'Frustrated' Solid Sucks Up Gases

A familiar calixarene has revealed a hidden talent. Normally, the crystalline material is unable to absorb gases. But researchers at the University of Missouri, Columbia, showed that with a little tweaking, it can rapidly and reversibly absorb gases such as acetylene and CO₂. The finding opens the possibility of using calixarenes and similar materials to separate or store gases.

The material "exhibits a new mode of gas sorption," comments professor Mike Zaworotko, who studies crystal engineering at the University of South Florida. Its behavior "should cause us to rethink the conventional wisdom concerning molecular organic solids that suggests they have little potential usage as adsorbent materials."

The crystalline solid consists of p-tert-butylcalix[5]arenes (TBC5), bowl-shaped molecules that can hold guests such as toluene. University of Missouri chemistry professor Jerry L. Atwood and colleagues found that when the solid is saturated with toluene, it's unable to absorb any gas. Nor can it hold any gas when it's completely rid of toluene, following mild heating. This procedure changes the solid's crystal structure into a "collapsed" form in which the tert-butyl groups of some calixarenes fill the bowls of others.

Atwood and his team discovered that even a few toluene molecules remaining in the crystal can "frustrate" its structural collapse, much like trapped cars holding up a freeway overpass that has partially collapsed in an earthquake. The remaining toluenes prop open the structure, leaving plenty of empty calixarene bowls that are ready to accept gas molecules (J. Am. Chem. Soc. 2006, 128, 15060).

The frustrated solid readily soaks up acetylene and can absorb CH₄ or CO₂ to a lesser extent, but it won't absorb O₂ or N₂. That selectivity means the material could be used to separate one gas from another, Atwood says.

Atwood's team has already unearthed another similar adsorbent organic solid—though it's not based on calixarenes—and expects to find several more. "We've discovered an entire lost world of sorptive materials," he declares.