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Carbon capture gets a boost from aromatic rings

Linking phenyls to the amines used to scrub CO2 from power plant emissions could make process more efficient

by XiaoZhi Lim
April 13, 2017 | A version of this story appeared in Volume 95, Issue 16

Structural schematic of m-xylylenediamine in a bilayer with carbon dioxide molecules bound to the amines.
Credit: J. Am. Chem. Soc.
m-Xylylenediamine traps CO2 in a bilayer, seen here from two directions, keeping water out.

Capturing and using carbon dioxide produced by power plants and other sources could help meet climate emissions goals. In theory, CO2 is easy to capture. Because it’s acidic, it reacts readily with simple bases such as amines. But in practice, amine scrubbing, the CO2-capture method used by some power plants to clean flue gases, gets bogged down because it traps the greenhouse gas in water-based solutions. Heating these large amounts of water to release the captured CO2 and regenerate the amines requires a great deal of energy.

To address this challenge, Fuyuhiko Inagaki and his research team at Kanazawa University report a family of amines that absorb CO2 but not water, potentially reducing the amount of energy needed to run the scrubbing process (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b01049).

Inagaki and colleagues, who are medicinal chemists, developed the amines while trying to create a dry stream of CO2 from air for use as a building block for pharmaceutical synthesis. Amines absorb water in addition to CO2, yielding “wet” CO2, which can interfere in chemical reactions and corrode pipes.

The researchers hypothesized that adding hydrophobic side groups, such as phenyls, onto amines used to capture CO2 could help reduce water absorption. So they tested the CO2 absorption ability of a variety of benzene-linked amines, ranging from simple benzylamine to xylylenediamines. Then the group exposed the best-performing amines to open air for two weeks to determine the amount of water absorbed relative to CO2. The researchers discovered that the xylylenediamines were the most promising: They absorbed no water at all. In comparison, monoethanolamine, the standard compound used for amine scrubbing, absorbed three molecules of water for every molecule of CO2. When the researchers dissolved the best-performing xylylenediamine in water and exposed the solution to air, the xylylenediamine-CO2 product precipitated as a white solid that contained no water.

Investigating the solid’s structure revealed a bilayer with the CO2 molecules sandwiched between sheets on the inside and the hydrophobic phenyl groups interacting with adjacent sheets on the outside. “One part of the molecule keeps the water away, while the other part is just right for capturing carbon dioxide,” says Vanda A. Glezakou of Pacific Northwest National Laboratory, who was not involved in the study.

The researchers released the CO2 by heating the solid product to 120 °C and showed that it could be used directly in the notoriously moisture-sensitive Grignard reaction. Inagaki and his team are now testing the xylylenediamine’s stability and exploring the feasibility of preparing it on a large scale for real-world applications in collaboration with industry and government agencies in Japan.



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