Framework Structures of Zeolites

Solid lines represent T—O—T linkages, whereas dotted lines indicate nonconnected T—O bonds found in the interrupted frameworks. 

The concept of coordination sequences (CSQ) was originally introduced by Bmnner and Laves,[37] and first applied to zeolite frameworks by Meier and Moeck.[381 In a typical zeolite framework, each T-atom is connected to N 4 neighboring T-atoms through oxygen bridges. These neighboring T-atoms are then linked in the same manner to A S T-atoms in the next shell. The latter are connected with N3 T-atoms, and so forth. Each T-atom is counted only once. In this way, a coordination sequence can be determined for each T-atom of the 4-connected net of T-atoms. It follows that  

Coordination sequences have been listed in the Atlas[3] from N up to Nm for each topologically distinct T-atom in every framework structure. For example, the coordination sequences in FAU are as follows  

The site multiplicity and the site symmetry are both given in parenthesis. 

The coordination sequences and the vertex symbol are unique for a particular framework topology, i.e. they can be used to distinguish between different zeolite framework types unambiguously. In this way, frameworks with the same topologies can be easily identified. Currently, it is easier to calculate the coordination sequences and vertex symbol using computer program based on crystallographic data. 

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Zeolites are crystalline aluminosilicates with porous, framework structures made up of linked [Si04] and [A104] tetrahedra that form channels and cages of discrete size [24]. The framework structures of zeolites bear a net negative charge, which must be balanced by positively charged species, typically alkali or alkaline earth metal cations these cations maybe exchanged for one another under appropriate experimental conditions. Zeolites are capable of... 

The isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

Zeolitic materials have been prominent amongst those so far studied by high resolution powder diffraction using synchrotron X-rays [36]. High definition synchrotron PXD data has been helpful in a number of framework structure determinations and has facilitated studies of planar faulting (see below). Successful Rietveld refinements of the framework structures of zeolite ZSM-11 [37, 38] and silica-ZSM-12 [39], and of the complete structures of zeolite Y containing cadmium sulfide [40] and cadmium selenide [41] clusters have been described. 

Copper ions are supported with atomic dispersion due to the ion-exchange properties of zeolites and are difficult to collect owing to the framework structure of zeolite. 

H. van Koningsveld, J. C. Jansen, and H. van Bekkum, Zeolites, 10, 235 (1990). The Monoclinic Framework Structure of Zeolite H-ZSM-5. Comparison with the Orthorhombic Framework of As-Synthesized ZSM-5. 

Aluminosilicate zeolites are normally synthesized under basic conditions. The introduction of OH- ions to the synthetic system will necessarily lead to the introduction of correlated cations. These positively charged cations play an important role in the polymerization of polysilicates and aluminates by affecting the polymeric state and their distribution, and have an important effect on the colloidal chemistry of aluminosilicate as well. In addition, cations existing in the synthetic system also have important effects on the formation of the framework structure of zeolites. For example, plenty of synthetic data indicated that a tight correlation between the formation of the SBU cages of zeolites and the charge and size of the cations existed, and this was named the templating effect of cations by R.M. Barrer.[191... 

Figure 9,11 Framework structure of zeolite L. (a) Projection along the c axis (b) projection perpendicular to the main channel direction, with A-E representing the positions of the cations (c) 12-membered-ring main channel (d) scanning electron microscopoe image of zeolite L crystals (e) structures of pyronine (left) and oxonine (right). Reproduced with permisson from [66], Copyright (2003) Wiley-VCH...

The constitutional or "framework" structure of zeolites is based on an infinitely extending three-dimensional network of AIO4 and Si04 tetrahedra that are linked to each other by shared oxygen atoms and may be represented as ... 

If there are two or more types of T-atoms and these are ordered (i.e. not randomly distributed over all T-siles), the ideal symmetry of the framework type is likely to be reduced. For example, Al and Si alternate in the framework structure of zeolite A (LTA). To illustrate the effect of this ordering on the symmetry, the LTA framework type with all nodes identical and with alternating nodes marked are shown in Figs. 20a and b, respectively. The lattice constant (repeat distance) a and one of the mirror planes for the former is shown in Fig. 20a. In Fig. 20b, the symmetry reduction dictated by the ordering of Si and Al is readily apparent. Two obvious effects of the alternation are that (I) the mirror planes between sodalite cages arc gone, and (2) the unit cell has to be doubled along each of the axes. Similar elfects are observed in other materials in which the T-aloms are ordered. 

Infrared spectroscopy has been a valuable technique for exploring zeolite structures. It is useful for studying the nature of hydroxyl groups in zeolites, the interaction of cations with adsorbed molecules, and the fundamental framework structures of zeolites. 

A collection of stereopairs showing the presently known framework structures of zeolites is presented. Only well-established structures have been incorporated in this survey which includes crystal data, information on channel geometry, and possible fault planes. 

Bennett and Smith (1969) determined the crystal structure of La-exchanged faujasite. In the structure (fig. 62), lanthanum atoms and water molecules are located in the large spaces between framework structures of zeolitic type, with partial occupancies. However, they mentioned that the electron density was insufficient to account for all the cations and water molecules, implying that they might move frequently from site to site. The R factor of their analysis was high, i.e. 0.15 for all reflections, and further analysis of the structure is necessary. 

Figure 8.13 Three-dimensional representations of the framework structure of zeolite types A, XY, L, and a two-dimensional framework structure of mordenite. The vertices of lines in the above structures indicate the Si or Al atoms, with bridging oxygen slightly displaced from the lines. Reproduced from reference (102) with permission. Copyright 1990 American Chemical Society.

In addition to dynamics associated with atomic and molecular motions of the adsorbate molecule and zeolite framework that induce chemical changes and affect diffusion, temperature can also affect the desorption of chemisorbed and physisorbed species and even collapse of the framework structure of zeolite ( above 800°K the structure of zeolite Y, with Si/Al=2.5, collapses to an amorphous residue ). Effects associated with the thermal redistribution of the... 

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]...

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