CN112899711A

CN112899711A - Method for improving yield of hexanitrostilbene prepared by one-step method

Info

Publication number: CN112899711A
Application number: CN202110052919.5A
Authority: CN
Inventors: 王鹏程; 邹洁冬; 陆明; 林秋汉; 许元刚
Original assignee: Nanjing University of Science and Technology
Current assignee: Nanjing University of Science and Technology
Priority date: 2021-01-15
Filing date: 2021-01-15
Publication date: 2021-06-04

Abstract

The invention discloses a method for improving the yield of HNS prepared by a one-step method, which comprises the steps of firstly feeding raw materials into a static mixer for mixing, reacting mixed liquor for 1.5 hours to obtain most products, then carrying out electrocatalysis on the reacted solution to ensure that a byproduct, namely Hexanitrobibenzyl (HNBB), is dehydrogenated and converted into a product, namely Hexanitrostilbene (HNS), and improving the yield of HNS prepared by the one-step method to 53 percent. The static mixer adopted by the invention enables the materials to be mixed more fully; in addition, by-products are converted into products by adopting an electrocatalysis method, so that the reaction process is efficient and environment-friendly. The method fully combines the advantages of simple reaction process of synthesizing the target product by the one-step method and easy large-scale continuous production, and simultaneously has high yield.

Description

Method for improving yield of hexanitrostilbene prepared by one-step method

Technical Field

The invention belongs to the technical field of energetic materials, and relates to a method for synthesizing hexanitrostilbene by an electrochemical method.

Background

Hexanitrostilbene (HNS) is used as a heat-resistant explosive with low vapor pressure, low sensitivity and higher thermal stability, and is widely applied to petroleum exploration, space and deep sea research. At present, the method for producing HNS mainly comprises a one-step method and a two-step method, wherein the two-step method becomes the main domestic method for producing HNS due to higher yield and product purity. Compared with the two-step method, the one-step method has the main defects of low yield and more byproducts, but the one-step method is simple in reaction operation and easy to realize serialization, and is very beneficial to large-scale continuous production of HNS.

According to previous reports, Shipp et al rapidly added sodium hypochlorite solution to tetrahydrofuran/methanol solution cooled to 0 ℃ and maintained the reaction temperature, and then washed with acetone to obtain crude product, the method has a low preparation yield, typically only 30-35% (J.org.chem.1964,29, 2620-2623). Salter et al improved the above method by adding a nitrogenous base, preferably the hydrochloride salt of an organic amine, to the reaction mixture to increase the yield of HNS to about 45%, but the use of a nitrogenous base has caused inconvenience in the work-up (Def Sci., Vol 31,1980,4(10): 305-308). Golding et al improved the above method by adding calcium oxide to the reaction to reduce side reactions. They claim that the yield of HNS is 46%, but the reaction time of this method is several tens of hours and acid treatment is required to obtain the crude product (U.S. Pat. No. 8, 5023386,1991-07-11).

Although there are many reports in the literature of the use of a one-step process for the preparation of hexanitrostilbene, most of these processes suffer from one or more of the following disadvantages: long reaction time, low product yield, high reaction cost, more side reactions, difficult post-treatment and the like. Meanwhile, the strong heat release of hexanitrostilbene in the synthesis process causes the temperature change of the system to be large, and the yield of the product and the safety of the experiment are greatly influenced, and the problem is not solved all the time.

Disclosure of Invention

The invention aims to provide a method for improving the yield of hexanitrostilbene prepared by a one-step method.

In order to achieve the purpose, the technical scheme of the invention is as follows: a method for synthesizing hexanitrostilbene by an electrochemical method comprises the following specific steps:

(1) mixing trinitrotoluene (TNT) and sodium hypochlorite through a static mixer in the presence of a mixed solvent of tetrahydrofuran and methanol, collecting the mixed reaction solution, reacting for a period of time,

(2) adding the solution obtained in the step (1) into a solvent, an electrolyte and a dehydrogenating agent, continuing the electrocatalytic reaction for a period of time to obtain a target product,

further, in the step (1), trinitrotoluene (TNT) and sodium hypochlorite are pre-cooled to 0 ℃ and then mixed by a static mixer, and the mixed reaction liquid is collected and reacted for 1-1.5 hours.

Further, in the step (1), the volume ratio of tetrahydrofuran to methanol is 2: 1.

