Method for synthesizing glyoxalic acid by photo-fenton oxidation of glyoxal

Abstract

The application discloses a method for synthesizing glyoxylic acid by photo-Fenton oxidation of glyoxal, and the technical scheme comprises two parts of photo-Fenton catalyst preparation and oxidation process condition implementation. 2‑ The mass composition of the photo-Fenton catalyst SO4 / TiO2-Fe3O4 is as follows: TiO2 10.2-52.6%, Fe3O4 45.4%-88.5%, and Si 1.2%-2.1%, the photo-catalytic efficiency of the photo-Fenton catalyst is 10 times that of a TiO2 or Fe3O4 photo-catalyst, the preparation method is simple, and the photo-Fenton catalyst can be recovered by magnetic adsorption. When the conversion rate of glyoxal in the reaction solution reaches more than 99%, the molar selectivity of glyoxylic acid in the reaction solution is 78%-82%, the molar selectivity of oxalic acid is 8%-12%, and the molar selectivity of CO2 is 8%-12%. In the photo-Fenton oxidation process for synthesizing glyoxylic acid from glyoxal, no pollutants are generated, the process is safe and environmentally friendly, and a new green synthesis method for glyoxylic acid is provided. The problems of low oxidation efficiency of the photo-catalytic method and high H2O2 consumption of the Fenton oxidation method are solved, and the method has an industrial application prospect.

Description

Technical Field

[0001] The invention relates to the technical field of new materials and chemical pharmaceuticals, and in particular to a method for synthesizing glyoxylic acid by photo-Fenton oxidation of glyoxal. Background Art

[0002] Glyoxylic acid is a widely used fine chemical, primarily used in the production of pharmaceutical and chemical products such as the antibiotic amoxicillin intermediate, the spice vanillin, and the plant growth regulator allantoin. While there are numerous synthetic routes for glyoxylic acid, many remain deficient in terms of safety, environmental protection, and technical and economic feasibility. With the growing market demand for glyoxylic acid, research and development of new processes are gaining renewed attention.

[0003] Based on domestic and international data, professional companies first selected the simple and easy-to-use Fenton oxidation method. Chinese patent CN101003474B (2012-04-18) discloses a method for preparing glyoxylic acid. This method uses a glyoxal solution as the raw material and undergoes an oxidation reaction with hydrogen peroxide in the presence of ferrous sulfate as a catalyst and ammonia as a co-catalyst. This method increases the yield of the glyoxylic acid product and improves the operating environment. However, the method has the following disadvantages: the oxidation reaction must be carried out at 2-10°C, which makes the reaction temperature difficult to control. The byproduct iron sludge is relatively heavy, and the glyoxal product is easily further oxidized and degraded into oxalic acid, formic acid, and carbon dioxide.

[0004] Li Yuxin, Yan Shenghu, Zhang Yue, et al. Continuous flow process for the synthesis of glyoxylic acid by oxidation of glyoxal with hydrogen peroxide [J]. Fine Chemicals, 2019, 36(07): 1483-1487. A continuous flow process for the synthesis of glyoxylic acid by liquid phase oxidation was investigated in a microchannel reactor using glyoxal and hydrogen peroxide as raw materials. The effects of material ratio, catalyst dosage, hydrogen peroxide concentration, residence time, and temperature on the reaction were investigated. The optimal process conditions were n(glyoxal)∶n(H2O2)∶n(FeSO4)=1.0∶1.0∶0.13, H2O2 concentration 1.67 mol / L, residence time 10 min, reaction temperature 30℃, glyoxal conversion rate reached 94.7%, and glyoxylic acid selectivity reached 85.4%. However, there are technical difficulties such as high residual glyoxal content in the product, complex continuous flow equipment, large amount of iron sludge pollutants, difficulty in separation, and high hydrogen peroxide consumption.

[0005] Chinese patent CN113789529B (2022-11-25) discloses a synthesis method for photoelectrocatalytic oxidation of glyoxal to glyoxylic acid, using WO3, TiO2, and BiVO4 semiconductor materials as photoanodes. The reaction conditions are mild, the process is simple and easy to control, and the selectivity reaches 95%. Although it can solve the problem of iron sludge pollution, it is difficult to overcome the problems of low raw material conversion rate and complex and difficult to expand photoelectrocatalytic oxidation equipment.

