Fluoride route, zeolite synthesis

From that time, the synthesis of zeolites by the so-called fluoride route has developed to a large extent, with great contributions by Guth et It has been successfully appHed to pure-silica, alumi-... 

However, there are at least two additional important structure-direction factors interplaying with that of fluoride in the synthesis of pure-silica zeolites by the fluoride route the OSDA and the degree of dilution of the synthesis gel. Villaescusa and Camblor have shown that the general concepts outlined above regarding structure-direction by OSD As can be successfully used in the search for new pure-silica materials, as shown for instance by the synthesis strategies that finally lead to the three-dimensional large pore zeolite ITQ-7. Nonetheless, we would like to point out at least one peculiarity, in this respect, of the fluoride route small cations may show by this route a rather specific structure-direction effect (like 1,3,5-trimethylimidazolium, which has a (C- -N)/N ratio of 8 but shows a rather large specificity towards zeolite ITQ-12 in fluoride aqueous medium). 

Synthesis may also be carried out at a lower pH using fluoride-containing media, wherein F ions are thought to act as structure directors via strong interactions with framework Si atoms. Consequently, the nucleation rate is decreased, which yields larger crystals relative to standard alkaline hydrothermal routes.The fluoride route under neutral/acidic pH conditions is also extremely useful to synthesize zeolite-like materials called zeotypes, which contain elements other than silicon and aluminum (e.g., titanosilicates, zirconosilicates, etc.). Under alkaline conditions, the precursors would be preferentially precipitated as hydroxide species rather than ordered arrays. 

Finally, the ionothermal synthesis route developed since 2004 by Morris et al. [187] was not successful for the silica-based materials but the ionic liquids themselves can act as classical OSDAs in aqueous media. Thus, by using the fluoride route, Zones et al. synthesized pure silica zeolites of TON, ITW and MTT topologies using the OSDAs l,3-dimethyl-37/-imidazol-l-ium, l,2,3-trimethyl-3//-imidazol-l-ium and 1,3-diisopro-pyl-3//-imidazol-l-ium, respectively [12]. Then, in 2009, we reported the synthesis of IM-16 (UOS) a germanosilicate prepared with the ionic liquid 3-ethyl-l-methyl-3//-imidazol-l-ium as OSD A in aqueous media. This microporous material possesses a new topology built from d4r and mtw composite building units [165]. The ionothermal synthesis route, up to now, was only successful in the formation of new phosphate-based microporous materials, as mentioned in the next paragraph. 

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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