Chemical Dealumination of Y-Zeolites

High-silica Y-zeolites can also be prepared by chemical extraction of alumina despite the practical problems involved. De-aluminated Y-zeolite may be stabilised by replacement of aluminium atoms in the zeolite stmcture by some silicon atoms from the solvent, or, depending upon the preparative procedure, the zeolite may sinqrly be left in a hy ogen-deleted form. 

In practice the only chemically produced HSY-zeolite to be used on a large scale was made by reaction of NH4Y-zeohte with ammonium fluorosilicate 

NH4Y-zeolite + (NH4)2SiF6 — AFSY-zeolite + (NH4)3A1F6 

High-silica AFSY-zeolites have a silica/alumina ratio of about 12 and good hydrothermal stability. The low active site density, however, gives a low intrinsic activity requiring a higher proportion of the zeolite in a catalyst. AFSY-zeolite is expensive to use because of high production costs and the need to dispose of toxic effluent. The main advantage in chemical dealumination is that the formation of non-framework alnmina is avoided. However, AFSY-zeolite is rarely used, because of the other difficulties involved. 

Nonframework alumina can also be removed from USY-zeolite catalysts by solution in ammonium nitrate, at pH 2.5, and washing out the aluminum nitrate formed. This does lead to further dealumination and, although defects remain, the zeolite is still very crystalline. The NFA-free catalysts have increased stability, produce higher-octane gasoline, have better coke selectivity, and are widely used. Most catalyst suppliers can provide dealuminated USY-zeolite with reduced nonframework alumina. 

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