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. Scientists from the University of Illinois Urbana-Champaign, University of California, 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 reduce greenhouse gas emissions (GHG).
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. 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. 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.
The authors 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. The new study published in the Journal of the American Chemical Society announces a series of coupled catalytic reactions that transform PE, which is #2 and #4 plastic that make up 29% of the world’s plastic consumption, into the building block propylene that is the key ingredient to produce PP, also known as #5 plastic that accounts for close to 25% of the world’s plastic consumption.
The new study establishes a proof-of-concept for upcycling PE plastic with more than 95% selectivity into propylene. The researchers have built a reactor that creates a continuous flow of propylene that can be converted into PP easily using current technology making this discovery scalable and rapidly implementable. The authors’ 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 GHG emissions comparable to taking 3 million cars off the road. The goal is to cut each very long PE molecule many times to obtain many small pieces, which are the propylene molecules. 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.
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. The research 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.
Nicholas M. Wang et al, Chemical Recycling of Polyethylene by Tandem Catalytic Conversion to Propylene, Journal of the American Chemical Society (2022). DOI: 10.1021/jacs.2c07781