RU2803219C2 - Method for producing sodium borohydride - Google Patents

Abstract

FIELD: production of sodium borohydride.

SUBSTANCE: group of inventions can be used as a carrier and source of hydrogen. The method for producing sodium borohydride includes the reaction of sodium borate and aluminum powder in an atmosphere of hydrogen gas with stirring at a temperature not lower than 400°C and not higher than 560°C. The molar ratio of sodium contained in sodium borate to boron contained in sodium borate is more than 1.0. Embodiments of the method for obtaining sodium borohydride are proposed.

EFFECT: group of inventions makes it possible to simplify the production of sodium borohydride using aluminum as a reducing agent.

7 cl, 12 dwg, 6 tbl, 26 ex

Description

Field of technology to which the invention relates

The present invention relates to a method for producing sodium borohydride and, more particularly, to a method for producing sodium borohydride from sodium metaborate.

Prior Art

Hydrogen fuel has now attracted attention as an alternative energy source to hydrocarbon fuels. Sodium borohydride (SBH) is a promising hydrogen carrier for hydrogen storage and transport, and as a source of hydrogen. The widespread use of sodium borohydride as a hydrogen carrier requires the creation of an optimal method for producing sodium borohydride with the prospect of developing a technology for mass production.

The processes for producing sodium borohydride have been studied by traditional methods. For example, Patent Reference 1 proposes a method for producing sodium borohydride by reacting a trialkyl borate with sodium aluminum hydride.

Patent Reference 2 proposes a method for producing sodium borohydride, comprising the step of reacting sodium metaborate with granulated aluminum under a hydrogen atmosphere, while simultaneously rolling and grinding the sodium metaborate with granulated aluminum using a grinding medium to obtain sodium borohydride.

In addition, Non-Patent Literature 1 describes a method for preparing sodium borohydride, in which a mixture of sodium diborate (Na ₄ B ₂ O ₅ ) and sodium metaborate reacts with aluminum and hydrogen at high temperature (855 K (581°C), preferably 873 K (599°C)) and with sodium diborate (Na ₄ B ₂ O ₅ ) in the molten state to obtain sodium borohydride.

List of cited sources

Patent literature

Patent source 1: US 3471268

Patent source 2: WO 2015/190403

Non-patent literature

Non-Patent Source 1: Bin Hong LIU, et al., "Sodium Borohydride Synthesis by Reaction of _Na2O contained Sodium Borate with Al and Hydrogen", Energy & Fuels, 2007, Vol. 21, No. 3, pp. 1707-1711.

Non-patent source 2: The Journal of Physical Chemistry C 114(15)7173-7177) March 2010.

Brief summary of the invention

According to the method proposed in Patent Literature 1, sodium borohydride is synthesized by reducing anhydrous borax in a hydrogen atmosphere, and thus sodium hydride or sodium metal is first used as a reducing agent. Sodium metal is expensive.

According to the method proposed in patent reference 2, dry anhydrous sodium metaborate (NaBO ₂ ) and aluminum granules with a diameter of about 3 mm are loaded into the reaction section, which can rotate inside a cylindrical reactor vessel; a grinding medium (steel balls with a diameter of about 30 mm) is placed inside the reactor vessel; and react these materials through a rolling and grinding process to produce sodium borohydride. Along with stretching during rolling and grinding, aluminum particles are ground to such an extremely small thickness and size that further grinding becomes difficult and the reaction stops.

According to the method proposed in Non-Patent Literature 1, sodium diborate (Na ₄ B ₂ O ₅ ) needs to be converted into a molten state. Thus, this method requires the use of equipment capable of withstanding high temperatures (855 K (581°C), preferably 873 K (599°C)) and high hydrogen pressure of 2.3 MPa, and this requirement leads to larger reactor sizes and higher production costs . At a pressure of 1 atm. sodium borohydride decomposes at temperatures of 534°C and above (see non-patent reference 2). The temperature (599°C) which is achieved according to the above method is higher than this decomposition temperature and will cause partial decomposition even in a hydrogen atmosphere at high pressure.

The present invention has been developed in light of the existing problems in the prior art discussed above. Therefore, the purpose of the present invention is to create a method for the preparation of sodium borohydride, allowing the synthesis of sodium borohydride in a simple manner using inexpensive aluminum as a reducing agent.

The method for producing sodium borohydride, according to the first aspect, includes reacting sodium borate and aluminum powder in an atmosphere of hydrogen gas with stirring and a temperature of not lower than 400°C and not higher than 560°C, and at a molar ratio of sodium contained in sodium borate to boron , contained in sodium borate, more than 0.5.

According to a first aspect, the method for producing sodium borohydride may further comprise adding an alkali metal oxide or an alkaline earth metal oxide, wherein the molar ratio of the number of moles of any alkali metal and alkaline earth metal plus any alkali metal and alkaline earth metal contained in the sodium borate to the number of moles of boron contained in sodium borate, may be more than 0.5 and not more than 3.

According to the first aspect, the method for producing sodium borohydride may further include adding an alkali metal hydroxide, wherein the mole ratio of any alkali metal contained in the alkali metal hydroxide and sodium borate to the number of moles of boron contained in the sodium borate may be more than 0.5 and not more than 3.

The method for producing sodium borohydride according to the second aspect includes: dehydrating a mixture comprising alkali metal hydroxide, sodium borate and aluminum powder in a static state at a temperature of not less than 400°C in an atmosphere of hydrogen gas; oxidizing aluminum powder by reacting with water to form a layer of sodium and aluminum oxide extending from the surface to the interior of the aluminum particles, this layer of sodium and aluminum oxide is represented by chemical formula (I) below; and carrying out the mixture reaction at a temperature not lower than 400°C and not higher than 560°C with stirring,

xNa ₂ O Al ₂ O ₃ … (I)

where 0 < x < 1.

The method for producing sodium borohydride according to the third aspect includes: dehydrating a mixture comprising alkali metal hydroxide, sodium borate and aluminum powder in a static state in a reactor, heating the reactor to a temperature of not less than 400°C under reduced pressure; oxidizing aluminum powder by reacting with water to form a layer of sodium and aluminum oxide located from the surface to the interior of the aluminum particles, this layer of sodium and aluminum oxide is represented by chemical formula (I) below; and carrying out the reaction of the mixture at a temperature of not lower than 400°C and not higher than 560°C in an atmosphere of hydrogen gas with stirring,

xNa ₂ O Al ₂ O ₃ … (I)

where 0 < x < 1.

According to the first, second and third aspect, the mole ratio of aluminum to boron contained in sodium borate may be not less than 1.33.

The first or second aspects may provide a method for producing sodium borohydride that does not require the use of any large or complex equipment.

In addition, the third aspect may provide a method for producing sodium borohydride that does not require the use of any large or complicated equipment even when the ingredients contain a lot of water.

Brief description of drawings

Fig. 1 is a flow chart for the synthesis of sodium borohydride (SBH) from sodium borate as an ingredient;

fig. 2 is a flow chart for the synthesis of sodium metaborate from borax as an ingredient and the synthesis of sodium borohydride (SBH) from sodium metaborate as an ingredient;

fig. 3 is a partial sectional view illustrating an example of a sealed reactor used in one embodiment;

fig. 4 is a partial sectional view illustrating another example of a sealed reactor used in one embodiment;

fig. 5A is a partial sectional view illustrating another example of a sealed reactor used in one embodiment;

fig. 5B is a partial sectional view illustrating another example of a sealed reactor used in one embodiment;

fig. 6 is a flow chart for the synthesis of sodium borohydride (SBH) from sodium metaborate as an ingredient, including removal of water under a hydrogen atmosphere;

fig. 7 is a flow chart for the synthesis of sodium borohydride (SBH) from sodium metaborate as an ingredient, including removing water by drying under reduced pressure without introducing hydrogen, followed by reacting under a hydrogen atmosphere;

fig. 8A is a simplified reaction scheme illustrating one of the reaction steps for the synthesis of sodium borohydride (SBH);

fig. 8B is a simplified reaction scheme illustrating one of the reaction steps for the synthesis of sodium borohydride (SBH);

fig. 8C is a simplified reaction scheme illustrating one of the reaction steps for the synthesis of sodium borohydride (SBH);

fig. 8D is a simplified reaction scheme illustrating one of the reaction steps for the synthesis of sodium borohydride (SBH);

fig. 8E is a simplified reaction scheme illustrating one of the reaction steps for the synthesis of sodium borohydride (SBH);

fig. 8F is a simplified reaction scheme illustrating one of the reaction steps for the synthesis of sodium borohydride (SBH);

fig. 8G is a simplified reaction scheme illustrating one of the reaction steps for the synthesis of sodium borohydride (SBH);

fig. 9A is an image illustrating an analysis result (SEM) of a granule prepared by mixing sodium metaborate and aluminum;

fig. 9B shows an electron backscatter image obtained from the reaction in Test Example 12 involving the addition of sodium hydroxide along with the result of SEM analysis;

fig. 9C is an electron backscatter image illustrating the state after completion of the reaction in Test Example 12 including addition of sodium hydroxide;

fig. 10A is an electron backscatter image obtained from the reaction in Test Example 12 prior to SEM elemental analysis;

fig. 10B is an image after SEM analysis illustrating positions of analysis points and determination results of chemicals;

fig. 10C is a table illustrating the results (atomic fractions (%)) of the SEM analysis points obtained after image analysis at the points corresponding to the numbers in FIG. 10A;

fig. 11A is an electron backscatter image at the completion of the reaction in Test Example 12 prior to elemental image analysis;

fig. 11B is an SEM image after image analysis;

fig. 11C is a table illustrating results (atomic fractions (%)) for SEM analysis points prior to image elemental analysis; And

Fig. 12 is a graph illustrating the production process according to the production method of sodium borohydride in Test Example 11.

Description of implementations

First implementation

Fig. 1 is a flow chart for the synthesis of sodium borohydride SBH) from sodium borate as an ingredient.

In the method for producing sodium borohydride, according to the first embodiment, sodium borate is mixed with aluminum powder in a sealed reactor filled with hydrogen gas, and the mixture is reacted at a temperature of not lower than 400°C and not higher than 560°C. Sodium borate powder and aluminum powder each react in a solid state. As shown in FIG. 1, the method for producing sodium borohydride according to the first embodiment includes the steps of a first step (S-11) to a third step (S-13).

Sodium borate 51 as the borate ingredient in the first step is a double oxide obtained from borax as a starting material by adding sodium hydroxide. Specifically, sodium borate is a double oxide comprising diboron trioxide (B ₂ O ₃ ) and sodium oxide (Na ₂ O) with a Na/B ratio in the range of 0.5 to 3.0. Specific examples of sodium borates include, but are not limited to, borax (sodium tetraborate: Na ₂ B ₄ O ₇ ), sodium metaborate (NaBO ₂ (= Na ₂ B ₂ O ₄ )) and sodium diborate (Na ₄ B ₂ O ₅ ). , as will be described below.

