Method for synthesizing ammonia borane

Abstract

To provide a novel method for synthesizing ammonia borane, which is a hydrogen storage material.SOLUTION: The present invention provides a method for synthesizing ammonia borane by reacting borohydride such as NaBH4 with ammonium salts such as NH4Cl and (NH4)2SO4 in ammonia water or by reacting borohydride with ammonia.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for synthesizing the hydrogen storage material ammonia borane. [Background technology]

[0002] High-pressure hydrogen is used in the fuel tanks of commercially available fuel cell vehicles, but there are still issues with cost and hydrogen density. Hydrogen storage materials are expected to be a high-density hydrogen storage method, and ammonia borane (NH3BH3) in particular has attracted attention because of its extremely high gravimetric hydrogen density.

[0003] As a method for synthesizing such ammonia borane, a method in which borohydride (M(BH4)n: M = metal) and an ammonium salt are stirred in an organic solvent (see Non-Patent Document 1), and a method in which sodium borohydride is reacted with an acid or an ammonium salt in an organic solvent (see Patent Document 1) have been proposed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-001419 [Non-patent literature]

[0005] [Non-Patent Document 1] Petit, JF, Miele, P., Demirci, UB, (2016) Ammonia borane H3N-BH3 for solid-state chemical hydrogen storage: Different samples with different thermal behaviors International Journal of Hydrogen Energy. 41, 15462-15470. Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, ammonia borane has attracted attention as a hydrogen storage material, but the method described in Non-Patent Document 1 has problems such as the number of steps, cost, and production time, as well as the inability to directly synthesize it from ammonia recovered from nature. Furthermore, the method described in Patent Document 1 has the problem of high cost because it requires a large amount of organic solvent. A new method for synthesizing ammonia borane that overcomes these problems and is suitable for practical use is needed.

[0007] An object of the present invention is to provide a novel method for synthesizing ammonia borane, a hydrogen storage material. [Means for solving the problem]

[0008] The inventors have been focusing on and studying the usefulness of ammonia borane as a hydrogen storage material. While searching for a new synthesis method for ammonia borane, they discovered that ammonia borane can be synthesized using borohydride and an ammonia salt in water (in water) without using an organic solvent, leading to the completion of this invention. In particular, they found that ammonia borane can be efficiently synthesized by carrying out the reaction in an aqueous ammonia solution.

[0009] That is, the present invention is as follows. [1] A method for synthesizing ammonia borane, characterized by reacting borohydride with an ammonium salt in water. [2] The method for synthesizing ammonia borane according to [1] above, characterized in that the reaction is carried out in an aqueous ammonia solution. [3] The method for synthesizing ammonia borane according to [2] above, characterized in that the reaction is carried out by adding at least one additive selected from KH2PO4, Na2HPO4, NaHCO3, and NaOH. [4] The method for synthesizing ammonia borane according to any one of [1] to [3] above, wherein the ammonium salt is at least one selected from (NH4)2HPO4, NH4NO3, (NH4)2CO3, NH4Cl and (NH4)2SO4. [5] The method for synthesizing ammonia borane according to any one of the above [1] to [4], characterized in that the reaction is carried out by adding carbon dioxide. [6] A method for synthesizing ammonia borane, which comprises reacting borohydride with ammonia in water. [7] The method for synthesizing ammonia borane according to the above [6], characterized in that the reaction is carried out by adding carbon dioxide. [8] The method for synthesizing ammonia borane according to [6] or [7] above, characterized in that the reaction is carried out by adding at least one additive selected from KH2PO4, Na2HPO4, NaHCO3, and NaOH. [Effects of the Invention]

[0010] According to the present invention, a novel method for synthesizing ammonia borane, which is a hydrogen storage material, can be provided. [Brief explanation of the drawings]

