Fluorination process

Other limitations of electrochemical fluorination ate that compounds such as ethers and esters ate decomposed by hydrogen fluoride and cannot be effectively processed. Branching and cross-linking often take place as a side reaction in the electrochemical fluorination process. The reaction is also somewhat slow because the organic reactant materials have to diffuse within 0.3 nm of the surface of the electrode and remain there long enough to have all hydrogen replaced with fluorine. The activated fluoride is only active within 0.3 nm of the surface of the electrode. 

The nonbonding electron clouds of the attached fluorine atoms tend to repel the oncoming fluorine molecules as they approach the carbon skeleton. This reduces the number of effective coUisions, making it possible to increase the total number of coUisions and stiU not accelerate the reaction rate as the reaction proceeds toward completion. This protective sheath of fluorine atoms provides the inertness of Teflon and other fluorocarbons. It also explains the fact that greater success in direct fluorination processes has been reported when the hydrocarbon to be fluorinated had already been partiaUy fluorinated by some other process or was prechlorinated, ie, the protective sheath of halogens reduced the number of reactive coUisions and aUowed reactions to occur without excessive cleavage of carbon—carbon bonds or mnaway exothermic processes. 

To achieve the very low initial fluorine concentration in the LaMar fluorination process initially a helium or nitrogen atmosphere is used in the reactor and fluorine is bled slowly into the system. If pure fluorine is used as the incoming gas, a concentration of fluorine may be approached asymptotically over any time period (Fig. 3). It is possible to approach asymptotically any fluorine partial pressure in this manner. The very low initial concentrations of fluorine in the system greatiy decreases the probabiUty of simultaneous fluorine coUisions on the same molecules or on adjacent reaction sites. 

Thus, for a successful fluorination process involving elemental fluorine, the number of coUisions must be drasticaUy reduced in the initial stages the rate of fluorination must be slow enough to aUow relaxation processes to occur and a heat sink must be provided to remove the reaction heat. Most direct fluorination reactions with organic compounds are performed at or near room temperature unless reaction rates are so fast that excessive fragmentation, charring, or decomposition occurs and a much lower temperature is desirable. 

R. M. Flynn, T. A. Kestner, and G. G. 1. Moore, "Stereochemistry of the Direct Fluorination Process," Tenth Winter Fluorine Conference, St. Petersburg, Fla., 1991. 

The synthesis of tantalum and niobium fluoride compounds is, above all, related to the fluorination of metals or oxides. Table 3 presents a thermodynamic analysis of fluorination processes at ambient temperature as performed by Rakov [51, 52]. It is obvious that the fluorination of both metals and oxides of niobium and tantalum can take place even at low temperatures, whereas fluorination using ammonium fluoride and ammonium hydrofluoride can be performed only at higher temperatures. 

Ammonium complex fluorometalates are generally susceptible to hydrolysis by water formed in the course of the fluorination process (see 25). Such interactions, resulting in the formation of oxyfluoride metalates can be described as follows ... 

In other cases, if the fluorination process leads to cardinal changes in the crystal structure of the initial oxide compounds, new compounds with polar structures can be obtained. A demonstrative example of such materials are compounds that belongs to the system Na5(W3 xNbx)09..xF5+x and that have chiolite-type structures, when neither pure fluoride nor oxide display any ferroelectric properties [393 - 395]. 

The fluorination process aims to decompose the material and convert tantalum and niobium oxides into complex fluoride compounds to be dissolved in aqueous solutions. The correct and successful performance of the decomposition process requires a clear understanding of the oxygen-fluorine substitution mechanism of the interaction itself. 

The optimal temperature range for the fluorination process was found to be about 230-290°C. The resulting cake was leached with water. The prepared solution was separated from the precipitate by regular filtration and the separated insoluble precipitate was identified as lithium fluoride, LiF. The solution contained up to 90 g/1 Ta205. Solution acidity was relatively low, with a typical pH = 3-4, and was suitable for the precipitation of potassium heptafluorotantalate, K2TaF7, tantalum hydroxide or further purification by liquid-liquid extraction after appropriate adjustment of the solution acidity [113]. 

The optimal temperature range for the interaction was found to be 150-230°C. The cake resulting from the fluorination process was also successfully leached with water, dissolving ammonium oxyfluoroniobate, (NH4)3NbOF6. The solution was separated from the precipitate of lithium fluoride. The main parameters of the solution were a niobium concentration of about 75 g/1 Nb205, pH = 3—4. 

In order to increase the efficiency of the fluorination process, the tantalum-niobium-containing raw material must be milled and well mixed with ammonium hydrofluoride. It is assumed that the process temperature can be set between 200-350°C. 

Ammonium hydrofluoride is relatively stable, even in the molten state. In addition to being in contact with tantalum or niobium oxide, the compound will initiate the fluorination process yielding complex tantalum or niobium fluoride compounds. There is no doubt that thermal treatment of the hydroxides at high temperatures and/or at a high temperature rate leads to the enhancement of the defluorination processes, which in turn results in an increase in fluorine content of the final oxides. 

