Preparation method of magnesium borohydride based on solvothermal reaction

Abstract

The invention discloses a preparation method of magnesium borohydride based on solvothermal reaction, and relates to the technical field of hydrogen storage materials, and the preparation method specifically comprises the following steps: S1, carrying out ball milling pretreatment on magnesium hydride according to a certain ball-to-material ratio; s2, adding the pretreated magnesium hydride and triethylamine borane into a self-made hydrothermal reaction kettle, then adding a solvent, starting heating and stirring procedures, carrying out a reaction for a certain time, and carrying out centrifugal separation to obtain a solid product; and S3, transferring the solid product into a tubular furnace, and carrying out vacuum drying to obtain magnesium borohydride. The method provided by the invention has the advantages of simple process, low price, one-step reaction, high safety, high yield and high purity.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrogen storage materials, and in particular to a method for preparing magnesium borohydride based on a solvent thermal reaction. Background Art

[0002] Hydrogen has 142MJ Kg -1 The high calorific value of H2 is about the energy density of gasoline (44MJ Kg -1 Hydrogen is three times more powerful than hydrogen (H2), produces no pollution (its oxidation product is water), and is abundant in reserves, making it considered one of the most promising energy carriers to replace fossil fuels. The development of the hydrogen economy primarily involves hydrogen production, storage, and utilization. For large-scale commercial application of hydrogen energy, hydrogen storage is a key issue that urgently needs to be addressed: finding a safe, efficient, and economical method for storing hydrogen.

[0003] Metal borohydrides (M[BH4] n )‌ is a representative new type of high-capacity hydrogen storage material. Its properties are relatively stable and it has advantages such as large hydrogen storage capacity, light weight, and low price. Therefore, it is considered to be one of the most promising hydrogen storage materials. Among them, magnesium borohydride has extremely high mass hydrogen storage density (14.9wt.%) and volume hydrogen storage density (~146Kg cm -1 ) is considered a very promising hydrogen storage material. It can release hydrogen through different methods such as pyrolysis and hydrolysis, thus meeting the needs of different applications. However, at present, magnesium borohydride is expensive due to its complicated preparation process, which greatly limits its promotion and application as a hydrogen storage material. Therefore, it is of great significance to develop a low-cost, simple, efficient, fast, safe and reliable preparation method for magnesium borohydride.

[0004] The currently widely used synthesis method of magnesium borohydride is based on the ion exchange reaction between magnesium chloride and borohydride by heating, ball milling and other methods to synthesize magnesium borohydride. The main reaction equation is as follows: MgX2+2MBH4→Mg(BH4)2+2MX (X=Cl, Br; M=Li, Na); When lithium borohydride and magnesium chloride are used in solid-phase ball milling, the yield of Mg(BH4)2 is only about 1.2%. When sodium borohydride and magnesium chloride are used in mixed ball milling, the yield can reach 50%. It can then be purified by dissolving and desolventizing with ether solvent. The yield can be increased to 77% by wet ball milling. However, the long-term mechanical ball milling process may produce Li-Mg or Na-Mg binary borohydride, resulting in an impure product. When magnesium bromide is used instead of magnesium chloride as a reactant, the ball milling time can be shortened, but the byproduct magnesium bromide is difficult to remove, making it difficult to promote and apply.

[0005] Currently, magnesium borohydride is prepared by wet chemical method using diethyl magnesium and diborane in ether. The reaction equation is as follows: 3MgR2+B2H6→3MgH2+2BR3 MgH2+B2H6→Mg(BH4)2; However, the dibutyl magnesium used in this method is expensive, and the second step reaction is extremely slow. Excessive borane must be used to prevent incomplete reaction. Diborane is highly toxic, unstable and flammable, making this method difficult to apply in practice.

[0006] Other reported methods for synthesizing magnesium borohydride include direct hydrogenation of its hydrogen release product, MgB2, or direct hydrogenation of metallic magnesium and boron. However, these methods often require extremely high hydrogen pressures and temperatures, resulting in high risks, high energy consumption, and difficulty in large-scale production.

