SYNTHESIS IN FLUORIDE MEDIUM

The development of the synthesis in fluoride medium has been studied in connection with the new possibilities opened by this method (incorporation in the framework of elements sparingly soluble in alkaline medium, synthesis without alkaline cations, new possiblility to directly incorporate cations such as NH4+, divalent cations, the good stability of usual templates in this medium,...). 

Finally, while most frequently an OSDA acts as a single molecule it is also possible to use a supramolecular assembly. Most interestingly, a n-n supramolecular assembly of two identical cations afforded the synthesis in fluoride medium at high concentration of the pure-silica analogue of zeolite LTA, which had defied synthesis for half a century. 

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... 

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... 

J. Patarin, H. Kessler, and J.L. Guth, Iron Distribution in Iron MFI-type Zeolite Samples Synthesized in Fluoride Medium - Influence of the Synthesis Procedure. Zeolites, 1990, 10, 674-679. 

Patarin, J., Kessler, H. and Guth, J.L. (1990) "Iron distribution in iron MFI-type zeolite samples synthesized in fluoride medium. Influence of the synthesis procedure". Zeolites, 10 (7), 674-679. 

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... 

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,... 

An aqueous Friedel-Crafts reaction has also been used in polymer synthesis. The acid-catalyzed polymerization of benzylic alcohol and fluoride functionality in monomeric and polymeric fluorenes was investigated in both organic and aqueous reaction media. Polymeric products are consistent with the generation of benzylic cations that participate in electrophilic aromatic substitution reactions. Similar reactions occurred in a water-insoluble Kraft pine lignin by treatment with aqueous acid. A Bisphenol A-type epoxy resin is readily emulsified in aqueous medium with an ethylene oxide adduct to a Friedel-Crafts reaction product of styrene and 4-(4-cumyl)phenol as emulsifier.Electrophilic substitution reaction of indoles with various aldehydes and ketones proceeded smoothly in water using the hexamethylenetetramine-bromine complex to afford the corresponding Z A(indolyl)methanes in excellent yields.InFs-catalyzed electrophilic substitution reactions of indoles with aldehydes and ketones are carried out in water.Enzymatic Friedel-Crafts-type electrophilic substitution reactions have been reported. ... 

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). 

The synthesis of Sn-beta (68,151) with isolated single tin sites was accomphshed in a fluoride medium TEOS was hydrolyzed in a stirred aqueous solution of TEAOH. Then a solution of SnC 5H2O in water was added and the mixture stirred until the ethanol formed by hydrolysis of the TEOS had evaporated. HF was added to the resulting clear solution, and a thick paste formed. Then a suspension of nanocrystalline seeds (20 nm in diameter) of dealuminated zeolite beta in water was added. The crystallization was carried out in aTeflon-hned stainless-steel autoclave, which was heated to a temperature of 140 °C and continuously rotated for 20 days. After drying and calcining of the resultant sohd, XRD showed that a highly crystalline material with BEA structure was obtained, which contained 1.6 wt% Sn according to chemical analysis. Further characterization by XRD and UV, IR, and Sn MAS NMR spectroscopies verified the structure and the isomorphous substitution of siHcon by tin. 

In this rather new field of zeolite synthesis [5], preliminary information on precursors in solution has been obtained by measuring the free fluoride ion concentration with a specific fluoride anion electrode. The mean number N of F bonded to one Si or other T atom (T = Al, Fe, Ga) has been computed and results are summarized in Table 2. No information on the exact nature of the species in solution is available discussed in the case of OH" type synthesis. Such a study has the potential of revealing some interesting information given the slower nucleation/crystal growth processes, the smaller number of metastable phases observed and the crystallization of almost defect-free zeolites in this medium [5]. F NMR could be added to the other spectroscopic characterization techniques for such solutions. 

The present chapter reports on the hydrodiermal synthesis in an acid and fluoride medium, and the characterization of virtually pure and well-crystallized products. The development of such synthesis has been studied in connection with the new possibilities presented by this method (higher thermal stability, easier control of substitutions in tetrahedral layers by the cation templating effect, easier... 

The development of synthesis in an acid and fluoride medium has been attempted in order to obtain dioctahedral 2 1 layer silicates, containing Si and A1 as fiamework-forming elements, under smooth synthesis conditions, industrially easy, and as near as possible to zeolites (low-temperature and low-pressure). 

Synthesis of Dioctahedral 2 1 Layer Silicates in an Acid and Fluoride Medium... 

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