Method for preparing adiponitrile by coupling electrochemical hydrogen evolution with electrochemical oxidation of waste PA-66 plastic

Abstract

The invention relates to a method for preparing adiponitrile by coupling electrochemical hydrogen evolution with electrochemical oxidation of waste PA-66 plastic, and belongs to the field of electrochemical hydrogen evolution and waste plastic recycling and upgrading. The method comprises the following steps: simply depolymerizing industrial waste PA-66 plastic to obtain an adipic acid crystal and a hexamethylenediamine solution with the purity reaching the standard; a hexamethylenediamine solution is directly used as an electrolyte, a catalyst V-Ni2P is adopted, continuous long-time electrolysis is carried out in an established two-electrode electrolytic tank, hydrogen is obtained at a cathode through electrolysis, adiponitrile is obtained at an anode through electrolysis, and the selectivity and the yield of the adiponitrile are both close to 100%. And after complete reaction, the adiponitrile and the aqueous solution are layered, and a high-purity adiponitrile product is directly obtained. The process is simple to operate, low in cost, high in return and capable of efficiently producing green hydrogen and upgrading waste plastics into chemicals, and renewable energy sources are utilized to the maximum extent. The method opens up a way for commercially and sustainably producing high-value bulk chemicals and clean H2 fuel from PA-66 waste plastics, and has a wide application prospect.

Description

Technical Field

[0001] The invention relates to the field of electrochemical hydrogen evolution and waste plastic recycling and upgrading, and in particular to a method for preparing adiponitrile by coupling electrochemical hydrogen evolution with electrochemical oxidation of waste PA-66 plastic. Background Art

[0002] The rapid depletion of fossil fuels and the increase in carbon emissions have become global problems facing mankind today, and there is an urgent need to use alternative and sustainable energy sources. Hydrogen (H2) is an ideal renewable energy source with high calorific value and environmental friendliness. The production of high-purity H2 through integrated water splitting technology using renewable electricity is a zero-carbon process. However, the high overpotential (1.23 V) and four-proton coupled electron transfer limit the efficiency of the oxygen evolution reaction (OER), resulting in the instability of the catalyst at high energy consumption and high potential. Therefore, it has become very common to replace OER with thermodynamically more favorable oxidation reactions, which is of great significance to reduce power consumption and improve the corresponding energy conversion efficiency. Moreover, coupling these electrosynthetic reactions with HER can produce high value-added products at the anode and cathode simultaneously. Continuous exploration and innovation in catalyst design, reaction mechanism and device development are expected to improve the economic feasibility and competitiveness of this hybrid water electrolysis strategy in industrial production.

[0003] Plastics have become an indispensable part of modern society due to their light weight, good chemical stability and low cost. However, the continued production and use of plastics around the world has led to a large amount of unmanaged waste plastics polluting the environment at an alarming rate. Plastics are usually composed of polymers and have high stability and resistance to degradation. Plastic waste has an extremely long residence time, causing many serious problems from production to disposal, causing great harm to the environment, ecosystems and human health. In the face of the urgency of the hazards of plastic waste, it is necessary to develop effective plastic waste disposal strategies and take positive actions to protect the environment and ecosystems from plastic pollution.

[0004] Polyamide-66 (PA-66), commonly known as nylon 66, is a polyamide material made by polycondensation of hexamethylenediamine and adipic acid. It is one of the five major engineering plastics, with an annual global demand of 1.3 million tons. Currently, PA-66 waste is mainly disposed of through landfills, but because PA-66 is not biodegradable, it will accumulate in the environment. For example, 10% of the garbage in the ocean is composed of PA-66. In addition to landfill, PA-66 can also be disposed of by incineration, but it will emit a large amount of carbon dioxide. Mechanical recycling is another option for the treatment of scrapped PA-66, but it is not popular due to the huge cost of waste separation and cleaning. Summary of the invention

[0005] In order to alleviate the plastic crisis and environmental pressure, the present invention discloses a method for electrochemical oxidation of waste PA-66 plastic to produce adiponitrile coupled with green hydrogen production. Due to its advantages of high utilization rate, low energy consumption, easy separation, low investment, high economic value and green environmental protection, it is expected to provide a new feasible path for alleviating the plastic crisis and the industrial development of electrolysis of water to produce hydrogen. The waste PA-66 is used to produce valuable AC, ADN and H2. This work may open a way for the commercial and sustainable production of high-value bulk chemicals and clean H2 fuel from PA-66 waste.

[0006] In order to achieve the above object, the present invention provides the following technical solutions: A method for preparing adiponitrile by electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastics comprises the following steps: (1) Recycling discarded PA-66 plastics, sorting and pre-treating them, and then grinding and crushing them into powder in a ball mill to obtain PA-66 plastic powder; (2) PA-66 plastic powder is placed in a reaction device, deionized water and an acidic solution are added at the same time, and the mixture is heated and stirred at a set temperature, so that the PA-66 plastic powder is depolymerized and converted into hexamethylenediamine and adipic acid; The molar ratio of the PA-66 plastic powder to deionized water and the acidic solution is 1:(1-5):(20-40); The heating and stirring temperature is 100-130°C, and the continuous stirring time is 2-6 h; (3) After the reaction is completed, the mixture is cooled to room temperature, a precipitate is separated by filtration, and the filter cake is the product adipic acid; potassium hydroxide solution is added to the filtrate, and after the reaction is complete, the bottom precipitate is filtered again to obtain potassium sulfate, and the filtrate is a hexamethylenediamine solution; (4) The depolymerized hexamethylenediamine solution is directly used as an electrolyte, and electrolysis is performed in a two-electrode electrolytic cell using a nickel foam or a V-doped nickel foam catalyst, and hydrogen is obtained at the cathode and adiponitrile is obtained at the anode through electrolysis; The electrolysis operating temperature is 20-40°C and the operating pressure is 1-3Mpa.

