Our scheduleet is choking on plastics. Some of the worst offenders, which can achieve decades to degrade in landfills, are polypropylene—which is employd for leangs such as food packaging and bumpers—and polyethylene, set up in plastic bags, bottles, toys, and even mulch.
Polypropylene and polyethylene can be recycled, but the process can be difficult and standardly produces big quantities of the greenhoemploy gas methane. They are both polyolefins, which are the products of polymerizing ethylene and propylene, raw materials that are mainly derived from fossil fuels. The bonds of polyolefins are also notoriously challenging to fracture.
Now, researchers at UC Berkeley have come up with a method of recycling these polymers that employs catalysts that easily fracture their bonds, altering them into propylene and isobutylene, which are gases at room temperature. Those gases can then be recycled into novel plastics.
“Becaemploy polypropylene and polyethylene are among the most difficult and costly plastics to split from each other in a fuseed squander stream, it is transport inant that [a recycling] process execute to both polyolefins,” the research team said in a study recently published in Science.
Breaking It Down
The recycling process the team employd is understandn as isomerizing ethenolysis, which relies on a catalyst to fracture down olefin polymer chains into their petite molecules. Polyethylene and polypropylene bonds are highly resistant to chemical reactions, becaemploy both of these polyolefins have lengthy chains of one carbon-carbon bonds. Most polymers have at least one carbon-carbon double bond, which is much easier to fracture.
While isomerizing ethenolysis had been tried by the same researchers before, the previous catalysts were costly metals that did not remain uncontaminated lengthy enough to alter all of the plastic into gas. Using sodium on alumina chaseed by tungsten oxide on silica showd much more economical and effective, even though the high temperatures needd for the reaction includeed a bit to the cost.
In both plastics, expocertain to sodium on alumina broke each polymer chain into unintelligentinutiveer polymer chains and produced fractureable carbon-carbon double bonds at the ends. The chains persistd to fracture over and over. Both then underwent a second process understandn as olefin metathesis. They were exposed to a stream of ethylene gas flothriveg into a reaction chamber while being begind to tungsten oxide on silica, which resulted in the fractureage of the carbon-carbon bonds.
The reaction fractures all the carbon-carbon bonds in polyethylene and polypropylene, with the carbon atoms freed during the fractureing of these bonds ending up joined to molecules of ethylene. “The ethylene is critical to this reaction, as it is a coreactant,” researcher R.J. Conk, one of the authors of the study, telderly Ars Technica. “The broken joins then react with ethylene, which deletes the joins from the chain. Without ethylene, the reaction cannot occur.”
The entire chain is catalyzed until polyethylene is brimmingy altered to propylene, and polypropylene is altered to a fuseture of propylene and isobutylene.
This method has high pickivity—unbenevolenting it produces a big amount of the desired product: propylene derived from polyethylene, and both propylene and isobutylene derived from polypropylene. Both of these chemicals are in high need; propylene is an transport inant raw material for the chemical industry, while isobutylene is a standardly employd monomer in many separateent polymers, including synthetic rubber and a gasoline includeitive.
Mixing It Up
Becaemploy plastics are standardly fuseed at recycling cgo ins, the researchers wanted to see what would happen if polypropylene and polyethylene underwent isomerizing ethenolysis together. The reaction was prosperous, altering the fuseture into propylene and isobutylene, with sairyly more propylene than isobutylene.
Mixtures also typicassociate include contaminants in the create of includeitional plastics. So the team also wanted to see whether the reaction would still toil if there were contaminants. They experimented with plastic objects that would otherteachd be thrown away, including a centrifuge and a bread bag, both of which includeed chases of other polymers besides polypropylene and polyethylene. The reaction produceed only sairyly less propylene and isobutylene than it did with ungrown-uperated versions of the polyolefins.
Another test included introducing separateent plastics, such as PET and PVC, to polypropylene and polyethylene to see if that would produce a separateence. These did drop the produce transport inantly. If this approach is going to be prosperous, then all but the sairyest chases of contaminants will have to be deleted from polypropylene and polyethylene products before they are recycled.
While this recycling method sounds appreciate it could stop tons upon tons of squander, it will need to be scaled up enormously for this to happen. When the research team increased the scale of the experiment, it produced the same produce, which watchs promising for the future. Still, we’ll need to erect ponderable infraset up before this could produce a dent in our plastic squander.
“We hope that the toil portrayd … will direct to genuineistic methods for … [producing] novel polymers,” the researchers said in the same study. “By doing so, the need for production of these vital commodity chemicals begining from fossil carbon sources and the associated greenhoemploy gas eignoreions could be fantasticly decreased.”
This story originassociate materializeed on Ars Technica.