[0006] The applicant's Chinese patent application CN113976103B (2022-03-01) discloses a highly active visible light photocatalyst for wastewater treatment and its preparation method. The method comprises a visible light photocatalyst precursor aqueous sol coated on fly ash beads, which is dried to form a gel film, which is then sintered at 500-700°C. The method is used to degrade organic pollutants in wastewater, essentially employing the photo-Fenton process to degrade refractory organic matter in wastewater. Inspired by this invention patent, the applicant creatively applied the photo-Fenton process to the oxidation of glyoxal aqueous solution to synthesize glyoxylic acid aqueous solution. Summary of the Invention

[0007] (1) Technical problems solved

[0008] The Photo-Fenton method, developed based on the Fenton method, is essentially a combined photocatalytic-Fenton process, primarily used to degrade organic pollutants in wastewater to CO₂. Extensive research and development efforts have been conducted both domestically and internationally, with considerable success. Professionals aim to design and select reaction conditions for the Photo-Fenton method, applying the principles of Photo-Fenton oxidation of organic matter to photochemical synthesis, particularly glyoxylic acid synthesis. The key technical considerations lie in the design of the Photo-Fenton catalyst and the selection of oxidation process conditions to ensure that the Photo-Fenton oxidation of glyoxal remains at the intermediate stage of glyoxylic acid, inhibiting the formation of oxalic acid or CO₂, thereby yielding a high-value-added glyoxylic acid product.

[0009] (2) Principles of the Method

[0010] Under the combined action of sunlight, photocatalyst and H2O2, the photosensitive complex on the surface of the photocatalyst and in the solution works synergistically to produce a large number of hydroxyl radicals ·OH, which oxidize the glyoxal aqueous solution into glyoxylic acid aqueous solution, which is further oxidized to produce oxalic acid and carbon dioxide.

[0011] When there's no H2O2 in the reaction solution, sunlight shines on the photocatalyst, which absorbs photons and generates electron-hole pairs. The holes react with water to produce hydroxyl radicals (·OH). The highly oxidizing ·OH gradually oxidizes the organic matter in the reaction solution, ultimately degrading it into CO2 and H2O. Because the large number of electrons and holes generated by photocatalysis easily recombine and annihilate, the efficiency of photocatalytic oxidation is relatively low.

[0012] Even in the absence of sunlight, when photocatalyst and H2O2 are present in the reaction solution, the photocatalyst and H2O2 undergo a Fenton-like reaction to produce a large amount of hydroxyl radicals ·OH. ·OH can also accelerate the oxidation of glyoxal aqueous solution into glyoxylic acid aqueous solution, which is further oxidized to produce oxalic acid and carbon dioxide.

[0013] When the reaction liquid simultaneously meets the three conditions of H2O2, photocatalyst and sunlight irradiation, the photo-Fenton reaction can not only occur on the surface of the photocatalyst, accelerating the speed and efficiency of H2O2 decomposition to produce highly oxidizing hydroxyl free radicals ·OH, but also greatly reducing the consumption of H2O2, and the efficiency of oxidizing organic matter is doubled.

[0014] (3) Technical solution

[0015] The technical solution of the present invention includes SO 2- 4 / TiO2-Fe3O4 photo-Fenton catalyst design and preparation and oxidation process conditions design and implementation part 2, the SO 2- The preparation method of 4 / TiO2-Fe3O4 photo-Fenton catalyst specifically comprises the following steps:

[0016] S1. Hydrolyze titanyl sulfate to form a white precipitate of TiO(OH)2, wash with deionized water until there is no sulfate ion in the precipitate, and filter and separate;

[0017] S2. Disperse the white precipitate in an oxalic acid aqueous solution and heat in a water bath at 60-80°C for 0.5-2h to form a transparent nano-TiO2 hydrosol;

[0018] S3, adding magnetic Fe3O4 powder with a particle size of 300 mesh to the nano-TiO2 aqueous sol, controlling the molar ratio of TiO2 to Fe3O4 to be 1:0.3-3, and mixing them evenly to form a TiO2-Fe3O4 composite material to effectively inhibit the dissolution of Fe3O4 in the oxidation reaction solution;

[0019] S4, drying the TiO2-Fe3O4 composite material in a blast oven at 250-300° C. to decompose the oxalic acid organic matter therein, thereby obtaining a TiO2-Fe3O4 composite material powder having a porous surface;

[0020] S5, immersing the TiO2-Fe3O4 composite material powder in an aqueous solution of ammonium sulfate with a concentration of 0.25-0.5 mol / L for 2-4 hours, filtering and separating the solid powder, and drying it in an oven at 110-150° C. for 2-4 hours;

[0021] S6. The dried TiO2-Fe3O4 solid powder adsorbed with ammonium sulfate was transferred into an alumina crucible and calcined in a high-temperature furnace at 500-600°C for 3-6 hours. The crucible and the sample were cooled to room temperature along with the furnace. Ammonium sulfate combined with TiO2-Fe3O4 to obtain the photo-Fenton catalyst SO 2- 4 / TiO2-Fe3O4, its mass composition is: TiO210.2-52.6%, Fe3O4 45.4%-88.5%, S 1.2%-2.1%.

