Synthetic method of maleic acid

Abstract

The invention belongs to the field of catalysis and bio-based chemicals, and discloses a synthesis method of maleic acid, which is characterized by comprising the following steps: adding furfural, an electrocatalyst 4-acetamido-2, 2, 6, 6-tetramethylpiperidine-1-oxy radical and a photocatalyst 5, 10, 15, 20-tetra (4-carboxyphenyl) porphyrin into a carbonate buffer solution, and placing in an anode chamber; adding p-nitrophenol into a potassium hydroxide and / or sodium hydroxide aqueous solution, and placing in a cathode chamber; and continuously introducing oxygen into the reaction liquid in the anode chamber, and synchronously carrying out a photocatalytic reaction and an electrolytic reaction under an illumination condition to prepare maleic acid in the anode chamber. The method overcomes the defects of harsh chemical catalytic reaction conditions, high energy consumption and the like, and has the advantages of mild reaction conditions, environment friendliness, high synthesis efficiency, excellent selectivity, easiness in amplification and the like.

Description

Technical Field

[0001] The invention belongs to the field of catalysis and bio-based chemicals, and in particular relates to a method for synthesizing maleic acid by converting furfural through photo / electrocatalysis. Background Art

[0002] Furfural is an important bio-based platform compound, usually produced by hydrolysis / dehydration of agricultural and forestry byproducts rich in hemicellulose. Furfural molecules contain highly reactive furan rings and aldehyde groups, which can be synthesized into a series of high value-added chemicals such as maleic acid through oxidation, hydrogenation and amination reactions (ACS Catal. 2016, 6, 7621). Maleic acid is an important industrial C4 chemical and is widely used in food, chemical, pharmaceutical and polymer fields. For example, maleic acid is a key raw material or chemical intermediate for the manufacture of unsaturated polyester resins, vinyl copolymers, surface coatings, printing and dyeing auxiliaries, preservatives, plasticizers, food additives and drugs. It is also used to synthesize products such as fumaric acid, tartaric acid and succinic acid.

[0003] At present, maleic acid is mainly obtained by chemical catalytic oxidation of petroleum-based raw materials such as benzene or butane under harsh reaction conditions and environmentally unfriendly. From the perspective of green and sustainable chemistry, it is more attractive to synthesize bio-based maleic acid using renewable furfural as a raw material. Currently, various methods for synthesizing maleic acid by chemical catalysis, photocatalysis, and electrocatalysis have been developed. For example, Li et al. used a bimetallic oxide catalyst (Mo4VO 14 ), in acetic acid solution, 120℃ and 2 MPa O2 conditions, after 16h of reaction, the furfural conversion rate was >99%, and the maleic anhydride yield was 62%; maleic anhydride was completely converted into maleic acid after hot water hydrolysis (Green Chem., 2016, 18, 2976). Ren et al. designed a highly dispersed CuO x The Nb2O5 photocatalyst can selectively oxidize furfural to maleic anhydride and 5-hydroxy-2(5 H )-furanone (Chem. Asian J., 2023, 18, e202300732). Thiyagarajan et al. constructed a photo / bio(electro)catalytic oxidation system for the oxidation of furfural to maleic acid, with a final yield of 90% (ACS Sustain. Chem. Eng., 2020, 8,10626). Although the yield of maleic acid is high, it involves intermediates (5-hydroxy-2(5 HThe separation and purification of )-furanone increases the complexity of the catalytic process. In short, the existing synthesis methods have problems such as harsh reaction conditions (high temperature and high pressure, use of organic solvents, strong acids and strong bases, etc.), use of metal catalysts, low conversion rate or poor product selectivity; and the biocatalytic pathway still cannot achieve the direct synthesis of maleic acid. Summary of the invention

[0004] In view of the problems existing in the prior art, the object of the present invention is to provide a green and efficient method for synthesizing maleic acid by photo / electrocatalytic oxidation of furfural using bio-based furfural as a substrate under mild conditions.