Table 1 provides a list of compositions that contain diboron trioxide (B ₂ O ₃ ) and sodium oxide (Na ₂ O). As shown in Table 1, each of sodium borate ingredients A - F (hereinafter also referred to as "ingredients A - F") includes sodium borate, consisting of diboron trioxide (B ₂ O ₃ ) and sodium oxide (Na ₂ O), which has place in a given composition when the ratio of sodium oxide (Na ₂ O) to borax (sodium tetraborate: Na ₂ B ₄ O ₇ ) as a starting material increases.

Table 1 Ingredients A B C D E F Sodium borate Borax Na ₂ B ₄ O ₇ Sodium metaborate (NaBO ₂ ) Sodium diborate (Na ₄ B ₂ O ₅ ) Na/B Na/B = 0.5 0.5 < Na/B < 1 Na/B = 1 1 < Na/B < 2 Na/B = 2 2 < Na/B Configurations B ₂ O ₃ -Na ₂ OB ₂ O ₃ Mixture
B ₂ O ₃ -Na ₂ OB ₂ O ₃ and Na ₂ O _{B2O3 - Na2O} Mixture
Na ₂ OB ₂ O ₃ -Na ₂ O and B ₂ O ₃ -Na ₂ O Na ₂ OB ₂ O ₃ -Na ₂ O Mixture
Na ₂ OB ₂ O ₃ -Na ₂ O and Na ₂ O

In Table 1, ingredient A is brown. Borax exists in the forms of sodium tetraborate (anhydrous), sodium tetraborate (pentahydrate), and sodium tetraborate (decahydrate). Borax decahydrate occurs naturally in the form of crystals. Thus, ingredient A can be isolated into crystals by crystallizing the hydrate from a solution and isolating and drying the hydrate.

Ingredient C is sodium metaborate (NaBO ₂ (= Na ₂ B ₂ O ₄ )), which is sodium borate with Na/B (in moles) = 1.0, prepared by adding sodium hydroxide (NaOH (Na ₂ O⋅ H ₂ O)) to borax. Sodium metaborate, which is ingredient C, can be isolated into crystals by crystallizing the hydrate from a solution and isolating and drying the hydrate.

Here, Na/B (in moles) is the molar ratio of the content of any alkali metals plus sodium in the sodium borate composition to the content of boron (hereinafter also referred to as “Na/B”).

Ingredient E is sodium diborate (Na ₄ B ₂ O ₅ ), which is a sodium borate with Na/B = 2.0, obtained by adding sodium hydroxide (NaOH(Na ₂ O⋅H ₂ O)) to sodium metaborate. Sodium diborate, which is ingredient E, cannot be crystallized from hydrate solution and therefore cannot be isolated as crystals from aqueous solution.

Additionally, ingredient B is sodium borate with 0.5 < Na/B < 1.0. This borate initially consists mainly of borax and gradually comes to an increased proportion of sodium oxide (Na ₂ O). This ingredient B is a mixture of borax (Na ₂ B ₄ O ₇ ) with sodium oxide (Na ₂ O).

Ingredient D is sodium borate with 1.0 < Na/B < 2.0. This borate initially consists mainly of sodium metaborate and gradually comes to an increased proportion of sodium oxide (N ₂ O) to become a mixture of sodium metaborate (NaBO ₂ ) and sodium diborate (Na ₄ B ₂ O ₅ ). This ingredient D is a liquid containing a hydrate, but this hydrate cannot be crystallized and therefore cannot be isolated as crystals.

Ingredient F is a sodium borate with 2.0 < Na/B. This borate initially consists mainly of sodium diborate (Na ₄ B ₂ O ₅ ) and gradually comes to an increased proportion of sodium oxide (Na ₂ O) to become a mixture of sodium diborate (Na ₄ B ₂ O ₅ ) and sodium oxide (Na ₂ O). Sodium diborate is the main crystal constituent of this ingredient F, but it cannot be crystallized from aqueous solution and therefore cannot be isolated as sodium diborate crystal. At higher Na/B ratios, excess sodium oxide (Na ₂ O) occurs. When the Na/B ratio is not less than 2, ingredient F exists in a free state.

From the above it follows that sodium borate is a double oxide containing diboron trioxide (B ₂ O ₃ ) and sodium oxide (Na ₂ O) with a given ratio. As described above, for example, sodium metaborate (NaBO ₂ ) is ingredient C (Na/B (moles) = 1.0; Na ₂ O:B ₂ O ₃ = 1:1), and sodium diborate (Na ₄ B ₂ O ₅ ) is ingredient E (Na/B (moles) = 2.0; Na ₂ O:B ₂ O ₃ = 2:1). However, other mixtures of Na ₂ O and B ₂ O ₃ with a given ratio, although not specifically named, can also be used as ingredients in the production of sodium borohydride (SBH).

The Na/B ratio (in moles) of sodium borate is preferably in the range of greater than 0.5 and not greater than 6, and preferably in the range of greater than 1.0 and not greater than 3.

The steps will be described in detail below. The sodium borate used in the steps below is isolated crystals of sodium metaborate.

First stage

As shown in FIG. 1, in the first step (S-11), sodium borate 51, which is a sodium metaborate, and aluminum powder 52 are charged into a sealed reactor, and then non-oxidizing gas 53 is introduced into the sealed reactor to fill the interior of the reactor with a non-oxidizing gas atmosphere. The sealed reactor may already be filled with non-oxidizing gas before loading the ingredients.

The first stage mainly involves preparing the ingredients and loading them into the reactor. At the first stage, the internal space of the sealed reactor is filled with non-oxidizing gas to prevent the absorption of water from the air by sodium metaborate and the oxide film on the surface of the aluminum. The sealed reactor may be filled with non-oxidizing gas before or after loading the ingredients into the sealed reactor.

Here, the non-oxidizing gas 53 may be, for example, hydrogen gas or an inert gas (such as helium gas or argon gas, for example). Alternatively, the first step may be a step in which the sealed reactor is charged with sodium metaborate and aluminum and the sealed reactor is evacuated to create a vacuum after loading the ingredients.

The sealed reactor used has sufficient heat resistance and pressure resistance to withstand high temperatures (eg 560° C.) and high pressures (eg 10 MPa), and has a sealed interior to be filled with gas. A reactor equipped with at least a stirring device can be used.

The sealed reactor will be described in detail later.

Sodium metaborate powder is soft and most of its particles are broken down into smaller particles by collisions during the mixing step. When the particle size is less than 100 μm, the frequency of contact with aluminum powder during the mixing process increases, and therefore the reaction yield also increases. Therefore, it is preferable that the particle size of the sodium metaborate powder is no more than 100 μm. Sodium metaborate powder tends to absorb water during storage, so that direct use of this powder results in a decrease in the reaction yield. Therefore, the humidity during storage of sodium metaborate should preferably not be more than 10%, so that the amount of water absorbed during short-term storage under such conditions will be 2 - 3% by weight. (hereinafter will be denoted by "%"). In particular, sodium diborate under the same conditions will absorb 5% water or more.

According to the present embodiment, sodium metaborate powder is an ingredient obtained by grinding sodium metaborate to a certain particle size, sifting through a 100 μm sieve, and collecting the particles passing through the sieve. To ensure good contact of sodium metaborate with aluminum, it is preferable to use sodium metaborate powder with smaller particle sizes. Such sodium metaborate powder can be obtained by, for example, grinding the sodium metaborate using a grinding medium in a sealed reactor or sifting the sodium metaborate powder through a sieve having an opening size of less than 100 μm (for example, a sieve having an opening size of 50 μm or less).

In the first step, sodium metaborate powder can be loaded in an amount suitably determined according to the required amount of sodium borohydride to be synthesized. In particular, it is necessary to add a higher amount of metaborate than estimated in light of the fact that ground and dried sodium metaborate is prone to adsorption of water and mass of this water will be lost.

The aluminum used as an ingredient may be in the form of, for example, fine particles such as powder or waste. For example, such fine aluminum particles may be waste materials such as shavings or sawdust. Where possible, aluminum should preferably be selected that contains only small amounts of metal impurities that are more noble than aluminum.

Preferably, the aluminum charged into the reactor has an average particle size, for example, of at least 1 μm and a maximum particle size of not more than 10 mm. If the average particle size of aluminum is less than 1 micron, these particles are difficult to handle due to the fact that such small particles contribute to explosions of the dust-air mixture, and these particles easily stick together to form aggregates in some cases. If the average particle size is greater than 10 mm, the specific surface area per unit mass is reduced so much that the reacting surface area is reduced and can cause a significant reduction in the reaction rate in the initial stage of the reaction. The average particle size is more preferably not less than 10 µm and not more than 5 mm.

The average particle size is obtained as a circular equivalent diameter determined by a laser diffraction particle size distribution analyzer.

At the first stage, the temperature in the sealed reactor while adding ingredients to it is not specifically limited. No special heating is required in a sealed reactor, so its temperature can be equal to ambient temperature. To prevent sodium metaborate from reacting with water from the air, it is advisable to load the ingredients in an anhydrous atmosphere. It is necessary that the reactor is tightly closed immediately after loading the ingredients.

Second stage (water removal stage)

As shown in FIG. 1, the second step (S-12) is a step in which the interior of the sealed reactor is heated to a temperature of not lower than 280°C and not higher than 560°C, and the sodium metaborate and aluminum are dried by removing the residual water contained therein.

This drying step to remove residual water can be carried out using either of two water removal methods.

The first method of water removal causes water from ingredients heated in a sealed reactor to react with aluminum, thereby converting the water into hydrogen gas and aluminum oxide.

More specifically, the first water removal method eliminates water from the reaction system by evaporating residual water in a sealed reactor and causing the water vapor to react with aluminum.

The reaction of water with aluminum is shown in reaction equation (1) below.

2Al + 3H ₂ O → Al ₂ O ₃ + 3H ₂ ... (1)

The second water removal method is a vacuum heat drying process where the interior of a sealed reactor is heated under vacuum and the water is removed to dehydrate the ingredients. This vacuum heating drying method can remove a large amount of water from the system and can reduce aluminum consumption, so this method is therefore suitable for the situation where the ingredients contain a large amount of water, although vacuum drying takes a long time.

In the water removal step, as a second step, heating may be carried out to a temperature of 280°C or higher to ensure removal of water from the sodium metaborate hydrate, and preferably may be carried out at a temperature of 400°C or higher due to the fact that the separation of water from the sodium borate powder continues up to high temperatures.

This water removal step can eliminate the influence of water present in the sodium metaborate on the synthesis of sodium borohydride, thereby making it possible to prevent deterioration in the synthesis efficiency of sodium borohydride.