[0007] Furthermore, the mesh size of the waste PA-66 plastic powder is 16 to 500 meshes.

[0008] Furthermore, the acidic solution includes hydrochloric acid solution, sulfuric acid solution, and nitric acid solution.

[0009] Furthermore, the preparation method of the V-doped nickel foam catalyst in step (4) is: a. Ultrasonic cleaning of nickel foam material in hydrochloric acid solution; b. Dissolve nickel nitrate hexahydrate, ammonium fluoride, urea and vanadium chloride in deionized water and stir to mix evenly; then soak the treated nickel foam material in the mixed solution and transfer it to a high-pressure hydrothermal autoclave for reaction; after the reaction, wash and dry to obtain a precursor material; c. Sodium hypophosphite is placed upstream of the tube furnace, and the precursor material is placed downstream of the tube furnace. After purging with N2, it is heated to 250-350°C for annealing, and then naturally cooled to room temperature. It is washed and dried to obtain a V-doped Ni2P catalyst.

[0010] Furthermore, the usage of nickel nitrate hexahydrate: ammonium fluoride: urea: vanadium chloride is 1.5:4:10: (0.2-0.6).

[0011] Furthermore, the annealing time in step c is 1-3h.

[0012] A V-doped nickel foam catalyst is prepared by the above method.

[0013] Specifically, a method for preparing adiponitrile by electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic comprises the following steps: (1) Recycled PA-66 plastics are sorted and classified, and pre-treated to remove labels, surface pigments and other impurities that may affect the purity of subsequent products. Then they are put into a ball mill for grinding and crushing into powder. The mesh size is determined according to the actual situation. The finer the powder, the more thorough the subsequent depolymerization.

[0014] (2) Weighing a certain amount of treated PA-66 powder and placing it in a reaction device, adding an appropriate amount of deionized water and an excess of acidic solution, heating and stirring the reaction at a set temperature for a certain period of time to achieve full depolymerization of the PA-66 plastic powder and convert it into hexamethylenediamine and adipic acid; (3) After the reaction is completed, the reaction device is placed in water and cooled to room temperature. A precipitate appears at the bottom, and the reaction product adipic acid is separated by filtration. Then, an excess of potassium hydroxide solution is added to the separated upper clear liquid. After the reaction is complete, the bottom precipitated potassium sulfate is filtered again, and the upper clear liquid is the hexamethylenediamine solution; (4) The depolymerized hexamethylenediamine solution is directly used as an electrolyte, and a non-precious metal bifunctional catalyst is used to perform continuous long-term electrolysis in a two-electrode electrolytic cell. Through electrolysis, hydrogen with a purity of more than 99% is obtained at the cathode, and the depolymerized hexamethylenediamine can be oxidized and upgraded to adiponitrile at the anode; (5) After the electrolysis reaction is completed, the electrolyte needs to be separated and purified. The cathode hydrogen enters the hydrogen / water separator to remove the water vapor carried by the gas, and then passes through the dryer for further dehumidification. The pressure regulator valve and the regulating valve are adjusted to the rated pressure output and transported to the required location. The anode reaction product adiponitrile is slightly soluble in water. Therefore, after the electrolyte is fully reacted, the adiponitrile product and the electrolytic solution are separated, and a high-purity adiponitrile product can be directly obtained. Specifically, the mesh size of the PA-6 plastic powder in step (1) is 16 to 500 meshes, preferably 500 meshes.

[0015] Specifically, the acidic solution in step (2) includes hydrochloric acid solution, sulfuric acid solution, and nitric acid solution, and preferably the acidic solution is sulfuric acid solution.

[0016] Specifically, in step (2), the molar mass ratio of the PA-66 powder to deionized water and 5 mol / L sulfuric acid solution is 1:(1-5):(20-40).

[0017] Specifically, the heating and stirring temperature range of step (2) is 80-140° C., the stirring speed is maintained above 200 r / min, and the continuous stirring time is 2-6 h.

[0018] Specifically, the electrolysis operation temperature in step (4) is 20-40° C., and the operating pressure is 1-3 Mpa.

[0019] It can be seen from the above technical solution that, compared with the prior art, the process for preparing adiponitrile by electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic provided by the present invention has the following excellent effects: 1. The depolymerized hexamethylenediamine solution is directly used as an electrolyte. The non-precious metal bifunctional catalyst V-Ni2P is used to perform continuous long-term electrolysis in a two-electrode electrolytic cell. Through electrolysis, hydrogen with a purity of more than 99% is obtained at the cathode, and the anode can oxidize and upgrade the depolymerized hexamethylenediamine to adiponitrile. The selectivity and yield of adiponitrile are close to 100%. After the complete reaction, adiponitrile and the aqueous solution are separated, and a high-purity adiponitrile product can be directly obtained.

[0020] 2. Compared with traditional landfill and incineration processes, the present invention is more environmentally friendly and will not cause additional impact on the environment. Physical recycling methods such as mechanical recycling have high costs and low returns. The electrochemical oxidation method can convert waste plastics into valuable adipic acid and adiponectin, and efficiently produce green hydrogen at the cathode, which has higher economic benefits.

