Turning carbon dioxide into ethylene
A technique that can turn waste carbon dioxide into ethylene - the high-value gas used to make polythene and a range of other products - has been unveiled by scientists in Canada. The system uses a clever, new, inexpensive copper-based catalyst that can selectively grab - and align - carbon dioxide molecules so that they react together - and with water - to make pure ethylene. Previously, the main industrial source of ethylene was oil, so this new approach has the potential to cut CO2 emissions, and reduce oil consumption. Toronto University’s Ted Sargent took Chris Smith through the new process...
Ted - We've got a system that eats carbon dioxide, so it consumes CO2. It also uses renewable energy, so energy from a solar cell or a wind farm, and it uses those two ingredients to produce ethylene, which is a major commodity chemical.
Chris - And that's the stuff we turn into polyethylene, otherwise known as polythene?
Ted - That's right. So this is a big global industry. It's about a $60 billion industry and it's got a major carbon footprint today. And people have become very interested in capturing CO2 and sequestering it. But the problem there is that you don't get value out of it. You just sort of bury it underground. And so there's a whole community - ourselves included - that has been trying to figure out ways to upgrade or utilise CO2 into a valuable product, to generate a pull on consuming and utilising CO2.
Chris - The problem here is, in chemistry, when we burn stuff that's got carbon in it, we release energy, and the waste product is CO2 - carbon dioxide - which we throw away, and the planet's been a convenient place to put that up until now. So, in order to get something useful back out of the CO2, you've got to do some work. And, hitherto, it's been very difficult to make these equations actually stack up in a financially viable way, that it was worth doing. So how have you solved this?
Ted - So in order to make something valuable, you also need to make it in a fairly pure form. Previously, we and other groups had made what are called heterogeneous catalysts. So these are basically pieces of metal, and on the metal, the carbon dioxide and the electrons from the electricity land, they mix up with water. And there the reaction occurs to produce the ethylene. What we found was that some brilliant scholars who became our collaborators from Caltech had come up with a way to put molecules onto a catalyst, where the molecules kind of made the carbon dioxide that you were trying to react with, stand up in a special and controlled way onto the catalyst. It allowed us to control the orientation of the carbon dioxide, so that we could then turn it selectively into ethylene. So when the reaction proceeds, two carbon dioxides end up coupled together, they form this two carbon molecule ethylene, and then we simply collect the gas that mixes with the water that's involved in the reaction.
Chris - And how does the whole catalytic process work? Do you feed in carbon dioxide gas?
Ted - Yep. So we feed in CO2. It interacts with the catalyst, and it does so right at the same place that there's water. And water's providing the hydrogen to produce these hydrocarbons.
Chris - And how fast is this? Is this actually viable? Because some of these catalysts, they turn something that happens excrutiatingly slowly into something that happens a little bit less excruciatingly slowly, but it's not industrially viable. So is your process fast?
Ted - So there's one measure that we use in this field to describe the activity or the speed of the reaction and it's a current density. I'll just give you a sense of what it was previously using these molecules, it was about 1 milliamp per square centimetre. Now we built a system in this project that's designed to work at industrial productivities, and most people in industry will agree that you need to get to about a hundred milliamp per square centimetre in order to have an industrially interesting process. We managed to get above 200 milliamps, so we achieve more than a factor of a hundred increase. And we did get into the range of industrial interest.
Chris - So would it be feasible to couple something like this up to the flue stream from say, a big power plant, And deal with what a coal fired power station, 30,000 tons of CO2 in an afternoon might chuck out?
Ted - So there's a lot of additional work we need to do on scaling, but we've done the calculations, and we know how much CO2 comes out of say, a steel plant or out of a cement factory. We know how much electricity is available and we also need to supply the water. That's actually the most straight forward of those three feeds, and it should be possible to couple these altogether. And we believe that with a bit more progress on the energy efficiency of our system, it will even become economically compelling to make ethylene that is renewable ethylene instead of fossil ethylene.
Chris - So you've effectively got carbon neutral plastic?
Ted - That's right. You end up actually taking CO2 that would otherwise have been emmitted and you sequester it into the plastic product.