Chemists from the United States have synthesized a covalent organic framework capable of selectively absorbing carbon dioxide from the air. When this substance was heated, the carbon dioxide was desorbed back. As the authors of the study write in the journal Nature, the resulting material withstood more than 100 cycles of adsorption-desorption of carbon dioxide from the outside air.
Covalent organic frameworks (COFs) are crystalline and porous structures built from organic molecules. They are often used to selectively absorb gases, including carbon dioxide CO2. However, chemists have not yet come up with frameworks that can selectively and reversibly absorb CO2 from the air because its concentration in the air is very low - about 400 parts per million.
But recently, scientists led by Omar M. Yaghi of the University of California, Berkeley, came up with a framework that could selectively absorb CO2 from the air. Their idea was to fill the pores of the framework with amino groups that could covalently bind carbon dioxide molecules into carbamates.
To obtain the desired structure, the chemists first synthesized a framework with a large number of azide groups from modified biphenyldicarbaldehyde and 1,3,5-tris(4-cyanomethylphenyl)benzene, which entered into a Knoevenagel condensation. The scientists reduced the resulting substance using triphenylphosphine in water - in this case, the azide groups turned into amino groups. The chemists mixed the product of this reaction with aziridine, which was opened by the amino groups of the framework to form branched carbon chains containing a large number of new amino groups. In this way, the scientists obtained the polyamine framework COF-999, which they characterized using solid-state NMR spectroscopy.
Next, the chemists examined the ability of COF-999 to adsorb various components of air - nitrogen, oxygen, argon and carbon dioxide. They found that the first three were practically not absorbed by the framework, but carbon dioxide was absorbed quickly and in large quantities. At zero humidity, COF-999 adsorbed up to 0.96 millimoles of carbon dioxide per gram of framework. And when the humidity increased to 50 percent, the material's capacity increased to two millimoles of CO2 per gram.
The researchers then tested COF-999 in outdoor conditions. Over the course of 20 days, the scientists conducted 100 consecutive adsorption-desorption cycles of CO2 from Californian outdoor air. On average, the frame's capacity was 1.28 millimoles of CO2 per gram. Moreover, after the 20-day experiment, the structure and activity of COF-999 did not change.
Thus, chemists have synthesized the first covalent organic framework capable of selectively absorbing up to two millimoles of carbon dioxide per gram of absorbent within an hour. In the future, the researchers plan to synthesize a set of similar frameworks and compare their ability to absorb CO2.
Previously, we talked about how chemists synthesized a covalent organic framework with topological bonds.