Zeolite silicalite

Fig. 8.22 Schetnatic structure of the zeolite silicalite showing the straight and zig-zag chaimels. (Figure adapted fron Smit B and JI Siepmann 2994. Simulating the Adsorption of Alkanes in Zeolites. Science 264 1118-1120.)...

This paper describes the morphological and transport properties of a composite zeolite (silicalite) - alumina membrane. Some advantages obtained in combining the membrane with a conventional fixed-bed catalyst are also reported. 

Ail these data confirm that a well-defined zeolite silicalite-type crystalline phase has been formed in the presence of the alumina porous tube (which seems indispensable for the zeolite synthesis, as no material is formed in its absence). 

Comparison of intracrystalline diffusivities between Si-MCM-41 and zeolites (silicalite and NaX) ... 

Unlike ordinary zeolites, silicalite is organophilic and hydrophobic and can remove from water a variety of dissolved organic compounds. Both ZSM-5 and silicalite display remarkable shape selectivity because of the geometry of the channels only certain reactants may enter and diffuse through the crystals, and only certain products may diffuse out of the intracrystalline space. 

Figure6.4 Simulated structure ofthe MFI-type zeolite Silicalite-1 (O atoms in dark gray and Si atoms light gray), projected on the be plane and showing the zigzag channels. The broken lines indicate the periodic cell boundaries. Alkane molecules (not drawn to scale) are indicated by black circles. Thanks to Dr. Merijn Schenk for the zeolite picture.

Figure 2. Zeolite silicalite membrane [34], Separation behavior of a n-hexane 2-2-dimethylbutane mixture (1 1) as a function of temperature.

Of interest with respect to this hypothesis is the significant difference in heat of paraffin adsorption between the medium-pore zeolite silicalite and large-pore, de—aluminated faujasite. The heat of paraffin adsorption is much smaller in the case of the de-aluminated faujasite, which has so far had to be prepared by an indirect route, than for silicalite, which can be synthesized direct in the presence of an organic molecule. The difference, which increases with chain length, is of the order of 5 kJ/mol per CH2 unit, and may be ascribed to the optimum fit of hydrocarbon and channel in the case of the medium-pore zeolite (H, 12). ... 

In 1978, the same year that the structure of ZSM-5 was first described, Flanigen and her co-workers reported the synthesis, structure and properties of a new hydrophobic crystalline silica molecular sieve (Flanigen et al., 1978). The new material, named Silicalite (now generally called Silicalite-I), has a remarkably similar channel structure to that of ZSM-5 but contains no aluminium. It was pointed out by the Union Carbide scientists that, unlike the aluminium-containing zeolites, Silicalite has no cation exchange properties and consequently exhibits a low affinity for water. In addition, it was reported to be unreactive to most acids (but not HF) and stable in air to over 1100°C. 

R. C. Runnebaum and E. J. Maginn, /. Phys. Chem. B, 101, 6394 (1997). Molecular Dynamics Simulations of Alkanes in the Zeolite Silicalite Evidence for Resonant Diffusion Effects. 

This novel route involves the retention of colloidal dispersions of zeolites (silicalite-1 described here) onto the surface of macroporous ceramic substrates. Silicalite-1 colloids were synthesised as described by Schoemann [14]. These were characterised by SEM and filtered through ceramic alumina discs. 

PFG (pulsed field gradient) NMR data determined the diffusion coefficients of methane in 3 MFI-type siliceous zeolite silicalite samples.640 The rotational motion of butane and pentane molecules adsorbed on zeolite ZK-5... 

Zeolite (silicalite) on steel 1-3x10 100400 Bakkeret al. [71] Thickness 50 pm... 

There has been considerable interest on the influence of zeolites on several aspects of the photochemical behaviour of ketones. Turro et al. have carried out further work on the influence of zeolites on the photochemical activity of dibenzyl ketones. In this study pentasil zeolites were used. In other work Turro has reported the use of triplet-triplet energy transfer as a probe of surface diffusion rates. The luminescent properties of some aromatic ketones in the presence of the hydrophobic zeolite Silicalite have been examined and a spectral study of trsmsients formed by the irradiation of aryl ketones in zeolites has been reported. ... 

Zeolites Silicalite and zeolite NaY ZEOLR HEN- ERGIE Only van der Waals terms... 

Based on the quite remarkable results obtained with TS-1 we expect that many more examples of microporous solids modified by isomorphous substitution with redox metal ions in the crystal lattice will be described in the future (see Fig. 12). Indeed, the scope for developing unique oxidation catalysts based on the concept of site-isolation in zeolites, silicalites, alpos and sapos is enormous [37], In addition to varying the redox metal the size and hydrophobicity of the cavity can be tuned by, for example, varying Si/Al ratios to provide a variety of unique heterogeneous catalysts for liquid phase oxidation. 

In the relentless quest for ever faster computer circuitry, the dielectric constant of the insulating layers between conductors on the chip is becoming a major issue. This constant should be as small as possible while the mechanical properties of the dielectric material must withstand the subsequent processing steps and ensure the integrity of the computer microprocessor. Nanoscale zeolite crystals, particularly pure silica zeolites, have been proposed as candidates for thin films with low dielectric constant (low k). As an example, suspensions of nanoscale crystals of the pure zeolite silicalite-1 (MFI-typc) were used for spin-on deposition of thin dielectric layers.[101] The as-deposited films were subsequently calcined at 450 °C in order to remove organic molecules and to consolidate the films. The authors report low dielectric constants (although the adsorption of humidity must be controlled) and satisfactory mechanical properties of their films. 

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. 

Fig.1 The structure of the medium-pore zeolite silicalite-1 (left) and a schematic representation of the pore system which consists of straight and zigzag channels...

Other substances exhibit the characterisations of zeolites. Silicalite is the zeolitic form of silica, and an hexagonal mesoporous silica (Hi-Si02) has been prepared and characterised. Aluminium phosphate gives rise to a family of ALPO zeolites, but they lack useful acidity. Much interest has been shown in the... 

H2O, CO2 and N2 removed from air by non-cryogenic pressure swing adsorption over LiX zeolite, leaving > 95% O2 High silica zeolites adsorb unburnt hydrocarbons and desorb them as the engine and the catalytic converter warm up Shape selective Ca-A zeolite (5A) adsorbs linear but not branched hydrocarbons Shape selective MFI type zeolites (silicalite) adsorb para- but not ortho- or meta-xylenes. FAU type zeolites are also effective for this separation under simulated moving bed conditions... 

Kong C, Lu J, Yang J, Wang J. Catalytic dehydrogenation of ethylbenzene to styrene in a zeolite silicalite-1 membrane reactor. J Membr Sci 2(X)7 306 29-35. 

Figure A2.3 (a) Structure of an all-silica zeolite silicalite, MFI framework (b) Speed-up of the total energy and wavefunction calculation with the number of processors in a cluster of AMD Athlon 1.9-GHz single-processor, 1-GB RAM, 512-KB cache, EIDE disks. The code was compiled with PGE90 v4.2 and -01 -tp athlon options.

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