Further, in step (1), in terms of molar ratio, sodium hypochlorite: the trinitrotoluene is controlled to be between 1:1 and 1.5:1, and the effective chlorine content in the sodium hypochlorite is between 2 and 8 percent.

Further, in the step (1), the pH of the reaction solution is controlled to be 10-10.5, and the temperature of the reaction solution is controlled to be 10-20 ℃.

Further, in the step (2), the solvent is any one selected from diethyl ether, DMSO, acetone, benzene and 1, 4-dioxane.

Further, in the step (2), the dehydrogenating agent is selected from triethylene Diamine (DABCO), triethylamine and FeCl₂TEMPO, DIPEA and the like, and the addition amount of the compound is the same as the molar amount of a by-product HNBB in the solution after the reaction in the step (1).

Further, in the step (2), the electrolyte is selected from any one of tetramethylammonium tetrafluoroborate, tetrabutylammonium perchlorate, tetrabutylammonium tetrafluoroborate, ammonium perchlorate and tetrabutylammonium hexafluorophosphate.

Further, in the step (2), the electrocatalytic reaction is carried out by taking a graphite rod as an anode and a platinum sheet as a cathode and electrifying under constant current of 4-12 mA.

Further, in the step (2), the electrocatalytic reaction temperature is 10-20 ℃, and the reaction time is 0.5-1 hour.

Compared with the prior art, the invention has the advantages that: the invention adopts the static mixer, effectively overcomes the problem that the temperature change of the system is larger due to strong heat release in the synthesis process, and improves the yield and the safety of the reaction; in addition, an electrocatalysis mode is adopted, the reaction process is simple, safe and environment-friendly, the yield of the generated hexanitrostilbene is high, and the method is expected to become a novel method for preparing HNS.

Drawings

FIG. 1 is a diagram of an apparatus for an electrochemical catalytic system including a static mixer.

FIG. 2 is a schematic diagram of the reaction mechanism.

FIG. 3 is a diagram showing a liquid phase analysis of a reaction product after purification.

FIG. 4 shows nuclear magnetic resonance of the reaction product¹And (H) diagram.

FIG. 5 shows the NMR of the reaction product after refining¹And (H) diagram.

FIG. 6 shows nuclear magnetic resonance of the reaction product¹³And (C) diagram.

Detailed Description

The technical solution of the present invention is further specifically described below with reference to the accompanying drawings and examples.

Referring to FIG. 1, the apparatus for the reaction system of the present invention includes a static mixer and an electrochemical reaction system. The reaction materials are conveyed to the mixing tee joint through the polytetrafluoroethylene tube by two peristaltic pumps. The mixing tee joint is directly connected to the top of the static mixer, and the material flowing out of the bottom of the static mixer is conveyed to a subsequent electrochemical reaction system through a polytetrafluoroethylene pipe. The material is pre-cooled before entering the static mixer. The time for mixing the materials in the static mixer does not exceed 3 minutes.

The method for preparing HNS by one-step method comprises the steps of firstly feeding raw materials into a static mixer for mixing, reacting mixed liquor for 1.5 hours to obtain most of products, and then passing the reacted solution through an electrocatalysis methodThe method comprises the following steps of dehydrogenating a byproduct Hexanitrobibenzyl (HNBB) to convert the hexanitrobibenzyl into a product Hexanitrostilbene (HNS): collecting the mixed solution from the static mixer, namely the reaction solution, by using a three-neck flask, adjusting the pH of the system by using a pH meter at the temperature of 10-20 ℃, and keeping the pH of the system at 10-10.5 (by adding 25 wt% of H)₂SO₄And 6 wt% NaOH), and reacting the reaction solution for 1-1.5 hours. Then, by-products after reaction are further converted into target products by an electrocatalysis method, and the specific process is as follows: adding electrolyte, dissolving HNBB in the by-product by using DMSO, keeping the experimental temperature at about 20 ℃, applying a certain current, taking a graphite rod as an anode, taking a platinum sheet as a cathode, taking triethylene Diamine (DABCO) as a dehydrogenating agent, and reacting and stirring for 0.5-1 hour. And after the reaction is finished, filtering the mixed solution, washing the mixed solution with water for three times, and drying the mixed solution in a 60 ℃ drying oven to obtain a HNS crude product. The crude product is digested by acetone, recrystallized, filtered, washed by methanol and water to obtain yellow powdery solid. The purity of the product was determined by means of liquid phase analysis and melting point measurement.

