Zeolite crystal, large

Large zeolite crystals with dimensions of tens and hundreds of micrometers have proven to be irreplaceable as model materials for reactivity and diffusion studies in the field of zeolite science and heterogeneous catalysis [1-3], These large crystallites often possesses complex structures consisting of several intergrown subunits and since the pore orientations of the different elements are not always aligned, this phenomenon can have a considerable effect on the accessibility of the pores in different crystallite regions [4]. 

In order to illustrate the general applicability of the methodology we have extended our approach to other large zeolite crystals, such as SAPO-34, SAPO-5 and ZSM-5. Our study on the rhombic SAPO-34 crystals reveals a four-pointed star fluorescence pattern at 445 K, which is transformed into a square-shaped feature at 550 K. This is illustrated in Figure 4a. Confocal fluorescence slices, summarized in Figures 4b-d, recorded at different temperatures show the cubical pattern, which proceed from the exterior of the crystal inwards. Both observations are consistent with a model which involves six components of equal tetragonal pyramids as illustrated in Figure 3b. 

Synthesis procedures to obtain large zeolite crystals are well developed (1,2). In particular much attention has been paid to the synthesis of ZSM-5 crystals (3-6). Elongated prismatic (Fig. la) and cubic-shaped orthorhombic (Fig. lb) ZSM-5 crystals of sizes between 2-50 /tm were reported in the first recipes (7) in the patent literature. Later on, systematic studies have led to excellent synthesis prescriptions for the growth of large crystals of the prismatic (8) as well as of the orthorhombic form (9). The synthesis parameters which are dominant in the crystallization of pure ZSM-5 single crystals, are still under study (10,11). 

Those results indicate two st lges of zeolite crystallization in silica pellets formation of small zeolite ZSM-5 crystals (0.025 pm) in the silica mesopores and large zeolite crystals (1-4 pm) in the silica macropores and on the outer surface of silica pellets followed by faster crystallization in the interior of the carriers pellets. At this second stage the mesopores and small zeolite crystals inside them do not grow but the pellets structure fails forming large slabs of amorphous silica with incapsulated zeolite crystals not accessible for adsorption and catalysis. 

First-of-its kind results [14,15] with electron tomography for the study of zeolites have been obtained with Ag/Na-Y. The location of the Ag particles of about 10 nm could be unequivocally established with respect to the surface and the interior of the crystals. The moderate resolution of 5 nm of the reconstructed images in this study related to the rather large zeolite crystals (500 nm) involved. Hereafter, we first discuss in more detail the use of electron tomography to study mesopores in zeolite crystals and, secondly, case studies involving metal particles and pore architecture of ordered mesoporous materials. 

Measurements by interference microscopy are, under favorable conditions, capable of yielding both internal diffusivities and apparent diffusivities based on overall sorption rates. The former tend to approach the values obtained from microscopic measurements while the latter yield values similar to those obtained by other macroscopic methods. Of necessity these studies have been carried out in large zeolite crystals. One may expect that smaller crystals may be less defective, although the influence of surface resistance may be expected to be greater. The extent to which these conclusions are applicable to the small zeolite crystals generally used in commercial zeolite catalysts and adsorbents remains an open question. 

At the present state of development the mesoscopic techniques are applicable only to relatively large zeolite crystals (> 100 p,m), so the extent to which surface and internal transport barriers are important in small commercial crystals is still uncertain. 

All measurements in the present contribution have been performed on large crystals of silicalite-1 and H-ZSM-5 zeolites. Use of large zeolite crystals simplifies significantly the modeling and experimental procedure because it allows one to avoid pelletizing of the crystals. This has the important advantage that no macropore diffusion has to be included in the hydrocarbon transport models and intracrystalline diffusion is the dominating process. 

Table 5.4 Summary of routes to large zeolite crystals (after Lethbridge...

Desilicated Fe-ZSM-5 samples have shown enhanced NjO decomposition activity [187]. This has been assigned to the full exchange of iron in desilicated ZSM-5 samples because of its enhanced accessibility without forming iron oxides. In contrast, for large zeolite crystals, Fe exchange is diffusionally controlled and leads to the deposition of inactive hydrolysis-formed iron species. 

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