[0011] [Figure 1] Figure showing an example of the results of 11B-NMR measurement of ammonia borane solution [Figure 2] FIG. 1 shows the production rate and selectivity of ammonia borane in Example 1. [Figure 3] FIG. 1 shows the production rate and selectivity of ammonia borane in Example 2. [Figure 4] FIG. 1 shows the production rate and selectivity of ammonia borane in Example 3. [Figure 5] FIG. 1 shows the production rate and selectivity of ammonia borane in Example 4. [Figure 6] FIG. 1 shows the production rate and selectivity of ammonia borane in Example 5. [Figure 7] FIG. 1 shows the production rate and selectivity of ammonia borane in Example 6. [Figure 8]FIG. 10 shows the production rate and selectivity of ammonia borane in Example 7. DETAILED DESCRIPTION OF THE INVENTION

[0012] [First invention] The method for synthesizing ammonia borane of the present invention is characterized by reacting borohydride with an ammonium salt in water. That is, the method of the present invention performs the reaction, which has conventionally been carried out in an organic solvent, in water, eliminating the need for an organic solvent. This makes it possible to produce ammonia borane inexpensively.

[0013] Here, in the present invention, carrying out a reaction in water means carrying out the reaction in a solvent mainly composed of water, and does not preclude the reaction from containing an organic solvent as a part of the solvent. 。

[0014] Conventionally, organic solvents have been used as solvents because borohydrides such as NaBH4 are hydrolyzed to water. However, in the present invention, it has been discovered that ammonia borane can be synthesized even in water.

[0015] The pH of the reaction solution is preferably 7 or higher, more preferably 10 or higher, and even more preferably 12 or higher. In the present invention, it is particularly preferable to use an aqueous ammonia solution as the solvent. This suppresses hydrolysis of borohydride and enables efficient synthesis of ammonia borane. The ammonia concentration of the aqueous ammonia solution may be, for example, 1% by mass or higher, preferably 5% by mass or higher, more preferably 15% by mass or higher, even more preferably 20% by mass or higher, particularly preferably 25% by mass or higher, and most preferably 30% by mass or higher. The upper limit is not particularly limited, but is, for example, 50% by mass, preferably 35% by mass. To obtain a highly concentrated aqueous ammonia solution, it is particularly preferable to use a liquid ammonia-water mixture. It is believed that the use of liquid ammonia produces ammonia borohydride hydrate (e.g., Na(BH4)·nNH3), accelerating the reaction.

[0016] In addition, Patent Document 2 above describes the use of concentrated aqueous ammonia as part of the solvent (10 to 30 mass%) for the reaction, but the main solvent in such a reaction is always an organic solvent (tetrahydrofuran), and the basic concept is fundamentally different from that of the present invention, which uses water as the main solvent.

[0017] The borohydride used in the method of the present invention is represented by M(BH4)n (M = metal), where M includes Group 1 elements (alkali metals) such as lithium, sodium, and potassium, and Group 2 elements (alkaline earth metals) such as magnesium and calcium. Sodium is particularly preferred from the standpoints of reactivity and stability in water. That is, sodium borohydride (NaBH4) is particularly preferred.

[0018] Examples of ammonium salts that can be used in the method of the present invention include (NH4)2HPO4, NH4NO3, (NH4)2CO3, NH4Cl, (NH4)2SO4, etc., with NH4Cl and (NH4)2SO4 being preferred from the standpoint of reactivity.

[0019] In the method of the present invention, it is preferable to add a predetermined additive (reaction accelerator) to the reaction solution. Examples of additives include KH2PO4, Na2HPO4, NaHCO3, and NaOH, and these may be used in combination. For example, a phosphate buffer solution containing KH2PO4 and Na2HPO4 may be used. While the details of the mechanism are unclear, the use of such an additive is thought to suppress the generation of protons and increase the ionic strength of NH4, thereby suppressing the decomposition of the intermediate NH4BH4 and the generated ammonia borane.

[0020] The reaction of borohydride with ammonium salt in the synthesis method of the present invention can be carried out without heating, and is preferably carried out with cooling. That is, the reaction of the present invention is an exothermic reaction, and if the temperature of the reaction solution rises above 40°C, the generated ammonia borane may decompose, so it is preferable to carry out the reaction with cooling. Furthermore, when liquid ammonia is used, it is preferable to carry out the reaction under pressure.

