Method for catalytically synthesizing hexamethylenediamine from caprolactam

Abstract

The invention belongs to the field of chemical catalysis, and provides a method for catalytically synthesizing hexamethylenediamine from caprolactam, which is characterized in that cobalt-loaded hollow nitrogen-containing carbon spheres are used as a catalyst, the caprolactam is used as a raw material, and the hexamethylenediamine is synthesized by an electro-catalytic reaction in an electrolyte; the electrolyte comprises caprolactam, ammonia water and a potassium sulfate solution, and the reaction potential is-0.5 to-1.1 V vs. RHE. The traditional high-temperature and high-pressure limitation is broken through, the reaction can be carried out at normal temperature and normal pressure, the reaction condition is mild, and the catalyst is high in activity, good in stability, recyclable, free of performance attenuation, free of pollution emission and environmentally friendly.

Description

Technical Field

[0001] This invention belongs to the field of chemical catalysis technology, specifically relating to a method for synthesizing hexamethylenediamine from caprolactam via electrocatalytic reaction, and particularly to a cobalt-supported hollow nitrogen-containing carbon sphere catalyst and its process for efficiently catalyzing the conversion of caprolactam to hexamethylenediamine under mild conditions. Background Technology

[0002] Hexamethylenediamine (HDMA) is an important basic chemical raw material and one of the three major raw materials for nylon. At room temperature, it is a colorless and transparent crystal with strong alkalinity and some toxicity. HDMA has a wide range of applications. It can react with adipic acid to produce nylon 66, and with sebacic acid to produce nylon 610. It can also be synthesized through a photochemical reaction into hexamethylene diisocyanate, which is then used to manufacture various nylon resins, nylon fibers, and engineering plastic products.

[0003] Currently, the production methods for hexamethylenediamine can be divided into the adiponitrile method, the caprolactam method, and other methods. The adiponitrile method is the main method for preparing hexamethylenediamine, using Raney Co and Rancy Ni type catalysts as raw materials and synthesizing hexamethylenediamine through a hydrogenation reaction. With the increasing production capacity of caprolactam, researchers have gradually shifted the raw material for hexamethylenediamine production from adiponitrile to caprolactam. The caprolactam method first converts caprolactam into the intermediate 6-aminohexanonitrile, and then uses hydrogen to reduce the intermediate to prepare hexamethylenediamine. Other methods include reacting hexanediol, aminohexanol, adipic aldehyde, adipic acid, etc., to produce hexamethylenediamine.

[0004] The existing traditional thermocatalytic synthesis of hexamethylenediamine requires high temperature (200~500 ℃) and high pressure (>2 MPa) reaction conditions. In addition, a large amount of alkaline solution is generated during the reaction, which pollutes the environment and requires post-treatment, increasing production costs. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for the catalytic synthesis of hexamethylenediamine from caprolactam. Using caprolactam as raw material, hexamethylenediamine is prepared by electrocatalytic reaction. The reaction conditions are mild, the operation is simple and easy, and it has the advantages of being green and environmentally friendly and having high economic benefits. It is a promising method.

[0006] This invention is implemented as follows: A method for the catalytic synthesis of hexamethylenediamine using caprolactam as a catalyst, wherein the method uses cobalt-supported hollow nitrogen-containing carbon spheres as a catalyst and caprolactam as a raw material to carry out an electrocatalytic reaction to synthesize hexamethylenediamine in an electrolyte; wherein the electrolyte contains caprolactam, ammonia and potassium sulfate solution, and the reaction potential is -0.5 ~ -1.1 V vs. RHE.