Here, the term “water” refers to water of hydration, water adsorbed on the surface, and water absorbed by the reaction of anhydrous sodium metaborate powder with water. For example, sodium metaborate can be prepared by hydrolysis of sodium borohydride. Sodium metaborate is usually a tetrahydrate, but can also be a hydrate containing water of hydration in other proportions. In the present embodiment, sodium metaborate can be used regardless of whether water is absorbed on the surface of the sodium metaborate or not.

Absorbed water is present not only on the sodium metaborate, but also, although only in trace amounts, on the oxide film covering the aluminum. This water can be evaporated during the water removal step. According to thermogravimetric analysis, sodium metaborate at a pressure of 1 atm. releases water of hydration at a temperature of 280°C. Thus, the heating temperature for separating hydrate water from sodium metaborate is preferably not lower than 280°C. The holding time is preferably at least 10 minutes. The release of hydrate water can be accelerated and the required holding time can be reduced by controlling the heating temperature to a range of not less than 300°C and not more than 560°C.

Third stage

In the present embodiment, as shown in FIG. 1, the second step (S-12) described above is followed by the third step (S-13) described below, in which sodium borohydride (SBH) is synthesized. When water removal is not performed, the second step (S-12) may be omitted, and the first step (S-11) may be immediately followed by the third step (S-13).

Mixing to enhance the sodium borohydride synthesis reaction can be accomplished using a single mixing device, but preferably also uses a grinding medium to enhance the required contact between aluminum and sodium borate and homogeneously mix the solid ingredients with the products of the synthesis reaction.

Examples of grinding media used in the invention include forms such as balls and rods, with balls being preferred. The diameter of the balls is preferably larger than the particle size of the loaded aluminum. Materials for the grinding medium can be selected accordingly from existing materials such as ceramics and stainless steel. In particular, ceramic balls, which are lightweight and which do not contain metal contaminants, are preferably used as the grinding medium. In particular, the ceramic balls are preferably alumina balls or mullite balls. Alumina is a product formed in synthesis reactions, but aluminum oxide or alumina ceramics annealed at high temperature are stable and do not adversely affect synthesis reactions.

In the specification, the diameter of the grinding medium may be less than 30 mm, preferably not less than 2 mm and not more than 20 mm, and more preferably not less than 3 mm and not more than 10 mm.

When mixing is carried out using a grinding medium, a suitable mixing speed may range from low rotation speed, when the speed at the periphery of the mixing device is about 13 cm/s, to high rotation speed, when the speed at the periphery of the mixing device is 90 cm/s or more, and the grinding medium acquires collision energy, which can deform, as well as roll out and crush the aluminum. If, however, the speed at the periphery of the mixing device is 90 cm/s or more, the aluminum is rolled out and ground into a thin and elongated profile which tends to adhere to the walls of the reactor along with the ingredients and products. As a result, less material is mixed and the reaction yield is correspondingly reduced. Thus, the stirring speed is preferably such that the speed at the periphery of the stirring device is no more than 70 cm/s, which does not cause deformation of the aluminum. Below, such mixing under conditions of low speeds, when the speed along the periphery of the mixing device is no more than 70 cm/s, will be called “soft mixing”.

In the sodium borohydride synthesis reaction in the present embodiment, the surface of the aluminum particle beyond the diffusion distance near the aluminum surface can be considered as a shore. During mixing by stirring, the ingredients present offshore or "offshore" (a region away from the surface of the aluminum particle, hereinafter referred to as "offshore") diffuse through the alumina film and the reaction product layer and reach the surface of the aluminum, which is the site of the reaction, and there a reaction occurs between aluminum, hydrogen and sodium borate to form sodium hydride. Reaction products, i.e. the sodium borohydride and aluminum oxide and sodium borate ingredient have concentration gradients within a diffusion distance near said surface, and the synthesis reaction proceeds while each of these materials migrates by diffusion within a diffusion distance near said surface to create an average concentration. The reaction rate depends on the distance and the rate of diffusion of sodium, boron, aluminum, oxygen and hydrogen ions. Hydrogen gas has little effect on the rate of reaction because it has a small molecular size and can quickly move through the compounds to the surface where the reaction occurs.

In the third step (S-13), the reaction is initiated through two mechanisms in a sealed reactor under an atmosphere of hydrogen gas.

According to the first mechanism that initiates the reaction, the aluminum is scratched during the initial mixing stage, during the collision and capture of particles between the grinding media balls or between the grinding media ball and the mixing device, and the oxide film covering the surface breaks, exposing the fresh aluminum surface, which then enters contact with dehydrated sodium metaborate and reacts with it under a hydrogen atmosphere.

When the surface of the oxide film on aluminum breaks through and microscopic reaction products are formed, the surface of the aluminum is coated with the product and alumina containing sodium oxide.

In the first mechanism that initiates the reaction, the sodium borate ingredient involves excess sodium oxide. In this case, sodium oxide binds to the aluminum oxide film on the surface of the aluminum, so that this oxide film, which has barrier effects to metal ions, is transformed into sodium aluminum dioxide (NaAlO ₂ ), through which the metal ions pass, thereby allowing the reaction to begin. .

The amount of hydrogen in the sealed reactor decreases as the synthesis of sodium borohydride proceeds. The reaction rate can be increased by increasing the pressure of the hydrogen gas. When the ingredient is a sodium metaborate, this reaction follows equation (2) below.

4Al + 6H ₂ + 3NaBO ₂ → 3NaBH ₄ + 2Al ₂ O ₃ ... (2)

In the third step (S-13), the sealed reactor may be the same sealed reactor used in the first step (S-11) and the second step (S-12), or it may be a different sealed reactor. That is, the steps from the first step (S-11) to the third step (S-13) can be carried out in the same sealed reactor or in separate sealed reactors.

In the third step (S-13), the hydrogen gas pressure is preferably maintained in a range of not less than 0.3 MPa and not more than 10 MPa, and more preferably in a range of not less than 1 MPa and not more than 10 MPa. By maintaining the hydrogen gas pressure not less than 0.3 MPa and not more than 10 MPa, sodium borohydride can be synthesized with suitable efficiency, and the cost of equipment can be reduced without the need for reactors, equipment, etc., resistant to ultra-high pressures.

To ensure that the reactions in the third step (S-13) proceed sufficiently, the heating temperature is preferably set to not lower than 400°C and not higher than 560°C. Such heating temperatures of not lower than 400°C and not higher than 560°C provide a sufficient reaction rate to obtain a suitable efficiency for the synthesis of sodium borohydride, and prevent decomposition of the synthesized sodium borohydride, and recover the borohydride to a sufficient extent.

Sodium borohydride (SBH) can be synthesized through the first step (S-11) to the third step (S-13) described above.

In the preferred embodiment discussed above, the sodium metaborate and aluminum may be charged to the sealed reactor separately and sequentially, or they may be charged to the reactor as a mixture.

When these ingredients are charged as a mixture, the first step (S-11) is preferably preceded by a step of mixing sodium metaborate with aluminum to form a mixture and then preferably loading the resulting mixture of sodium metaborate and aluminum into a sealed reactor in the first step (S-11) .

By mixing aluminum and sodium metaborate in advance and using these ingredients as a mixture, the frequency of contact between aluminum and sodium borate can be increased and the reaction rate can accordingly be increased.

When aluminum and sodium metaborate are used as a mixture, they can be dispersed and mixed in advance and then compacted to form granules. Granules offer advantages such as less hygroscopicity and greater convenience and ease of handling. The granules are crushed into smaller particles during the mixing process and then behave similarly to powder.

While the above description illustrates the synthesis of sodium borohydride (SBH) using powdered sodium metaborate as the sodium borate, sodium borohydride (SBH) can be synthesized by, as already described above, adding sodium hydroxide to borax as the sodium borate ingredient to form sodium metaborate, which is then converted to sodium borohydride (SBH).

Fig. 2 is a flow chart for the synthesis of sodium metaborate from borax as an ingredient and the synthesis of sodium borohydride (SBH) from sodium metaborate as an ingredient.

During this process, as shown in FIG. 2, the first step (S-11) includes two steps, namely, the thermal calcination step (S-11A), in which sodium hydroxide is added to borax to form crystals (double oxide) of diboron trioxide (B ₂ O ₃ ) and sodium oxide (Na ₂ O), and a mixing and contacting step (S-11B) in which the resulting product is brought into contact with aluminum through mixing. As a result of these two steps, a double oxide of diboron trioxide (B ₂ O ₃ ) and sodium oxide (Na ₂ O) can be formed with a given Na/B ratio in the range of 1.0 – 3.0 by changing the proportion of sodium hydroxide.

As indicated in Table 1 above, sodium borate with a ratio of Na/B = 1 is sodium metaborate (NaBO ₂ (= Na ₂ B ₂ O ₄ )), and sodium borate with a ratio of Na/B = 2 is sodium diborate (Na ₄ B ₂ O ₅ ).

In the thermal calcination step (S-11A) of the first step (S-11), the heating temperature is preferably not higher than 400°C. The sodium borate resulting from this thermal calcination is cooled to room temperature and then ground. In the mixing and contacting step (S-11B), aluminum is introduced and brought into contact with sodium borate by stirring at room temperature.

After these steps, water is removed in a second step (S-12) as described above, and sodium borohydride (SBH) is synthesized in a third step (S-13) as described above.

The resulting products include sodium borohydride (SBH), as well as sodium oxide (Na ₂ O), aluminum oxide (Al ₂ O ₃ ) and sodium hydride.

Next, some examples of sealed reactors that can be used in the present embodiment will be described, but the sealed reactors in the present embodiment are not limited to this.

Fig. 3 is a partial sectional view illustrating an example of a sealed reactor used in the present embodiment.

As shown in FIG. 3, the sealed reactor 10A has a cylindrical round-bottom main body 12, and a removable disc-shaped cover 14 that tightly seals the reactor main body 12. A temperature-controlled heater 16 is located externally at the bottom of the main reactor body 12, so that the heater 16 is used to heat the contents of the main reactor body 12. In addition, an O-ring 18 is located on the upper end surface of the main reactor body 12 to ensure the airtightness of the internal space of the reactor when this ring is in close contact with the cover 14. When the cover 14 is closed, this cover is in close contact with the main reactor body 12 due to this O-ring 18.

The cover 14 has a central hole and a cylindrical portion projecting from this hole. Above this cylindrical portion is a motor 20. The stirrer consists of a motor 20, a stirrer rod 22 connected to the rotating shaft of the motor 20, and a plurality of stirrer segments 22A projecting in a direction perpendicular to the axis of the stirrer rod 22. When the cover 14 is attached to the main reactor body 12, the end of the stirrer rod 22 reaches a lower region in the interior of the main reactor body 12. That is, the operating motor 20 rotates the stirrer rod 22 together with the stirrer segments 22A to mix the contents of the main reactor body 12 .

