FLUORIDE SYNTHESIS MEDIUM

Table 1 summarises the list of ferrisllicate zeolites that have been prepared to-date by direct synthesis in basic media. Ferrisllicate pentasll zeolites have also been synthesised hydrothermally in an acidic, fluoride-containing medium [18,19]. Such samples, however, sometimes,... 

The yields are highly dependent on the synthesis method ALPO-5 samples synthesized in a fluoride-containing medium give lower conversions. The lower content of lattice defects in the iatter material is thought to be responsible for this effect. 

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). 

Direct Fluorination. This is a more recently developed method for the synthesis of perfluorinated compounds. In this process, fluorine gas is passed through a solution or suspension of the reactant in a nonreactive solvent such as trichlorotrifluoroethane (CFC-113). Sodium fluoride may also be present in the reaction medium to remove the coproduct hydrogen fluoride. There has been enormous interest in this area since the early 1980s resulting in numerous journal pubHcations and patents (7—9) (see Fluorine compounds, organic-direct fluorination). Direct fluorination is especially useful for the preparation of perfluoroethers. 

The Si-CHA samples were synthesized in fluoride medium and using /V.iV.iV-trimethyl adamantammonium (TMAda+) as structure-directing agent. The starting gel of molar composition 1 Si02 0.5 TMAdaOH 0.5 HF 3 H20 was introduced in a Teflon-lined stainless-steel autoclave and heated at 150°C during 90 h. After synthesis, the product was filtered, washed with distilled water and dried at 60°C overnight. The solid was... 

Table I illustrates the utility of DRUV-visible data in determining the surface structures involving Ti. Samples of TS-1 were prepared by three different methods or treatments. Samples 1 and 2 were prepared by conventional hydrothermal synthesis and sample 3 by synthesis in a fluoride medium. TS-2 was synthesized as reported (7). At least five bands could be discerned by deconvolution (Fig. 3), at 205, 228, 258, 290, and 330 nm. Band 1 at 205 nm is assigned to tetrahedral, tetrapodal Ti present in TS-1, TS-2, and Ti-beta. Band 5 at 330 nm is assigned to an...

Blasco et al. (12,13) developed a novel method for the synthesis of Al-free Ti-beta zeolite in a fluoride medium. The Ti-beta zeolite thus obtained (Ti-beta(F)) was free of connectivity defects and was hydrophobic. The typical unseeded synthesis of Al-free Ti-beta zeolite (Ti-beta(F)) involves hydrolysis of TEOS in aqueous solutions of TEAOH (35%) and H202, followed by hydrolysis of TEOT and evaporation of ethanol and water. The water lost in the evaporation and... 

The reaction of ferrocene and formaldehyde in either concentrated sulfuric acid or liquid hydrogen fluoride, followed by reduction, produces a compound containing two ferrocenyl and two methylene groups (57, 98, 123). After several incorrect assignments had been proposed for the structure of this condensation product, Rinehart and coworkers showed by an unequivocal synthesis that the product was 1,2-diferrocenylethane (XIX) (104). The mechanism of the reaction presumably involves the initial formation of ferrocenylcarbinol (XX) followed by ionization in the strongly acidic medium to the ferrocenylmethyl-carbonium ion (XXI). Conversion to radical ion XXII followed by dimerization and subsequent reduction produces the product. 

The synthesis of MCM-41 structure in fluoride medium has been reported by Silva and Pastore [10], who used sodium silicate as silica source and carried out crystallization at 150 °C. They suggested the behaviour of Si02-CTMA+-F system was significantly different from that reported previously on the mesophase system. It is known that fluoride ions do influence the nature, activity and polymerizing capacity of silica precursors, and a fluorinated silica surface is much more hydrophobic and more resistant to the attack of water molecules than a silanol silica surface [11]. 

In order to optimize the synthesis parameters, a series of experiments were carried out to study in detail the factors influencing the formation of integrated MCM-41 structure in fluoride medium, such as F7Si molar ratio, pH value, Na+/Si molar ratio, CTMABr/Si molar ratio, temperature as well as Al/Si molar ratio. In these experiments, the crystallization was performed at 100 °C using the molar composition of SiC 0.54 NaOH 0.50 CTMABr 0.34 HF 100 H20. 

Since it is known that the introduction of Al species leads to the synthesis of acidic Al-substituted MCM-41 materials, which are useful catalyst supports [2-4,13,14], the formation of Al-substituted MCM-41 mesoporous structure in fluoride medium is... 

The electrolytic method of synthesis has been used to prepare K2[Pt(CN)4](FHF)o.3o,JtH20. The medium used is a very acidic HF medium, and the added fluoride ion is present as FHF-(equation 100).293 The structure consists of columnar chains of square planar Pt(CN)1-60-groups separated by a Pt-Pt distance of 2.798(1) A.294 This distance is very close to that of 2.77 A in Pt metal. 

The primary thrust of the material in this chapter will be to indicate to researchers not already familiar with the details of superacid chemistry that, for the synthesis of many inorganic fluorides and other compounds containing fluorine, a favourable reaction medium can frequently be found by careful selection of an appropriate superacid, by deliberate control of the acidity or basicity of that medium and by use of suitable redox and other reactants and precipitants. 

---