Secondary building units zeolite frameworks

Fig. 2.11 Different types of linkages of tetrahedra in the secondary building units of framework structures of zeolite groups, a, c, d Analcime group, b Heulandite and Moidenite groups, e Phillipsite group, f Pentasil and g Chabazite group [8]. Variation in channel shapes and dimensions of common zeolites, h Analcime—8R, i CUnoptilolite—8R and j Faujasite—12R [8]...

Many structures of zeolites can be described by certain structural units called building units, such as double 4 or 6 membered rings (D4R or D6R), structural sheets and rods. The main requirement for stmcture analysis of zeolites at present is not the refinement of electron charge distribution in the unit cell but the determination of the framework t5q)e stmctures, manner of arrangements of secondary building units or characterizing their lattice defects. 

Framework infrared has also been used to look for the formation of a zeolite during synthesis. Since many of the secondary building units can be detected in the infrared spectrum, it is possible to see zeolite formation at very early stages in the synthesis. In fact, zeolite formation can be detected in the infrared before crystallinity is observed by X-ray diffraction. Most of the reported work has been done by sampling the zeolite synthesis at various stages, isolating the soUds and measuring infrared spectra of the dried samples. 

What is the nature of the Al- and Si-bearing species in the mixture Are aluminosilicate ions present Are the secondary building units found in zeolitic frameworks already present in solution ... 

In most zeolite structures. Ihe primary structural units, telrahedru. are assembled into secondary building units, which may he simple polyhedra such as cubes, hexagonal prisms, or truncated octahedra. The tinal framework structure consists of assemblages of the secondary units. 

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 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]... 

The pore diameter, pore volume, framework density, and secondary building units (SBU) of the typical microporous zeolites crystallized from this system are listed in Table 3.3 for further study of the correlation between crystallization temperature and the structures of the corresponding microporous zeolitic crystallines. 

In recent years, molecular sieves have found new uses as hosts for the preparation of small metal- and semiconductor clusters that can be grown in confined zeolite spaces and are envisioned for uses in photo-catalysis, non-linear optics, sensors, flat panel displays, etc..[31] The framework-type structures of zeolites can be described with the presence of the n- rings, e.g., 4-, 6-, 8-, 12-rings or double 5-, 6-, 8- rings, or secondary building units, e.g. the sodalite cavity, super cages, and others. These confined spaces can be used for the preparation of optical and electronic materials with desired properties. 

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