Secondary Building Units SBUs

The SBUs for various structure types of zeolites given in the database of zeolite structures141 are summarized in Table 2.2. One type of framework can comprise several SBUs. For example, the LTA framework contains five types of SBUs, including 4, 8, 4-2, 4-4, and 6-2 units, any of which can be used to describe its framework structure. In some instances, combinations of SBUs have been encountered, i.e., the framework cannot be generated by only one type of SBU. Examples include LOV, MEP, and other clathrasil-type frameworks. 

It should be noted that the SBUs are only theoretical topological building units and should not be considered to be or equated with species that may be in the solution/gel during the crystallization of a zeolitic material. 

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It is convenient to visualize zeolites in terms of secondary building units (SBUs). Several of the SBUs currently recognized by the IZA are depicted in Fig. 3 along with other useful structural units, the sodalite and pentasil cages. The sodalite unit is... 

The role of the template in the synthesis is not merely as a porogen on the contrary, it is also responsible for many key functions [5, 9, 10]. The template (typically cationic) balances the negative charge that characterizes zeolitic framework, due to the isomorphic substitution of Si(IV) by Al(III), prearranges the secondary building units (SBUs) toward the zeolitic framework, improves the gel synthesis conditions, especially the solubility of the silica precursors, and favors the thermodynamics of the reaction by stabilizing the porous zeolite framework. 

The FAU framework structure is composed of sodalite cages, linked by 6-6 secondary building units (SBUs) like the carbon atom in the structure of diamond. The formed cube has an axis, a 24.3 A, and the framework produced by the union of these cubes contains windows, which lead to an approximately spherical cavity with a radius, R 6.9 A, known as the supercage or (1-cagc [108,109], These supercages have tetrahedral symmetry, and are opened through four 12-member ring (MR) windows, each with a diameter of d 1A A [108,109],... 

Figure 2.5.3 Frequent tin-containing secondary building units (SBUs) that can be used for the generation of macrocyclic or extended supramolecular assemblies (a) Sn202 dimers (b) Sn402X2Y2-distannoxanes (c) Sn Oe drums...

In addition, in the field of dewaxing (gas oils, HDC residues, lubricating oil, etc.), synthesis of novel molecular sieves with better adsorption and separation abilities is highly desired. In the past 20 years, thanks to the discovery of many molecular sieves with new compositions and structural features [secondary building units (SBUs) and pores], there have appeared a number of new application fields for molecular sieves, such as basic catalysis, extra-large microporous molecular sieve catalysis, redox catalysis, asymmetric catalysis, and dual- and multi-functional catalysis.1-201 All of these will lay a further solid foundation for the development of molecular sieves in catalysis, adsorption, and separation. 

The framework of a zeolite can be thought of as being made of finite component units or infinite component unit-like chains and layers. The concept of infinite component units, such as secondary building units (SBUs), was introduced by Meier[9,10]and Smith.[11] 18 kinds of SBUs that have been found to occur in tetrahedral frameworks[3] are shown in Figure 2.2. These SBUs, which contain up to 16 tetrahedrally coordinated atoms (T-atoms) are derived by assuming that the entire framework is made up of one type of SBU only. It should be noted that SBUs are invariably nonchiral. A unit cell always contains an integral number of SBUs.[12]... 

Figure 2.2 Secondary building units (SBUs). The SBU codes are given below the figures. Reproduced with permission from [3], Copyright (2001) Elsevier...

Figure 2 Secondary building units (SBUs) of zeolite structures. (Reproduced by permission of Butterworth-Heinemann from Baeriocher, Meier, and Olson")...

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