[0007] Furthermore, the catalyst is prepared using the following steps: S1. Thoroughly mix 30-50 mL of deionized water, 110-130 mL of anhydrous ethanol, and 4.6-4.4 mL of concentrated ammonia. Add 5.2-6.0 mL of tetraethyl orthosilicate and stir for 50-70 min. After the tetraethyl orthosilicate is hydrolyzed, add 0.56-0.64 g of F127, 0.76-0.84 g of resorcinol, and 1.08-1.16 mL of formaldehyde solution, and stir for 25-35 min. Add 0.620-0.640 g of melamine and 0.80-0.88 mL of formaldehyde solution, and stir for 22-26 h. Pour the above mixture into a reaction vessel, add a steel sleeve, and carry out a hydrothermal reaction. After the reaction is complete, obtain the solid material in the reaction vessel by centrifugation, rinse the solid with ultrapure water, and then dry it in a vacuum drying oven. The dried material is then heat-treated under an argon atmosphere for 1.5-2.5 minutes. h, cooled to room temperature, to obtain a solid nitrogen-containing carbon sphere support; S2. The obtained solid nitrogen-containing carbon sphere carrier is etched with hydrofluoric acid solution; the etched carbon spheres are rinsed with ultrapure water by centrifugation, and then dried in a vacuum drying oven to obtain the hollow nitrogen-containing carbon sphere carrier; S3. Dissolve polyvinylpyrrolidone in 35-45 mL of anhydrous ethanol, add the hollow nitrogen-containing carbon spheres from S2, and stir for 22-26 h; filter the solution using a vacuum filter, wash with anhydrous ethanol, and dry the resulting solid in a vacuum drying oven to obtain the nitrogen-containing carbon spheres modified with polyvinylpyrrolidone; S4. Place the nitrogen-containing carbon spheres obtained after step S3 in a mixed solution of ethanol and deionized water to fully disperse the carbon spheres in the solution; add cobalt nitrate hexahydrate, stir for 8-12 min, then add sodium citrate dihydrate and hexamethyleneimine, stir for 5-15 min, and then reflux. After the reaction is complete, wash with anhydrous ethanol and then dry in a vacuum drying oven to obtain the catalyst precursor. S5. The dried catalyst precursor is heat-treated under a nitrogen atmosphere for 1.5-2.5 h, cooled to room temperature, and then reduced in a 10% H2 / Ar atmosphere for 3.5-4.5 h to obtain a cobalt-supported hollow carbon sphere support.

[0008] Furthermore, in step S1, a hydrothermal reaction is carried out at 90-110 ℃, with a heating rate of 10 ℃ / min and a hydrothermal reaction time of 22-24 h; in step S1, the drying time is 8-12 h and the drying temperature is 75-85 ℃.

[0009] Furthermore, in step S1, the heat treatment is pre-purged with argon gas for 30-50 min, the heat treatment temperature is 700-800℃, and the heating rate is 2-5℃ / min.

[0010] Furthermore, the hydrofluoric acid solution in S2 is prepared by uniformly mixing 40% hydrofluoric acid with ultrapure water at a volume ratio of 3:1.

[0011] Furthermore, the drying time in S2 and S3 is 8-12 h, and the drying temperature is 75-85 ℃.

[0012] Further, in S4, the volume ratio of ethanol to deionized water is 1:4; the molar ratio of cobalt nitrate hexahydrate, hexamethyleneimine, and trisodium citrate in S4 is 2~8:10:1; the reflux temperature in S4 is 80-100 ℃, and the reflux time is 4-8 h; the drying time in S4 is 8-12 h, and the drying temperature is 75-85 ℃.

[0013] Furthermore, in step S5, the heat treatment is pre-purged with nitrogen for 30-50 min, the heat treatment temperature is 780-820℃, and the heating rate is 2-5℃ / min; the reduction treatment is pre-purged with 10% H2-Ar for 30-50 min, the reduction treatment temperature is 530-570℃, and the heating rate is 2-5℃ / min.

[0014] Furthermore, in the electrocatalytic synthesis of hexamethylenediamine, the volume ratio of each raw material in the electrolyte is caprolactam: ammonia: 0.5 mol / L potassium sulfate solution = 1-3: 2-4: 23-27.

[0015] The present invention has the following advantages: The method of this invention uses a cobalt-supported hollow nitrogen-containing carbon sphere catalyst to achieve efficient and mild preparation of hexamethylenediamine through the following steps: a hollow nitrogen-containing carbon sphere support is synthesized by a template method, followed by HF etching and PVP modification; a cobalt precursor and an organic ligand self-assemble on the support surface, and an active catalyst is formed by heat treatment; electrocatalytic reduction is carried out in an electrolyte (caprolactam + ammonia + K2SO4) at room temperature and pressure (potential -0.5 ~ -1.1 V vs. RHE).

