RU2356879C1 - Method of producing tantalum alcoholates - Google Patents

Abstract

FIELD: nanotechnologies, electrochemistry.

SUBSTANCE: invention relates to electrochemical synthesis of tantalum alcoholates representing promising forerunners of high-purity highly-dispersed mono- and mixed oxide compounds for nanotechnologies. The proposed method consists in electrochemical dissolving of tantalum in its anode polarisation by constant current at increased temperature in the solution of isopropyl or butyl alcohol, followed by extraction of isopropoxitantalum or butoxitantalum from appropriate electrolysis solution. Note here that the synthesis is performed in electrolysis of isopropyl or butyl alcohol solution in methanol, ammonium chloride being used as electrically conducting additive in amount of 1.0 to 2.0 wt %. Note that the content of appropriate alcohol in methanol makes 30 to 70 vol. %.

EFFECT: simplified synthesis of tantalum alcoholates, lower power consumption.

1 tbl, 14 ex

Description

The invention relates to the field of electrochemical synthesis of tantalum alcoholates. Interest in these products is based on the prospect of their use as precursors of highly pure and highly dispersed mono- and mixed oxide compositions for nanotechnological processes. Ceramic materials based on them have a high dielectric constant and a low dielectric loss tangent, which led to the creation of high-quality dielectric resonators for the purposes of modern microwave microelectronics. Tantalum oxide (Ta ₂ O ₅ ) is used as a dielectric in precision capacitors, integrated circuits.

The widespread use of tantalum alcoholates, especially on a large scale, is hindered by the imperfection of their synthesis - the difficulty of carrying out reactions and purifying the final products, and the low availability of starting reagents.

Known methods for producing tantalum alcoholates are mainly based on the reaction between the Ta (V) halide and the corresponding alkyl alcohol in the presence of gaseous ammonia (NH ₃ ), which is necessary for the binding of the halogen-containing acid released during the reaction. These are multi-stage and very laborious processes that do not meet modern environmental requirements. Thus, according to the Japanese patent (No. 1045325, 1989), Ta (V) halide is subjected to multistage purification to obtain the corresponding tantalum alcoholate, including its extraction with a mixed organic solvent from an aqueous solution of HF + H ₂ SO ₄ , subsequent extraction, precipitation of the halide with a KOH mixture + KF, dissolving the precipitate at a controlled pH and finally extracting the desired tantalum halide with an ether type solvent and separating the solvent.

Known methods for producing metal alcoholates, including tantalum, with a relatively high molecular weight, which are based on the transesterification reaction of lower molecular weight alcoholates, for example, isopropoxy and butoxytantal from methoxy or ethoxytantal according to the following schemes:

Ta (OCH ₃ ) ₅ + 5C ₃ H ₇ OH → Ta (OS ₃ H ₇ ) ₅ + 5CH ₃ OH

Ta (OS ₂ Н ₅ ) ₅ + 5С ₃ Н ₇ OH → Ta (OS ₃ Н ₇ ) ₅ + 5С ₄ Н ₉ OH

Ta (OS ₂ Н ₅ ) ₅ + 5С ₄ Н ₉ OH → Ta (OS ₄ Н ₉ ) ₅ + 5С ₄ Н ₉ OH

Ta (OCH ₃ ) ₅ + 5C ₄ H ₉ OH → Ta (OS ₄ H ₉ ) ₅ + 5CH ₃ OH

(D.C. Bradley, R.C. Mehrotra, D.P. Gaur, Metal Press, London, 1978).

A significant drawback of these known methods is the inaccessibility of low molecular weight tantalum alcoholates (methoxy and ethoxy) due to the lack of their production.

A promising direction for the production of isopropoxytantalum and butoxytantalum, as well as lower molecular weight alcoholates that can be used in these transesterification reactions, is the use of electrochemical methods for the synthesis of these compounds based on the anodic dissolution of tantalum in the corresponding C ₁ -C ₄ aliphatic alcohols.

Known methods for producing tantalum alcoholates by electrochemical method relate mainly to the electrosynthesis of ethoxytantalum. So, according to the method described in US Pat. England No. 1307581 (1973), the electrochemical dissolution of tantalum was carried out in an ethanol solution containing an electrically conductive additive of the type [N (R) ₄ ] X, where R is alkyl or hydrogen, X is halogen. The electrolysis was carried out at a boiling point of the solvent, a voltage of 40-60 V and a direct current of 0.5-2.0 A. The yield of alcoholate by current was 91%. The current density and yield of the product substance are not indicated.

The disadvantage of this method is its low efficiency, associated with a decrease in the current load from 2.0 A to 0.7 A and an increase in voltage during electrolysis to 80 V.

