Rayn Lakha
Image from https://phys.org/news/2013-11-metal-organic-framework-action.html. Credit: National Academy of Sciences
I entered this article, without the above image, into the 2023 New York Times STEM writing competition.
You’re hurrying down a polluted pathway, enveloped in the stench of sulfur, when a towering edifice begins to take shape through the haze. Peering closer, you discern myriad rows of metal-studded cages and, with a gasp, espy scores of your peers imprisoned within. Turning away from the rattling of their enclosures, you try to flee, but the tendrils of smog surrounding you impel you into a cell.
A horrific occurrence - but a useful one for humanity, if you happen to be a CO2 molecule.
Although renewable energy sources are rightfully lauded, they cannot remove CO2 emissions from the innate chemical reactions upon which heavy industries depend. Instead, these fumes must be caught as they leave factories, stopping them from contributing to global warming. This process, known as carbon capture, is “unavoidable if net zero … emissions are to be achieved” according to the Intergovernmental Panel on Climate Change, and our molecular prisons (called metal-organic-frameworks, or MOFs) could be our best bet for fulfilling this urgent need.
MOFs are perhaps best thought of as 3-dimensional grids, consisting of metal ions acting as corners which are linked by organic molecules. The resulting structure is tremendously porous, and so has a remarkably large surface area: according to Professor Long at UC Berkeley, “just one gram of a MOF … can have a surface area greater than a football field”. This means that there are many sites for gas molecules to bind to, enabling the MOF to act “like a sponge … soaking up vast quantities of … carbon dioxide”.
Using super-porous materials to influence gases is nothing new though - similar compounds called zeolites have been used for this purpose for decades. So what makes MOFs so special? The answer, Professor Telfer, Dr Qazvini, and Dr Babarao of the MacDiarmid Institute for Advanced Materials explain, is “high selectivity” for CO2 over competing gases. MOFs’ “pore shape, size, and chemical environment can be systematically designed [which] allows interactions between [the MOF] and molecular guests to be tailored”, they note in their new paper in the journal Nature. In practice, this means that MOFs can be customised to only attract and capture CO2 molecules instead of connecting to many molecules of a similar size, so that the MOF has to be emptied (regenerated) less often, requiring less energy.
MOFs’ use of weak physical binding mechanisms to stick to their guests (including the arrangement of differently charged atoms in organic linkers complementing the charge distribution within CO2 molecules) rather than strong chemical bonds further reduces their regeneration energy. This makes them more economic than traditional solvent-based systems, which require 35% of a power station’s output to clean its exhaust.
According to an EU research programme, there is one more hurdle for MOFs: completing thorough performance testing under harsh industrial conditions, after which commercial adoption is viable. It’s not all bad news for our incarcerated molecular buddies though - captured carbon dioxide can be recycled in industrial processes to start a new life as a useful product.
Sources
https://prometheanparticles.co.uk/carbon-capture/
https://www.nature.com/articles/s41467-020-20489-2
https://www.energy.gov/science/articles/sponge-soak-carbon-dioxide-air
https://www.moftechnologies.com/mofs-for-co2
https://www.moftechnologies.com/post/mofs-the-solution-for-maintaining-good-indoor-air-quality
https://www.youtube.com/watch?v=EWSSYN9kh-E
https://www.mdpi.com/2073-4344/10/11/1293
https://www.nytimes.com/2009/12/08/science/08obgas.html
https://www.nytimes.com/2010/09/07/science/07nano.html
https://www.catf.us/2022/04/what-does-latest-ipcc-report-say-about-carbon-capture/
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