

This study establishes a proof of concept for upcycling PE with more than 95 percent selectivity into propylene, the researchers say. They have built a reactor that creates a continuous flow of propylene that can be converted into PP using current technology, which they say makes this discovery scalable and rapidly implementable. 

“Our preliminary analysis suggests that if just 20 percent 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 3 million cars off the road," says 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, according to a news release about the process. 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 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 explains. 

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

The news release notes that this process is complementary to a paper published in Science last week by researchers associated with the University of California at Berkeley and the Division of Chemical Sciences at the Lawrence Berkeley National Laboratory that was funded by the National Institutes of Health and the U.S. Department of Energy. Both groups used virgin plastics and similar chemistries. However, the Science team used a different process in an enclosed batch reactor, requiring much higher pressure—which is energy intensive—and the need 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 says. “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.” 

Researchers at Dow, headquartered in Midland, Michigan, also were 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," says 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."