A method for photocatalytic in-situ preparation of low-concentration hydrogen peroxide by using a gas-liquid microreactor with a flow field structure

Abstract

The present invention discloses a method for photocatalytic in-situ preparation of low-concentration hydrogen peroxide by using a gas-liquid microreactor with a flow field structure. In a photo-microreactor equipped with a microbubble generator, an acidic aqueous solution containing benzyl alcohol reacts with oxygen, and the photocatalytic reaction process is catalyzed by a g-C3N4 catalyst treated with a mixture of different types of molten salts to in-situ prepare an aqueous solution of low-concentration hydrogen peroxide. Benzyl alcohol is converted into benzaldehyde as a sacrificial agent, and the catalyst can be recycled. In addition, the present invention uses a gas-liquid microgenerator to strengthen the photocatalytic process, which can effectively solve the problems of complicated steps, long reaction time, poor gas-liquid mixing effect, etc. in the traditional reaction process, and can improve the photocatalytic efficiency, and is suitable for the in-situ preparation of an aqueous solution of low-concentration hydrogen peroxide.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical synthesis, and particularly relates to a method for photocatalytic in-situ preparation of low-concentration hydrogen peroxide by using a gas-liquid microreactor with a flow field structure. Background Art

[0002] Currently, the energy crisis is a global problem faced by mankind. With the continuous promotion of economic development in various countries, greenhouse gas emissions have soared, and it has become particularly important to establish and improve a green, low-carbon and circular development system and improve the energy structure. Energy and environment are two important aspects for the sustainable development of human society. In particular, photocatalytic conversion, achieved by using abundant and sustainable biomass and inexhaustible solar energy as the sole energy input, effectively promotes carbon cycle utilization, not only reducing CO2 emissions but also helping to mitigate the energy crisis.

[0003] Photocatalytic technology has the advantages of environmental friendliness, mild reaction conditions, low cost, cleanliness and high efficiency, and is widely used in water splitting, carbon dioxide conversion, degradation of organic pollutants, bacterial disinfection, and selective synthesis of organic compounds. Carbon-based materials, such as graphene, carbon nanotubes and activated carbon, are a widely used type of substrate. Considering the cost of carbon-based materials, biomass-derived carbon materials have attracted increasing attention due to their low cost, easy synthesis and high physicochemical stability. Carbon is abundant in the earth's surface, low in cost, non-toxic and has good biocompatibility. The photocatalytic oxidation performance depends on two major factors: the physicochemical properties of the catalyst (specific surface area, pore structure, active site distribution, etc.) and the photoelectrochemical properties (visible light absorption ability, photogenerated charge separation and transport performance). Researchers expect to prepare photocatalysts with advantages such as high specific surface area, high stability, inhibition of electron-hole recombination, and utilization of visible light through modification of semiconductor materials. Common modification methods include heterostructure construction, atomic doping, morphology control, and noble metal deposition. g-C3N4 is a 2D sheet material developed in recent years and has wide applications in the fields of materials, energy, catalysis, etc. g-C3N4 is a planar 2D layered structure similar to graphene, with better flexibility than 3D porous materials, and the repeating basic unit structure is 3-s-triazine ring.

[0004] As is well known, hydrogen peroxide is an important chemical raw material, but its safe manufacturing, transportation, and storage are important issues that need to be urgently solved. For example, the "7·16" explosion accident in Dalian, the "7·22" explosion accident in Guangdong, and the "4·21" explosion accident in Lianyungang. Currently, hydrogen peroxide on the market is mainly produced by the anthraquinone process, which requires the sequential hydrogenation reaction of alkyl anthraquinone and oxidation in an organic solvent (such as heavy aromatic hydrocarbons) using Ni or Pd as a catalyst. The anthraquinone process involves complex operation units such as hydrogenation, oxidation, separation, concentration, and purification, and belongs to an energy and capital-intensive process for large-scale centralized production of 50wt% hydrogen peroxide solution. However, the alkaline environment during the production process easily causes the decomposition of hydrogen peroxide, leading to explosions, and the risk of explosion is also increased during storage and transportation. Low-concentration hydrogen peroxide solution is mainly prepared by diluting 50wt% hydrogen peroxide solution.