[0021] The raw materials, borohydride and ammonium salt, can be mixed in the form of 1) aqueous solutions, 2) one solution and the other solid (powder) mixed together, or 3) both solids can be mixed with a solvent. Method 1 is preferred because it provides a stable reaction environment.

[0022] The blending ratio (molar ratio) of borohydride to ammonium salt is not particularly limited as long as it can produce ammonia borane, but from the viewpoint of reactivity, it is preferably 0.5 to 5:1, more preferably 0.8 to 4:1, and even more preferably 1 to 3:1.

[0023] In the synthesis method of the present invention using aqueous ammonia, it is preferable to carry out the reaction by adding CO2. Examples of methods for adding CO2 include introducing CO2 into a sealed reaction system and bubbling CO2. By adding CO2, (NH4)2CO3 is generated by the reaction of CO2, water, and ammonium ions or ammonia, and it is thought that the synthesis of ammonia borane is promoted by the reaction with borohydride.

[0024] [Second Invention] The method for synthesizing ammonia borane of the present invention is characterized by reacting borohydride with ammonia in water. The synthesis method of the present invention does not require ammonium salt, and ammonia borane can be synthesized using water and ammonium ions or ammonia. The raw materials such as borohydride, additives, reaction conditions, etc. are the same as those of the first invention, so explanations are omitted. [Example]

[0025] [Example 1] Ammonia borane was synthesized by reacting borohydride with ammonium salt in water or aqueous ammonia.

[0026] An aqueous borohydride solution was prepared by dissolving 1.23 g (0.04 mol) of NaBH powder in 50 ml of distilled water. An aqueous ammonium salt solution was prepared by dissolving 2.15 g (0.016 mol) of (NH)SO powder in 50 ml of distilled water or 50 ml of 28% by weight aqueous ammonia (concentrated aqueous ammonia). The aqueous borohydride solution and the aqueous ammonium salt solution were mixed in a flask (pH 5-7 for distilled water, pH 12 for aqueous ammonia), and the flask was stopped to prevent reaction with air. The synthesis reaction was carried out while cooling using a water bath.

[0027] (Determination of boron content by synthesis of ammonia borane) After 8 hours of reaction, 11 The boron content was quantified by B-NMR. 11An example of the B-NMR measurement results is shown in Figure 1. Specifically, 11 Using the results of B-NMR measurement, the production rate (yield) and selectivity of NH3BH3 were calculated according to the following formulas.

[0028]

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[0029]

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[0030] Figure 2 shows the production rate and selectivity of ammonia borane in the synthesis in water and in aqueous ammonia. As shown in Figure 2, ammonia borane was obtained in both the synthesis in water and in aqueous ammonia. When synthesis was performed in aqueous ammonia, the production rate and selectivity of ammonia borane were improved compared to the synthesis in water.

[0031] [Example 2] Next, to confirm the ammonium salt dependence, ammonia borane was synthesized in aqueous ammonia using different ammonium salts. The ammonium salt aqueous solution used was prepared by dissolving 4.28 g (0.032 mol) of (NH4)2HPO4 powder, 2.59 g (0.032 mol) of NH4NO3 powder, 3.11 g (0.032 mol) of (NH4)2CO3 powder, 1.73 g (0.032 mol) of NH4Cl powder, and 4.28 g (0.032 mol) of (NH4)2SO4 powder in 50 ml of 28% by mass ammonia water. The aqueous borohydride solution used was prepared by dissolving 1.23 g (0.04 mol) of NaBH4 powder in 50 ml of distilled water. Other than that, ammonia borane was synthesized in the same manner as in Example 1, and the boron component was quantified. The results are shown in Figure 3.

[0032] As shown in Figure 3, ammonium borane was obtained in high yields and selectivities exceeding 2 mol% in all synthesis examples. The highest yields and selectivities were obtained when NH4Cl was used as the ammonium salt.

