JP4099350B2 - Method for producing alkali metal borohydride - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a novel method for producing an alkali metal borohydride, in particular, an alkaline earth metal oxide produced as a by-product when producing an alkali metal boron hydride using an alkali metal metaborate as a raw material. It provides a method for converting to a derivable compound to improve the efficiency of the alkali metal borohydride.
[0002]
[Prior art]
Alkali metal borohydrides, such as sodium borohydride, have been widely used as reducing agents or hydrogenating agents, but recently they have also been tried to be used as hydrogen generators or fuel cells.
[0003]
Up to now, as a method for producing an alkali metal borohydride, for example, a method in which two molecules of alkali metal hydride and one molecule of borax are heated and reacted under substantially anhydrous conditions (Japanese Patent Publication No. 42-27256). A method of reacting hydrogen and sodium or sodium hydride or aluminum or an alloy thereof with anhydrous borax at a temperature of 200 to 600 ° C. and a hydrogen pressure of 1 × 10 ² to 50 × 10 ² kPa (Japanese Patent Publication No. 43-2221) ), An alkali metal or alkaline earth metal borate and an alkali metal hydride or a mixture of alkali metal and hydrogen at a ratio different from the molar ratio of the metal oxide and boron oxide in the metaborate. Dry methods such as a method of reacting at ˜1000 ° C. (Japanese Examined Patent Publication No. 47-48117) are known.
[0004]
However, in these dry methods, it is necessary to use raw materials that must be prepared by complicated operations, raw materials that are dangerous in handling, such as metallic sodium, or reaction under hydrogen pressure or high temperature. It was unsuitable for such a method.
[0005]
By the way, as a method for improving the drawbacks of such conventional methods, the present inventor reacted a previously available alkali metal metaborate salt with an easily handled alkaline earth metal hydride using mechanochemical technology. Proposed a method (Japanese Patent Application No. 2000-372093).
However, in this method, since the by-product alkaline earth metal oxide, particularly the oxide after releasing hydrogen from MgH ₂ , is chemically stabilized, it is hydrogenated to regenerate MgH _2. The problem remains that it cannot be reused as a raw material, and the alkali metal borohydride cannot be produced industrially efficiently.
[0006]
[Problems to be solved by the invention]
Under such circumstances, the present invention is capable of reacting under mild conditions using raw materials that are easily available and easy to handle, and alkali metal borohydride using alkali metal metaborate as a raw material. The purpose of the present invention is to provide a method for producing an alkali metal borohydride in a high yield by effectively using a by-product in the production of.
[0007]
[Means for Solving the Problems]
As a result of various studies on a method for efficiently producing an alkali metal borohydride by a simple operation using easily available raw materials, the inventor has carried out electrolysis after alkaline earth metal oxides have been halogenated. By converting the metal halide into a metal halide and a halogen, an alkaline earth metal hydride that can easily lead to the production of an alkali metal boron hydride can be obtained using the hydrogen halide generated in this process, and as a raw material, It has been found that it is effective to use an alkaline earth metal oxide produced as a by-product in the production of an alkali metal boron hydride by hydrogenating an acid alkali metal salt with an alkaline earth metal hydride or an alkaline earth metal and hydrogen. Based on this finding, the present invention has been made.
[0008]
That is, the present invention comprises (1) a step of reacting an alkaline earth metal oxide with a hydrogen halide to produce an alkaline earth metal halide,
(2) electrolyzing the alkaline earth metal halide obtained in step (1) into alkaline earth metal and halogen;
(3) a step of hydrogenating the alkaline earth metal obtained in step (2) with hydrogen to produce an alkaline earth metal hydride, and (4) the alkaline earth metal hydride obtained in step (3). And a method for producing an alkali metal boron hydride by reacting an alkali metal metaborate with an alkali metal borate, and an alkali metal borate as the alkaline earth metal oxide of the raw material The present invention provides a method using a by-product in producing an alkali metal boron hydride by hydrogenating a metal salt with an alkaline earth metal hydride or an alkaline earth metal and hydrogen.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
As described above, the method of the present invention includes the steps (1) to (4).
