Polyethylene is one of the most common types of plastic in the world.
While easy to make, it’s devilishly difficult to break down again, making re-use and recycling tricky.
But a team of US chemists have figured out a new way to return polyethylene into component parts – a discovery which could one day make it fully circular.
“To the extent they get recycled, a lot of polyethylene plastics get turned into low-grade materials. You can’t take a plastic bag and then make another plastic bag with the same properties out of it,” says Professor John Hartwig, an organometallic chemist at the University of California, Berkeley, US.
Hartwig is senior author on a paper in Science, describing how the long molecular chain (polymer) of polyethylene can be broken down into propylene, also called propene: a useful feedstock for a variety of different things.
“If you can take that polymer bag back to its monomers, break it down into small pieces and repolymerize it, then instead of pulling more carbon out of the ground, you use that as your carbon source to make other things — for example, polypropylene,” says Hartwig.
“We would use less shale gas for that purpose, or for the other uses of propene, and to fill the so-called propylene gap.”
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Polyethylene is made from carbon atoms connected to each other in a long chain, with hydrogen atoms branching off to the side.
The bond between carbon atoms – the carbon-carbon bond – is difficult to break, and even tricker to break in a systematic way.
The researchers’ innovation was to use a couple of different metals to catalyse two different reactions.
The first relies on a catalyst made from iridium, or platinum and zinc, to modify the hardy carbon-carbon bond.
“We take a saturated hydrocarbon — all carbon-carbon single bonds — and remove a few molecules of hydrogen from the polymer to make carbon-carbon double bonds, which are more reactive than carbon-carbon single bonds,” says Hartwig.
“A few people had looked at that process, but nobody had achieved it on a true polymer.”
Then, the researchers found that a palladium-based catalyst could pounce on that bond and use it to progressively break up the polymer with a substance called ethylene.
“Once we have a long chain with a carbon-carbon double bond at the end, our catalyst takes that carbon-carbon double bond and isomerises it, one carbon in,” says Hartwig.
“Ethylene reacts with that initial isomerised product to make propylene and a nearly identical, just shorter, polymer with a double bond at the end.
“And then it does the same thing again and again. It walks one step in, cleaves; walks in, cleaves; walks in and cleaves until the whole polymer is cut into three-carbon pieces. From one end of the chain, it just chews down on the chain and spits off propylenes until there is no chain left.”
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They were able to convert 80% of their polyethylene into propylene: little molecules with three carbon atoms each.
There’s still plenty of work to be done before the process can be industrialised.
At the moment, for instance, both catalysts need to be in liquid form – the researchers are hoping to find solid catalysts instead, because they’re easier to re-use.
Hartwig says that the technique is “far from commercialisation”.
“But it is easy to see how this new process would convert the largest amount of plastic waste to a huge chemical feedstock — with much further development, of course.”