Scientists from the University of Missouri-Columbia and the University of Stellenbosch, South Africa have discovered that a nonporous organic molecular crystal can be exploited for the purification of hydrogen gas.

The crystal, consisting of pairs (called dimers) of bowl-like calixarene molecules joined together, absorbs molecules such as carbon dioxide, carbon monoxide, oxygen, and nitrogen into the space within each dimer. The figure to the right shows a section (outlined in yellow) through a calixarene dimer revealing the hourglass-shaped void within where other molecules can be stored.

When the researchers exposed the crystals to a stream of gas containing equal amounts of hydrogen and carbon dioxide, the crystals selectively absorbed carbon dioxide, leaving the hydrogen behind. As Science News reported: “It’s like going through a turnstile,” [Chemist Jerry Atwood of University of Missouri] said.

The immediate practical importance of this discovery comes to play in producing hydrogen for use in fuel cells.

Current techniques for reforming natural gas or methanol into hydrogen require a final separation process to remove impurities -- primarily carbon dioxide. Filtering hydrogen through calixarene crystals could offer a relatively inexpensive and more efficient alternative.

The research is published in the Angewandte Chemie International Edition, Volume 43, Issue 22 (p 2948-2950), and can be reached at the link above.