The cover 14 is further provided with a first pipe 24 and a second pipe 30 connected directly to the interior of the main reactor body 12. The first pipe 24 is connected to a source of hydrogen gas (not shown) through a hydrogen gas supply valve 26 and to a vacuum pump (not shown) through an outlet valve 28. That is, hydrogen gas enters the main reactor body 12 when the hydrogen gas supply valve 26 is open, and the main reactor body 12 is evacuated when the outlet valve 28 is opened. In addition, the second pipe 30 is connected to a pressure gauge 32 indicating the pressure inside main reactor body 12.

In fig. 4 is partial sectional views illustrating other examples of sealed reactors that may be used in the present embodiment.

The sealed reactor 10B is different from the sealed reactor shown in FIG. 3, in that, as shown in FIG. 4, the sealed reactor 10B contains a large amount of grinding medium 40 inside the main reactor body 12. When the ingredients are loaded inside the main reactor body 12 and the stirring rod 22 is rotated, the grinding medium 40 is agitated to enhance the migration of the sodium borate ingredient and the sodium hydride intermediate. The amount of grinding medium 40 can be increased or decreased accordingly, so that the reaction rate will be increased.

In the description, the ratio of the mass of the grinding medium 40 to the mass of the ingredients is preferably not less than 1 and less than 110.

In fig. 5A is a partial cross-sectional view illustrating other examples of sealed reactors that may be used in the present embodiment.

The sealed reactor 10C is different from the sealed reactor shown in FIG. 3, in that, as shown in FIG. 5A, this sealed reactor 10C has a J-shaped stirrer segment 22B at the lower end of the stirrer rod 22 rotating inside the main body of the reactor 12. This J-shaped stirrer segment 22B is curved along the inner peripheral surface of the bottom 12a of the main body of the reactor 12. When With the ingredients loaded inside the reactor main body 12 and the stirrer rod 22 rotating, the ingredients are less likely to stick to the inner wall of the reactor during mixing due to the stirrer segment 22B being curved in a J-shape along the inner peripheral surface of the bottom 12a. A grinding medium 40 such as that shown in FIG. 4 can be added accordingly. In a sealed reactor 10C, as shown in FIG. 5B, two grinding media elements 40 are added inside the main reactor body 12.

Second exercise

The method for producing sodium borohydride according to the second embodiment includes the steps of the first step to the third step described in relation to the method for producing sodium borohydride according to the first embodiment. Descriptions of features of steps that are the same as in the first embodiment will be omitted, and features different from those of the first embodiment will be described.

In the first step of the method for producing sodium borohydride according to the second embodiment, an alkali metal hydroxide or an alkali metal oxide is additionally charged into a sealed reactor. In the second stage, the mixture of sodium borate powder, aluminum powder and alkali metal hydroxide is dehydrated through a reaction in a sealed reactor filled with hydrogen gas at a temperature not lower than 400°C and not higher than 560°C. Alternatively, the ingredients are dehydrated by heating under vacuum conditions and then in a third step, sodium metaborate powder, aluminum powder and alkali metal oxide are mixed in a sealed reactor filled with hydrogen gas and the mixture is reacted at a temperature of not less than 400°C and not above 560 °C.

Examples of alkali metal hydroxides include sodium hydroxide (NaOH), potassium hydroxide (KOH), and lithium hydroxide (LiOH).

Examples of alkali metal oxides include sodium oxide (Na ₂ O), lithium oxide (Li ₂ O), sodium aluminum dioxide (NaAlO ₂ ), calcium oxide (CaO) and strontium oxide (SrO). Calcium oxide can be added to increase alkalinity.

When sodium hydroxide (NaOH) is used as the alkali metal hydroxide, sodium hydroxide reacts with aluminum and releases hydrogen into a sealed reactor, as shown by reaction equation (3) below. Thus, it is preferable to increase the amount of aluminum added to compensate for the loss of aluminum due to this interaction.

6NaOH + 2Al → 2NaAlO ₂ + 2Na ₂ O + 3H ₂ ... (3)

The method for producing sodium borohydride according to the second embodiment, including introducing a metal hydroxide (eg, NaOH), can produce sodium borohydride mainly in two ways. When a metal oxide (eg CaO) is used, this method is preferably carried out according to the first scheme, since the water content is low.

Below, a method for preparing sodium borohydride using the introduction of a metal hydroxide (eg, NaOH) will be described with reference to FIG. 6 - 7.

Fig. 6 is a flow chart for the synthesis of sodium borohydride (SBH) from sodium metaborate as an ingredient, including removal of water under a hydrogen atmosphere.

Fig. 7 is a flow chart for the synthesis of sodium borohydride (SBH) from sodium metaborate as an ingredient, including removing water by drying under reduced pressure without introducing hydrogen, followed by reacting under a hydrogen atmosphere.

As shown in FIG. 6, the first method for producing sodium borohydride according to the second embodiment includes a drying step (S-21), in which sodium metaborate is dried at a temperature of 280°C or higher, a mixing and contacting step (S-22), in which an alkali metal hydroxide, sodium borate and aluminum is brought into contact and mixed with the dried borate at room temperature, water removal step (S-23), in which the mixture reacts by dehydration under static conditions at a temperature of not less than 400°C in an atmosphere of hydrogen gas, thereby removing water, and the SBH borohydride formation step (S-24) in which the aluminum particles undergo an oxidation reaction with the water generated in the water removal step (S-23) to form a layer of alumina and sodium, represented by the chemical equation (I) below and passing from the surface towards the inside of the aluminum particles, and then reacting the mixture at a temperature of not lower than 400°C and not higher than 560°C, with stirring, to form sodium borohydride (SBH).

xNa ₂ O⋅Al ₂ O ₃ ... (I)

where 0 < x < 1.

Moreover, as shown in FIG. 7, the second method for producing sodium borohydride according to the second embodiment includes a drying step (S-31) in which the sodium metaborate is dried at a temperature of 280°C or higher, a mixing and contacting step (S-32) in which an alkali metal hydroxide, sodium borate and aluminum is brought into contact and mixed with the dried borate at room temperature, a water removal step (S-33) by drying under reduced pressure, in which the mixture is heated at a temperature of not less than 400°C under reduced pressure and reacted by dehydration in a static state, thereby removing water, and the SBH borohydride formation step (S-34), in which the aluminum particles are subjected to an oxidation reaction with water generated in the water removal step (S-33) to form a sodium aluminum oxide layer represented by the chemical equation of formula (I) and passing in the direction from the surface into the aluminum particles, after which the reaction is carried out in the mixture at a temperature of not lower than 400°C and not higher than 560°C with stirring in an atmosphere of hydrogen gas to form sodium borohydride (SBH).

xNa ₂ O⋅Al ₂ O ₃ ... (I)

where 0 < x < 1.

In the second method for producing sodium borohydride according to the second embodiment shown in FIG. 7, first remove water from the system by drying under reduced pressure. Thus, this second method is suitable when the ingredients contain a lot of water (for example, 2% or more). When dehydration is provided only by aluminum metal, as in the case of the first method for producing sodium borohydride according to the second embodiment shown in FIG. 6, Ingredients cannot be completely dehydrated if the water content is high (such as 10% or more). In this case, the second method is preferable.

The invention will further describe a first method for producing sodium borohydride according to the second embodiment shown in FIG. 6. The alkali metal hydroxide water is heated to reaction temperature and removed by reacting with the aluminum for at least 10 minutes without contact with the grinding medium, maintaining the reaction temperature. In the following description, such dehydration will be referred to as “reactive dehydration.”

Dehydration will be described below using sodium hydroxide (NaOH) as a typical example of alkali metal hydroxides. When the water generated from sodium hydroxide is maintained at high temperature under non-collision conditions with the grinding media, it reacts with the aluminum to produce hydrogen. The resulting aluminum oxide coordinates with sodium oxide (Na ₂ O), which is a metal oxide formed at the same time, and neutralizes sodium oxide to form sodium and aluminum dioxide (NaAlO ₂ = Na ₂ O⋅Al ₂ O ₃ ). When held at high temperature under static conditions, this dioxide releases sodium oxide (Na ₂ O) and turns into stable β "-aluminum oxide (0.17 _{Na 2} O⋅Al ₂ O ₃ ) or β-aluminum oxide (0.09 Na ₂ O ·Al ₂ O ₃ ), which as a result has the formula xNa ₂ O⋅Al ₂ O ₃ (0.09 < x < 1). As a result of crystallization, xNa ₂ O⋅Al ₂ O ₃ has a more stable structure than sodium dioxide and aluminum, and achieves high strength due to strong bonds in Al ₂ O _3. In the following, in the description, the xNa ₂ O⋅Al ₂ O ₃ layer with increased strength will be called the “hard shell”.

In the description, layers of sodium and aluminum oxide (xNa ₂ O⋅Al ₂ O ₃ ; 0 < x < 1) formed from β"-aluminum oxide (0.17Na ₂ O⋅Al ₂ O ₃ ) or β-aluminum oxide (0 .09Na ₂ O⋅Al ₂ O ₃ ) are called beta-alumina solid electrolytes. Such beta-alumina solid electrolytes are superionic conductors having sodium ions distributed between two-dimensional layers formed by alumina blocks, and these Sodium ions move at high speed between these layers.

The layer of sodium and aluminum dioxide is soft immediately after formation. This soft layer of sodium dioxide and aluminum will be referred to hereafter as the "soft shell". When the grinding medium is agitated, this soft shell is separated from the aluminum particles, even under mild agitation conditions.

The sodium and aluminum dioxide separated from the aluminum particles are small in size and arrive at the site with high kinetic energy and are therefore less likely to crystallize into β-alumina, releasing some sodium oxide. Once released, sodium oxide is located away from the aluminum surface and cannot be reduced by aluminum.

The purpose of adding alkali metal hydroxide is to increase the amount of metal oxide (sodium oxide (Na ₂ O)) on the surface of the aluminum, and therefore it is preferable to keep the mixture at heating temperature without stirring until dehydration is complete.

The hard shell obtained by reaction dehydration protects the reaction site on the aluminum surface from gentle mixing. In addition, the β"-alumina (xNa ₂ O⋅Al ₂ O ₃ ) hard shell has suitable ion permeability and allows sodium, boron, aluminum, oxygen and hydrogen ions to move within the hard shell through diffusion. In particular, ions sodium can move with a high migration rate. At the aluminum boundary, where the reduction reaction occurs, sodium oxide (Na ₂ O), released from sodium dioxide and aluminum as a result of the continued formation of β"-alumina and β-alumina, is reduced by aluminum to metallic sodium The resulting sodium metal has low solubility in solid phase aluminum and accumulates on the surface and in defects such as crystal grain boundaries.