[0005] The purpose of the present invention is achieved through the following technical solutions: A method for synthesizing maleic acid comprises the following steps: adding furfural, an electrocatalyst 4-acetylamino-2,2,6,6-tetramethylpiperidin-1-oxyl free radical (ACT), and a photocatalyst 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) into a carbonate buffer solution, and placing the solution in an anode chamber; adding p-nitrophenol into an aqueous solution of potassium hydroxide and / or sodium hydroxide, and placing the solution in a cathode chamber; continuously introducing oxygen into a reaction solution in the anode chamber, and simultaneously carrying out a photocatalytic reaction and an electrolytic reaction under light conditions, so as to obtain maleic acid in the anode chamber; The photocatalytic reaction is to pump the reaction liquid in the anode chamber into a transparent plastic spiral tube after oxygen is introduced, and the transparent plastic spiral tube is placed under light, and the liquid after the reaction in the spiral tube flows back to the anode chamber to achieve a continuous cycle photocatalytic reaction.

[0006] Preferably, the concentration of ACT is 5-60 mol%; and the concentration of TCPP is 0.5-5 mol%.

[0007] Preferably, the concentration of ACT is 20±10 mol; the concentration of TCPP is 1-2 mol; the concentration of furfural is 100-300 mM; and the concentration of p-nitrophenol is 10-40 mM.

[0008] Preferably, the conditions of the electrolysis reaction are: initial reaction liquid temperature 20-40° C., rotation speed 600±200 r / min, reaction time 3±1 h, and oxygen flow rate 13±5 mL / min.

[0009] Preferably, the working electrode, reference electrode and counter electrode are respectively a mesh glassy carbon electrode, a silver / silver chloride electrode and a nickel foam electrode; the applied voltage is 0.7-0.9 V; and the cathode chamber and the anode chamber are separated by a Nafion117 membrane.

[0010] Preferably, the concentration of the potassium hydroxide and / or sodium hydroxide aqueous solution is 1±0.5 M; the concentration of the carbonate buffer solution is 0.25-1 M, and its pH is 8.5-10; carbonate needs to be added during the reaction to maintain the pH of the system unchanged.

[0011] Preferably, the carbonate buffer is a solution of Na2CO3 and / or NaHCO3.

[0012] Preferably, the conditions of the photocatalytic reaction are: the initial reaction liquid temperature is 20-40°C, and oxygen is continuously introduced; the reaction liquid and oxygen flow rates are 11.5±3.5 mL / min and 13±5 mL / min, respectively; and the reaction time is 3±1 h.

[0013] Preferably, the length (optical path) of the transparent plastic spiral tube (spiral section) is 175-235 cm, the inner diameter is 2±0.5 mm, and the outer diameter is 2.4±0.5 mm.

[0014] Preferably, the light source of the illumination is a green LED lamp, a blue LED lamp, a purple LED lamp, a red LED lamp, a white LED lamp and sunlight, and the power of the LED lamp is 30±20W.

[0015] Compared with the prior art, the present invention has the following advantages: (1) The present invention uses clean energy (electricity and light) to achieve photo / electricity synchronous catalytic oxidation under mild conditions, overcoming the defects of chemical catalytic reactions such as harsh conditions and high energy consumption. The continuous cycle photocatalytic reaction not only shortens the reaction time, but also makes it difficult for the active furfural to undergo side reactions during the reaction, thereby greatly improving the yield of maleic acid.

[0016] (2) The present invention establishes a green pathway for synthesizing maleic acid using bio-based furfural as a substrate, which not only avoids dependence on petroleum-based resources but also achieves the goal of carbon neutrality.

[0017] (3) The reaction process of the present invention is simple, easy to control and scale up, with high synthesis efficiency and excellent selectivity. Not only is the product quality high, but it is also conducive to simplifying the subsequent separation and purification of the target product. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the reaction device for batch electrocatalysis and continuous flow photocatalysis furfural oxidation.

[0019] Figure 2 furfural, furoic acid, 5-hydroxy-2(5 H )-furanone and maleic acid structural formula.

[0020] Figure 3This is the HPLC chromatogram for quantitative analysis of maleic acid in Example 4. DETAILED DESCRIPTION

[0021] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto.

