EDTA-dealuminated Y zeolites

X-ray studies carried out by Gallezot et al. (46) on a 53 percent EDTA-dealuminated Y zeolite, have shown that the aluminum extraction does not leave any vacancies in the framework after calcination at 400°C in flowing, dry oxygen and nitrogen (46). It was suggested that a local re-crystall-ization of the framework occurs even in the absence of steam. The silica necessary for the process presumably originates in the destroyed surface layers of the crystallite and diffuses into its interior. 

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... 

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. 

In toluene disproportionation, two toluene molecules react to yield benzene and xylenes by transferring a methyl group from one toluene to the other. This reaction has to be done at moderate temperatures to avoid cracking and dealkylation of toluene. Therefore, reactions are mostly carried out at around 400 °C. Rhodes and Rudham [154] studied this reaction over steam-and EDTA-dealuminated Y zeolites and found initially an increase of conversion with time, followed by a maximum after 5 to 20 min. Subsequently a complete deactivation of the catalysts occurred after 10 to 25 minutes. This behavior resulted from the formation of catalytically active coke during the reaction. The maximum activity over all catalysts was linearly related... 

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. 

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. 

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]. 

Recently, liu et al. [223, 224] investigated the effects of 0.025-2.0 M KOH solutions on the structure of Y zeolites previously dealuminated by ultrastabi-Hzation and by extraction with EDTA and (NH4)2[SiFg]. As for ultrastabilized... 

Y zeolite, the re-insertion mechanism involving the refilling of lattice vacancies was again confirmed. The samples dealuminated by exctraction with both EDTA and (NH4)2[SiFg] were foimd to be practically free of extra-framework aluminum. Nevertheless, a significant increase in the framework aluminum concentration was observed upon treatment of both samples with KOH solutions. Mainly based on the treatment effects on crystal morphology and concentration of silanol groups, this phenomenon was attributed to the dissolution of the outer silicon-enriched layer in the case of the EDTA-treated sample and to the removal of framework silicon in the zeolite dealuminated with (NH,USi ,l... 

Zeolite Y was dealuminated with EDTA according to a published procedure (4). Boron oxide (0.15 g) was dissolved in 140 mL of 0.22 H KOH. Zeolite Y (5 g, Si/Al = 9.1) was suspended in the solution. The suspension was heated at 80°C for 24 h in a Teflon vessel. Products were washed thoroughly with deionized water and dried at 120°C. The chemical composition of products prepared by direct synthesis and by post-synthetic methods are listed in Table 1. 

Zeolite Y. We also substituted boron into dealuminated zeolite Y. We dealuminated zeolite Y by EDTA treatment using standard methods (4). The presence of hydroxyl nests in the product was confirmed using 29Si CPMAS NMR spectroscopy. The dealuminated material incorporated 33 times more boron than zeolite Y when treated with KOH/B2O3. These data are summarized in Table 3. The boron substituted faujasite exhibits a single sharp resonance in the NMR spectrum, consistent with structural substitution. Since the substitution level was low and would not be expected to cause large shifts in the diffraction pattern, no corrected XRD data were obtained on substituted zeolite Y. 

For the dealuminated zeolites produced by exposing Na-Y to an SiCU stream, an enhanced concentration of extra-framework aluminum species (EFAL) inside the zeolite crystal was discovered, and heats > 130kjmol were observed in the SiCU-treated samples. This was not the case on faujasites dealuminated with EDTA, which showed a lower concentration of strong... 

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