The Fluoride Route

In silicate synthesis performed at high pH, the hydroxide ion acts as a miner-aliser, increasing the solubility of silica and catalysing condensation reactions. Fluoride ions can also perform this function, particularly in the synthesis of pure silica polymorphs of zeolites. The use of fluoride was recognised by Flanigen and Patton in 1978, and was developed further by Guth and Kessler in the early 1990s for silicates and phosphates. 

The most spectacular successes with the method were achieved by Camblor and co-workers in the mid 1990s with the synthesis of a series of silica polymorphs in the ITQ series of solids using the following general composition as a starting point  

The fluoride ions remain in the structure upon crystallisation so that the product solids have the general formula  

In one synthetic germanosilicate-fluoride system, it is possible to prepare mixtures of the A-, B- and C-polymorphs of zeolite Beta. In this case, the fluoride MAS NMR provides a powerful tool to establish the relative amounts of the C-polytype, in which the fluoride occupies D4Rs that are in the structure, compared to the A- and B-polymorphs, which have no D4Rs and in which the fluoride resides in [4 5 ] cages, with a clearly diflerent chemical shift. 

Zeolites with the FAU structure type can be prepared by direct synthesis with Si/Al ratios over a much narrower range (from 1 to ca. 5). The higher values refer to syntheses that use the crown ether 15-erown-5 as an additive. In these cases the sodium complex of 15-crown-5 is included in the structure. The lower charge density of the included complex compared to hydrated sodium cations requires less framework charge for charge balancing. Zeolite Y with much higher Si/Al ratios, required for catalytic applications, must be prepared by post-synthetic treatment (Chapter 6). 

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MFI-type zeolitee with Si PARTLY SUBSTITUTED by TIII(T=B. Al, Fe. Qa). The fluoride route can be used to prepare MFI-type zeolitea with... 

The fluoride route of synthesis seems very rich and already provided four among the six most open frameworks described up to now in the litterature in the category of microporous compounds (0 < A) VSB-1 [22] cloverite [11], ULM-5 [23] and ULM-16 [24] with tunnels limited by 24, 20,16 and 16 polyhedra respectively. These four solids are really porous, the porosity being generated by the elimination of the template either by thermal or chemical methods which preserve the inorganic framework. 

In the fluoride route, which was primitively applied to silica based and A1P04 and GaP04 materials, the first chemical parameter is the nature of the metallic species, since the reactivity of Si toward F is completely different from that of A1 or Ga. 

The fluoride route also works under "dry" conditions, i.e., in systems to which no water has been added [24, 25]. A typical synthesis system then consists only of silica, the SDA and NH4F, This solid mixture is filled into the autoclave and heated. Of course, the reaction of silica with NH4F to form fluoride-containing Si complexes liberates water, but the amount formed is very small. This may be important for SDAs that are sensitive to water. 

As for all other zeolite crystals prepared with the fluoride route, the crystals of the structure types FER TON and HTT are of very large size in the absence of seeds. The FER-type crystals are aggregates of plates, the size of which can be larger than 200 pm. The other structure types (TON and HTT) show a fibrous aspect, the length being over 100 pm for a diameter below 1 pm (Figure 5). 

Structure-Direction Issues in the Fluoride Route to Pure-Silica Zeolites... 

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). 

Villaescusa L, Camblor M (2003) The fluoride route to new zeolites. Recent Res Dev Chem 1 93-141 Wagner T, Waitz T, Roggenbuck J, Froba M, Kohl C-D, Tiemann M (2007) Ordered mesoporous ZnO for gas sensing. Thin Solid Films 515 8360-8363... 

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. 

The Fluoride Route A Good Opportunity for the Preparation of 2D and 3D Inorganic Microporous Frameworks... 

In addition to the materials listed in Table 16.1, several silica-based microporous materials were also synthesized using the fluoride route CJS-3[64] (certainly a clathrasil), ITQ-10 [81] (beta like) and ITQ-14 [81] (BEA-BEC overgrowth), PREFER (a lamellar precursor of ferrierite) [82], SSZ-61 (Standard Oil Synthetic Zeolite-61) [12, 83], Nu-86 [12], ZSM-48 [68, 86], ITQ-28 [85], and SSZ-70 (aborosilicate) [86]. 

Table 16.1 Silica-based ((Si) or (Si,AD) microporous materials prepared via the fluoride route in the alphabetic order of their code assigned by the Structure Commission of the International Zeolite Association (SC-IZA) [25]...

Table 16.2 Phase selectivity for pure silica zeolites prepared under the fluoride route as a function of the water content with three different OSDAs...

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