Molecular cage traps rare gases

Organic compound could cull valuable xenon from the air and detect cancer-causing radon

With a squeeze, an organic molecule can snatch rare gases from the air.

The compound contains cavities that are just the right size to nab atoms of xenon, krypton and radon. These noble gases range from valuable to radioactive; they’re all largely inert, usually present in low or negligible concentrations in the air and extremely difficult to capture. Researchers hope the new molecular trap could one day be used to sort radioactive waste gas, monitor home air and collect valuable resources.

“I would die to have these compounds in my hands,” says organic chemist Siegfried Waldvogel of the Johannes Gutenberg University Mainz, in Germany, who works on chemical sensors.

But the new chemical cage was a bit of a “happy accident,” admits materials chemist Andrew Cooper of the University of Liverpool in the England.

Cooper and his colleagues had set out to create a polymer. Instead, they produced a 3-D cage by reacting four aldehyde molecules with six nitrogen-containing molecules. The individual cages clumped together to form a multichambered atomic jail.

GAS GRABBER Molecular cages such as the one illustrated in gray and blue have a central gap (purple) perfectly sized to hold a rare gas atom. L. Chen et al/Nature Materials
The researchers found that the cages are around 0.44 nanometers wide. That size is well fitted to hold the three noble gases: a krypton atom is 0.37 nanometers wide, xenon is 0.41 and radon 0.42.

However, openings to the cages are only around 0.36 nanometers, small enough to possibly prevent the gas atoms from slipping into the cages. But like all organic compounds, the cage molecules wiggle. Those fluctuations cause the entryways to expand and contract. The researchers used computer simulations to estimate that a door would open to allow xenon to slip in and out just 7 percent of the time; for radon, it would open for 3 percent. Smaller krypton would see an open door for much of the time and could come and go more easily.

In experiments, researchers confirmed that the molecular trap could catch atoms. They then tested the molecule’s selectivity, blowing a mix of common elements in air and trace amounts of xenon and krypton through the molecular trap for 45 minutes. The nitrogen, oxygen and carbon dioxide passed right through. Xenon got trapped for around 15 minutes, while krypton passed through more quickly. The results appearJuly 20 in Nature Materials.

The finding suggests that the molecular trap can strip out valuable xenon from krypton and other gases. Xenon, which sells for around $5,000 per kilogram, is useful in lighting, medical imaging and anesthesia. The molecular cage could help clean up waste gas from spent nuclear fuel. Waste gas can contain a combination of radioactive krypton and nonradioactive xenon.

Additionally, Cooper and colleagues think the molecules could detect radon in homes. The radioactive gas causes approximately 21,000 lung cancer deaths in the United States each year.

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