Primary Building Units

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. 

2 Structural Building Units of Zeolites 2.2.1 Primary Building Units 

Zeolite comprises of T04 tetrahedra through corner sharing giving rise to a three-dimensional four-connected framework. Framework T atoms generally refer to Si, Al, or P atoms. In some cases, other atoms such as B, Ga, Be, and Ge, etc., are also involved. 

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

The structures of aluminosilicate zeolites obey the Lowenstein s Rule171 with an avoidance of Al—O—A1 linkages. Similarly, the linkages P—O—P, P—O—Si, Al—O—Al, Me—O—Al, and Me O Me (where Me = metal) appear to be unlikely in aluminopho-sphate-based molecular sieves.[8] 

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The way of the best choice to model PS s structure on both molecular and supramolecular levels begins with allocation of primary building units (PBUs), which without gaps and overlaps would fill a 3D space occupied by a PS. An universal method for allocation of such PBUs in both ordered and randomly arranged PSs, formed of packings of convex particles (or pores), is based on the construction of the assembles of Voronoi polyhedra (V-polyhedra) and Delaunay simplexes (or D-poly-hedra), which form Voronoi-Delaunay tessellation [100],... 

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... 

Among the over thirty framework compounds, at least 6 have been structurally determined by means of single crystal X-ray diffraction. These include GaP04-C3, -C4, -C7, AIASO4-I, -2 and GaAs04-2. The details of the cell parameters and the primary building units (PBU) of their structures are given in Table 4. 

Table 4. Crystallographic parameters and primary building units (PBU) of selected MXOi-type structures...

Figure 4.7 Simplified three-dimensional representation of a zeolite structure. Each Si or A1 atom is surrounded by four oxygen atoms, forming tetrahedron elementary strucUires (primary building units).

Neeraj, S, Natarajan, S. Rao, C. N, R, Isolation of a zinc phosphate primary building unit 1C eN2Hi8]2+lZn HP04] H2P04)2l2 and its transformation to open-framework phosphate 1C el Hid lZnr Solid State Chem. 2000, 15Q 417,... 

Phosphoric Acid Esters as Primary Building Units... 

N, R, Isolation of a Zinc Phosphate Primary Building Unit. [C6N2Hi8]z+[Zn(HP04)(H2P04)2]2-, and its Transformation to an Open-Framework Phosphate. [CsN2HiB]2+[Zn3(H20)4(HP04)4]2-. J, Solid State Chem. 2000. 750. 417-422,... 

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. 

The investigation of ferrisilicates under reducing conditions clearly reveal the difference of Fe(lll)-Td and Fe(lll)-Oh coordinations. Isomorphously substituted ions in Td sites withstand reduction in a broad temperature range and preserve their oxidation number. The decrease in the portion is more rapid for the Fe(lll)-Oh state (characteristic probably for the extra-framework emplacement) in dependence on temperature, and the component disappears already at 770 K even in 4 kPa hydrogen, as ESR data attest. This is in accordance with the extension of Loewenstein s rule to iron (there are no adjacent [Fe04/2] primary building units in the lattice). The extra-framework Fe(lll)-Oh components are more accessible to hydrogen, thus they can be reduced to ferrous state more easily. 

The frameworks are constructed by metal-centered primary building units with various coordination states, such as [A104], [A106], [Ga04], and [Ga04(0H)2] ... 

Figure 4. Simulated small-angle scattering curve from fractal geometry theorems (16). The small-angle scattering curve represents scattering from a fictitious fractal aggregate (R = 250 nm, D = 2.25, and the size of the primary building unit ro was 3 nm).

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