Pentasil framework structure

Pentasils constitute a well known family of porotectosilicates. The framework structure of the members of this family, based on five membered rings of tetrahedra, can be described in terms of two different stackings of layer pairs related by an inversion center (i-type) and mirror simmetry (o-type), respectively ( 1 ). ZSM-5 aluminosilicate, the parent borosilicate BOR-C ( 2 ), and the pure silica analog Silicalite-1 ( 3 ), represent the most important... 

This cation favors the formation of BOR-A (Nu-l-type framework), when used alone in the reaction mixture (2). In the above binary mixtures, it behaves differently, leading to the formation of pentasil-type structure, even when its concentration greatly exceeds that of the other alkylammonium cation in the reaction mixture. 

FIGURE 1.9. Schematic diagram of one layer of a pentasil zeolite structure showing how the framework is built up from the D5R units. (From ref. 27, reprinted with permission.)... 

A family of titanium-containing molecular sieves with pentasil-type framework structures is represented by titanivun-silicalite-3 (TS-3) [74], characterized by a framework topology similar to that of TS-2 but with a different degree of stacking faults. In fact, as found by Perego et al. for the boron-substituted MFI/MEL molecular sieves, the frequency of stacking faults may be controlled by choosing the appropriate pair of tetraalkylammonium ions (e.g., TMAOH/TPAOH, TMAOH/TBAOH, TEAOH/TBAOH) [72]. 

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]...

As mentioned in Section 4.2 earlier in this chapter, infrared spectroscopy was used to provide information about the structural units present in UZM-5. The framework IR spectrum of a UZM-5 sample is shown in Figure 4.18. The characteristic vibrational bands for double four-rings (D4R) and pentasil rings (S5) are present. This provided some valuable information about the types of linkage units present and combined with data from other techniques such as XRD and TEM allowed the structure of UZM-5 to be solved. 

It is necessary to mention that observed VIS spectra of the Co those reported for pentasil containing zeolites (with deformed six-member rings present in the framework) and different from the spectra of the Co2+ ions located in A, X and Y zeolites [1,5,8,9], It indicates that deformed six-member rings are present in the structure of MCM-41, but the confirmation of this suggestion requires further detailed study. 

The frequency of o-type stackings is lower than 0.25, the value determined for the structure of H-BOR-D, in all of the samples obtained in the present investigation. The excess of i-type stackings commonly found in the framework of pentasils, suggests that this type of stacking is favoured with respect to the o-type at least under the conditions usually adopted for the synthesis of these materials. 

A number of structural features (cages, channels, chains, sheets) arc common to several different zeolite framework types, so designations such as a-cavity and (i-cage, pentasil unit, crankshaft and double crankshaft chain, and 4.82 sheet or net have crept into common usage. To help the reader, some of these subunits are shown in Figs. 1, 2 and 3. In these drawings, oxygen atoms have been omitted for clarity. 

Pentasil MFI (ZSM-5) type materials exhibiting different crystallite sizes and AEL (SAPO-11) type materials have been studied. Structural investigations using X-ray diffraction techniques, developed for polycrystalline powdered samples, allow one to determine precisely the structure and crystallinity of the samples and the effect of adsorbates (e.g., p-xylene, n-hexane, etc) on the framework topology (monoclinic towards orthorhombic for MFI) and unit-cell dimensions. 

Infrared Spectroscopy. Infrared spectroscopy has been used to study borosilicate molecular sieves (22.25.33-361. Vibrational bands associated with trigonal framework boron occur near 900 cm"1 and 1400 cm 1 (22.25.331. The presence of the Si-0-B asymmetric stretching vibration, indicative of tetrahedral framework boron incorporation, cannot be observed directly because it is masked by the strong Si-0-Si band near 1100 cm-1 in the pentasil structures. The tetrahedral Si-O-B vibration has been observed for the borosilicate mineral danburite (.33). Shifts of bands in the framework vibrational region at 550 cm"1 and 560 cm"1 to higher frequencies as a function of boron content has been used to study boron incorporation in the framework of AMS-1B borosilicate (36). 

For analysis of the state of iron were employed EPR, FTTR, and Mossbauer spectroscopies. For structural interpretation of these results the concept of divalent transition metal cation siting was used as recently established for pentasil ring zeolites in a wide range of metal concentrations and Si/Al compositions. With help of UV-Vis and FTIR this approach evidenced three zeolite coordination of divalent cations in similar six-membered rings of framework local structures. Three cationic fiiamework sites were thus suggested, denoted as a, p and y. (For details see [7-11]). 

Nowadays, the term zeolite includes all microporous solids based on silica and exhibiting crystalline walls, as well as materials where a fraction of Si atoms has been substituted by another element, T, such as a trivalent (T = Al, Fe, B, Ga,. ..) or a tctravalent (T = Ti, Ge,...) metal. Crystalline microporous phosphates are known as zeotypes or as related microporous solids (14, 54). At present, there are 179 confirmed zeoHtc framework types. For the structure types, three-letter codes are used, which were adopted from the name of the first material reported with a specific stmcturc. As an example, FAU is given for the structure of faujasite and its synthetic equivalents X and Y, and MFl for the stracture of ZSM-5 or silicalite-1 (105). Figure 9.11 shows prominent examples of zeolite firameworks, for example, FAU, LTA, and MFI types (pentasil). 

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