Dealumination Y zeolites

Reactions with acids. Hydrochloric acid was used in the dealumination of clinoptilolite (1), erionite (14) and mor-denite (2,3,15,92). In the case of Y zeolite, dealumination with mineral acids was successful only after conversion of the zeolite into the ultrastable form (vide infra). Barrer and Makki (1) were the first to propose a mechanism for the removal of aluminum from mordenite by mineral acids. It involves the extraction of aluminum in a soluble form and its replacement by a nest of four hydroxyl groups as follows ... 

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. 

The objective of this work is to identify relationships between structure and catalytic performance in the specific case of hydrocarbon cracking over dealuminated Y zeolites. Dealuminated zeolites are prepared using chemical and hydrothermal methods and the effect of dealumination method on structure-performance... 

The main objective of this work is to characterise the textural properties of a series of Y zeolites dealuminated by ammonium hexafluorosilicate treatment. It was observed that the fluorosilicate treatment produced a highly crystalline product with a contracted unit cell. Both textural and XRD analysis confirmed the samples to be at least 95% crystalline for dealumination degrees <50%. According to N2 adsorption only a minimal contribution of mesopores was observed. However, a notable loss of micropores accompanied by the formation of mesopores was noted for severe dealumination levels, along with a considerable structural degradation. 

This was attributed to the increase in acidity due to the completely isolated framework Al. On the other hand, the hydrogen transfer reactions, which are believed to be responsible for olefin saturation and, consequently, for the parallel decrease in the RON observed, have been related to the density of acid sites. These reasonable assumptions can not fully explain, however, the product distribution observed during the cracking of gas-oil on a series of Y zeolites dealuminated at different levels and by different procedures. This is due to the presence, besides the framework-associated Bronsted sites, of Bronsted and Lewis sites which are associated with extraframework aluminium (EFAL) and which can catalyze carbonium ion as well as radical cracking reactions. 

The gas phase alkylation reaction of isobutane with 1-butene has been carried out on a series of Y zeolites dealuminated by treatment with HtEDTA solution. Dealuminated zeolites found to contain no extra-ftamework aluminium as evidenced by Al MAS NMR. However, only limited dealumination was achieved as attempts to further dealuminate results in structural collapse. Si MAS NMR spectra show the presence of amorphous siliceous species in highly dealuminated samples. Dealuminated Y zeolites are active for alkylation of isobutane with 1-butene but deactivated rapidly within first few minutes reaction. Mild dealumination improves the selectivity for trimethylpentanes but further dealumination results in the increase of cracked products and faster deactivation. The catalysts are active in the 60-80 C temperature range but beyond that catalyst deactivation was considerable. 

FIGURE 27. Mesopore volume for Y zeolites dealuminated by different procedures. 

Macedo et aL [151], also studying H-Y zeolites dealuminated by steaming, foimd that the strength of intermediate sites decreased with increased dealumination for Si/Al ratios from 8 to greater than 100. For comparison, isomorphously substituted H-Y, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [151]. This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

Shi et al. [158] measured the strength of H-Y zeolites dealuminated hy treatment with SiCU for Si/Al ratios ranging from 4.2 to 37.1, and they also found a decrease in the number of sites possessing intermediate strength with increasing dealumination. As the Si/Al ratio increased, the initial heat of adsorption, presumably corresponding to a few Lewis sites present in the samples, first increased and then passed through a maximum at an Si/Al... 

Zi et al. [91] studied Y zeolites dealuminated with H4EDTA and (NH4)2SiF6. The surface acidity of the zeolites was determined by n-butyl-amine titration, and the amount of strong (Ho < -3.0) acid sites was correlated with the cumene conversion. Both values showed a volcano-type curve if plotted against the amount of Al in the framework. The maximum in these plots was found to be around 30 alumimuns per unit cell, which agrees with the results pubhshed by DeCanio et al. [82]. 

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]. 

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. 

