Preparation zeolites

Table 3. Some Synthetic Zeolites Prepared from Sodium Aluminosilicate Gels...

Volume 12 Metal Microstructures in Zeolites. Preparation - Properties -Applications. Proceedings of a Workshop, Bremen, September 22-24,1982 edited by P.A. Jacobs, N.l. Jaeger, P. Jitu and G. Schulz-Ekloff... 

Table 2.1 Benefits of mesoporous zeolites prepared by desilication in catalytic applications with respect to the purely microporous counterparts.

Volume 12 Metal Microstructures in Zeolites. Preparation - Properties - Applications. 

Selective catalytic reduction of NOx by NH3 on V-Mo-zeolite prepared by solid-state ion exchange method... 

Model fuel and zeolite preparation n-dodecane (Lancaster, 99+%) is used as model fuel. 

Sodium aluminate, 2 345t, 358-359 analysis, 2 275-276 economic aspects, 2 275 health and safety factors, 2 276 manufacture, 2 274-275 neutralization, 2 424 physical and chemical properties of, 2 273-274 uses of, 2 276-277 in water treatment, 26 111 Sodium aluminosilicate gels, synthetic zeolites prepared from, 16 831t Sodium aluminosilicates, 12 578 Sodium aluminum hydride, 13 621, 623-624... 

The discovery of the new class of high-silica zeolites "pentasil" during the last decade has attracted considerable interest due to the important applications of these zeolites in catalysis. The best known member of this family of zeolites is ZSM-5, developed in the Mobil laboratories. The unusual properties of pentasil zeolites have rekindled the interest in other high-silica zeolites, prepared by dea-lumination of low-silica zeolites. In this paper we shall review the preparation methods of aluminum-deficient zeolites, and shall discuss the properties of these materials, with emphasis on recent advances in their characterization. 

ALUMINUM DEFICIENT Y ZEOLITES PREPARED BY STEAM/ACID TREATMENT (L8)a... 

It has already been mentioned that the formation of ultrastable Y zeolites has been related to the expulsion of A1 from the framework into the zeolite cages in the presence of steam (8,9), and the filling of framework vacancies by silicon atoms (11,12). This results in a smaller unit cell size and lower ion- exchange capacity (6). It also results in a shift of X-ray diffraction peaks to higher 20 values. Ultrastable Y zeolites prepared with two calcination steps (USY-B) have a more silicious framework than those prepared with a single calcination step (USY-A). Furthermore, since fewer aluminum atoms are left in the USY-B framework, its unit cell size and ion-exchange capacity are also lower and most of the nonframework aluminum is in neutral form (18). 

Using Si-NMR spectra to calculate the SiO /Al O ratio in the framework and chemical analysis to determine the overall SiO /Al O ratio, Lippmaa et al. (38) concluded that in their sample of USY-A zeolite, 33 Al/u.c. are in the framework and 24 Al/u.c. are in non-framework positions (42 percent framework dealumination). In the USY-B zeolite prepared from the same parent NaY, Lippmaa et al. found 4 Al/u.c. in the framework and 53 Al/u.c. in non-framework positions (93% framework dealumination). 

Thomas et al. (39,41) recorded the Si-NMR spectrum of an aluminum-deficient Y zeolite prepared by reacting NaY zeolite with SiCl vapors. The spectrum showed a single sharp peak, characteristic of Si(0 Al) groupings, and indicative of an essentially luminum-free faujasite structure. 

Aluminum-deficient Y zeolites prepared by reacting Y zeolites with SiCl vapors at 500°C also showed an enrichment of the surface in aluminum (44). The X-ray data show a shift of diffraction peaks to higher 20 values, consistent with a more silicious framework (27). However, the X-ray pattern also indicates some structural differences between this material and the one prepared by the steam/acid treatment. 

Stability. Ultrastable Y zeolites, prepared by the hydrothermal treatment of ammonium Y zeolites, have considerable thermal and hydrothermal stability (6) The high... 

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. 

The unusually high stability of DAY zeolites prepared from USY-B and having SiO /Al O ratios over 100 indicates that the non-framework aluminum species present in USY-B play no role in enhancing the stability of this zeolite. It is the highly silicious framework, in which most of the aluminum has been replaced by silicon atoms, that is responsible for the high stability of USY-B zeolites and of corresponding DAY zeolites. In zeolites with a lesser degree of framework dealumination (i.e. in USY-A), the non-framework aluminum species appear to play a role in the stabilization of the zeolites, since their removal results in materials of lesser stability (28). 

