Chabazite unit cell

The Chabazite unit cell contains only 12 tctrahedra and its stoichiometric fonnula is Ca2 [ (AI02)4 (Si02)8] I3H20. 

Commercially significant zeolites include the synthetic zeolites type A (LTA), X (FAU), Y (FAU), L (LTL), mordenite (MOR), ZSM-5 (MFI), beta ( BEA/BEC), MCM-22 (MTW), zeolites E (EDI) andW (MER) and the natural zeolites mordenite (MOR), chabazite (CHA), erionite (ERl) and clinoptiloUte (HEU). Details of the structures of some of these are given in this section. Tables in each section lists the type material (the common name for the material for which the three letter code was established), the chemical formula representative of the unit cell contents for the type material, the space group and lattice parameters, the pore structure and known mineral and synthetic forms. 

It has already been mentioned that zeolites are shape selective with respect to molecular adsorption. This property relates to their micropores stmcture. The zeolite framework shows a limited flexibility, which is essential. For instance, Yashonath et al. have shown in their classical dynamic simulations study of molecular diffusion within zeolite micropore that the zeolite framework flexibility affects significantly diffusion when the molecules have a size comparable with the micropore size. To get an idea of the order of magnitude of this flexibility, one can consider the hybrid semi-empirical DFT periodic study of chabazite zeolite of Ugliengo et al. V They introduced in the unit cell of chabazite Br0nsted acidic sites which are known to induce an increase of the volume of around 10 This increase of the volume relates with the difference of volume between a Si04 tetraheron and a... 

Increasing attention has been given to the structures of zeolitic crystals, not only on account of their practical value as selective sorbents, but also because of the remarkable pore systems which have been revealed. As a result considerable new information exists about the anionic frameworks, although the disposition of the relatively mobile intracrystalline water and cations is intrinsically more difficult to determine. Four structures which have certain related features, and which are of interest as molecular sieves, are those of chabazite, gmelinite, levynite and erionite, for which hexagonal unit cells may be given as follows ... 

Fig. 5(0).— The orientation of diffusion channels in chabazite, in relation to hexagonal and rhombic unit cells.

Of the cation forms of chabazite, Cs-chabazite contains about 20% of residual Ca while the other modifications should contain less than 10% of this ion (8). In zeolite X, the Rb, Cs, Mg, and Ba forms contain about 30, 30, 27, and 23 residual Na" ions per unit cell (14) out of a total in our... 

Figure 1. Eig ht unit cells of pure silica chabazite illustrating the connectivity of the D6R...

I restrict my attention here to work that has applied periodic DFT calculations to structure or reactivity of zeolites. The great majority of work on this topic has focused on a small number of materials that have small unit cells, including chabazite (structure code CHA), which has 36 framework atoms per unit cell, mordenite (structure code MOR), which has 146 atoms per unit cell, ZSM-22 (structure code TON), with 72 atoms per unit cell, and ferrierite (structure code FER), with 108 atoms per unit cell. Notably, this list does not include ZSM-5, a material with 288 framework atoms per unit cell, which is the... 

Rozanska et al. compared plane wave LDA and GGA calculations for the chemisorption of propylene in chabazite.282 They concluded that allowing zeolite atoms to relax upon chemisorption made a significant impact on the computed results, but relaxation of the zeolite s unit cell shape and volume had considerably less effect. The same group performed plane wave GGA calculations for the chemisorption of isobutene in three different zeolites, chabazite, ZSM-22 (TON), and mordenite.283... 

Eig. 1. (a) A trigonal unit cell of the zeolite chabazite, consisting of 36 atoms. This unit cell is also chosen as the supercell, (b) A perspective view down a channel of chabazite 54 unit cells are displayed. 

In AIPO4-34, the fluoride ions interact with a chabazite-precursor framework (Fig. 17). The interaction causes a triclinic deformation of the chabazite rhombohedral unit cell (48). The deformation is clearly observable in the X-ray diffraction pattern (a, triclinic, b rhombohedral structure) (Fig. 17). 

Figure 4.5 shows the energies of the initial weak hydrogen-bonded adsorbed state of propylene, the proton-activated transition state and the final alkoxy product state of the protonated propylene. The structures and energies are established from DFT cluster calculations using the model structure shown in Fig. 4.5a and periodic DFT calculations using the unit cell of chabazite and the zeolitic protons (Fig. 4.5b). The cluster used in Fig.

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