[0022] Example 1 Nickel foam electrode (NiB x @NF) was prepared according to the reference (Angew. Chem. Int. Ed., 2019, 58, 9155). Nickel foam (purchased from Tianjin Yingke United Technology Co., Ltd.) was cut into 20 mm × 25 mm × 1 mm pieces, cleaned in an ultrasonic cleaner with acetone and anhydrous ethanol for 30 min, and then dried at room temperature for use. Subsequently, 25 mL of 125 mM NiCl2 aqueous solution was prepared, placed in an ice water bath, and ethylenediamine with a final concentration of 750 mM was slowly added under stirring; 25 mL of 55 mM NaBH4 solution was prepared in 0.2 M NaOH aqueous solution; the two solutions prepared above were mixed evenly, the pH was adjusted to 13.5 with 10 M NaOH, and the volume was made up to 100 mL with distilled water. Finally, the washed and dried nickel foam sheet was suspended in the above mixed solution and allowed to stand at 90 °C for 2 h; then it was rinsed with distilled water and dried at 60 °C for later use, thus obtaining NiB x @NF electrode.

[0023] Example 2 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (1 M Na2CO3 and 1 M NaHCO3, pH 10) and 16 mL of catholyte (1M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane (DuPont, USA). 100 mM furfural, 5 mol% ACT and 1 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 10 with Na2CO3 powder. It was then connected to the O2 pipeline and a transparent polyfluorinated ethylene propylene plastic spiral tube (optical path length of 175 cm, inner diameter of 2 mm, outer diameter of 2.4 mm) continuous flow photoreaction device through a plastic pipeline and with the help of a peristaltic pump, and then the reaction solution mixed with O2 was pumped back to the anode chamber under the action of the peristaltic pump, and the reaction solution continued to circulate under the action of the peristaltic pump. The continuous flow photoreaction device was placed under green light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was photocatalytically reacted. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was 25°C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 h. Cathode chamber: 10 mM p-nitrophenol. Among them, mesh glassy carbon (10 × 10 × 5 mm, 100 PPI, purchased from ERG Aerospace), NiB x @NF and silver / silver chloride were the working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction was 600 r / min, the reaction temperature was 25°C, and the voltage was 0.7 V. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 50 mM, and the yield was 50%.

[0024] Example 3 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (1 M Na2CO3 and 1 M NaHCO3, pH 10) and 16 mL of catholyte (1 M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane. 100 mM furfural, 60 mol% ACT and 1 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 10 with Na2CO3 powder. It was then connected to the O2 pipeline and a transparent polyfluorinated ethylene propylene plastic spiral tube (optical path length of 200 cm, inner diameter of 2 mm, outer diameter of 2.4 mm) continuous flow photoreaction device through a plastic pipeline and with the help of a peristaltic pump, and then the reaction solution mixed with O2 was pumped back to the anode chamber under the action of the peristaltic pump, and the reaction solution continued to circulate under the action of the peristaltic pump. The continuous flow photoreaction device was placed under irradiation of purple light (LED lamp, 30 W), and the reaction solution mixed with O2 was photocatalytically reacted. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was 25°C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 h. Cathode chamber: 10 mM p-nitrophenol. x @NF and silver / silver chloride were the working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction was 600 r / min, the reaction temperature was 25°C, and the voltage was 0.9 V. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 72 mM, and the yield was 72%.

[0025] Example 4 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (1 M Na2CO3 and 1 M NaHCO3, pH 10) and 16 mL of catholyte (1 M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT and 1 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 10 with Na2CO3 powder. It was then connected to the O2 pipeline and a transparent polyfluorinated ethylene propylene plastic spiral tube (optical path length of 235 cm, inner diameter of 2 mm, outer diameter of 2.4 mm) continuous flow photoreaction device through a plastic pipeline and with the help of a peristaltic pump, and then the reaction solution mixed with O2 was pumped back to the anode chamber under the action of the peristaltic pump, and the reaction solution continued to circulate under the action of the peristaltic pump. The continuous flow photoreaction device was placed under green light (LED lamp, 50 W) irradiation, and the reaction solution mixed with O2 was photocatalytically reacted. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was 25°C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 h. Cathode chamber: 10 mM p-nitrophenol. x @NF and silver / silver chloride were the working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction was 600 r / min, the reaction temperature was 25°C, and the voltage was 0.8 V. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 78 mM, and the yield was 78%.

