Method for selectively oxidizing ethylene glycol into glycollic acid based on TEMPO mediation photoelectrocatalysis

Abstract

The invention discloses a method for selectively oxidizing ethylene glycol into glycollic acid based on photoelectrocatalysis mediated by 2, 2, 6, 6-tetramethylpiperidine oxide (TEMPO), which comprises the following steps: (I) preparation of a photo-anode and (II) photoelectrocatalysis oxidation reaction. According to the method disclosed by the invention, the ethylene glycol is efficiently and highly selectively converted into the glycolic acid with high additional value by taking the PET plastic recycled material ethylene glycol as a raw material through a green and mild photoelectrocatalysis technology under the condition that a noble metal catalyst is not used. A new strategy is provided for green synthesis of glycollic acid, and compared with a traditional chemical oxidation method (harsh reaction conditions), a thermal catalysis method (high energy consumption) and a biomass catalysis method (complex process), the method has the remarkable advantages of being easy and convenient to operate, high in efficiency, environmentally friendly and the like, and has wide application prospects in the fields of fine chemical engineering and renewable resource utilization.

Description

Technical Field

[0001] The present invention relates to the field of photoelectrocatalytic organic synthesis, and particularly to a method for photoelectrocatalytic selective oxidation of ethylene glycol to glycolic acid mediated by 2,2,6,6-tetramethylpiperidine oxide (TEMPO). Background Art

[0002] As a mature and low-cost biomass platform compound, ethylene glycol (EG) has a market price of approximately 4,100 yuan per ton (purity 99.9%). As an important chemical raw material, ethylene glycol can be selectively oxidized into various high-value-added products, such as formic acid, glyoxal, and glycolic acid. Among them, glycolic acid (GA) has significant economic value due to its wide applications in biodegradable materials (such as polyglycolic acid PGA), cosmetics, and fine chemicals. Currently, the market price of industrial-grade crude glycolic acid (purity 70%) is approximately 12,000 yuan per ton, while the price of high-purity products (≥95%) can reach 100,000 - 200,000 yuan per ton, indicating great market potential. Currently, the main methods for industrial preparation of glycolic acid by oxidizing ethylene glycol include chemical oxidation, biomass oxidation, and new catalytic technologies. Chemical oxidation methods usually use strong oxidants such as nitric acid and potassium dichromate, which have problems such as the use of highly toxic chemical reagents, the generation of a large amount of wastewater containing heavy metals or nitrogen oxides, serious environmental pollution, and poor product selectivity. Biological oxidation methods usually rely on microbial fermentation, and the main defects include a long reaction period (usually 48 - 72 hours), cumbersome product separation and purification steps, and harsh strain culture conditions. Although new catalytic technologies such as electrocatalysis, thermocatalysis, and photocatalysis developed in recent years have solved some environmental problems, they still have significant limitations, such as a high dependence on noble metal catalysts such as platinum (Pt) and palladium (Pd). The use of noble metals results in the catalyst cost accounting for more than 60% of the total production cost, and high requirements for reaction condition control (such as precise control of potential in electrocatalysis). Therefore, developing a method for oxidizing ethylene glycol to glycolic acid by photoelectrocatalysis without noble metal catalysts can not only avoid the defects of mainstream preparation methods but also significantly reduce the production cost of glycolic acid. Summary of the Invention

[0003] The object of the present invention is to solve the problem that the current selective oxidation of ethylene glycol to glycolic acid highly depends on noble metals, and to propose a method for selective oxidation of ethylene glycol to glycolic acid based on non-noble metals. Its technical advantages include: using a transition metal-based semiconductor catalyst (such as TiO 2 ), completely avoiding the use of noble metals; achieving efficient oxidation under mild conditions (room temperature and atmospheric pressure) through photoelectrochemical co-catalysis.