[0021] 3. From the perspective of practical application and production, the separation of the products after the reaction of waste plastic PA-66 is simple and low-cost, which makes the entire recycling process lower in cost and more efficient. Most importantly, the electrochemical oxidation of PA-66 can be used as a substitute for the anode oxygen evolution reaction and coupled with the cathode electrolysis of water to produce hydrogen. From the perspective of energy utilization, the lower starting potential and operating potential make the energy consumption required at the same hydrogen production rate lower, and the cost of electricity can be further reduced; in addition, compared with the low value of oxygen, it is obviously more economically valuable to convert waste plastics into high-value-added adiponitrile and adipic acid.

[0022] In general, the electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic to produce adiponitrile disclosed in the present invention is not only expected to achieve the recycling and upgrading of waste plastics, but can also be combined with water electrolysis to produce hydrogen, which may open a way for the commercial and sustainable production of high-value bulk chemicals and clean H2 fuel from PA-66 waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0024] Figure 1 is the XRD pattern of the adipic acid product prepared in Example 1.

[0025] Figure 2 This is the XRD pattern of the potassium sulfate precipitate obtained in Example 1.

[0026] Figure 3 is the NMR chart of the supernatant prepared in Example 1.

[0027] Figure 4 This is a high-resolution photograph of the electrolyte after full electrolysis in Example 1.

[0028] Figure 5 The electrocatalytic performance comparison of V-Ni2P / NF, Ni2P and NiV-Pre in Example 2 is shown in FIG. Figure 6 These are the LSV curves, Tafel spectra and electrochemical impedance spectra of the catalysts with different vanadium doping amounts in Example 3.

[0029] Figure 7 These are the LSV curves, Tafel spectra and electrochemical impedance spectra of the catalysts with different annealing times in Example 4.

[0030] Figure 8This is the LSV curve of V-Ni2P in Example 5 as a bifunctional electrocatalyst in a dual-electrode HER||GOR and HER||OER electrolysis system.

[0031] Fig. 9 This is a diagram of power consumption at different current densities in Example 5.

[0032] Fig.10 This is a graph showing the yield and conversion rate of the hydrolysis reaction at different amounts of water and H2SO4 in Example 6.

[0033] Fig.11 It is a graph of the hydrolysis reaction yield and conversion rate at different reaction temperatures in Example 7.

[0034] Fig.12 It is a graph of the hydrolysis reaction yield and conversion rate at different reaction times in Example 8.

[0035] Fig.13 It is the LSV curve diagram at different reaction temperatures in Example 9. Fig.14 It is a process flow chart. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present invention will be described clearly and completely below in combination with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] The embodiment of the present invention discloses a green hydrogen production process coupled with electrochemical oxidation of waste PA-66 plastic to produce adiponitrile.

[0038] In order to better understand the present invention, the present invention is further specifically described below through the following examples, but it should not be understood as a limitation of the present invention. Some non-essential improvements and adjustments made by technicians in this field based on the above invention content are also considered to fall within the protection scope of the present invention.

[0039] The technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0040] Example 1 The implementation steps of a process for producing adiponitrile by electrochemical oxidation of waste PA-66 plastic coupled with green hydrogen production are as follows: (1) The recycled PA-66 plastic is sorted and classified, and pre-treated to remove labels, surface pigments and other impurities that may affect the purity of subsequent products. It is then placed in a ball mill for grinding and pulverization to obtain 500 mesh PA-66 plastic powder.

[0041] (2) Weigh 10 g of PA-66 powder and place it in a 50 mL round-bottom flask. Add 22.7 mL of deionized water and 5.7 mL of 5 mol / L sulfuric acid solution. Place it in an oil bath and heat to 110 °C. Set the speed to 250 r / min and stir continuously for 4 hours. The PA-66 plastic powder is fully depolymerized into hexamethylenediamine and adipic acid.

[0042] (3) After the reaction is completed, the reaction device is placed in water and cooled to room temperature. A precipitate appears at the bottom. The adipic acid in the mixed solution is rapidly crystallized due to its acid insolubility. The lower precipitate is filtered, washed and dried to obtain a product adipic acid ( Figure 1 is the XRD diagram of adipic acid product). Then, 5 mol / L potassium hydroxide solution was added to the separated supernatant until the pH was 13-14. During this period, a large amount of K2SO4 precipitated ( Figure 2 is the XRD pattern of potassium sulfate precipitation), the supernatant obtained after filtering the precipitation is hexamethylenediamine solution ( Figure 3 is the NMR graph of the supernatant); (4) The depolymerized hexamethylenediamine solution was directly used as an electrolyte, and pretreated commercial nickel foam was used as a bifunctional catalyst. The pretreatment steps were as follows: the purchased 1 mm thick, 110 ppi commercial nickel foam was ultrasonically cleaned in a 2 mol / L HCl solution for 30 minutes to remove the surface oxide layer, and then washed three times with ethanol and water respectively to remove grease and impurities on the NF surface. Continuous long-term electrolysis was carried out in the assembled two-electrode electrolytic cell. The operating temperature of the electrolysis device was 30°C and the operating pressure was 1 MPa. Through electrolysis, hydrogen with a purity of more than 99% was obtained at the cathode, and the anode could oxidize the depolymerized hexamethylenediamine to adiponectin ( Figure 3 ); (5) After the electrolysis reaction is completed, the electrolyte needs to be separated and purified. The cathode hydrogen enters the hydrogen / water separator to remove the water vapor carried by the gas, and then passes through the dryer for further dehumidification. It is then adjusted to the rated pressure output by the pressure regulating valve and the regulating valve, and transported to the required location. The anode reaction product adiponitrile is slightly soluble in water. Therefore, after the electrolyte is fully reacted, the adiponitrile product and the electrolytic solution are separated, and a high-purity adiponitrile product can be directly obtained ( Figure 4 High-resolution photo of the electrolyte after full electrolysis).