[0016] It is evident that the electrocatalytic synthesis of hexamethylenediamine using caprolactam as a raw material in this invention can be carried out under normal temperature and pressure conditions, with mild reaction conditions, green and environmentally friendly, while reducing production costs.

[0017] Furthermore, the caprolactam electrocatalyst for the synthesis of hexamethylenediamine used in this invention—cobalt-supported hollow nitrogen-containing carbon spheres—features well-dispersed active metals on the nitrogen-containing carbon spheres, a large number of active sites, and excellent catalytic performance. In addition, this catalyst exhibits good stability, is recyclable, and its performance does not decline after regeneration, demonstrating promising application prospects. Attached Figure Description

[0018] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0019] Figure 1 The X-ray powder diffraction patterns are those of the catalysts described in Examples 1-3 of this invention. Figure 2 This is a transmission electron microscope image of the catalyst described in Example 1 of the present invention; Figure 3 This is an elemental distribution diagram of the catalyst described in Example 1 of the present invention; Figure 4 This is a schematic diagram illustrating the catalytic efficiency of the catalyst described in Example 1 of the present invention in the electrocatalytic synthesis of hexamethylenediamine from caprolactam. Figure 5 This is a comparison chart of the catalytic efficiency of the catalyst described in Example 2 of the present invention and that in Example 1; Figure 6 This is a comparison chart of the catalytic efficiency of the catalyst described in Example 3 of the present invention and that in Example 1; Figure 7 This is a comparison chart of the catalytic efficiency of the catalyst described in Example 1 of the present invention and that of Comparative Example 1; Figure 8 This is a comparison chart of the catalytic efficiency of the catalyst described in Example 1 of the present invention and that of Comparative Example 2.

[0020] Figure 9 This is a catalytic efficiency graph showing the catalyst described in Example 1 of the present invention after being recycled 7 times. Detailed Implementation

[0021] The following will be combined with the appendix Figure 1-9 The technical solution of the present invention will be clearly and completely described in detail with specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used, unless otherwise specified, are all commercially available conventional products.

[0022] Example 1 A method for preparing hexamethylenediamine by caprolactam catalysis includes the following steps: Catalyst preparation: S1. Mix 40 mL of deionized water, 120 mL of ethanol, and 5 mL of ammonia thoroughly and stir for 60 min. Add 5.6 mL of tetraethyl orthosilicate and stir for 60 min. Add 0.6 g of F127, 0.8 g of resorcinol, and 1.12 mL of formaldehyde solution and stir for 30 min. Add 0.63 g of melamine and 0.84 mL of formaldehyde solution and stir for 24 h. Pour the solution into a reaction vessel, add a steel sleeve, and perform a hydrothermal reaction at 100 °C for 24 h. After the reaction is complete, rinse with ultrapure water by centrifugation and then dry in a vacuum drying oven at 80 °C for 10 h. The obtained product is heat-treated under an argon atmosphere for 2 h and then naturally cooled to room temperature to obtain a solid nitrogen-containing carbon sphere carrier.

[0023] S2. The obtained solid nitrogen-containing carbon sphere carrier is etched with hydrofluoric acid solution; the carbon spheres are rinsed with ultrapure water by centrifugation, and then dried in a vacuum drying oven at 80 °C for 10 h to obtain the hollow nitrogen-containing carbon sphere carrier.

[0024] S3. Dissolve 4 g of polyvinylpyrrolidone in 40 mL of ethanol, add 0.2 g of the hollow nitrogen-containing carbon sphere carrier prepared in S2, and stir for 24 h. Filter the solution using a vacuum filter, wash the obtained solid with anhydrous ethanol, and then dry it in a vacuum drying oven at 80 °C for 10 h to obtain the modified nitrogen-containing carbon sphere carrier.

[0025] S4. Place 0.2 g of nitrogen-containing carbon spheres obtained after step S3 in a mixed solution of ethanol and deionized water to fully disperse the carbon spheres in the solution; add 4 mmol of cobalt nitrate hexahydrate, stir for 10 min, then add 1 mmol of sodium citrate dihydrate and 10 mmol of hexamethyleneimine, stir for 10 min, and then reflux at 90 °C for 6 h while stirring. After the reaction is complete, wash with anhydrous ethanol and then dry in a vacuum drying oven at 80 °C for 10 h to obtain the catalyst precursor. S5. The dried catalyst precursor was heat-treated under a nitrogen atmosphere for 2 h, cooled to room temperature, and then reduced in a 10% H2 / Ar atmosphere for 4 h to obtain the cobalt-supported hollow carbon sphere support.