[0004] To achieve the above object, the present invention is implemented according to the following technical scheme:

[0005] A method for the selective electro-optical catalysis of glycol oxidation to glycolic acid mediated by TEMPO, comprising the following steps:

[0006] (Ⅰ) Preparation of the photoanode:

[0007] (1) Take one of hydrochloric acid, nitric acid, and sulfuric acid and mix it with water in proportions of 1:1, 2:3, 4:3, etc., and stir evenly to prepare a mixed solution A;

[0008] (2) Take 0.1 - 0.5 milliliters of one of titanium tetrachloride, tetrabutyl titanate, and titanium isopropoxide, add it to the mixed solution A in step (1), and stir for 10 - 40 minutes;

[0009] (3) First, put the commercially purchased and cut FTO film into water and ultrasonically clean it for 10 - 30 minutes, then put it into ethanol and ultrasonically clean it for 10 - 30 minutes, then put it into acetone solvent and ultrasonically clean it for 10 - 30 minutes, and finally dry the cleaned FTO film with high-purity nitrogen or argon;

[0010] (4) Take 20 - 40 milliliters of the mixed solution B from step (3), add it to a 50-milliliter reaction kettle with a stainless steel lining of polytetrafluoroethylene, and then put the cleaned FTO film in step (4) into the above reaction kettle, where the conductive surface of the FTO film faces down and leans against the wall of the reaction kettle at an angle of 30 - 60 degrees;

[0011] (5) Transfer the reaction kettle in step (4) to an electrothermal blast drying oven, control the reaction temperature at 150 - 190 degrees Celsius, and the reaction time at 8 - 12 hours. After the reaction is completed and the temperature of the reaction kettle drops to room temperature, take it out, rinse it 3 - 6 times with deionized water and ethanol, and then put it into an electrothermal blast drying oven at 50 - 80 degrees Celsius and dry it for 1 - 3 hours to obtain a TiO 2 photoanode precursor.

[0012] (6) Calcinate the TiO 2 photoanode precursor prepared in step (5) in a high-temperature furnace at 350 - 550 degrees Celsius for 1 - 5 hours to obtain a TiO 2 photoanode;

[0013] (Ⅱ) Electro-optical catalysis of glycol oxidation to glycolic acid

[0014] Assemble the TiO 2 photoanode prepared in step (Ⅰ) with a reference electrode, a photocathode, and an electrolyte into a catalytic device, and oxidize glycol to glycolic acid under the conditions of light irradiation and an externally applied voltage of an electrochemical workstation.

[0015] In the above technical solution, the electrolytic cell in step (Ⅱ) needs to use quartz material and the anode and cathode are separated by a proton membrane.

[0016] In the above technical solution, the electrolyte solution in step (II) is an aqueous mixed solution of any one of sodium hydroxide, potassium hydroxide, sodium sulfate, sodium nitrate, sodium chloride, potassium chloride and ethylene glycol, TEMPO.

[0017] In the above technical solution, the concentration of the electrolyte in step (II) is 0.1 to 0.5 mol / L, the concentration of ethylene glycol is 0.1 to 1 mol / L, and the concentration of TEMPO is 1 to 20 mmol / L.

[0018] In the above technical solution, the range of the applied voltage in step (II) is 0 to 2 volts, relative to the saturated silver / silver chloride electrode.

[0019] The present invention provides a method for photocatalytic selective oxidation of ethylene glycol to prepare glycolic acid, which combines solar energy and electric energy to convert ethylene glycol with lower economic benefits into glycolic acid with higher economic benefits through a green and mild photocatalytic method. In addition, the present invention does not use precious metals such as platinum and palladium, which can greatly reduce the production cost and provide a new idea for the green and cheap preparation of glycolic acid. Description of the Drawings

[0020] Figure 1 X-ray diffraction pattern of the TiO 2 photoanode prepared by the invention;

[0021] Figure 2 TiO prepared by the invention 2 Scanning electron micrograph of the photoanode;

[0022] Figure 3 High performance liquid chromatography of the results of photocatalytic oxidation of ethylene glycol;

[0023] Figure 4 Faraday efficiency diagram of the results of photocatalytic oxidation of ethylene glycol; Detailed Description of the Invention

[0024] The present invention will be further described below in conjunction with specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but not to limit the present invention.

