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MOFs are compelling

Picture a set of assembled tinker-toys, rigid struts connecting central hubs, a lightweight framework containing copious open space. Shrink this down to the molecular level and you might have an MOF, or metal-organic framework material, where the metal atoms are the hubs and rigid organic ligands are the struts. MOFs have contain open spaces that are roughly one molecule in width, and if you tune the size of the hubs and struts just like that open space will be exactly the same size as a molecule of CO2.

Nature abhors a vacuum, so given a choice these MOFs will want their pores filled precisely. They can be tuned to bind selectively to CO2 over nitrogen and oxygen molecules, which allows them to pluck CO2 out of a stream of flue gas. There are many MOFs that can do this, but most have some affinity for water, which is present in large amounts in flue gas streams, and can get in the way of CO2 sorption. As a result, even they have a similar capacity for CO2 sorption to the best amine sorbents, they don't function very well outside the lab.

The startup Svante uses an MOF developed at the University of Calgary called CALF-20 that is unusual for several reasons:

  • It can tolerate water, absorbing CO2 in environments up to 40% relative humidity

  • It is stable to steam treatment, which can be used to release the bound CO2

  • Svante has developed a high throughput process, with promising operational economics.

  • The MOF is super-cheap to make, and the synthesis scales

This makes Svante's MOF in particular compelling for flue gas capture. It probably is not a good candidate for direct air capture, because its binding energy is less than half of that of a typical amine. But that's ok - providing a path to <$40/ton capture is reason enough for interest. Svante's MOF does this, and it will be interesting to see if others using this approach can keep pace.

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