Fluorination of ammonia

The direct fluorination of ammonia by fluorine in the gas phase is not feasible for technical use, cf. p. 176. However, the direct fluorination in the presence of molten ammonium hydrogen difluoride is feasible and was commercialized in a production plant by Air Products and Chemicals, Inc. [1, 2]. 

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Difluoroamine (fluorimide) was discovered by Kennedy and Colburn as a by-product accompanying tetrafluorohydrazine in the reduction of NF3 by arsenic1 and arsine. It was found to be a product of hydrolysis of fluorinated urea80 and was detected in trace amounts among the products of fluorination of ammonia.8 Reactions to be considered for preparative purposes include the following (1) reduction of tetrafluorohydrazine,4 (2) hydrolysis of difluorourea, (3) protonation of trityldifluoroamine, (4) hydrolysis of difluorosulfamide,7 and (5) hydrolysis of isopropyl difluorocarbamate.8... 

At first sight the direct fluorination of ammonia might appear to be a useful alternative method for obtaining the trifluoride, but this reaction, if uncontrolled, yields mainly nitrogen and hydrogen fluoride with only about 6% of NF3 (258). The yield is greatly improved when the fluorine is diluted with N2 and this gas is allowed to mix with ammonia in a reactor packed with copper turnings (212). Under these conditions... 

NF3 is synthesized by the melt electrolysis of NH4F/HF mixtures, or by the fluorination of ammonia catalyzed by copper. 

Nitrogen trifluoride, first synthesized in 1928 by electrolysis of molten NH4F-HF, is the only nitrogen fluoride which has attained some commercial importance. It is presently produced by the direct fluorination of ammonia with fluorine in the presence of molten ammonium fluoride. Since about 1975, NF3 has been used as the fluorine source in high-power HF/DF chemical lasers. 

Although NF3 has been known since 1928, it first became of commercial interest in the late 1950s. Only two processes are of technical and economic importance for the large-scale production of NF3 (1) the electrolysis of molten ammonium hydrogen difluoride, and (2) the direct fluorination of ammonia in the presence of molten ammonium hydrogen difluoride. The electrolytic process has also been recommended and widely used for the preparation of NF3 in the laboratory. 

Nitrogen trifluoride can be formed from a wide variety of chemical reactions. The commercial process for production involves direct fluorination of ammonia with fluorine gas in the presence of ammonium fluoride. 

Nitrogen forms halides with the elements of group 17. Nitrogen trifluoride, NF3, can be made by the fluorination of ammonia in the presence of a Cu catalyst. 

Ammonia. Ammonia (qv) reacts with excess fluorine ia the vapor phase to produce N2, NF, N2F2, HF, and NH F. This reaction is difficult to control ia the vapor phase because of the iatense heat of reaction, and ia some cases only N2 and HF are produced. Nitrogen trifluoride was obtained ia 6% yields ia a gas-phase reaction over copper (42). Yields of ca 60% are achieved by the reaction of fluorine and ammonia ia a molten ammonium acid fluoride solution (43,44). 

CoF is used for the replacement of hydrogen with fluorine in halocarbons (5) for fluorination of xylylalkanes, used in vapor-phase soldering fluxes (6) formation of dibutyl decalins (7) fluorination of alkynes (8) synthesis of unsaturated or partially fluorinated compounds (9—11) and conversion of aromatic compounds to perfluorocycHc compounds (see Fluorine compounds, organic). CoF rarely causes polymerization of hydrocarbons. CoF is also used for the conversion of metal oxides to higher valency metal fluorides, eg, in the assay of uranium ore (12). It is also used in the manufacture of nitrogen fluoride, NF, from ammonia (13). 

Manufacture and Economics. Nitrogen tritiuoride can be formed from a wide variety of chemical reactions. Only two processes have been technically and economically feasible for large-scale production the electrolysis of molten ammonium acid fluoride and the direct fluorination of the ammonia in the presence of molten ammonium fluoride. In the electrolytic process, NF is produced at the anode and H2 is produced at the cathode. In a divided cell of 4 kA having nickel anodes, extensive dilution of the gas streams with N2 was used to prevent explosive reactions between NF and H2 (17). 

