Zeolite aluminum-deficient

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

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. 

The discovery of the new class of high-silica zeolites "pentasil" during the last decade has attracted considerable interest due to the important applications of these zeolites in catalysis. The best known member of this family of zeolites is ZSM-5, developed in the Mobil laboratories. The unusual properties of pentasil zeolites have rekindled the interest in other high-silica zeolites, prepared by dea-lumination of low-silica zeolites. In this paper we shall review the preparation methods of aluminum-deficient zeolites, and shall discuss the properties of these materials, with emphasis on recent advances in their characterization. 

General Preparation Methods of Aluminum-Deficient Zeolites... 

High-silica, aluminum-deficient zeolites have been prepared by the following methods (Table I) ... 

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. 

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. 

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... 

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. 

Dry air pretreatment of the aluminum-deficient zeolite does not change significantly the acidity, while wet air pretreatment decreases the total acidity by a factor of 2-2.5. 

This increase in activity was attributed to a lower diffusion resistance of the aluminum-deficient zeolites, which resulted from the removal of amorphous material from the zeolite channels. However, the hydroisomerization of n-pentane... 

Scherzer, J. (1984) The Preparation and Characterization of Aluminum Deficient Zeolites, AGS Symposium Series 248, American Ghemistry Society, Washington, DG, p. 157. 

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... 

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