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Aerogels aim for purification role

ChemicalProcessing.com
08/31/2007

The first aerogels based on chalcogenides — compounds of the group of elements on the periodic table that includes sulfur, selenium and tellurium — readily remove mercury from water and also could be used to eliminate contaminants from hydrogen, say researchers at Argonne National Laboratory, Argonne, Ill.

In laboratory tests, sulfur-based aerogels reduced mercury levels in water to 0.04 pm from 650 ppm in less than 15 minutes, notes Mercouri Kanatzidis, a senior scientist at Argonne and professor at Northwestern University, Evanston, Ill. Metal sulfide binds the mercury onto the aerogel via chemical adsorption. The mercury then can be recovered. The aerogels should work reasonably well for lead and cadmium removal, too, he says. The materials also might be able to eliminate aromatic organic compounds such as polychlorinated biphenyls and dioxin from water, he believes, adding that tests should take place within the next year.

Kanatzidis, together with Peter Chupas at Argonne and graduate student Santanu Bag at Northwestern, now have created seven such aerogels. They boast surface areas of 300 m2 to 400 m2/gram, which are as good as those of silica aerogels on a volume basis (the chalcogenide aerogels have a higher density), he says. They have an amorphous, random structure like those of silica aerogels.

“Making the aerogels from gels is easy,” explains Kanatzidis. “The trick is creating the gels in the first place.” The researchers have relied on platinum chemistry to form the gels. To reduce costs, they now are making gels without using the precious metal, he notes, and the aerogels derived from these maintain the performance of platinum versions.

The next step is to make many more variants of the aerogels for evaluation. Perhaps as many as 100 will be produced within the next year, hopes Kanatzidis.

The aerogels also may be able to play a role in purification of gases such as hydrogen. Here, though, the mechanism would be selective permeability rather than chemical adsorption, says Kanatzidis. “The aerogels may offer unique permeability.”

Hydrogen purification will be the main focus of such efforts, which should take place over the next year. “When people talk about the hydrogen economy, one of the big questions they’re asking is ‘Can you make hydrogen pure enough that it doesn’t poison the catalyst?’,” Chupas explains.

“The high surface area also makes you think of catalysis,” adds Kanatzidis. It’s possible to include active sites in the structure of the aerogel, he notes.

So far, work has been done on a laboratory scale, but there’s no limitation to scale up of the aerogels, he says, adding the researchers already have received some inquiries from industry.