The static mixer adopted by the invention is an SK static mixer, and is provided by Changjiang chemical machinery Co.

Example 1: optimization of one-step synthesis of hexanitrostilbene by using static mixer alone

First, a three-neck flask (150mL) was taken out of an oven-dried flask with a stirrer, and the three-neck flask was placed in a water bath to facilitate temperature adjustment. 2.0g of trinitrotoluene (TNT) was weighed out, dissolved in 30mL of Tetrahydrofuran (THF) and 15mL of methanol (MeOH) and pumped into the mixing tee by means of a peristaltic pump. 16mL of sodium hypochlorite is weighed, added with water and diluted to 50mL, and pumped into a mixing tee joint through a peristaltic pump. And (3) placing the two reaction materials after preparation in ice water for precooling, opening an inlet valve of the mixing tee after precooling is finished, setting parameters of a peristaltic pump according to a preset flow rate, and simultaneously connecting the two pumps. The mixture leaving the outlet tube was collected in a prepared three-necked flask (magnetic stirring), and when the final reaction left the mixer, both pumps were turned off, at which time the collection of the mixture was stopped. The mixer system was then flushed. The outlet pipe is first moved to a waste liquid collecting device, then the pump is started and dissolved by TNTThe liquid inlet system was flushed with 20mL of methanol solution, 30mL of water, through the NaClO inlet system with 50mL of water, and the pump was turned off after the cleaning solution drained. After the mixed solution reacts for 2.5 hours, the mixed solution is filtered by a Buchner funnel under reduced pressure, then the mixed solution is washed by deionized water for three times and then is dried in an oven at 60 ℃ to obtain a crude product (the yield is 38 percent, and the purity is lower). The crude product is digested by acetone and refined by recrystallization. The purity of the product was determined by means of liquid phase analysis and melting point measurement. The nuclear magnetic characterization of the refined product (see figure 4) is as follows:¹H NMR(500MHz,DMSO-d₆)δ＝3.42-3.39(s,4H,CH₂),7.15-7.13(s,2H),9.10-9.04(s,4H,aryl H)，9.20-9.11(s,4H)。

in order to improve the yield of the process, Lg (3) was used as a factor in the orthogonal experiment, taking the reaction temperature, the reaction system pH, the molar ratio of the reactants (n (NaClO): n (TNT)), and the available chlorine of sodium hypochlorite as the factors⁴) The test was performed on an orthogonal table, with the following results:

TABLE 1 orthogonal level factor Table for affecting yield

Table 2 table of orthogonal levels experiment affecting yield

As can be seen from the above table, the extreme differences of the factor A, B, C, D are 3, 5.2, 5.7 and 4.2 respectively, and it is known that the pH of the reaction system has the greatest influence on the yield of HNS; in addition, it can be known that the optimal reaction condition for the one-step synthesis of hexanitrostilbene by using a static mixer alone is A₂B₂C₃D₂Verification experiments were performed based on the results of orthogonal experiments by controlling the appropriate n (naclo): n (TNT), effective chlorine content of sodium hypochlorite,Reaction temperature, reaction system PH. The optimal reaction conditions of the process of synthesizing hexanitrostilbene by using a static mixer one-step method can be obtained, wherein the feeding ratio of a solvent is tetrahydrofuran to methanol is 2:1, the feeding molar ratio of TNT to a sodium hypochlorite solution is n (TNT), n (NaClO) is 1:1.2 (2 g is weighed by TNT, and 16mL is weighed by the sodium hypochlorite solution); the reaction temperature is kept at about 15 ℃, and the reaction duration is preferably 2-3 hours; the content of available chlorine in the sodium hypochlorite is 5 percent; after the start of the reaction 25 wt% H was used₂SO₄And 6 wt% NaOH to adjust the pH of the system to about 10.5. Under these conditions, the crude yield of the reaction was 47%. The crude product was subjected to liquid phase analysis and found to contain significant amounts of HNBB as a by-product. Then, the crude product was purified, and it was confirmed by melting point (308.4 ℃) and nuclear magnetic resonance analysis that it was the

target product

2,2',4,4',6,6' -Hexanitrodiphenylethylene (HNS).