[0005] There are the following problems in the preparation of current low-concentration hydrogen peroxide: 1. The safety during the manufacturing, transportation, and storage of hydrogen peroxide solution is poor, and the process energy consumption is high; 2. There is no reported green and efficient process for the immediate preparation and use of low-concentration hydrogen peroxide solution. In recent years, researchers have developed a variety of photocatalysts and reactors for the preparation of hydrogen peroxide, but most of them have problems such as cumbersome catalyst preparation process, slow photocatalytic process, and poor safety. For example, CN117735483A discloses using tetra-(4-aminophenyl)ethylene and 2,5-dihydroxyterephthalaldehyde as raw materials, dissolving them in dioxane and ultrasonicating, then adding 6mol / L acetic acid to the system and ultrasonicating, sealing and evacuating the system, reacting at 120°C for 3 days, and finally repeatedly washing with ethanol and ethyl acetate and drying overnight in a vacuum drying oven at 80°C to obtain the material TAE-DaOH. This method has problems such as cumbersome catalyst preparation process and large environmental pollution. CN220502741U discloses a tower-type device for promoting photocatalytic reaction, including a reaction tower. The bottom of the reaction tower is provided with an ozone aeration unit, and the inside of the reaction tower is integrally provided with an ultraviolet radiation unit, a photocatalytic unit, and a hydrogen peroxide dosing unit. The top of the reaction tower is provided with a pressure monitoring table, a two-way breathing valve, and a tail gas collection pipe. One side wall of the reaction tower is provided with a water inlet at the lower end and a reflux port at the upper end, and the other side wall of the reaction tower is provided with a water outlet at the upper end. It integrates high-intensity ultraviolet radiation, H2O2 multi-point dosing oxidation field, and micro-nano high-concentration ozone water, and makes the substances such as O3, H2O2, UV, and organic substances undergo multiple combined reactions in a completely uniform mixture of gas, solid, and liquid phases using high-intensity UV photons. This method has problems such as slow photocatalytic reaction process and poor safety. Summary of the Invention

[0006] Objective of the Invention: The technical problem to be solved by the present invention is to provide a method for photocatalytic in-situ preparation of low-concentration hydrogen peroxide by using a gas-liquid microreactor with a flow field structure in view of the deficiencies of the prior art. In a reaction device equipped with a microbubble generator, an acidic aqueous solution containing benzyl alcohol reacts with oxygen, and a g-C3N4 catalyst treated by mixing different types of molten salts is used to catalyze the photocatalytic reaction process to in-situ prepare an aqueous solution of low-concentration hydrogen peroxide. At the same time, benzyl alcohol is converted into benzaldehyde as a sacrificial agent, and valuable conversion can also be achieved through separation.

[0007] To solve the above technical problem, the present invention discloses a method for photocatalytic in-situ preparation of low-concentration hydrogen peroxide by using a gas-liquid microreactor with a flow field structure, and the method comprises the following steps:

[0008] (1) Add a photocatalyst to a mixed solution of benzyl alcohol and water, adjust the pH of the solution to 3-4, and stir under dark conditions. Preferably, stir for 0.5 h to obtain an acidic mixed solution;

[0009] (2) Carry out a photocatalytic reaction on the acidic mixed solution and O2 in a photo-microreactor, collect the effluent, and centrifuge the effluent to separate the aqueous phase and the organic phase to obtain hydrogen peroxide in the aqueous phase. Wherein, the photo-microreactor is a microfluidic chip provided with an internal member of a spiral baffle in the channel, and a microbubble generator is arranged at the O2 inlet of the microfluidic chip.

[0010] Wherein, the photocatalyst is prepared by the following method: Mix and grind a catalyst precursor and a molten salt of the catalyst, put it into a muffle furnace and calcine at 520-550 °C, take it out after cooling, wash to remove salts and then carry out suction filtration. Preferably, wash with hot water multiple times to remove salts and then carry out suction filtration, and then rinse with ethanol and dry for standby. Wherein, the catalyst precursor is any one or more of melamine, urea, thiourea, monocyanamide or dicyandiamide; the molten salt of the catalyst comprises sodium chloride, potassium chloride and a third type of molten salt, and the third type of molten salt is selected from at least one of zinc chloride, magnesium chloride, cobalt chloride, lithium chloride. Preferably, the third type of molten salt is zinc chloride or magnesium chloride; the photocatalyst is separated and washed after the reaction, and after drying, it is reused in step (1). The final calcined product is generally collectively referred to as g-C3N4.

[0011] Preferably, the mass ratio of the catalyst precursor to the molten salt of the catalyst is 1:(0.6 - 1.5). Among them, in the molten salt of the catalyst, the mass ratio of sodium chloride, potassium chloride, and the third type of molten salt is (2 - 5):1:(0.05 - 0.1). An ideal high-temperature molten salt material requires a lower melting point and higher thermal stability. There have been reports on the related applications of binary metal molten salt systems. The addition of the third molten salt should not affect its performance but can increase the production of hydrogen peroxide. In addition, some metals have the effect of decomposing hydrogen peroxide. In order to retain the low melting point and high stability of the molten salt material, when the ratio of sodium chloride, potassium chloride, and the third molten salt is preferably in the range of (2 - 5):1:(0.05 - 0.1), the photocatalytic effect shows a trend of increasing first and then decreasing with the addition amount of the third molten salt. After exceeding the amount, the metal has a certain impact on the decomposition of hydrogen peroxide.