[0033] [Example 3] Next, the production rate and selectivity of ammonia borane were investigated when an additive was added to aqueous ammonia. As the additives of the present invention, KH2PO4, Na2HPO4, phosphate buffer solution (KH2PO4 + Na2HPO4), NaHCO3, and NaOH were used. In addition, NaBH4 was used as the borohydride, and (NH4)2SO4 was used as the ammonium salt.

[0034] KH2PO4 was prepared by adding 1.00 g of KH2PO4 to 100 ml of distilled water to prepare a solution (pH = 5.6). Na2HPO4 was prepared by adding 7.19 g of Na2HPO4 to 100 ml of distilled water to prepare a solution (pH = 8.8).

[0035] Phosphate buffer solution was prepared by adding 1.00 g of KH2PO4, 7.19 g of Na2HPO4, and 200 ml of distilled water (pH = 6.8). NaHCO3 was prepared by adding 4.20 g of NaHCO3 to 100 ml of distilled water to prepare a solution (pH = 7.9). NaOH was prepared by adding 4.92 g of NaOH to 100 ml of distilled water to prepare a solution (pH = 12).

[0036] Ammonia borane was synthesized in aqueous ammonia in the same manner as in Example 2, except that an additive was added, and the boron component was quantified. The results are shown in Figure 4. For reference, an example using only water is also shown.

[0037] As shown in Figure 4, synthesis using ammonia water containing KH2PO4 resulted in the highest yield (17.1 mol%) and the highest selectivity (20.4 mol%). Na2HPO4 also showed improved yield and selectivity compared to synthesis using ammonia water alone (yield 13.9 mol%, selectivity 17.8 mol%). Furthermore, other additives also showed improved yield and selectivity compared to synthesis using ammonia water alone.

[0038] [Example 4] Ammonia borane was synthesized using various ammonia sources. (4-1) Ammonia gas was used as the ammonia. An aqueous borohydride solution was prepared by dissolving 1.23 g (0.04 mol) of NaBH powder in 10 ml of distilled water. An aqueous ammonium salt solution was prepared by dissolving 2.15 g (0.016 mol) of (NH)SO powder in 20 ml of distilled water. The aqueous borohydride solution and the aqueous ammonium salt solution were mixed in a flask, covered with aluminum foil with a hole in it, and allowed to stand for 8 hours while introducing ammonia gas at 0.2 MPa. (4-2) Liquid ammonia was used as the ammonia. An aqueous borohydride solution was prepared by dissolving 1.23 g (0.04 mol) of NaBH powder in 10 ml of distilled water. An aqueous ammonium salt solution was prepared by dissolving 2.15 g (0.016 mol) of (NH)SO powder in 10 ml of distilled water. The aqueous borohydride solution and the aqueous ammonium salt solution were mixed in a flask, 1.5 ml of liquid ammonia was added, and the flask was covered with aluminum foil with a hole in it and allowed to stand for 8 hours. (4-3) As the ammonia, aqueous ammonia and liquid ammonia were used. 1.23 g (0.04 mol) of NaBH4 powder, 2.15 g (0.016 mol) of (NH4)2SO4 powder, 1 ml of 28 mass % ammonia water, and 1.5 ml of liquid ammonia were allowed to stand in a metal cell for 8 hours. (4-4) Ammonia borane was synthesized in the same manner as in (4-3), except that 2.15 g (0.022 mol) of (NH4)2CO3 powder was used as the ammonium salt instead of 2.15 g (0.016 mol) of (NH4)2SO4 powder.

[0039] The boron content of the ammonia borane synthesized in (4-1) to (4-4) was determined in the same manner as in Example 1. The results are shown in Figure 5.

[0040] As shown in Figure 5, when liquid ammonia was used, the production rate and selectivity of ammonia borane increased. It is thought that the synthesis of ammonia borane proceeds more easily when the ammonia concentration in the reaction solution is higher.