[0007] In summary, currently reported methods for synthesizing magnesium borohydride suffer from high costs, demanding process conditions, low safety, and low product yield and purity. Therefore, providing a simple, safe, and high-purity method for preparing magnesium borohydride is a challenge that needs to be addressed in the field. Summary of the Invention

[0008] In view of the above problems, the present invention provides a method for preparing magnesium borohydride based on a solvent thermal reaction, which solves the problems of complex process, low safety, low yield and purity of magnesium borohydride in the prior art.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is as follows: The present invention provides a method for preparing magnesium borohydride based on a solvothermal reaction, which specifically comprises the following steps: S1: Pre-processing magnesium hydride by ball milling according to a certain ball-to-material ratio; S2: adding the pretreated magnesium hydride and triethylamine borane to a hydrothermal reactor, followed by adding a solvent, starting the heating and stirring process, reacting for a certain time, and centrifuging to obtain a solid product; S3: The solid product is transferred to a tube furnace and vacuum dried to obtain magnesium borohydride. The XRD pattern of the magnesium borohydride prepared by the present invention is compared with the PDF standard card. Figure 1 .

[0010] Furthermore, in the step S1, the ball mill used for the ball milling pretreatment is a planetary ball mill.

[0011] Furthermore, in step S1, the ball-to-material ratio is 300:1, the ball milling speed is 500 rpm, and the ball milling time is 10 h.

[0012] Furthermore, in step S2, the weight to volume ratio of the treated magnesium hydride, triethylamine borane, hydrothermal reactor, and solvent is 1 g:9 g:150 mL:50 mL.

[0013] Furthermore, in step S2, the solvent used is n-hexane.

[0014] Furthermore, in step S2, the reaction temperature is 120-180° C., and the reaction time is 6-12 hours.

[0015] Furthermore, in step S3, the vacuum drying temperature is 160° C. and the time is 12 h.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. The magnesium borohydride of the present invention is prepared by a simple hydrothermal method using magnesium hydride as a magnesium source and triethylamine borane as a boron source. The process is simple, the price is low, the reaction is one-step, and the safety is high.

[0017] 2. The present invention avoids the problems in the prior art, such as the high price of borohydrides (such as sodium borohydride, lithium borohydride, etc.) used in ion exchange methods, and the high-risk problems of using flammable and explosive hydrogen or borane in high-temperature and high-pressure synthesis methods.

[0018] 3. Compared with existing technologies, such as solid-phase ball milling and direct hydrogenation, the preparation time of magnesium borohydride in the present invention is greatly shortened, and the yield is significantly improved, which has the application potential of large-scale production.

[0019] 4. The magnesium borohydride prepared by the present invention can avoid the problem in the prior art that the purity of the synthesized magnesium borohydride is low due to substitution reactions between elements such as Li, Na, and Cl and ions in the magnesium borohydride during ion exchange reactions. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a comparison chart of the XRD pattern of the magnesium borohydride prepared by the present invention and the PDF standard card; Figure 2 The XRD patterns of the pre-milled magnesium hydride in Experimental Example 1 and the unmilled magnesium hydride in Comparative Example 1 are compared; Figure 3 The figure is a comparison of the temperature-dependent hydrogen release curves of pre-milled magnesium hydride and unmilled magnesium hydride in Experimental Example 1; Figure 4 is a comparison of the XRD patterns of the products prepared in Examples 2-5; Figure 5 is a comparison of FTIR spectra of the products prepared in Examples 2-5; Figure 6 is a comparison of the temperature-dependent hydrogen release curves of the products prepared in Examples 2-5; Figure 7 is a comparison of the XRD patterns of the products prepared in Example 4 and Examples 6 to 8; Figure 8 FTIR spectra of the products prepared in Example 4 and Examples 6 to 8 are compared; Figure 9 The temperature-dependent hydrogen release curves of the products prepared in Example 4 and Examples 6 to 8 are compared; Figure 10 is a comparison of the XRD patterns of the products prepared in Example 4 and Example 9; Figure 11 The figure is a comparison of the hydrogen release curves of the products prepared in Example 4 and Example 9. DETAILED DESCRIPTION

[0021] In order to make the purpose and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0022] Unless otherwise specified, the instruments, reagents, and materials used in the following examples are all conventional instruments, reagents, and materials available in the prior art and can be obtained through regular commercial channels. The experimental methods and detection methods used in the following examples are all conventional experimental methods and detection methods available in the prior art, unless otherwise specified.

