Y zeolite aluminum-deficient

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. 

ALUMINUM DEFICIENT Y ZEOLITES PREPARED BY STEAM/ACID TREATMENT (L8)a... 

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). 

Aluminum-deficient Y zeolites. The properties of aluminum-deficient Y zeolites, including ultrastable zeolites, have been reviewed in several papers (9,33-35). During the last several years, new techniques have been applied to study these materials. This led to a better understanding of their structural characteristics and of the correlations between structure and properties. We shall discuss the structure and properties of aluminum-deficient Y zeolites, with the emphasis on more recently published results. 

Thomas et al. (39,41) recorded the Si-NMR spectrum of an aluminum-deficient Y zeolite prepared by reacting NaY zeolite with SiCl vapors. The spectrum showed a single sharp peak, characteristic of Si(0 Al) groupings, and indicative of an essentially luminum-free faujasite structure. 

Aluminum-deficient Y zeolites prepared by reacting Y zeolites with SiCl vapors at 500°C also showed an enrichment of the surface in aluminum (44). The X-ray data show a shift of diffraction peaks to higher 20 values, consistent with a more silicious framework (27). However, the X-ray pattern also indicates some structural differences between this material and the one prepared by the steam/acid treatment. 

Aluminum-deficient Y zeolites prepared by partial removal of aluminum with a chelating agent (e.g. EDTA) also show improved thermal and hydrothermal stability compared to the parent zeolite. The optimum stability was found in the range of 25 to 50 percent of framework A1 extraction (8). However, the maximum degree of dealumination is also affected by the SiO /Al O ratio in the parent zeolite a higher ratio appears to allow more advanced dealumination without loss of crystallinity (8,25,45). Above 50 or 60 percent dealumination, significant loss of crystallinity was observed. Calcination of the aluminum-deficient zeolite resulted in a material with a smaller unit cell size and lower ion-exchange capacity compared to the parent zeolite. 

The mid-infrared spectra of aluminum-deficient Y zeolites prepared by the reaction of Y zeolites with SiCl are similar to those prepared by steam/acid treatment (27). As in the... 

Acid properties. The acid properties of zeolites, including those of aluminum-deficient zeolites, have been described in several reviews (e.g. 33-35). The methods used to study the acidity of aluminum-deficient Y zeolites include infrared spectroscopy (primarily pyridine and ammonia sorption studies), n-butylamine titrations in the presence of Hammett or arylmethanol indicators, and to a lesser extent potentiometric titrations and calorimetric measurements. 

Pyridine sorption studies on EDTA-dealuminated Y zeolites at various temperatures (54,58), as well as measurements of differential heats of adsorption of ammonia on aluminum-deficient Y zeolites (57,59) have led to the conclusion that aluminum-deficient Y zeolites have stronger acid sites than the parent zeolite. 

A fairly large number of patents has been issued describing the application of aluminum-deficient Y zeolites in different areas of catalysis. Ultrastable Y zeolites have been used in the preparation of catalysts applied in hydrocarbon cracking, e.g. (94,95) hydrocracking, e.g. (96,97) hydrotreating, e.g. (98) and disproportionation, e.g. (99). 

Catalysts containing aluminum-deficient Y zeolites prepared by Al extraction with chelating agents have been used for cracking and hydrocracking hydrocarbons, e.g. (100). 

Correlations between preparation method and properties. A review of the physico-chemical characteristics of aluminum-deficient Y zeolites has shown that certain characteristics are common to all DAY zeolites, regardless of preparation method, while other characteristics are strongly affected by the preparation method used. 

These are some of the more important factors that should be considered in order to explain the catalytic activity of aluminum-deficient mordenite zeolites. In general, the same factors will affect the catalytic properties of aluminum-deficient Y zeolites, although fewer data are available with regard to the catalytic properties of these materials. 