[0041] [Example 5] 1.23 g (0.04 mol) of NaBH4 powder was dissolved in 10 ml of 28 mass % aqueous ammonia, and 10 ml of the same phosphate buffer solution as in Example 3 was added. The solution was covered with aluminum foil with a hole in it and allowed to stand for 8 hours. The results are shown in Figure 6.

[0042] As shown in Figure 6, ammonia borane was obtained by using aqueous ammonia without using an ammonium salt.

[0043] [Example 6] An aqueous borohydride solution was prepared by dissolving 1.23 g (0.04 mol) of NaBH powder in 50 ml of distilled water, and 50 ml of a 28% by mass ammonia solution was also prepared.

[0044] The aqueous borohydride solution and the aqueous ammonia solution were mixed in a flask, the flask was sealed with a stopper, and CO2 was blown into the reaction solution. The CO2 was generated using sodium bicarbonate and citric acid and slowly introduced. The results are shown in Figure 7.

[0045] As shown in Figure 7, by introducing CO2, ammonia borane was obtained without using an ammonium salt.

[0046] [Example 7] An aqueous borohydride solution was prepared by dissolving 1.23 g (0.04 mol) of NaBH powder in 50 ml of distilled water, and 50 ml of a 28% by mass ammonia solution was also prepared. As additive solutions, KH2PO4 solution (pH = 5.6) was prepared by adding 1.00 g of KH2PO4 to 100 ml of distilled water, and Na2HPO4 solution (pH = 8.8) was prepared by adding 7.19 g of Na2HPO4 to 100 ml of distilled water.

[0047] The aqueous borohydride solution, aqueous ammonia solution, and aqueous additive solution were mixed in a flask, and the flask was sealed with a stopper. CO2 was then blown into the flask in the same manner as in Example 4. The results are shown in Figure 8.

[0048] As shown in FIG. 8, it was revealed that the production rate and selectivity of ammonia borane were improved by using KH2PO4 or KH2PO4 compared to the case of using only an aqueous ammonia solution (Example 6), and ammonia borane could be synthesized efficiently. [Industrial Applicability]

[0049] The method for synthesizing ammonia borane of the present invention is industrially useful because it enables ammonia borane, a hydrogen storage material, to be synthesized efficiently through a simple operation.

Claims

1. A method for synthesizing ammonia borane, characterized by reacting borohydride with an ammonium salt in a molar ratio of 0.5 to 5:1 in a solution consisting of water with an ammonia concentration of 1 mass% or more, a pH of 7 or more, and a temperature of less than 40°C.

2. A method for synthesizing ammonia borane according to claim 1, characterized in that the reaction is carried out in a mixed solution of liquid ammonia and water.

3. KH ２ P.O. ４ , Na ２ HPO ４ , NaHCO ３ 3. The method for synthesizing ammonia borane according to claim 1, wherein the reaction is carried out by adding at least one additive selected from the group consisting of ammonium borane, ...

4. The ammonium salt is (NH ４ ) ２ HPO ４ , N.H. ４ NO ３ , (NH ４ ) ２ CO ３ , N.H. ４ Cl and (NH ４ ) ２ SO ４ The method for synthesizing ammonia borane according to any one of claims 1 to 3, characterized in that the compound is at least one selected from the group consisting of:

5. 5. The method for synthesizing ammonia borane according to claim 1, wherein the reaction is carried out by adding carbon dioxide.

6. A method for synthesizing ammonia borane, characterized by reacting borohydride with ammonia in a solution consisting of water having an ammonia concentration of 1 mass% or more, pH 7 or more, and below 40°C.

7. 7. The method for synthesizing ammonia borane according to claim 6, wherein the reaction is carried out by adding carbon dioxide.

8. KH ２ P.O. ４ , Na ２ HPO ４ , NaHCO ３ 8. The method for synthesizing ammonia borane according to claim 6, wherein the reaction is carried out by adding at least one additive selected from the group consisting of ammonium borane, ...

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