Sodium metal present on the surface of aluminum reduces hydrogen gas, and sodium metal accumulated in defects in aluminum reduces hydrogen atoms, thereby forming sodium hydride (NaH). Sodium hydride easily passes through the hard shell and diffuses into and out of the hard shell. When boron oxide (B ₂ O ₃ ) diffuses into the area near sodium hydride (NaH), this sodium hydride is stripped of hydrogen (hydride ion) by boron oxide to form sodium borohydride (SBH), as shown by reaction equation (4) below .

8NaH + B ₂ O ₃ → 3Na ₂ O + 2NaBH ₄ ... (4)

This reduction reaction produces sodium borohydride, and partially returns sodium oxide. The migration of ions in the solid shell occurs mainly with sodium ions, but a small number of boron ions also move into the solid shell, so that the reaction to form sodium borohydride also occurs within the solid shell. The sodium oxide required for the reaction is supplied from outside through the solid shell, and the reaction continues continuously.

This reduction reaction occurs in regions of aluminum enriched in sodium metal and consequently continues on the surface of the aluminum and in crystalline defects in the aluminum. At the same time, the substitution reaction occurs in a wide range of regions as a result of the migration of sodium hydride.

The reactions will now be described in detail with reference to simplified reaction schemes (FIGS. 8A to 8G) illustrating the reaction steps for the formation of sodium borohydride (SBH) using sodium hydroxide (NaOH) as the alkali metal hydroxide to produce an increased amount of metal oxide (sodium oxide (Na ₂ O)).

In fig. 8A - fig. 8G shows simplified reaction schemes illustrating the reaction steps for the synthesis of sodium borohydride (SBH).

On the surface of the aluminum (Al) particles 101, as shown in FIG. 8A, there is a film of oxide (Al ₂ O ₃ ·H ₂ O) 102 formed as a result of natural oxidation. This oxide 102 film, obtained by natural oxidation, is a dense layer with a thickness of 0.01 μm.

As shown in FIG. 8B, the oxide film (Al ₂ O ₃ ⋅H ₂ O) 102 on the surface of the aluminum particle 101 is dissolved in water generated from sodium hydroxide 103 and in sodium hydroxide (NaOH) 103. As a result of this dissolution, sodium hydroxide (NaOH) begins to react directly with the surface (with the newly exposed surface) of the aluminum 101 particle.

When the aluminum particle 101 is heated and the temperature is in the range of 300°C and below, oxidation is caused by water generated from sodium hydroxide (NaOH) 103, and aluminum reacts with sodium hydroxide as shown by reaction equation (5) below to form a soft layer of boehmite ( Al ₂ O ₃ ⋅xH ₂ O) 104, sodium oxide (Na ₂ O) 105 and hydrogen (H ₂ ) 106.

4NaOH + 2Al + H ₂ O → Al ₂ O ₃ ⋅H ₂ O + Na ₂ O + 2H ₂ ... (5)

The boehmite layer (Al ₂ O ₃ ⋅H ₂ O) 104 releases most of its water at a temperature of 230°C – 300°C or about 500°C. In addition, sodium hydroxide (NaOH) melts at about 320°C and reacts directly with aluminum (Al) to form Na ₂ O, Al ₂ O ₃ and H ₂ .

As shown in FIG. 8C, sodium oxide (Na ₂ O) 105 and aluminum oxide (Al ₂ O ₃ ) combine with each other, as shown by reaction equations (6) - (9) below, to form a soft shell 107 consisting of sodium dioxide and aluminum ( Na ₂ O Al ₂ O ₃ ).

6NaOH + 2Al → Na ₂ O⋅Al ₂ O ₃ + 2Na ₂ O + 3H ₂ ... (6)

Al ₂ O ₃ ·H ₂ O → Al ₂ O ₃ + H ₂ O ... (7)

3H ₂ O + 2Al → Al ₂ O ₃ + 3H ₂ ... (8)

Na ₂ O + Al ₂ O ₃ → Na ₂ O⋅Al ₂ O ₃ ... (9)

As shown in FIG. 8D, the soft shell layer 107 formed from sodium aluminum dioxide (Na ₂ O·Al ₂ O ₃ ) is gradually converted into beta-prime aluminum oxide (β"-alumina) 0.17Na ₂ O·Al ₂ O ₃ by heating to a high temperature of at least 400°C over time. During the formation of this beta-alumina, the soft shell releases part of Na ₂ O (0.8 Na ₂ O) 105 and forms a hard shell 108 consisting of beta"-alumina . The solid shell 108 resulting from the formation of β"-alumina is a superionically conductive solid electrolyte permeable to ions such as, for example, H ⁺ , Na ⁺ , O ^{2 -} , B ³⁺ and Al ³⁺ .

As shown in FIG. 8E, the liberated portion of sodium oxide (Na ₂ O) 105 comes into contact with aluminum and is partially reduced by aluminum to form sodium metal (Na) 109 and aluminum oxide (Al ₂ O ₃ ). Aluminum oxide combines with sodium oxide to form sodium aluminum dioxide, which is then converted to β"-alumina. Sodium metal (Na) 109 has low solubility in solid phase aluminum and accumulates on the aluminum surface and in crystalline defects.

As shown in FIG. 8F, hydrogen gas (H ₂ ) passes through the solid shell of β"-aluminum oxide and dissociates, breaking up into hydrogen atoms, which then enter the interior of the aluminum 101 particle. The hydrogen atoms react with sodium metal (Na), the concentration of which is increased at the surface aluminum and in crystalline defects of aluminum, thereby forming sodium hydride (NaH) 110. The resulting sodium hydride (NaH) 110 leaves the aluminum particle 101, passing into the product phase, and diffuses through this product phase. In addition, sodium hydride (NaH ) 110 accumulates and also remains in the crystalline defects of the aluminum (Al) 101 particle.

As shown in FIG. 8G, sodium hydride (NaH) 110 reacts with diboron trioxide (B ₂ O ₃ ) 111 to form sodium borohydride (SBH) 112 and sodium oxide (Na ₂ O) 105. In addition, sodium hydride (NaH) accumulated in crystal defects aluminum particles 101, reacts with diboron trioxide (B ₂ O ₃ ) 111, diffused into these defects, thus forming sodium borohydride (SBH) 112 and sodium oxide (Na ₂ O) 105.

In addition, sodium hydride (NaH) 110, diffusing in the solid shell 108 of β"-alumina, meets diboron trioxide (B ₂ O ₃ ) 111 and reacts with it to form sodium borohydride (SBH) 112 and sodium oxide ( Na ₂ O) 105.

As described above, sodium hydride (NaH) 110 reacts on the surface and within the aluminum (Al) particle 101, and also in the product phase. The reaction products are grouped at the boundaries of crystal grains and in defects of the aluminum (Al) particle 101, which are enriched with sodium nitride (NaH). The microstructure of the aluminum (Al) particle 101 is sometimes broken into thin segments by the reaction product formed within the β"-alumina solid shell 108.

Sodium hydride (NaH) has a small molecular size and can diffuse more easily than diboron trioxide (B ₂ O ₃ ). Thus, sodium hydride, although in a solid state, can move in and away from the aluminum (Al) particle 101 (regions located away from the surface of the aluminum particle) towards the reaction zones where the ingredient still remains. As a result, the reaction rate and reaction yield increase.

In fig. 9A is an image illustrating the analysis result (SEM) of a granule prepared by mixing sodium metaborate and aluminum. Here, the sodium-aluminum metaborate granule has a Na/B ratio of 1 and an Al particle size of 30 µm. As shown in FIG. 9A, the aluminum 101 particles are slightly flattened grains, and the sodium metaborate forms a uniform structure.

In fig. 9B is an electron backscatter image captured at a portion indicating the progress of the reaction in Test Example 12 described later involving the addition of sodium hydroxide, along with the result of the SEM analysis. The image magnification is approximately 3 times higher relative to the size of Fig. 9A.

In fig. 9B, Na and B are probably included in NaBH ₄ (SBH); O, B and Na are probably included in sodium metaborate (NaBO ₂ ); Na, Al, B and O are probably included in NaBH ₄ (SBH) + sodium oxide (Na ₂ O) + diboron trioxide (B ₂ O ₃ ) + aluminum oxide (Al ₂ O ₃ ); O, Al, Na and B are probably included in aluminum oxide (Al ₂ O ₃ ) + sodium oxide (Na ₂ O) + diboron trioxide (B ₂ O ₃ ) + NaBH ₄ (SBH); both Na and O are probably included in sodium oxide (Na ₂ O).

As shown in FIG. 9B, a hard shell 108 is formed in the aluminum particle 101 shown in FIG. 8D above, and the synthesis reaction of sodium borohydride (SBH) occurs inside the solid shell 108. The image shows that inside the solid shell 108, part of the sodium oxide (Na ₂ O) (0.8 Na ₂ O) 105 was released and formed sodium borohydride (SBH ) 112.

In addition, sodium borohydride (SBH) 112 melts at the reaction temperature (about 500°C) and the melts are combined. This sodium borohydride (SBH) 112, which is the result of the accumulation of melts, collects in the cracks of the aluminum particle 101. Moreover, the reaction continues along this crack, and the yield of the product of this reaction increases.

In fig. 9C is an electron backscatter image illustrating the state after completion of the reaction in the following Test Example 12 involving the addition of sodium hydroxide. The image magnification is approximately 6 times higher relative to the size of Fig. 9A.

As shown in FIG. 9C, which shows the structure of the β"-alumina hard shell 108, this shell contains sodium borohydride (SBH). This hard shell is fragmented into thin segments.

In fig. 10A is an electron backscatter image captured during the reaction of Test Example 12 described below prior to SEM elemental analysis. In fig. 10B is an image after SEM analysis illustrating the positions of analysis points and the evaluation results of chemical contents. Enlargement of Fig. 10B is about 2 times the size of FIG. 10A. In fig. 10C is a table illustrating the results (atomic percentages (%)) for the SEM analysis points performed after the analysis at the points corresponding to the numbers in FIG. 10A. The isolated substances visible in the electron backscatter image will be discussed next following the analysis illustrated in FIG. 10B. For example, section 6 in FIG. 10B corresponds to point No. 6 analysis in Fig. 10C and is considered as sodium hydride (NaH). Sodium hydride (NaH) melts under intense electron beam irradiation and is thus likely released. Based on the assumption that these precipitates were sodium hydride, much sodium hydride (NaH) was found at the boundaries of xNa ₂ O·Al ₂ O ₃ structures. Sodium hydride (NaH) was also found in small quantities in the layers of xNa ₂ O·Al ₂ O ₃ structures. At the same time, sodium hydride (NaH) was not detected at all in areas outside the solid shell layers, where sodium metaborate (NaBO ₂ ) is still present.

