Recycling Polythene

A new way to break down and recycle plastic has been announced by scientists in the US this week. This is welcome news since the world produces millions of tonnes of plastic every year, yet less than 20% of it is recycled. Most ends up in landfill, some is burned and a significant amount ends up in the ocean where it degrades into potentially harmful microplastics. Part of the problem with plastic is that it's an unnatural chemical, which nature has no easy way to attack, so it just hangs around. But John Hartwig and his colleagues have developed a relatively low temperature chemical process that can break into the long chains of carbon atoms that make up polythene - or polyethylene - and break them into short molecules of the gas "propene", which is a valuable raw material for making many other materials. He told me how it works...

John - Polyethylene is multiple units of ethylene put together through carbon/carbon bonds, which are very stable bonds. So what we've done is a three step process where first we introduce a change to the chain that provides kind of an Achilles' heel where we can then cleave that chain. And maybe a good analogy is if you think about the polymer chain being actually a physical chain like you'd buy at the hardware store that has these links that are very stable and you can't pull them apart. But if we make a chemical change to our polymer chain that transforms one of the linkages to say a clasp like you have on a necklace, that could then be opened. Now we have a way to break apart those chains under milder conditions. But then there's two additional steps to rearrange the change and attach a very small molecule to break those down into very, very small pieces.

Chris - I was going to ask how many of these so-called "clasps" one would need to insert in order to get this thing to fall apart? And under what sort of conditions do you need to do this? Because one of the criticisms of recycling processes is that, if you're not careful, you end up with a bigger carbon footprint than the one you're saving!

John - Right. Fewer than 1% of those chains need to be transformed into a clasp. That transformation of the chain to the clasp, we do that at 200 degrees, which is a pretty low temperature for chemical processes.

Chris - And how do you actually do this? What is the mechanism by which you change those chains so that you can then do this modification and make them fall apart?

John - The first step is a reaction called dehydrogenation, where we remove hydrogen: therefore the name. We do that with a catalyst. The catalyst has platinum in it. It's a platinum zinc catalyst. That's the best one we've found. And then once that chain link has been turned into a clasp, we use two catalysts. One that is the subject of a Nobel Prize in 2005, Olefin metathesis catalyst it's called. And so that basically would take two chains that would have a clasp, undo the clasp, and put the chains back together. And then we have another reaction that would take that clasp. We walk that clasp bin just a little bit into the chain and then break the chain at that point to make a small piece off the end and a shorter chain. And then we do that about a thousand times to turn the solid polyethylene into a gas propene. That's our final product.

Chris - Is this a single reaction that you can do those three reactions in one place? Or do you have to do one thing, feed into the next, and then feed into the next? How practical is this?

John - Right. We actually run it as two. We run the de hydrogenation first and then this moving of the class. But opening and closing of it, those two reactions are run at the same time. There's also a very closely related work that has just appeared also in publication and they run it together, but the yields are lower than in our case. So in principle, all three steps could be run together and practice. Our highest yields come when we run them separately.

Chris - And what sorts of yields can you get? Is this a practical and viable, as in industry ready technique? Could we see this digesting plastic bags anytime soon?

John - Well, the answer to the last question is it will take a lot of time to develop, but the answer to the first question about the yields and whether it's practical, the yields are very high. Almost 90% gets converted into propene. And even on waste plastic, the yields are around 50% and they're probably higher because those plastics aren't all polyethylene. They contain fillers, colours, dyes and various other additives to them. But it takes decades between the time of a first paper like this and when it becomes practical. So we need to have the catalyst be very, very stable to be able to run for a long period of time, make many molecules of the product maybe a hundred thousand times. And right now we're at maybe a hundred instead of a hundred thousand. So we have a lot of development to do, but we hope this provides motivation to do the work and demonstration that it could become feasible in the future.

Chris - The problem plastic has at the moment - it is a wonderful material when we first make it and first use it - it becomes a pain when we get rid of it. It's valueless almost, isn't it? When it's a waste material. Does your process mean that potentially we return value to waste plastic? And does that solve our problem in the sense that it incentivizes people to pick it up?

John - Well, that's exactly the idea. So what we want to do is to take aims back apart to the small molecules like propene that has only three carbon atoms in it and is a feedstock chemical to make all sorts of other things. That's the idea of this kind of recycling, we call it chemical recycling or advanced recycling.