[0012] The pH of the acidic mixed solution is adjusted by an acidic reagent, and the acidic reagent is a 0.1 - 1M HCl solution.

[0013] The volume ratio of water to benzyl alcohol in the acidic mixed solution is 1:(0.1 - 5); the concentration of the catalyst in the acidic mixed solution is 0.5 - 5mg / mL.

[0014] The rate of pumping the acidic mixed solution into the photomicroreactor is 0.05 - 1mL / min; the ratio of the rate of pumping O2 into the photomicroreactor to the rate of pumping the acidic mixed solution into the photomicroreactor is (1 - 10):1. Preferably, it is (5 - 10):1.

[0015] The light source of the photomicroreactor is 20 - 40cm away from the microfluidic chip, so that the center of the light spot irradiates on the microfluidic chip; the inner diameter of the microfluidic chip is 2 - 10mm, and the reaction volume is 1 - 15mL. Preferably, the inner diameter range is 2 - 5mm, and the volume range is 5 - 10mL.

[0016] The light source is a xenon lamp, model XHA350w xenon lamp, and the reaction wavelength is above 400nm; the temperature of the photoreaction is 20 - 30°C; the reaction time is 5 - 180min.

[0017] The microbubble generator consists of a group of circular sieve holes with a diameter of 0.1mm - 0.5mm. In a specific embodiment, the sieve holes can be arranged in a 2 - 4 - 4 - 2 pattern in each row, and the number is 12.

[0018] In order to enable the light spot emitted by the light source to cover the entire fluid channel of the microfluidic chip, in one embodiment, the fluid channel of the microfluidic chip is a bent square tube structure in an M shape. The spiral baffle inner member is arranged in sections inside the square tube structure. The spiral baffle is composed of two built-in baffles. Each baffle is obtained by rotating a rectangular parallelepiped baffle around the fluid channel by a certain angle, and the two baffles form a spiral structure. By forming a spiral structure with the two baffles, the mixing effect of the liquid and the catalyst can be enhanced, the flow pressure of the mixed liquid in the inner tube can be reduced, and the problems of uneven solid-liquid mixing and large fluid flow pressure in the solid-supported reactor can be weakened, greatly shortening the reaction time. Of course, the structural form of the spiral baffle inner member is not limited to the above structure. However, those skilled in the art should be aware that the channel arrangement method, type, etc. of the microfluidic chip are not limited to the above method, as long as the coverage of the light spot can be satisfied and the mixing effect of the liquid and the catalyst can be improved at the same time.

[0019] Preferably, the thickness of the spiral baffle is 2 - 3 mm, and the length of each group of spiral baffles is 20 - 25 mm.

[0020] In a specific embodiment, the fluid channel of the microfluidic chip is a bent square tube structure in an M shape, and a total of 8 groups of spiral baffles are provided inside the fluid channel.

[0021] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0022] (1) The photocatalyst in the reaction system of the present invention is prepared by mixing and calcining a precursor with three molten salts. The preparation process is simple and can be recycled. Compared with bulk-phase carbon nitride, it has a higher specific surface area, a faster electron-hole separation rate, and better photoelectrochemical performance.

[0023] (2) The reaction system of the present invention adopts a photocatalytic system, in which oxygen and water react in a solution containing benzyl alcohol in the presence of a photocatalyst, which is not likely to cause environmental pollution and operation safety hazards, and the subsequent separation operation is simple.

[0024] (3) The device using the combination of photocatalysis and microreactor in the present invention has a small reaction volume and a high reaction rate, and the microreactor is a microbubble generator combined with a flow field structure, which can achieve the purpose of expanding the contact area between the liquid and the catalyst and enhancing the mixing effect of the liquid and the catalyst.

[0025] (4) In the method provided by the present invention, the yield of hydrogen peroxide is as high as 125 mM / h. After cyclic experiments, the concentration of hydrogen peroxide can reach 998 mM, corresponding to a mass fraction of 3% concentration, and the yield of benzaldehyde is 54%. The photocatalytic process in the system of the method is simple and environmentally friendly, and the reaction conditions of the method are mild and safe.

[0026] Generally speaking, the present invention uses a gas-liquid microgenerator to enhance the photocatalytic process, which can effectively solve the problems of complex steps, long reaction time, poor gas-liquid mixing effect, etc. in the traditional reaction process, and can improve the photocatalytic efficiency, being suitable for on-site preparation of low-concentration hydrogen peroxide aqueous solution. Description of the Drawings

[0027] The following further specific description of the present invention will be made in conjunction with the drawings and specific embodiments, and the above and / or other advantages of the present invention will become clearer.