[0043] Example 2 The core of the present invention is to upgrade plastic monomers into chemicals at the anode and efficiently produce green hydrogen at the cathode to maximize the use of renewable energy. Therefore, in order to better reflect the industrial prospects of the present invention, a V-doped Ni2P catalyst with a nanoflower structure composed of nanosheets with higher catalytic performance and higher stability was designed and synthesized, and the necessity of catalyst modification was explored. The specific contents are as follows: (1) Measure 10 mL of 2M hydrochloric acid solution, cut the nickel foam material into pieces with a thickness of 1 mm, a width of 20 mm, and a length of 40 mm, and ultrasonically clean the nickel foam material in the solution for 30 min to remove the surface oxide layer. Then, the nickel foam material after ultrasonic cleaning is cleaned three times with ethanol and deionized water respectively to remove surface grease and impurities.

[0044] (2) Weigh 1.5mmol (436mg) nickel nitrate hexahydrate, 4mmol (148.2mg) ammonium fluoride, 10mmol (600.6mg) urea, and 0.4mmol vanadium chloride and dissolve them in 30mL deionized water. Stir for 30min and mix them evenly. Then soak a piece of treated nickel foam material in the mixed solution, transfer it to a 50mL high-pressure hydrothermal autoclave, and heat it in an oven to 120℃ for 12h. Then rinse the obtained precursor material with deionized water and ethanol several times, and then dry it in an oven at 60℃ overnight to obtain the precursor material NiV-Pre; (3) Weigh 0.8 g of sodium hypophosphite and place it upstream of the tube furnace. Place the synthesized precursor material downstream of the tube furnace and purge with N2 for 20 min to remove air. Then, heat to 300 °C at a rate of 5 °C / min under N2 atmosphere and maintain for 2 h. Then, naturally cool to room temperature. Rinse the obtained catalyst material with deionized water and ethanol several times, and then dry it in an oven at 60 °C overnight to obtain the final V-doped Ni2P catalyst V-Ni2P.

[0045] At the same time, the undoped vanadium catalyst Ni2P is synthesized, and the synthesis steps are the same as those of the vanadium-doped nickel phosphide, except that vanadium chloride is not added during the synthesis process. And the unphosphated catalyst NiV-Pre.

[0046] (4) The hexamethylenediamine solution obtained in the manner provided in Example 1 was used as the electrolyte, and the three synthesized catalysts V-Ni2P, Ni2P and NiV-Pre were directly used as anode catalysts. First, in a three-electrode system, CV scanning was performed to activate the catalysts using a Coster CS350MA electrochemical workstation. Then, a series of electrochemical tests were performed in the assembled three-electrode electrolytic cell. The operating temperature of the electrolysis device was 30°C and the operating pressure was 1 MPa. The obtained electrochemical data were analyzed to observe the changes in the activity of the modified catalysts.

[0047] (5) After the electrolysis reaction, the electrolyte needs to undergo subsequent product separation and purification. The hydrogen gas at the cathode enters the hydrogen / water separator to remove the water vapor carried by the gas. Then, after further dehumidification through a dryer, it is adjusted to the rated pressure output through a pressure stabilizing valve and a regulating valve, and transported to the required location. The anodic reaction product adiponitrile is slightly soluble in water. Therefore, after the electrolyte fully reacts, the adiponitrile product and the electrolytic solution are stratified, and high-purity adiponitrile product can be directly obtained. Compared with Ni2P and NiV-Pre, the synthesized V-Ni2P / NF catalyst has better catalytic activity for anodic oxidation. The potentials (i.e., E100) of all samples to reach 100 mA cm -2 are V-Ni2P < Ni2P < NiV-Pre in sequence, indicating that the catalytic performance decreases in sequence, which shows that V doping and phosphating are indispensable in improving the catalytic performance. At the same time, the lower Tafel further confirms the best AOR performance of V-Ni2P. It has the lowest Tafel slope (23.6 mV dec -1 )), which indicates that V-Ni2P is most favorable for the catalytic kinetics of AOR. At the same time, it has the lowest Rct value, proving the decreasing nature of AOR kinetics, which is in good agreement with the above Tafel results. To further study the excellent AOR activity of V-Ni2P, its double-layer capacitance value (Cdl) was calculated according to the relevant cyclic voltammetry curves (it should be noted that the maximum Cdl value of V-Ni2P is 6.5 mF cm-2, which is 1.7 times that of Ni2P. This indicates that doping V into Ni2P helps to improve the catalytic ability, as Figure 5 shown (comparison diagram of the electrocatalytic performance of V-Ni2P / NF, Ni2P and NiV-Pre).

