It’s not easy to capture carbon dioxide. I’ve written before about some of the more ambitious efforts currently underway – be it sucking it directly out of the urban air, or transforming it into rock. But now, researchers from Rice University have found another option – asphalt – and they’ve used it to suck carbon dioxide out of natural gas. Now, it’s important to say at this point that this research was funded by an oil and gas exploration company, so there’s a risk of ‘spin’. However, having read the paper(s), it’s clear to me that the technique itself is interesting, and I suspect it’ll find much wider usage. So let’s look into how it works.

When natural gas emerges from the ground, it’s composed of hydrocarbons, and up to 10% carbon dioxide (CO₂). Before the gas can be sold to the market, the CO₂ plus any other impurities need to be removed, and this cleanup process is expensive. Generally, the ‘raw’ gas is filtered through a series of liquid compounds called amines, which extract only the CO₂, while letting the ‘clean’ gas through. Amines have a limited capacity though – they can absorb around a fifth (between 15 – 20%) of their own weight in CO₂ – and recycling them for reuse is incredibly energy-intensive. So, lots of research groups have been looking into alternative options that could reduce this cost.

Enter Rice University and their asphalt…though, I prefer to call it bitumen, so no doubt I’ll switch between the two terms. Anyway, asphalt/bitumen is the black, sticky, petroleum-based substance that’s used to build roads. There, it holds together the small bits of rock that are known as aggregate, to form a dense, tough surface for road vehicles to drive on. But if it’s to be used for carbon storage, you need to do a bit of chemistry first.

The team, led by Prof James Tour, started with a naturally-occurring form of bitumen called Gilsonite, which is found in various locations across the US, and used in everything from cement to inks. This, they heated to 400°C to remove the volatile (‘evaporate-able’) components. What’s left is then heated to 900°C in the presence of potassium hydroxide, transforming it into a porous form of asphalt. These tiny holes give the asphalt an ultra-high surface area – so high, in fact, that a single gram of it has a surface area equivalent to that of two ice hockey rinks. And in the same way that a bath sponge can hold a lot of water, this asphalt sponge could be used to store gas… albeit temporarily. This sponge relies on high pressures, already present at gas wells, to hold the carbon dioxide within it pores. Once the pressure drops, the CO₂ is released – either to be pumped back into the ground, keeping it out of the atmosphere, or stored for use elsewhere.

The paper, published in Advanced Energy Materials (£), isn’t the first from this team – they’ve been working on carbon sequestration for years. In 2014, they wrote about transforming gaseous carbon dioxide into solid polymers (there’s also a video about that work here) and in 2015, they produced the first version of this porous asphalt. In those initial tests, they showed that their sponge could adsorb (store on its surface) 114% of its weight in carbon dioxide. But in this latest paper, thanks to the increase in surface area (i.e. they made more space in which to store the gas), the asphalt could manage 154% of its weight…. that’s ten times more than the amines currently in use.

There are other benefits too. The raw Gilsonite is readily available, and unlike amines, the final porous sponge can also be reused immediately. “[We’ve shown] we can take the least expensive form of asphalt and make it into this very high surface area material to capture carbon dioxide,” Prof Tour said in the press release. I admit that I’m no great fan of the oil and gas industry, and I hope that we move away from it sooner rather than later. But anything that makes it cleaner and more energy-efficient in the short-term is a positive step, so I’ll be keeping an eye on this area.

David Fuller's view

I have no idea how close this is to commercial application, and presumably Laurie Winkless of Forbes does either, or it would have been mentioned.  Nevertheless, what impresses me is that we live in an era where the combination of investment capital for privately funded research facilities, or grants for university research, combining highly qualified personnel, plus rapidly developing technologies, are solving problems and producing useful products at a faster rate than ever before.

These are the most important and enduring stories of our era, rather than the current uncertainty caused by low interest rates and slow GDP growth which have temporarily lowered confidence, not least in the developed world.