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How zeolites hold tight to metal ions.

The petroleum industry would be on its knees without zeolites, porous solids that refineries use in the processing of oil and gas. Zeolites grab metal ions and hold them tightly in complicated networks of channels and pores. The ions can catalyze a wide variety of chemical reactions that break large oil molecules into smaller, more useful ones.

Researchers at the University of Cambridge in England have used a computer analysis to explore how zeolites grasp metal ions so tenaciously. At some locations, they find, the zeolite structure distorts to accommodate the ions. As well as helping scientists understand existing zeolites better, the computer program may also make possible the creation of structures that catalyze particular reactions. The findings appear in the May 12 Physical Review Letters.

"To a large extent, [zeolites] are used on a trial-and-error basis," says study coauthor Kenton D, Hammonds. Although scientists have long thought that flexibility in zeolite structure was important, "there haven't been that many ways of actually calculating what's going on."

Most zeolites have a framework of small tetrahedral molecular units, triangular pyramids in which four oxygen atoms enclose a central aluminum or silicon atom. Using their computer model, Hammonds and his colleagues have discovered that when an incoming metal ion situates itself within a zeolite pore, the group of surrounding tetrahedrons twists slightly, closing in on the ion like a camera shutter. That movement optimizes the distance between the ion and the oxygen atoms, enhancing the strength of the bonds.

The tetrahedrons move as a unit, which requires much less energy than rearranging the individual atoms. "In zeolites, the framework distorts but these [tetrahedrons] remain undistorted," says Hammonds. "On a large scale, the framework can flex, but on a much smaller scale, it's not flexible."

The flexibility of the framework may underlie zeolites' ability to catalyze reactions, the researchers suggest, because the structure can adjust to wrap around incoming molecules.

The computer, program calculates, from the symmetry of the zeolite structure, where distortions in a framework would occur and analyzes what would happen if low-frequency vibrations, or phonons, propagated through that framework.

Like waves in the sea, a large number of phonons can converge and either add together or cancel each other out. Where the phonons add, the greatest structural deformations occur. "Our calculations were based on coming up with the best way of predicting what this constructive interference would be," Hammonds says.

Six years ago, another group used X-ray spectroscopy to pinpoint the locations of metal ions bound to a zeolite called faujasite. The Cambridge researchers found "a close correlation" between their predictions and those previous findings, Hammonds says.

The group is now looking at how a zeolite's size affects its flexibility--larger zeolites appear to be more flexible than smaller ones. The program can also analyze how phonons distort other materials with tetrahedral structures, such as quartz and feldspar.

Although the program allows, researchers to "play around" with zeolites on the computer, Hammonds says, it's no substitute for working with the actual material. "You'd still have to go to the lab and test it all out for real," he says.
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Author:Wu, Corinna
Publication:Science News
Date:May 24, 1997
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