Ultrastable material

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. 

In order to understand the structural changes which occur, 4 different samples, each from different stages in the preparation of the ultrastable material, were studied. The first sample was selected from step 2 of Procedure A outlined by McDaniel and Maher (9). This sample was obtained from NaY which had undergone ammonium sulfate exchange to reduce the sodium oxide content to 2.5% and then had been calcined at 540°C for 3 hours in a muffle furnace. Analyses of this sample showed that its unit cell composition was Na9(A102)53(Si02)i39 (Structure I). 

The third sample (Structure III) was the ultrastable Y material itself, Nao.5(A102)53(Si02)i39. It was obtained by calcination of the second sample at 870°C for 5 hours in a muffle furnace. The fourth sample studied (Structure IV) was a portion of the ultrastable material which was subjected to 2 additional cycles of 100 °C ammonium sulfate treatment followed by calcination for 5 hours at 870°C. By chemical analysis, this material had the same silica and alumina content as the second and third samples. 

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. 

The Mobility of Silica in Steam. The reactivity of silica and silica-containing materials to steam has been assumed in the literature to explain several phenomena, a few of which are the sintering of silica (35), the aging of amorphous silica alumina cracking catalysts (36) and the formation of ultrastable molecular sieves (37). The basis of all these explanations is the interaction of siliceous materials with water to form mobile, low molecular weight silicon compounds by hydrolysis (38) such as ... 

Zeolites are crystalline aluminosilicates with a regular pore structure. These materials have been used in major catalytic processes for a number of years. The application using the largest quantities of zeolites is FCC [102]. The zeolites with significant cracking activity are dealuminated Y zeolites that exhibit greatly increased hydrothermal stability, and are accordingly called ultrastable Y zeolites (USY), ZSM-5 (alternatively known as MFI), mordenite, offretite, and erionite [103]. 

Later, Cattanach, Wu, and Venuto did an elaborate thermogravi-metric study on the calcination of ammonium zeolite Y and the resulting products (19). They found that the hydrogen zeolite reacted with anhydrous ammonia to yield an ammonium zeolite identical in ammonia content with the initial ammonium zeolite. Further, these workers reported that after loss of chemical water ( dehydroxylation according to Uytter-hoeven, Christner, and Hall or decationization according to Rabo, Pickert, Stamires, and Boyle) the sample became amorphous when exposed to moisture. This observation conflicted with the statement of Rabo et al. (16) in which they emphasized the extreme stability of their decationized Y. The data of Cattanach, Wu, and Yenuto prove, beyond any doubt, that they obtained the expected normal hydrogen zeolite Y prior to the loss of chemical water above 450°. Rabo et al., however, did not prove that the material from which they removed chemical water, was in fact, the hydrogen zeolite. They probably prepared, unknown to them at the time, the ultrastable zeolite described below. 

Influence of Temperature. Data concerning the thermal stability of the catalytic activity are given in Figure 2 and Table I. The thermal stability of the starting materials Na-8.7 and D.Na-5.4 is discussed first. The limit of stability of the Na-8.7 sample appears to be higher than for the NaHY zeolites studied previously (3, 6, 27, 28). Nevertheless, this sample cannot be considered ultrastable since neither its structural data nor the thermal stability of its OH groups are characteristic of ultrastable zeolites (17). This increase in the stability may be explained by dry air heating and subsequent rehydration. 

The unit cell dimension of the ultrastable zeolite is smaller than in the parent material. 

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. 

The main objective in FCC catalyst design is to prepare cracking catalyst compositions which are active and selective for the conversion of gas-oil into high octane gasoline fraction. From the point of view of the zeolitic component, most of the present advances in octane enhancement have been achieved by introducing low unit cell size ultrastable zeolites (1) and by inclusion of about 1-2 of ZSM-5 zeolite in the final catalyst formulation (2). With these formulations, it is possible to increase the Research Octane Number (RON) of the gasoline, while only a minor increase in the Motor Octane Number (MON) has been obtained. Other materials such as mixed oxides and PILCS (3,4) have been studied as possible components, but there are selectivity limitations which must be overcome. 

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. 

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). 

Kerr et al. (35) suggested that the term decationated not be used. They distinguished between ammonium zeolite Y hydrogen zeolite Y produced by controlled deammination and dehydration dehydroxylated form produced by heating above 450°C at 10"6 torr or above 600°C at 760 torr, loss of 1 water per pair of Na+-free A102 giving negative 4-co-ordinated Al and positive 3-coordinated Si ultrastable zeolite for material of McDaniel and Maher (41), with some framework Al turned into cationic Al. 

McDaniel and Maher have found that Step 3 of their procedure for making ultrastable Y is extremely important. They noted that treatment of the 540°C calcined, low-sodium oxide material (3% Na20) with a solution of ammonium sulfate at 100 °C for a prolonged period was essential to the stabilization step. In repeated experiments in which this material was subjected to only 2 cycles of 15 to 20 minutes each of boiling ammonium sulfate treatment, the molecular sieve collapsed upon high-temperature (810° to 927°C) calcination (9). 

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 ... 

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