Crystal could generate pure hydrogen fuel

The bizarre behavior of an organic crystal called calixarene could help drive a hydrogen economy, suggests a new study.

Researchers describe a crystal that, when exposed to air, absorbs molecules such as carbon dioxide, carbon monoxide, oxygen, and nitrogen. The crystal consists of two calixarene molecules joined together, resembling a pair of cups attached at a their open ends to leave a cavity inside.

Chemist Jerry Atwood of the University of Missouri-Columbia and his colleagues discovered calixarene’s behavior after adding a drop of nitrobenzene to a layer of crystal material on a glass slide. Suddenly, the crystal began to bubble vigorously, says Atwood, indicating that molecules of nitrobenzene somehow had migrated into the crystal and forced out gas molecules.

When the researchers exposed the crystals to a stream of gas containing equal amounts of hydrogen and carbon dioxide, the calixarene did something even more surprising: It selectively absorbed carbon dioxide, leaving the hydrogen behind.

This separation method could be a boon for fuel cells that rely on hydrogen, such as those proposed for cars. Impurities in hydrogen, including carbon dioxide, decrease a fuel cell’s efficiency and durability. Hydrogen-production plants use costly separation technologies to remove the impurities. Filtering hydrogen through calixarene crystals could offer a relatively inexpensive and more efficient alternative, says Atwood.

Just how the carbon dioxide gets into the crystal’s cavity remains unknown. Atwood suspects that the crystal’s lattice structure shifts slightly upon contact with carbon dioxide and lets the molecule enter. “It’s like going through a turnstile,” he says.

The researchers describe the crystals in the May 24 Angewandte Chemie.