Example 2 optimization of the one-step Synthesis of Hexanitrostilbene by the method of electrocatalysis alone

2.0g of TNT (about 8.8mmol) was first weighed and dissolved in a predetermined amount of 15mL of methanol and 30mL of tetrahydrofuran. And (3) placing the prepared TNT solution in an ice water bath to cool to 0-5 ℃. Measuring 16mL of sodium hypochlorite solution with the effective chlorine content of 5%, adding water to dilute the solution to 50mL, pouring the solution into a round-bottom flask (150mL) fixed on a low-temperature cooling device, and pre-cooling the solution to 0-5 ℃. The TNT solution in the beaker was quickly poured into the flask with appropriate stirring speed. The color of the solution in the flask is quickly changed from light yellow to purple red, the reading of the temperature agent is quickly increased to about 25 ℃, the low-temperature reaction device is removed when the temperature falls back stably, and the proper reaction temperature is ensured to be about 1-2 hours. After continued stirring, it was evident that solids had precipitated from the flask. At this point n-Bu is added₄NBF₄And (3) dissolving HNBB in the by-product (HNBB can be dissolved in the solution for electrolytic reaction) by using a solvent such as DMSO and the like, wherein the experimental temperature is kept at about 25 ℃. A graphite rod (Φ 6mm) was attached to the three-necked flask as an anode, and a platinum sheet electrode (10mm × 10mm) was attached to the flask as a cathode. And triethylenediamine (DABCO) was used as a dehydrogenating agent. Electrolyzing the reaction mixture for 0.5-1 hour under the constant current of 4-12 mA. After the reaction is finished, the reaction solution is added,the mixture was filtered under reduced pressure using a buchner funnel, washed three times with deionized water and dried in an oven at 60 ℃ to give the crude product (37.2% yield). The crude product is digested by acetone and refined by recrystallization. The purity of the product was determined by liquid phase analysis and melting point (285.4 ℃ C.) measurement.

Next, the reaction process is optimized in terms of the kind of electrolyte, the kind of solvent, the kind of dehydrogenating agent, the constant current/voltage of the reaction system, and the like, respectively, in order to improve the yield of the process.

(1) The specific reaction steps are as described above, after the reaction mixture reacts for 1-1.5 h, electrochemical catalysis is carried out, a proper solvent (dissolving HNBB) and a proper dehydrogenating agent are selected, the reaction current/voltage is constant in a proper range, different electrolytes are added into the reaction system, the products of each experiment are analyzed, and the influence of the electrolyte type on the reaction yield is shown in Table 3. As is clear from table 3, the purity and purity of the obtained product were low if no electrolyte was added during the electrocatalysis process. The electrolyte is mostly quaternary ammonium salt, and the quaternary ammonium salt is mainly used as a common organic supporting electrolyte and has the advantages of good conductivity, stable chemical property, low price, easy preparation and the like. Under the condition that other conditions are not changed, the yield of the reaction is improved under the action of the electrolyte, mainly because HNBB in the side product is converted into HNS as a target product.

TABLE 3 Effect of different types of electrolytes on yield

Note: the melting point of the pure HNS is 312.4 ℃, the melting point of the obtained product can roughly reflect the purity of the product, and the accurate purity of the product can be obtained only by means of liquid phase analysis and the like.

(2) The specific reaction steps are as described above, after the reaction mixed solution reacts for 1-1.5 h, electrochemical catalysis is carried out, appropriate electrolyte and dehydrogenating agent are selected, the reaction current/voltage is constant in an appropriate range, different solvents are added into the reaction system, products of each experiment are analyzed, and the influence of the solvents on the reaction yield is shown in table 4. It was found that the yield of the reaction was higher when DMSO was used as solvent, mainly due to the complete dissolution of HNBB in DMSO (higher dielectric constant and more polar of DMSO), which made the electrolysis reaction easier. A clear comparison is that although HNBB is also soluble in 1, 4-dioxane, its dielectric constant is small, which makes it difficult for HNBB to participate in electrolytic reactions. In addition, acetone has a boiling point that is too low to be suitable as a solvent. When using amide (DMF, hexamethylphosphoramide) as the reaction solvent, there are a large number of undefined by-products in the liquid phase detection product.

