Faujasite ultrastable

Samples of Y faujasites were prepared by sodium exchange of a starting ultrastable Y zeolite (H form, denoted in the following as USY). Global Si/Al ratio is 16 according to X fluorescence measurements framework Si/Al is 21 as measured by 29Si MAS NMR. 

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). 

The Nature of Ultrastable Faujasite and Aluminum-Deficient Zeolite Y... 

Jacobs and Uytterhoeven studied the nature of deep-bed calcined ammonium zeolite Y and aluminum-deficient zeolite Y, the latter prepared by the H4EDTA technique 30). They concluded that the stability of ultrastable faujasite was imparted only by cationic aluminum and that aluminum deficiency in itself did not contribute to stability. Their conclusion regarding aluminum deficiency was made on the basis of H4EDTA-... 

The Three General Methods for Preparing Ultrastable Faujasites... 

Perhaps an overly simple mechanism has been proposed by Kerr to explain the formation of the nonframework aluminum found in ultrastable faujasites prepared by the two methods just described 22,26). 

Ultrastable faujasitic catalysts are a cornerstone of the petroleum industry, and it is not surprising that much effort has been devoted to the study of their properties amd methods of preparation. The following summarizes the most important observations ... 

Beyerlein et al. (33) studied the catalytic properties of a series of ultrastable synthetic faujasites dealuminated by steaming and by acid extraction to determine catalytic acidity as a function of framework characteristics. They found that carbonium-ion activity in isobutane conversion is proportional to framework-Al content, and comparing results obtained by using hydrothermally and AHF-dealuminated synthetic faujasite, they found that the steamed material, which contains extra-framework Al, gave a large increase in carbonium-ion activity compared with the AHF-treated material, which had a relatively clean framework. This indicates that strong acidity exhibited by mildly steamed synthetic faujasite, while directly related to framework-Al content, depends on a balance between framework and extra-framework Al, and that this extraframework Al contributes greatly towards catalytic performance. 

Catalytic isomerizations of ethylbenzenes and xylenes over zeolites are commercial processes and have been used as test reactions of acid catalysts. Corma and Sastre26 have recently suggested that xylenes can form via transalkylation of trimethylbenzene which is believed to be an intermediate in the isomerization of p-xylene. A general scheme as that shown in Eq. 626 was proposed on the basis of kinetic and mass spectrometric data. The reactant p-xylene was believed to produce m-xylene as a primary product but also rearranges in the pores of ultrastable faujasite zeolites to form o-xylene which appears as a primaiy product. In addition, trimethylbenzenes were formed along with toluene. 

Further isomerization of m-xylene as well as transalkylation of trimethylbenzene and toluene to form m-xylene can occur. Evidence for the bimolecular transalkylation mechanism was provided by observation of a peak at m/e 109 in the mass spectra for CD3 substitution of toluene. These data rule out unimolecular 1,2-methyl shifts as the sole means of formation of xylenes. The higher the A1 content of the ultrastable faujasite the greater the extent of bimolecular transalkylation. These observations have significant implications for unimolecular kinetic models that have been proposed as well as reported activation energies and turnover frequencies. 

K in 6RuNa faujasites with different A1 contents (A1 per unit cell). The 2 samples with lowest A1 content are obtained via dealumination with SiCl. The open point correspond to a steamed zeolite support (ultrastable zeolite). 

Clearly the success of the catalyst depends upon its ability to withstand these extreme conditions and this is one of the advantages of the ultrastable synthetic faujasite (USY) and ZSM-5 materials and is always a consideration in the other applications listed in Table 31. It is, however, only one of the known advantages acquired when zeohtes are considered as industrial catalysts. Other benefits come from the zeohte... 

Molecular sieve zeolites have become established as an area of scientific research and as commercial materials for use as sorbents and catalysts. Continuing studies on their synthesis, structure, and sorption properties will, undoubtedly, lead to broader application. In addition, crystalline zeolites offer one of the best vehicles for studying the fundamentals of heterogeneous catalysis. Several discoveries reported at this conference point toward new fields of investigation and potential commercial utility. These include phosphorus substitution into the silicon-aluminum framework, the structural modifications leading to ultrastable faujasite, and the catalytic properties of sodium mordenite. 