In fig. 11A is an electron backscatter image at the completion of the reaction in Test Example 12 described below, prior to elemental analysis. In fig. 11B is an SEM image after analysis. In fig. 11C is a table illustrating the results (atomic percentages (%)) for SEM analysis points obtained prior to elemental analysis. These drawings show that sodium hydride (NaH) is generated where xNa ₂ O·Al ₂ O ₃ is present. In addition, sodium metaborate (NaBO ₂ ) is present at point 13 indicated in FIG. 11B, and this point contains no precipitates, indicating that there was no sodium hydride (NaH) present.

Third implementation

The method for producing sodium borohydride in the third embodiment includes the steps of the first step to the third step described in the method for producing sodium borohydride in the first embodiment. Descriptions of features of steps that are the same as in the first embodiment will be omitted, and descriptions of features different from those of the first embodiment will be given. The third embodiment differs from the first embodiment in that instead of sodium borate in the first step, the third embodiment uses sodium diborate (Na ₄ B ₂ O ₅ ).

In the present embodiment, sodium diborate (Na ₄ B ₂ O ₅ ) is prepared in advance by adding sodium hydroxide to sodium borate. Na/B (in moles) in sodium diborate (Na ₄ B ₂ O ₅ ) is 2.

Sodium diborate (Na ₄ B ₂ O ₅ ) is prepared as follows. Sodium tetraborate (hydrate) or sodium metaborate (hydrate) and sodium hydroxide are taken in such quantities that the Na/B ratio is 2, and loaded into an alkali-resistant vessel, such as a nickel vessel. The vessel is then heated to produce a molten sodium borate hydrate and the melt is maintained in a molten state by heating while avoiding boiling to evaporate the water and reduce the water content. After this, the melt solidifies. The resulting solid material containing the hydrate is heated to 400°C and maintained at this temperature for 1 – 4 hours to obtain a solid mixture of sodium oxide and sodium diborate. When Na/B = 2 or less, the product is a mixture of sodium diborate and sodium metaborate. At higher ratios the product is a mixture of sodium diborate and sodium oxide. The resulting solid is highly hygroscopic so that it is desirably stored in a container containing no or very little water, or it is desirably ground into powder in an anhydrous atmosphere immediately before use.

In the method according to the third embodiment, the reaction system consists of aluminum powder (solid), sodium diborate (solid) and hydrogen (gaseous). To ensure proper reactions in this reaction system, it is important to remove the water adsorbed in the second step from the sealed reactor by vacuum heat treatment, given that sodium diborate powder is prone to adsorbing water during handling.

When the ingredients do not contain water, stirring can be started as soon as the reaction temperature is reached. If, however, these ingredients contain residual water, mixing is started after the aluminum oxidation reaction. Stirring creates cracks in the oxide film on the surface of the aluminum to expose a fresh surface where the ingredients come into contact with the aluminum to begin the sodium borohydride synthesis reaction.

When the water has been completely removed and the reaction initiated is only the reaction of sodium borohydride, oxides, including aluminum oxides, are formed, but do not form an oxide film on the surface of the aluminum, since at the same time sodium borohydride (SBH) is formed as a reaction product. When no oxide film is formed, reactions continue according to the diffusion rates of the ingredients through the solid. When sodium diborate is used as an ingredient, the amount of sodium oxide produced is large enough to avoid a shortage of sodium oxide at the interfaces where the reaction takes place, thereby allowing the reactions to continue uninterrupted.

In a third embodiment, using sodium diborate powder, aluminum is reacted with water from the sodium diborate powder. In light of this, aluminum is loaded into a sealed reactor in the first stage at a molar ratio of aluminum to boron in sodium diborate of at least 4/3. In particular, it is preferable to add aluminum in such an amount that the molar ratio of aluminum, excluding the amount of aluminum that will react with water, to boron in the sodium diborate is not less than 4/3.

It is preferable that the amount of aluminum powder added as an ingredient is 20 wt%. more taking into account the loss of aluminum due to reaction with water and the formation of aluminum oxide.

Test Cases and Comparison Cases

The present embodiment will be described in more detail by presenting test examples illustrating the beneficial effects of the present embodiment. However, it should be understood that the present implementation is not limited to these examples.

Further, in test examples 1 – 3, sodium metaborate (NaBO ₂ ) is used.

Test examples 4 – 7 use sodium diborate (Na ₄ B ₂ O ₅ ).

Test Examples 8 use sodium metaborate (NaBO ₂ ) and sodium diborate (Na ₄ B ₂ O ₅ ).

Test Example 9 uses sodium diborate (Na ₄ B ₂ O ₅ ) and borax (sodium tetraborate (Na ₂ B ₄ O ₇ )).

Test Examples 10 - 12 use sodium metaborate powder and include alkali metal hydroxide (NaOH) as an additive.

Test Example 13 uses sodium metaborate (NaBO ₂ ) and borax (sodium tetraborate (Na ₂ B ₄ O ₇ )), and includes alkali metal hydroxide (NaOH) as an additive.

Test Examples 14 - 16 use sodium metaborate powder and include alkali metal hydroxide (NaOH) as an additive. In Test Examples 14 – 16, heating in a vacuum (drying under reduced pressure) is performed before adding hydrogen.

In Test Example 17, sodium metaborate powder is used, and alkali oxide (CaO) is included as an additive.

In Comparative Example 1, the same conditions as in Test Example 1 are used, but the temperature in the third stage is increased (583°C).

Comparative Example 2 reproduces Test Example 1 at a higher temperature (510°C) and lower maximum pressure (0.53 MPa) in the third stage.

Comparative Example 3 reproduces Test Example 1 at a lower temperature (230°C) in the third step.

Comparative Example 4 reproduces Test Example 1 at a higher temperature (673°C) in the third step without stirring.

Comparative Example 5 reproduces Test Example 1, changing the ratio of sodium borate to borax (Na/B = 0.5).

Comparative examples 6 – 9 reproduce test example 11, with a second stage of dehydration with the introduction of hydrogen and without a grinding medium during the heating process.

Test case 1

The sodium borate used as an ingredient in Test Example 1 is sodium metaborate powder.

(a) First stage

The mixture is prepared by mixing 1.94 g of sodium metaborate, ground and sifted through a 100 µm sieve, and 1.06 g of aluminum powder (average particle size: 30 µm), which must react with the above mass of sodium metaborate in a molar ratio of 4/ 3, as shown by reaction equation (10) below. The mixture is charged into a sealed reactor 10B shown in FIG. 4, at room temperature. Next, the sealed reactor is connected to a vacuum pump and pumped out, after which it is filled with hydrogen gas (non-oxidizing gas).

4Al + 6H ₂ + 3NaBO ₂ → 3NaBH ₄ + 2Al ₂ O ₃ ... (10)

(b) Second stage

After the first stage, the ingredients are dehydrated in the second stage (water removal stage), in which the interior of the sealed reactor is heated to 300°C, pumping out the sealed reactor through an associated vacuum pump to remove water adsorbed by sodium metaborate and aluminum powder, and hydration sodium metaborate water.

(c) Third stage

Hydrogen gas (non-oxidizing gas) is introduced into the sealed reactor to a pressure of 0.5 MPa at room temperature, after which the interior of the sealed reactor is heated to 512°C. Immediately thereafter, the stirrer in the sealed reactor is turned on to rotate the stirrer segments 22A, and the mixture is stirred at a peripheral speed of 40 cm/s of the stirrer for 4.0 hours while maintaining a heating temperature of 512°C. At the end of the third stage, mixing is completed when the pressure increase in the sealed reactor stops and the contents of the reactor are cooled. The initial pressure of hydrogen gas in the above process is 0.93 MPa, and the final pressure of hydrogen gas is 0.48 MPa. The grinding medium used 40 is alumina ceramic balls (diameter 5 mm), and the weight ratio of these balls to the weight of the ingredients is 55.

Sodium borohydride (SBH) was obtained using the method described above.

After the completion of the third step, the reaction yield is calculated based on the amount of pressure reduction of the hydrogen gas. This amount of pressure reduction is determined by subtracting the minimum pressure at the reaction conditions from the maximum pressure of the gas and converting the resulting difference to the volume (number of moles) of hydrogen gas. As a result, the reaction yield (SBH yield) is 33.6%.

In addition, the content of sodium borohydride in the reaction products was determined to be 12.0% by the iodometric titration method described below. The reaction yield obtained from this content is 33.7%.

Iodometric titration method

(1) Take a sample (reaction products) in a beaker in the amount of 50 mg and measure the weight accurate to the first decimal place (accurate to 0.1 mg).

(2) The sample weighed in step (1) is transferred to a closed 200 ml conical flask. 40 ml of 20 g/l NaOH solution is added to a closed conical flask and the mixture is heated in a water bath to completely decompose the unreacted aluminum powder.

(3) The decomposition product from step (2) is cooled to room temperature and 20.0 ml of 0.05 M iodine solution is added using a graduated pipette. After this, the flask is closed and left in a dark place for 15 minutes.

(4) After step (3), 3 ml of hydrochloric acid is added to the flask and the flask is shaken vigorously. The mixture is then titrated with 0.1 M sodium thiosulfate.

(5) The titration is completed when the purple color of the iodine disappears.

(6) The control test is carried out without a sample. The sodium borohydride content is calculated using the following equation.

Equation for calculating sodium borohydride content

NaBH ₄ (wt.%) = {(A - B) × 0.1 × f × 37.83/8}/C × 100

The variables and constants in this equation are described below.

A: Volume (ml) of 0.1 M sodium thiosulfate solution used for titration of the blank experiment

B: Volume (ml) of 0.1 M sodium thiosulfate solution used for titration of the sample solution

f: Titer of 0.1 M sodium thiosulfate solution

C: Sample weight (mg)

37.83: Molar mass (g/mol) of sodium borohydride

8: Normality (N) of 1 mol/l sodium borohydride solution

Test case 2

In Test Example 2, sodium borohydride (SBH) is prepared by repeating the procedure of Test Example 1 except that in the third step, unlike Test Example 1, the heating temperature is changed to 520°C and stirring is carried out at a lower speed (peripheral speed mixing device: 13 cm/s) for 1.9 hours. As a result, the reaction yield (SBH yield) is 26.3%.

Test case 3

Sodium borohydride (SBH) was prepared by repeating the procedure of Test Example 1 except that the sodium borate as an ingredient in Test Example 1 was replaced by 0.7 g sodium metaborate and 1.48 g borax, the heating temperature was changed to 516°C, and mixing time changed to 2.9 hours. As a result, the reaction yield (SBH yield) is 21.6%.

