Crystallization silicalite

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. 

Based on the crystal size, yield and pH data, it is possible to postulate a model for silicalite crystallization in a high gravity environment. The primary effect of applying high gravity during the crystallization is to segregate the collodial silica... 

K.A. Carlsson, J. Warzywoda, and A. Sacco, Jr., Modeling of Silicalite Crystallization from Clear Solution. Stud. Surf. Sci. Catal., 2001, 135 (Zeolites and Mesoporous Materials at the Dawn of the 21st Century), 255-262. 

Original materials containing organic template were calcined for 3 h at 550°C. A Si/Al > 2000, silicalite, crystal size 370 nm. 

The dimensions of the silicalite which is crystallized are strongly influenced by changes in reactant concentrations (Fig. 2 and 3, Table 3). These differences are most pronounced in the aspect ratios (i.e. length/width) of crystals synthesized. Aspect ratios were found to differ considerably for the S- and T-series where the alkalinity was varied over an extensive range. For constant alkalinity, a rather constant aspect ratio is observed (B-series). Constant values of aspect ratios were observed for batches with identical alkalinities but where the source of silica was changed from precipitated silica to Ludox. High aspect ratios have been reported previously for silicalite crystallized at low alkalinities (19,20). 

The diffusion and equilibrium adsorption of aqueous alcohols in silicalite crystals have been studied using a novel HPLC technique. With a nonlinear mathematical model, the adsorption isotherms and intracrystalline diffusivities have been determined at 10, 30, 50, 70°C for ethanol, i-propanol, i-butanol, and at 30°C for n-propanol and n-butanol. The liquid intracrystalline diffusivities are found to be in the range of 10- to 10 11 cm /s and decrease in the folowing order n-butanol> n-propanol> ethanol >i-propanol> i-butanol. 

The mathematical model for the mass transfer of an adsorbate in the LC column packed with the silicalite crystal particles is based on the assumptions of (1) axial—dispersed plug—flow for the mobile phase with a constant interstitial flow velocity (2) Fickian diffusion in the silicalite crystal pore with an intracrystalline diffus— ivity independent of concentration and pressure and (3) spherical silicalite crystal particles with a uniform particle size distribution. A detailed discussion of these assumptions can be found in (13). The differential mass balances over an element of the LC column and silicalite crystal result in the following two partial differential equations ... 

The spherical symmetrical condition is applied to the silicalite crystal particle ... 

As the mass transfer resistance at the external surface of the silicalite crystal particle has negligible effects on the overall mass transfer in the present LC system (13), local equilibrium at the external surface thus can be assumed to simplify the mathematics involved in the numerical solution. Since the adsorption isotherm data of alcohols from dilute aqueous solution in silicalite, as reported by Milestone and Bibby (2), can be described by a Langmuir type equation, the following Langmuir type equation is used as the boundary condition at the external surface of the silicalite crystal particle ... 

An HPLC experimental technique has been developed to determine the intracrystalline diffusivity and nonlinear adsorption isotherm for the adsorption of liquids in molecular sieve crystals. The application of the experimental technique was demonstrated by the determination of the adsorption isotherms and diffusivities of the five alcohols (ethanol, i-propanol, n-propanol, i-butanol and n-butanol) from aqueous solutions in silicalite crystals. 

The addition of increasing amount of NaOH to a solution of high purity tetrapropylammonium hydroxide, was showed to produce an increase of the amount of Ti detected in the solid recovered at the end of the synthesis and a decrease of the 960 cm UR band [21]. At the highest level of sodium, the presence of anatase beside the silicalite crystals, was revealed by X-ray diffraction analysis. The presence of sodium in the reaction mixture seems to favour the formation of insoluble titanium-silicate species which reduce the amount of titanium available for the formation of the TS-1 crystals. 

Ga-MFI zeolites have been reported to exhibit catalytic activity in conversion of light paraffins to aromatics [3] and therefore the diffusion data of aromatics are of interest to explain the mechanism of the catalytic reaction.There has been done much of both theoretical and experimental investigations on the motion of aromatics (benzene, toluene, xylene etc.) in ZSM-5 zeolite and silicalite crystals cf.e.g. 

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