[0023] Example 1 At room temperature, in an argon atmosphere glove box with a pressure of 0.1 MPa, 1 gram of magnesium hydride was added to a ball mill. Tungsten carbide balls of different sizes were loaded into the stainless steel ball mill according to a ball-to-material ratio of 300:1. The ball mill was filled with hydrogen to a pressure of 50 bar. The ball mill was then placed on a planetary ball mill and ball milled at a speed of 500 rpm for 10 hours to obtain pre-milled magnesium hydride.

[0024] Experimental Example 1 The results of comparing pre-milled magnesium hydride with unmilled magnesium hydride are as follows.

[0025] Figure 2 Comparison of XRD patterns of magnesium hydride before and after ball milling. Figure 3 The temperature-dependent hydrogen release curves of magnesium hydride before and after ball milling are compared. The amorphization degree of magnesium hydride increases after ball milling, and the initial hydrogen release temperature of magnesium hydride decreases significantly after ball milling, indicating that the pre-ball milled magnesium hydride is activated.

[0026] Example 2 1 g of pre-milled magnesium hydride and 9 g of triethylamine borane were added to a 150 mL hydrothermal reactor equipped with stirring, followed by 50 mL of n-hexane. The reaction was heated to 90° C. for 6 h, and the solid product was obtained by centrifugation.

[0027] The solid product was transferred to a tube furnace and dried under vacuum at 160° C. for 12 h to obtain magnesium borohydride.

[0028] Example 3 The difference from Example 2 is that the hydrothermal reaction temperature is 120° C. and the hydrothermal reaction time is 6 h.

[0029] Example 4 The difference from Example 2 is that the hydrothermal reaction temperature is 150° C. and the hydrothermal reaction time is 6 h.

[0030] Example 5 The difference from Example 2 is that the hydrothermal reaction temperature is 180° C. and the hydrothermal reaction time is 6 h.

[0031] Experimental Example 2 The products prepared in Examples 2-5 were compared, and the results are as follows.

[0032] Figure 4 The XRD comparison of the products shows that the reaction was not complete at the hydrothermal reaction temperature of Example 2, and the product was still magnesium hydride. The reaction products at the temperatures of Examples 3 to 5 were all magnesium borohydride.

[0033] Figure 5 The FTIR spectra of the products show that: the product of Example 2 does not have a BH bond, indicating that magnesium borohydride is not generated. The products of Examples 3 to 5 have a BH bond, indicating that magnesium borohydride is successfully prepared.

[0034] Figure 6 The hydrogen release curves of the products are shown as a function of temperature. The results show that the magnesium borohydride synthesized under the conditions of Example 3 has an initial hydrogen release temperature of 270°C and a hydrogen release rate of 10.09 wt% at 500°C. The magnesium borohydride synthesized under the conditions of Example 4 has an initial hydrogen release temperature of 270°C and a hydrogen release rate of 10.8 wt% at 500°C. The magnesium borohydride synthesized under the conditions of Example 5 has an initial hydrogen release temperature of 240°C and a hydrogen release rate of 10.09 wt% at 500°C.

[0035] Example 6 The difference from Example 4 is that the hydrothermal reaction temperature is 150° C. and the hydrothermal reaction time is 3 h.

[0036] Example 7 The difference from Example 4 is that the hydrothermal reaction temperature is 150° C. and the hydrothermal reaction time is 9 h.

