Our planet is drowning in plastics. Some of the worst offenders, which can take decades to degrade 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 methane, a greenhouse gas. Both are polyolefins, which are products of the polymerization of ethylene and propylene, raw materials that are primarily derived from fossil fuels. The bonds in polyolefins are also very difficult to break.
Now, UC Berkeley researchers have devised a method to recycle these polymers that uses catalysts that easily break their bonds, converting them into propylene and isobutylene, which are 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 crucial that a (recycling) process be applied to both polyolefins,” the research team said. in a study recently published in Science.
breaking it
The recycling process the team used is known as isomerizing ethenolysis, which relies on 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 carbon-carbon single bonds. Most polymers have at least one carbon-carbon double bond, which is much easier to break.
Although the same researchers had tried isomerizing ethenolysis before, 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 was much more economical and effective, although the high temperatures required for the reaction increased the cost somewhat.
In both plastics, exposure to sodium on alumina broke each polymer chain into shorter polymer chains and created breakable carbon-carbon double bonds at the ends. The chains continued to break again and 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 tungsten oxide was introduced onto silica, causing the carbon-carbon bonds to break.
The reaction breaks all the carbon-carbon bonds in polyethylene and polypropylene, and the carbon atoms released during the breaking of these bonds end up attached to ethylene molecules. “Ethylene is essential for this reaction, since it is a coreactant,” researcher RJ Conk, one of the authors of the study, told Ars Technica. “The broken links react with ethylene, which removes the links from the chain. Without ethylene, the reaction cannot occur.”
The entire chain is catalyzed until the polyethylene is completely converted to propylene and the 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: propylene derived from polyethylene and both propylene and isobutylene derived from polypropylene. Both chemicals are in high demand; Propylene is an important raw material for the chemical industry, while isobutylene is a monomer frequently used in many different polymers, including synthetic rubber and a gasoline additive.
Mixing it up
Because plastics are often mixed at recycling centers, the researchers wanted to see what would happen if polypropylene and polyethylene were subjected to isomerizing ethenolysis together. The reaction was successful, converting the mixture to propylene and isobutylene, with slightly more propylene than isobutylene.
Mixtures also often include contaminants in the form of additional plastics. So the team also wanted to see if the reaction would still work if contaminants were present. They experimented with plastic objects 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 with the unadulterated versions of the polyolefins.
Another test involved introducing different plastics, such as PET and PVC, polypropylene and polyethylene to see if that made a difference. These significantly reduced performance. For this approach to be successful, it will then be necessary to remove all but the most traces of contaminants from polypropylene and polyethylene products before recycling them.
While this recycling method seems like it could prevent tons and tons of waste, it will need to be greatly scaled up for this to happen. When the research team scaled up the experiment, they achieved the same performance, which looks promising for the future. Still, we will need to build considerable infrastructure before this can affect our plastic waste.
“We hope that the work described… will lead to practical methods for… (producing) new polymers,” the researchers said in the same report. study. “Doing so could significantly reduce the demand for production of these essential chemicals from fossil carbon sources and the associated greenhouse gas emissions.”
This story originally appeared on Ars Technique.