This monograph compiles the latest research on the chemistry of complex fluorides and oxyfluorides of tantalum and niobium, and covers synthesis and fluorination processes, crystal structure peculiarities and crystal chemical classification, as well as the behavior of complex ions in fluorine solutions and melts. 

We did not feel any of these methods would work reliably on a commercial scale at current densities in the range of 300 mA cm"2 or for commercial periods (at least 4000 hr). Rudge s work9,10 with porous carbon anodes was a very elegant solution to the problem (and formed the basis for the Phillips Electrochemical Fluorination process), but the high electrical resistance of the porous carbon limited it to small anodes at high current densities or lower current densities on large anodes. 

The direct fluorination of inorganic,1,2 organometallic,3 5 and organic compounds,6-8 employing the LaMar9,10 and Exfluor-Lagow" methods, has impacted the synthesis of fluorinated compounds over the past 25 years. Among the most important applications of direct fluorination are the synthesis of fluoropolymers from hydrocarbon polymers and the conversion of the surface of the hydrocarbon polymers to fluoropolymer surfaces.12,13 The direct fluorination process is an excellent approach to the synthesis of fluoropolymers. 

The best known aspect, and the first one to find commercialization in the direct fluorination area, was the fluorination of polymer surfaces. This Lagow-Margrave invention, trademarked Fluorokote, involved many types of polymeric materials in various forms e.g., polyethylene bottles, polypropylene objects, and rubber gloves. Polyethylene bottles are easily given fluorocarbon surfaces (>0.1 mm), and this has been commercialized. Air Products has at least 20 licenses for what is known as their Aeropak process and Union Carbide has a Linde Fluorination process as well. Applications in chemical, pharmaceutical, and cosmetic storage are widespread. 

Owing to the product diversity of the South African market, the AEC has developed its surface fluorination technology based on the more versatile post-fluorination process. This decision has led to numerous novel applications and improvements to existing products, some of which will be discussed in this chapter. 

The electrofluorination of acetophenone and benzophenone takes place in anhydrous HF and in the presence of solvents such as chloroform and acetonitrile [38]. The fluorination of the aromatic rings occurred to various extent. Further uses of anhydrous hydrogen fluoride as a liquid environment for electrofluorination processes have been reported, for example, by Matalin etal. [39]. In particular, systems with low conductivity in liquid hydrogen fluoride and nonselective processes have been studied and optimized. The fluorination of benzene and halobenzenes in the presence of Et4NF—(HF) in an undivided cell has been studied by Horio et al. [40] Cathodic dehalogenation is observed to accompany the anodic fluorination process. 

A carbon-deposited film was prepared from the alumina film with 30-nm channels by the CVD technique using propylene. Fluorination was carried out by direct reaction of the film with dry fluorine gas (purity 99.7%). The film was placed in a nickel reactor and was allowed to react with 0.1 MPa of fluorine gas for 5 days at a predetennined temperature in the range of 50 to 200°C. Then the fluorinated carbon nanotubes were separated by dissolving the alumina film with HF. A schematic drawing of the fluorination process is given in Fig. 10.1.15. 

Fig. 10.1.15 Schematic diagram of the fluorination process of carbon nanotube.

Other nucleophilic fluorination processes in heterocycles have included displacement of bromine with potassium, cesium, or silver fluorides, but these failed to work for 4-bromo-5-nitroimidazole and ethyl 4-bromoimid-azole-5-carboxylate (73JA4619). Xenon hexafluoride was reported to convert 2,4,5-tribromoimidazole into the trifluoro analogue (79JGU1251). 

In the LaMar Fluorination process [11] (referred to as LaMar in the following discussion) the substrates are condensed at low temperature into a tube packed with copper turnings through which fluorine, initially highly diluted in either helium or nitrogen, is passed. The concentration of fluorine and the reaction temperature are slowly increased over a period of several days to permit perfluorination. This is a batch process that requires relatively long reaction times to perfluorinate samples of material. 

The Aerosol Fluorination process [42] (Aerosol) is operated on the principle that the substrate is absorbed onto the surface of fine sodium fluoride particles in the fluorination apparatus in which the fluorine concentration and the temperature increases along the length of the reaction vessel. A U.V. photo-fluorination finishing stage completes the perfluorination process which has the advantage that it is a continuous flow method. 

Primary alkyl chlorides give the corresponding perfluoroalkylchlorides (Fig. 10) [58] in good yield demonstrating that the carbon-chlorine bond resists the fluorination process. However, secondary alkyl chlorides are susceptible to rearrangement processes (Fig. 11). 

The acidity and dielectric constant of the reaction media can have a profound effect on the fluorination process. Studies concerning the fluorination of a model substrate, 4-fluorobenzoic acid, in a variety of solvents showed that conversion of the substrate to 3,4-difluorobenzoic acid (Table 5) rose as the acidity of the solvent increased, due to the increased interaction between fluorine and the reaction medium (Fig. 56) [147]. 

Fluorine reacts with N-methylpyrrole and thiophene to give mixtures of mono-fluorinated isomers, consistent with an electrophilic fluorination process (Fig. 58) [152, 153], whilst furan gives a mixture of products largely resulting from addition of fluorine to the ring. The conversions of the reactions reported... 

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