[0028] Figure 1 Schematic structural diagram of the microreaction integration device used in the present invention;

[0029] Figure 2 Schematic structural diagram of the optical microreactor, wherein, (a) is the top view of the spiral baffle inner member; Figure 2 (b) is the structural diagram of the spiral baffle inner member; (c) is the physical diagram of the optical microreactor;

[0030] Figures 3-6 IR, EIS, I-T, and M-S diagrams of the catalyst in Example 3;

[0031] Figure 7 CFD (Computational Fluid Dynamics) simulation gas-liquid mixing diagram in Example 6;

[0032] Figure 8 True gas-liquid mixing diagram in the camera in Example 6;

[0033] Figure 9 Standard curve diagram of the product hydrogen peroxide;

[0034] Figure 10 UV test diagram of the product hydrogen peroxide diluted 20 times in Example 10;

[0035] Figure 11 Standard curve diagram of the product benzaldehyde;

[0036] Figure 12 Liquid chromatography diagram of the product benzaldehyde in Example 10. Specific Embodiments

[0037] In the following embodiments, the experimental methods are all conventional methods unless otherwise specified; the reagents and materials can be obtained from commercial channels unless otherwise specified.

[0038] Analysis method:

[0039] Determination of H2O2 concentration: 30% hydrogen peroxide was selected to prepare a 1 mmol / L H2O2 aqueous solution as the stock solution, which was diluted into 6 gradients of 0.2, 0.3, 0.4, 0.6, 0.7, and 0.8 mmol / L respectively. 0.1 mL of 5% titanium sulfate solution and 0.2 mL of concentrated ammonia water were added to the samples respectively. After shaking well, centrifugation was carried out to obtain the precipitate. The precipitate was dissolved in 5 mM sulfuric acid, and the absorbance was measured at 412 nm. Based on these data, a standard curve of absorbance vs. H2O2 concentration was established, and the linear equation was obtained: y = 0.1083x + 0.0151, with a correlation coefficient of 0.9972. The standard curve is as Figure 9 shown.

[0040] The analytical method for benzaldehyde and in the oxidation products of benzyl alcohol was high performance liquid chromatography (HPLC), and the external standard method was used for content determination. Model of HPLC: Agilent; Chromatographic column: XDB-C18; Detector: DAD; The maximum absorption wavelength of benzaldehyde was 254 nm, the injection volume was 5 μL, the mobile phase was acetonitrile and water, the flow rate was 1 mL / min, and the column oven temperature was 25 °C. The specific operation method for quantitative analysis of benzaldehyde was: 0.2 ml of the separated solution was taken into a centrifuge tube, diluted to 1 ml with water, filtered through an organic membrane, and then subjected to high performance liquid chromatography analysis. The standard curve of benzaldehyde is as Figure 11 shown.

[0041] As Figure 1 shown, the present application provides a method for photocatalytic in-situ preparation of low-concentration hydrogen peroxide using a gas-liquid microreactor with a flow field structure, which is characterized in that the method comprises the following steps:

[0042] (1) Add a photocatalyst to the mixed solution of benzyl alcohol and water, adjust the pH of the solution to 3-4, and stir under dark conditions to obtain an acidic mixed solution;

[0043] (2) Carry out a photoreaction on the acidic mixed solution and O2 in a photo-microreactor, collect the effluent, and centrifuge the effluent to separate the water phase and the organic phase to obtain hydrogen peroxide in the water phase.

[0044] Among them, the photo-microreactor is a microfluidic chip with a spiral baffle internal component arranged in the channel, and a microbubble generator is arranged at the O2 inlet of the microfluidic chip.

[0045] The structure of the photo-microreactor used in the embodiment of the present invention is as Figure 1 and 2As shown, in order to enable the light spot of the light source to cover the entire optical microreactor, the fluid channel of the microfluidic chip can be a bent square tube structure in an "M" shape. A total of 8 sets of spiral baffle internal components 1 are provided in the fluid channel. Each set of spiral baffle internal components is composed of two built-in baffles (1-1 and 1-2). Each baffle is obtained by rotating a rectangular parallelepiped baffle around the fluid channel by a certain angle. The two baffles form a spiral structure, forming an internal rotation flow field structure. In this embodiment, it is obtained by rotating a rectangular parallelepiped baffle around the fluid channel by 180°. The thickness of the spiral baffle (i.e., the thickness of the rectangular parallelepiped baffle) is 2-3 mm, and the length of each set of spiral baffles is 25 mm. Among them, Figure 2 (a) is a top view of the spiral baffle internal component, Figure 2 (b) is a structural schematic diagram of the spiral baffle internal component. Figure 2 (c) is a physical diagram of the optical microreactor.

[0046] The microbubble generator 3 is arranged at the oxygen inlet for generating microbubbles of uniform size, which is composed of a group of circular sieve holes (at the bubble sieve holes in the figure), with a diameter of 0.2 mm. The sieve holes can be arranged in 2-4-4-2 per row, and the number is 12.

[0047] Example 1: Preparation of photocatalyst TCN-Na / K / Mg.