[0026] Performance testing: The X-ray powder diffraction pattern of the cobalt-supported hollow carbon sphere catalyst prepared in Example 1 is shown below. Figure 1 As shown, the transmission electron microscope image is as follows: Figure 2 As shown, the element distribution map is as follows: Figure 3 As shown, from Figure 2 It can be seen from this that the catalyst has a hollow spherical structure with a particle size of 100~200 nm. Figure 3 It can be seen from the data that the catalyst has a uniform Co / N / C dispersion.

[0027] The catalyst prepared in Example 1 was used in the electrocatalytic synthesis of hexamethylenediamine. A typical three-electrode system was used, including a platinum sheet counter electrode, a saturated Ag / AgCl reference electrode, and carbon paper loaded with the catalyst as the working electrode. The electrolyte consisted of 2 mL caprolactam, 3 mL ammonia, and 25 mL of 0.5 mol / L K₂SO₄ solution. The electrocatalytic synthesis of hexamethylenediamine from caprolactam was tested using an H-type reaction cell under ambient conditions and with a chronoamperometry method at a reaction voltage of -0.5 to -1.1 V vs. RHE. After 2 h of reaction, samples were taken and analyzed by liquid chromatography. The reaction results are shown in Figure 4. As can be seen from the figure, when the reaction potential is -0.5 V, the yield is only 123 µmol·h⁻¹. -1 ·g -1 cat The FE was 27%, while the yield reached a maximum of 237 µmol·h⁻¹ when the reaction potential was -0.8 V. -1 ·g -1 cat FE is 23%.

[0028] The catalyst prepared in Example 1 was used for the electrocatalytic synthesis of hexamethylenediamine at a reaction voltage of -0.8 V vs. RHE, with the remaining parameters the same as in Example 1; for details on the catalytic efficiency after the catalyst was recycled 7 times, please refer to [reference needed]. Figure 9 As can be seen from the figure, the performance of the catalyst does not decrease significantly after multiple cycles of regeneration, and it has strong stability.

[0029] Implement column 2 The difference between Example 2 and Example 1 lies in the amount of cobalt nitrate hexahydrate used in the catalyst preparation step, which includes the following steps: Catalyst preparation: S1. Mix 40 mL of deionized water, 120 mL of ethanol, and 5 mL of ammonia thoroughly and stir for 60 min. Add 5.6 mL of tetraethyl orthosilicate and stir for 60 min. Add 0.6 g of F127, 0.8 g of resorcinol, and 1.12 mL of formaldehyde solution and stir for 30 min. Add 0.63 g of melamine and 0.84 mL of formaldehyde solution and stir for 24 h. Pour the solution into a reaction vessel, add a steel sleeve, and perform a hydrothermal reaction at 100 °C for 24 h. After the reaction is complete, rinse with ultrapure water by centrifugation and then dry in a vacuum drying oven at 80 °C for 10 h. The obtained product is heat-treated under an argon atmosphere for 2 h and then naturally cooled to room temperature to obtain a solid nitrogen-containing carbon sphere carrier.

[0030] S2. The obtained solid nitrogen-containing carbon sphere carrier is etched with hydrofluoric acid solution; the carbon spheres are rinsed with ultrapure water by centrifugation, and then dried in a vacuum drying oven at 80 °C for 10 h to obtain the hollow nitrogen-containing carbon sphere carrier.

[0031] S3. Dissolve 4 g of polyvinylpyrrolidone in 40 mL of ethanol, add 0.2 g of the hollow nitrogen-containing carbon sphere carrier prepared in S2, and stir for 24 h. Filter the solution using a vacuum filter, wash the obtained solid with anhydrous ethanol, and then dry it in a vacuum drying oven at 80 °C for 10 h to obtain the modified nitrogen-containing carbon sphere carrier.