DAY zeolites prepared from Y zeolites and SiCl also show high stability, a smaller unit cell and resistance to mineral acids (27). 

Beyer and Belenykaia (27) have investigated the sorption properties of DAY zeolites prepared from Y zeolite and SiCl vapors. They reported a very low adsorption capacity for water and ammonia, similar to that of the almost aluminum-free silicalite (49). The low adsorption capacity for water is indicative of a hydrophobic zeolite surface. The adsorption isotherms for n-butane, benzene and n-hexane obtained on the aluminum-deficient zeolite have a shape similar to those obtained on NaY zeolite and are characteristic for micropore structures. They show the absence of secondary pores in this DAY zeolite. 

The mid-infrared spectra of aluminum-deficient Y zeolites prepared by the reaction of Y zeolites with SiCl are similar to those prepared by steam/acid treatment (27). As in the... 

It was also shown than DAY zeolites prepared by different methods have different acid strength distributions (18). This was related to structural differences between these zeolites. 

Catalysts containing aluminum-deficient Y zeolites prepared by Al extraction with chelating agents have been used for cracking and hydrocracking hydrocarbons, e.g. (100). 

Sorption. The sorption properties of aluminum-deficient mordenite are strongly affected by the dealumination procedure used and by the degree of dealumination. Materials prepared by procedures that do not involve high temperature treatments show a relatively high sorption capacity for water (15,70), due to the presence of silanol groups, which are hydrophilic centers. However, aluminum-deficient mordenite zeolites prepared by methods requiring heat treatment show a lower sorption capacity for water due to fewer silanol groups. This was shown by Chen (71), who studied the sorption properties of aluminum-deficient mordenite prepared by the two-step method. 

The formation of such bonds during the heat treatment of dealuminated mordenite has also been suggested by Rubinshtein et al. (72-74), in some instances without the intermediate formation of SiOH groups. The hydrophobic nature of the zeolite also increases with progressive dealumination. Chen (71) has shown that aluminum-deficient mordenite zeolites with SiO /Al O ratios over 80 absorb little or no water at low pressure. These highly silicious zeolites are truly hydrophobic and in this respect are similar to highly silicious zeolites prepared by direct synthesis (e.g. ZSM-5) (75). 

Aluminium distribution in A and B1 intermediate and final phases Various compositional zonings have been reported in silica-rich ZSM-5 zeolites prepared under particular conditions (11-13,19,26, 27,31,32,59,68) while other studies provided evidence for homogeneous A1 distribution throughout the crystallites (69-71). Obviously, the distribution of aluminium in ZSM-5 must depend on its mechanism of crystallization. 

In view of the difficulty of measuring the diffusivity of o-xylene at the reaction temperature, 482°c, we have used the diffusivity determined at 120°C. For a series of ZSM-5 catalysts, the two D-values should be proportional to each other. Para-xylene selectivities at constant toluene conversion for catalysts prepared from the same zeolite preparation (constant r) with two different modifiers are shown in Figure 11. The large effect of the modifier on diffusivity, and on para-selectivity, is apparent. 

In order to compare a number of different zeolite preparations we have found it convenient to determine not the diffusivity of o-xylene per se, but to characterize the samples by measuring the time (tQ 3) it takes to sorb 30% of the quantity sorbed at infinite time. The characteristic diffusion time, t0 3, is a direct measure of the critical mass transfer property r2/D ... 

Butadiene oxidation to furan was also performed on ZSM zeolites prepared in the presence of v3+ [42-44]. Non-oxidative activation prevents collapse of the zeolite structure to cristoballite and improves both the activity and selectivity in furan synthesis. 

Lunsford and coworkers reported on the epoxidation of propene and cyclohexene using a molybdenum zeolite, prepared by introducing either Mo(CO)g or M0CI5 into the zeolite, which showed initial good activity and selectivity for the epoxidation reaction in the presence of The activity declined over a period of minutes, which was... 

Decationated zeolites. We start by considering decationated zeolites since they do not contain any metal ions extrinsic to the silica-alumina framework. This type of zeolite is obtained by pretreating, above 350°C, a NH4Y zeolite prepared by exchanging the sodium form of a Y-type zeolite with ammonium ions. Ammonia is evolved leaving a decationated or HY zeolite ... 

TABLE 2. SOME SYNTHETIC ZEOLITES PREPARED FROM SODIUM ALUMINOSILICATE OELS... 

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