[0048] Example 3 The core of the present invention lies in upgrading plastic monomers to chemicals at the anode, while efficiently producing green hydrogen at the cathode, maximizing the utilization of renewable energy. Therefore, in order to better reflect the industrialization prospect of the present invention, a V-doped Ni2P catalyst with a nano-flower-like structure composed of nanosheets with higher catalytic performance and higher stability was designed and synthesized, and at the same time, the influence of different vanadium doping amounts on the catalyst activity was explored. The specific content is as follows: (1) Measure 10 mL of 2 M hydrochloric acid solution, cut a foam nickel material with a thickness of 1 mm * width of 20 mm * length of 40 mm and ultrasonically clean it in the solution for 30 min to remove the surface oxide layer. Then, wash the ultrasonically treated foam nickel material three times each with ethanol and deionized water to remove the surface grease and impurities.

[0049] (2) Weigh 1.5mmol (436mg) nickel nitrate hexahydrate, 4mmol (148.2mg) ammonium fluoride, 10mmol (600.6mg) urea, and a certain amount of vanadium chloride and dissolve them in 30mL deionized water. Stir for 30min and mix them evenly. Then soak a piece of treated nickel foam material in the mixed solution, transfer it to a 50mL high-pressure hydrothermal autoclave, and heat it in an oven to 120℃ for 12h. Then rinse the obtained precursor material with deionized water and ethanol several times, and then dry it in an oven at 60℃ overnight. The doping amount of vanadium chloride is 0.2mmol (31.5mg), 0.4mmol (62.9mg), and 0.6mmol (94.5mg) respectively. (3) Weigh 0.8 g of sodium hypophosphite and place it upstream of the tube furnace. Place the synthesized precursor material downstream of the tube furnace and purge with N2 for 20 minutes to remove air. Then, heat to 300 °C at a rate of 5 °C / min under N2 atmosphere and maintain for 2 hours. Then, naturally cool to room temperature. Rinse the obtained catalyst material with deionized water and ethanol several times, and then dry it in an oven at 60 °C overnight to obtain the final V-doped Ni2P catalyst, which is named 0.2-V-Ni2P, 0.4-V-Ni2P, and 0.6-V-Ni2P according to different vanadium doping amounts. (4) The hexamethylenediamine solution obtained in the manner provided in Example 1 was used as the electrolyte, and the V-doped Ni2P catalyst was directly used as the anode catalyst. First, in a three-electrode system, CV scanning was performed to activate the catalyst using a Coster CS350MA electrochemical workstation. Then, a series of electrochemical tests were performed in the assembled three-electrode electrolytic cell. The operating temperature of the electrolysis device was 30°C and the operating pressure was 1 MPa. The obtained electrochemical data were analyzed to observe the effect of different vanadium doping amounts on the catalyst activity.

[0050] (5) After the electrolysis reaction is completed, the electrolyte needs to be separated and purified. The cathode hydrogen enters the hydrogen / water separator to remove the water vapor carried by the gas, and then passes through the dryer for further dehumidification. The pressure regulator valve and the regulating valve are adjusted to the rated pressure output and transported to the required location. The anode reaction product adiponitrile is slightly soluble in water. Therefore, after the electrolyte is fully reacted, the adiponitrile product and the electrolytic solution are separated, and a high-purity adiponitrile product can be directly obtained. Experimental data show that the 0.4-V-Ni2P / NF catalyst exhibits better catalytic activity in the anodic oxidation of hexamethylenediamine, such as Figure 6 0.4-V-Ni2P-2 has the best electrocatalytic performance and the lowest onset potential. The corresponding Tafel slope shows that it is comparable to 0.2-V-Ni2P-2 (43.2 mV dec -1) and 0.6-V-Ni2P-2 (125.6 mV dec -1 ), 0.4-V-Ni2P-2 showed the lowest Tafel slope (23.6 mV dec -1 ), indicating that it has the most favorable AOR kinetics. The electrochemical impedance spectroscopy (EIS) of 0.4-V-Ni2P-2 also demonstrated its highest catalytic ability, with the smallest radius, which determines the smallest charge transfer resistance (Rct) at the catalyst / electrolyte interface and the fastest charge transfer rate.

[0051] Example 4 The core of the present invention is to upgrade plastic monomers into chemicals at the anode and efficiently produce green hydrogen at the cathode to maximize the use of renewable energy. Therefore, in order to better reflect the industrial prospects of the present invention, a V-doped Ni2P catalyst with a nanoflower structure composed of nanosheets with higher catalytic performance and higher stability was designed and synthesized, and the effect of different phosphating times on the catalyst activity was explored. The specific contents are as follows: (1) Measure 10 mL of 2M hydrochloric acid solution, cut the nickel foam material into pieces with a thickness of 1 mm, a width of 20 mm, and a length of 40 mm, and ultrasonically clean the nickel foam material in the solution for 30 min to remove the surface oxide layer. Then, the nickel foam material after ultrasonic cleaning is cleaned three times with ethanol and deionized water respectively to remove surface grease and impurities.