[0032] S4. The nitrogen-containing carbon spheres obtained after step S3 are placed in a mixed solution of ethanol and deionized water to fully disperse the carbon spheres in the solution. 12 mmol of cobalt nitrate hexahydrate is added, and after stirring for 10 min, 2 mmol of sodium citrate dihydrate and 20 mmol of hexamethyleneimine are added. After stirring for 10 min, the mixture is refluxed at 90 °C for 6 h while maintaining stirring. After the reaction is completed, the mixture is washed with anhydrous ethanol and then dried in a vacuum drying oven at 80 °C for 10 h to obtain the catalyst precursor. S5. The dried catalyst precursor was heat-treated under a nitrogen atmosphere for 2 h, cooled to room temperature, and then reduced in a 10% H2 / Ar atmosphere for 4 h to obtain the cobalt-supported hollow carbon sphere support. Its X-ray powder diffraction pattern is shown below. Figure 1 As shown.

[0033] Performance testing: The performance testing procedure was the same as in Example 1, using the catalyst prepared in Example 2. After 2 hours of reaction, samples were taken and analyzed using liquid chromatography. The results are compared with those of Example 1 in the appendix. Figure 5 As shown in the figure, increasing the amount of cobalt nitrate hexahydrate from 4 mmol to 12 mmol during the preparation of the catalyst precursor will decrease the yield at different reaction potentials.

[0034] Example 3 The difference between Example 3 and Example 1 lies in the amount of cobalt nitrate hexahydrate used in the catalyst preparation step, which includes the following steps: Catalyst preparation: S1. Mix 40 mL of deionized water, 120 mL of ethanol, and 5 mL of ammonia thoroughly and stir for 60 min. Add 5.6 mL of tetraethyl orthosilicate and stir for 60 min. Add 0.6 g of F127, 0.8 g of resorcinol, and 1.12 mL of formaldehyde solution and stir for 30 min. Add 0.63 g of melamine and 0.84 mL of formaldehyde solution and stir for 24 h. Pour the solution into a reaction vessel, add a steel sleeve, and perform a hydrothermal reaction at 100 °C for 24 h. After the reaction is complete, rinse with ultrapure water by centrifugation and then dry in a vacuum drying oven at 80 °C for 10 h. The obtained product is heat-treated under an argon atmosphere for 2 h and then naturally cooled to room temperature to obtain a solid nitrogen-containing carbon sphere carrier.

[0035] S2. The obtained solid nitrogen-containing carbon sphere carrier is etched with hydrofluoric acid solution; the carbon spheres are rinsed with ultrapure water by centrifugation, and then dried in a vacuum drying oven at 80 °C for 10 h to obtain the hollow nitrogen-containing carbon sphere carrier.

[0036] S3. Dissolve 4 g of polyvinylpyrrolidone in 40 mL of ethanol, add 0.2 g of the hollow nitrogen-containing carbon sphere carrier prepared in S2, and stir for 24 h. Filter the solution using a vacuum filter, wash the obtained solid with anhydrous ethanol, and then dry it in a vacuum drying oven at 80 °C for 10 h to obtain the modified nitrogen-containing carbon sphere carrier.

[0037] S4. The nitrogen-containing carbon spheres obtained after step S3 are placed in a mixed solution of ethanol and deionized water to fully disperse the carbon spheres in the solution. 0.1 mmol of cobalt nitrate hexahydrate is added, and the mixture is stirred for 10 min. Then, 0.25 mmol of sodium citrate dihydrate and 5 mmol of hexamethyleneimine are added, and the mixture is stirred for 10 min. The mixture is then refluxed at 90 °C for 6 h with stirring. After the reaction is complete, the mixture is washed with anhydrous ethanol and then dried in a vacuum drying oven at 80 °C for 10 h to obtain the catalyst precursor. S5. The dried catalyst precursor was heat-treated in a nitrogen atmosphere for 2 h, cooled to room temperature, and then reduced in a 10% H2 / Ar atmosphere for 4 h to obtain the cobalt-supported hollow carbon sphere support. Its X-ray powder diffraction pattern is shown in Figure 1.

