Zeolites atoms, types

Zeolites are aluminosilicates characterized by a network of silicon and aluminum tetrahedra with the general formula Mx(A102)x(Si02)Y. The M are cations that are necessary to balance the formal negative charge on the aluminum atoms. The tetrahedra are linked to form repeating cavities or channels of well-defined size and shape. Materials with porous structures similar to zeolites but with other atoms in the framework (P, V, Ti, etc.), as a class are referred to as zeotypes. The structure committee of the International Zeolite Association (IZA http //www.iza-online.org/) has assigned, as of July 1st 2007, 176 framework codes (three capital letters) to these materials. These mnemonic codes do not depend on the composition (i.e. the distribution of different atom types) but only describe the three-dimensional labyrinth of framework atoms. 

When NH4Y is activated at 400°C to form the decationated or HY zeolite, two types of V centers are observed after y irradiation in vacuo (274). Both types give a rather broad EPR line with the same g tensor, x 2.045, g2 = 2.005, and g3 = 2.002, but only one exhibits a six-line hyperfine structure (Ax not determined, A2 = 8.0, A3 = 7.5 G) due to an interaction with 27A1 nuclei (I = f) which is superimposed on the broad EPR line. These V centers are attributed to a positive hole (denoted by the symbol ) trapped either on an oxygen adjacent to both an A1 and a Si atom (V,) or on an oxygen adjacent to two Si atoms (V2). 

Reference cites the literature from which the crystal data, atomic coordinates, and displacement factors were obtained. In many cases there are multiple refinements of the same zeolitic material, but because of space limitations not all refinements could be included. We would be appreciative if authors and users would inform us of any errors or omissions. A listing of the references for isotypic species can be found in the Atlas of Zeolite Framework Types (Baerlocher, McCusker and Olson (2007)). A list of references to structure analyses of zeolites with different cations, up to 1982, is given in the Compilation of Extra Framework Sites in Zeolites, Mortier (1982). 

A zeolitic structure can be described in various crystallographic terms. For many systems it is now possible to specify the following structural features the SBUs, the framework density, the coordination sequences, the unit cell dimensions and composition, the direction of the channels and the aperture (window) dimensions (Atlas of Zeolite Structure Types, 1992 Thomas et al., 1997). The framework density, FD, is defined as the number of T atoms per 1000 A1 (i.e. per 1 nm3) of the structure. 

X-ray powder diffraction has been the primary tool used in zeolite structure research. With new high-flux sources, the size requirement of useful single-crystals for structure determination studies has decreased significantly. In addition, refinements of atomic coordinates of known structures using Rietveld powder techniques have become common (24). The solution of a dozen or more new zeolite structure types within the last several years has added to our knowledge base for looking at unknowns (for examples, see references 25-31), and has made us better able to characterize catalyst materials and to correlate synthesis, sorptive, catalytic, and process parameters to their structures (32,33). 

Zeolites are now defined as solids that possess a framework of tetrahedra which are all corner-sharing and include a degree of opermess such as channels or cavities. Each framework type is issued a unique three letter code by the Structure Commission of the International Zeolite Association. " At the end of February 2007, there were 176 framework types. This definition does not specify atom types. 

Using thallium—sodium zeolites of type X (n = 2.30), we observed an increase of adsorption capacity for water vapor and benzene at 38% replacement of sodium ions by thallium ions, and then its decrease with an increase in the degree of exchange. As thallium ions replace sodium ions in the position Sm (25) we may assume redistribution of cations at dehydration under the conditions of high vacuum and thermal treatment at 350°C, and stronger chemical bonds of thallium atoms in screened positions in comparison with sodium atoms. 

An initial set of coordinates and velocities of all particles in the system must be known to start a simulation. For the atoms of a zeolite framework, those initial coordinates are usually taken from experimental data, for example, from X-ray or neutron diffraction measurements collected in the Atlas of Zeolite Structure Types A... 

Acetyiene and its derivatives also turned out to be suitable probe-molecules for basic centers in zeolites. Figure 2 depicts IR spectra of acetylene adsorbed on the NaX, Cs/NaX, and Na/Y zeolites. The most intense bands at 3300-2900 cm belong to the stretching vibrations of the C-H bond. Obviously, in the case of alkaline forms of zeolites, two types of complexes with acetylenes may be formed (1) 7t-complexes with metal cations (complex 3) and (2) o-complexes with basic oxygen atoms of the framework (complex4) ... 

The PBUs are built from smaller units composed of a limited number of T-atoms, by applying a simple operation to the smaller unit, e.g. translation, rotation.[4] The zeolite framework types can be analysed in terms of these component PBUs. The infinite PBUs, such as (multiple) chains, tubes, and layers, and the finite PBUs, such as (double) 4-rings, (double) 6-rings, and cages are far from unique. However, they are common to several zeolite framework types and allow an easy description of the frameworks to be made. Infinite PBUs and finite PBUs can be used to build the zeolite frameworks (see details in the database of zeolite structures Schemes for Building Zeolite Structure Models).141 Here only an example from AFI is presented to show the building of zeolites. 

The channels of zeolites are delimited by the rings formed by n T-atoms. Besides some small channel systems like 6-rings, the pore openings of zeolites contain 8-, 9-, 10-, 12-, 14-, 18-, and 20-rings. A summary of the largest rings of various zeolite structure types is presented in Table 2.6. 

The coordinates of crystallographically different atomic types of Si-0(H)-Al Bronsted centers within five H-form aluminosilicate frameworks have been optimized using a full periodic ab initio Hartree-Fock scheme at the STO-3G level. Single-point calculations have been carried out to obtain the H, Al, and respective QCCs and e.f.g. anisotropies. The latter have been discussed and compared to the experimental values measured for different zeolites. 

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. 

Fig. 16.2 Stereoviews of (al sodafile. Na Al Sv0.4-2H 0 [ring sizes are 4 and 6 1220 pml lb) ZSM-5. NasAI Siy)0,ra-l6H,0 [ring sizes are 4. 5. 6. 7. 8. 10 (J80 pml] lc> mordenile. Na AIHSiwOw-24H.O [ring sizes are 4. 5. 6. 8. 12 (760 pm>. See also Fig. 1.3. Lines represent oxygen bridges intersections of lines show positions of the aluminum and silicon atoms. Note increasing size of pore aperture (largest diameter given in parentheses). [From Meier. W. M. Olson. D. H. Atlas af Zeolite Structure Types, 2nd ed. Buttcrworths London. 1987. Reproduced with permission.]...

Calculations with point-charge arrays representing zeolite unit cells have, indeed, confirmed that the BE trends described above are closely mirrored by the trends of fhe average Madelung potential felt by the emitting atom type (Si, 0, Al,... 

The ability of crystalline microporous solids to adsorb molecules selectively on the basis of their size is a consequence of the well-defined pore windows that limit access of molecules to the internal surface area. The size and shape of these windows are determined mainly by the structure of the framework, which is specific to each type of material. For low silica zeolites it can often be modified by ion exchange, where the cations can adopt sites that partially block pore windows. The Atlas of Zeolite Framework Types gives the dimensions of the free diameters of these pores for each structure type, taking into account the radii of the framework oxygen atoms (taken as 1.35 A) and neglecting the effect of extra-framework cations. 

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