METHODS FOR THE SYNTHESIS OF VANADIUM DIOXIDE AND VANADYL BROMIDE CRYSTAL HYDRATE

G. N. KHODALEVICH, E. T. LABYKINA, L. P. GEVLICH, B. N. BERZIN*

(Submitted by the scientific seminar of the Department of General and Inorganic Chemistry)

Vanadium oxides are used as catalysts in oxidation–reduction catalysis. The use of vanadium pentoxide in sulfuric-acid production is generally known; vanadium oxide is used as a catalyst in the hydrogenation process [1]. There are no data on the use of vanadium dioxide in industrial catalysis, but it may quite evidently be assumed that in oxidation–reduction catalysis vanadium dioxide should occupy a suitable place. Vanadium dioxide is also a starting product for the synthesis of vanadium preparations.

In the present work we have specified the conditions for the synthesis of vanadium dioxide. The synthesis of vanadyl bromide crystal hydrate was developed by us in connection with the recommendation of the Soyuzkhimreaktiv trust concerning the need to manufacture this preparation for research purposes [2].

Vanadium dioxide

Several methods are known for obtaining vanadium dioxide: 1) by cautious oxidation of vanadium oxide with atmospheric oxygen [3], 2) by prolonged heating of a mixture of the oxide with vanadium pentoxide [4], 3) by decomposition of vanadyl oxalate upon heating [4]. Obtaining the product by the first and second methods is disadvantageous, since it is associated with a large expenditure of time—8 and 3 days, respectively.

We used the third method [4], which is given in the handbook without specific synthesis conditions. The synthesis proceeds according to the reaction

\[ \mathrm{V_2O_5 + H_2C_2O_4 = 2VO_2 + 2CO_2 + H_2O} \]

upon heating, in a stream of carbon dioxide. The synthesis conditions are given in Table 1.

Experiment showed that single heating of vanadium pentoxide with oxalic acid does not lead to obtaining pure dioxide. Reduction proceeds to completion only with twofold heating. On the basis of the investigations carried out, we recommend the following method for synthesizing vanadium dioxide.

Into a quartz tube with \(d = 20\text{—}25\) mm we load a well-ground mixture of 10 g of vanadium pentoxide with a threefold amount of oxalic acid, previously dried in a desiccator. The tube is placed in an electric furnace and a stream of carbon dioxide is passed through it. The furnace temperature is raised to \(500^\circ\) (over 20–25 min) and it is held

* Student L. Ya. Khamitova took part in the analytical part of the work.

Table 1

Percentage reduction of vanadium pentoxide to dioxide as a function of synthesis temperature

60 min. The tube is periodically rotated. The product obtained is again mixed with a twofold amount of oxalic acid, and the heating operation is repeated. The final product is vanadium dioxide, finely crystalline sintered flakes of black-blue color. The product yield is 96%. When small quantities are obtained (in a boat), the yield is 100%.

Vanadyl bromide crystal hydrate

Methods are known for obtaining vanadyl bromide in the anhydrous state and in solution [1, 5]. Vanadyl bromide in the form of a crystal hydrate has not been obtained, and its composition has not been studied.

The following reactions were used by us as the basis for methods of obtaining vanadyl bromide crystal hydrate:

\[ \text{a)}\quad \mathrm{VOSO_4 + BaBr_2 = VOBr_2 + BaSO_4,} \]

\[ \text{b)}\quad \mathrm{VO_2 + 2HBr = VOBr_2 = H_2O.} \]

By method a), 81.5 g of vanadyl sulfate were dissolved in 200 ml of water, and to the heated solution was added a solution of 148 g of barium bromide (calculated for anhydrous salts). The precipitate of barium sulfate was filtered off on a Büchner funnel; the filtrate was tested for \(Ba^{2+}\) and \(SO_4^{2-}\) ions, and, if they were detected, additional precipitation was carried out. The filtrate, slightly acidified with hydrobromic acid, was evaporated to a syrupy black-blue solution and then crystallized in a desiccator over phosphorus anhydride. The yield was 90–92%.

By method b), 20 g of vanadium dioxide were dissolved with heating in a 1.5-fold amount of 20% hydrobromic acid. The solution obtained was filtered through a No. 4 glass filter, and then the process was carried out as in method a), before crystallization over phosphorus anhydride holding it in a desiccator over potassium hydroxide for 48 hours. The product yield was 95–98%.

