Zeolite crystals, preparation

As for all other zeolite crystals prepared with the fluoride route, the crystals of the structure types FER TON and HTT are of very large size in the absence of seeds. The FER-type crystals are aggregates of plates, the size of which can be larger than 200 pm. The other structure types (TON and HTT) show a fibrous aspect, the length being over 100 pm for a diameter below 1 pm (Figure 5). 

The mesoporous zeolite crystals are larger (1 pm, Fig. 2b) than the zeolite crystals prepared without carbon black (0.5 pm. Fig. 2a). The sample ZSM-5/IMP prepared by the second method contains crystals in a broad range of size (Fig. 2c). The cofBn-like shape, which is typical for ZSM-5 zeolites, was observed for each sample. 

In the late 1940s zeolites were synthesized according to the procedure shown in Fig. 3.24. First an amorphous alumino-silicate gel is formed. This process is completely analogous to the production of alumina and silica gels described before. Subsequently this gel is crystallized into zeolite. The preparation of zeolites has drawn tremendous attention of the scientific and industrial community. A wide variety of zeolites have been synthesized, and reproducible synthesis procedures have been reported (often in the patent literature). Natural zeolites also exist massive deposits have been discovered in many places in the world. 

Table 1 shows the dimensions of the zeolite L crystals prepared by varying the water content in the preparation. As the water content in the reactant gel is increased the aspect ratio of crystals was found to increase. Figure 1 shows SEM images of the three types of crystals, from which this increase in aspect ratio with water content can be observed. 

Figure 2 Error signal AFM images of the three different zeolite L preparations where i) and iv) have aspect ratios of 1.5, ii) and v) have aspect ratios of 2.3 and iii) and vi) have aspect ratios of 5.1, and where i)-iii) were taken on the hexagonal (001) face of the crystal and iv)-vi) were taken on a portion of the side walls (100).

This can be related to the fact that the Si atoms substituting Al in the framework during the SiCl treatment originate outside the zeolite (i.e. from SiCl,), while in the steam/ acid treatment the corresponding silicon atoms originate in other parts of the zeolite crystals. This can also explain the absence of "secondary" pores in the material prepared with SiCl, as shown by sorption isotherms for different hydrocarbons (27). 

We also prepared a wide range of Py loading ranging from 0.007 to 0.182 for zeolite crystals with a length of approximately 650 nm. The crystals were modified with Ox+ acceptors as before. Figure 13b shows the emission spectra upon excitation at 470 nm. The spectra are scaled to the same height at the Py emission maximum, which allows better comparison. The Ox emission has its... 

Depending on the size of an incorporated dye, the angle of the transition dipole moment to the c axis lies between 0° for long molecules and 72° for smaller ones. Therefore, if a small molecule is inserted into the channels of zeolite L, part of the emission will be parallel to the c axis. Due to the flat and parallel ends of appropriately prepared zeolite crystals, one can envisage to arrange crystals between two mirrors or to add a reflecting layer on individual crystals. This might lead to a microlaser with a plane-parallel resonator. Apart from experimental difficulties, the realization of a dye-loaded zeolite L microlaser appears to be feasible. 

Kubo et al. (3) and Boudart et al. (4) showed the effectiveness of the zeolites for preparing well dispersed platinum catalysts. Ni(I)-zeolites were also subjected to hydrogen reduction the data gave strong evidence that nickel was not atomically dispersed and that metal atoms diffuse out of the pores to form crystals at the external surface of the zeolite (5,6). 

Two categories of mesoporous solids are of special interest M41S type materials and pillared or delaminated derivatives of layered zeolite precursors (pillared zeolites in short). The M41S family, first reported in early 1990 s [1], has been extensively studied [2,3]. These materials exhibit broad structural and compositional diversity coupled with relative ease of preparation, which provides new opportunities for applications as catalysts, sorption and support media. The second class owes its existence to the discovery that some zeolite crystallizations can produce a lamellar intermediate phase, structurally resembling zeolites but lacking complete 3-dimensional connectivity in the as-synthesized form [4]. The complete zeolite framework is obtained from such layered zeolite precursor as the layers become fused, e.g. upon calcination. The layers posses zeolitic characteristics such as strong acidity and microporosity. Consequently, mesoporous solids derived from layered zeolite precursors have potentially attractive characteristics different from M41S and the zeolite species... 