Removal of framework silicon from Y zeolite dealuminated with (NH4)3 [ AlFg] and virtually free of extra-framework aluminum spedes was also suggested by Liu et al. [224], who observed a gradual decrease of the framework Si/Al ratio upon treatment with 0.25 M KOH solution at temperatures between 40 and 100°C and the appearence of soluble silicate in the fluid. Moderate desiUcation (about 10%) of H-ZSM-5 upon treatment with 0.08 N NaOH at reflux temperature was observed by Lietz et al. [227]. 

An amorphous component such as silica-alumina is added to the catalyst, for a sort of pre-cracking of the large molecules (greater than about C25), which cannot enter the zeolite pores. The smaller fragments may then react in the zeolite. Middle distillates maximum yield is achieved by the use of dealuminated Y zeolites. 

Keywords Two-Dimensional correlation, Infrared spectroscopy, Dealuminated H-Y zeolite, Bronsted acidity, MQ-MAS NMR. 

Liquid-phase breakthrough experiments were also developed in order to characterize mesopores. The principle of the methodology relied on the analysis of the diffusion and adsorption of molecular probes with various molecular dimensions and adsorption strength. The relative proportion of occluded and accessible mesopores in the studied dealuminated Y zeolite could then be estimated. To allow this estimation, it is necessary to use molecular probes that can or cannot penetrate into the microporosity of the Y zeolite (see Figure 2). 

It was found that more than 70 % of the mesopore volume of the highly dealuminated Y zeolite was externally accessible [7]. Interestingly these results are in accordance with... 

These data clearly indicate that the NiMCM-36 catalyst exhibits very interesting properties for ethylene oligomerization, by comparison with the microporous NiMCM-22 zeolite at both reaction temperatures (70 and 150°C, respectively). Compared with other catalysts, the NiMCM-36 behaviour is intermediate between Ni-exchanged dealuminated Y zeolite and Ni-exchanged mesoporous materials. Taking into account that the amount of Ni2+ sites is near the same for all samples (Table 1), in order to explain these differences in catalytic behaviors, two mains categories of properties could be considered (i) the concentration and strength of acid and nickel sites and (ii) the diffusional properties (determined by the size and the architecture of pores). 

The microwave technique has also been found to be a potential method for the preparation of the catalysts containing highly dispersed metal compounds on high-porosity materials. The process is based on thermal dispersion of active species, facilitated by microwave energy, into the internal pore surface of a microporous support. Dealuminated Y zeolite-supported CuO and CuCl sorbents were prepared by this method and used for S02 removal and industrial gas separation, respectively [5], The results demonstrated the effective preparation of supported sorbents by micro-wave heating. The method was simple, fast, and energy-efficient, because the synthesis of both sorbents required a much lower temperature and much less time compared with conventional thermal dispersion. 

Figure 15, Dealumination of zeolite Na-Y using SiClk vapor studied by 29Si-MAS NMR at 79.8 MHz. Key top, parent Na-Y zeolite and bottom, after treatment with SiClk. (Reproduced from Ref. 30. Copyright 1982, American Chemical Society.)...

Zeolite dealumination was first reported by Barrer and Makki (1), who progressively removed aluminum from clinoptilolite by treating the zeolite with hydrochloric acid of different strengths. Subsequent dealumination studies were carried out primarily on mordenite (2-5) and Y zeolites. 

An example of such thermal dealumination is the formation of ultra-stable Y zeolites (USY zeolites). McDaniel and Maher (6) reported the preparation of two types of ultrastable Y zeolites (a) one type prepared by the hydrothermal... 

Kerr (7-9) has shown the critical role of the calcination environment and bed geometry in the formation of USY zeolites ("deep bed" vs."shallow bed"calcination). Ward (10) prepared USY zeolites by calcining ammonium Y zeolites in flowing steam. The work done by Kerr and Maher et al. (11) has clearly demonstrated that USY zeolites are formed as a result of aluminum expulsion from the framework at high temperatures in the presence of steam. The nature of the non-framework aluminum species has not been completely clarified. Obviously, their composition will be strongly affected by the preparation procedure of the USY zeolite. Table II shows different oxi-aluminum species assumed to be formed during thermal dealumination of the zeolite framework. 

Figure 1. Reaction mechanism for hydrothermal dealumination and stabilization of Y zeolites.

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). 

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). 

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. 

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. 

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. 

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