New process vaporizes plastic wastes to make new, recycled plastics

The catalytic process efficiently reduces polymers to chemical precursors.

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A new catalytic process developed at the University of California, Berkeley, has the potential to revolutionize plastic waste management. The process can efficiently break down the two main types of post-consumer plastic waste – polyethylene, the component of most single-use plastic bags, and polypropylene, the stuff of hard plastics, from microwavable dishes to luggage – into hydrocarbon building blocks that can be used to create new plastics.

This breakthrough could lead to a circular economy for many throwaway plastics, reducing our reliance on fossil fuels to create new plastics.

“We have an enormous amount of polyethylene and polypropylene in everyday objects, from lunch bags to laundry soap bottles to milk jugs — so much of what’s around us is made of these polyolefins,” said John Hartwig, a UC Berkeley professor of chemistry who led the research. “What we can now do, in principle, is take those objects and bring them back to the starting monomer by chemical reactions we’ve devised that cleave the typically stable carbon-carbon bonds. By doing so, we’ve come closer than anyone to giving the same kind of circularity to polyethylene and polypropylene that you have for polyesters in water bottles.”

Polyethylene and polypropylene plastics are a major source of post-consumer plastic waste, with a staggering 80% ending up in landfills, incinerators, or harmful microplastics in the environment. This disturbing trend is not only polluting our planet but also contributing to the growing greenhouse gas emissions from petroleum-based plastic production.

Researchers worldwide have been tirelessly working to find innovative solutions to convert plastic waste into valuable resources, such as the essential building blocks for creating new, high-quality plastics. The ultimate goal is to establish a sustainable circular polymer economy that alleviates the reliance on petroleum and reduces the environmental impact of plastic production.

Examples of the types of plastics the new process can handle. Left to right, a jug made of high density polyethylene, a test tube of polypropylene and a low density polyethylene bread bag. The numbers below each image are the percentage yield of monomers that can be used to make new plastic polymers.
Examples of the types of plastics the new process can handle. Left to right, a jug made of high density polyethylene, a test tube of polypropylene and a low density polyethylene bread bag. The numbers below each image are the percentage yield of monomers that can be used to make new plastic polymers. Credit: John Hartwig and RJ Conk, UC Berkeley

Just two years ago, a breakthrough was made by Hartwig and his UC Berkeley team, who developed a groundbreaking process to transform polyethylene plastic bags into the valuable monomer propylene. This innovative chemical process utilized bespoke heavy metal catalysts to convert the polyethylene polymer into propylene, which can then be reused to create high-quality polypropylene plastics.

Despite this remarkable achievement, challenges persist, particularly with regard to the recovery and reusability of the catalysts. The need of the hour is to further refine and optimize this process to ensure the efficient recovery and longevity of the catalysts, thereby paving the way for a sustainable and effective solution to plastic waste.

In a groundbreaking new process, costly soluble metal catalysts have been replaced with more economical solid ones commonly used in the chemical industry for continuous flow processes that allow for catalyst reuse. This innovation opens up the potential for scalable production, handling large volumes of material.

The catalyst development stemmed from Conk’s collaboration with Bell, an expert in heterogeneous catalysts, within the Department of Chemical and Biomolecular Engineering.

Conk’s experimentation with a sodium-on alumina catalyst demonstrated its exceptional ability to efficiently crack various polyolefin polymer chains, leaving one piece with a reactive carbon-carbon double bond. Subsequently, a tungsten oxide on silica catalyst facilitated the addition of a carbon atom to ethylene gas, which continuously streamed through the reaction chamber, resulting in the formation of a propylene molecule.

This transformative process, known as olefin metathesis, left behind a reusable double bond accessible to the catalyst until the entire chain had been converted to propylene.

The reaction with polypropylene produces a mix of propene and a hydrocarbon known as isobutylene. This isobutylene has widespread use in the chemical industry, being utilized for creating polymers used in a variety of products, from sports equipment to beauty products, as well as in the production of high-octane gasoline additives.

Unexpectedly, the tungsten catalyst demonstrated even greater effectiveness than the sodium catalyst in breaking down polypropylene chains.

“You can’t get much cheaper than sodium,” Hartwig said. “And tungsten is an earth-abundant metal used in the chemical industry on a large scale, as opposed to our ruthenium metal catalysts that were more sensitive and more expensive. This combination of tungsten oxide on silica and sodium on alumina is like taking two different types of dirt and having them together disassemble the whole polymer chain into even higher yields of propene from ethylene and a combination of propene and isobutylene from polypropylene than we did with those more complex, expensive catalysts.”

The new catalysts offer a breakthrough advantage by avoiding the need to remove hydrogen to form a breakable carbon-carbon double bond in the polymer, a step required in the researchers’ earlier process to deconstruct polyethylene. These double bonds serve as an Achilles heel of a polymer, similar to the way reactive carbon-oxygen bonds in polyester or PET make the plastic easier to recycle. Unlike polyethylene and polypropylene, which lack this Achilles heel, their long chains of single-carbon bonds are exceptionally strong.

“Think of the polyolefin polymer like a string of pearls,” Hartwig said. “The locks at the end prevent them from falling out. But if you clip the string in the middle, now you can remove one pearl at a time.”

The combination of two catalysts has revolutionized the recycling process for plastic polymers. This dynamic duo efficiently converts polyethylene and polypropylene into propylene and isobutylene, with an impressive yield of nearly 90%. What’s more, individual polyethylene or polypropylene processing results in even higher yields.

Intriguingly, when plastic additives and different types of plastics were introduced into the reaction chamber, the catalytic reactions proved resistant to contaminants. However, the presence of PET and polyvinyl chloride (PVC) notably diminished the efficiency. Nevertheless, the existing recycling methods effectively segregate plastics by type, potentially mitigating this issue.

While many researchers aspire to redesign plastics for improved recyclability, the hard-to-recycle plastics of today are bound to pose a significant challenge for decades to come.

“One can argue that we should do away with all polyethylene and polypropylene and use only new circular materials. But the world’s not going to do that for decades and decades. Polyolefins are cheap, and they have good properties, so everybody uses them,” Hartwig said. “People say if we could figure out a way to make them circular, it would be a big deal, and that’s what we’ve done. One can begin to imagine a commercial plant that would do this.”

Journal reference:

  1. Richard J. Conk, Jules F. Stahler, Jake X Shi, Ji Yang, Natalie G. Lefton, John N. Brunn, Alexis T. Bell, John F. Hartwig. Polyolefin waste to light olefins with ethylene and base-metal heterogeneous catalysts. Science, 2024; DOI: 10.1126/science.adq7316
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