Capturing carbon from the air just got easier

Capturing and storing human-produced carbon dioxide is essential for reducing atmospheric greenhouse gases and slowing global warming, but current carbon capture technologies only work well for concentrated sources of carbon, such as exhaust gases from power plants. The same methods cannot effectively capture carbon dioxide from ambient air, where concentrations are hundreds of times lower than those in flue gases.

Yet we rely on direct air capture, or DAC, to reverse the rise in CO₂ levels, which reached 426 parts per million (ppm), 50% higher than pre-industrial revolution levels. Without it, according to the Intergovernmental Panel on Climate Change, we will not meet humanity’s goal of limiting warming to 1.5°C (2.7°F) above pre-existing global averages.

A new type of absorbent material developed by chemists at the University of California, Berkeley could help move the world to negative emissions. The porous material – a covalent organic structure (COF) – captures CO₂ ambient air without degradation by water or other contaminants, one of the limitations of existing DAC technologies.

“We took a powder of this material, put it in a tube, and we passed Berkeley air – just outside air – through the material to see how it would perform, and it was beautiful It completely purified the air of CO.₂. Everything,” said Omar Yaghi, the James and Neeltje Tretter Professor of Chemistry at UC Berkeley and lead author of a paper that will appear online Oct. 23 in the journal Nature.

“I am excited about this project because there is nothing comparable in terms of performance. It opens new perspectives in our efforts to solve the climate problem,” he added.

According to Yaghi, the new material could easily be replaced in carbon capture systems already deployed or being tested to remove CO.₂ refinery emissions and capture atmospheric CO₂ for underground storage.

Zihui Zhou, a UC Berkeley graduate student and first author of the paper, said just 200 grams of material, or a little less than half a pound, can absorb that much CO₂ in a year – 20 kilograms (44 pounds) – as a tree.

“Capturing combustion gases is a way to slow down climate change because we try not to release CO₂ in the air. Direct air capture is a method that takes us back to what it was 100 years ago or more,” Zhou said. “Currently, CO₂ The concentration in the atmosphere is above 420 ppm, but it will increase perhaps to 500 or 550 before we can fully develop and use flue gas capture. So if we want to lower the concentration and get back to maybe 400 or 300 ppm, we need to use direct air capture. »

COF vs. MOF

Yaghi is the inventor of COFs and MOFs (metal-organic structures), which are both rigid crystalline structures with regularly spaced internal pores that provide a large surface area for gases to stick or adsorb. Some MOFs he and his lab have developed can adsorb water from the air, even in arid conditions, and when heated, release the water for drinking. He’s been working on MOFs to capture carbon since the 1990s, long before DAC was on most people’s radar screens, he said.

Two years ago, his laboratory created a very promising material, MOF-808, which adsorbs CO₂but the researchers found that after hundreds of adsorption and desorption cycles, the MOFs degraded. These MOFs were decorated internally with amines (NH₂ groups), which effectively bind CO₂ and are a common component of carbon capture materials. In fact, the dominant method of carbon capture is to bubble exhaust gases through liquid amines that capture carbon dioxide. Yaghi noted, however, that the energy-intensive regeneration and volatility of liquid amines hamper their further industrialization.

Working with colleagues, Yaghi discovered why some MOFs degrade for DAC applications: they are unstable under basic conditions, as opposed to acidic, and amines are bases. He and Zhou worked with colleagues in Germany and Chicago to design a stronger material, which they call COF-999. While MOFs are held together by metal atoms, COFs are held together by carbon-carbon and carbon-nitrogen covalent double bonds, among the strongest chemical bonds in nature.

As with MOF-808, the pores of COF-999 are decorated internally with amines, allowing the absorption of more CO₂ molecules.

“Trapping CO₂ of air is a very difficult problem,” Yaghi said. “It’s energy demanding, you need a material that has a high carbon dioxide capacity, that is highly selective, that is stable at water, stable to oxidation and recyclable. It must have a low regeneration temperature and must be scalable. This is a big challenge for a material. And in general, what has been deployed today are amine solutions, which are energy intensive because they are based on the presence of amines in the water, and the water requires a lot of energy to heat, or solid materials that eventually degrade over time. “

Yaghi and his team have spent the past 20 years developing COFs with a structure strong enough to resist contaminants, ranging from acids and bases to water, sulfur and nitrogen, that degrade other materials porous solids. COF-999 is assembled from an olefin polymer backbone with an amine group attached. Once the porous material is formed, it is flushed with more amines which attach to NH.₂ and form short amine polymers inside the pores. Each amine can capture approximately one CO₂ molecule.

When 400 ppm CO₂ air is pumped through the COF at room temperature (25°C) and 50% humidity, it reaches half capacity in about 18 minutes and fills in about two hours. However, this depends on the sample shape and could be accelerated to a fraction per minute once optimized. Heating to a relatively low temperature – 60°C or 140°F – releases CO₂and the COF is ready to adsorb the CO₂ Again. It can contain up to 2 millimoles of CO₂ per gram, distinguishing itself from other solid absorbents.

Yaghi noted that not all amines in the internal polyamine chains currently capture CO₂so it may be possible to enlarge the pores to bind more than twice as much.

“This COF has a strong skeleton, is chemically and thermally stable, requires less energy and we have shown that it can withstand 100 cycles without loss of capacity. No other material has demonstrated such performance,” said Yaghi. “It’s basically the best material available for direct air capture.”

Yaghi is optimistic that artificial intelligence can help accelerate the design of even better COFs and MOFs for carbon capture or other purposes, including identifying the chemical conditions required to synthesize their crystal structures. He is the scientific director of a research center at UC Berkeley, the Bakar Institute of Digital Materials for the Planet (BIDMaP), which uses AI to develop cost-effective and easily deployable versions of MOFs and COFs to help limit and respond to climate impacts. change.

“We are very, very excited about blending AI with the chemistry that we do,” he said.

The work was funded by King Abdulaziz City for Science and Technology in Saudi Arabia, Yaghi’s carbon capture startup Atoco Inc., Fifth Generation’s Love, Tito’s and BIDMaP. Yaghi’s collaborators include visiting scholar Joachim Sauer of Humboldt University in Berlin, Germany, and computer science specialist Laura Gagliardi of the University of Chicago.