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RESEARCHERS at the University of Southern California, US, have developed a polyethylenimine-based, easy-to-regenerate adsorbent which captures CO₂ directly from air.

The team, led by chemistry professor Sri Narayanan, speculates that the material could be used for direct air capture to remove excess atmospheric CO₂ resulting from small-scale sources such as transport and home heating

“[The adsorbent] could be utilised to purify gas streams from CO₂ in submarines and other closed environments as well as for applications such as alkaline fuel cells for which it is important to have an air source free of CO₂ to avoid the formation of carbonates by reaction with a strongly basic electrolyte,” say the researchers.

In tests, the solid adsorbent, which consists of fumed silica impregnated with polyethylenimine (PEI), absorbed all the CO₂ from samples of air, even where the concentration was very low. The team reports that under humid conditions, the carbon capture values were among the highest ever recorded. The PEI-based material absorbed 1.74 mmol/g of CO₂. This compares favourably with other solid adsorbents – for example hyperbranched silica with a comparable amine content of 36%, absorbs 1.44 mmol/g of CO_2. Hyperbranched silica with a higher amine content of 42.5% absorbs 1.72 mmol/g of CO₂, while triamine-grafted pore-expanded mesoporous silica absorbs 0.98 mmol/g.

Narayanan and his team used a high molecular weight PEI, mixed with methanol and fumed silica, an inexpensive material consisting of small silica particles often used as a thickener or as an abrasive in toothpaste. The methanol was then evaporated, leaving behind the PEI on the surface of the silica. The resulting solid absorbent was then tested at 25°C with various CO₂ concentrations. To regenerate the absorbent, the researchers heated it under vacuum for three hours at 85°C, which removed adsorbed CO₂ and water. They carried out four regeneration cycles without any noticeable drop in efficiency.

Liquid CO₂ scrubbers based on amine solutions, such as those proposed for use in industrial smokestacks, are not suitable for direct air capture, as in the presence of oxygen, they tend to degrade quickly. They are also costly to regenerate due to their high heat capacity. Using amines and polyamines deposited on solid supports, such as fumed silica, avoids these problems. The researchers add that the necessary infrastructure for direct air carbon capture could be placed in areas where it would do least damage to the environment, or near to facilities using CO₂ as a chemical feedstock.

Journal of the American Chemical Society doi: 10.1021/ja2100005