Also, with a current efficiency of 91%, ethoxytantalum was obtained by VAShreider et al, Direct Electrochemical Synthesis of Metal Alkoxides (Inorg. Chimica Acta, 53, pp.73-76, 1981). The process was conducted at a current load of 0.2 A and a total voltage of 30 V; tetrabutylammonium bromide (Bu) ₄ NBr was used as an electrically conductive additive. After 10 hours of electrolysis, 5.5 g of ethoxytantalum Ta (OS ₂ H ₅ ) ₅ was obtained, which was purified by vacuum distillation.

The disadvantage of this method is the high energy intensity of the process (the calculated energy consumption based on the data presented in the work was ~ 11.0 kWh / kg; as a conductive additive, inaccessible and difficult to clean (Bu) ₄ NBr are used.

A known method of producing ethoxytantalum with a current efficiency of 97% (US Pat. US No. 3730857, 1973). Electrolysis was carried out in hot ethanol saturated with ammonium chloride. A direct current of 0.25-0.4 A was applied to the electrodes after their preliminary alternating current polarization for 20 minutes. After passing 6 amp hours at a voltage of 50 V, the electrolysis solution contained 17.6 g of Ta (OS ₂ H ₅ ) ₅ .

The disadvantage of this method is the complexity of the electrochemical stage associated with the need for preliminary alternating current polarization of the electrodes, the high energy intensity of the process, apparently due to the low solubility of NH ₄ Cl in ethanol (the calculated energy consumption is ~ 17.0 kWh / kg) the proposed method to obtain isopropoxybutoxytantalum by direct electrochemical method due to the practical insolubility of ammonium chloride in isopropanol and butanol.

A known method (ed. Certificate No. 1008282, 1981) of producing tantalum alcoholates under the influence of alternating current of industrial frequency in C ₁ -C ₄ -alkyl alcohols containing an inert diluent (acetonitrile, dimethyl sulfoxide, dimethylformamide) and an electrically conductive additive of the general formula KX, where K - cation of an alkali metal, ammonium or quaternary ammonium base, X - halogen, ClO ₄ -, BF ^- in an amount of 2-20%. The electrolysis was initially conducted by direct current until the deposition of metal oxides ceased, and then by alternating current of 0.5 A and voltage of 220 V. The yield of ethoxytantalum by current and substance was 68% and 71%, respectively.

The disadvantage of this method is the complex electrolysis system, high energy consumption due to high voltage and low current outputs and the substance of the target product. The use of solvents of various nature and an increase in the concentration of the electrically conductive additive are additional sources of contamination of the target product.

The method closest in content to the invention is described in (N.Ya. Turova, AVKorolev, DETchebukov and AIBelokon, Tantalum (V) Alkoxides: Electrochemical Synthesis, Mass spectral Investigation and oxoalkoxocomplexes. Polyhedron, Vol.15, No.21, pp. 3869-3880, 1996).

According to this method, the electrochemical synthesis of tantalum alcoholates was carried out in a glass cell equipped with a thermostatic jacket and a reflux condenser, with an tantalum electrode bag and a cathode made of platinum or stainless steel. As the electrolysis solution, we used the corresponding alcohol with lithium chloride dissolved in it, which was previously dried in vacuum at 80–100 ° C, and the alcohols were dehydrated with Al (OR) ₃ and zeolites to a moisture content of 0.01–0.03 mass%. . The electrolysis was carried out by direct current (U = 110-220 V, I = 0.05-0.20 A) at an average density of 0.05 A / cm ² with the return of solvent vapor and the removal of hydrogen generated at the cathode. After electrolysis was completed (10-20 hours), the alcohol was distilled off under reduced pressure, the alcoholates were extracted with hexane or toluene, and thus they were separated from LiCl, which was practically insoluble in hydrocarbons. After centrifugation, the extractant was separated from the tantalum alcoholates by evaporation under reduced pressure. In some cases, to purify tantalum alcoholates from trace amounts of LiCl, they were subjected to vacuum distillation or, as in the case of Ta (OS ₂ H ₅ ) _{5, was} extracted with hexane. In addition, it was noted that the electrosynthesis of butoxytantal proceeded at rates much lower than in the case of other alcoholates, and at the end of electrolysis the surface of the Ta-anode was covered with a layer of resinous products.

The disadvantage of this method is the high energy intensity of the process, the use of LiCl as an electrically conductive additive, contaminating the target product with lithium compounds, low efficiency, especially in the case of butoxytantal electrosynthesis.

Thus, an analysis of the above methods for the production of tantalum alcoholates shows that chemical methods are ineffective either because of their multi-stage, labor-intensive and environment that does not meet modern requirements, or due to the lack of acceptable methods for obtaining the starting products, as in the case of transesterification of methoxy and ethoxy - derivatives of tantalum with isopropyl and butyl alcohols. In turn, the electrochemical method allows one to obtain the considered alcoholates, but is characterized by high energy intensity and low efficiency in the synthesis of relatively high molecular weight tantalum alcoholates, as well as using compounds that are inaccessible and polluting the target product as an electrically conductive additive.

