(fw) In collaboration with scientists of several other universities, researchers at the Crystallography Laboratory of ETH Zurich have succeeded in decoding the biggest zeolite structure prepared up to now. As the scientists write in the latest issue of the scientific journal Nature (1), they succeeded in reconstructing the crystal structure thanks to a novel combination of proven investigation methods.

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Crystallographers often use what is known as the X-ray powder diffraction method to determine the structure of unknown materials. When a powdered material is irradiated with X-rays, the crystal structure of the sample being studied can be determined from the diffraction pattern. However, the method comes up against its limits in the case of complex compounds. Because of the numerous overlaps in the diffraction pattern, important information in the recorded signal, without which the required structure cannot be determined directly, is lost.

The group of researchers has now combined diffractometry data with phase information from electron microscope images. Using special software already developed at ETH previously, the scientists were finally able to characterise the unknown material. The compound, named TNU-9, has 24 silicon and aluminium atoms in tetrahedral positions in the smallest unit in which such crystals are described – which is 8 atoms more than in a crystal of ITQ-22, previously the most complicated zeolite structure known. "The new technique is a clear step forwards,” explains Fabian Gramm, the first of the study’s authors.

The calculations required considerable computing power. A G5 Macintosh computed non-stop for 16 days before it had determined the first suggested solution for the new structure.

An image of one layer of the crystal structure of the zeolite TNU-9. Only the Si atoms are shown; the oxygen atoms between the Si atoms are omitted for reasons of clarity. The channels formed by the 10-member rings are clearly visible. Another channel running within the layer is marked in red. large

Crystals in demand

Gramm explains that the results of the study are interesting from various points of view. Zeolites have hollow channel-shaped cavities in their crystal structure and because of their special construction they are used for various applications. For example they are used in the chemical industry as molecular sieves and ion exchangers. However, their main use is as catalysts in the petrochemicals industry.

Gramm says that a whole series of research groups were involved in the project. The material was originally prepared by Korean scientists. After that it was examined at the University of St. Andrews (GB). Next the British researchers brought in scientists from Stockholm University, who contributed the electron microscopy data. Finally the Zurich researchers were also involved, and they finally succeeded in making the breakthrough thanks to their novel method.