Silicalite-1 crystal structure

Mono-dispersed silicalite and ZSM-5 type zeolite nanocrystals with a diameter of 80-120 nm were successfully prepared in a surfactant-oil-water solution. The ionicity of the surfactants used in the preparation affected the crystallinity and structure of the silicalite crystals, and silicalite nanocrystals could he obtained when using a nonionic sur ctant. By adding an A1 source into the synthetic solution, ZSM-5 type zeolite nanocrystals with strong acid sites could be obtained. 

From this modelling approach it seems that the combined surface difiusion and activated gas translational difiusion can describe the observed single component permeation behaviour. The interpretation for the latter type of diffusion is not well-crystallized at present. It might have to do with an increasing deformation of the silicalite-1 structure with increasing temperature that causes this apparently activated process. This aspect has not been considered up to now, the silicalite-1 has been considered as a rigid structure. 

Another approach to minimizing the problem of metal peroxide dissolution was to incorporate a redox metal into a zeolite by replacing some or all of the aluminum in the framework. Of these redox zeolites, the most common catalysts for peroxide oxidations are the titanium silicalites, TS-1 and TS-2 (Chapter 10). 14,15,19-23 TS-1, the more generally used material, has a crystal structure analogous to ZSM-5 with two dimensional channels of 0.55-0.60 nm in... 

The work of Sanders et al. (1984), Jackson and Catlow (1988) and Tomlinson et al. (1990) showed that with Born model potentials it is possible accurately to reproduce the crystal structures of silicates including zeolites. A typical example is shown in Plate I which illustrates the calculated and experimental structures of a purely siliceous (i.e. pure Si02 polymorph) zeolite, silicalite. (A closely related, isostructural material, ZSM-5, which contains a low concentration of Al is an effective isomerization and hydrocarbon synthesis catalyst.) The agreement between theory and experiment is evidently good more discussion follows in Chapter 9. 

The zeohte overgrowth has been reported for FAU on EMT zeohte [44] and MCM-41 on FAU zeohte [45]. On the other hand, in this study, zeohte layers were grown on the zeohte with the same framework structure, resulting in high coverage of ZSM-5 crystals with silicalite layers and high para-selectivity. The zeohte crystals with oriented thin layer on their external surface are expected to form a new class of shape-selective catalysts. 

Nature of Vanadium Species. The data on the characterization of V- containing silicalite indicate the presence of various types of vanadium species (i) a polynuclear V-oxide containing V in various valence states (V , V and V ), (ii) octahedral sites, preferentially interacting with OH groups localized inside the pore structure of the zeolite crystals, (iii) nearly symmetrical tetrahedral species, attributed to a V species directly interacting with the zeolite framework, and (iv) after reduction, V species in a nearly tetrahedral environment. 

TS-1 has been obtained by the hydrothermal crystallization of a gel obtained from TEOS and TEOT in the presence of tetrapropylammonium hydroxyde (TPA-OH). The structure of TS-1 has been demonstrated by X-ray diffraction (XRD), energy dispersive X-ray (EDX), microprobe analysis, and 29Si magic-angle spinning (MAS) NMR spectroscopy. Furthermore, an absorption band in the IR spectrum at 960 cm-, present in TS-1 and absent from that of silicalite, was initially considered a fingerprint for the characterization of TS-1. However, later work (discussed below) has shown that this band is also present in many other silica compounds, and therefore its relation to framework Tilv is not straightforward. 

Titanium Silicalite-2 (TS-2), structurally similar to TS-1, could be prepared likewise using tetrabutylammonium hydroxide as the template [13, 14]. Titanium aluminum Beta (Ti,Al-[3) was prepared by hydrothermal synthesis from amorphous silica, sodium aluminate, tetraethyltitanate and tetraethylammonium hydroxide [15]. The presence of A1 was necessary for the crystallization of the product. Al-free Titanium Beta (Ti-[3) could be obtained in the presence of particular templates, such as dibenzyldimethylammonium hydroxide [16]. Titanium Mordenite (Ti-MOR), conversely, was obtained by post-synthesis insertion of Ti to dealuminated Mordenite [17]. Ti-MWW (Ti-MCM-22) was obtained by the synthesis of the lamellar precursor of Ti,B-MCM-22 followed by acid treatment to remove most of the boron and extra-framework Ti and finally calcination to burn out the template and bring about the condensation of lamellae into the three-dimensional MWW structure [18]. Ti is present in a number of different environ-... 

Various redox metals, including Ti, V, Cr, Mn, Fe, Co, Cu, Zn, As, Zr and Sn, have been incorporated into microporous materials such as silicalites through hydrothermal synthesis by the addition of the respective cations to the synthesis gel. The disadvantages of this method include the time-consuming optimization of synthesis procedure for each metal-zeolite combination and the necessity of A1 for crystallization of certain structures. The presence of A1 leads to Bronsted acidity... 

Finally, zeolite nanoparticles have been used as building blocks to construct hierarchical self-standing porous stmctures. For example, multilayers of colloidal zeolite crystals have been coated on polystyrene beads with a size of less than 10 p,m [271,272]. Also, silicalite-1 membranes with a thickness ranging from 20 to several millimeters and controlled mesoporosity [273] have been synthesized by the self-assembly of zeolite nanocrystals followed by high-pressure compression and controlled secondary crystal growth via microwave heating. These structures could be useful for separation and catalysis applications. 

Two-dimensional DQ H MAS-NMR spectra were used to investigate the local structure of a surfactant-templated silicate thin film.498 Silicalite-1 powders (20-1000 nm. crystals) were characterised by 29Si MAS-NMR spectroscopy.499 13C and 29Si solid-state NMR data were used to characterise organic-inorganic mesoporous silica-based materials.500... 

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