TABLE 4 Effect of different kinds of solvents on yield

(3) The specific reaction steps are as described above, after the reaction mixed solution reacts for 1-1.5 h, electrochemical catalysis is carried out, appropriate electrolyte and solvent are selected, the reaction current/voltage is constant in an appropriate range, different dehydrogenating agents are added into the reaction system, products of each experiment are analyzed, and the influence of the solvent on the reaction yield is shown in table 5. It can be found that most of the added dehydrogenating agents are tertiary amines, mainly because the tertiary amines are used as the dehydrogenating agents, the method has the advantages of stable chemical properties, low price, easy preparation, good dehydrogenation effect (the tertiary amines are easy to be combined with carbon atoms connected with a benzene ring and methylene on HNBB, so that hydrogen atoms on the methylene are removed to form divalent anion HNS) and the like. Under the condition that other conditions are not changed, the reaction yield is improved under the action of the dehydrogenating agent, mainly because HNBB in the side product is converted into HNS as a target product. When DABCO (triethylene diamine) is used as an electrolyte, the yield of the product is high, and the purity of the product can be found to be high by measuring the melting point and liquid phase analysis. This is mainly because one DABCO contains two tertiary N atoms, and in the same amount of dehydrogenating agent, DABCO more easily removes two protons from HNBB to form intermediate HNS^2-(see the figure), in addition, the alkali strength of DABCO is moderate(Pk_aAbout.9) has little influence on the pH of the reaction system. FeCl₂TEMPO as a dehydrogenating agent needs to be carried out in an oxygen atmosphere, and the temperature of a reaction system is required, so that TEMPO is not suitable for being used as the dehydrogenating agent.

TABLE 5 Effect of different kinds of dehydrogenating agents on yield

(4) The specific reaction steps are as described above, after the reaction mixture reacts for 1-1.5 h, electrochemical catalysis is carried out, appropriate electrolytes, solvents and dehydrogenating agents are selected, the current/voltage of the reaction system is kept constant in an appropriate range, products of each experiment are analyzed, and table 6 shows that when the constant current of the reaction system is lower than 2mA and higher than 12mA, the reaction yield is rapidly reduced. From the above figure, it can be found that the yield of HNS in the product is higher when the constant current of the reaction system is between 6-10 mA, and the purity of HNS in the range is also higher through liquid phase analysis.

TABLE 6 influence of constant Current of the reaction System on the reaction yield

(4) The optimum process conditions for the one-step synthesis of the hexanitrostilbene by the electrocatalysis method can be obtained by selecting proper electrolyte, dehydrogenating agent and solvent, keeping the reaction current/voltage constant in a proper range, taking tetrabutyl ammonium perchlorate as the electrolyte, dissolving HNBB in the by-products by using solvents such as DMSO (dimethyl sulfoxide) and the like, and keeping the experimental temperature at about 25 ℃. A graphite rod (Φ 6mm) was attached to the three-necked flask as an anode, and a platinum sheet electrode (10mm × 10mm) was attached to the flask as a cathode. And the reaction mixture was electrolyzed at a constant current of 8mA for 0.5 to 1 hour using triethylenediamine (DABCO) as a dehydrogenating agent. Under these conditions, the crude yield of the reaction was 48.5%. And performing liquid phase analysis on the crude product to find that the product purity is higher. Then, the crude product was purified, and it was confirmed by melting point (310.4 ℃) measurement and nuclear magnetic resonance analysis that it was the

target product

2,2',4,4',6,6' -Hexanitrodiphenylethylene (HNS).

Example 3

Optimization of one-step method for synthesizing hexanitrostilbene by comprehensively applying static mixer and electrochemical system

