Faujasite cages, synthetic

Figure 7. Diagram of synthetic faujasite cages showing cation positions (SI, SF, Sir, SII, sill) and pore openings...

The linking pattern of two zeolites is shown in Fig. 16.24. They have the /I-cage as one of their building blocks, that is, a truncated octahedron, a polyhedron with 24 vertices and 14 faces. In the synthetic zeolite A (Linde A) the /3-cages form a cubic primitive lattice, and are joined by cubes. j3-Cages distributed in the same manner as the atoms in diamond and linked by hexagonal prisms make up the structure of faujasite (zeolite X). 

Egerton and Stone (29), taking into account that synthetic sodalite zeolites did not adsorb CO molecules, concluded that CO does not enter the sodalite cages of the Y zeolites. However, the strong electric fields present in zeolites could also produce changes in the adsorptive properties of the solids thus the energies associated with the cationic sites in crystalline zeolites must be considered. From our IR results, we concluded that CO molecules were located in the volume of the sodalite cages. Thus, the steric effect alone cannot explain the different adsorptive properties exhibited by sodalite and faujasite. 

West (231) observed no 23Na resonance in dehydrated synthetic faujasites, suggesting that the EFG at the cationic site is larger than in hydrated samples because of the displacement of the cations away from their high-symmetry positions. The signal appeared when seven H20 molecules per cage were present. Fully hydrated Na-X and Na-Y had Tf of 100 and 140 fj.sec, respectively, while in dehydrated samples much faster transverse relaxation was observed. 

It is clear that the Wacker cycle in a CuPdY zeolite incorporates the traditional features of the homogeneous catalysis combined with typical effects of a zeolite (303, 310). It also follows that whereas other cation exchangers in principle will show Wacker activity after cation exchange with Cu/Pd ions, the cage and pore architecture will probably be less suitable for Wacker chemistry than those of the faujasite structure. This is the case for fluoro-tetrasilicic mica, a synthetic layer silicate that swells under reaction conditions and allows access to the interlayer space (311). 

The detailed structures of zeolites are varied and complex, but the common pseudo-cubic basket-like frameworks of linked tetrahedra are depicted in Figure 1.7. These typify the unit cage structures found for the mineral faujasite and the synthetic zeolites whose structures are generically described as types A, X, and Y. The constitution, channel diameters, and ion exchange capacities for various natural and synthetic zeolites are shown in Table 1.4. 

Type Y zeolite is a synthetic analogue of the mineral faujasite the Site-A1 ratio is between 1.5 and 3. The structure consists of a three-dimensional array of spherical supercages 13 A in diameter. These spheres are connected by 7.4 A windows. The sodalite cages are truncated octahedra of diameter 7.6 A. The INS spectra of NaY and H-NaY, which are typical zeolite spectra, are shown in Fig. 7.26 and the peaks are listed and assigned in Table 7.16 [108,109]. 

During the preparation of R-exchanged zeolites (R=rare-earth atoms) it was found that reversible ion-exchange isotherms could be obtained in the La,Na-X and La,Na-Y systems, and these depend on the temperature and the zeolite type. Thus, rare earths cannot diffuse into the small cages of synthetic faujasites at 25°C within a reasonable time,... 

One of the most elegant synthetic approaches to using the internal cavities of zeolites as nanometre size reactors is that of the ship-in-a-bottle synthesis of metal complexes within zeolite cages, which are then too large to escape through the cage windows. The term was initially coined by Herron to describe metal complexes, such as those with salen-(bis(salicylidene)ethylendiamine-) " or phthalocyanine (Scheme 6.9) that were formed in the supercages of faujasitic zeolites. Zeolites X and Y are most commonly used, but the fully... 

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