New absorbing material could help get the world to negative emissions

Capturing and storing the carbon dioxide produced by humans is essential for reducing atmospheric greenhouse gases and slowing global warming. Current carbon capture technologies are effective for concentrated carbon sources like power plant emissions, but they struggle to capture carbon dioxide from the ambient air, where concentrations are much lower.

However, direct air capture (DAC) is seen as crucial for reversing the increasing levels of CO₂, which have now reached 426 parts per million (ppm), 50% higher than pre-Industrial Revolution levels. Without DAC, we may not achieve the goal of limiting global warming to 1.5 °C (2.7 °F) above preexisting global averages, as outlined by the Intergovernmental Panel on Climate Change.

A novel CO₂-absorbing material created by chemists at the University of California, Berkeley, has the potential to contribute to achieving negative emissions. This porous material, known as a covalent organic framework (COF), can capture CO₂ from the ambient air without being degraded by water or other contaminants, addressing a major limitation of existing DAC technologies.

According to Omar Yaghi, the James and Neeltje Tretter Professor of Chemistry at UC Berkeley and senior author of a paper, the new material could seamlessly replace existing carbon capture systems and effectively remove CO₂ from refinery emissions while capturing atmospheric CO₂ for underground storage. According to UC Berkeley graduate student Zihui Zhou, just 200 grams of this material, slightly less than half a pound, can absorb as much CO₂ in a year—20 kilograms (44 pounds)—as a tree.

“Flue gas capture is a way to slow down climate change because you are trying not to release CO₂ to the air. Direct air capture is a method to take us back to like it was 100 or more years ago,” Zhou said. “Currently, the CO₂ concentration in the atmosphere is more than 420 ppm, but that will increase to maybe 500 or 550 before we fully develop and employ flue gas capture. So if we want to decrease the concentration and go back to maybe 400 or 300 ppm, we have to use direct air capture.”

Yaghi is the pioneering mind behind COFs and MOFs (metal-organic frameworks), both known for their rigid crystalline structures with regularly spaced internal pores, providing a large surface area for gases to adhere to. He and his team have successfully developed MOFs capable of extracting water from the air, even in dry environments. When heated, these MOFs release the collected water for consumption. Yaghi has been focused on using MOFs to capture carbon since the 1990s, well before DAC gained widespread attention.

Two years ago, Yaghi’s lab created a highly promising material, MOF-808, capable of adsorbing CO₂. However, after numerous cycles of adsorption and desorption, the MOFs began to deteriorate. These particular MOFs were internally decorated with amines (NH₂ groups) that effectively bind CO₂, a common component of carbon capture materials.

Despite the prevalent use of liquid amines to capture carbon dioxide, Yaghi highlighted the energy-intensive regeneration and volatility of liquid amines, which hinder their industrial application.

Collaborating with peers, Yaghi identified the reason for the degradation of some MOFs in DAC applications — their instability under basic, rather than acidic, conditions, with amines being bases. Yaghi and Zhou, along with colleagues in Germany and Chicago, designed a more robust material, named COF-999. Unlike MOFs, COFs are held together by covalent carbon-carbon and carbon-nitrogen double bonds, which are among the strongest chemical bonds in nature.

Similar to MOF-808, the interior of COF-999 is adorned with amines, enabling the uptake of a greater number of CO₂ molecules.

“Trapping CO₂ from air is a very challenging problem,” Yaghi said. “It’s energetically demanding, and you need a material that has high carbon dioxide capacity, that’s highly selective, that’s water stable, oxidatively stable, and recyclable. It needs to have a low regeneration temperature and needs to be scalable. It’s a tall order for a material. In general, what has been deployed as of today are amine solutions, which are energy intensive because they’re based on having amines in water, and water requires a lot of energy to heat up, or solid materials that ultimately degrade with time.”

Yaghi and his team have dedicated two decades to perfecting COFs with an incredibly resilient backbone capable of withstanding a wide range of contaminants that would break down other porous materials, such as acids, bases, water, sulfur, and nitrogen. The COF-999 is constructed from a base of olefin polymers with an attached amine group. After the porous material is formed, it is infused with additional amines that bond to NH₂ and create short amine polymers within the pores. Each amine has the capability to capture approximately one CO₂ molecule.

When 400 ppm CO₂ air is passed through the COF at room temperature (25 °C) and 50% humidity, it reaches half capacity in around 18 minutes and becomes fully saturated in about two hours. However, this time frame is dependent on the form of the sample and could be significantly reduced with optimization.

By heating to a relatively low temperature of 60 °C (140 °F), the CO₂ is released, and the COF is ready to adsorb CO₂ once more. It can retain up to 2 millimoles of CO₂ per gram, setting it apart from other solid sorbents.

Yaghi mentioned that not all the amines in the internal polyamine chains currently capture CO₂, implying that there may be potential to expand the pores in order to bind more than twice as much.

“This COF has a strong chemically and thermally stable backbone, it requires less energy, and we have shown it can withstand 100 cycles with no loss of capacity. No other material has been shown to perform like that,” Yaghi said. “It’s basically the best material out there for direct air capture.”

Yaghi is confident that artificial intelligence can accelerate the development of enhanced COFs and MOFs for carbon capture and other applications, particularly by pinpointing the chemical conditions required to produce their crystalline structures.

Journal reference:

1. Zihui Zhou, Tianqiong Ma, Heyang Zhang, Saumil Chheda, Haozhe Li, Kaiyu Wang, Sebastian Ehrling, Raynald Giovine, Chuanshuai Li, Ali H. Alawadhi, Marwan M. Abduljawad, Majed O. Alawad, Laura Gagliardi, Joachim Sauer & Omar M. Yaghi. Carbon dioxide capture from open air using covalent organic frameworks. Nature, 2024; DOI: 10.1038/s41586-024-08080-x