[0026] Example 5 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (1 M Na2CO3 and 1 M NaHCO3, pH 10) and 16 mL of catholyte (1 M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT and 2 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 10 with Na2CO3 powder. It was then connected to the O2 pipeline and the transparent polyfluorinated ethylene propylene plastic spiral tube (optical path length of 235 cm, inner diameter of 2 mm, outer diameter of 2.4 mm) continuous flow photoreaction device through a plastic pipeline and with the help of a peristaltic pump, and then the reaction solution mixed with O2 was pumped back to the anode chamber under the action of the peristaltic pump, and the reaction solution continued to circulate under the action of the peristaltic pump. The continuous flow photoreaction device was placed under red light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was photocatalytically reacted. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was 30°C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 h. Cathode chamber: 10 mM p-nitrophenol. x @NF and silver / silver chloride were the working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction was 600 r / min, the reaction temperature was 30°C, and the voltage was 0.8 V. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 72 mM, and the yield was 72%.

[0027] Example 6 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (0.5 M Na2CO3 and 0.5 M NaHCO3, pH 10) and 16 mL of catholyte (1 M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT and 2 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 10 with Na2CO3 powder. It was then connected to the O2 pipeline and a transparent polyfluorinated ethylene propylene plastic spiral tube (optical path length of 235 cm, inner diameter of 2 mm, outer diameter of 2.4 mm) continuous flow photoreaction device through a plastic pipeline and with the help of a peristaltic pump, and then the reaction solution mixed with O2 was pumped back to the anode chamber under the action of the peristaltic pump, and the reaction solution continued to circulate under the action of the peristaltic pump. The continuous flow photoreaction device was placed under green light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was photocatalytically reacted. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was 25°C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 h. Cathode chamber: 40 mM p-nitrophenol. x @NF and silver / silver chloride were the working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction was 600 r / min, the reaction temperature was 25°C, and the voltage was 0.8 V. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 80 mM, and the yield was 80%.

[0028] Example 7 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (0.25 M Na2CO3 and 0.25 M NaHCO3, pH 8.5) and 16 mL of catholyte (1 M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT and 1 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 8.5 with Na2CO3 powder. It was then connected to the continuous flow photoreaction device of the O2 pipe and the transparent polyfluorinated ethylene propylene plastic spiral tube (optical path of 235 cm, inner diameter of 2 mm, outer diameter of 2.4 mm) through a plastic pipe and with the help of a peristaltic pump, and then the reaction solution mixed with O2 was pumped back to the anode chamber under the action of the peristaltic pump, and the reaction solution continued to circulate under the action of the peristaltic pump. The continuous flow photoreaction device was placed under green light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was photocatalytically reacted. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was 25°C. The pH of the reaction solution was adjusted to 8.5 with Na2CO3 powder every 1 h. Cathode chamber: 20 mM p-nitrophenol. x @NF and silver / silver chloride were the working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction was 600 r / min, the reaction temperature was 25°C, and the voltage was 0.8 V. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 67 mM, and the yield was 67%.

[0029] Example 8 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (0.25 M Na2CO3 and 0.25 M NaHCO3, pH 10) and 16 mL of catholyte (1 M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT and 1 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 10 with Na2CO3 powder. It was then connected to the O2 pipeline and a transparent polyfluorinated ethylene propylene plastic spiral tube (optical path length of 235 cm, inner diameter of 2 mm, outer diameter of 2.4 mm) continuous flow photoreaction device through a plastic pipeline and with the help of a peristaltic pump, and then the reaction solution mixed with O2 was pumped back to the anode chamber under the action of the peristaltic pump, and the reaction solution continued to circulate under the action of the peristaltic pump. The continuous flow photoreaction device was placed under green light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was photocatalytically reacted. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was 30°C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 h. Cathode chamber: 40 mM p-nitrophenol. x @NF and silver / silver chloride were the working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction was 600 r / min, the reaction temperature was 30°C, and the voltage was 0.8 V. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 90 mM, and the yield was 90%.

