Our planet is choking on plastic. Some of the worst offenders, which take decades to break down in landfills, are polypropylene (used for things like food packaging and bumpers) and polyethylene, found in plastic bags, bottles, toys, and even mulch.
Polypropylene and polyethylene can be recycled, but the process can be difficult and often produces large amounts of the greenhouse gas methane. They are both polyolefins, which are the products of polymerizing ethylene and propylene, feedstocks that are primarily derived from fossil fuels. The bonds of polyolefins are also notoriously difficult to break.
Now, researchers at the University of California, Berkeley, have devised a way to recycle these polymers, using catalysts that easily break their bonds and convert them into propylene and isobutylene, gases at room temperature. Those gases can then be recycled into new plastics.
“Because polypropylene and polyethylene are among the most difficult and expensive plastics to separate from each other in a mixed waste stream, it is critical that [a recycling] “The research team said in a study recently published in Science that the process applies to both polyolefins.”
The breaking down
The recycling process the team used is known as isomerizing ethenolysis, which requires a catalyst to break down olefin polymer chains into their small molecules. Polyethylene and polypropylene bonds are very resistant to chemical reactions because both polyolefins have long chains of single carbon-carbon bonds. Most polymers have at least one carbon-carbon double bond, which are much easier to break.
Although isomerizing ethenolysis had been attempted previously by the same researchers, the previous catalysts were expensive metals that did not remain pure long enough to convert all the plastic to gas. Using sodium on alumina followed by tungsten oxide on silica proved to be much more economical and effective, although the high temperatures required for the reaction added slightly to the cost
In both plastics, exposure to sodium on alumina broke each polymer chain into shorter polymer chains and created brittle carbon-carbon double bonds at the ends. The chains continued to break over and over again. Both then underwent a second process known as olefin metathesis. They were exposed to a stream of ethylene gas flowing into a reaction chamber while they were introduced into tungsten oxide on silica, resulting in the breaking of the carbon-carbon bonds.
The reaction breaks all of the carbon-carbon bonds in polyethylene and polypropylene, with the carbon atoms released when these bonds are broken ultimately being attached to molecules of ethylene. “Ethylene is crucial to this reaction because it's a co-reactant,” study author RJ Conk told Ars Technica. “The broken links then react with ethylene, which removes the links from the chain. Without ethylene, the reaction can't happen.”
The entire chain is catalyzed until polyethylene is completely converted to propylene, and polypropylene is converted to a mixture of propylene and isobutylene.
This method has high selectivity, meaning it produces a large amount of the desired product. That means propylene derived from polyethylene, and both propylene and isobutylene derived from polypropylene. Both chemicals are in high demand, as propylene is a key feedstock for the chemical industry, while isobutylene is a common monomer in many different polymers, including synthetic rubber and a gasoline additive.
The mixing
Because plastics are often mixed together in recycling centers, the researchers wanted to see what would happen if polypropylene and polyethylene were to undergo isomerizing ethenolysis together. The reaction was successful, converting the mixture into propylene and isobutylene, with slightly more propylene than isobutylene.
Mixtures also typically contain contaminants in the form of additional plastics. So the team wanted to see if the reaction would still work if there were contaminants. So they experimented with plastic items that would otherwise be thrown away, including a centrifuge and a bread bag, both of which contained traces of other polymers besides polypropylene and polyethylene. The reaction produced only slightly less propylene and isobutylene than when the polyolefins were unadulterated.
Another test involved introducing different plastics, such as PET and PVC, into polypropylene and polyethylene to see if that would make a difference. These reduced yields significantly. If this approach is to be successful, all but the smallest traces of contaminants must be removed from polypropylene and polyethylene products before they are recycled.
While this recycling method sounds like it could prevent tons and tons of waste, it will need to be scaled up massively for this to happen. When the research team scaled up the experiment, it achieved the same yield, which seems promising for the future. However, we will need to build significant infrastructure before this can make a dent in our plastic waste.
“We hope that the work described…will lead to practical methods for…[producing] “New polymers,” the researchers said in the same study. “This could significantly reduce the demand for the production of these essential commodity chemicals, derived from fossil carbon sources, and the associated greenhouse gas emissions.”
Science, 2024. DOI: 10.1126/science.adq731
