Silicalite procedure

Indeed, several interesting procedures based on three families of active catalysts organometallic complexes, phase-transfer compounds and titanium silicalite (TS-1), and peroxides have been settled and used also in industrial processes in the last decades of the 20th century. The most impressive breakthrough in this field was achieved by Katsuki and Sharpless, who obtained the enantioselective oxidation of prochiral allylic alcohols with alkyl hydroperoxides catalyzed by titanium tetra-alkoxides in the presence of chiral nonracemic tartrates. In fact Sharpless was awarded the Nobel Prize in 2001. 

V-containing silicalite (Al- and Na-free) samples were prepared hydrothermally and then treated with an ammonium acetate solution at room temperature in order to remove extralattice vanadium. Three samples with Si02A 203 ratios of 117, 237 and 545, respectively, were prepared. Hereinafter these samples will be referred to as follows V-SH117, V-SU237 and V-SH545. Details on the preparation procedure, and characterization of the samples have been reported previously (7,2). 

N2 adsorption-desorption isotherms and pore size distribution of sample II-IV are shown in Fig. 4. Its isotherm in Fig. 4a corresponds to a reversible type IV isotherm which is typical for mesoporous solids. Two definite steps occur at p/po = 0.18, and 0.3, which indicates the filling of the bimodal mesopores. Using the BJH procedure with the desorption isotherm, the pore diameter in Fig. 4a is approximately 1.74, and 2.5 nm. Furthermore, with the increasing of synthesis time, the isotherm in Fig. 4c presents the silicalite-1 material related to a reversible type I isotherm and mesoporous solids related to type IV isotherm, simultaneously. These isotherms reveals the gradual transition from type IV to type I. In addition, with the increase of microwave irradiation time, Fig. 4c shows a hysteresis loop indicating a partial disintegration of the mesopore structure. These results seem to show a gradual transformation... 

Comparison with I in other zeolites. The same procedure for inclusion was used with B phenylpropiophenone on Silicalite and on molecular sieves (MS) 3A, 5A, 10X and 13X. No emission was observed on MS-3A or MS-5A, while the emission on MS-10X and MS-13X (under nitrogen) was 9% and 54% of that in Silicalite. 

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

NaOH 450 H2O at 95 °C, four identical non-stirred syntheses were carried out these syntheses were terminated after different time intervals. The composition of the solution was quantified at 95 °C by chemical trapping and the solid phase by XRD and elemental analyses (wt % Si, C, H, N). In this way a Si mass balance over the solution and the solid phase during the silicalite synthesis was obtained. The results are summarized in Table I, which also gives the results of a similar procedure for an analogous synthesis in the presence of DMSO. In this latter case, however, we were not able to filter the solution as thoroughly as in the first one, so that at the beginning of the synthesis a minor trace of amorphous material was present. 

Molecular mechanics (MM), molecular dynamics (MD), and Monte-Carlo (MC) methods were employed to simulate the adsorption of methane, ethane, propane and isobutane on silicalite and HZSM-5. The silicalite was simulated using the same cluster-model adopted in the diffusion calculations. The H-ZMS-5 structure was constructed according to the procedure suggested by Vetrivel et al. [32], which consists in replacing one atom at the channel intersection by and protonating the oxygen atom bridging the Ta and Tg sites in order to preserve the lattice neutrality. 

From our earlier experiences, we have learned that the zeolite seed hydrothermal approach is rather difficult to reproduce. Consequently, a dvee-step-synthesis procedure involving the preparation of zeolite nanoprecursors (NPs) by a short hydrothermal step, the flocculation of these NPs using a sui ctant, and the steaming of the NPs/surfactant composite to produce the final material was developed. We have recently demonstrated that aggregates of less than 30 nm silicalite nanocrystals can be prepared from this procedure. We further discovered that the nature of the as-collected NPs was very much dependent on the stirring time of NPs/CTAMeBr flocculants. Under identical steaming condition, the 3 h-stirred NPs were converted into nanocrystals of silicalite-1, whereas. 

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... 

The membrane should be a continuous layer, free of defects. Fortunately, due to the synthesis procedure, this can be easily checked for the silicalite-1 membrane. After synthesis the template molecule, tetrapropyl ammonium hydroxyde, is still present and blocks all the pores. This is removed by thermal decomposition in air ( calcination ) at 673 K. During calcination the membrane is placed in the cell and a mixture of Kr in air is used for calcination. A good membrane does not permeate Kr at room temperature and should develop permeability for Kr during the calcination procedure. Ths is illustrated by Figure 13 which shows the permeation development of Kr as a function of time, at a heating rate of 1 K/min. Already around 500 K some permeation is observed, but the large breakthrough is observed above 600 K, until a steady-state permeation level is reached. 

Figure 4.40 The procedure for preparing silicalite-l microspheres using anion-exchange resins as macrotemplates. Reproduced with permission from [147]. Copyright (2000) Elsevier...

Jia/Noble and coworkers [87,88] reported the successful synthesis of silicalite membranes on y-alumina composite supports using an interesting modification of the in situ crystallisation method. The support consisted of a short a-alumina tube coated on the inside with a 5 pm thick y-alumina film with an average pore diameter of 5 nm, commercially available from US Filter. The precursor solution was put into the support tube after plugging both ends with teflon and the filled tube was then placed in a teflon-lined autoclave. Hydrothermal treatment was carried out at 180°C for 12 h. After removal from the autoclave and washing the formed zeolite layer with water, the procedure was repeated with the tube inverted from its previous orientation to obtain a uniform coating. As reported by Vroon et al. [82,84,98], Jia/Noble [88] also concluded that at least two synthesis steps are necessary to obtain defect-free membranes. 

The procedure of Zhdanov and Samulevich enables the calculation of isothermal nucleation rate profiles from determinations of growth rate and crystal size distribution [16,82]. Originally implemented in analyses of zeolite Na-A [83] and Na-X [82] crystallisation, the method has subsequently been applied to other zeolite systems, including silicalite [84,85]. If it is supposed that all the crystals in a batch have the same (known) growth rate behaviour, the total growth time of each crystal can be calculated. Assuming also that the nuclcation point for each crystal can be obtained by linear extrapolation to zero time, the nucleation profile for the whole batch can be determined from their final sizes. 

Extruded silicalite zeolite honeycombs were also prepared by well known procedures (9,10). Silicalite zeolite powder (Union Carbide Corp. S-115) was extruded with a boehmite alumina (Vista Chemical Catapal D) which forms a gamma alumina binder phase after heat treatment. The final composition is 84% silicalite + 16% gamma alumina. These extruded honeycomb samples were ion-exchanged and evaluated in the same way as the washcoated ones. 

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