The purpose of the invention is to reduce the energy intensity of the electrochemical synthesis of tantalum alcoholates and to simplify the technological scheme of this process.

This goal is achieved in that the synthesis of the alcoholate is carried out during the electrolysis of a solution of the corresponding alcohol in methanol using NH ₄ Cl as an electrically conductive additive. This makes it possible to carry out electrosynthesis of the target product in one reaction space, both due to the electrochemical reaction of anodic dissolution of tantalum and the transesterification reaction of methoxytantal formed in this process.

With regard to the electrosynthesis of isopropoxytantalum and butoxytantalum, the essence of the proposed method lies in the fact that the methanol-isopropanol or methanol-butanol solution was subjected to electrolysis with an alcohol content in them in the range of 30-70 vol.% In the presence of NH ₄ Cl in an amount of 1.0-2, 0 wt.% At a temperature of 60-70 ° C in the first case and 70-75 ° C in the second. Then, from the reaction masses obtained in the first or second cases, using effective distillation, methanol was completely removed first at atmospheric pressure, and then an excess of isopropanol or butanol was removed under reduced pressure. As a result, bottoms were obtained with a composition representing the corresponding alcoholate mixed with an electrical conductive additive. Hexane was added to the obtained residue and the alcoholate was extracted, while NH ₄ Cl, insoluble in hydrocarbons, precipitated. The extract was decanted from it, the solvent was distilled off under reduced pressure, and the obtained isopropoxytantalum or butoxytantalum was purified by recrystallization or distillation, respectively.

It is significant that if, at the same current density, electrolysis in isopropyl or butyl alcohols in the presence of an electrically conductive additive proceeds with high voltage and energy consumption, then the electrolysis of a solution containing 30-70 vol.% Isopropanol or butanol in methanol proceeds at a substantially lower voltages and specific energy consumption. It is also significant that in this case, instead of the relatively expensive and hygroscopic lithium chloride, which also pollutes the target product with lithium compounds, it was possible to use cheap and affordable ammonium chloride that is easy to clean and dry.

The concentration of NH ₄ Cl in the range of 1.0-2.0 wt.% For solutions of the corresponding alcohol is optimal. When its content in the electrolysis solution is less than 1.0 wt.%, The specific energy consumption noticeably increases. The upper limit of the concentration of conductive additives is due to the solubility of NH ₄ Cl in a methanol-alcohol solution of the specified composition.

The amounts of isopropanol or butanol included in the electrolysis solution are also optimal. The use of solutions containing more than 70 vol.% Isopropanol or butanol is impractical due to a significant increase in specific energy consumption. The use of these alcohols in amounts of less than 30 vol.% Is also impractical, since in this case the specific energy consumption practically does not decrease, and the efficiency of the transesterification reaction decreases, which leads to a decrease in the yield of the corresponding alcoholate.

The invention is illustrated by the following examples.

Example 1

In a glass electrolyzer with a Ta-anode and a stainless steel cathode, equipped with a cooling jacket, a reflux condenser, a thermometer and an inert gas supply device, 200 ml of an electrolysis solution containing the corresponding alcohols and up to 2.0 wt.% NH ₄ Cl were loaded. Electrolysis was carried out at a current load of 1.5 A and an average current density of 5 A / dm ² . The electrolysis mode is galvanostatic. After passing 20 A · h, electrolysis was stopped. Then, methanol was distilled off from the post-electrolysis solution on a distillation column at atmospheric pressure, and then, under reduced pressure, an excess of isopropanol or butanol was distilled off. ~ 100 ml of hexane was added to the residue, which was isopropoxytantalum or butoxytantalum, which was extracted with the corresponding alcoholate and NH ₄ Cl insoluble in hexane was separated. The extract was decanted, and hexane was separated from it under reduced pressure on a rotary-film evaporator. As a result, alcoholates were obtained with current yields of 91% and 95% for the substance. In this case, the specific energy consumption amounted to 3.0 kW h / kg.

Examples 2-11 were carried out under similar conditions, but with a different ratio of methanol and isopropanol, methanol and butanol, as well as with different contents of the conductive additive NH ₄ Cl.

The synthesis results are shown in Table 1.

Examples 12-14 comparative prototype.

Thus, the present invention allows to simplify the process of electrochemical synthesis of tantalum alcoholates with a significant reduction in the energy intensity of this process.

Claims (1)

A method of producing tantalum alcoholates by electrolysis by electrochemical dissolution of tantalum by direct current anodic polarization at an elevated temperature in a solution of isopropyl or butyl alcohol, in the presence of an electrically conductive additive, followed by isolation of isopropoxytantalum or butoxytantalum from the corresponding electrolysis solution by extraction, characterized in that the synthesis is carried out at electrolysis of a solution of isopropyl or butyl alcohol in methanol, and as an electrically conductive avki ammonium chloride is used in an amount of 1.0-2.0 wt.%, the content of methanol in the corresponding alcohol is 30-70 vol.%.