Covalent organic framework captures carbon from air

As the levels of carbon dioxide in the environment rapidly rise and the climate consequences of that rise become increasingly apparent, research and investment into carbon capture strategies have also picked up momentum.

“Carbon capture is the problem of our society today . . . it’s also a fantastic chemistry problem,” says Omar Yaghi of the University of California, Berkeley, who has been working on networked materials for carbon capture and water harvesting since the 1990s.

Direct air capture (DAC) aims to remove CO₂ from ambient air using weak bases. The captured CO₂ can then be sequestered or used for industrial applications. But current materials for DAC tend to require a lot of energy to recover the captured CO₂, a problem Yaghi and many other researchers are working to address.

Yaghi and his team have now invented a new covalent organic framework (COF) for open-air carbon capture that combines high stability and moisture tolerance with low-temperature release of CO₂ (Nature 2024, DOI: 10.1038/s41586-024-08080-x).

This new material, COF-999, is based on a durable, double-bonded COF network that the group developed a few years ago (J. Am. Chem. Soc. 2019, DOI: 10.1021/jacs.9b02848). Graduate student Zihui Zhou tethered amine polymers into the material’s pores to snatch up CO₂ via an acid-base reaction.

Carbon capture materials based on porous materials tend not to be stable when exposed to moisture, but a little bit of water is actually helpful in this COF because it creates carbamates and bicarbonates that boost the carbon capture capacity. Zhou carefully designed the material with a hydrophobic scaffold to enhance those benefits while preventing water from getting stuck in the pores.

In dry air, 1 g of COF-999 can take up 1 mmol of CO₂ from a gas mixture containing 400 ppm CO₂; at 50% humidity, the capacity is just over 2 mmol/g. Heating the COF to 60 °C will release the captured CO₂, freeing up the material to capture more carbon. Other materials have greater capacity, but they tend to be less tolerant to moisture and require more energy to regenerate, says Zhou.

Zhou also tested COF-999 with air piped in from outside of the university’s chemistry building. He carried out 100 catch-and-release cycles over 20 days without seeing any significant drop in performance.

Yaghi and Zhou say they’d like to make some additional tweaks to boost how much CO₂the material can take up. They also plan to work on a more cost-effective and scalable way to make the material so that they can advance it toward commercialization.

Natalia Shustova, a materials chemist researching COFs and MOFs at the University of South Carolina who was not involved in the work, praised the researchers’ clever design strategy, saying that the work “clearly demonstrates the exceptional versatility and modularity of COF structures.”

Chemical engineer Christopher Jones of the Georgia Institute of Technology, who studies carbon capture and was also not involved in the work, agrees that the paper is a creative and thorough academic materials study. But he cautions that there are more challenges that must be met to prove that this COF would work for real-world carbon capture. For example, the desorption experiments in the paper didn’t include recovery of pure CO₂, which a full DAC system would need to do.

Yaghi is confident that his team and their COFs will prove themselves for effective carbon capture. “We’re not going to stop here,” he says.