[0052] (2) Weigh 1.5mmol (436mg) nickel nitrate hexahydrate, 4mmol (148.2mg) ammonium fluoride, 10mmol (600.6mg) urea, and 0.4mmol (62.9mg) vanadium chloride and dissolve them in 30mL deionized water. Stir for 30min and mix well. Then soak a piece of treated nickel foam material in the mixed solution, transfer it to a 50mL high-pressure hydrothermal autoclave, and heat it in an oven to 120℃ for 12h. Then rinse the obtained precursor material with deionized water and ethanol several times, and then dry it in an oven at 60℃ overnight. (3) Weigh 0.8g of sodium hypophosphite and place it upstream of the tube furnace. Place the synthesized precursor material downstream of the tube furnace. Purge with N2 for 20 minutes to remove air. Then heat to 300 °C at a rate of 5 °C / min in N2 atmosphere and maintain for a certain time. Then cool naturally to room temperature. Rinse the obtained catalyst material with deionized water and ethanol several times, and then dry it in an oven at 60 °C overnight to obtain the final V-doped Ni2P catalyst. The annealing time is 1h, 2h, and 3h, respectively. According to the different annealing times, they are named V-Ni2P-1, V-Ni2P-2, and V-Ni2P-3 respectively; (4) The hexamethylenediamine solution obtained in the manner provided in Example 1 was used as the electrolyte, and the V-doped Ni2P catalyst was directly used as the anode catalyst. First, in a three-electrode system, CV scanning was performed to activate the catalyst using a Coster CS350MA electrochemical workstation. Then, a series of electrochemical tests were performed in the assembled three-electrode electrolytic cell. The operating temperature of the electrolysis device was 30°C and the operating pressure was 1 MPa. The obtained electrochemical data were analyzed to observe the effect of different annealing times on the catalyst activity.

[0053] (5) After the electrolysis reaction is completed, the electrolyte needs to be separated and purified. The cathode hydrogen enters the hydrogen / water separator to remove the water vapor carried by the gas, and then passes through the dryer for further dehumidification. The pressure regulator valve and the regulating valve are adjusted to the rated pressure output and transported to the required location. The anode reaction product adiponitrile is slightly soluble in water. Therefore, after the electrolyte is fully reacted, the adiponitrile product and the electrolytic solution are separated, and a high-purity adiponitrile product can be directly obtained. The experimental data show that compared with the samples with annealing time of 1h and 3h, the sample with annealing time of 2h (i.e., V-Ni2P-2) shows the smallest onset potential and the lowest Tafel slope, indicating that it has the best catalytic activity, such as Figure 7 (LSV curves, Tafel spectra and electrochemical impedance spectroscopy of catalysts with different annealing times).

[0054] Example 5 In order to fully illustrate the advantages of low power consumption and high energy conversion efficiency of the electrochemical hydrogen evolution coupled PA-66 waste plastic monomer oxidation process in the present invention, the difference between the present electrolysis device and the water electrolysis hydrogen production device was studied in detail.

[0055] The hexamethylenediamine solution obtained in the manner provided in Example 1 was used as the electrolyte, and the bifunctional catalyst V-Ni2P was directly used as the cathode and anode catalysts. First, in a three-electrode system, CV scanning was performed to activate the catalyst with the aid of a Coster CS350MA electrochemical workstation, and then a polarization curve LSV scan was performed in the assembled two-electrode electrolytic cell. The operating temperature of the electrolysis device was 30°C, the operating pressure was 1 MPa, the scanning rate was 10 mV / s, the scanning range was 1-2.6 V, the iR compensation was set to 85%, and the scanning was continued for multiple times until it stabilized.

[0056] At the same time, 1 mol / L potassium hydroxide was used as the electrolyte, and the same bifunctional catalyst V-Ni2P was used directly as the cathode and anode catalyst. First, in the three-electrode system, CV scanning was performed on the Coster CS350MA electrochemical workstation to activate the catalyst, and then the polarization curve LSV scanning was performed in the assembled two-electrode electrolytic cell. The operating temperature of the electrolysis device was 30°C, the operating pressure was 1 MPa, the scanning rate was 10 mV / s, the scanning range was 1~2.6 V, the iR compensation was set to 85%, and the scanning was continued for multiple times until it stabilized.

[0057] Figure 8 The LSV curve of V-Ni2P as a bifunctional electrocatalyst in the dual-electrode HER||GOR and HER||OER electrolysis systems. By comparing the two LSV curves, compared with the traditional water electrolysis hydrogen production, the cathode HER coupled anode AOR (HER||AOR) device only needs 1.58, 1.65, 1.69 and 1.74 V to reach 100, 200, 300 and 400 mA cm, respectively. -2 The current density of the HER||AOR system is much lower than that of conventional water splitting (HER||OER, 1.89, 1.99, 2.08, and 2.15 V). The HER||AOR system can also achieve higher industrial-grade current densities (≥ 500 mA cm-2). More importantly, the V-Ni2P-based HER||AOR system requires only 1.88 V to achieve an industrial-relevant current density of 800 mA cm-2. -2 The current density is at least 500 mV lower than that of water splitting. Such a low cell voltage of the HER||AOR system verifies the corresponding energy-saving performance and efficient energy conversion, which is better than the recently reported hybrid electrolysis system. It is worth noting that Fig. 9 The power consumption at different current densities is shown in Figure 2, which shows the calculated power consumption of the HER||AOR and HER||OER systems, where the power consumption increases gradually with increasing current density. -2 and 800 mA cm -2 When HER||AOR system produces 1 kg H2, the power consumption is 46.66 kWh and 50.55 kWh (kWh kg -1 H2) consumes less electricity than overall water electrolysis. This result further reveals the advantages of the coupled process of hydrogen evolution reaction and hexamethylenediamine oxidation cycle in the practical electrosynthesis of high value-added chemicals.

[0058] Example 6 Exploration of the solvent conditions for the hydrolysis of PA-66 plastic in the process of electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic to produce adiponitrile.

