Crystal Structure of the Zeolite

The crystal structure of the zeolite hydrogen faujasite. /. Catal., 13, 221-231. 

R. Rouse and D. Peacor, Crystal Structure of the Zeolite Mineral Goosecreekite, CaA12 Si6016-5H20. Am. Mineral., 1986, 71, 1494-1501. 

Dempsey et al. (25) restricted the use of the terms Y and X zeolite to the composition ranges 53-64 and 80-96 Al atoms per cell. The intermediate range was denoted the Transition type (Figure 3). The nomenclature proposed by Kerr et al. (35) for decationated zeolites was described earlier. Hopefully, further experimental data will reduce the uncertainty in the experimental data on the cell dimensions and the crystal structures of the zeolites, and provide a firmer basis for nomenclature. 

This process is probably accompanied by a partial release of framework aluminium resulting in minor local lattice damages. Interaction of the so-formed oxidic extraframework aluminium with Fe(ll) results in species characterized by the Mbssbauer parameters of Fe II)ocf-2- Nevertheless, it could be shown by XRD that the crystal structure of the zeolitic component is retained even at 720 K. 

Figure 3. Exploded and normal representation of the proposed intergrowth structures of the zeolite crystals under study a) CrAPO-5 (front subunits are not shown) b) SAPO-34 c) SAPO-5 (front subunits are not shown) and d) ZSM-5.

B. (1994) Rietveld refinement of the crystal structure of the synthetic porous zincosihcate VPl-7. Zeolites, 14,... 

How exactly the molecules are oriented inside the channels depends on their specific shape and on the adsorption interaction between the dyes and the channel walls or charge compensating cations. Because of the dye s oblongness, a double-cone-like distribution in the channels is a reasonable model. This distribution is illustrated in Fig. 19a. The arrows represent the transition moments of the dyes and a describes the half-opening angle of the double cone. The hexagonal structure of the zeolite L crystal hence allows six equivalent positions of the transition moments on this double cone with respect to the channel axis. 

Fig. 2.17 Schematic representation of the structure of the zeolite natrolite [Na2Al2Si30io 2H2OI. (A) The (Si04, AIO4) chains, viewed parallel to c (along the chain length) and down c. The striped tetrahedra are AIO4. (B) The structure of natrolite and dehydrated natrolite. Solid circles are Na" , open circles are H2O, = axis of symmetry a/2 and b/2 indicate vector direction in the crystal structures. Note the rotation of tetrahedra and shift of the Na" positions in the dehydrated structure. Dehydration changes the configuration of the open areas between chains.

The aluminum content of the as-synthesized zeolites also influences their X-ray powder diffraction pattern. The height of the main peak in the patterns decreases with decreasing Si/Al ratio in the zeolite, but their width increases simultaneously so that the area remains practically constant for all samples. On the other hand, the dhki distance corresponding to the diffraction peak at 43 of 20 correlated linearly with the aluminum content of the zeolite (Figure 4). However, the lack of knowledge of the crystal structure of Beta zeolite makes it impossible to correlate the Al content and unit cell parameters. 

Nature of Vanadium Species. The data on the characterization of V- containing silicalite indicate the presence of various types of vanadium species (i) a polynuclear V-oxide containing V in various valence states (V , V and V ), (ii) octahedral sites, preferentially interacting with OH groups localized inside the pore structure of the zeolite crystals, (iii) nearly symmetrical tetrahedral species, attributed to a V species directly interacting with the zeolite framework, and (iv) after reduction, V species in a nearly tetrahedral environment. 

Tlhe importance of zeolites in research on heterogeneous catalysis is A based mainly on the fact that the structure of the active surface is a defined part of the crystal structure and does not represent a more or less severe lattice defect as most catalyst surfaces do. The crystal structure, and therefore the structure of the zeolite surface, can be determined by x-ray diffraction. Knowledge of this structure allows the construction of simple models of the distribution of electric fields in the holes of the zeolite by which wide ranges of experimental results can be explained, as is shown by the pioneering work of Barrer 1-5) and Kiselev 6-9) on calculation of the heats of adsorption of various substances. 

Zeolite formation depends on reaction conditions 2-4). It is generally believed that most zeolites are formed as metastable phases. According to Barrer (3), the course of the synthesis, beginning with the type of starting material, determines the structure of the zeolite formed. The studies of Zhdanov 2, 5) on the composition of liquid and solid phases of hydrogels indicate that the kind and composition of the zeolite formed depend on the hydrogel composition and that the results of crystallization of aluminosilicate gels obtained in the same way are reproducible. 

Tn recent experimental work in this laboratory (1-3) we have studied the kinetics and equilibria of sorption of light hydrocarbons and some other simple non-polar molecules, in 5A zeolite. The crystal structure of the A-type zeolites is among the simplest of the zeolitic lattices (4). In this paper we show that many features of the sorption kinetics and equilibria may be explained by simple theoretical considerations. 

Figure 10.12. The crystal structures of four zeolites mordenite, heulandite, laumonite, and chabazite.

The very last example here refers to the, so-called, organic zeolites. There are several structures which belong to this class of inclusion compounds and their physicochemical properties are remarkable, being of particular interest to separation science. An example of crystal structure of the compounds is given in Fig. 11.10 [9]. 

Fig. II.10 Crystal structure of organic zeolite formed by Ni(NCS)2(4-methylpyridine)4. Yellow spheres represent symbolically the empty space available for guest species...

Carbide s patent department had reservations because in their experience, composition of matter claims had to be drawn very narrowly to be valid and as such were frequently easy to circumvent. We convinced them that the unique properties that make A and X useful are singular results of their specific chemical composition and the arrangement of atoms in the crystal lattice of the zeolites. To our knowledge, the use of powder x-ray data as a finger print to uniquely identify a specific crystal structure was a new concept in patent protection. 

Since the original determination of the crystal structure of Linde zeolite 4A(1), the refinement of the structure of the hydrated variety in space group Fm3c with a unit cell edge of 24.6 instead of space group Pm3m with a pseudo-cell edge of 12.3A,(2) showed that the Si and A1 alternate in the framework. 

The mode of coke deposition is closely related to the pore structure of the zeolite (5-8). Figure 1 shows how coke deposits on typical zeolites. In the case ofZSM-5, coke deposits at intersections of the straight and zigzag channels, and also on the outer surface of the crystal. Whereas, Y type zeolites and mordenites have supercages whose sizes are almost equal to the molecular sizes of aromatic compounds composed of a few benzene rings, and coke is easily formed in the supercages. These differences in the manner of the coke formation reflect on mode of the deactivation... 

J. Felsche, S. Luger, and Ch. Baerlocher, Crystal Structures of the Hydro-sodalite Na6 [AlSi04]6 8H20 and of the Anhydrous Sodalite Na6[AlSi04]6. Zeolites, 1986, 6, 367-372. 

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