A kind of synthetic method of maleic acid

Abstract

The invention belongs to the field of catalysis and bio-based chemicals, discloses a synthetic method for maleic acid, it is characterised in that furfural, electrocatalyst 4 acetylamino 2,2,6,6 tetramethylpiperidine 1 oxygen radical, photocatalyst 5,10,15,20 tetra (4 carboxylphenyl) porphyrin are added to carbonate buffer and placed in the anolyte compartment;p-nitrophenol is added to potassium hydroxide and / or sodium hydroxide aqueous solution and placed in the cathode compartment;Oxygen is continuously passed into the reaction solution of the anolyte compartment, photocatalytic reaction and electrolytic reaction are carried out synchronously under illumination conditions, and maleic acid is obtained in the anolyte compartment. The present invention overcomes the defects such as harsh chemical catalytic reaction conditions, high energy consumption, and has the advantages of mild reaction conditions, environmental friendliness, high synthesis efficiency, excellent selectivity and easy amplification.

Description

Technical Field

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

[0002] Furfural is an important biobased platform compound, typically produced by hydrolysis and dehydration of hemicellulose-rich agricultural and forestry byproducts. Furfural molecules contain highly reactive furan rings and aldehyde groups, which can be synthesized into a range 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 widely used in the food, chemical, pharmaceutical, and polymer industries. For example, maleic acid is a key raw material or chemical intermediate in the manufacture of unsaturated polyester resins, vinyl copolymers, surface coatings, printing and dyeing auxiliaries, preservatives, plasticizers, food additives, and pharmaceuticals. It is also used in the synthesis of products such as fumaric acid, tartaric acid, and succinic acid.

[0003] Currently, maleic acid is mainly obtained from petroleum-based raw materials such as benzene or butane through harsh and environmentally unfriendly chemical catalytic oxidation. From the perspective of green and sustainable chemistry, the synthesis of bio-based maleic acid using renewable furfural as a raw material is more attractive. 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 (Mo4VO4) to synthesize maleic acid. 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 an intermediate (5-hydroxy-2(5 HThe isolation and purification of )-furanone complicates the catalytic process. In summary, existing synthesis methods suffer from harsh reaction conditions (high temperature and pressure, the use of organic solvents, strong acids and bases, etc.), the use of metal catalysts, low conversion rates, and poor product selectivity. Furthermore, biocatalytic pathways remain incapable of directly synthesizing maleic acid. Summary of the Invention

[0004] In view of the problems existing in the prior art, the purpose 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:

[0006] A method for synthesizing maleic acid comprises 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) to a carbonate buffer solution and placing the solution in an anode chamber; adding p-nitrophenol to an aqueous solution of potassium hydroxide and / or sodium hydroxide and placing the solution in a cathode chamber; continuously introducing oxygen into the reaction solution in the anode chamber, and simultaneously carrying out a photocatalytic reaction and an electrolytic reaction under light conditions to produce maleic acid in the anode chamber.

[0007] The photocatalytic reaction is to pump the reaction liquid in the anode chamber into a transparent plastic spiral tube after introducing oxygen, and the transparent plastic spiral tube is placed under light. The liquid after the reaction in the spiral tube flows back to the anode chamber to achieve a continuous cycle photocatalytic reaction.

[0008] Preferably, the concentration of ACT is 5-60 mol%; and the concentration of TCPP is 0.5-5 mol%.

[0009] 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.

[0010] Preferably, the conditions of the electrolysis reaction are: initial reaction liquid temperature of 20-40° C., rotation speed of 600±200 r / min, reaction time of 3±1 h, and oxygen flow rate of 13±5 mL / min.

[0011] Preferably, the working electrode, reference electrode and counter electrode are a mesh glassy carbon electrode, a silver / silver chloride electrode and a nickel foam electrode, respectively; the applied voltage is 0.7-0.9 V; and the cathode chamber and the anode chamber are separated by a Nafion 117 membrane.

[0012] 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.