[0037] Example 8 The difference from Example 4 is that the hydrothermal reaction temperature is 150° C. and the hydrothermal reaction time is 12 h.

[0038] Experimental Example 3 The products prepared in Examples 4 and 6 to 8 were compared, and the results are as follows.

[0039] Figure 7 The XRD comparison of the products shows that the reaction was not complete at the hydrothermal reaction time of Example 6, and the product was still magnesium hydride. The reaction products at the reaction time of Examples 4, 7-8 were magnesium borohydride.

[0040] Figure 8 The FTIR spectrum of the product shows that the product of Example 6 does not have a BH bond, indicating that magnesium borohydride was not generated. The products of Examples 4, 7, and 8 have a BH bond, indicating that magnesium borohydride was successfully prepared.

[0041] Figure 9 The hydrogen release curves of the products are shown as a function of temperature. The results show that the magnesium borohydride synthesized under the conditions of Example 4 has an initial hydrogen release temperature of 270°C and a hydrogen release rate of 10.8 wt% at 500°C. The magnesium borohydride synthesized under the conditions of Example 7 has an initial hydrogen release temperature of 240°C and a hydrogen release rate of 10.8 wt% at 500°C. The magnesium borohydride synthesized under the conditions of Example 8 has an initial hydrogen release temperature of 240°C and a hydrogen release rate of 10.8 wt% at 500°C.

[0042] Example 9 The difference from Example 4 is that the vacuum drying temperature is 25° C. and the vacuum drying time is 12 h.

[0043] Experimental Example 4 The products prepared in Example 4 and Example 9 were compared, and the results were as follows.

[0044] Figure 10 The XRD comparison of the products shows that the purity of the reaction product magnesium borohydride at the vacuum drying temperature of Example 9 is not high. The reaction product at the vacuum drying temperature of Example 4 is magnesium borohydride.

[0045] Figure 11 The hydrogen release curve of the product over temperature shows that the product synthesized under the conditions of Example 9 releases only 6.6 wt % hydrogen, indicating low product purity. The magnesium borohydride synthesized under the conditions of Example 4 has an initial hydrogen release temperature of 270°C and a hydrogen release rate of 10.8 wt % at 500°C.

[0046] In summary, the preparation method of magnesium borohydride in this embodiment is simple, safe, and has high yield and purity, and has the potential for industrial production of synthesized magnesium borohydride.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing magnesium borohydride based on a solvothermal reaction, characterized in that: The specific steps include: S1: Pre-processing magnesium hydride by ball milling according to a certain ball-to-material ratio; S2: adding the pretreated magnesium hydride and triethylamine borane to a hydrothermal reactor, followed by adding a solvent, starting the heating and stirring process, reacting for a certain time, and centrifuging to obtain a solid product; S3: The solid product was transferred to a tube furnace and dried under vacuum to obtain magnesium borohydride.

2. The method for preparing magnesium borohydride based on a solvothermal reaction according to claim 1, wherein: In the step S1, the ball mill used for the ball milling pretreatment is a planetary ball mill.

3. The method for preparing magnesium borohydride based on a solvothermal reaction according to claim 1, wherein: In step S1, the ball-to-material ratio is 300:1, the ball milling speed is 500 rpm, and the ball milling time is 10 h.

4. The method for preparing magnesium borohydride based on a solvothermal reaction according to claim 1, wherein: In step S2, the weight-to-volume ratio of the treated magnesium hydride, triethylamine borane, hydrothermal reactor, and solvent is 1 g:9 g:150 mL:50 mL.

5. The method for preparing magnesium borohydride based on a solvothermal reaction according to claim 1, wherein: In step S2, the solvent used is n-hexane.

6. The method for preparing magnesium borohydride based on a solvothermal reaction according to claim 1, wherein: In step S2, the reaction temperature is 120-180° C., and the reaction time is 6-12 hours.

7. The method for preparing magnesium borohydride based on a solvothermal reaction according to claim 1, characterized in that: In step S3, the vacuum drying temperature is 160° C. and the time is 12 h.

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