[0059] (1) Weigh a certain amount of treated PA-66 powder and place it in a reaction device. Add an appropriate amount of deionized water and an excess of acid solution. Place it in an oil bath and heat it to 110°C. Set the speed to 250 r / min and stir continuously for 4 hours. The PA-66 plastic powder is depolymerized into hexamethylenediamine and adipic acid. The molar mass ratio of PA-66 powder to deionized water and 5 mol / L sulfuric acid solution is 1: (1-5): (20-40). The yield and conversion rate of the hydrolysis reaction under different amounts of water and H2SO4 are shown in Figure 2. Fig.10 shown.

[0060] The study found that when there is less sulfuric acid solution, the conversion rate and yield of the product are low. With the increase of the amount of sulfuric acid solution, the conversion rate and yield of the product increase significantly. The addition of sulfuric acid solution provides a good acidic environment for the depolymerization reaction of PA-66, which is conducive to the depolymerization reaction. However, when there is too much sulfuric acid solution, the conversion rate and yield of the product do not increase significantly, and more potassium hydroxide solution will be needed later, resulting in waste.

[0061] At the same time, the amount of deionized water will also have a significant effect on the depolymerization reaction. When the molar mass ratio of PA-66 powder to deionized water is less than 1:30, the conversion rate and yield of the product increase with the increase of the amount of deionized water. If the amount of deionized water continues to increase, the conversion rate and yield of the product begin to decrease. Therefore, the optimal molar mass ratio of PA-66 powder to deionized water and 5 mol / L sulfuric acid solution is 1:2.5:30. Fig.10 .

[0062] Example 7 Exploration of the temperature conditions for hydrolysis of PA-66 plastic in the process of electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic to produce adiponitrile.

[0063] (1) Weigh 10 g of PA-66 powder and place it in a 50 mL round-bottom flask. Add 22.7 mL of deionized water and 5.7 mL of 5 mol / L sulfuric acid solution. Place the flask in an oil bath and heat to 80°C, 90°C, 100°C, 110°C, 120°C, 130°C and 140°C. Set the speed to 250 r / min and stir continuously for 4 hours. The PA-66 plastic powder is fully depolymerized into hexamethylenediamine and adipic acid.

[0064] The depolymerization temperature has a significant effect on the depolymerization reaction of PA-66. As the reaction temperature increases, the conversion rate and yield of the product increase significantly. This is because as the temperature increases, the PA-66 powder will swell or dissolve in water, increasing the contact area with water and accelerating the depolymerization reaction rate. When the temperature rises from 80°C to 110°C, the conversion rate and yield rise sharply until it is completely depolymerized at 110°C. When the temperature continues to rise, the conversion rate and yield of the product do not increase significantly. In order to maintain low energy consumption, the optimal reaction temperature is 110°C. The yield and conversion rate of the hydrolysis reaction at different reaction temperatures are shown in the figure below. Fig.11 .

[0065] Example 8 Exploration of the hydrolysis time conditions of PA-66 plastic in the process of electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic to produce adiponitrile.

[0066] (1) Weigh 10 g of PA-66 powder and place it in a 50 mL round-bottom flask. Add 22.7 mL of deionized water and 5.7 mL of 5 mol / L sulfuric acid solution. Place the flask in an oil bath and heat to 110 °C. Set the speed to 250 r / min and stir continuously for 1, 2, 3, 4, 5, and 6 hours. The PA-66 plastic powder is fully depolymerized into hexamethylenediamine and adipic acid.

[0067] The depolymerization time also has a significant effect on the PA-66 depolymerization reaction. As the reaction time increases, the conversion rate and yield of the product increase significantly. When the time reaches 4 hours, the conversion rate and yield of the product are the highest, and the PA-66 powder is completely depolymerized. Continuing to extend the time, the conversion rate and yield of the product do not increase significantly. In order to maintain low energy consumption, the optimal reaction time is 4h. The yield and conversion rate of the hydrolysis reaction at different reaction times are shown in the figure below. Fig.12 .

[0068] Example 9 Exploration of the operating conditions of PA-66 plastic electrolysis in the process of electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic to produce adiponitrile.

[0069] (1) The hexamethylenediamine solution obtained in the manner provided in Example 1 was used as the electrolyte, and the bifunctional catalyst V-Ni2P was directly used as the cathode and anode catalysts. First, in a three-electrode system, a CV scan was performed with the aid of a Coster CS350MA electrochemical workstation to activate the catalyst, and then a polarization curve LSV scan was performed in a two-electrode electrolytic cell. The scan rate was 10 mV / s, the scan interval was 1-2.6 V, and the iR compensation was set to 85%. The scan was continued for multiple times until it stabilized. The operating temperature of the electrolysis device was 20°C, 30°C, and 40°C.

[0070] Electrochemical tests were conducted under different operating conditions of the electrolysis device. Through the test of LSV curve and quantitative analysis of the product, it was found that the reaction temperature has a significant effect on the catalytic activity and catalytic rate. As the temperature increases, the transfer rate of electrons and ions accelerates, and the catalytic reaction rate also increases significantly. However, high temperature also brings problems, such as faster solution evaporation rate, higher energy consumption, and greater test on the stability of catalyst materials. After comprehensive consideration, the optimal temperature can be set to 30°C. The LSV curves at different reaction temperatures are shown in the figure below. Fig.13 The effect of pressure on the electrolysis reaction is not obvious and can be set to 1 MPa without special requirements.

