Primary building unit, zeolite frameworks

Of the zeolitic materials, AlPO s cut a conspicuous figure because of their structural diversity and the incorporation of other elements into their frameworks. The recently developed VPI-5 (refs. 2, 3) announced the feasibility of synthesis of micoporous structures with windows comprising rings of over 12-T. All AlPO s, SAPO s and MAPO s form a family of microporous structures constructed by or essentially by A1(I) and P(V). Some of them are isostructural with zeolites but a majority have novel structures. The primary building units (PBU) centred by P(V) are invariantly PO4 whereas those centred by A1(I) are AlO in most cases and AIO5 or even AlOs in a few cases. So far all AlPO s, SAPO s and MAPO s have been synthesized exclusively in the presence of amines or... 

As has been discussed already, the primary building unit of all zeolite and zeotype frameworks is the tetrahedral unit, which can be linked to produce the SBUs seen in Figure 2. In some cases, it is convenient to describe zeolite structure-type frameworks by the recognition of larger assemblages of tetrahedra. Examples of this approach will now be discussed to emphasize how it can be used to simplify complex frameworks. 

Besides zeolites, a diverse range of microporous materials with novel open-framework structures have been discovered. The framework atoms of microporous materials have expanded to include most of the elements in the periodic table.151 The framework elements are not limited to A1 and Si atoms alone, and the primary building units are not only confined to tetrahedra. This chapter will mainly describe the structural characteristics of zeolites and some zeolitic open-framework materials. 

These [Si04], [A104], or [P04] tetrahedra are the basic structural building units of a zeolite framework. The primary building units are T04 tetrahedra. 

Figure 2.4 Building units commonly found in zeolite frameworks. From left to right, (top row) the tetrahedral primary building unit, followed by the secondary building units 4-membered ring (4MR) 6MR (second row) double 4-membered ring (D4R) D6R (bottom row) cancrinite cage (e-cage) sodalite cage (P-cage).

In these SBUs only the position of tetrahedral Si and Al are shown. Oxygen atoms lie near the connecting solid lines, which are not intended to mean bonds. The classification used by Breck et al. (1974) is based on the framework topology of the zeolites for which the structures are known, and it consists of seven groups, within which zeolites have a common subunit of structure that is a specific array of (Al,Si)04 tetrahedra. In the classification, the Si-Al distribution is neglected. These subunits have been called SBUs by Meier (1968). The primary building unit for zeolites is the tetrahedron (Figure 6.1). 

The primary building unit of all zeolite frameworks is the TO4 tetrahedron. A regular TO4 tetrahedra will have four equal T-0 bonds of constant length t, and six equal tetrahedron edge distances, 0...0 of length E. If the distance from the T atom to the midpoint of 0...0 is represented as d, the following equations represent the relationships between E, t and d ... 

The second point that inorganic and hybrid solids have in common is their structural description. On a primary basis, their skeletons are based on the connection of polyhedral entities (considered primary building units), which can be described either in a ball-and-stick representation (often used by molecular chemists) or in terms of polyhedra (preferred by solid state scientists). The increasing complexity of the inorganic frameworks has incited researchers to simplify their representation, first in terms of connected polyhedra instead of linked atoms (Fig. 2), but even with that a further simplification was needed. The centres of the polyhedra were therefore assimilated to nodes. Their linkage defined clear figures of nets, which characterized the connectivity of the solid. This approach, initiated by Wells [52], was applied to zeolites by Smith [53] and now the structures of the latter are described using this way [54]. 

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. 

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