Bringing Circular Economy for Plastics One Step Closer to Reality

Can’t stand plastic waste? Just vaporize it. At least that’s the plan from a team at the University of California, Berkeley. The group’s new chemical process can essentially vaporize plastics that currently dominate the waste stream and turn them into hydrocarbon building blocks for new plastics.

“I guess the breakthrough that we had was using Earth-abundant metals and a heterogeneous catalyst that could be recycled,” said Research Lead John Hartwig, UC Berkeley Professor of Chemistry.

The catalytic process works equally well with the two dominant types of post-consumer plastic waste: polyethylene, used in the majority of single-use plastic bags, and polypropylene, found in hard plastics. In addition, the process also efficiently degrades a combination of these types of plastics.

A couple of years ago, Hartwig and Co. devised a process for breaking down polyethylene plastic bags into the monomer propylene — aka propene — that could then be reused to make polypropylene plastics. This process used three different bespoke heavy-metal catalysts. In the new process, the expensive, soluble metal catalysts were replaced by cheaper solid ones commonly used for continuous flow processes that reuse the catalyst.

“Two years ago, we published the concept of how you could take polyolefins — polyethylene, specifically, was all that worked — and be able to deconstruct one carbon by one carbon by this series of reactions,” Hartwig added. “We installed a reactive unit into the polymer that allows you to cleave at that position, and then we need to move that reactive unit up and down the chain and be able to then cleave it one carbon at a time.

“We showed that that could be done, but with expensive catalysts made from uredium, palladium, and ruthenium — all precious metals. Also, they are catalysts that can't be recycled. So, one-time use. We created very expensive propene from waste plastic.”

One advantage of the new catalysts is that they eliminate the hydrogen-removal process when trying to form a breakable carbon-carbon double bond in the polymer — a feature of the team’s earlier work.

“We can take a piece of waste plastic, like a milk jug, cut it up into pieces, and put it in a reaction vessel — something that can then also take pressure,” Hartwig said. “Then, we add to that the catalyst, and we add ethylene as a gas, ethylene has two carbons in it, and then heat it at 320 °C for 90 minutes and the product that comes out has three carbons in it.

“The three carbons are the two carbons of that ethylene that we put in and one carbon from the polymer. Each ethylene will come up and take one carbon away with the catalyst, one carbon away from the polymer. It’s kind of like taking one pearl off a necklace. You break the necklace in the middle and you might have two pearls, and you add one on, you get three, two, add one, and just all the way down a thousand times until the chain is completely deconstructed.”

If scaled up, the process could help reduce plastic waste — as it would be converted back into the monomers used to make polymers — and, thus, a reduction in fossil fuels.

However, there’s a lot of additional research to be done before the team can start thinking about scaling the process up, Hartwig said. For starters, the catalysts need to be more active and more long-lived. Also, they need to be able to be recycled better, as that’s the most expensive part — even if they’re from abundant and inexpensive.

“In the end, there's a number of ways that you could think about changing the overall process that would make it more practical — whether you first break the chains to get to a material that is more liquid and less solid, and then we would deconstruct that,” he said. “Then, when you think about which different types of plastics, is the milk jug the most important thing or is it packaging material?

According to him, the packaging can't be recycled right now. “Whether it's an Amazon plastic shipping bag or the plastic that's around whatever you got from Amazon or the plastic that's around your food — that's very clean. It's also flexible and flexible in a way that makes it so it can't be recycled,” Hartwig said.

Hartwig noted that while many researchers are hoping to redesign plastics from the ground up to be easily reused, today’s hard-to-recycle plastics will be a problem for decades.

This article was written by Andrew Corselli, Digital Content Editor at SAE Media Group. For more information, visit here  .

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