Dealuminated zeolites surface composition

SURFACE COMPOSITION AND DEPTH PROFILE OF DEALUMINATED ZEOLITES (43,44)... 

Using X-ray photoelectron spectroscopy. Gross et al. [95] studied the surface composition of Y zeolites dealuminated by hydrothermal treatment and by extraction with EDTA according to [24]. In hydrothermally dealiuninated samples, it was found that remaining alumimun accumulated at the outer crystal surface of zeolites in the form of oxidic clusters, but not of a dense layer. The existence of cationic aliunimun species could not be confirmed. Extraction of Y zeolite with EDTA favored the dealumination of the external crystal shell, probably resulting in an aluminum concentration gradient along the radii of the crystals [95]. 

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. 

Type II excess isotherms determined on Na-Y zeolite in the alcohol(l)+benzene(2) mixtures are presented in Fig. 19. It can be concluded from the isotherms that in a wide range of mixture compositions alcohols are preferentially adsorbed on the surface and micropores of the adsorbent. The adsorption capacities characterizing the preferentially adsorbed alcohols are listed in Table 4. Adsorption excess isotherms determined on dealuminated Y zeolite in the alcohol- -benzene mixtures are presented in Fig. 20. These isotherms may be classified as type IV, which is characterized by a linear region in a rather wide intermediate composition range (in the present case, xj = 0.15 - 0.55). The adsorption capacities determined on the basis of these linear regions by graphic extrapolation are given in Table 4. 

The aim of the present work is to investigate the alkylation activity of chemically dealiuninated Y zeolites with different framework compositions during the gas phase alkylation of isobutane with 1-butene. Zeolite Y was treated with H4EDTA solution to achieve different degree of dealumination. These samples were characterized by X-ray diffraction, ICP-AES, surface area measurements, Si and Al MAS NMR and temperature programmed desorption of ammonia. 

There are several sources for catalyst deactivation. It may be caused by sintering, change of the catalyst structure with rates depending on temperature and on gas composition (Roestrup-Nielsen and Hojlund-Nielsen, 1985) or the catalyst supr>ort may lose surface area. Zeolite catalysts may lose activity because of dealumination (Yurchak e( al., 1979 Joensen el al., 1985). 

He et al. [ 116] and Wan and Shu [117] reported on the influence of calcination and hydrothermal treatment on compositional characteristics and thermal stability of rare earth containing Y zeolites and their performance in catalytic cracking. The alkaline and hydrothermal stability of Y zeolites dealuminated via hydrothermal treatment and by the SiCl4 technique was studied by Lutz et al. [118]. Hydrothermal treatment was found to increase the chemical resistance of Y zeoHte to superheated water at 200°C as well as to alkaline solutions due to the formation of a protective layer of extra-lattice oxidic aluminum species on the external surface of the zeoHte crystals. The removal of this layer by acid leaching resulted in significantly less stable products. 

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