[0048] Weigh 10 g of thiourea, 10 g of sodium chloride, 5 g of potassium chloride, and 100 mg of magnesium chloride, mix and grind them, put them into a muffle furnace at 550 °C for calcination for 4 hours, take them out after cooling, put them into a flask, heat, stir, and let stand for 24 h. Wash with hot water 3 times to remove salts, then perform suction filtration, and then rinse with ethanol 3 times. Dry overnight at 80 °C in a vacuum drying oven. The yield is about 0.3 g, and store it after drying.

[0049] Example 2: Preparation of photocatalyst TCN-Na / K / Co.

[0050] Weigh 10 g of thiourea, 10 g of sodium chloride, 5 g of potassium chloride, and 100 mg of cobalt chloride, mix and grind them, put them into a muffle furnace at 550 °C for calcination for 4 hours, take them out after cooling, put them into a flask, heat, stir, and let stand for 24 h. Wash with hot water 3 times to remove salts, then perform suction filtration, and then rinse with ethanol 3 times. Dry overnight at 80 °C in a vacuum drying oven. The yield is about 0.1 g, and store it after drying.

[0051] Example 3: Preparation of photocatalyst TCN-Na / K / Zn.

[0052] Weigh 10 g of thiourea, 10 g of sodium chloride, 5 g of potassium chloride, and 100 mg of zinc chloride, mix and grind them, put them into a muffle furnace and calcine at 550 °C for 4 hours. After cooling, take them out, put them into a flask, heat, stir, and let stand for 24 h. Wash with hot water 3 times to remove salts, then perform suction filtration. Rinse with ethanol 3 times and dry overnight at 80 °C in a vacuum drying oven. The yield is about 0.2 g, and store it after drying.

[0053] Figures 3-6 The IR, EIS, I-T, and M-S diagrams of the catalyst prepared in Example 3. Compared with the TCN bulk catalyst, Figure 3 in which, the absorption peak at 906 cm -1 corresponds to the N-H bending vibration, and the absorption peak at 1127 cm -1 corresponds to the generation of nitrogen defects, indicating that the basic composition of the molten salt catalyst has changed to a certain extent, a large number of nitrogen vacancies have been generated, and there is coordination between the metal and N; Figure 4 It can be seen that the impedance value of the catalyst prepared by the molten salt method becomes smaller; Figure 5 in which, the photocurrent value of the catalyst prepared by the molten salt method is significantly larger, and the electron transfer and separation rate becomes faster; Figure 6 in which the flat band potential of the catalyst prepared by the molten salt method is -1.55 V (TCN corresponds to -1.45 V), indicating that the reduction ability of the catalyst is enhanced.

[0054] Example 4: The specific method for the photocatalytic preparation of hydrogen peroxide using TCN-Na / K / Mg prepared in Example 1.

[0055] Weigh 12.5 mg of the photocatalyst prepared in Example 1, place it in a centrifuge tube, add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol, and 0.2 mL of 0.1 mM HCl to the centrifuge tube, with pH = 3 - 4. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take this mixed solution as one phase and O2 as another phase in a photo-microreactor (inner diameter 5 mm), set the flow rate ratio to 5:1, the liquid flow rate to 0.1 mL / min, and the gas flow rate to 0.5 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the photo-microreactor for 1 h of photoreaction. Collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and test the yields of hydrogen peroxide and benzaldehyde respectively. Finally, it is measured that: the concentration of hydrogen peroxide is 97.2 mM, the corresponding mass fraction is 0.29%, and the yield of benzaldehyde is 4.0%.

[0056] Example 5: The specific method for the photocatalytic preparation of hydrogen peroxide using TCN-Na / K / Co prepared in Example 2.

[0057] Weigh 12.5 mg of the photocatalyst prepared in Example 3, place it in a centrifuge tube, add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol and 0.2 mL of 0.1 mM HCl to the centrifuge tube, with pH = 3 - 4. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take the mixed solution as one phase and O2 as another phase in a photo-microreactor (inner diameter 5 mm), set the flow rate ratio to 5:1, the liquid flow rate to 0.1 mL / min, and the gas flow rate to 0.5 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the photo-microreactor for 1 h of photoreaction. Collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and perform tests on the yields of hydrogen peroxide and benzaldehyde respectively. Finally, it is measured that the concentration of hydrogen peroxide is 64.4 mM, corresponding to a mass fraction of 0.19%, and the yield of benzaldehyde is 2.6%.

[0058] Example 6: The specific method for using the TCN-Na / K / Zn prepared in Example 3 for photocatalytic production of hydrogen peroxide.

