Silicalite

Stable nano/mesoporous materials with mixed oxides such as zeolites, clays, and other minerals are widely used in various fields catalysis, adsorption, ion-exchange, separation, etc. because of their catalytic activity in acid/base and redox (e.g., materials with titania phase) reactions, ability to sorb selective molecules of diverse types, participate in ion-exchange reactions, providing sieve effects, etc. (Tanabe 1970, Grandjean and Laszld 1989, Rocha and Anderson 2000, Cundy and Cox 2005, Tao et al. 2006). The most important processes that utilize the selective properties of these materials are alkylation and isomerization of aromatic hydrocarbons as well as conversion of methanol to hydrocarbons, and some other reactions. Silicalite is an extreme type of the materials with the ZSM-5 zeolite structure but whose aluminum content is negligible. Therefore, unlike conventional zeolites, silicalite does not possess ion-exchange properties, and its surface has a weak affinity to water. 

The XRD studies of the samples prepared were conducted with an automated diffractometer DRON-UMl using Ni-filtered CuKa radiation. The comparison of the XRD patterns of the samples prepared with the literature data on ZSM-5 (Argauer and Landolt 1972) shows that the x-ray spectrum lines for these samples correspond to the spectral lines of ZSM-5. The degrees of crystallinity of samples 1 and 2 were 100% and 66%, respectively. The specific surface area of the silicalite samples measured by the method of LT adsorption of nitrogen was 312 and 240 mVg, 

FIGURE 2.85 Pore size distributions of silicalites calculated using NLDFT (Quantachrome software, version 2.02) and modified Nguyen-Do (MND) methods with the model of cylindrical pores. 

Chemical shifts were measured with reference to TMS with a powdered sample of the hydrated adsorbent being introduced into the deuterochloroform medium containing a small additive of TMS. This procedure did not lead to any variations in the intensity and shape of the adsorbed water signaL which gave ground to consider the values of the chemical shift of adsorbed water in the air and in the CDCI3 medium to be identical. The accuracy of chemical shift determination was 0.05 ppm. 

Nuclear Magnetic Resonance Studies of Interfacial Phenomena 

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Vlugt T J H, Krishna R and Smit B 1999 Molecular simulations of adsorption isotherms for linear and branched alkanes and their mixtures in silicalite J. Phys. Ohem. B 103 1102-18... 

Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

Flanigen E M, Bennet J M, Grose R W, Cohen J P, Patton R L, Kirchner R M and Smith J V 1978 Silicalite a new hydrophobic crystalline silica molecular sieve Nature 271 512-16... 

Bibby D M, Milestone N B and Aldridge L P 1979 Silicalite-2 a silica analogue of the aluminosilicate zeolite ZSM-11 Nature 280 664-5... 

Bell R G, R A Jackson and C R A Catlow 1990. Computer Simulation of the Monoclinic Distortion in Silicalite. Journal of the Chemical Society Chemical Communications 10 782-783. 

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.)...

Titanium silicalite Titanium silicates Titanium-silicon alloy Titanium slag Titanium suboxides... 

Fig. 8. Adsorption isotherms of H2O, O2, and / -hexane on 2eolite NaX (open symbols) and silicalite (filled symbols). Oxygen is at — 183°C and water and...

Many studies on template thermal degradation have been reported on zeolites of industrial interest including ZSM5 [1-5], silicalite [1], and beta [6-8], as well as surfactant-templated mesostructured materials [9-13]. The latter are structurally more sensitive than molecular sieves. Their structure usually shrinks upon thermal treatment. The general practice is slow heating at 1 °C min (N2/air) up to 550 °C, followed by a temperature plateau. 

A well-known example of the latter type is titanium silicalite-1 (TS-1), a redox molecular sieve catalyst [7]. 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

The excellent high para-selectivity can be explained by the selective escape of p-xylene from the H-ZSM-5 catalyst and inhibition of isomerization on the external surface of catalysts by silicalite-1 coating. In addition to the high para-selectivity, toluene conversion was still high even after the silicalite-1 coating because the silicalite-1 layers on H-ZSM-5 crystals were very thin. 

High catalytic activity and selectivity of silicalite-l/H-ZSM-5 composites must be caused by the direct pore-to-pore connection between H-ZSM-5 and silicalite-l as revealed by Fe-SEM and TEM [43]. The silicalite-l crystals were epitaxially grown on the surface of the H-ZSM-5 crystals. 

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. 

Figure 10.7 Pora-selectivity as a function in the conversion of the alkylation of methanol and toluene to xylene by bare and silicalite-coated H—ZSM5 catalyst particles.

The zeolite nanocrystals have attracted the considerable attention of many researchers [1-5]. The syntheses of several types of zeolites with different nanometer sizes, such as silicalite-1, ZSM-5, A-type and Y-type, have been reported. Recently, micellar solutions or surfactant-containing solutions have been used for the preparation of zeolite nanoerystals [4,5], We have also successMIy prepared silicalite nanoerystals via hydrothermal synthesis using surfactants. In this study, we demonstrate a method for preparing mono-dispersed silicalite nanoerystals in a solution consisting of surfiictants, organic solvents and water. 

Effect of the types of surfactants on the morphology of silicalite samples... 

On the other hand, in.the case of the nonionic surfactants C-15, NP-15 and 0-15 (the nonionic surfactant/cyclohexane system), mono-dispersed silicalite nanocrystals were obtained as shown in Fig. 1(c), 1(d) and 1(e), respectively. The X-ray diffraction patterns of the samples showed peaks corresponding to pentasile-type zeolite. The average size of the silicalite nanocrystals was approximately 120 nm. These results indicated that the ionicity of the hydrophilic groups in the surfactant molecules played an important role in the formation and crystallization processes of the silicalite nanocrystals. 

Effect of concentration of the Si source on the size of silicalite nanocrystais... 

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. 

Natural zeolites may bear the name of the mineral (mordenite, faujasite, ferrier-ite, silicalite), or sometimes that of the discoverer, e.g. Barrerite after Professor Barrer, or the place where they were found, e.g. Bikitaite from Bikita, Zimbabwe. Synthetic zeolites are usually named after the industry or university where they were developed, e.g. VPI comes from Virginia Polytechnic Institute, and ZSM stands for Zeolite Socony Mobil. 

The spectrum in Figure Id is for a crystalline form of silica, silicalite (Union Carbide S-115, see ref. 13). The structure is comprised of twelve silica tetrahedra linked into five pentasil groups and one hexasil group. This building block is repeated... 

Infrared spectra of silicas, a) Aerosil dried at 350 C, b) Aerosil as received, c) Aerosil slurried in water and dried at 100 C, d) Silicalite as received. 

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. 

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