Dealuminated Faujasite-Type Zeolites

Since the thermal stability of hydrogen forms of faujasite-type zeolites turned out to be crucial in their important industrial application as cracking catalysts [401, 402], techniques were developed for stability improvement, as for instance the treatment via steaming of the catalysts [403-406]. This procedure of treatment of NH4- or H-forms of faujasite-type catalysts requires the presence of water vapor at elevated temperatures [404-407]. It proved to reduce the density of strong Bronsted sites (indicated by the bands at ca. 3640 and 3550 cm ), to increase the concentration of non-acid silanol groups (i.e., the intensity of the 3740 cm -band) and to generate additional OH groups different from those 

It was shown that the latter were due to non-acid hydroxy groups [399,409]. Since stabilization of the hydrogen form was essentially due to dealumination [397,407], the hydroxy groups newly formed upon steaming and indicated by the 3670-3690 cm band were ascribed to OHs attached to extra-framework Al-con-tainingspecies,i.e.,K -Oii species [408,410,413 (cf.also [414,417,419]). 

In agreement with Refs. [380, 399], however, Loeffler et al. [417] advanced arguments to relate the 3727 cm band to hydroxy groups associated with aluminum-deficient sites in the zeolite lattice. 

InSect. 5.2.2.10, the work by Kubelkovaetal. [271] onH-Y, Ce,Na-Y and dealuminated Ce, Na-Y was already mentioned, which was carried out employing the DRS/KBr pellet technique. These authors, however, obtained also DRS spectra of the powdered materials and compared their results with those of the conventional IR transmission spectroscopy of self-supporting wafers. They found that some of the OH bands observed by DRS were shifted to lower wavenumbers and attributed this effect to the higher temperature produced by the DRS beam in the powdered samples. In an investigation of hydrothermally treated and, thus, dealuminated H-ZSM-5 Martin et al. [429] observed by the pyridine/IR technique an elimination of Bronsted sites accompanied by a condensation of extra-frame-work Al-containing species (Lewis centers). 

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Chemical and thermal stability of dealuminated faujasite-type zeolites in gaseous and aqueous phases". Applied Catal., 2S. 71-87. 

Experiments were carried out with a Degussa P-25 TiOg (ca- 70% anatase, 30% rutile, specific surface area 56 m2.g-i, non porous). Organic substrates were reagent grade quality and employed as received. The zeolites used were Y-Faujasite type zeolites with different Si/AI ratios. HY2 5 was supplied by Zeochem and dealuminated HY10.15.20 by Zeocat. The silanated zeolites were modified by Chemical Vapor Deposition Technique as reported previously [16,17]. 

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). 

Attempts have been made to correlate this behavior with the ring sizes and average T-O-T angles present in zeolites of different topologies [300]. However, the observed frequency dependencies were not fairly uniform. In particular, for A-type zeolites a trend reversed to that for faujasites has been observed by changing the nsi/n i ratio systematically [290]. Hence, the frequency shifts obviously arise from different effects of structural changes, altered nature of normal modes, and electron density distribution and from combinations of these effects. Instead of the band shift observed for aliunimun-rich samples, for highly dealuminated faujasites the band width of the most prominent peak was discovered to reflect the aliuninum content at low levels [295]. 

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. 

It is obvious that dealumination of aluminum-rich zeolite frameworks resulting in the formation of high lattice defect concentrations should diminish the stability of the crystal structure. As early as 1958 it was reported [21] that the structure of faujasite-type zeoHtes collapsed completely upon treatment with strong mineral acids. However, Lee and Rees [22] have shown that the crystal structure of Y zeolite is not significantly affected if the amount of HCl appHed in aqueous solutions does not exceed 10 mmol/g Na-Y which results in the release of 56% of the framework aluminum atoms and in the complete exchange of the sodium cations. Thus, at least part of the aluminum in Na-Y zeoHte can be extracted without considerable lattice destruction if HCl is applied in amounts that do not yet cause too intense dealumination. 

In the first paper dealing with this method [ 147] it was reported that attempts to use this technique for the dealiunination of L zeolite in its as-synthesized K,Na-form and Na-mordenite failed. Later it was claimed [170] that synthetic large-pore Na-mordenite is partially (24%) dealuminated with SiCl4 vapor at 700 °C. That is for this type of reaction an extremely high temperature and resulted, at least in the case of faujasites, in complete destruction of the lattice. Though it was stated that the treated material retained high crystallinity, the crucial point was not pointed out in more detail, i.e., it was not evidenced that the relatively low dealumination was not accompanied by a similarly slight loss in crystallinity. 

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