[0013] Preferably, the carbonate buffer is a solution of Na2CO3 and / or NaHCO3.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] Compared with the existing technology, the present invention has the following advantages:

[0018] (1) The present invention uses clean energy (electricity and light) to achieve photo / electricity simultaneous catalytic oxidation under mild conditions, overcoming the defects of chemical catalytic reactions such as harsh conditions and high energy consumption. The use of continuous cycle photocatalytic reaction can not only shorten the reaction time, but also make it difficult for the active furfural to undergo side reactions during the reaction, thereby greatly improving the yield of maleic acid.

[0019] (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.

[0020] (3) The reaction process of the present invention is simple, easy to control, and easy to scale up, with high synthesis efficiency and excellent selectivity. It not only has high product quality, but also helps to simplify the subsequent separation and purification of the target product. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the reaction apparatus for batch electrocatalytic and continuous flow photocatalytic furfural oxidation.

[0022] Figure 2 Furfural, furoic acid, 5-hydroxy-2(5 H )-furanone and maleic acid structural formula.

[0023] Figure 3This is the HPLC chromatogram for quantitative analysis of maleic acid in Example 4. DETAILED DESCRIPTION

[0024] The present invention will be further described in detail below with reference to examples, but the embodiments of the present invention are not limited thereto.

[0025] Example 1

[0026] Nickel foam electrode (NiB x @NF) was prepared according to the literature (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 sheets, cleaned in acetone and then anhydrous ethanol in an ultrasonic cleaner for 30 minutes, and then air-dried at room temperature for later use. Subsequently, 25 mL of a 125 mM NiCl₂ aqueous solution was prepared and placed in an ice-water bath. Ethylenediamine was slowly added with stirring to a final concentration of 750 mM. 25 mL of a 55 mM NaBH₄ solution was prepared in a 0.2 M NaOH aqueous solution. The two solutions were mixed thoroughly, the pH was adjusted to 13.5 with 10 M NaOH, and the volume was brought to 100 mL with distilled water. Finally, the cleaned 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 standby use to obtain NiB x @NF electrode.

[0027] Example 2

[0028] 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). The anolyte and catholyte compartments were separated by a Nafion 117 membrane (DuPont, USA). 100 mM furfural, 5 mol% ACT, and 1 mol% TCPP were added to the anolyte compartment (O₂ was continuously bubbled into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 10 with Na₂CO₃ powder. The mixture was then connected to an O₂ line and a transparent poly(fluorinated ethylene propylene) spiral tubing (optical path length 175 cm, inner diameter 2 mm, outer diameter 2.4 mm) continuous flow photoreactor via plastic tubing connected by a peristaltic pump. The O₂-mixed reaction solution was then pumped back to the anolyte compartment by the peristaltic pump, where it continuously circulated. The continuous flow photoreactor 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 hour. The cathode chamber: 10 mM p-nitrophenol. Among them, the reticulated glassy carbon (10 × 10 × 5 mm, 100 PPI, purchased from ERG Aerospace), NiB x @NF and silver / silver chloride served as the working electrode, counter electrode, and reference electrode, respectively. The electrocatalytic reaction was stirred at 600 r / min, the reaction temperature was 25°C, and the voltage was 0.7 V. After 3 hours of reaction, the maleic acid concentration in the anode chamber reached 50 mM, and the yield was 50%.

[0029] Example 3

[0030] 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). The anolyte and catholyte compartments were separated by a Nafion 117 membrane. 100 mM furfural, 60 mol% ACT, and 1 mol% TCPP were added to the anolyte compartment (O₂ was continuously bubbled into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 10 with Na₂CO₃ powder. The mixture was then connected to an O₂ line and a transparent poly(fluorinated ethylene propylene) plastic spiral tube (optical path length 200 cm, inner diameter 2 mm, outer diameter 2.4 mm) continuous flow photoreactor via plastic tubing via a peristaltic pump. The O₂-mixed reaction solution was then pumped back to the anolyte compartment by the peristaltic pump, where it continuously circulated. The continuous flow photoreaction device was placed under violet light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was subjected to photocatalytic reaction. 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. The cathode chamber: 10 mM p-nitrophenol. x @NF and silver / silver chloride served as the working electrode, counter electrode, and reference electrode, respectively. The electrocatalytic reaction was stirred at 600 r / min, the reaction temperature was 25°C, and the voltage was 0.9 V. After 3 hours of reaction, the maleic acid concentration in the anode chamber reached 72 mM, with a yield of 72%.

