Niobium chloride, anhydrous

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Another solvent extraction scheme uses the mixed anhydrous chlorides from a chlorination process as the feed (28). The chlorides, which are mostly of niobium, tantalum, and iron, are dissolved in an organic phase and are extracted with 12 Ai hydrochloric acid. The best separation occurs from a mixture of MIBK and diisobutyl ketone (DIBK). The tantalum transfers to the hydrochloric acid leaving the niobium and iron, the DIBK enhancing the separation factor in the organic phase. Niobium and iron are stripped with hot 14—20 wt % H2SO4 which is boiled to precipitate niobic acid, leaving the iron in solution. 

Another method of purifying niobium is by distillation of the anhydrous mixed chlorides (29). Niobium and tantalum pentachlorides boil within about 15°C of one another which makes control of the process difficult. Additionally, process materials must withstand the corrosion effects of the chloride. The system must be kept meticulously anhydrous and air-free to avoid plugging resulting from the formation of niobium oxide trichloride, NbOQ. Distillation has been used commercially in the past. 

Weight gain data and oxidation-reduction titers (Table II) for the niobium (V) chloride and bromide reaction products both indicated the reduction of niobium (V) to niobium (IV). The niobium (IV) adducts were separated by washing the crude reaction mixtures with anhydrous acetonitrile. This solvent removed the various organic products of the initial reaction plus any unreduced niobium compounds. Analysis of the washed products gave ratios of Nb X py of 1 4 2 and agreed with analyses for samples of NbCl py2 and NbBr4 py2 which... 

Samples of both the bromide and chloride crude reaction mixtures were extracted with anhydrous chloroform to obtain a chloroform-soluble residue. Chloroform extraction was chosen here, because the niobium (IV) adducts appeared to be substantially insoluble in this solvent. Samples of the soluble residue were dissolved in anhydrous chloroform for infrared analysis. The resulting absorption spectra were then compared to those obtained in the same way for known samples of pyridinium chloride and bromide (Table VI). 

From these observations it was concluded that the major products of the reduction of niobium(V) chloride with anhydrous pyridine were tetrachlorodi-(pyridine)niobium(IV) and l-(4-pyridyl)pyridinium dichloride. Oxidation-reduction titrations indicated that this reduction accounted for approximately 70% of the reaction products. In view of the rapid reaction of tantalum(V) halides with pyridine to form 1 to 1 adducts, it was assumed that the remaining 30% of niobium (V) which was not reduced was present in the reaction mixture as pentachloro(pyridine)niobium(V). On this basis the following over-all reaction is proposed ... 

Atkinson, Steigman, and Hiskey have found distillation of anhydrous chlorides to be an effective method for separating tin from niobium and tantalums... 

In a separate series of experiments the speciation of niobium was studied using exchange reactions of metallic niobium with bismuth, silver or nickel ions in NaCl-KCl melts. The starting NaCl-KCl-BiC melt was prepared by chlorinating bismuth metal (>98%, Reahim) by chlorine gas in the molten salt mixture. Nickel and silver chloride-containing melts were prepared by dissolving anhydrous nickel chloride (98%, Aldrich) and silver chloride (99%, Aldrich), respectively, in NaCl-KCl with HCl gas bubbled through the electrolyte for 2 h to eliminate traces of absorbed moisture. 

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