Kerr, Plank, and Rosinski reported the preparation and catalytic properties of aluminum-deficient zeolite Y materials 35). Topchieva and co-workers studied the catalytic properties of cationic forms of aluminum-deficient Y zeolites, the aluminum deficiency being effected by the H4EDTA method 36-40). They found that up to 50% aluminum removal increased both stability and cumene cracking activity maximum activity was observed at the 50% removal level. Increased catalytic cracking activity was observed by Eberly and Kimberlin for mordenites from which about 80% aluminum had been removed (. 1). Weiss et al. removed over 99% of the aluminum from a hydrogen mordenite and found the zeolite retained catalytic activity of the type induced by Bronsted acids 42). Although the initial activity of this material was lower than that of more aluminum-rich mordenites, the aging rate was markedly reduced, and in a relatively short time the aluminum-deficient catalyst was the most active. 

Materials. Zeolites A, X, Y, and L were from Union Carbide Corporation, and Zeolite Z was a synthetic large port mordenite from Norton Company. Chabasite was a crystallographically very pure natural zeolite from an Hungarian deposit. Zeolite Y is an aluminum-deficient Y zeolite prepared by H4EDTA treatment. The hexammine complex of was from Strem Chemicals. The ruthenium-on-alumina catalyst was from Ventron. 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

Zhixiang et al. [107] compared typical characteristics of aliunimun-deficient Y zeolites prepared by steaming followed by add leaching and produced by the reverse sequence of the two steps. The sample obtained by the reverse method showed, in contrast to the other one, aluminum eiuichment on the external surface of the crystals and lower charge and add site density on the intracrystalline and mesopore surface. 

Kerr, G.T. (1973). Hydrogen zeolite Y, ultrastable zeolite Y, and aluminum-deficient zeolites. Adv. Chem. Ser. 121, 219-229... 

Combination of thermal and chemical dealumination. This is a two-step method which was applied in the preparation of aluminum-deficient mordenite (4,5) and Y zeolites (28,29). In some instances the two-step treatment was repeated on the same material, in order to obtain a higher degree of dealumination (5,28). 

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. 

Beyer and Belenykaia (27) have investigated the sorption properties of DAY zeolites prepared from Y zeolite and SiCl vapors. They reported a very low adsorption capacity for water and ammonia, similar to that of the almost aluminum-free silicalite (49). The low adsorption capacity for water is indicative of a hydrophobic zeolite surface. The adsorption isotherms for n-butane, benzene and n-hexane obtained on the aluminum-deficient zeolite have a shape similar to those obtained on NaY zeolite and are characteristic for micropore structures. They show the absence of secondary pores in this DAY zeolite. 

The infrared spectra of EDTA-dealuminated Y zeolites show bands in the OH stretching region similar to those encountered in HY zeolites at about 3750, 3640 and 3540 cm (50,54). However, the OH groups responsible for the 3640 and 3540 cm bands in the spectra of the aluminum-deficient zeolites... 

It is generally accepted that aluminum deficient structures derived from type Y zeolite alter the extent of hydrogen transfer reactions which ordinarily favor the formation of paraffins and aromatics at the expense of olefins and naphthenes. This octane reducing reaction is controlled principally by the silica/alumina ratio of the zeolite and its rare earth content(1). 

World-Wide there is approximately 1000 tons of fluid cracking catalyst manufactured each day. Of this, about 35% contains some form of aluminum deficient zeolite Y, one whose SiOz/AlaOa ratio exceeds 5.5 1, and whose performance is generally characterized by enhanced olefin formation and higher gasoline research and motor octane number. The aluminum deficient... 

Hydrogen Zeolite Y, Ultrastable Zeolite Y, and Aluminum-Deficient Zeolites... 

The Nature of Ultrastable Faujasite and Aluminum-Deficient Zeolite Y... 

Jacobs and Uytterhoeven studied the nature of deep-bed calcined ammonium zeolite Y and aluminum-deficient zeolite Y, the latter prepared by the H4EDTA technique 30). They concluded that the stability of ultrastable faujasite was imparted only by cationic aluminum and that aluminum deficiency in itself did not contribute to stability. Their conclusion regarding aluminum deficiency was made on the basis of H4EDTA-... 

---