Tetrahedral sites faujasitic framework

Figure 10a, Scheme of the aluminosilicate framework of a typical faujasitic zeolite Si/Al ratio of LI8 (arbitrarily chosen to illustrate the ordering among tetrahedral sites) before (left half) and after (right half) exposure to SiCls, which dealuminates the zeolite (see Figure 10b),... 

Figure 5. Possible tetrahedral environments of a silicon atom (marked with a cross) in the faujasite framework. First-order tetrahedral neighbours are separated from the second-order neighbours by the dotted line. Open circles denote silicon, closed circle aluminium arrows indicate Si(OAl) sites. The number of such sites eliminated by substituting the centred silicon by an aluminium is indicated under each diagram.

If the twin boundaries constitute true mirror planes, it can be shown that, across these planes Loewenstein1s rule must be contravened. If, however, the mirror planes merely reflect the tetrahedral sites of the faujasitic framework and not the actual tenants of such sites Loewenstein s rule need not be... 

The isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

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