Frameworks building units

By contrast, the mechanisms of the synthesis of ordered mesoporous solids are very dilferent from those of silicate and phosphate frameworks. For control over the synthesis of mesoporous solids, the key is to understand the interactions of micellar surfactants with condensable inorganic framework-building units. Synthetic routes are also being developed to prepare mesoporous silicates made up of nanoparticles of zeolites, with the aim of combining advantages of microporous and mesoporous solids. 

The traditional definition of a zeolite refers to microporous, crystalline, hydrated aluminosilicates with a tliree-dimensional framework consisting of comer-linked SiO or AlO tetrahedra, although today the definition is used in a much broader sense, comprising microporous crystalline solids containing a variety of elements as tetrahedral building units. The aluminosilicate-based zeolites are represented by the empirical fonmila... 

Fig. 7. Diagram illustrating the building units and structural relationship between the FAU and EMT frameworks, (a) Two (111) layers type A (A) and V (B) in twin orientation, (b) Hexagonal [100], (or cubic [1 — 1 0]) views of A and B. (c) Cubic FAU framework occurs when only type A or B stack, (d) Hexagonal EMT framework which occurs when A and B stack in alternation.

Zeolites form many different families in which common stmcture building units for the frameworks are found. They have a tendency to form intergrowths between different structures within the same families. 

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. 

Introductory Chapter 1 provides a historical overview of molecular sieve materials. Chapter 2 covers the definition of a zeolite and describes their basic and composite building units and how they are linked in zeolite frameworks. It defines pores, channels, cages and cavities and it gives references for finding detailed... 

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. 

Figure 1. Idealized projection of the building units of the faujasite framework...

Zeolite Structure Building Units Double Cation Specificity for Framework... 

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