In this step (1), the target alkali metal borohydride is separated from the product obtained by hydrogenating the alkali metal metaborate, and then the remaining alkaline earth metal oxide is reacted with hydrogen halide. To produce an alkaline earth metal halide, and crystallize, separate and dry the alkaline earth metal halide as necessary. The by-product alkaline earth metal oxide obtained by hydrogenating the alkali metal metaborate used in this step (1) is in the form of finely divided particles, particularly MgH ₂ or the like. When magnesium-based hydride is used, hydrogen is released from the particle surface and the hydrogenation reaction occurs. At the same time, an oxide is formed only near the surface of the particle. By using hydrogen halide, the oxide on the particle surface is easily removed, and magnesium halide MgX ₂ (where X is a halogen atom) is rapidly formed.
Examples of the hydrogen halide used in this step (1) include hydrogen fluoride, hydrogen chloride, hydrogen bromide, hydrogen iodide and the like, and an alkaline earth metal fluoride, chloride or bromide is obtained according to each. .
[0010]
Next, in step (2), using the alkaline earth metal halide obtained in step (1), the alkaline earth metal halide that has been made anhydrous together with the removal of impurities is electrolyzed by a molten salt electrolysis method. As a result, alkaline earth metal is deposited on the cathode and halogen is produced on the anode. For example, when obtaining magnesium Mg and chlorine Cl ₂ using anhydrous magnesium chloride MgCl ₂ , an electrolytic bath having a predetermined concentration prepared by adding an appropriate salt (such as NaCl) to lower the melting point as desired is used. When electrolysis is performed at a temperature of 700 ° C. and a cell voltage of 6 to 8 V, a current efficiency of 75 to 85% is obtained. When CaCl ₂ is used to obtain metallic Ca and chlorine Cl ₂ , the temperature is 780 ° C. and the cell voltage. When electrolyzed at 25V, a current efficiency of 65-80% is obtained.
[0011]
In addition, as described in JP-B-32-9052, when metal Mg and chlorine Cl ₂ are obtained using hydrous magnesium chloride MgCl ₂ , the following steps (4 By supplying hydrous magnesium chloride while blowing dry hydrogen chloride (HCl) gas obtained in (1), dehydration and electrolysis can be performed while preventing decomposition and generation of magnesium oxide.
[0012]
The alkaline earth metal obtained in this step (2) can be supplied with hydrogen as a raw material as it is in the production of the alkali metal borohydride, or the following alkaline earth metal chloride is produced ( It can also be supplied as a raw material of 3).
[0013]
In step (3), the alkaline earth metal obtained in step (2) is reacted with hydrogen to produce an alkaline earth metal hydride. Alkaline earth metals usually give stable hydrides when contacted with hydrogen under heating. For example, when metal calcium, metal barium, and metal strontium are heated to 350 ° C. or higher in a hydrogen stream, they quickly react to become calcium hydride, and metal magnesium is pulverized to 100 μm or less and hydrogen at 24-400 atmospheres. When heated to 300-400 ° C. under pressure, it becomes magnesium hydride.
[0014]
In the case of metallic magnesium, the surface is usually oxidized and the initial rate in the hydrogenation reaction is slow. Therefore, prior to hydrogenation, it is advantageous to fluorinate with an aqueous alkali fluoride solution in advance. This fluorination treatment increases the reaction rate, and a high yield of 80% or more can be obtained even at a low temperature of 100 ° C. or lower and a low pressure. As the aqueous alkali fluoride solution at this time, an aqueous solution containing an alkali fluoride such as sodium fluoride, potassium fluoride, or ammonium fluoride at a concentration of 0.2 to 10.0% by weight with hydrogen fluoride has a pH of 2.0. To 6.5, preferably adjusted to 4.5 to 6.0.
[0015]
In step (4) of the method of the present invention, the alkaline earth metal hydride obtained in step (3) is reacted with the alkali metal metaborate to produce the desired alkali metal borohydride.
In the method of the present invention, hydrogen halide is produced by direct reaction with hydrogen using the halogen produced in the above step (2), and further absorbed in water as necessary to a hydrogen halide of up to about 35% by mass. An acid is produced, and the hydrogen halide or hydrohalic acid thus obtained is stored in, for example, a storage tank, and after adjusting the concentration and supply amount as necessary, it is circulated to the above step (1) to obtain an alkali. It is preferably used as a raw material for producing an alkaline earth metal halide from an earth metal oxide.