[0059] Weigh 12.5 mg of the photocatalyst prepared in Example 3, place it in a centrifuge tube, add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol and 0.2 mL of 0.1 mM HCl to the centrifuge tube, with pH = 3 - 4. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take the mixed solution as one phase and O2 as another phase in a photo-microreactor 1 (inner diameter 5 mm), set the flow rate ratio to 5:1, the liquid flow rate to 0.1 mL / min, and the gas flow rate to 0.5 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the photo-microreactor for 1 h of photoreaction. Collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and perform tests on the yields of hydrogen peroxide and benzaldehyde respectively. Finally, it is measured that the concentration of hydrogen peroxide is 125 mM, corresponding to a mass fraction of 0.38%, and the yield of benzaldehyde is 5.7%.

[0060] Among them, Figure 7 is the CFD (Computational Fluid Dynamics) simulation gas-liquid mixing diagram in Example 6, Figure 8 is the real gas-liquid mixing diagram in the camera in Example 6.

[0061] Example 7: The specific method for using the TCN-Na / K / Zn prepared in Example 3 for photocatalytic production of hydrogen peroxide.

[0062] Weigh 12.5 mg of the photocatalyst prepared in Example 3, place it in a centrifuge tube, and add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol and 0.2 mL of 0.1 mM HCl into the centrifuge tube. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take the mixed solution as one phase and O2 as another phase in the photo-microreactor 1 (inner diameter 5 mm), set the flow rate ratio to 8:1, the liquid flow rate to 0.1 mL / min, and the gas flow rate to 0.8 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the photo-microreactor and carry out the photoreaction for 1 h. Collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and test the concentrations of hydrogen peroxide and benzaldehyde respectively. Finally, it is measured that the hydrogen peroxide concentration is 114 mM, corresponding to a mass fraction of 0.35%, and the benzaldehyde yield is 5.3%.

[0063] Example 8: The specific method for using the TCN-Na / K / Zn prepared in Example 3 for photocatalytic production of hydrogen peroxide (the problem is the same as that in Example 4).

[0064] Weigh 12.5 mg of the photocatalyst prepared in Example 3, place it in a centrifuge tube, and add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol and 0.2 mL of 0.1 mM HCl into the centrifuge tube. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take the mixed solution as one phase and O2 as another phase in the photo-microreactor 1 (inner diameter 5 mm), set the flow rate ratio to 10:1, the liquid flow rate to 0.1 mL / min, and the gas flow rate to 1 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the photo-microreactor and carry out the photoreaction for 1 h. Collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and test the concentrations of hydrogen peroxide and benzaldehyde respectively. Finally, it is measured that the hydrogen peroxide concentration is 105 mM, corresponding to a mass fraction of 0.32%, and the benzaldehyde yield is 4.4%.

[0065] Example 9: The specific method for using the TCN-Na / K / Zn prepared in Example 3 for photocatalytic production of hydrogen peroxide.

[0066] Weigh 12.5 mg of the photocatalyst prepared in Example 3, place it in a centrifuge tube, add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol and 0.2 mL of 0.1 mM HCl into the centrifuge tube. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take the mixed solution as one phase and O2 as another phase in a photo-microreactor 2 (inner diameter: 10 mm). Set the flow rate ratio to 5:1, the liquid flow rate to 0.2 mL / min, and the gas flow rate to 1 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the photo-microreactor for 1 h of photoreaction. Collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and test the concentrations of hydrogen peroxide and benzaldehyde respectively. Finally, it is measured that the concentration of hydrogen peroxide is 92.0 mM, the corresponding mass fraction is 0.28%, and the benzaldehyde yield is 3.3%.

[0067] Example 10: Specific method for using the TCN-Na / K / Zn prepared in Example 3 for photocatalytic production of hydrogen peroxide

[0068] Weigh 12.5 mg of the photocatalyst prepared in Example 3, place it in a centrifuge tube, add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol and 0.2 mL of 0.1 mM HCl into the centrifuge tube. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take the mixed solution as one phase and O2 as another phase in a photo-microreactor 1 (inner diameter: 5 mm). Set the flow rate ratio to 5:1, the liquid flow rate to 0.1 mL / min, and the gas flow rate to 0.5 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the photo-microreactor for 8 cycles of photoreaction (that is, the effluent from the photo-microreactor is continuously flowed using a peristaltic pump to extend the residence time in the reactor, and the catalyst continuously circulates inside). Collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and test the concentrations of hydrogen peroxide and benzaldehyde respectively. Finally, it is measured that the concentration of hydrogen peroxide is 998 mM, the corresponding mass fraction is 3.0%, and the benzaldehyde yield is 54.3%.

[0069] Figure 10 It is the UV test chart of the product hydrogen peroxide diluted 20 times in Example 10; Figure 12 It is the liquid chromatogram of the product benzaldehyde in Example 10.

