Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2004, 43 (22), 2948—2950
A Goblet for Gases
Calixarene crystals as new absorption agents for the purification of hydrogen
Air bubbling out of a crystal when it is submerged in nitrobenzene—this surprising and more or less accidental observation could form the basis of a new method for the purification of hydrogen. The unusual crystalline material, with which Jerry L. Atwood and Agoston Jerga (University of Missouri, Columbia, USA) as well as Leonard J. Barbour (University of Stellenbosch, South Africa) were experimenting, is a calixarene: a large cup-shaped molecule. Sublimation of the solid material allowed the team to obtain crystals consisting of double layers of molecules; pairs of slightly distorted cups are oriented with their open ends facing each other. The cups thus form the two half-shells of a large cavity. These cavities are closed off, with no pores or channels leading out. The researchers were thus that much more surprised by the abovementioned gas bubbles coming out of the crystals. Despite their initial doubts, the team was able to prove that the gas bubbles consist of air that seeps into the cavities when the calixarene crystals are stored in air. If the crystals are submerged in nitrobenzene, the nitrobenzene molecules are more strongly absorbed by the calixarene and push the gas molecules out of the cavities. The different components of air are also absorbed to different degrees; carbon dioxide (CO2) is absorbed especially effeciently.
Then came the next surprise, which truly electrified the researchers: hydrogen is not absorbed by the calixarenes, not even under high pressure! In an atmosphere of hydrogen and CO2, the crystals selectively absorb CO2 while the hydrogen stays in the gas phase. Why is this so exciting? Industrially produced hydrogen is impure as a result of the production process; it contains a number of other gases, particularly carbon dioxide and carbon monoxide. Their removal is a complicated and correspondingly expensive affair, but it is urgently needed for the generation of hydrogen for use in fuel cells. The purity of the hydrogen used is critical for the smooth operation, long life, and thus the efficiency of fuel cells. Calixarene crystals have the potential to be efficient absorption agents for the removal of impurities from hydrogen.
Despite all the experimentation, one puzzle remains unsolved: how does the model ship get inside the bottle, that is, how do the gas molecules actually get into the sealed cavities?