Dealumination of Y zeolites

Reaction with chelating agents. Such reactions have been used primarily for partial dealumination of Y zeolites. In 1968, Kerr (8,21) reported the preparation of aluminum-deficient Y zeolites by extraction of aluminum from the framework with EDTA. Using this method, up to about 50 percent of the aluminum atoms was removed from the zeolite in the form of a water soluble chelate, without any appreciable loss in zeolite crystallinity. Later work (22) has shown that about 80 percent of framework aluminum can be removed with EDTA, while the zeolite maintains about 60 to 70 percent of its initial crystallinity. Beaumont and Barthomeuf (23-25) used acetylacetone and several amino-acid-derived chelating agents for the extraction of aluminum from Y zeolites. Dealumination of Y zeolites with tartaric acid has also been reported (26). A mechanism for the removal of framework aluminum by EDTA has been proposed by Kerr (8). It involves the hydrolysis of Si-O-Al bonds, similar to the scheme in Figure 1A, followed by formation of a soluble chelate between cationic, non-framework aluminum and EDTA. 

A modification of the above cyclic method has proved more effective in the dealumination of Y zeolites. An almost aluminum-free, Y-type structure was obtained by using a process involving the following steps a) calcination, under steam, of a low-soda (about 3 wt.% Na O), ammonium exchanged Y zeolite b) further ammonium exchange of the calcined zeolite c) high-temperature calcination of the zeolite, under steam d) acid treatment of the zeolite. Steps a) and c) lead to the formation of ultrastable zeolites USY-A and USY-B, respectively. Acid treatment of the USY-B zeolite can yield a series of aluminum-deficient Y zeolites with different degrees of dealumination, whose composition depends upon the conditions of the acid treatment. Under severe reaction conditions (5N HC1, 90°C) an almost aluminum-free Y-type structure can be obtained ("silica-faujasite") (28,29). 

Dwyer et al. (43) have also reported that dealumination of Y zeolites by a steam/acid leaching process produces a more uniform composition than dealumination by EDTA. The later method caused a depletion of Al in the outermost surface layer, producing a compositional gradient in the zeolite crystals. The conclusions reached by J. Dwyer in his studies of aluminum-deficient zeolites using the FABMS method are summarized in Table IV. 

The shape of adsorption-desorption isotherms gives the first pieces of information on a porous solid texture. Existence of an inflexion point in the low-pressure region of the high-resolution isotherm is a sign of the microporous character of a solid [6]. NaY zeolite appears microporous (type I isotherm) regardless of probe molecule and temperature. Similarly, USY zeolite is mainly microporous,but a small hysteresis loop at high P/Po values reveals the presence of some mesopores created during dealumination of Y zeolite [8,9]. 

Dealumination of Y zeolite by H4EDTA removes aluminium not only from the framework but also from the interstitial spaces resulting in samples completely free from extraframework aluminium. However, the degree of dealumination without structural collapse was rather low. Mildly dealuminated Y zeolites are found to be more active and selective for isobutane alkylation. 

It is well known that silanol groups are formed upon dealumination of Y zeolites. The excellent catalytic performance of moderately dealuminated ultrastable... 

Wang, Q. L., Torrealba,M.,Giarmetto,G., Guisnet,M.,Perot, G., Cahoreau, M., and Casso, J. 1990. Dealumination of Y zeolite with ammonium hexa-fluorosilicate A SIMS-XPS study of the aluminum distribution. Zeolites 10 703-706. 

Influence of the method of dealumination of Y zeolites on its behaviour for cracking n-heptane and vacuum gas-oil", in P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff (eds). Innovation in Zeolite Materials Science, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 22, 495-503. 

Figure 5.6. Dealumination of Y zeolite. Reproduced with permission from Grace Davidson Refining Technologies.

Dealumination of Y zeolite was also performed with silicon chloroform, SiHClj, at reaction conditions usually used with the SiCl4 technique [147].The reaction is obviously similar to that described by Eq. (5) however, it is not yet known in detail. Special precautions must be taken to avoid the presence of any oxygen in the reactant stream since SiHClj is flammable at the reaction temperature. The only advantage offered by this reactant in comparison to silicon tetrachloride may be its potential applicability to the dealumination of zeolites with smaller pore diameters due to its smaller molecular size. 

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