[0038] Performance testing: The performance testing procedure was the same as in Example 1, using the catalyst prepared in Example 3. After 2 hours of reaction, samples were taken and analyzed using liquid chromatography. The results are compared with those of Example 1 in the appendix. Figure 6 As shown in the figure, reducing the amount of cobalt nitrate hexahydrate from 4 mmol to 0.1 mmol during the preparation of the catalyst precursor leads to a decrease in yield at different reaction potentials. Comparative Example 1 The difference between Comparative Example 1 and Example 1 lies in the proportion of reactants used in the performance testing steps, which include the following steps: Catalyst preparation: The catalyst preparation is the same as in Example 1.

[0039] Performance Testing: The catalyst prepared in Example 1 was used in the electrocatalytic synthesis of hexamethylenediamine. A typical three-electrode system was used, including a platinum sheet counter electrode, a saturated Ag / AgCl reference electrode, and carbon paper loaded with the catalyst as the working electrode. The electrolyte consisted of 1 mL caprolactam, 4 mL ammonia, and 25 mL of 0.5 mol / L K₂SO₄ solution. The electrocatalytic synthesis of hexamethylenediamine from caprolactam was tested using an H-type reaction cell under ambient conditions and a chronoamperometry method with a reaction voltage of 0.5–-1.1 V vs. RHE. After 2 hours of reaction, samples were taken and analyzed by liquid chromatography. The results are compared with those of Example 1 in the appendix. Figure 7 As shown in the figure, changing the ratio of caprolactam to ammonia in the electrolyte (reducing 2 mL of caprolactam to 1 mL and increasing 3 mL of ammonia to 4 mL) will decrease the yield at different reaction potentials.

[0040] Comparative Example 2 The difference between Comparative Example 2 and Example 1 lies in the different proportions of reactants used in the performance testing steps, including the following steps: Catalyst preparation: The catalyst preparation is the same as in Example 1.

[0041] Performance Testing: The catalyst prepared in Example 1 was used in the electrocatalytic synthesis of hexamethylenediamine. A typical three-electrode system was used, including a platinum sheet counter electrode, a saturated Ag / AgCl reference electrode, and carbon paper loaded with the catalyst as the working electrode. The electrolyte consisted of 3 mL caprolactam, 2 mL ammonia, and 25 mL of 0.5 mol / L K₂SO₄ solution. The electrocatalytic synthesis of hexamethylenediamine from caprolactam was tested using an H-type reaction cell under ambient conditions and a chronoamperometry method with a reaction voltage of 0.5–-1.1 V vs. RHE. After 2 hours of reaction, samples were taken and analyzed by liquid chromatography. The results are compared with those of Example 1 in the appendix. Figure 8 As shown in the figure, changing the ratio of caprolactam to ammonia in the electrolyte (increasing 2 mL of caprolactam to 3 mL and decreasing 3 mL of ammonia to 2 mL) will decrease the yield at different reaction potentials.

[0042] While specific embodiments of the present invention have been described above, those skilled in the art should understand that the specific embodiments described are merely illustrative and not intended to limit the scope of the present invention. Equivalent modifications and variations made by those skilled in the art in accordance with the spirit of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for the catalytic synthesis of hexamethylenediamine using caprolactam, characterized in that: The method uses cobalt-loaded hollow nitrogen-containing carbon spheres as catalysts and caprolactam as a raw material to synthesize hexamethylenediamine through an electrocatalytic reaction in an electrolyte; the electrolyte contains caprolactam, ammonia, and potassium sulfate solution, and the reaction potential is -0.5 ~ -1.1 V vs. RHE.