In order to utilize the absorption properties or the synthetic zeolite crystals in processes, the commercial materials arc prepared as pelleted aggregates combining a high percentage of the crystalline zeolite with an inert binder. The formation of these aggregates introduces macro pores in the pellet which may result in some capillary condensation at high adsorhate concentrations. In commercial materials, the inacropores contribute diffusion paths. However, the main pan of the adsorption capacity is contained in the voids within the crystals. 

Faujasite type zeolites were prepared from two NaY commercial samples (Si-to-Al ratio 2.4) with average crystal size of 0,80 (HY-100) or 0.30 Un by exchanging with solutions of ammonium acetate... 

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

In order to check this conclusion further, a number of (Si,B)-ZSM-5 zeolites were prepared via direct synthesis. The crystals of B-ZSM-5 are all very well developed with the length of about 200 pm (Figure 2). In the as-synthesized (Si,B)-ZSM-5 the chemical shift of boron is -3.95 ppm with FWMH=94 Hz [Figure 3 (b)]. The signal is narrow and symmetric because of small quadrupole interactions of tetrahedral B. The signals at -3.9 ppm in the spectra of boronated samples are also narrow (91 to 115 Hz, see Table I and Figure 1) and are clearly due to tetrahedrally coordinated framework boron. 

As shown in Fig. 2, the catalytic activity of the zeolite prepared by the direct heating method for methanol conversion was higher than that of the zeolite crystallization for 25 days by the standard preparation method. However, deactivation of the catalyst by carbon deposit occurred early in the reaction, just as with the catalyst prepared by the standard method. Differences in crystallite morphology between those prepared by the standard method and the direct heating method would be attributed to the stage of the precursor formation. Therefore, after the precursor formation the rapid heating was adopted as described below. 

Precursor heating method. The gel mixture was maintained at 100°C for 3 days for precursor formation. The precursor with the mother liquor was transferred to autoclaves, and the temperature was raised at a constant rate of 1.7°C,min 1 to 130, 160, 190, and 220°C. The temperature was maintained at each level for 0.5 h. The synthesized materials were also treated in the same manner as the standard preparation method. XRD patterns showed that the zeolites prepared at 190 and 220°C were ZSM-34 however, the zeolite prepared at 220°C contained some sodalite structure. The zeolites crystallized at 130 and 160°C had insufficient XRD intensity of ZSM-34 patterns and showed an activity of only DME formation. When the crystallization temperature was raised to 190°C, DME decreased to ca. 1/10, and C2-C, olefins increased dramatically. However, when the crystallization temperature was raised to 220°C, ethylene formation decreased markedly and DME increased. 

This paper describes chemical analyses at points across individual zeolite crystals in the size range 0.1-2.0pm. The technique employed was x-ray emission spectroscopy in the scanning transmission electron microscope (STEM). Two ZSM-5 preparations were made with Si Al ratios about 10 and 40. Many particles were examined carefully to detect chemical segregation. To check the analysis procedure, particles of NaA zeolite were examined as a control. 

Internal versus External and Extraframework Sites in Zeolite Acid Catalysis the Use of Hindered Basic Probes Catalytically active sites also exist on the external surface and at the pore mouth of zeolite crystals. These sites are considered to be responsible for unwanted non-selective catalysis. On the other hand, H-zeoUtes also catalyze reactions of molecules that do not enter the cavities because of their larger size. So, the external surface of zeolites is certainly active in acid catalysis. Additionally, the bulk and surface Si/Al compositions of a zeolite could be different and different preparation procedures can be chosen to modify this ratio. 

The entrapment-type nanocomposites can be prepared from zeolites and they are of two types zeolite-inorganic and zeolite-organic. Zeolite crystals are three-dimensionally linked network structures of aluminosilicate, aluminophosphate (ALPO), and silicoaluminophosphate (SAPO) composition and are porous, the pores being in the range of 2.8 to 10 A. Many of the highly siliceous, ALPO, and SAPO zeolites have been synthesized using organic templates such as tetrapropyl... 

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