Sodalite unit

Fig. 9. A stereoview of a sodalite unit containing an octahedral Ag, molecule within a cube of eight Ag ions is shown, using ellipsoids of 20% probability. The ions at Ag(3) occupy 8-ring sites and are not shown (165).

Figure 4.3. Infrared spectra of the HY sample upon TMA adsorption and NH3 saturation evidencing OH in the supercages at 3637 cm1, OH in the sodalite units at 3548 cm-1 and OH in the hexagonal prism at 3501 cm-1 [54].

It is convenient to visualize zeolites in terms of secondary building units (SBUs). Several of the SBUs currently recognized by the IZA are depicted in Fig. 3 along with other useful structural units, the sodalite and pentasil cages. The sodalite unit is... 

Recent sorption studies tend to support this interpretation. Based on sorption studies on USY-zeolites, Lohse et al. (36) concluded that whole sodalite units are destroyed during the dealumination and rebuilding of the framework in USY-zeolites. Such sodalite units provide the silica necessary to fill the large number of framework vacancies left by dealumination. The consumption of whole sodalite cages leads to the formation of "secondary" pores (vide infra). 

FIGURE 7.6 Zeolite frameworks built up from sodalite units (a) sodalite (SOD), (b) zeolite A (LTA), and (c) faujasite (zeolite X and zeolite Y)... 

FIGURE 7.7 (a) The zeolite A framework (b) a sodalite unit in zeolite A, illustrating the linkage to the truncated cuboctahedral cavity. 

Zeolites are aluminosilicates enclosing channels and cages of molecular dimensions (see Section 1.5). We have discussed the structure of zeolites derived from sodalite units (Fig. 1.22), which result in large cavities (cages) connected by apertures (pores) of... 

This picture is changed at high pressures and temperatures since the oxygen can then enter the sodalite units (239). If the temperature is rapidly lowered after this treatment, the oxygen remains trapped in the sodalite units. 

The logical next step in this sequence of calculations was to perform simulations that included the extraframework cations Na and Ca in the zeolite A structure (56). The cations were situated near the hexagonal faces of the sodalite units in zeolite A such that the geometrical hindrance to methane molecules passing through the 8-ring windows was small. Interactions between the methane molecule and the cations were derived from the data of Ruthven and Derrah (58), and the same two sets of parameters were used (A and B). Long simulations were performed, 25 ns with a time step of 10 fs. 

Zeolite A. The structure of zeolite A contains two types of voids (1) the a cage, 11.4 A in diameter, and (2) the P cage (or sodalite unit), 6.6 A in diameter (7). Table I compares experimentally determined pore volumes of zeolite A with the void volume as calculated from the structure (no influence of cations considered). Since the sodium zeolite A does not adsorb normal paraffins, data are included for the calcium-exchanged form. Also shown in column 5 is the void fraction, Vi, as calculated by... 

Results on the X and Y zeolites do not allow as clear an analysis. Less well defined extrapolations of the early portions of the curves of Figure 2c lead to a value of U.3 g/100 g zeolite for type NaX. This corresponds to about 35 water molecules per unit cell, or U.U per supercage (or per sodalite unit). It is tempting to identify the sites being titrated here as those within the sodalite units, which will accommodate four each 2h). 

Figures 3 and 4 show plausible sodalite unit and large cavity structures which are consistent with the structural parameters determined for hydrated Bag-A.

Figure 4. A stereoview of the sodalite unit of hydrated Bat-A. Ba1 ions are coordinated to 3 framework oxide ions, 0(3), and probably to some unlocated water molecules. Ellipsoids of 20% probability are used.

The copper content corresponds to 2 Cu11 ions per sodalite unit. 

Figure 1. (a) The sodalite -cage exhibiting the imbibed tetrahedral M4X cluster, (b) The bcc packing arrangement of -cages in the sodalite unit cell, (c) Quantum supralattices. 

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