Centre for Molecular Design (CMD)
Molecular Modelling of Zeolites
Research by Paul Cox
What are zeolites?
What are templates?
Why use computers?
What has been revealed?
Publications
What are zeolites and what is so important about them?
Occuring widely in nature, zeolites are cage-structured aluminosilicates which possess pores and channels of regular dimensions within the crystalline lattices. These 'holes' within the crystal structures are highly useful and can be exploited for many important applications as, for example, ion exchangers, molecular sieves and more recently stereo-selective catalysts.
Zeolites are based on the primary tetrahedral building unit, TO4, where the central tetrahedrally bonded (T) atom is usually either a silicon or aluminium atom, surrounded by four oxygen atoms. By linking these tetrahedra together, it is possible to build larger structures; this is demonstrated in figure 1. When these TO4 units are linked together to form a zeolite framework, they characteristically yield periodic cavities and channels throughout the structure. This is illustrated in figure 2.
Figure 1: The primary building unit TO4 is shown on the left. On the right, two TO4 units have been joined to demonstrate how they link together to form larger structures.
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Figure 2: This illustrates how the primary TO4 unit can be linked to create larger structures, or secondary building units (SBU's). Part a, shows the formation of a 6T ring (where T denotes the number of tetrahedral atoms) and part b reveals how this can then be combined to form a SBU (sodalite cage).
Figure 3: An illustration of how different zeolites structures can be constructed from different methods of linking the same secondary building units together (for those who care to know, the 3 lattices are Zeolite A, Y and Faujasite)
What are templates and what role do they play?
Zeolites are traditionally synthesised by hydrothermal techniques. This basically involves heating a synthesis gel of silica and alumina starting materials for a set period of time and then allowing it to cool. As the gel cools, the zeolite appears as micro-crystalline powder. The exact product that is synthesised, is very sensitive to the precise reaction conditions, composition, etc and the mechanism by which they crystallise is not well understood. However, it is assumed that hydrated cations or organic directing agents present in the reaction mixture are highly influential components, behaving as templating agents to direct the formation of the framework about them. The choice of template selected for a reaction can be critical in deciding which zeolite is produced, however, the relationship between template and zeolite product is a source of considerable debate.
Once the template has been occluded (or encapsulated) within the zeolite framework, the identification of the site that it occupies can not easily be determined by experimental techniques. To date, only a select few systems have been successfully resolved (combine to this, the fact that literally thousands of organic agents have been used in the past and you start to get the picture!) giving a limited opportunity to draw some conclusions.
So why do we use computers to model them?
Until quite recently, despite the wealth of experimental data available, there have been very few attempts at rationalising the role that these templates play in zeolite synthesis. In order to study the systems in greater detail, you need to be able to observe their relationships at the atomic level. Unfortunately, due to the limitations of diffraction techniques, this is not feasible experimentally. However, computational techniques can provide a chance to study the relationships, by applying mathematical models to represent the systems of interest.
So how did we go about modelling them and what has been revealed?
In this work, various molecular modelling techniques have been applied in an attempt to correlate the physical properties of many different organic compounds with the zeolite structures they form. Similarity methods have revealed that organic molecules with similar molecular dimensions facilitate the synthesis of the same zeolite. Molecular visualisation techniques have proved a successful method of showing the remarkable correlation between the shape of the organic molecules and the cavity of the zeolite's pore system. An automated docking approach based on the calculation of molecular energies has been developed enabling the determination of the conformation of the template when occluded in the zeolite framework. Results from this technique have further demonstrated the correlation with the zeolite and should prove valuable in helping to formulate a systematic approach to novel synthesis.
If you are interested in further information on this subject, then the two publications cited below will be of interest. Alternatively you could look in Adrian's thesis (Computational investigations of the role of organic templating agents in zeolites synthesis (1996), University of Portsmouth) which contains the work in much greater detail.
Recent Research Publications on Zeolites
'Molecular Modelling Studies of Zeolite Synthesis', P.A.Cox, A.P.Stevens, L.Banting and A.M.Gorman, in Zeolites and Related Microporous Materials: State of the Art 1994. Studies in Surface Science and Catalysis, Eds. J.Weitkamp, H.Pfiefer and W.Holderich, Elsevier Science B.V., Amsterdam, (1994), 84C, p 2115
'Postulated Mechanism For Faulting in Zeolite Beta', A.P.Stevens, P.A.Cox, J. Chem. Soc., Chem. Commun., 343 (1995).