永利集团官方网站入口

Science: Synthetic zeolites for designer catalysts

作者:仰掼    发布时间:2019-03-04 05:08:03    

By LIONEL MILGROM SWISS chemists are on the verge of a breakthrough that could lead to the first ‘designer’ catalysts. The research focuses on modifying a class of crystalline substances called zeolites. These materials have a molecular structure like a honeycomb, which makes them very porous. Recently, chemists have begun to use zeolites as catalysts, because they easily absorb and bind small molecules. The degree of porosity of the materials relates to how well they behave as adsorbents and catalysts. Now two Swiss chemists, G. Brunner and Walter Meier of the Swiss Federal Institute of Technology in Zurich, have discovered a simple relationship linking the porosity of a zeolite directly to its molecular structure (Nature, vol 337, p146). The discovery should enable chemists to ‘tune’ a zeolite’s adsorbing and catalytic properties by adjusting its molecular structure. Chemists should then be able to make zeolites with much larger pores. These could be used to ‘crack’ – or break down – the larger molecules of hydrocarbons in heavy oils, and so convert a greater proportion of a barrel of oil into the higher grade of hydrocarbons used in petrol. Such zeolites could also be used to separate large molecules important in medicine, such as sugars and steroids. Zeolites belong to a class known as aluminosilicates in which four oxygen anions surround silicon and aluminium cations to build up a honeycomb of molecules which carries a negative charge. The excess charge is the key to the adsorbing and catalytic powers of a zeolite (‘Minerals with a natural advantage’, New Scientist, 25 March). In order to maintain electrical neutrality within the zeolite lattice, more mobile cations are present. These cations fit into cavities made by the aluminosilicate’s rigid scaffolding, but they are easily displaced by other cations that fit the cavities better. Certain zeolites, which contain sodium cations, will exchange these for calcium ions in ‘hard’ water and so soften the water. Chemists have found that they can generate localised sites with large electric fields within the honeycomb structure of the zeolite. They do so by changing the mobile metal cations for ones that carry a high charge. Such highly charged sites can, therefore, catalyse molecular transformations by providing certain molecules with a snug fit and then ripping them apart. For example, the zeolite ZSM-5 turns methanol into simple hydrocarbons that are suitable as fuels. Until now, chemists have not been able to catalyse reactions with hydrocarbons such as sugars, peptides and steroids because the molecular pores in the zeolites that have been made are too small. Now, the Swiss chemists claim to have devised a way of making the zeolite’s pores bigger. The basic building-block in the honeycomb is a tetrahedral arrangement of the four oxygen anions surrounding the silicon and aluminium cations. Brunner and Meier call the cations, tetrahedral or T-atoms. These tetrahedra join up into rings which link further to form the complex network of pores within the zeolite. The size and the number of these rings determine the width of the pores. As the size of the rings decreases, more T-atoms are needed to maintain the zeolite’s structure, otherwise it would collapse. The T-atoms are then confined to a smaller volume, leaving more empty space. In other words, the greater the number of the smallest possible rings, the greater the width of the pore (see diagram). The Swiss chemists express the number of T-atoms per cubic nanometre (1 nanometre is a millionth of a millimetre) in terms of a number called the minimum framework density or FD. They are the first to have shown that the smaller the FD, the wider the zeolite’s pores. The situation is analogous to building a house. The walls should occupy the minimum possible amount of space in the house to keep the house upright, in order to obtain the maximum living space. The FD in this analogy represents the amount of space occupied by the walls: the smaller the FD, the larger the living space. The smallest FD achieved so far is 12.5, which corresponds to a zeolite in which more than half the volume is empty space. Brunner and Meier believe that, to get really large pores, the FD has to be below 12. If the rings that form the pores contain four T-atoms then the pores can only be one-dimensional. If the pores were two – or three-dimensional, calculations show that the zeolite would collapse. This structure cannot be used as a catalyst because it means that molecules can only get to catalytic sites within the zeolite from the two ends of the pore. This leads to a molecular traffic jam within the zeolite that would slow down the rate of catalysis; so the rings in the structure need to be smaller. Meier thinks that smaller rings will produce multidimensional pores that are much wider. Molecules will be able to enter and leave the catalytic sites much faster in these structures. A hypothetical zeolite that fits the bill is shown in the diagram. This has an FD of only 9.3. Even though chemists know how to make zeolites with larger pores, it will not be easy to do so, says Mark Davis, a chemist from Virginia State University, because naturally occurring zeolites that have three T-rings also contain beryllium oxides, which are highly toxic. Apart from helping to extract more energy from a barrel of oil, and separating important biomolecules, these still hypothetical zeolites could have other very interesting applications. Davis says: ‘The larger pore size that we will get by building our zeolites exclusively out of rings of three T-atoms will allow us to make crystals with fewer imperfections for doubling the frequency of laser light shone through them.’ This property is very useful in optical communications. ‘We could even support organometallic catalysts within these larger pores, which, because they would also be shape-selective, would turn the new zeolites virtually into inorganic enzymes,

 

Copyright © 网站地图