[0016]
Further, the hydrogen source supplied to the step (3) and used for hydrogenation of the alkaline earth metal, and the hydrogen source used for producing hydrogen halide from the halogen obtained in the step (2) are all particularly There is no restriction | limiting and a commercial item can be used.
[0017]
The alkaline earth metal oxide that is a raw material of the method of the present invention includes an alkali metal metaborate salt that is produced by hydrogenating an alkaline earth metal hydride or alkaline earth metal with hydrogen and producing an alkali metal boron hydride. It is advantageous to construct a series of alkali metal borohydride production cycles using the resulting alkali metal metaborate salt. Examples of the alkali metal borate used at this time include sodium metaborate (NaBO ₂ ), potassium metaborate (KBO ₂ ), and lithium metaborate (LiBO ₂ ). Among these, sodium metaborate which is obtained easily and in large quantities by reacting sodium hydroxide with natural borax, followed by dehydration and heat drying is preferable.
[0018]
In addition, the residual content after using alkali metal boron hydride as a hydrogen generator, fuel or reductant in a fuel cell that converts its chemical energy directly into electrical energy is dehydrated, and metaboric acid contained therein Alkali metal salts can also be recovered and used.
It is preferable to hydrogenate the alkali metal metaborate recovered in this manner after heating and drying in advance to make the crystal water 1 or less, preferably 0.
[0019]
Next, as the alkaline earth metal hydride used for hydrogenating the alkali metal metaborate, there are calcium hydride (CaH ₂ ), barium hydride (BaH ₂ ), and strontium hydride (SrH ₂ ). However, magnesium hydride (MgH ₂ ) is particularly preferable. Examples of alkaline earth metals used with hydrogen include magnesium (Mg), calcium (Ca), barium (Ba), and strontium (Sr).
[0020]
This hydrogenation with alkali metal borate and alkaline earth metal hydride or alkaline earth metal and hydrogen can be carried out under high temperature and high pressure conditions, but each is a fine powder of 500 μm or less, preferably 100 μm or less. It is advantageous to mix and bring the raw materials into contact with each other and react while adding mechanical energy such as grinding energy, grinding energy, impact energy, ultrasonic grinding energy at room temperature and atmospheric pressure. An alkali metal borohydride can be obtained in a yield.
[0021]
The mechanical energy at this time is applied by a ball mill, a tube mill, a rod mill, an attrition mill, a vibration mill, a jet mill, a micronizer, etc., and it is particularly preferable to use a ball mill. The energy applied at this time is generally at least 4 kJ / g or more, preferably 10 kJ / g or more. For example, the energy efficiency can be increased in a shorter time as ball milling is performed at a higher rotational speed. The reaction product is used by dissolving an alkali metal borohydride in an alkaline aqueous solution or by extracting and separating the alkali metal borohydride with anhydrous ethylenediamine or the like.
[0022]
FIG. 1 is a process diagram showing a production cycle when the method of the present invention is incorporated, taking as an example a method for producing sodium borohydride (NaBH ₄ ) from sodium metaborate (NaBO ₂ ) and magnesium hydride (MgH ₂ ). The reaction of NaBO ₂ and MgH ₂ to produce NaBH ₄ causes MgO by-product to react with HCl to produce MgCl ₂ [step (1)], and this MgCl ₂ is electrolyzed. Mg and Cl ₂ are generated [Step (2)], and this Mg is reacted with H ₂ to be converted to MgH ₂ [Step (3)]. Then, MgH ₂ obtained in the above step (3) is reacted with NaBO ₂ producing NaBH _4. The Cl ₂ obtained in the step (2) is reacted with H ₂ to form HCl, and this HCl is circulated to the step (1) and used for the reaction with MgO. Further, Mg obtained in the step (2) is mixed with H ₂ and used as a raw material for producing NaBH ₄ like the above MgH ₂ .
[0023]
In this way, by forming a series of NaBH ₄ production cycles including the method of the present invention, by-products can be used fully effectively, so that NaBH _4, that is, alkali metal boron hydride is produced in high yield. Can do.
The method of the present invention is particularly suitable for producing sodium borohydride or potassium borohydride.
[0024]
【Example】
EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited by this.
[0025]
EXAMPLE NaBH ₄ was produced according to the process shown in FIG.
