Fluorides reaction with silica

Brojmine(III) fluoride is a pale yellow liquid at room temperature, while chlo-rine(II]I) fluoride has to, be cooled below 12° to be available in the liquid state at atmospheric pressure. Both are highly reactive and explode in the presence of water and most organic matter, react with asbestos with incandescence, but can be manipulated in quartz apparatus because the reactions with silica in this form are slow. Several reviews are available - ... 

FIGURE 15-22 When a mixture of hydrofluoric acid and ammonium fluoride is swirled inside a glass vial (a i, reaction with the silica in the glass frosts the surface of the cover glass as well as the walls of the vial (b)... 

Modem production of elemental phosphoras uses a technique similar to the metallurgical processes described in Chapter 20. Apatite is mixed with silica and coke and then heated strongly in the absence of oxygen. Under these conditions, coke reduces phosphate to elemental phosphoms, the silica forms liquid calcium silicate, and the fluoride ions in apatite dissolve In the liquid calcium silicate. The reactions are not fully understood, but the stoichiometry for the calcium phosphate part of apatite is as follows ... 

The covalently bonded solids such as silica cannot be easily broken by aqueous solutions. For example, the strong Si-O bonds silica is not dissolvable by boiling with concentrated acids except hydrofluoric acid because of the formation of silicon fluoride which is a gas and expels otherwise else it may form fluosilicic acid by reaction with water. 

Beside SILP experiments with silica as support material, reports have also been made on the use of membranes coated with ionic liquid catalyst solution for the hydrogenation reaction of propene and ethene. The membranes were obtained by supporting various ionic liquids, each containing 16 to 23 mmol Rh(I) complex Rh(nbd)(PPh3)2 (nbd=norbornadiene), in the pores of poly(vinylidene fluoride) filter membranes [118]. 

Fused basic salts and basic oxides react with vitreous silica at elevated temperatures. Reaction with alkaline-earth oxides takes place at approximately 900°C. Halides tend to dissolve vitreous silica at high temperatures fluorides are the most reactive (95). Dry halogen gases do not react with vitreous silica below 300°C. Hydrogen fluoride, however, readily attacks vitreous silica. 

G. S. Serullas treated potassium chlorate with an excess of hydrofluosilicic acid the clear liquid was decanted from the sparingly soluble potassium fluosilicate, the soln. evaporated below 30°, and filtered through glass powder J. J. Berzelius evaporated the acid liquid mixed with finely divided silica below 30° in air, or over cone, sulphuric acid and potassium hydroxide in vacuo. The excess of hydrofluoric acid was volatilized as silicon fluoride, and the clear liquid was then filtered from the excess of silica. R. Bottger treated sodium chlorate with oxalic acid whereby sparingly soluble sodium oxalate was formed J. L. Wheeler, and T. B. Munroe treated sodium chlorate with hydrofluosilicic acid and M. Brandau treated potassium chlorate with aluminium sulphate and sulphuric acid and precipitated the alum so formed with alcohol. Chloric acid is formed in many reactions with hypochlorous and chlorous acid for example, it is formed when an aq. soln. of chlorine or hypochlorous or chlorous acid decomposes in light. It is also formed when an aq. soln. of chlorine dioxide stands in darkness or in light. A mixture of alkali chlorate and chlorite is formed when an aq. soln. of an alkali hydroxide is treated with chlorine dioxide. 

Reaction of organosUanes with lactones and enonesf Certain carbanions, generated by desilylation, have been found to react not only with aldehydes and ketones, but also with some lactones and a,/ -enones. Cesium fluoride or tetrabutylam-monium fluoride supported on silica can be used as catalysts. 

Xenon difluoride reacts also with some halosubstituted alkanes18,19 at room temperature in chloroform, carbon tetrachloride, dichloromethane or bromoform, and depending on the nature of the solvent used chloro or bromo derivatives are isolated with imidazo-(l,2-fr)-pyridazine18. Carbon tetrachloride reacts with xenon difluoride at 180°C, while room-temperature transformations are achieved when various catalysts [antimony trifluoride, tantalum(V) fluoride or silica dioxide] are used, whose structure also influences the product distribution20. Tris(fluorosulfonyl)methane gives a fluoro-substi-tuted product in its reaction with xenon difluoride in difluorodichloromethane21. 

Anhydrous reagent. Tetraalkylammonium fluorides are useful catalysts in various synthetic reactions, but their use is hampered by their extreme ability to retain water, which reduces the effectiveness. Clark has found that silica gel (60-120 mesh) can give completely anhydrous salts. Thus a 20% aqueous solution of tetra-n-butylammonium fluoride is shaken with silica gel water is then partially removed under reduced pressure. Methanol is added and evaporation is continued to... 

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. 

If the silica is treated with fluoride prior to titanation, which converts many of the silanol groups into Si-F surface groups, the reaction with titanium alkoxide is inhibited and the treatment is less effective. The data in Table 34 illustrate this outcome. Silica samples were treated (or not) with two fluoride compounds in aqueous solution, then they were dried at 260 °C in the normal way prior to titanation. Titanium isopropoxide was added to make the catalyst contain 5 wt% Ti. Each sample was then calcined at 815 °C in air. Chromium was applied (0.5 wt%) as bis(f-butyl) chromate) in hexane solution (two-step activation, see Section 12). After final activation in air at 315 °C, each sample was tested at 102 °C, and the polymer MI values obtained are listed in the table. The change in MI shows that the titanium did not attach well to the carrier in the presence of fluoride. As more fluoride was added, the polymer MI dropped. 

Another tetravalent compound, tetrakis-(neopentyl)chromium, behaved similarly [295]. It had no activity by itself, but did adsorb on various carriers to yield active catalysts. It reacted slowly with all carriers. Even in contact with fluorided silica-coated alumina, it had to be heated to about 65 °C to start its reaction with the support. Then, gradually, the purple solution adsorbed on the carrier to yield a brown catalyst. Again, the polymerization activity paralleled the reactivity with the support, and the fluorided silica-coated alumina provided catalysts with unusually high polymerization activities. 

In Figure 6, column (1) represents the initial chemical composition of the leachate observed prior to contact with the sandstone. Column (5) summarizes the solution composition observed after reaction with the sandstone for five days. Columns (2)-(4) represent intervening steps in the reaction-path simulation. The major changes in chemistry observed between columns (1) and (5) are an increase of three orders of magnitude in the concentration of Mg and significant decreases in total dissolved carbonate, fluoride, and silica. 

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