[0030] Example 9 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (0.25 M Na2CO3 and 0.25 M NaHCO3, pH 10) and 16 mL of catholyte (1 M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane. 300 mM furfural, 20 mol% ACT and 1 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 10 with Na2CO3 powder. It was then connected to the O2 pipeline and a transparent polyfluorinated ethylene propylene plastic spiral tube (optical path length of 235 cm, inner diameter of 2 mm, outer diameter of 2.4 mm) continuous flow photoreaction device through a plastic pipeline and with the help of a peristaltic pump, and then the reaction solution mixed with O2 was pumped back to the anode chamber under the action of the peristaltic pump, and the reaction solution continued to circulate under the action of the peristaltic pump. The continuous flow photoreaction device was placed under green light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was photocatalytically reacted. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was 35°C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 h. Cathode chamber: 40 mM p-nitrophenol. x @NF and silver / silver chloride were the working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction was 600 r / min, the reaction temperature was 35 ℃, and the voltage was 0.8 V. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 255 mM, and the yield was 85%.

[0031] Example 10 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (0.25 M Na2CO3 and 0.25 M NaHCO3, pH 10) and 16 mL of catholyte (1 M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT and 1 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 10 with Na2CO3 powder. It was then connected to the O2 pipeline and a transparent polyfluorinated ethylene propylene plastic spiral tube (optical path length of 235 cm, inner diameter of 2 mm, outer diameter of 2.4 mm) continuous flow photoreaction device through a plastic pipeline and with the help of a peristaltic pump, and then the reaction solution mixed with O2 was pumped back to the anode chamber under the action of the peristaltic pump, and the reaction solution continued to circulate under the action of the peristaltic pump. The continuous flow photoreaction device was placed under solar irradiation to conduct photocatalytic reaction on the reaction solution mixed with O2. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was about 35 °C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 h. Cathode chamber: 40 mM p-nitrophenol. x @NF and silver / silver chloride were the working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction was 600 r / min, the reaction temperature was 35 ℃, and the voltage was 0.8 V. After 2 h of reaction, the concentration of maleic acid in the anode chamber was 90 mM, and the yield was 90%.

[0032] Comparative Example 1 The photo / electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL anolyte (0.25 M Na2CO3 and 0.25 M NaHCO3, pH 10) and 16 mL catholyte (1 MKOH), in which the anode chamber and the cathode chamber were separated by a Nafion117 membrane. 100 mM furfural, 20 mol% ACT and 1 mol% TCPP were added to the anode chamber, and after being fully mixed, the H-type glass electrolytic cell was placed under green light (LED lamp, 30 W) irradiation, and a one-pot photo / electrocatalytic reaction of furfural to MA was carried out at 25 °C, 400 r / min and 0.8V. Among them, mesh glassy carbon, Pt silk and Ag / AgCl were used as working electrode, counter electrode and reference electrode, respectively. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 18 mM, and the yield was 18%.

[0033] Comparative Example 2 The photo / electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL anolyte (0.25 M Na2CO3 and 0.25 M NaHCO3, pH 10) and 16 mL catholyte (1 MKOH), in which the anode chamber and the cathode chamber were separated by a Nafion117 membrane. 100 mM furfural, 20 mol% ACT and 1 mol% TCPP were added to the anode chamber, and after thorough mixing, the H-type glass electrolytic cell was placed under green light (LED lamp, 30 W) irradiation, and a one-pot photo / electrocatalytic reaction of furfural to MA was carried out under the conditions of oxygen flow (13 mL / min), 25 °C, 400 r / min and 0.8 V. Among them, mesh glassy carbon, Pt silk and Ag / AgCl were used as working electrode, counter electrode and reference electrode, respectively. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 34 mM, and the yield was 34%.

[0034] Comparative Example 3 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (0.25 M Na2CO3 and 0.25 M NaHCO3, pH 10) and 16 mL of catholyte (1 M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT and 1 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 10 with Na2CO3 powder. It was then connected to the O2 pipeline and a transparent polyfluorinated ethylene propylene plastic spiral tube (optical path length of 235 cm, inner diameter of 2 mm, outer diameter of 2.4 mm) continuous flow photoreaction device through a plastic pipeline and with the help of a peristaltic pump, and then the reaction solution mixed with O2 was pumped back to the anode chamber under the action of the peristaltic pump-the reaction solution continued to circulate under the action of the peristaltic pump. The continuous flow photoreaction device was placed under green light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was photocatalytically reacted. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was 30°C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 h. Cathode chamber: 40 mM p-nitrophenol. Among them, mesh glassy carbon, Pt wire and silver / silver chloride were working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction was 600 r / min, the reaction temperature was 30°C, and the voltage was 0.8 V. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 45 mM, and the yield was 45%.

