A groundbreaking catalytic process developed by the University of California, Berkeley, effectively converts the most common plastic waste – polyethylene and polypropylene – into basic hydrocarbon monomers. This innovative technology utilizes solid catalysts, offering higher efficiency and potential scalability, enabling the reuse of plastic materials. By doing so, it reduces dependence on fossil fuels for new plastic production and promotes the development of a circular economy.
The process is equally effective for two major post-consumer plastic waste types: polyethylene (the main component of most single-use plastic bags) and polypropylene (the component of hard plastics, from microwave dishware to luggage). It can also effectively degrade these types of mixed plastics.
If this technology is promoted, it will help achieve a circular economy for many discarded plastics, converting plastic waste into monomers for manufacturing polymers. This will reduce the need for fossil fuels in the production of new plastics, as fossil fuels produce greenhouse gases.
In the 1980s, clear plastic water bottles made from polyester-based polytetrafluoroethylene (PET) were designed for recycling in this way. However, compared to polyethylene and polypropylene plastics, which are known as olefins, the amount of polyester plastic used is negligible.
John Hartwig, a chemistry professor at the University of California, Berkeley, leading this research, said: Our everyday items contain a large amount of polyethylene and polypropylene, from lunch bags to laundry soap bottles to milk bottles – many things around us are made of these olefins. What we can do, in principle, is to break down the usually stable carbon-carbon bonds through the chemical reactions we have designed, and convert these objects back into starting monomers. By doing so, we are closer than anyone else to providing the same recyclability for polyethylene and polypropylene as that of the polyester in water bottles.
Hartwig, graduate student Richard J. RJ Conk, chemical engineer Alexis Bell (a professor at the University of California, Berkeley Graduate School of Education), and their colleagues published the details of the catalytic process in the journal Science on August 29.
Polyethylene and polypropylene plastics account for about two-thirds of global post-consumer plastic waste. About 80% of plastics are eventually buried, burned, or directly discarded on the streets, often forming microplastics in streams and oceans. The rest are recycled as low-value plastics, becoming decorative materials, flower pots, and forks.
To reduce this waste, researchers have been searching for ways to convert plastics into more valuable substances, such as monomers for the production of new plastics. This will create a circular polymer economy for plastics, reducing the need for crude oil in the production of new plastics, as crude oil produces greenhouse gases.
Two years ago, Hartwig and his University of California, Berkeley team proposed a process for decomposing polyethylene plastic bags into monomer propylene (also known as propylene), which can be reused to produce polypropylene plastics. This chemical process used three different customized heavy metal catalysts: one catalyst added carbon-carbon double bonds to polyethylene polymers, and the other two catalysts broke the chains at the double bonds, repeatedly shearing a carbon atom with ethylene to generate propylene (C3H6) molecules until the polymer disappeared. However, the catalysts dissolved in the liquid reaction had a short lifespan, making it difficult to recover them in an active form.
Graduate student RJ Conk adjusted the reaction chamber in the reaction chamber, where mixed plastics were degraded into reusable new polymer components. Source: Robert Sanders/University of California, Berkeley
In the new process, expensive soluble metal catalysts have been replaced with cheaper solid catalysts commonly used in the chemical industry for continuous flow processes that can be reused. Continuous flow processes can be scaled up proportionally to handle large amounts of material.
Conk first tried using these catalysts after consulting with heterogenous catalyst experts Alexis Bell of the Chemical and Bio molecular Engineering Department. Conk synthesized a sodium catalyst on alumina and found that it could effectively break or cleave various polyolefin polymer chains, leaving one end of the two parts with an active carbon-carbon double bond. The second catalyst is tungsten oxide on silica, which adds carbon atoms to ethylene gas at the end of the chain, which continuously flows through the reaction chamber, forming propylene molecules. This process is called olefin polymerization, which leaves a double bond, allowing the catalyst to enter the double bond repeatedly until the entire chain
Views: 0