The elucidation of the structures involved in producing ultrastable faujasite reported by Maher, Hunter, and Scherzer. Although the discovery of the ultrastable faujasite was reported at the 1967 London meeting, these subsequent studies (and work by G. T. Kerr) are pointing to an unexpected method of altering the crystalline framework. 

The catalytic activity and stability of aluminosilicate zeolites is strongly influenced by removal of Al from the framework sites into extraframework positions. This can be accomplished by dehydroxylation or calcination followed by steam treatment, the latter process producing ultrastable zeolites. Early Al NMR studies indicated the appearance of the 30 ppm resonance attributed to Al when faujasite and other zeolites are steamed (Gilson et al. 1987). Double rotation Al NMR studies at two magnetic fields (Ray and Samoson 1993) indicated that the nature of the Al species depends on the method of dealumination the 30 ppm resonance in zeolite-Y samples treated... 

At 150 °C with zeolites such as the ultrastable faujasite H-USY, conversions up to about 40% and selectivities of up to about 80% can be achieved.47 The best selectivities are observed in phenol as solvent. The main by-product is phenol, which may be formed via the decomposition of the starting material or the deacetylation of the product. This inevitably seems to result in catalyst deactivation. 

SAPO-37 molecular sieve which has the crystalline structure of faujasite differs from this zeolite by the presence of phosphorus in the structure (1). It was shown that this element increases the thermal and hydrothermal stability of the structure (2). With regards to acidity, the SAPO-37 materials have acidic properties (1,3,4) with two OH groups very similar to those of faujasites (1,4). It was also observed that the SAPO-37 materials have besides acid centers of medium strength a small number of protonic sites stronger than in HY or even than those of an ultrastable LZY-82 (4). 

The crystal structures of 4 ammonium exchanged, heat-treated faujasites were determined from x-ray powder data. Structure I, often called decationated Y, has lost 15 framework aluminum atoms and 21 framework 0(1) atoms (bridging oxygen atoms) per unit cell, and 15 Al(OH)2+ ions are present in the sodalite cages. Structure 11, called ammonium-aluminum Y hydrate, shows a complete rehydroxyla-tion of the vacant 0(1) positions. Structure III, called ultrastable Y, shows the same 15 framework aluminum atoms absent, and the removal of 25 0(3) and 13 0(h) framework oxygen atoms. Structure TV, which is a repetitive exchanged and heat-treated version of Structure 111, has a mean Si-O bond length of 1.610 A, which indicates that little framework aluminum is present. 

Ambs and Flank (2) have concluded on the basis of limited data that the thermal stability of synthetic faujasite is dependent only on the level of sodium present. They further assert that no difference exists between decationated Y and ultrastable materials. 

D. H. Olson (Mobil Research Development Corp., Princeton, N. J.) The results reported here are the best structural data to date pertaining to the structural transformations occurring during the preparation of ultrastable faujasite. However, I feel the warning presented earlier by J. V. Smith should be kept in mind. The large standard deviations of the population parameters which give site occupancies and vacancies should be considered while deriving a mechanistic picture from the structural data. 

D. A. Hickson (Chevron Research Co., Richmond, Calif. 94802) Please contrast and compare the dielectric behavior of decationated and ultrastable faujasite. How does the dielectric behavior of these materials change on filling the cavities with polar molecules such as water ... 

R. Schoonheydt Our dielectric measurements are performed on tightly compacted pellets. Therefore, we believe that the heating process, producing H-zeolites and removing NH3, gives a sample which is more likely to be an ultrastable than a decationated faujasite. From our results, a distinction is not possible. 

Ultrastable Y zeolite with varied silica-alumina ratios, 6.5-60, faujasite, mordenite, and pentasil... 

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