Table 2 below shows the results of test cases 1 – 3.

Comparative example 1

Sodium borohydride (SBH) was prepared by repeating the procedure of Test Example 1 except that in the third step of Test Example 1, the heating temperature was increased (583°C) and the stirring time was changed to 3.0 hours. As a result, the reaction yield (SBH yield) is 7.2%.

Comparative example 2

Sodium borohydride (SBH) is prepared by repeating the procedure of Test Example 1 except that in the third step, unlike Test Example 1, the heating temperature is changed to 510°C, the maximum pressure is lower (0.53 MPa), and the stirring time is changed to 4.1 hours As a result, the reaction yield (SBH yield) is 18.2%.

Comparative example 3

Sodium borohydride (SBH) was obtained by repeating the procedure of Test Example 1, except that in the third stage, unlike Test Example 1, the heating temperature was reduced (230°C) and the stirring time was changed to 10.6 hours. As a result, the reaction yield (SBH yield) is 8.7%.

Comparative example 4

Sodium borohydride (SBH) was prepared by repeating the procedure of Test Example 1 except that the amounts of ingredients were halved compared to Test Example 1 and the third step was performed at a heating temperature of 673°C without stirring. As a result, the reaction yield (SBH yield) is 2.1%.

Comparative Example 5

Sodium borohydride (SBH) was prepared by repeating the procedure of Test Example 1 except that the sodium borate as an ingredient in Test Example 1 was replaced by 1.98 g of borax instead of sodium metaborate, the heating temperature was changed to 514°C, and the stirring time was changed to 2 ,5 hours. At the same time, Na/B in comparative example 5 5 is 0.5. As a result, the reaction yield (SBH yield) is 4.8%.

Table 3 shows the results of comparative examples 1 – 5.

As shown in Table 2, Test Cases 1 – 3 using heating temperatures of 512°C – 520°C achieved suitable SBH borohydride conversions of 33.6%, 26.3% and 21.6%. Moreover, as shown in Test Example 3, sodium borohydride was synthesized with adequate efficiency even when the ingredient was a mixture of sodium metaborate and borax.

In contrast, as shown in Table 3, in Comparative Example 1 using a high heating temperature of 583°C, the reaction stops at high pressure of hydrogen produced by the decomposition of sodium borohydride, resulting in a low SBH borohydride conversion of 7.2%. Comparative Example 2 uses a high temperature (510°C) and a low maximum pressure (0.53 MPa) in the third step and therefore does not achieve increased synthesis of sodium borohydride, resulting in a low SBH borohydride conversion of 18.2 %. Comparative Example 3 involves a low temperature (230° C.) in the third step, and as a result, increased synthesis of sodium borohydride is not achieved, resulting in a low SBH borohydride conversion of 8.7%. In Comparative Example 4, the temperature is 673°C and stirring is not performed, whereby the synthesis of sodium borohydride was not increased and the conversion rate of SBH borohydride was only 2.1%. Comparative Example 5 includes borax (Na/B = 0.5) as the sodium borate, and as a result, increased sodium borohydride synthesis is not achieved, resulting in a low SBH borohydride conversion of 4.8%.

Test case 4

Sodium borohydride (SBH) was prepared by repeating the procedure of Test Example 1 except that the sodium borate as an ingredient in the first step of Test Example 1 was replaced by 2.94 g of sodium diborate (Na ₄ B ₂ O ₅ ) and heating was carried out at a temperature of 416 °C for 7.0 hours, and while mixing under rolling and grinding conditions (peripheral speed of the mixing device: 140 cm/s). In test case 4, Na/B (in moles) is 2.00.

In Test Example 4, water was removed by drying by heating under vacuum. Sodium diborate is synthesized in an aqueous solution and the water is evaporated to dryness. The resulting solid is ground and dried at 400°C. However, the powder absorbs water during storage in a desiccator and contains about 5% water. To avoid a decrease in the reaction yield due to this 5% water, drying was carried out by heating under vacuum in a sealed reactor before the mixing operation (the same applies to Test Example 5 and Test Example 6).

As a result, the reaction yield (SBH yield) is 32.6%.

Test Case 5

Sodium borohydride (SBH) was prepared by repeating the procedure of Test Example 4 except that in the third step in Test Example 4, heating was carried out at a heating temperature of 509°C for 7.1 hours while stirring (peripheral speed of the stirrer: 40 cm/s ). In test example 5 Na/B (in moles) is 2.00. As a result, the reaction yield (SBH yield) is 60.4%.

Test case 6

Sodium borohydride (SBH) is prepared by repeating the procedure of Test Example 4 except that in the third step in Test Example 4, heating is carried out at a heating temperature of 560°C for 4.0 hours while stirring (peripheral speed of the stirrer: 40 cm/s ). In Test Example 6 Na/B (in moles) is 2.00.

As a result, the reaction yield (SBH yield) is 57.6%.

Test case 7

Sodium borohydride (SBH) is prepared by repeating the procedure of Test Example 4 except that the amount of aluminum powder as an ingredient in Test Example 4 is increased by 20 wt%. to 1.27 g, and in the third stage, heating is carried out at a heating temperature of 495°C for 7.1 hours with stirring (peripheral speed of the stirrer: 40 cm/s). In Test Example 7, Na/B (in moles) is 2.00. As a result, the reaction yield (SBH yield) is 73.2%.

Test Case 8

Sodium borohydride (SBH) was prepared by repeating the procedure of Test Example 4 except that the sodium borate as an ingredient in Test Example 4 was replaced by 0.82 g of sodium metaborate and 1.31 g of sodium diborate, and heating was carried out at a heating temperature of 496°C for 12.8 hours with stirring (peripheral speed of the stirring device: 40 cm/s). As a result, the reaction yield (SBH yield) is 40.7%.

Test case 9

Sodium borohydride (SBH) was prepared by repeating the procedure of Test Example 4 except that the sodium borate as an ingredient in Test Example 4 was replaced by 0.98 g of borax and 1.0 g of sodium diborate, and heating was carried out at a heating temperature of 495°C at for 7.6 hours with stirring (peripheral speed of the stirring device: 40 cm/sec). As a result, the reaction yield (SBH yield) is 37.5%.

Table 4 shows the results of test cases 4 – 9.

As shown in Table 4, Test Examples 4 – 7 resulted in suitable SBH borohydride conversions of 32.6%, 60.4%, 57.6% and 73.2%. In particular, in Test Example 7, a high conversion rate of SBH borohydride of 73.2% was achieved due to 20 wt%. an increase in the amount of aluminum powder, which compensates for the loss of aluminum due to the oxidation reaction. The conversion rate of SBH borohydride is also high, up to 40.7% and 37.5%, in Test Example 8, including sodium metaborate and sodium diborate as sodium borates, and in Test Example 9, including borax and sodium diborate as sodium borates.

Test case 10

In the first step, in Test Example 1, aluminum powder was added in an amount of 1.33 g, and sodium hydroxide powder was added in an amount of 1.18 g. Thus, sodium borohydride (SBH) was obtained. In the test example, 10 Na/B (in moles) is 2.00. As a result, the reaction yield (SBH yield) is 40.7%.

Test case 11

In Test Example 11, the system was held at a predetermined temperature prior to mixing to remove water by reacting with aluminum.

In fig. 12 is a graph illustrating the production process according to the production method of sodium borohydride in Test Example 11.

In the first step, a mixture is prepared by mixing 1.94 g of sodium metaborate, sifted through a 100 μm sieve, 2.36 g of sodium hydroxide powder and 1.59 g of aluminum powder (average particle size: 30 μm), which includes 1. 06 g, corresponding to 4/3 mol of boron in sodium metaborate, and 0.53 g, corresponding to the amount of aluminum that must react with the water contained in 2.36 g of sodium hydroxide. This mixture is loaded into a sealed reactor, the sealed reactor is pumped out at room temperature, then filled with hydrogen gas to a pressure of 0.25 MPa and tightly closed.

In the second step, the interior of the sealed reactor is heated to 515°C, after which it is maintained at this temperature for 15 minutes to evaporate the water contained in the sodium metaborate powder, sodium hydroxide powder and aluminum powder. This water is removed by reaction with aluminum. The pressure inside the sealed reactor rises to 0.88 MPa as a result of the elevated temperature and the generation of hydrogen gas from the water reaction. During this procedure, the mixing device did not work, i.e. The hydrogen generation reaction took place without stirring.

In the third stage, the interior of the sealed reactor is heated to 520°C, and the stirrer is started at a rotation speed of 300 rpm (peripheral speed of the stirrer: 40 cm/s). The pressure inside the sealed reactor increases to 0.88 MPa. The pressure of hydrogen gas begins to drop sharply 0.5 hours after the start of stirring and decreases to 0.43 MPa after 0.95 hours of stirring. At this point, hydrogen gas is introduced to raise the pressure inside the sealed reactor to 0.5 MPa.

After this, stirring is carried out for 0.3 hours, but there is no decrease in the pressure of hydrogen gas. Then stirring and heating are completed. In this way sodium borohydride (SBH) is obtained. In Test Example 11 Na/B (in moles) is 3.00. As a result, the reaction yield (SBH borohydride conversion rate) calculated based on the reduction amount of hydrogen gas is 49.8%. The grinding medium used is alumina ceramic beads (5 mm in diameter), and the weight ratio of these beads to the weight of ingredients is 55, similar to Test Example 1.

Test case 12

In the first step, in Test Example 11, sodium hydroxide powder was added in an amount of 2.35 g; in the second stage, the conditions were changed to a temperature of 515°C and a time of 95 minutes; and in the third stage, the heating temperature is changed to 515°C. The sealed reactor is replaced by a sealed reactor 10C including a stirring device shown in FIG. 5B (J-shaped segment of the mixing device 22B), and having two columnar mullite balls as the grinding medium 40 (3.1 g×2, weight ratio to ingredients: 1.16). The rotation speed is such that the peripheral speed of the stirring device is 130 cm/s, and the duration of the stirring time is changed to 4.0 hours. The rest of the procedure is the same as in Test Example 11. Sodium borohydride (SBH) is thus obtained. In the test example, 12 Na/B (in moles) is 2.95. As a result, the reaction yield (SBH yield) is 64.2%.

Test case 13

Sodium borohydride (SBH) is prepared by repeating the procedure of Test Example 10, except that the sodium borate as an ingredient in Test Example 10 is replaced by 0.95 g of sodium metaborate and 1.56 g of borax, and 1.60 g of aluminum powder is added, sodium hydroxide powder was added in an amount of 2.35 g, the heating temperature was changed to 495°C, and the stirring time was changed to 8.5 hours. In Test Example 13, the Na/B ratio (in moles) is 1.95. As a result, the reaction yield (SBH yield) is 59.8%.