[0070] Comparative Example 1

[0071] Weigh 12.5 mg of the photocatalyst TCN-Na / K (for the catalyst preparation process, refer to Example 3 and delete zinc chloride), place it in a centrifuge tube, and add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol, and 0.2 mL of 0.1 mM HCl to the centrifuge tube, with pH = 3 - 4. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take the mixed solution as one phase and O2 as another phase in the photo-microreactor 1 (inner diameter is 5 mm), set the flow rate ratio to 5:1, the liquid flow rate to 0.1 mL / min, and the gas flow rate to 0.5 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the photo-microreactor for 1 h of photoreaction. Collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and respectively conduct the yield tests for hydrogen peroxide and benzaldehyde. It is measured that: the concentration of hydrogen peroxide is 59.1 mM, the corresponding mass fraction is 0.18%, and the yield of benzaldehyde is 2.3%. Comparative Example 6 shows that the addition of the third type of molten salt effectively improves the performance of photocatalytic hydrogen peroxide preparation.

[0072] Comparative Example 2

[0073] Weigh 12.5 mg of the photocatalyst TCN-Na / K / Li (for the catalyst preparation process, refer to Example 3 and replace zinc chloride with lithium chloride), place it in a centrifuge tube, and add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol, and 0.2 mL of 0.1 mM HCl to the centrifuge tube, with pH = 3 - 4. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take the mixed solution as one phase and O2 as another phase in the photo-microreactor 1 (inner diameter is 5 mm), set the flow rate ratio to 5:1, the liquid flow rate to 0.1 mL / min, and the gas flow rate to 0.5 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the photo-microreactor for 1 h of photoreaction. Collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and respectively conduct the yield tests for hydrogen peroxide and benzaldehyde. It is measured that: the concentration of hydrogen peroxide is 84.3 mM, the corresponding mass fraction is 0.26%, and the yield of benzaldehyde is 3.1%. Comparative Example 6 shows that compared with common Li salts, Zn salts can more effectively improve the performance of photocatalytic hydrogen peroxide preparation.

[0074] Comparative Example 3

[0075] Weigh 12.5 mg of the photocatalyst TCN-Na / K / Zn prepared in Example 3, place it in a pressure-resistant tube, add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol and 0.2 mL of 0.1 mM HCl into the tube, place the pressure-resistant tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Continuously introduce O2 into the tube, use a 300 W xenon lamp as the light source to continuously irradiate the pressure-resistant tube, carry out the photoreaction for 1 h, centrifuge to separate the aqueous phase and the organic phase, and test the concentrations of hydrogen peroxide and benzaldehyde respectively. It is measured that the concentration of hydrogen peroxide is 82.5 mM, corresponding to a mass fraction of 0.25%, and the yield of benzaldehyde is 3.1%. Carry out the photoreaction for 8 h continuously, collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and finally it is measured that the concentration of hydrogen peroxide is 609 mM, corresponding to a mass fraction of 1.8%, and the yield of benzaldehyde is 36.8%. Comparative Examples 6 and 10 illustrate that the optical microreactor of the present invention can significantly improve the performance of preparing hydrogen peroxide during the photocatalytic process, whether it is for a short time (1 h) or a long time (8 h).

[0076] Comparative Example 4

[0077] Weigh 12.5 mg of the photocatalyst TCN-Na / K / Zn, place it in a centrifuge tube, add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol and 0.2 mL of 0.1 mM HCl into the centrifuge tube, with pH = 3 - 4. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take its mixed solution as one phase and O2 as one phase in the optical microreactor 3 (inner diameter is 2 mm, without a microbubble generation device and a flow field structure), set the flow rate ratio to 5:1, the liquid flow rate is 0.1 mL / min, and the gas flow rate is 0.5 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the optical microreactor, carry out the photoreaction for 1 h, collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and test the yields of hydrogen peroxide and benzaldehyde respectively. It is measured that the concentration of hydrogen peroxide is 99.7 mM, corresponding to a mass fraction of 0.30%, and the yield of benzaldehyde is 4.2%. Comparative Example 6 illustrates that the flow field structure optical microreactor of the present invention overcomes the adverse effects brought by the scale-up effect, and can achieve excellent photocatalytic effects beyond the original small scale (2 mm) while realizing scale-up (5 mm).

[0078] Comparative Example 5

[0079] Weigh 12.5 mg of the photocatalyst TCN-Na / K / Zn, place it in a centrifuge tube, add 2.5 mL of deionized water, 2.5 mL of benzyl alcohol and 0.2 mL of 0.1 mM HCl into the centrifuge tube, with pH = 3 - 4. Place the test tube in the dark and stir for 0.5 h until the photocatalyst is evenly dispersed. Take the mixed solution as one phase and O2 as another phase in the photo-microreactor 1 (inner diameter is 5 mm, without a microbubble generating device). Set the flow rate ratio to 5:1, with the liquid flow rate of 0.1 mL / min and the gas flow rate of 0.5 mL / min. Use a 300 W xenon lamp as the light source to continuously irradiate the photo-microreactor for 1 h of photoreaction. Collect the effluent, centrifuge to separate the aqueous phase and the organic phase, and respectively test the yields of hydrogen peroxide and benzaldehyde. It is measured that the concentration of hydrogen peroxide is 92.3 mM, corresponding to a mass fraction of 0.28%, and the yield of benzaldehyde is 3.7%. Comparative Example 6 illustrates that the reason for the improvement of the photocatalytic performance of the photo-microreactor for hydrogen peroxide production in the present invention lies in the synergistic effect of the flow field structure and the bubble generator in the photo-microreactor.