[0071] Example 10 In order to fully illustrate the advantages of low cost and high return of the process for preparing adiponitrile by electrochemical hydrogen evolution coupled with electrochemical oxidation of PA-66 waste plastic monomer in the present invention, the separation cost in this process is explained.

[0072] (1) After the depolymerization reaction is completed, the reaction device is placed in water and cooled to room temperature. A precipitate appears at the bottom. The adipic acid in the mixed solution is rapidly crystallized due to its acid insolubility. The lower precipitate is filtered, washed and dried to obtain adipic acid with a purity that meets the standard. Then, 5 mol / L potassium hydroxide solution is added to the separated upper clear liquid until the pH is 13-14. During this period, a large amount of K2SO4 is precipitated. After filtering the precipitate, the upper clear liquid obtained is a hexamethylenediamine solution.

[0073] (2) The electrolyte after the electrolysis reaction needs to be separated and purified. The cathode hydrogen enters the hydrogen / water separator to remove the water vapor carried by the gas, and then passes through the dryer for further dehumidification. The pressure regulator valve and the regulating valve are adjusted to the rated pressure output and transported to the required place. The anode reaction product adiponitrile is slightly soluble in water. Therefore, after the electrolyte is fully reacted, the adiponitrile product and the electrolytic solution are separated, and a high-purity adiponitrile product can be directly obtained. Unlike traditional commercial water electrolysis systems, which are dominated by electricity consumption costs, the cost of the separation system in the electrochemical hydrogen evolution coupled with the electrochemical oxidation process of waste plastics cannot be ignored. The process design in the present invention allows the products to be separated at low cost. In particular, during the depolymerization of PA-66 powder, adipic acid crystallizes due to its acid insolubility. At the same time, benefiting from the excellent performance of the electrochemical catalyst V-Ni2P, at 1.40 V (vs. RHE), the selectivity and yield of adiponitrile are close to 100%, which allows all the hexamethylenediamine in the solution to be converted into an adiponitrile liquid that is stratified from the aqueous solution ( Fig.14 ).

[0074] In summary, from the analysis of the above experimental results, it can be seen that this process of electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic to produce adiponitrile opens a way for the commercial and sustainable production of high-value bulk chemicals and clean H2 fuel from PA-66 waste, and has broad application prospects.

[0075] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing adiponitrile by electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic, characterized in that: The steps include: (1) Recycling discarded PA-66 plastics, sorting and pre-treating them, and then grinding and crushing them into powder in a ball mill to obtain PA-66 plastic powder; (2) PA-66 plastic powder is placed in a reaction device, deionized water and an acid solution are added at the same time, and the mixture is heated and stirred at a set temperature, so that the PA-66 plastic powder is depolymerized and converted into hexamethylenediamine and adipic acid; The molar ratio of the PA-66 plastic powder to deionized water and the acidic solution is 1:(1-5):(20-40); The heating and stirring temperature is 100-130°C, and the continuous stirring time is 2-6 h; (3) After the reaction is completed, the mixture is cooled to room temperature, a precipitate is separated by filtration, and the filter cake is the product adipic acid; potassium hydroxide solution is added to the filtrate, and after the reaction is complete, the bottom precipitate is filtered again to obtain potassium sulfate, and the filtrate is a hexamethylenediamine solution; (4) The depolymerized hexamethylenediamine solution is directly used as an electrolyte, and electrolysis is performed in a two-electrode electrolytic cell using a nickel foam or a V-doped nickel foam catalyst, and hydrogen is obtained at the cathode and adiponitrile is obtained at the anode through electrolysis; The electrolysis operating temperature is 20-40°C and the operating pressure is 1-3Mpa.

2. The method for preparing adiponitrile by electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic according to claim 1, characterized in that: The mesh number of the A-66 plastic powder is 16 to 500 meshes.

3. The method for preparing adiponitrile by electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic according to claim 1, characterized in that: The acidic solution includes hydrochloric acid solution, sulfuric acid solution and nitric acid solution.

4. The method for preparing adiponitrile by electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic according to claim 1, characterized in that: The preparation method of the V-doped nickel foam catalyst in step (4) is as follows: a. Ultrasonic cleaning of nickel foam material in hydrochloric acid solution; b. Dissolve nickel nitrate hexahydrate, ammonium fluoride, urea and vanadium chloride in deionized water and stir to mix evenly; then soak the treated nickel foam material in the mixed solution and transfer it to a high-pressure hydrothermal autoclave for reaction; after the reaction, wash and dry to obtain a precursor material; c. Sodium hypophosphite is placed upstream of the tube furnace, and the precursor material is placed downstream of the tube furnace. After being purged with N2, it is heated to 250-350°C for annealing, and then naturally cooled to room temperature. It is washed and dried to obtain a V-doped Ni2P catalyst.

5. The method for preparing adiponitrile by electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic according to claim 4, characterized in that: The dosage of the nickel nitrate hexahydrate: ammonium fluoride: urea: vanadium chloride is 1.5:4:10: (0.2-0.6).

6. The method for preparing adiponitrile by electrochemical hydrogen evolution coupled with electrochemical oxidation of waste PA-66 plastic according to claim 4, characterized in that: The annealing time in step c is 1-3 hours.

7. A V-doped nickel foam catalyst, characterized in that: The method is prepared by any one of claims 4 to 6.

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