Faujasites arrangements

Fig. 4. Model of the ciystal structure of zeolites X, Y, and the mineral faujasite. At the tight is shown the tetrahedral arrangement of tmncated octahedra surrounding one large cavity. On the left the packing model of zeohte X is shown, containing three types of Na cations.

Figure 4- Probable Si, Al ordering schemes in double 6-ring unit of faujasite-type structures. Al positions are marked by circles. B requires 96 AVs and C and D require 64 AVs per unit cell. The space group symmetry of the framework is given for each arrangement of the Al atoms. Numbering refers to the nonequivalent T atoms in the common subgroup F222...

Recent work by Rabo et al. (57) opens new possibilities for controlling the activity and selectivity of zeolite catalysts. Occlusion of various guest molecules into the sodalite cavities of Y zeolites can significantly change the catalytic properties of the zeolites for carbonium-type reactions. Anions of occluded salts are located close to the center of the sodalite cavity and strongly influence the arrangement of cations in the faujasite lattice and hence the catalytic activity. 

Double 6-ring arrangements considered for the formation of a faujasite framework. Shown in the figure are the number of Al-0-Si-0-Al pairs across 4-rings ( ) and across 6-rings (5 ) for each D6R moiety as well as the 3-neighbor distributions... 

Formulation of local Si,Al ordering in faujasite in terms of pathway I, 6R - SC - faujasite, represents an extension and refinement of our previous treatment in terms of ordered SC sub-units (4). As we have noted, pathway I for K p = 0 corresponds to formulation of faujasite from the SC arrangements of Figure 1. Actually the set of SC units specified by Kjjp = 0 is more exclusive than that specified previously (4) since there are minimum Al, A1 next nearest neighbor SC units which cannot be formed using only and p2 6R s. Moreover, consideration of 6R ensembles with Kjjp f 0 provides a straightforward approach to the study of deviations from complete local order. 

On the other hand, if the sodalite units are bridged across six-membered faces, the resulting structure (Fig. 4.23) has a 0.73-nm opening to a 12-nm cavity, forming faujasite. Still another arrangement gives the parallel pores of mordenite (Fig. 4.23). 

There are over 70 known different kinds of building arrangements, each resulting in a distinct structure. Some ofthese occur as natural minerals some are synthetic. The search for new combinations goes on. Catalytically important zeolites are listed in Table 4.11. More details on exact structures will be found in the references.In recent years, much attention has been directed toward a new zeolite, ZSM 5, whose structure is shown in Fig. 4.24. This is a zeolite with openings of 0.S4 0.56 nm, intermediate between the zeolite A (0.4 nm) and faujasite X and Y (0.74 nm). It has been successfully applied to the production of gasoline from methanol and for other specialized purposes. ... 

Figure 4. Perspective views of the arrangement of truncated octahedra (sodalite cages) in faujasite.

There is no evidence to prove that the mineral faujasite has the same Si,Al arrangement as synthetic Y with the same Si/Al ratio. Furthermore, the mineral contains variable amounts of Na, Ca, Mg, and other cations. Hence, it should not be casually equated with synthetic Y grown from a pure Na-bearing system. Even when the mineral has been ion-exchanged to the fully Na-exchanged form it should not be equated... 

D-6 ring, and (right) 2 linked D-4 rings as in the A-type framework, and (b) tetrahedral arrangement of T.O. units linked with D-6 ring found in faujasite type frameworks... 

The dealumination process is associated with a change in the porosity within the crystals and may sometimes cause a drastic loss of crystallinity. The microporous adsorbents of the faujasite type are so arranged that the Si/Al ratio increases as the munber of cations and the average electrostatic field within the framework decrease. To assess the effect of the Si/Al ratio on the activity and acidity of Y zeolites, it is desirable to compare samples with similar extents of exchange, since the degree of exchange has a significant influence on the catalytic and acidic properties of faujasites. 

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