Test case 14

In the second step of Test Example 10, water was removed from the system by drying at 400°C under reduced pressure for 4 hours. In the third stage, hydrogen is introduced, the interior of the reactor is heated, and then the stirring device in the sealed reactor is immediately turned on to rotate the stirring device at a peripheral speed of 40 cm/s, and stirring is carried out for 6.8 hours, maintaining the heating temperature at 501°C. In this process, the amount of sodium hydroxide powder added is 2.34 g. Sodium borohydride (SBH) is thus obtained. In the test example, 14 Na/B (in moles) is 2.97. As a result, the reaction yield (SBH yield) is 53.2%.

Test case 15

In Test Example 14, stirring was carried out for 6.1 hours while maintaining the heating temperature at 522°C. The amount of sodium hydroxide powder added is 2.36 g. Sodium borohydride (SBH) is thus obtained. In the test example, 15 Na/B (in moles) is 2.98. As a result, the reaction yield (SBH yield) is 58.6%.

Test case 16

In Test Example 14, stirring was carried out for 7.6 hours while maintaining the heating temperature at 505°C. The amount of added sodium hydroxide powder is 1.17 g. Thus, sodium borohydride (SBH) is obtained. In the test example, 16 Na/B (in moles) is 1.99. As a result, the reaction yield (SBH yield) is 60.8%.

Test case 17

Sodium borohydride (SBH) was prepared by repeating the procedure of Test Example 1, except that in the first step, in Test Example 1, sodium hydroxide was replaced by 0.55 g of calcium oxide powder, and heating was carried out at a heating temperature of 495°C for 18.0 hours. . In test example 17 Na (including Ca)/B (in moles) is 1.33. As a result, the reaction yield (SBH yield) is 42.7%.

Table 5 below lists the results of test cases 10 – 17.

As shown in Table 5, Test Example 10 involving the addition of sodium hydroxide produced an SBH borohydride conversion of 40.7%, and a remarkable increase was achieved compared to the SBH borohydride conversion of 26.3% in Test Example 2, which does not include the addition of sodium hydroxide.

In Test Examples 11 to 13, a large amount of water released from sodium hydroxide was successfully removed by the dehydration reaction before stirring, and thereby the suitable SBH borohydride conversion rates of 49.8%, 64.2% and 59.8% were achieved.

As noted above, Test Example 12 performs SEM analysis. The SEM elemental analysis presented in FIG. 1 will be discussed next. 9A – 11B. Mixing is carried out under the following conditions: a J-segment mixing device, two ceramic balls with a mass ratio to ingredients of 1.16 and a peripheral speed of 130 cm/s of the mixing device. Under these conditions, where the volume of ingredients is greater than the volume of the grinding medium, the likelihood of collisions between the balls and the grinding medium is low and a large volume of ingredients will be trapped between them during collisions. So this stirring is gentle and leaves the hard shells intact. The shape of the remaining aluminum changes from the original to an irregular shape as a result of the reaction along the defects. The reaction also occurs outside of solid shells as a result of diffusion of sodium hydride, and this likely contributes to the increased yield of the reaction. In Test Examples 14 – 16, a large amount of water released from sodium hydroxide was successfully removed by drying under reduced pressure. In Test Examples 14 – 16, borohydride SBH is synthesized by introducing hydrogen after removing water. By this removal of water before the synthesis reaction, sodium borohydride is efficiently synthesized with suitable SBH borohydride conversions of 53.2%, 58.6% and 60.8%. Test Example 17, including calcium oxide instead of sodium hydroxide, similarly results in efficient synthesis of sodium borohydride with an SBH conversion of 42.7%.

Comparative Example 6

In the second step, in Test Example 11, the stirrer is turned on to start stirring under a hydrogen atmosphere at a rotation speed of 90 rpm (peripheral speed of the stirrer: 12 cm/s), and water is removed while performing gentle stirring.

The grinding medium 40 used is stainless steel balls (5 mm in diameter), and the ratio of the mass of the balls to the mass of the ingredients is 110. The sealed reactor is replaced by a sealed reactor 10C including a mixing device shown in FIG. 5A (J-shaped stirrer segment 22B), and mixing is carried out at a rotation speed of 300 rpm (peripheral speed of the stirrer: 40 cm/s). The heating temperature is changed to 515°C, and the stirring time is 1.3 hours. The rest of the procedures are the same as in Test Example 11. Sodium borohydride (SBH) is thus obtained. As a result, the reaction yield (SBH yield) is 12.5%.

Comparative example 7

Sodium borohydride (SBH) was prepared according to the procedure of Test Example 11, but the heating temperature was changed to 321°C and the heating duration was changed to 5.0 hours. In the second step of this process, stirring is started under a hydrogen atmosphere at a rotation speed of 1150 rpm (peripheral speed of the stirrer: 140 cm/s), and water is removed during stirring. As a result, the reaction yield (SBH yield) is 22.5%.

Comparative Example 8

Sodium borohydride (SBH) was prepared by repeating the procedure of Comparative Example 6 except that in the first step of Comparative Example 6, sodium hydroxide powder was added in an amount of 5.89 g, and in the third step, the heating temperature was changed to 520°C and the stirring time was changed to 5 o'clock. In test case 8 Na/B (in moles) is 6.00. As a result, the reaction yield (SBH yield) is 16.7%.

Comparative Example 9

Sodium borohydride (SBH) is prepared by repeating the procedure of Comparative Example 6 except that the sodium borate as an ingredient in Comparative Example 6 is replaced by 1.97 g of sodium metaborate and 1.50 g of borax, the heating temperature is changed to 519°C, and the duration mixing time was changed to 3.6 hours. The grinding medium used was alumina ceramic beads (5 mm in diameter), and the ratio of the mass of the beads to the mass of the ingredients is 55. As a result, the reaction yield (SBH yield) is 13.3%.

Table 6 below shows the results of Comparative Examples 6 – 9.

As shown in Comparative Examples 6 to 9 in Table 6, the conversion (%) of sodium borohydride (SBH) was reduced because mixing using the grinding medium was carried out at such an early stage as during heating to remove water in the second stage.

In particular, although the mixing is gentle mixing, early mixing at the start of heating and the use of a grinding medium results in grinding of the soft sodium metaborate. The water generated from sodium hydroxide is adsorbed on the fine sodium metaborate particles and is present on these fine sodium metaborate particles even when kept at 321°C - 520°C and is slowly separated over a long period of time. As a result, the oxidation reaction of aluminum occurs over time, and soft shells are formed continuously. In addition, gentle mixing simultaneously with the generation of water destroys the already formed soft shells, and the destroyed soft shells remain in a mixed state in the area remote from the aluminum particles (in areas distant from the surface of the aluminum particles). Thus, less sodium oxide ( _Na2O ) released from the soft shells is present in the vicinity of the aluminum, so that the reaction cycle from sodium oxide, through sodium, sodium hydride, sodium borohydride (SBH) and back to sodium oxide is not established , which leads to low reaction yield.

Comparative Examples 6, 8 and 9 use gentle stirring, which results in a lower reaction yield (SBH yield) than the test examples. Comparative Example 7 necessarily involves mixing the ingredients and rolling and grinding these ingredients, which results in large deformations of the aluminum, and the newly exposed contact surfaces increase the likelihood of contact with other ingredients present in the remote area. Therefore, the reaction yield is slightly higher than other comparative examples.

List of reference items

10A, 10B, 10C Sealed reactor

12 Main reactor vessel

14 Cover

16 Heater

18 O-ring

20 Engine

22 Stirring rod

22a Mixer segment

22b J-shaped agitator segment

24 First trumpet

26 Hydrogen gas supply valve

28 Outlet valve

30 Second trumpet

32 Pressure gauge

40 Grinding medium

51 Sodium borate

52 Aluminum powder

53 Non-oxidizing gas

101 Aluminum particle

102 Dense oxide film

103 Sodium hydroxide (NaOH)

104 Boehmite layer (Al ₂ O ₃ ⋅xH ₂ O)

105 Sodium oxide (Na ₂ O)

106 Hydrogen (H ₂ )

107 Soft shell

108 Hard shell

109 Sodium metal (Na)

110 Sodium hydride (NaH)

111 Diboron trioxide (B ₂ O ₃ )

112 Sodium borohydride (SBH).

Claims (17)

1. A method for producing sodium borohydride, including the reaction of sodium borate and aluminum powder in an atmosphere of hydrogen gas with stirring at a temperature of not lower than 400°C and not higher than 560°C, in which the molar ratio of sodium contained in sodium borate to boron contained in sodium borate is more than 1.0. 2. The method for producing sodium borohydride according to claim 1, which further includes adding an alkali metal oxide or an alkaline earth metal oxide, the molar ratio of the number of moles of any alkali metal and alkaline earth metal plus any alkali metal or alkaline earth metal contained in the sodium borate to the number of moles boron contained in sodium borate is more than 1.0 and not more than 3. 3. The method for producing sodium borohydride according to claim 1, which further includes adding an alkali metal hydroxide, wherein the molar ratio of the number of moles of any alkali metal contained in the alkali metal hydroxide and in sodium borate to the number of moles of boron contained in sodium borate is more than 1.0 and no more than 3. 4. The method for producing sodium borohydride according to claim 2, in which the molar ratio of the number of moles of any alkali metal and alkaline earth metal plus any alkali metal and alkaline earth metal contained in sodium borate to the number of moles of boron contained in sodium borate is from 1. 33 to 3. 5. A method for producing sodium borohydride, comprising dehydration of a mixture comprising alkali metal hydroxide, sodium borate and aluminum powder in a static state at a temperature of at least 400°C in an atmosphere of hydrogen gas; oxidation of aluminum powder by reaction with water to form a layer of sodium and aluminum oxide extending from the surface into the aluminum particles, having the chemical formula (I) xNa ₂ O⋅Al ₂ O ₃ , (I) where 0 < x < 1; And interaction of this mixture at a temperature not lower than 400°C and not higher than 560°C with stirring. 6. A method for producing sodium borohydride, comprising dehydration of a mixture containing alkali metal hydroxide, sodium borate and aluminum powder in a static state in a reactor when heated to a temperature of at least 400°C under reduced pressure; oxidation of aluminum powder by reaction with water to form a layer of sodium and aluminum oxide extending from the surface into the interior of the aluminum particles, having the chemical formula (I): xNa ₂ O⋅Al ₂ O ₃ , (I) where 0 < x < 1; And interaction of this mixture at a temperature not lower than 400°C and not higher than 560°C in an atmosphere of hydrogen gas with stirring. 7. A method for producing sodium borohydride according to any one of claims. 3, 5 or 6, wherein the molar ratio of the number of moles of any alkali metal plus any alkali metal contained in the sodium borate to the number of moles of boron contained in the sodium borate is from 1.33 to 3.0.