[0080] The experimental results show that due to the high transfer rate of the photo-microreactor in the present invention, the preparation rate and selectivity of hydrogen peroxide in the present invention are improved. In addition, the device combining the bubble generator and the photo-microreactor used in the present invention has a small reaction volume and a high photocatalytic rate, and can achieve the purpose of expanding the contact area between the gas, liquid and catalyst and strengthening the mixing effect of the gas, liquid and catalyst.

[0081] The present invention provides a method for photocatalytically preparing low-concentration hydrogen peroxide of 0.1 - 3% by using a photo-microreactor. There are many methods and ways to specifically implement this technical solution. The above is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention. Each component not clearly defined in this embodiment can be realized by the prior art.

Claims

1. A method for photocatalytic in-situ preparation of low-concentration hydrogen peroxide by using a gas-liquid microreactor with a flow field structure, characterized in that, The method includes the following steps: (1) Add a photocatalyst into a mixed solution of benzyl alcohol and water, adjust the pH of the solution to 3-4, and stir under dark conditions to obtain an acidic mixed solution; (2) Carry out a photoreaction on the acidic mixed solution and O2 in a photomicroreactor, collect the effluent, and centrifuge the effluent to separate the water phase and the organic phase to obtain hydrogen peroxide in the water phase; where the photomicroreactor is a microfluidic chip with several helical baffle internal components arranged in the channel, and a microbubble generator is arranged at the O2 inlet of the microfluidic chip; The photocatalyst is prepared by the following method: Mix and grind a catalyst precursor and a molten salt of the catalyst, put it into a muffle furnace and calcine at 520-550 °C, take it out after cooling, wash and remove the salt, then carry out suction filtration, and then rinse with ethanol and dry for standby; where the catalyst precursor is any one or several of melamine, urea, thiourea, monocyanamide or dicyandiamide; the molten salt of the catalyst includes sodium chloride, potassium chloride and a third type of molten salt, and the third type of molten salt is selected from at least one of zinc chloride, magnesium chloride, cobalt chloride, lithium chloride; the photocatalyst is separated and washed after the reaction, and after drying, it is reused in step (1); The microbubble generator is composed of a group of circular sieve holes with a diameter of 0.1 mm - 0.5 mm; the fluid channel of the microfluidic chip is a bent square tube structure in an M shape, and the helical baffle internal component is arranged inside the square tube structure, and the helical baffle internal component is composed of two built-in baffles, and each baffle is obtained by rotating a cuboid baffle by a certain angle around the fluid channel, and the two baffles form a helical structure.

2. The method according to claim 1, characterized in that The mass ratio of the catalyst precursor to the molten salt of the catalyst is 1:(0.6~1.5), where, in the molten salt of the catalyst, the mass ratio of sodium chloride, potassium chloride and the third type of molten salt is (2-5):1:(0.05-0.1).

3. The method according to claim 1, wherein The pH of the acidic mixed solution is adjusted by an acidic reagent, and the acidic reagent is a 0.1-1 M HCl solution.

4. The method according to claim 1, characterized in that The volume ratio of water to benzyl alcohol in the acidic mixed solution is 1:(0.1-5); the concentration of the catalyst in the acidic mixed solution is 0.5-5 mg / mL.

5. The method according to claim 1, wherein The rate of pumping the acidic mixed solution into the photomicroreactor is 0.05-1 mL / min; the ratio of the rate of pumping O2 into the photomicroreactor to the rate of pumping the acidic mixed solution into the photomicroreactor is (1-10):

1.

6. The method according to claim 1, wherein The light source of the photomicroreactor is 20-40 cm away from the microfluidic chip, so that the center of the light spot irradiates on the microfluidic chip; the inner diameter of the microfluidic chip is 2-10 mm, and the reaction volume is 1-15 mL.

7. The method according to claim 6, characterized in that, The light source is a xenon lamp, the model is XHA350w xenon lamp, the reaction wavelength is above 400 nm; the temperature of the photoreaction is 20-30 °C; the reaction time is 5-180 min.

8. The method according to claim 1, wherein The thickness of the helical baffle internal component is 2-3 mm, and the length of each group of helical baffle internal components is 20-25 mm.

Images