Some properties of vanadium dioxide and vanadyl bromide. Analytical data

Since vanadium dioxide was synthesized for preparative purposes, we determined the rate of its dissolution in 20% hydrobromic acid.

The dioxide obtained at 500° has the greatest solubility, with simultaneously the highest yield of pure product (Table 1).

The amount of vanadium in the dioxide was determined by permanganatometric titration, after first dissolving the dioxide in sulfuric acid (1:4). Analysis showed a vanadium content of 61.44%; theoretical, 61.42%.

Vanadium dioxide is readily oxidized by atmospheric oxygen; therefore it should be stored in sealed ampoules. The vanadyl bromide crystal hydrate obtained by us has the composition $\mathrm{VOBr_2\cdot 3.5H_2O}$. It is a finely crystalline black-green substance (Fig. 1), which deliquesces in air and is readily oxidized by atmospheric oxygen. It can be stored only in sealed ampoules.

Fig. 1. Microphotograph of crystals of vanadyl bromide $\mathrm{VOBr_2\cdot 3.5H_2O}$, $\times 60$.

The analysis of vanadyl bromide crystal hydrate was carried out by two methods: 1) gravimetric, in combination with permanganatometric; 2) photometric.

In the first method, about one gram of vanadyl was dissolved in 25 ml of water previously weighed in a weighing bottle. The weight of the sample was determined from the difference between the weights of the water and the solution. Bromine ions were precipitated with an excess of silver nitrate. The bromine content was determined from the weight of silver bromide. The vanadium content in the solution was determined by permanganatometric titration. The content in the product obtained (calculated on an anhydrous basis) was: vanadium, 22.47%; bromine, 70.48%; theoretical vanadium content, 22.46%; bromine, 70.49%. Photometric methods for determining vanadium are used rather widely [6], but there are no recommendations for determining vanadium in vanadyl bromide.

Vanadyl bromide solutions have a bright blue color, which provides a basis for applying colorimetry. Our preliminary studies in this area showed that the photometric method can be applied.

The determination was carried out as follows: into a series of 100-ml volumetric flasks were introduced 2 ml of 20% $\mathrm{H_2SO_4}$ and, respectively, 5, 6, 7, 8, 10, 15, 20, and 25 ml of a vanadyl bromide solution containing 3.724 mg of vanadium per 1 ml; the flasks were made up to volume with water. After 10–15 min the optical density of the solutions was measured on a photoelectric colorimeter (relative to a blank solution containing no $V^{4+}$ ions) and a calibration curve was constructed (Fig. 2). A 30-mm cuvette was used. The experiment showed that the most sensitive was a red light filter with a maximum light absorption at 680 mμ.

Fig. 2. Calibration curve for determining vanadium by the photometric method.

The optical density of the solutions is proportional to the concentration in the interval studied by us from 0.15 to 2 mg of vanadium/ml. The determinations were carried out on an FEK-M photocolorimeter,

Conclusions

1. A procedure has been developed in detail for obtaining vanadium dioxide by reducing vanadium pentoxide with oxalic acid.

2. Vanadyl bromide crystal hydrate has been obtained, and a method for its synthesis is given.

3. The composition of the vanadyl bromide crystal hydrate has been established.

4. The possibility of determining vanadium in vanadyl bromide by a photometric method has been demonstrated.

REFERENCES

1. H. Remy, Course of Inorganic Chemistry. Vol. II, p. 112, Mir, Moscow, 1966.

2. List of Chemical Reagents and Preparations Recommended to Institutes and Experimental Enterprises for Priority Manufacture under Economic Contracts with the Soyuzreaktiv Trust, Moscow, 1963.

3. O. A. Cook, J. Am. Chem. Soc., 69, 331 (1947).

4. Manual of Preparative and Inorganic Chemistry, ed. by G. Brauer. Vol. II, Moscow, 1956.

5. B. V. Nekrasov, Course of General Chemistry. Goskhimizdat, Moscow, 1960.

6. A. I. Busev, V. G. Tiptsova, V. M. Ivanov, Practical Guide to the Analytical Chemistry of Rare Elements, p. 176. Khimiya, Moscow, 1966.