Aluminum-deficient zeolites characteristics

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. 

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. 

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. 

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. 

The Hb NMR spectrum of this sample contains a single narrow resonance centered at -3.2 ppm, which is characteristic of boron in a tetrahedral coordination environment in the framework structure. The Si nmr spectra of a synthetically prepared siliceous mordenite with the same Si/Al ratio is shown in Figure 8. No CP resonances are present, Which indicates that hydroxyl nest concentration in this material is very low compared to the acid treated sample. These data confirm that hydroxyl nests, generated by the removal of A1 from the zeolite structure, are reactive sites for isomorphous substitution. Aluminum deficient, preformed zeolites which do not contain hydroxyl nests, i.e. synthetically prepared samples, do not undergo isomorphous substitution when treated in a similar fashion. 

The theory of structural acids is largely due to Pauling (22). In any crystal lattice involving both n ative and positive ions, a net negative charge can be created by the isomorphous substitution of a positive ion of a valence lower than that of the substituted podtive ion. Thus, if an aluminum ion is substituted for a silicon ion in a silica lattice made up of silica tetrahedra, a tiivalent ion has been substituted for a quadrivalent ion and there results a positive valence deficiency of one for each aluminum ion so isomorphously introduced. In many naturally-occurring silica-alumina structures, this type of substitution has taken place. In all these systems the valence deficiency or net n ative charge in the crystal lattice is made up or satisfied by a positive ion at or near the point in the structure at which the substitution has taken place. Materials typical for these structural characteristics are natrolite and other natural zeolites, montmorillonites, and feldspars. 

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. 

---