Silicalite from clear solution

The quantification of D5R and other silicates during a silicalite synthesis at 95 °C from clear solution. 

Silicalite Synthesis from Clear Solution. In order to check whether the above findings on the presence of polymeric species in solution and on the fast rate of exchange between the smaller silicates are also applicable in a real zeolite synthesis mixture, we have studied a silicalite synthesis in more detail. Starting from a clear, filtered solution of molar composition 25 Si02 9 TPAOH ... 

Second, the elemental analyses of the ZSM-5 samples (20-261 prepared via standard synthesis routes do not point to missing T sites defects since these samples contain the normal (1 ) value of 3.5 - 4 TPA entities per 96 T sites (see Table IV). This observation has been confirmed by thermogravimetric measurements. Therefore, the defects in these materials are more likely to originate from hydrolysed SiOSi linkages. Interestingly, the silicalite sample prepared from clear solution at low temperature (sample 27, Table IV) most probably does contain missing T sites defects. This assertion is based on the observed high -103 ppm NMR intensity and the occlusion of more than 4 TPA entities per 96 T sites (see also Table I, samples 1-4). 

In the past 10 years, many characterization techniques have been used in the in situ study of the formation mechanism of zeolite from clear solution. For example, Honssian et al. studied the TPA-silicalite-1 system by using the small-angle X-ray scattering technique [34] Carlsson et al. performed a modeling study on the crystallization of silicalite-I from a liquid-phase system [35] Smaihi and colleagues applied in situ 27A1-,... 

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. 

P-l6 - Modeling of silicalite crystallization from clear solution... 

Fig. 7. Schematic illustration of the model for nucleation of silicalite from clear synthesis mixtures a TPA-silicate clusters in solution, b primary fractal aggregates formed from the TPA-silicate clusters, c densification of the fractd aggregates from b above, d combination of densified aggregates into a second fractal aggregate structure, and e densification of the second fractal aggregates followed by crystal growth. Figure redrawn with permission from [58]...

Table 7. Comparable studies from the literature, regarding the crystallization of ultra-small silicalite-1 crystals from clear solutions [59,60] ...

Batch oxide molar chemical composition of the reaction mixture (hydrogel) for the synthesis was 1.0 AI2O3/IOO Si02/28 Na20/4000 H2O. A series of silicalite-1 nanocrystals having different mean diameters (90, 180, 220, 260 and 690 nm Fig. 2) were prep>ared by synthesis from clear solution and used as seeds for the further growth of ZSM-5 nanocrystals. 

Samples 1-7 (silicalite) were prepared in a Teflon bottle at 95 °C, under static conditions from a homogenized, filtered, and clear solution of molar composition 25 SiC>2 (ex silicic acid),... 

Schoeman BJ, Erdem-Senatalar A, Hedlund J, and Sterte J. The growth of sub-micron films of TPA-silicalite-1 on single crystal silicon wafers from low-temperature clear solutions. Zeolites 1997 19(l) 21-28. 

The high preference of the crystalline silicic acids for methanol and ethanol from water is clearly expressed when the alcohol molar fraction in the adsorption layer, xi, is plotted against this fraction, xi, in the equilibrium solution (Fig. 27). Such a high preference for methanol and ethanol from aqueous solutions was also found for silicalite and ZSM-5 zeolites [171-174]. The parent compound K2Si2o04i XH2O also takes up high amounts of ethanol from dilute ethanol/ water mixtures [108]. 

It is possible to formulate a conceptual model of the mechanism for the zeolite crystal growth process, based on the various pieces of information that have been reported in research studies to date for silicalite synthesis in clear solutions. It is speculated that this system is representative of zeolite crystal growth, that the knowledge will be applicable to other systems, and that seemingly different features of other systems stem from differences in degree rather than mechanism. Points which must be kept in mind are ... 

The most common hydrophobic adsorbents are activated carbon and silicalite. The latter is of particular interest since the affinity for water is very low indeed the heat of adsorption is even smaller than the latent heat of vaporization (3). It seems clear that the channel structure of silicalite must inhibit the hydrogen bonding between occluded water molecules, thus enhancing the hydrophobic nature of the adsorbent. As a result, silicalite has some potential as a selective adsorbent for the separation of alcohols and other organics from dilute aqueous solutions (4). 

The origin of the catalytic oxidative activity of the the Sn-siljcalites is not clear at the moment. It may be due to the reduction of isolated Sn " to Sn, which is then oxidised back with H2O2. Also, many hydroperoxides of tin have been known from the action of H2O2 upon solutions of Sn2+ and Sn4+. With our Sn-silicalites, however, there was no evidence for the dissolution of Sn under the reaction conditions as they have been regenerated after the reaction and reused several times without significant loss of catalytic activity. Surface tin hydroperoxides may be the active species but further detailed studies are required before possible mechanisms of oxidation involving Sn could be discussed. 

---