[0035] Comparative Example 4 The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL anolyte (0.25 M Na2CO3 / 0.25 M NaHCO3, pH 10) and 16 mL catholyte (1 M KOH), in which the anode chamber and the cathode chamber were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT and 1 mol% TCPP were added to the anode chamber (O2 was continuously introduced into the anode chamber at a flow rate of 13 mL / min) and mixed thoroughly, and then the pH of the mixed solution was adjusted to 10 with Na2CO3 powder. The electrolytic cell was placed under green light (LED lamp, 30 W) irradiation for the reaction, and the pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 h. Cathode chamber: 40 mM p-nitrophenol. Among them, mesh glassy carbon, NiBx@NF and silver / silver chloride were the working electrode, counter electrode and reference electrode, respectively. The stirring speed at the electrocatalytic reaction site was 600 r / min, the reaction temperature was 25°C, and the voltage was 0.8 V. After 3 h of reaction, the concentration of maleic acid in the anode chamber was 28 mM, and the yield was 28%; after 24 h of reaction, the concentration of maleic acid in the anode chamber was 43 mM, and the yield was 43%.

[0036] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for synthesizing maleic acid, characterized in that: Furfural, an electrocatalyst 4-acetylamino-2,2,6,6-tetramethylpiperidin-1-oxyl free radical, and a photocatalyst 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin are added to a carbonate buffer solution and placed in an anode chamber; p-nitrophenol is added to an aqueous solution of potassium hydroxide and / or sodium hydroxide and placed in a cathode chamber; oxygen is continuously introduced into the reaction solution in the anode chamber, and a photocatalytic reaction and an electrolytic reaction are simultaneously carried out under light conditions to obtain maleic acid in the anode chamber; The photocatalytic reaction is to pump the reaction liquid in the anode chamber into a transparent plastic spiral tube after oxygen is introduced, and the transparent plastic spiral tube is placed under light, and the liquid after the reaction in the spiral tube flows back to the anode chamber to achieve a continuous cycle photocatalytic reaction.

2. The method according to claim 1, characterized in that The concentration of ACT is 5-60 mol %; the concentration of TCPP is 0.5-5 mol %.

3. The method according to claim 2, characterized in that The concentration of ACT is 20±10 mol; the concentration of TCPP is 1-2 mol; the concentration of furfural is 100-300 mM; and the concentration of p-nitrophenol is 10-40 mM.

4. The method according to claim 3, characterized in that The conditions of the electrolysis reaction are as follows: initial reaction liquid temperature 20-40°C, rotation speed 600±200r / min, reaction time 3±1h, oxygen flow rate 13±5 mL / min.

5. The method according to claim 4, characterized in that The working electrode, reference electrode and counter electrode are respectively a mesh glassy carbon electrode, a silver / silver chloride electrode and a foamed nickel electrode; the applied voltage is 0.7-0.9 V; the cathode chamber and the anode chamber are separated by a Nafion117 membrane.

6. The method according to claim 5, characterized in that The concentration of the potassium hydroxide and / or sodium hydroxide aqueous solution is 1±0.5M; the concentration of the carbonate buffer solution is 0.25-1M, and its pH is 8.5-10; carbonate needs to be added during the reaction to keep the pH of the system unchanged.

7. The method according to claim 6, characterized in that The carbonate buffer is a solution of Na2CO3 and / or NaHCO3.

8. The method according to any one of claims 1 to 7, characterized in that: The conditions of the photocatalytic reaction are as follows: the initial reaction liquid temperature is 20-40°C, and oxygen is continuously introduced; the reaction liquid and oxygen flow rates are 11.5±3.5 mL / min and 13±5 mL / min, respectively; and the reaction time is 3±1 h.

9. The method according to claim 8, characterized in that The transparent plastic spiral tube has a tube length of 175-235 cm, an inner diameter of 2±0.5 mm, and an outer diameter of 2.4±0.5 mm.

10. The method according to claim 9, characterized in that The light sources of the illumination are green LED lamp, blue LED lamp, purple LED lamp, red LED lamp, white LED lamp and sunlight, and the power of the LED lamp is 30±20W.

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