Chabazite framework structure

Other framework structures based on zeolites have also been synthesized which contain atoms other than aluminium and silicon, such as boron, gallium, germanium, and phosphorus, which are tetrahedrally coordinated by oxygen. Such compounds are known as zeotypes. Pure aluminium phosphate, commonly called ALPO, and its derivatives, can take the same structural forms as some of the zeolites such as sodalite (SOD), faujasite (FAU), and chabazite (CHA) (e.g., ALPO-20 is isostructural... 

Figure 7.7. Schematic representation showing framework structures of (a) zeolite A, (b) zeolites X and Y, (c) erionite and (d) chabazite.

It should be noted that zeolites will form from reactant gels with very low water contents ) although reaction rates are low. Also, the conversion of the zeolite chabazite in the absence of added water to give a range of other more compact framework structures has been reported. Particularly relevant was the conversion of a silicaceous sodium-form of chabazite to nosean (sodalite) at ca. 300°C. 1 )... 

The overwhelming majority of the theoretical studies were performed on cluster models of the catalytic site, hi spite of the fact that the role of space confinement and the secondary interactions with the framework atoms is well-known, there are only a few electronic structure calculations on lattice models involving hydrocarbons, using either periodic DFT calculations, or embedding methods. In this brief account of the subject we attempt to overview some of the recent computational results of the literature and present some new data obtained from ab initio DFT pseudopotential plane wave calculations on Cl - C4 alkanes in the chabazite framework. 

Figure 12. Transition structures of propane and n-butane cracking in the chabazite framework.

The 34 ( 8), 44 (19), and 47 (19) types are all now known to be structures with the chabazite framework topology (as determined by single-crystal methods) but prepared with different templates, TEAOH, cyclohexyl ami ne, or N,N-di ethyl-ethanol ami ne, respectively. As initially prepared, there was sufficient variation in the x-ray powder patterns of the as-synthesized forms for different framework topologies to be considered. Once calcined, the three species exhibit essentially identical powder patterns, consistent with the framework topology. Other properties vary substantially among the three species. There still remains the possibility of unique, template mediated, framework siting for the metal in these and other tempiated forms of the chabazite topology. 

J.V. Smith, R. Rinaldi, and L.S.D. Glasser, Crystal Structures with a Chabazite Framework. II. Hydrated Ca-chahazite at Room Temperature. Acta Crystallogr., 1963, 16, 45-53. 

The nomenclature used throughout this paper designates the structural type by the letter of the most closely related phosphorus-free synthetic zeolite as used in the Union Carbide laboratories (4, 5). Table II in Ref. 5 contains a description of the synthetic zeolites described here. For the aluminosilicophosphate zeolites, these letter designations are prefixed by P to indicate that the zeolite framework contains phosphorus substituted in the tetrahedral (Si, Al) site. For example, the phosphate zeolite structurally related to Type A zeolite is designated P-A P-R zeolite is the phosphate zeolite related to zeolite R, a synthetic sodium aluminosilicate zeolite with a chabazite type framework structure (5). The aluminosilicophosphate zeolites will be abbreviated to phosphate zeolites or phosphorus-substituted zeolites throughout this paper. 

The chabazite framework is a sequence of D6R units joined, via S4Rs, into the same sequence as that in cubic close packing, that is, ABCABCABC (see Figure 6, also Figure 2 and Table 1). This means that an easy way to describe the chabazite structure is that of a NaCl-type arrangement with a D6R unit at each corner of a simple cubic assemblage. 

The AIPO4-I8 has a new framework structure. The framework can be described as stacking layers of tilted double six rings and is related to the chabazite framework. It contains a 3-D eight-ring channel system with a slightly elliptical cross section. The aperture of the channel in the 001 direction is 0.38 x 0.38 nm (42). 

The transition states for the proton-activated cleavage of propane and butane are showm in Fig. 4.7. These transition-state intermediates can be considered as protonated propane and butane and hence are the analogues of classical carbonium ions. The structures showm in Fig. 4.7 have been computed in the chabazite cavity The calculated activation energies for proton activation are quite high, between 170 and 200 kJ/mol. The n-butane molecule does not fit well in the small pore of the chabazite framework. Therefore, it must adopt a gauche conformation. 

Figure 4.7. Transition-state structures of propane and n-butane cracking in the chabazite framework. Only the lattice atoms in contact with the substrate molecules are clearly visible, adapted from Angyan et al.I l.

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]...

M. Calligaris, A. Mezzetti, G. Nardin and L. Randaccio. Cation sites and framework deformations in dehydrated chabazites. Crystal structure of a fully silver-exchanged chabazite. Zeolites 4, 1984, 323-328. 

Diaz-Cabanas, M.J., Barrett, P.A., and Camblor, M.A. (1998) Synthesis and structure of pure Si 02 chabazite the Si 02 polymorph with the lowest framework density. Chem. Commun., 1998, 1881-1882. 

The combined results of EXAFS and Mossbauer effect spectroscopies have been used to deduce the oxidation state and local structure of iron in iron containing Na, K- and H-chabazites. In the mineral Na,K(Fe) chabazite iron is found to be present as oxo- or hydroxo-bridged Fe(III) oligimers, while in the H-chabazite it is found to be present as Fe(III) ions bridged by oxo- or hydroxo-groups to the zeolite frameworks... 

Chabazite, gmelinite, levynite and erionite form a group of porous crystals which exhibit certain similarities. Structures have previously been proposed for the anionic frameworks of chabazite,5 gmelinite and erionite. A structure is now proposed for levynite. On the basis of the crystallographic data, dilfusion anisotropy and molecular sieve behaviour of the four types of framework have been examined and the channel systems described and compared. 

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 ... 

Anionic frameworks have been proposed for chabazite. gmelinite 5 and erionite. In this paper we suggest an anionic framework for levynite, and compare, on the basis of the proposed frameworks, the ease and degree of anisotropy of molecule diffusion, and the possible molecular sieve behaviour, for the four zeolites. It has already been shown that diverse intracrystalline channel systems can arise in structures such as analcite, nosean-sodaUte minerals, cancrinite, faujasite, and Linde Sieve A.9> 10... 

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