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Dutch chemists have visualised how the porous structure of a zeolite catalyst depends on the production method. Zeolite made with carbon fibres as a template, has particles with straight canals that act as highways for the oil components which must be converted into benzene components.
Zeolite is normally given a steam treatment to improve its catalytic properties. As a result of this the mineral acquires a more sponge-like structure. The canals formed ensure that the zeolite crystal becomes more easily accessible. At least, that is generally thought to be the case.
Ries Janssen from Utrecht University demonstrated that about a quarter of all canals were closed cavities which did not contribute to an improved accessibility. This was shown using electron entomography, a special form of electron microscopy that provides a three-dimensional image.
Subsequently the researcher tried to make better canals in the zeolite. He used carbon powder as a template for this. The zeolite particles crystalise out on the carbon. After the carbon has been burnt away porous zeolite crystals remain. The carbon structure therefore determines the size and shape of the canals in the zeolite.
Under the electron microscope it could be seen that the new method provided a good open structure with twisting canals.
An even better structure was achieved when carbon fibres were used a template. That produced a zeolite with straight canals which acted as highways transporting reacting substances to and from the zeolite.
Zeolite is a natural mineral. It consists mainly of silicon oxide in which the silicon atoms are sometimes replaced by aluminium atoms, which have one electron less per atom. When this charge difference is compensated for with a proton, the zeolite is active for acid catalysis. Zeolites have molecule-sized pores. These micropores are part of the crystal structure. That makes zeolites suitable for use as selective catalysts. Only molecules which fit through the pores can react. The petrochemical industry uses different types of zeolite for various conversions, including the preparation of benzene from crude oil.
For further information please contact Ries Janssen (Faculty of Chemistry, Utrecht University), tel 31-302-536-765 (work), 31-302-723-625 (home), e-mail: a.h.janssen@chem.uu.nl. The doctoral thesis will be defended on 17 March 2003. Mr Janssen's supervisor is Prof. K.P. de Jong.
Image at www.nwo.nl/news.
Electron entomographic image of the canals in a zeolite particle.
The research was funded by the Netherlands Organisation for Scientific Research (NWO).