[0031] Example 4

[0032] 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). The anolyte and catholyte compartments were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT, and 1 mol% TCPP were added to the anolyte compartment (O₂ was continuously bubbled into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 10 with Na₂CO₃ powder. The mixture was then connected to an O₂ line and a transparent poly(fluorinated ethylene propylene) plastic spiral tube (optical path length 235 cm, inner diameter 2 mm, outer diameter 2.4 mm) continuous flow photoreactor via plastic tubing via a peristaltic pump. The O₂-mixed reaction solution was then pumped back to the anolyte compartment by the peristaltic pump, where it continuously circulated. The continuous flow photoreaction device was placed under green light (LED lamp, 50 W) irradiation, and the reaction solution mixed with O2 was subjected to photocatalytic reaction. 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. The cathode chamber: 10 mM p-nitrophenol. x @NF and silver / silver chloride served as the working electrode, counter electrode, and reference electrode, respectively. The electrocatalytic reaction was stirred at 600 r / min, the reaction temperature was 25°C, and the voltage was 0.8 V. After 3 hours of reaction, the maleic acid concentration in the anode compartment reached 78 mM, with a yield of 78%.

[0033] Example 5

[0034] 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). The anolyte and catholyte compartments were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT, and 2 mol% TCPP were added to the anolyte compartment (O₂ was continuously bubbled into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 10 with Na₂CO₃ powder. The mixture was then connected to an O₂ line and a transparent poly(fluorinated ethylene propylene) plastic spiral tube (optical path length 235 cm, inner diameter 2 mm, outer diameter 2.4 mm) continuous flow photoreactor via plastic tubing via a peristaltic pump. The O₂-mixed reaction solution was then pumped back to the anolyte compartment by the peristaltic pump, where it continuously circulated. The continuous flow photoreaction device was placed under red light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was subjected to photocatalytic reaction. 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. The cathode chamber: 10 mM p-nitrophenol. x @NF and silver / silver chloride served as the working electrode, counter electrode, and reference electrode, respectively. The electrocatalytic reaction was stirred at 600 r / min, the reaction temperature was 30°C, and the voltage was 0.8 V. After 3 hours of reaction, the maleic acid concentration in the anode chamber reached 72 mM, with a yield of 72%.

[0035] Example 6

[0036] 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). The anolyte and catholyte compartments were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT, and 2 mol% TCPP were added to the anolyte compartment (O₂ was continuously bubbled into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 10 with Na₂CO₃ powder. The mixture was then connected to an O₂ line and a transparent poly(fluorinated ethylene propylene) plastic spiral tube (optical path length 235 cm, inner diameter 2 mm, outer diameter 2.4 mm) continuous flow photoreactor via plastic tubing via a peristaltic pump. The O₂-mixed reaction solution was then pumped back to the anolyte compartment by the peristaltic pump, where it continuously circulated. The continuous flow photoreaction device was placed under green light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was subjected to photocatalytic reaction. 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. The cathode chamber: 40 mM p-nitrophenol. x @NF and silver / silver chloride served as the working electrode, counter electrode, and reference electrode, respectively. The electrocatalytic reaction was stirred at 600 r / min, the reaction temperature was 25°C, and the voltage was 0.8 V. After 3 hours of reaction, the maleic acid concentration in the anode chamber reached 80 mM, with a yield of 80%.

[0037] Example 7

[0038] 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). The anolyte and catholyte compartments were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT, and 1 mol% TCPP were added to the anolyte compartment (O₂ was continuously purged into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 8.5 with Na₂CO₃ powder. The mixture was then connected to an O₂ line and a transparent poly(fluorinated ethylene propylene) plastic spiral tube (optical path length 235 cm, inner diameter 2 mm, outer diameter 2.4 mm) continuous flow photoreactor via plastic tubing via a peristaltic pump. The O₂-mixed reaction solution was then pumped back to the anolyte compartment by the peristaltic pump, where it continuously circulated. The continuous flow photoreaction device was placed under green light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was subjected to photocatalytic reaction. 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. The cathode chamber: 20 mM p-nitrophenol. x @NF and silver / silver chloride served as the working electrode, counter electrode, and reference electrode, respectively. The electrocatalytic reaction was stirred at 600 r / min, the reaction temperature was 25°C, and the voltage was 0.8 V. After 3 hours of reaction, the maleic acid concentration in the anode compartment reached 67 mM, with a yield of 67%.

