Dealumination of faujasite

For the conversion of m-xylene, the activity of H-faujasite depends strongly on its degree of dealumination. Maximum catalytic activity is obtained for aluminum T-atom fractions equal to 0.10. Surprisingly, in contrast to both theoretical predictions and to the behaviour of H-ZSM-5, for dealuminated H-faujasites the turnover frequency per protonic aluminum site exhibits a pronounced maximum when the aluminum T-atom fraction is 0.09. The present results can be rationalized if, besides the classical predictions on zeolite acidity, a new concept of "hidden acid sites" is handled. Changes of the m-xylene isomerisation and disproportionation selectivities with the degree of dealumination of faujasite are in agreement with this concept. 

In a study by Brueva et al. [ 147], the calorimetric values obtained by ammonia adsorption at 573 K over hydrogen forms of H-Y zeohtes suggested that in parent H-Y the strong sites were mainly in the range 105-110 kJ mol , whereas dealumination of faujasites resulted in the appearance of stronger acid sites with q > 120 kJ mol [ 147). Relationships between addity and catalytic activity were also discussed. 

The dealumination of faujasite,mazzite and offretite with ammonia hexaflu-orosilicate and the characterization of the products with various techniques have been reported [192]. The maximum level of dealiunination, which could be achieved without loss of X-ray crystallinity, corresponded to 50% for faujasite and 30% for mazzite and offretite. The dealiunination capability was foimd to depend on the texture of the crystals, which may have indicated that the process was diffusion-controlled. 

THE CHEMISTRY OF THE DEALUMINATION OF FAUJASITE ZEOLITES WITH SILICON TETRACHLORIDE... 

The zeolite-catalyzed glucosylation of n-butanol was also investigated by Chapat et al.38 using dealuminated HY faujasites. The thermodynamically more-stable butyl glucopyranosides were predictably the final reaction products, resulting from... 

Besides di- and poly-saccharides, zeolites have been applied for hydrolysis of simple glycosides as described by Le Strat and Morreau.132 Methyl a- and /i-D-glucopyrano-sides were treated with water in the presence of dealuminated HY faujasite with an Si/Al ratio of 15, at temperatures ranging between 100 and 150 °C. It was observed that the reaction rate for the (i glycoside was about 5-6 times higher than that for the oc anomer, a result that might arise from the shape-selective properties of the zeolite and stereoelectronic effects on the surface of the solid. 

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). 

The use of traditional and new techniques to elucidate the structure of synthetic faujasites with different silica alumina ratios, dealuminated by steaming and chemical treatment, and with and without faulting will be described. The migration and fixation of cations and the role of aluminum in the dealumination of the zeolite will be discussed. 

Beyerlein et al. (33) studied the catalytic properties of a series of ultrastable synthetic faujasites dealuminated by steaming and by acid extraction to determine catalytic acidity as a function of framework characteristics. They found that carbonium-ion activity in isobutane conversion is proportional to framework-Al content, and comparing results obtained by using hydrothermally and AHF-dealuminated synthetic faujasite, they found that the steamed material, which contains extra-framework Al, gave a large increase in carbonium-ion activity compared with the AHF-treated material, which had a relatively clean framework. This indicates that strong acidity exhibited by mildly steamed synthetic faujasite, while directly related to framework-Al content, depends on a balance between framework and extra-framework Al, and that this extraframework Al contributes greatly towards catalytic performance. 

Of great interest is the question of the role of trigonal aluminum, which is usually assumed to act as a LAS. Such a center should be quite typical of A1203, where it may appear as a result of surface oxygen vacancy formation. These vacancies may either develop due to dehydroxylation or be of a biographical nature. A similar situation may take place in the case of such mixed oxides as amorphous aluminosilicates. Uytterhoeven, Cristner, and Hall 123) have concluded that trigonal aluminum could also appear as a LAS upon dehydroxylation of H forms of zeolites. Their scheme was criticized, however, by Kiihl 124), who has undertaken X-ray fluorescence studies of the dehydroxylated forms of faujasites and found that the dehydroxylation was accompanied by dealumination of a zeolite framework with formation of extralattice aluminum which could also exhibit the Lewis acidity. 

Reports on the thermal stabilities of faujasites and mordenites are largely confined to their resistance to collapse at elevated temperatures. There is, however, a need to extend these works to the investigations of reactions which occur during the thermal treatment of hydrogen zeolites. These include aluminum migration, dehydroxylation and formation of new active sites. The present study is concerned with the effect of calcination temperature on the crystallinity, the extent of thermal dealumination, concentration of hydroxyl groups and catalytic activity of hydrogen faujasites and mordenites with different Si/Al framework ratios. 

These dealumination procedures result in Al deficient zeolites of the high thermal stability. In the case of faujasite Y, the so called ultra-stable Y zeolite (US-Y) is used as a catalyst for cracking catalysis (47). It was reported that the total acidity of (48) Al-deficient zeoITtes was less than that of the parent zeolite buT with stronger acidic sites (49). For mordenite, this acidity decreases linearly with Al content (50, 51). However calorimetric measurement of the NH- heat of adsorption has shown that when the total number of acidic sites decreases regularly with dealumination, as could be reasonably expected, the strength of the strongest acid sites is enhanced (52). 

In faujasite, the A1 content must necessarily be varied by dealumination of a low-silica zeolite, as no synthesis recipe for high-silica faujasite is available. Because the aim of this work was to study the acidity of the zeolite itself, the SiCl4 dealumination method was selected in order to minimise the formation of extralattice aluminum (24.). ... 

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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