That is, 3.0 g of a mixture of anhydrous sodium metaborate fine powder (average particle size 75 nm) and magnesium hydride fine powder (average particle size 100 nm) in a molar ratio of 1: 2 was mixed with a planetary ball mill pulverizer [Fritsch. The product symbol “P-5 type” manufactured by the company was charged at 60 ° C. under atmospheric pressure at 20 ° C. with a ball acceleration of 86.5 m / s ² for 60 minutes to react with sodium borohydride, 0. 8 g (45% yield based on anhydrous sodium metaborate) and 1.65 g magnesium oxide were obtained.
Next, 15 ml of 35% strength hydrochloric acid was added to 1.65 g of this magnesium oxide and reacted at 50 ° C. for 30 minutes to obtain 3.75 g of magnesium chloride.
Next, 0.1 g of sodium chloride was added to this magnesium chloride, heated and melted at 700 ° C., and electrolyzed at a cell voltage of 7 V to obtain 0.95 g of magnesium metal and 2.70 g of chlorine gas.
This magnesium metal is placed in 10 ml of 5% strength by weight potassium fluoride aqueous solution adjusted to pH 5.5 with hydrogen fluoride, treated at 50 ° C. for 30 minutes, dried, and then at 80 ° C. in a hydrogen stream for 10 hours. By processing, 0.6 g of magnesium hydride was obtained.
After finely pulverizing this magnesium hydride to an average particle size of 100 nm, it is mixed with newly supplied magnesium hydride fine powder and anhydrous sodium metaborate fine powder in the same ratio as above to produce sodium borohydride. When used, it was able to be processed without any problems.
[0026]
【The invention's effect】
According to the method of the present invention, an alkali metal borohydride can be produced in a high yield by using raw materials that are easily available and easy to handle. By incorporating it into the production method, all the components of the alkaline earth metal oxide, which is a by-product, can be circulated and used for the hydrogenation reaction. By separately supplying only the hydrogen required for hydrogenating the alkali metal salt, the desired alkali metal borohydride can be produced efficiently.
[Brief description of the drawings]
FIG. 1 is a process diagram showing an example of a production cycle of an alkali metal borohydride incorporating the method of the present invention.

Claims (10)

(1) a step of reacting an alkaline earth metal oxide with a hydrogen halide to produce an alkaline earth metal halide,
(2) electrolyzing the alkaline earth metal halide obtained in step (1) into alkaline earth metal and halogen;
(3) a step of hydrogenating the alkaline earth metal obtained in step (2) with hydrogen to produce an alkaline earth metal hydride, and (4) the alkaline earth metal hydride obtained in step (3). And a method of producing an alkali metal borohydride by reacting an alkali metal metaborate with an alkali metal borate. Alkaline earth produced as a by-product in the production of alkali metal boron hydride by hydrogenating alkali metal metaborate with alkaline earth metal hydride or alkaline earth metal and hydrogen as alkaline earth metal oxide in step (1) The method for producing an alkali metal borohydride according to claim 1, wherein a metal oxide is used. The method for producing an alkali metal borohydride according to claim 1, wherein hydrogen is reacted with the halogen obtained in step (2) to produce hydrogen halide, which is circulated to step (1). The method for producing an alkali metal borohydride according to claim 2 or 3, wherein the alkali metal metaborate salt is reacted with an alkaline earth metal hydride or an alkaline earth metal with hydrogen while applying mechanical energy. The method for producing an alkali metal borohydride according to claim 2, 3 or 4, wherein the alkali metal metaborate is sodium metaborate. 6. The method for producing an alkali metal borohydride according to claim 5, wherein the sodium metaborate is sodium metaborate which is reacted with borax and a sodium hydroxide aqueous solution, dehydrated and dried by heating. The alkali metal boron hydride according to claim 2, 3 or 4, wherein the alkali metal salt of metaborate is obtained by hydrolyzing the alkali metal boron hydride to generate hydrogen to dehydrate and heat dry the residue. Manufacturing method. The method for producing an alkali metal borohydride according to claim 2, 3 or 4, wherein the alkali metal metaborate is recovered from spent effluent as fuel for a fuel cell. The method for producing an alkali metal borohydride according to any one of claims 1 to 8, wherein the alkali metal borohydride is sodium borohydride or potassium borohydride. The method for producing an alkali metal borohydride according to any one of claims 1 to 9, wherein the alkaline earth metal is magnesium or calcium.

Images