[0025] Example 1

[0026] (1) Take 9 ml of hydrochloric acid and mix it with water in a ratio of 4:3, and stir evenly to prepare a mixed solution A

[0027] (2) Take 0.25 ml of tetrabutyl titanate, add it to the mixed solution A in step (1) and stir for 10 minutes;

[0028] (3) First, put the cut FTO wafers obtained by commercial purchase into water and ultrasonically clean them for 30 minutes, then put them into ethanol and ultrasonically clean for 30 minutes, then put them into acetone solvent and ultrasonically clean for 30 minutes, and finally dry the cleaned FTO wafers with high-purity nitrogen or argon;

[0029] (4) Take 25 mL of the mixed solution B from step (3) and add it to a 50 mL reaction kettle with a stainless steel lining of polytetrafluoroethylene. Then put the cleaned FTO wafers from step (4) into the above reaction kettle, where the conductive surface of the FTO wafers is placed downward and inclined against the wall of the reaction kettle at an angle of 45 degrees;

[0030] (5) Transfer the reaction kettle in step (4) to an electrothermal blast drying oven, control the reaction temperature at 150 °C, and the reaction time at 12 hours. After the reaction is completed and the temperature of the reaction kettle drops to room temperature, take out the TiO 2 photoanode precursor, rinse it with deionized water and ethanol 3 - 6 times, and then dry it in an electrothermal blast drying oven at 50 - 80 °C for 1 - 3 hours;

[0031] (6) Calcinate the TiO 2 photoanode precursor in a high-temperature furnace at 350 - 550 °C for 1 - 5 hours to obtain the TiO 2 photoanode

[0032] (7) Prepare an electrolyte solution by mixing 120 mL of an aqueous solution of potassium hydroxide and ethylene glycol with a concentration of 1 mol / L.

[0033] (8) Assemble the prepared electrolyte solution, photoanode, cathode, reference electrode, proton membrane, and quartz electrolytic cell into a photoelectrocatalytic device, and add TEMPO with a concentration of 10 mmol / L to the anode.

[0034] (9) Connect the photoelectrocatalytic device to an electrochemical workstation and oxidize it for 2 hours under a light intensity of 100 mW / cm² and an applied voltage of 0 V (relative to the saturated silver chloride reference electrode).

[0035] (10) After the reaction is completed, take 2 mL of the anode electrolyte and detect it using high-performance liquid chromatography. The results are as Figure 3 shown. After calculation, the selectivity of glycolic acid is 79.74%.

[0036] Example 2

[0037] Steps (1) - (8) are the same as in Example 1

[0038] (9) Connect the photoelectrocatalytic device to an electrochemical workstation and oxidize it for 2 hours under a light intensity of 100 mW / cm² and an applied voltage of 0 - 0.4 V (relative to the saturated silver chloride reference electrode).

[0039] (10) After the reaction is completed, 2 mL of the anolyte is taken for detection by high performance liquid chromatography (HPLC). Combining with the charge amount passed through the device during the reaction process by the electrochemical workstation, the Faraday efficiency of glycolic acid at each voltage is calculated as Figure 4 shown below.

[0040] Figure 1 : X-ray diffraction pattern (XRD) of the TiO 2 photoanode. It is proved that the prepared photoanodes are all composed of SnO 2 (from the FTO substrate) and TiO 2 .

[0041] Figure 2 : Scanning electron microscopy image of the TiO 2 photoanode. It shows that the morphology of the prepared TiO 2 photoanode is a nanorod array.

[0042] From Figure 1 and 2 , it can be concluded that the TiO 2 photoanode is successfully prepared.

[0043] Figure 3 : HPLC result chart of ethylene glycol after 2-hour oxidation. The main products are oxalic acid, glycolic acid and formic acid. After calculation, the selectivity of glycolic acid is 79.74%.

[0044] Figure 4 : Faraday efficiency of glycolic acid at different voltages after 2-hour oxidation.

[0045] Based on the above preparation examples, we draw the following further conclusions:

[0046] (1) Under the mediation of TEMPO, the photoelectrocatalytic method with TiO 2 as the photoanode successfully oxidizes ethylene glycol to glycolic acid, and its selectivity is about 79.74%;

[0047] (2) The Faraday efficiency of glycolic acid after 2-hour oxidation at different voltages is slightly different, but all are above 60%.

