A better way to capture carbon

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In this carbon-capturing sponge, carbon dioxide molecules don’t simply fill the material’s pores – they become part of its walls. X-ray snapshots show the material before (left) and after (right) exposure to carbon dioxide. – Thomas McDonald – UC Berkeley

Capturing carbon is effortless if you’re a plant. But for humans, scrubbing carbon dioxide out of the air is not so easy. Jeffrey Long and his colleagues at the University of California in Berkeley recently stumbled upon a particularly promising carbon-capture material – and have just discovered why it works so well. Their sponge-like material sucks up CO2 in a cascade, each absorbed CO2 molecule popping open the next part of the network for the next CO2 molecule to slot into.

“It’s a real breakthrough,” says Chris Sumby, who works on carbon-capture technology at the University of Adelaide. The findings have been published in Nature.

Power plants burning fossil fuels emit more than 13 billion tonnes of carbon dioxide into the atmosphere each year – stoking global warming. Last year, the world’s first industrial-scale carbon capture and storage power plant opened in Canada. At Boundary Dam, the plant’s emissions – 10% are comprised of CO2 – pass through tanks of liquid amines (a nitrogen-based solvent) to trap the carbon. Unfortunately, releasing the CO2 from the solution for storage guzzles more than 20% of the power station’s energy output.

So there’s great interest in finding an alternative. Metal-organic frameworks (MOFs) have the potential to play a leading role. In 2012, Jeffrey Long and his team at Berkeley stumbled upon a particularly promising magnesium MOF that not only soaked up CO2 effectively, but only required gentle warming to squeeze the CO2 back out again.

The magnesium MOF became a “benchmark material,” says Sumby. At the molecular scale it resembles a honeycomb. The researchers assumed the CO2 filled up the hexagonal tunnels. But why the material worked so well was a mystery.

"It was something we had no idea was possible."

Long and his team have now solved it. When the researchers took X-ray snapshots of the material’s molecular structure, they saw that instead of sitting in the tunnels of the honeycomb, the CO2 became part of the framework, squeezing itself into a bond between a magnesium and a nitrogen atom. “It was something we had no idea was possible,” says Thomas McDonald, the lead author of the study.

When the CO2 nudges its way into the framework it sets off a chain reaction. Once one CO2 molecule is inserted, the neighbouring magnesium-nitrogen bond is destabilised, helping the next CO2 molecule slip in, and so on. This domino effect cascades down the honeycomb, explaining how the MOF absorbs the CO2 so swiftly.

The researchers also found that once the MOF is warmed by 50ºC, the same effect happens in reverse. As one CO2 molecule is kicked back out, the next CO2 is less stable in the framework.

The temperature at which the MOF inhales and exhales CO2 is also adjustable. The team found that switching the magnesium in the MOF to manganese shifted the effective CO2 absorbing temperature range down.

So how soon can we expect to see MOFs deployed at power plants?

Matthew Hill, MOF materials scientist at the CSIRO, speculates that completely sequestering all CO2 from a power plant would require around 100 tonnes of MOF pellets – although it is not yet known how often that store would need to be replaced. They cost $100 per kilogram, so price remains a significant hurdle (although production costs are falling). On the other hand, the MOFs used by the Berkeley team need 35% less energy than liquid amines to regenerate, so they should be a more economical method of carbon capture.

McDonald says he is excited about testing the material in real power plant flue gas. He’s optimistic the material will soon find its way into industry. Hill sees MOFs quickly implemented at power plants around the world as soon as the formula is right. “We’re at a tipping point,” he says.

One interesting aside is that Long’s magnesium MOF has a similar structure to the magnesium-containing active pocket in rubisco, a plant enzyme that grabs CO2 ready for conversion to glucose. So far, plants remain the benchmark for efficiently drawing CO2 out of the air. But, as Hill points out, “we’ve only had a few years in the chemistry lab. Evolution’s had millions of years of head start.”

More on this topic from Cosmos: Has carbon capture’s time finally come?

The crystals that can clean the planet

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