Our planet is choked with plastic. Some of the most harmful substances include polypropylene, which is used in things like food packaging and bumpers, and polyethylene, which is used in plastic bags, bottles, toys, and even mulch. It can take decades to decompose in a landfill.
Polypropylene and polyethylene can be recycled, but the process is difficult and often produces large amounts of the greenhouse gas methane. Both of these are polyolefins, which are the products of polymerizing ethylene and propylene, which are raw materials obtained primarily from fossil fuels. Polyolefin bonds are also known to be difficult to break.
Now, researchers at the University of California, Berkeley have devised a way to recycle these polymers using a catalyst that easily breaks their bonds and converts them to propylene and isobutylene, which are gases at room temperature. Those gases can be recycled into new plastic.
“Polypropylene and polyethylene are among the most difficult and expensive plastics to separate from each other in mixed waste streams, so it is important to apply the (recycling) process to both polyolefins.” The research team said in a study recently published in the journal Science. .
break it down
The recycling process the researchers used is known as isomerization ethenolysis, which uses a catalyst to break down olefin polymer chains into smaller molecules. The bond between polyethylene and polypropylene is 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 very easy to break.
Isomerization ethenolysis had been attempted by the same researchers before, but previous catalysts were expensive metals that could not stay pure long enough to convert all the plastic into gas. . The use of sodium on alumina followed by tungsten oxide on silica was found to be much more economical and effective, although the high temperatures required for the reaction slightly increased cost.
For both plastics, exposure to sodium on alumina broke each polymer chain into shorter polymer chains, creating fragile carbon-carbon double bonds at the ends. The chain kept breaking. Both then underwent a second process known as olefin metathesis. When these were introduced into tungsten oxide on silica, they were exposed to a flow of ethylene gas flowing into the reaction chamber, resulting in the breaking of carbon-carbon bonds.
This reaction breaks all the carbon-carbon bonds in polyethylene and polypropylene, and the carbon atoms released during the breaking of these bonds eventually bond into molecules of ethylene. “Ethylene is important for this reaction because it is a co-reactant,” researcher RJ Conk, one of the study’s authors, told Ars Technica. “The broken link reacts with the ethylene, causing the link to come off the chain. Without ethylene, the reaction cannot occur.”
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 is highly selective and produces large quantities of the desired products (propylene derived from polyethylene and both propylene and isobutylene derived from polypropylene). Both of these chemicals are in high demand. Propylene is an important raw material for the chemical industry, while isobutylene is a monomer frequently used in a variety of polymers, including synthetic rubber and gasoline additives.
Try mixing it up
Since plastics are often mixed together at recycling centers, the researchers wanted to see what happens when polypropylene and polyethylene undergo isomerization ethenolysis together. The reaction was successful and the mixture was converted to propylene and isobutylene, with slightly more propylene than isobutylene.
The mixture usually also contains contaminants in the form of additional plastics. So the research team also wanted to see if the reaction would work in the presence of contaminants. They experimented with plastic objects that would normally be thrown away, such as a centrifuge and bread bags, but both contained trace amounts of polymers other than polypropylene and polyethylene. The amount of propylene and isobutylene obtained in this reaction was slightly less than for pure polyolefin.
In another test, different plastics such as PET and PVC were introduced into polypropylene and polyethylene to see if they made a difference. These significantly reduced yields. If this approach is to be successful, all but the smallest traces of contaminants should be removed from polypropylene and polyethylene products before they can be recycled.
Although this recycling method appears to be able to prevent large amounts of waste, it would need to be scaled up significantly to make this a reality. When the research team scaled up the experiment, they obtained the same yield, which seems promising for the future. Still, significant infrastructure will need to be built before this can reduce plastic waste.
In the same study, the researchers said, “We hope that the work described…will lead to practical methods of (manufacturing) new polymers.” “Doing so could significantly reduce the demand and associated greenhouse gas emissions for the production of these essential chemicals starting from fossil carbon sources.”
This story was originally Ars Technica.
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