[0039] Example 8

[0040] 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). The anolyte and catholyte compartments were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT, and 1 mol% TCPP were added to the anolyte compartment (O₂ was continuously bubbled into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 10 with Na₂CO₃ powder. The mixture was then connected to an O₂ line and a transparent poly(fluorinated ethylene propylene) plastic spiral tube (optical path length 235 cm, inner diameter 2 mm, outer diameter 2.4 mm) continuous flow photoreactor via plastic tubing via a peristaltic pump. The O₂-mixed reaction solution was then pumped back to the anolyte compartment by the peristaltic pump, where it continuously circulated. 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. The cathode chamber: 40 mM p-nitrophenol. x @NF and silver / silver chloride served as the working electrode, counter electrode, and reference electrode, respectively. The electrocatalytic reaction was stirred at 600 r / min, the reaction temperature was 30°C, and the voltage was 0.8 V. After 3 hours of reaction, the maleic acid concentration in the anode chamber reached 90 mM, with a yield of 90%.

[0041] Example 9

[0042] 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). The anolyte and catholyte compartments were separated by a Nafion 117 membrane. 300 mM furfural, 20 mol% ACT, and 1 mol% TCPP were added to the anolyte compartment (O₂ was continuously bubbled into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 10 with Na₂CO₃ powder. The mixture was then connected to an O₂ line and a transparent poly(fluorinated ethylene propylene) plastic spiral tube (optical path length 235 cm, inner diameter 2 mm, outer diameter 2.4 mm) continuous flow photoreactor via plastic tubing via a peristaltic pump. The O₂-mixed reaction solution was then pumped back to the anolyte compartment by the peristaltic pump, where it continuously circulated. The continuous flow photoreaction device was placed under green light (LED lamp, 30 W) irradiation, and the reaction solution mixed with O2 was subjected to photocatalytic reaction. 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. The cathode chamber: 40 mM p-nitrophenol. x @NF and silver / silver chloride served as the working electrode, counter electrode, and reference electrode, respectively. The electrocatalytic reaction was stirred at 600 r / min, the reaction temperature was 35°C, and the voltage was 0.8 V. After 3 hours of reaction, the maleic acid concentration in the anode chamber reached 255 mM, with a yield of 85%.

[0043] Example 10

[0044] 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). The anolyte and catholyte compartments were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT, and 1 mol% TCPP were added to the anolyte compartment (O₂ was continuously bubbled into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 10 with Na₂CO₃ powder. The mixture was then connected to an O₂ line and a transparent poly(fluorinated ethylene propylene) plastic spiral tube (optical path length 235 cm, inner diameter 2 mm, outer diameter 2.4 mm) continuous flow photoreactor via plastic tubing via a peristaltic pump. The O₂-mixed reaction solution was then pumped back to the anolyte compartment by the peristaltic pump, where it continuously circulated. The continuous flow photoreactor was placed under solar irradiation, and the reaction solution mixed with O2 was subjected to a photocatalytic reaction. The reaction solution flow rate was 11.5 mL / min, the O2 flow rate was 13 mL / min, and the temperature was approximately 35°C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every 1 hour. The cathode chamber contained 40 mM p-nitrophenol. x @NF and silver / silver chloride served as the working electrode, counter electrode, and reference electrode, respectively. The electrocatalytic reaction was stirred at 600 r / min, the reaction temperature was 35°C, and the voltage was 0.8 V. After 2 hours of reaction, the maleic acid concentration in the anode chamber reached 90 mM, with a yield of 90%.

[0045] Comparative Example 1

[0046] The photo / 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). The anodic and cathodic compartments were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT, and 1 mol% TCPP were added to the anodic compartment and thoroughly mixed. The H-type glass electrolytic cell was then irradiated with green light (LED, 30 W). A one-pot photo / electrocatalytic reaction of furfural to MA was carried out at 25 °C, 400 rpm, and 0.8 V. Reticulated glassy carbon, Pt silk, and Ag / AgCl served as the working electrode, counter electrode, and reference electrode, respectively. After 3 h of reaction, the concentration of maleic acid in the anodic compartment reached 18 mM, with a yield of 18%.