2. The method for catalytic synthesis of hexamethylenediamine using caprolactam according to claim 1, characterized in that: The catalyst is prepared using the following steps: S1. Thoroughly mix 30-50 mL of deionized water, 110-130 mL of anhydrous ethanol, and 4.6-4.4 mL of concentrated ammonia. Add 5.2-6.0 mL of tetraethyl orthosilicate and stir for 50-70 min. After the tetraethyl orthosilicate is hydrolyzed, add 0.56-0.64 g of F127, 0.76-0.84 g of resorcinol, and 1.08-1.16 mL of formaldehyde solution, and stir for 25-35 min. Add 0.620-0.640 g of melamine and 0.80-0.88 mL of formaldehyde solution, and stir for 22-26 h. Pour the above mixture into a reaction vessel, add a steel sleeve, and carry out a hydrothermal reaction. After the reaction is complete, obtain the solid material in the reaction vessel by centrifugation, rinse the solid with ultrapure water, and then dry it in a vacuum drying oven. The dried material is then heat-treated under an argon atmosphere for 1.5-2.5 minutes. h, cooled to room temperature, to obtain a solid nitrogen-containing carbon sphere support; S2. The obtained solid nitrogen-containing carbon sphere carrier is etched with hydrofluoric acid solution; the etched carbon spheres are rinsed with ultrapure water by centrifugation, and then dried in a vacuum drying oven to obtain the hollow nitrogen-containing carbon sphere carrier; S3. Dissolve polyvinylpyrrolidone in 35-45 mL of anhydrous ethanol, add the hollow nitrogen-containing carbon spheres from S2, and stir for 22-26 h; filter the solution using a vacuum filter, wash with anhydrous ethanol, and dry the resulting solid in a vacuum drying oven to obtain the nitrogen-containing carbon spheres modified with polyvinylpyrrolidone; S4. Place the nitrogen-containing carbon spheres obtained after step S3 in a mixed solution of ethanol and deionized water to fully disperse the carbon spheres in the solution; add cobalt nitrate hexahydrate, stir for 8-12 min, then add sodium citrate dihydrate and hexamethyleneimine, stir for 5-15 min, and then reflux. After the reaction is complete, wash with anhydrous ethanol and then dry in a vacuum drying oven to obtain the catalyst precursor. S5. The dried catalyst precursor is heat-treated under a nitrogen atmosphere for 1.5-2.5 h, cooled to room temperature, and then reduced in a 10% H2 / Ar atmosphere for 3.5-4.5 h to obtain a cobalt-supported hollow carbon sphere support.

3. The method for catalytic synthesis of hexamethylenediamine using caprolactam according to claim 2, characterized in that: In step S1, a hydrothermal reaction is carried out at 90-110 ℃ with a heating rate of 10 ℃ / min and a hydrothermal reaction time of 22-24 h; the drying time in step S1 is 8-12 h and the drying temperature is 75-85 ℃.

4. The method for catalytic synthesis of hexamethylenediamine using caprolactam according to claim 2, characterized in that: In S1, the heat treatment is pre-purged with argon gas for 30-50 min, the heat treatment temperature is 700-800 ℃, and the heating rate is 2-5 ℃ / min.

5. The method for catalytic synthesis of hexamethylenediamine using caprolactam according to claim 2, characterized in that: The hydrofluoric acid solution in S2 is prepared by uniformly mixing 40% hydrofluoric acid with ultrapure water at a volume ratio of 3:

1.

6. The method for catalytic synthesis of hexamethylenediamine using caprolactam according to claim 2, characterized in that: The drying time in S2 and S3 is 8-12 h, and the drying temperature is 75-85 ℃.

7. The method for catalytic synthesis of hexamethylenediamine using caprolactam according to claim 2, characterized in that: The volume ratio of ethanol to deionized water in S4 is 1:4; the molar ratio of cobalt nitrate hexahydrate, hexamethyleneimine, and trisodium citrate in S4 is 2~8:10:1; the reflux temperature in S4 is 80-100 ℃, and the reflux time is 4-8 h; the drying time in S4 is 8-12 h, and the drying temperature is 75-85 ℃.

8. The method for catalytic synthesis of hexamethylenediamine using caprolactam according to claim 2, characterized in that: In step S5, the heat treatment is pre-purged with nitrogen for 30-50 min, the heat treatment temperature is 780-820 ℃, and the heating rate is 2-5 ℃ / min; the reduction treatment is pre-purged with 10% H2-Ar for 30-50 min, the reduction treatment temperature is 530-570 ℃, and the heating rate is 2-5 ℃ / min.

9. The method for catalytic synthesis of hexamethylenediamine using caprolactam according to claim 1, characterized in that: In the electrocatalytic synthesis of hexamethylenediamine, the volume ratio of each raw material in the electrolyte is caprolactam: ammonia: 0.5 mol / L potassium sulfate solution = 1-3: 2-4: 23-27.

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