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Rubrik: Science Life

ETH Zurich crystallographers elucidate the structure of a new mineral
Zeolite reveals its secret

Published: 01.03.2007 06:00
Modified: 28.02.2007 21:08
A synthetic zeolite has guarded the secret of its structure for ten years. ETH Zurich scientists have now successfully elucidated the mineral’s complicated makeup. This feat will inspire research into other complex crystal structures.

Peter Rüegg

A research team led by Dr. Christian Baerlocher and Dr. Lynne McCusker from the Laboratory of Crystallography of ETH Zurich has succeeded in solving the structure of the zeolite IM-5, which was first synthesised about ten years ago. However its makeup is so complex that its structure could not be clarified until now. This was mainly because IM-5 is available only in powder form as crystals smaller than one thousandth of a millimetre. All that researchers were able to infer from catalytic test reactions in 2000 was a rough picture of IM-5’s pore system. The paper by Baerlocher and his colleagues was published in Science (1) on Friday 23 February 2007.

More complex than ever

Baerlocher’s studies show that IM-5 has a basic structure of 24 individual silicon atoms. A unit cell of the crystal consists of 864 atoms. However, because of various symmetries, the researchers “only” needed to determine just over 70 atomic positions. Baerlocher stresses that “Up to now the upper limit for polycrystalline materials was 20 to 30 atoms. In this case the structure is more than twice as big.” This means the structure of IM-5 is as complex as that of the zeolite TNU-9. So far the latter has the most complex structure that the same ETH Zurich research group was also able to solve recently.

Environmentally friendly alternatives to strong acids

(per) Zeolites are porous minerals, actually “thermally stable sponges “. In nature they occur in volcanic rocks, among other places, but large deposits are found in sediments. However, most zeolites are made synthetically nowadays, especially those used as catalysts in chemical reactions. For example zeolites can be used as cation exchangers in detergents instead of phosphate. This involves the exchange of calcium ions for sodium ions by the zeolite. However the porous minerals can also be used in water purification and detoxification. After the Chernobyl nuclear reactor accident, cows were fed zeolites which absorbed the radioactive caesium and caused the milk to become less radioactive. Zeolites are also used as a desiccant in the frames of double-glazed windows. They prevent condensation on the inside of the glass.

Zeolites such as IM-5 are particularly desirable as acid catalyst in the petrochemical industry. Christian Baerlocher says “Practically every drop of petrol burnt in engines has passed through a zeolite.” This involves breaking up the long carbon chains of the petroleum into smaller fragments and reforming them, thus increasing their octane number. The advantage of this “sponge” is that its pores have a restricted volume so that only very specific reactions can take place in them. This forms less unwanted by-products. The great advantage of these inorganic substances is that although they have an acidic reaction internally, they are not themselves corrosive. Therefore they are considered to be extremely environmentally friendly.

A model representation of the zeolite IM-5: the yellow skeleton outlines the position of the silicon-oxygen atoms and the red-blue tubes illustrate the unique pore system (Photo: C. Baerlocher / ETH Zurich Laboratory of Crystallography)

EOne of IM-5’s special features is its unusual pore and channel system. On the one hand this is two-dimensional, i.e. it has countless channels running in parallel, but on the other hand it has limited three-dimensionality as a result of cross-links between the parallel pores and dead end side-branches. Groups of three pore systems lying in a plane are interconnected. These nano-sized planes are separated by single walls.

A novel combination led to the breakthrough

Baerlocher/McCusker and their colleagues from Stockholm University broke new ground to solve the structure. Their structural determination was based on data from X-ray diffraction experiments, high resolution electron transmission microscopy images and a computer model (“Charge Flipping”) which they adapted to the specific needs in determining the complex crystal structures of powders.

This was because although the group had already cracked the complex structure of TNU-9, they could not use this method for IM-5. To elucidate the structure of TUN-9 the scientists used a method specific to zeolites, but they developed a general one for IM-5. Lynne McCusker says “That’s why our novel method is a breakthrough in studying the complex crystal structures of polycrystalline materials.” She says the method is probably as important to the research world as the knowledge of the structure of IM-5. In future the new method will allow the study of other polycrystalline materials such as catalysts, medicines, ceramics or complex metal alloys. A pre-condition for it is that images of the substance can be made by using transmission electron microscopy.

The petrochemical industry is extremely interested in the structure

However the industry, principally petrochemicals, was very interested in the solution of the structure of IM-5. Zeolites are used on a large scale as acid catalysts in petroleum refining. The petrochemical industry expects quite a lot from IM-5, including some very specific catalytic properties. Baerlocher says “Without knowledge about the structure, the industry cannot optimise syntheses and we do not understand exactly what catalysis is taking place.” However he says that at present it is still too soon to use IM-5 industrially.

(1 Baerlocher, C. et al. (2007): Structure of the Polycrystalline Zeolite Catalyst IM-5 Solved by Enhanced Charge Flipping. Science Vol. 315, pp 1113-1116

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