[0047] Comparative Example 2

[0048] The photo / 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). The anodic and cathodic compartments were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT, and 1 mol% TCPP were added to the anodic compartment and thoroughly mixed. The H-type glass electrolytic cell was then irradiated with green light (LED, 30 W). A one-pot photo / electrocatalytic reaction of furfural to MA was carried out under the following conditions: oxygen flow (13 mL / min), 25 °C, 400 rpm, and 0.8 V. Reticulated glassy carbon, Pt silk, and Ag / AgCl served as the working electrode, counter electrode, and reference electrode, respectively. After 3 h of reaction, the concentration of maleic acid in the anodic compartment reached 34 mM, with a yield of 34%.

[0049] Comparative Example 3

[0050] 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), with the anolyte and catholyte compartments separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT, and 1 mol% TCPP were added to the anolyte compartment (O₂ was continuously purged into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 10 with Na₂CO₃ powder. The mixture was then connected to an O₂ line and a transparent poly(fluorinated ethylene propylene) plastic spiral tube (optical path length 235 cm, inner diameter 2 mm, outer diameter 2.4 mm) continuous flow photoreactor via plastic tubing and a peristaltic pump. The peristaltic pump then pumped the O₂-mixed reaction solution back to the anolyte compartment, where the reaction solution continuously circulated. The continuous-flow photoreactor was irradiated with green light (LED, 30 W), and a photocatalytic reaction was carried out 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 30°C. The pH of the reaction solution was adjusted to 10 with Na2CO3 powder every hour. The cathode chamber contained 40 mM p-nitrophenol. Reticulated glassy carbon, Pt wire, and silver / silver chloride served as the working electrode, counter electrode, and reference electrode, respectively. The electrocatalytic reaction zone was stirred at 600 r / min, the reaction temperature was 30°C, and the voltage was 0.8 V. After 3 hours of reaction, the maleic acid concentration in the anode chamber was 45 mM, and the yield was 45%.

[0051] Comparative Example 4

[0052] The electrocatalytic reaction was carried out in an H-type glass electrolytic cell consisting of 20 mL of anolyte (0.25 M Na₂CO₃ / 0.25 M NaHCO₃, pH 10) and 16 mL of catholyte (1 M KOH). The anolyte and catholyte compartments were separated by a Nafion 117 membrane. 100 mM furfural, 20 mol% ACT, and 1 mol% TCPP were added to the anolyte compartment (O₂ was continuously bubbled into the anolyte compartment at a flow rate of 13 mL / min) and thoroughly mixed. The pH of the mixture was then adjusted to 10 with Na₂CO₃ powder. The cell was irradiated with green light (LED, 30 W), and the pH of the reaction mixture was adjusted to 10 with Na₂CO₃ powder every 1 h. The cathode compartment contained 40 mM p-nitrophenol. The reticulated glassy carbon, NiBx@NF, and silver / silver chloride served as 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 hours of reaction, the maleic acid concentration in the anode compartment was 28 mM, and the yield was 28%. After 24 hours of reaction, the maleic acid concentration in the anode compartment was 43 mM, and the yield was 43%.

[0053] 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 considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for synthesizing maleic acid, characterized in that: Furfural, an electrocatalyst 4-acetamido-2,2,6,6-tetramethylpiperidin-1-oxyl free radical (ACT), and a photocatalyst 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP) are added to a carbonate buffer solution and placed in an anode chamber; p-nitrophenol is added to an aqueous potassium hydroxide and / or sodium hydroxide solution 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 at an applied voltage of 0.7 to 0.9 V; and maleic acid is produced in the anode chamber. The photocatalytic reaction is to pump the reaction liquid in the anode chamber into a transparent plastic spiral tube after introducing oxygen, and the transparent plastic spiral tube is placed under light. 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 of 20-40° C., rotation speed of 600±200 r / min, reaction time of 3±1 h, and oxygen flow rate of 13±5 mL / min.

5. The method according to claim 4, characterized in that The cathode chamber and the anode chamber are separated by a Nafion 117 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±0.5 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.

7. The method according to claim 6, characterized in that The carbonate buffer solution 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 length of 175 to 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.

Images