According to the results obtained in the examples 1 and 2, the synthesis of hexanitrostilbene by one-step method is optimized by comprehensively using a static mixer and an electrochemical system, and the specific process is as follows: the two materials enter a static mixer through a peristaltic pump, the mixing time is less than 3min, a three-neck flask is used for collecting mixed liquid at an outlet pipe of the static mixer, the mixed liquid reacts for 1-1.5 h before the reaction, the feeding ratio of a solvent is tetrahydrofuran to methanol is 2:1, the feeding molar ratio of TNT to a sodium hypochlorite solution is n (TNT) to n (NaClO) is 1:1.2 (2 g is weighed by TNT, and 16mL is weighed by the sodium hypochlorite solution); the reaction temperature is kept at about 15 ℃; the content of available chlorine in the sodium hypochlorite is 5 percent; after the start of the reaction 25 wt% H was used₂SO₄And 6 wt% NaOH to adjust the pH of the system to 10-10.5. After 0.5-1 h of reaction, tetrabutylammonium perchlorate is used as electrolyte, solvents such as DMSO are used for dissolving HNBB in the byproducts, and the experimental temperature is kept at about 20 ℃. A graphite rod (Φ 6mm) was attached to the three-necked flask as an anode, and a platinum sheet electrode (10mm × 10mm) was attached to the flask as a cathode. And using triethylene Diamine (DABCO) as a dehydrogenating agent, and electrolyzing the reaction mixture for 0.5-1 hour under the constant current of 9 mA. After the reaction is finished, a Buchner funnel is used for decompressing and filtering, then deionized water is used for washing for three times, and then the mixture is placed in a 60 ℃ drying oven for drying, so that a crude product is obtained. The crude product is digested by acetone and refined by recrystallization. The purity of the product was determined by means of liquid phase analysis and melting point measurement. Under these conditions, the yield of the reaction was 53%. The refined product was analyzed in liquid phase (see FIG. 3) to find that the product was of high purity. Then, by measuring the melting point (312.8 ℃) and nuclear magnetic resonance analysis (see attached figures 5 and 6),¹h NMR (500MHz, DMSO-d6) δ 7.40-7.35(s,2H),9.20-9.15(s, 4H). The target product, 2',4,4',6,6' -hexanitrodiphenylethylene, was confirmed.

Based on experimental results, the invention provides a reaction mechanism for converting HNBB into HNS through electrocatalysis, and the specific reaction process is shown in figure 2.

Due to the influence of the six nitro groups on HNBB, the amine is first bound to the carbon atom of the benzene ring that is attached to the methylene group, and HNS dianion is generated mainly for the following reasons: 1) electron withdrawing ability of nitrosyl group, which can stabilize negative charge; (2) the steric crowding of the numerous nitro groups on the phenyl ring can be alleviated by the formation of a dianion. In addition, the structure of the dianion allows free rotation about a central carbon-carbon bond, resulting in an HNS dianion (HNS)^2-) Can exist relatively stably. Then, under the action of constant current/voltage, HNS^2-The HNS was generated electrolytically on a graphite (anode) electrode.

Claims

1. A method for improving the yield of hexanitrostilbene prepared by a one-step method is characterized by comprising

(1) Mixing trinitrotoluene and sodium hypochlorite through a static mixer in the presence of a mixed solvent of tetrahydrofuran and methanol, reacting the collected reaction liquid for a period of time,

(2) adding solvent, electrolyte and dehydrogenating agent into the solution obtained in the step (1), continuing the electrocatalytic reaction for a period of time to obtain a target product,

2. the method according to claim 1, wherein in the step (1), trinitrotoluene and sodium hypochlorite are pre-cooled to 0 ℃ and then mixed by a static mixer, and the mixed reaction solution is collected and reacted for 1-1.5 hours.

3. The method of claim 1, wherein in step (1), the volume ratio of tetrahydrofuran to methanol is 2: 1.

4. The method of claim 1, wherein in step (1), the molar ratio of sodium hypochlorite: trinitrotoluene is 1: 1-1.5: 1, and the content of available chlorine in the sodium hypochlorite is 2-8 wt%.

5. The method according to claim 1, wherein in the step (1), the pH of the reaction solution is 10 to 10.5 and the temperature of the reaction solution is 10 to 20 ℃.

6. The method of claim 1, wherein in the step (2), the solvent is selected from any one of diethyl ether, DMSO, acetone, benzene, and 1, 4-dioxane.

7. The method of claim 1, wherein in step (2), the dehydrogenating agent is selected from the group consisting of triethylenediamine, triethylamine, FeCl₂TEMPO or DIPEA in an amount equivalent to the molar amount of HNBB as a by-product in the reaction solution in the step (1).

8. The method according to claim 1, wherein in the step (2), the electrolyte is selected from any one of tetramethylammonium tetrafluoroborate, tetrabutylammonium perchlorate, tetrabutylammonium tetrafluoroborate, ammonium perchlorate and tetrabutylammonium hexafluorophosphate.

9. The method of claim 1, wherein in the step (2), the electrocatalytic reaction is performed by electrifying at a constant current of 4-12mA, with a graphite rod as an anode and a platinum sheet as a cathode.

10. The method of claim 1, wherein in the step (2), the electrocatalytic reaction temperature is 10-20 ℃ and the reaction time is 0.5-1 hour.