Other applications of zirconium tetrafluoride are in molten salt reactor experiments as a catalyst for the fluorination of chloroacetone to chlorofluoroacetone (17,18) as a catalyst for olefin polymerization (19) as a catalyst for the conversion of a mixture of formaldehyde, acetaldehyde, and ammonia (in the ratio of 1 1 3 3) to pyridine (20) as an inhibitor for the combustion of NH CIO (21) in rechargeable electrochemical cells (22) and in dental applications (23) (see Dentalmaterials). 

The high reactivity of N-H bonds has also been exploited to produce N-F denvatives without significant substitution on neighbonng C-H bonds, Diethyl-phosphoramidates of ammonia, alkylammes, and a,polar solvents to produce difluoroamine [57], N,N-difluoroalkylamines, and a,to-bis(At,7V-difluoroamino)alkanes [52] Acetamide undergoes fluonnation to give modest yields of N,N difluoroacetatnide and acetyl fluonde when fluorinated... 

Hexafluorometalates of ammonia are formed in the presence of trivalent metals [51, 52]. It was shown, for instance, that fluorination of chromium oxides using ammonium hydrofluoride yields (NFLi)3CrF6. It was also found that the oxidation degree of chromium does not depend on the initial valency of the original oxide used as a precursor [110]. 

Equations (141) and (142) describe the equilibrium between the hydrolysis of complex fluoride acids (shift to the right) and the fluorination of hydroxides (shift to the left). Near complete precipitation of hydroxides can be achieved by applying an excessive amount of ammonia. Typically, precipitation is performed by adding ammonia solution up to pH = 8-9. However, the precipitate that separates from the mother solution can be contaminated with as much as 20% wt. fluorine [490]. Analysis of niobium hydroxides obtained under different precipitation conditions showed that the most important parameter affecting the fluorine content of the resultant hydroxide is the amount of ammonia added [490]. Sheka et al. [491] found that increasing the pH to 9.6 toward the end of the precipitation process leads to a significant reduction in fluorine content of the niobium hydroxide. 

Application of an excessive amount of ammonia solution in the precipitation of tantalum and niobium hydroxides from strip solutions usually ensures good quality of the products. Nevertheless, the method has two general problems. First, hydroxides containing low levels of fluorine contamination... 

Uchino and Azuma [498] proposed a way in which to recycle the filtrate solutions. The process consists of adding calcium hydroxide, Ca(OH)2, to the filtrate, yielding a calcium fluoride, CaF2 precipitate and gaseous ammonia, NH3. The fluorine and ammonia are recovered in forms that are suitable for reutilization. 

The ionization energy of the hydrogen atom, 313.6 kcal/mole, is quite close to that of fluorine, so a covalent bond between these two atoms in HF is expected. Actually the properties of HF show that the molecule has a significant electric dipole, indicating ionic character in the bond. The same is true in the O—H bonds of water and, to a lesser extent, in the N—H bonds of ammonia. The ionic character of bonds to hydro-... 

Fluidized bed reactors were first employed on a large scale for the catalytic cracking of petroleum fractions, but in recent years they have been employed for an increasingly large variety of reactions, both catalytic and non-catalytic. The catalytic reactions include the partial oxidation of naphthalene to phthalic anhydride and the formation of acrylonitrile from propylene, ammonia, and air. The noncatalytic applications include the roasting of ores and Tie fluorination of uranium oxide. 

Also, nitrogen trifluoride can he prepared by reaction of ammonia with fluorine diluted with nitrogen in a reactor packed with copper. Other nitrogen fluorides, such as N2F2, N2F4, and NHF2 also are produced. The yield of major product depends on fluorine/ammonia ratio and other conditions. 

In similar transformations, agriculturally and medicinally significant fluorine-containing quinazolines 101, isolated as the hydrates, were produced in good yields from 100 after condensation with various aldehydes in the presence of ammonia followed by oxidation with DDQ <99H2471>. 

Conversion of the separated fluorides into the corresponding oxides is effected by boiling with concentrated sulphuric acid until free from fluorine, and then hydrolysing the product by boiling with water. Alternatively, the hydrated acids are precipitated by the addition of ammonia to the solutions of the double fluorides.4 Niobium pentoxide, Nbg05, or tantalum pentoxide, TaaOs, is obtained on ignition of the precipitated hydrate. 

Terminal alkenes with at least three fluorine atoms attached to the double bond react with ammonia to yield carbonitriles. This proceeds via addition of ammonia and subsequent elimination of two equivalents of hydrogen fluoride. In the case of trifluoroethene or halotri-fluoroethencs the resulting monofluoroacctonitriles trimerize to give the respective... 

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