[0048] The technical solution of the present invention is not limited to the limitations of the above specific embodiments. Any technical deformation made according to the technical solution of the present invention falls within the protection scope of the present invention.

Claims

1. A method for selectively oxidizing ethylene glycol to glycolic acid based on TEMPO-mediated photoelectrocatalysis, characterized in that: The following steps are involved: (I) Photoanode preparation: (1) mixing one of hydrochloric acid, nitric acid and sulfuric acid with water in a ratio of 1:1, 2:3, 4:3, etc., and stirring to form a mixed solution A; (2) taking 0.1-0.5 ml of one of titanium tetrachloride, tetrabutyl titanate, and titanium triisopropoxide, adding it to 20-30 ml of the mixed solution A in step (1) and stirring for 10-40 minutes; (3) The cut commercially purchased conductive glass (FTO) sheet is first placed in water for ultrasonic cleaning for 10 to 30 minutes, then placed in ethanol for ultrasonic cleaning for 10 to 30 minutes, and then placed in acetone solvent for ultrasonic cleaning for 10 to 30 minutes, and finally the cleaned FTO sheet is blown dry with high-purity nitrogen or argon; (4) Take 20-40 ml of the mixed solution B from step (3) and add it to a 50 ml polytetrafluoroethylene reactor with a stainless steel substrate, and then put the FTO wafer cleaned in step (4) into the above reactor, wherein the conductive surface of the FTO wafer is placed downward and tilted against the reactor wall at an angle of 30-60 degrees; (5) The reactor of step (4) is transferred to an electric blast drying oven, and the reaction temperature is controlled at 150 to 190 degrees Celsius for 8 to 12 hours. When the reaction is completed and the temperature of the reactor drops to room temperature, the reactor is taken out, rinsed with deionized water and ethanol for 3 to 6 times, and then placed in an electric blast drying oven at 50 to 80 degrees Celsius for 1 to 3 hours to obtain a TiO2 photoanode precursor. (6) Calcine the TiO2 photoanode precursor prepared in step (5) in a high temperature furnace at 350 to 550 degrees Celsius for 1 to 5 hours to obtain a TiO2 photoanode. (II) Photoelectrocatalytic oxidation of ethylene glycol to glycolic acid The TiO2 photoanode prepared in step (I) is assembled into a catalytic device with a reference electrode, a cathode catalyst, and an electrolyte, and ethylene glycol is oxidized to glycolic acid under the conditions of light irradiation and an applied voltage of an electrochemical workstation.

2. The method of claim 1 for selective photoelectrocatalytic oxidation of ethylene glycol to glycolic acid mediated by TEMPO, characterized in that: The electrolytic cell must be made of quartz material and the anode and cathode must be separated by a proton membrane.

3. The method of claim 1 for selective photoelectrocatalytic oxidation of ethylene glycol to glycolic acid mediated by TEMPO, characterized in that: The cathode catalyst is a high-purity platinum metal sheet, and the reference electrode is a saturated silver / silver chloride electrode.

4. The method of claim 1 for selective photoelectrocatalytic oxidation of ethylene glycol to glycolic acid mediated by TEMPO, characterized in that: The electrolyte in step (II) is a mixed aqueous solution of any one of sodium hydroxide, potassium hydroxide, sodium sulfate, sodium nitrate, sodium chloride, potassium chloride, ethylene glycol, and TEMPO.

5. The method of claim 1 for selective photoelectrocatalytic oxidation of ethylene glycol to glycolic acid mediated by TEMPO, characterized in that: The concentration of the electrolyte is 0.1 to 0.5 mol / L, the concentration of ethylene glycol is 0.1 to 1 mol / L, and the concentration of TEMPO is 1 to 20 mmol / L.

6. The method of claim 1 for selective photoelectrocatalytic oxidation of ethylene glycol to glycolic acid mediated by TEMPO, characterized in that: The applied voltage ranged from 0 to 2 volts relative to a saturated silver / silver chloride electrode.

7. The method of claim 1 for selective photoelectrocatalytic oxidation of ethylene glycol to glycolic acid mediated by TEMPO, characterized in that: The light intensity used for the catalytic process is 100 to 300 milliwatts per square centimeter.

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