Cages of zeolite

Figure 1.45 cluster in the sodalite cage of zeolite Y. On the right and... 

In the last few years, computer graphics with colour display are being more commonly used not only to visualize complex structures better, but also to examine unusual structural features, defects and transformations as well as reactions. In Fig. 1.45, we show the presence of a Nal" cluster within the sodalite cage of zeolite Y as depicted by computer graphics the cluster fits well within the cavity bounded by the van der Waals surface (net) of the framework atoms. The immense power of computer graphics has been exploited widely in recent years. Structural transitions in solids and sorbate dynamics in zeolites are typical areas where computer simulation and graphics have been used (Ramdas et al., 1984 Rao et al., 1992). 

Other simulations of the diffusion of methane in zeolite A have been performed by Cohen de Lara et al. (50), who reported calculations for a single methane molecule in an a-cage of zeolite A. They used a 7-cage array as a model for the zeolite, with cations fully occupying the SI sites, half-filling the SII sites, and occupying only 1 /12th of the Sill sites. Ionic... 

Fig. 6. Density profiles along aplane bisecting the cation-poor a-cage of zeolite A at a loading of 6 Xe atoms per cage. Increased localization about the minimum energy positions is observed at lower temperatures left, 300 K right, 100 K (from 119, with permission of Taylor Francis).

In Zeolite A. The location and distribution of methane within the a-cages of zeolite A were found to be very similar to those found in zeolite Y, consistent with the structural similarities of these two materials. Cohen de Lara et al. (50) complemented their extensive experimental work with MD calculations of a single methane molecule adsorbed in zeolite A. At... 

Based on host-guest interaction, microporous zeolites have been used as heterogeneous host for encapsulation of metal complexes and organometallic fragments. For zeolite-encapsulated photosensitizer, the steric and electrostatic constraint imposed on the complexes within the channels or cages of zeolites can alter the photochemical and photophysical properties of the guest complexes and diminish the photodegradation and undesirable electron transfer reactions [6]. But, the pore sizes (-13 A) of microporous zeolites are too small for... 

Figure 7. Chem-X model for a (WC>3)4 cubane-cluster encapsulated in an a-cage of zeolite Y, projected down a 3-fold axis of the host structure, displaying tungsten-dioxo anchoring to extraffamework site II Na+ cation six-ring binding sites.

Table I. 129Xe chemical shifts of mixed clusters XenKr in the alpha cages of zeolite Na/f (ppm) ...

The formation of the Al-0 complexes in the sodalite cages of zeolite Y is analagous to similar La-0 complexes previously described (15,16). These phenomena are under study with neutron- and synchrotron-X-ray diffraction. 

Barrer and Gibbons (1965) did calculations for the interaction potentials of C02 and NH3 moving along the axes running through the center of the 12-ring window of faujasite-type 26-hedral cage of zeolite X. The results are shown in Tables VII and VIII. The qualitative comparison with experimental... 

Various strategies have been pursued in order to immobilise chiral epoxidation catalysts and these encompass covalent attachment to solid supports,[41] steric occlusion in nanosized cages of zeolites,[42 44] entrapment in a polydimethylsiloxane membrane145,461 and fluorous biphasic systems.1471 However, these approaches frequently require tedious ligand modifications and often lead to a marked decrease in both selectivity and activity of the transition metal catalyst. 

Intermediates were also observed in the synthesis of a neutral cluster, Ir4(CO)i2, from Ir(CO)2(acac) in the cages of zeolite NaY these were characterized by IR and extended X-ray absorption fine structure (EXAFS) spectroscopies, the latter being a technique ideally suited to investigation of small, highly dispersed species present in small amounts in solids. The spectra indicated dimeric intermediates, possibly Ir2(CO)8 [ 16], when the reaction was carried out in the near absence of water in the zeohte in contrast, the reaction in the dehydrated zeolite was faster, and no evidence of intermediates was observed [16]. 

Fig. 1 Representation of an array of interacting Na4 clusters, located in the sodalite cages of zeolite Na Y. Na4 ions are formed within the zeolite according to the reaction ...

Aluminosilicate zeolites are normally synthesized under basic conditions. The introduction of OH- ions to the synthetic system will necessarily lead to the introduction of correlated cations. These positively charged cations play an important role in the polymerization of polysilicates and aluminates by affecting the polymeric state and their distribution, and have an important effect on the colloidal chemistry of aluminosilicate as well. In addition, cations existing in the synthetic system also have important effects on the formation of the framework structure of zeolites. For example, plenty of synthetic data indicated that a tight correlation between the formation of the SBU cages of zeolites and the charge and size of the cations existed, and this was named the templating effect of cations by R.M. Barrer.[191... 

Very small clusters (1 < m < 4) accommodated in the small cages of zeolites (such as the sodalite cage of faujasite) or in the straight channels of zeolites such as mordenite and zeolite L. 

In 1984, Edwards and coworkers reported the formation of Na43+ in zeolites Y and A and the formation of K43 1 in K+-exchanged zeolite Y. Later on, they found that if the zeolite was Na-Y, the formed metal ion clusters are Na43+ no matter what (Na or K) the reaction vapor was whereas if the zeolite used was K-Y, the obtained metal clusters will be K43+. That is, the formed metal cluster species is not related with the vapor but simply depends on the type of cation in the zeolite used. In fact, through variation of the reaction condition, various M,(i ion clusters can be prepared. If M = Na, n = 2 6, whereas if M = K, n 3,4. After the alkali metals enter the channels or cages of zeolites to form metal ion clusters, the electrons on the original metal atoms will be released to be shared by more than one metal atom. It has been confirmed that these free electrons actually occupy the holes formed by the metal atoms (ions). Therefore, these electrons are also called solid solvated electrons (in analogy with the solvated electrons formed by alkali metals in solvents such as liquid ammonia),[7] and the formed compounds are called solid electrides. 

Figure 9.6 Cluster crystals formed by Na43+ clusters located in the sodalite cages of zeolites with different structures, (a) Sodalite (SOD), (b) Zeolite Y (FAU). (c) Zeolite A (LTA). Although the array shown in (c) has not been prepared, the analogous potassium clusters shown in (d) is indeed available in (d) the cluster crystal is actually composed of K43+ clusters in all the sodalite cages and the larger K,24+ clusters in every other a-cages. Reproduced from [4], Copyright (2002) Springer-Verlag...

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