Fluoride-silicalite

For LZ-105 (28), there is very little observable change in the X-ray spectra before and after treatment. Full retention of crystallinity is evident. However, reduction in intensity for the peak at 20 = 24.4° is noted. Such an intensity change is also very prominent in the spectrum of fluoride silicalite, a silica polymorph synthesized in the presence of a fluoride salt (29). 

This review summarizes some highlights of recent X-ray structural analyses of varieties of zeolite A and of fluoride-silicalite. 

Figure 4. Difference-Fourier maps of electron density associated with non-framework atoms in the precursor to fluoride-silicalite (a) section in mirror plane at y 0.75 (b)...

Silicalite-1 was synthesised according to published recipe [5], Fumed silica was added to an aqueous solution of tetrapropylammonium bromide and ammonium fluoride, and... 

There is a = 4% difference between our value of the solution enthalpy of the highly siliceous calcined si] -MFI sample and the value published recently by Johnson et al. (29) for the dissolution in 24. i% HF of a sample of silicalite I prepared in alkaline medium in the absence of fluoride (-144.93X0.18 kJ/Si02 for a Si02/HF ratio equivalent to the ratio used here). Such a difference could be related to the nature of the samples and to morphology differences. Our calcined sample contained traces of fluorine whereas the silica-... 

Isotherms for H20 and 0-liexane adsorption at room temperature and for 02 adsorption at liquid oxygen temperature on 13X (NaX) zeolite and on the crystalline Si02 molecular sieve silicalite are are shown in Figure 8 (43). Silicalite adsorbs water very weakly. Further modification of silicalite by fluoride incorporation provides an extremely hydrophobic adsorbent, shown in Figure 9 (44). These examples illustrate the broad range of properties of crystalline molecular sieves. 

Figure 4 S l-picture of fluoride-syntheszed silicalite prisms, embedded as a monolayer in a thin cl film on a one layer a-ahimina support...

On the other hand, we have recently investigated the role played by alkali cations in the synthesis of silicalite-1 in presence of fluoride ions [12]. 

We report here the study of the influence of alkali cations (NH4+, Na+, K" " and Cs+) on the incorporation of boron in silicalite-1 structure in fluoride containing media. 

From this study, it can be concluded that higher than 4 B/u.c. can be introduced into silicalite-1 using a fluoride containig medium in presence of either K+ or Cs+ ions. We can see now that the maximxnn of 4B/u.c. observed in... 

Usually, the sources of fluoride are NH4F, NH4HF2, or HF. Sometimes, a framework element compact fluoride such as (NH4)SiF6, A1F3 H20, or NH4BF4 could be used as a fluoride source as well. The synthesis of heteroatom-substituted silicalite-I and ZSM-5 are described below as two typical examples ... 

J.M. Popa et al. described the preparation of a titanium containing silicalite from a reaction mixture having a very low pH (6.5 - 7.5) and containing fluoride anions [26]. The crystalline product obtained by this method had a monoclinic symmetry. No evidence was given to support the presence of titanium in lattice position. 

Pure silica end-members may be considered as special cases of aluminosilicate zeolites. They may be prepared directly from hydrothermal synthesis and in some cases from aluminosilicates by post-synthetic treatment. For example, the pure silica analogue of ZSM-5 (Silicalite-1) is readily prepared by direct synthesis, whereas purely siliceous zeolite Y can only be obtained by postsynthetic treatment (Chapter 6). The microstructures present in these solids depend on the way in which they are prepared. For direct preparation routes the presence or absence of fluoride as a mineraliser in the preparation (see Chapter 5) determines whether the framework is prepared defect-free or with high concentrations of terminal silanol (SiOH) hydroxyls, where silicon is attached to three bridging oxygen atoms and a hydroxyl group. Post-crystallisation preparation of pure silica zeolites can be achieved by treatment of appropriate starting materials with silicon tetrachloride or by removal of aluminium from the aluminosilicate framework by heating the ammonium form in steam (Chapter 6). 

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