[0022] The specific steps for implementing the oxidation process conditions are:

[0023] T1. Add 40% by mass concentration of glyoxal, oxalic acid, deionized water, and a photo-Fenton catalyst to a transparent oxidation reactor, so that the initial molar concentration of glyoxal in the reaction solution is 0.25-0.5 mol / L, the initial molar concentration of oxalic acid is 0.01-0.1 mol / L, and the photo-Fenton catalyst is 0.1-0.5 mol / L, calculated as Fe. Compressed air is introduced through a gas distributor for stirring, and a 30% by mass concentration of H2O2 aqueous solution is continuously added to control the initial molar concentration of H2O2 to 0.1-0.5 mol / L.

[0024] T2. After 0.1-0.5h of sunlight irradiation, H2O2 and the photo-Fenton catalyst generate hydroxyl radicals, and the reaction of oxidizing glyoxal to generate glyoxylic acid is initiated. The reaction temperature slowly rises and O2 and CO2 gases are generated. The reaction temperature is controlled at 10-20°C with cooling water. A 30% H2O2 aqueous solution is continuously added within 4-8h, and the molar ratio of the added H2O2 to glyoxal is controlled to be 0.5-1.0:1. The hydroxyl radical oxidant lacking in glyoxal is completely oxidized by photocatalysis;

[0025] T3, when the conversion rate of glyoxal in the oxidation reaction liquid reaches 85%-90%, no longer adding H2O2 aqueous solution, mainly relying on the hydroxyl radicals generated by light irradiation to oxidize glyoxal, and controlling the reaction temperature to 20-30°C; when the conversion rate of glyoxal in the reaction liquid reaches 99%, the oxidation process is terminated, and the molar selectivity of glyoxylic acid in the reaction liquid is 78%-82%, the molar selectivity of oxalic acid is 8%-12%, and the molar selectivity of CO2 is 8%-12%;

[0026] T4. Separate the photo-Fenton catalyst for recycling, and use a mixed anion and cation exchange resin to adsorb the metal ion complex dissolved in the oxidation solution; vacuum concentrate the oxidation reaction solution at 50-60°C to a glyoxylic acid mass concentration of 30%-40%, cool the concentrated mother liquor to 10-20°C with water in a crystallizer, and separate the precipitated oxalic acid crystals;

[0027] T5. Further concentrating the crystallization mother liquor under vacuum at 50-60° C. to a glyoxylic acid mass concentration of 49%-50%, followed by cooling and crystallization at -2 to 2° C. for 1-2 hours, separating oxalic acid crystals precipitated from the mother liquor, and obtaining a glyoxylic acid product with a mass concentration of 50%-51%, wherein the mass percentages of glyoxal and oxalic acid are less than 0.5%, and the mass percentages of oxalic acid are less than 0.5%.

[0028] The photo-Fenton catalyst SO of the present invention 2-The Fe3O4 component in 4 / TiO2-Fe3O4 exhibits excellent photocatalytic properties, and its composite with TiO2 to form TiO2-Fe3O4 can exert a synergistic photocatalytic effect. TiO2 coated on the Fe3O4 surface inhibits its dissolution in the acidic reaction solution, reducing the difficulty of subsequent iron ion separation. A small amount of iron doped into the TiO2 structure gives it better photocatalytic performance than both TiO2 and Fe3O4. The TiO2-Fe3O4 composite exhibits photocatalytic performance across the entire wavelength range of sunlight, with photocatalytic performance improved by more than 10 times; the photo-Fenton catalyst SO formed after sulfation treatment 2- 4 / TiO2-Fe3O4, compared with TiO2-Fe3O4 that has not been sulfated, has greatly improved surface acidity and further improved photocatalytic performance by 3-5 times, with outstanding substantial characteristics and significant technological progress.

[0029] The photo-Fenton catalyst SO of the present invention 2- The Fe₃O₄ component in the 4 / TiO₂-Fe₃O₄ is slightly soluble in the acidic oxidation reaction solution and combines with oxalic acid and glyoxylic acid in the solution to form a yellow-green photosensitizing complex, which promotes the photo-Fenton reaction. The liquid-phase photo-Fenton reaction in the oxidation reaction solution and the heterogeneous photo-Fenton reaction on the catalyst surface proceed simultaneously. Because the heterogeneous photo-Fenton reaction produces virtually no precipitate, the photocatalyst can be magnetically adsorbed and recycled. Small amounts of soluble iron ion complexes can be removed by adsorption using mixed anion and cation exchange resins.

[0030] During the glyoxylic acid synthesis process, the product glyoxylic acid is easily oxidized by oxidants to produce oxalic acid and decompose into carbon dioxide. The raw material glyoxal is also easily decomposed into carbon dioxide by oxidants, resulting in a low selectivity for the entire glyoxylic acid synthesis process. Some professionals have come to realize that the decomposition of glyoxal into CO2 is the primary factor affecting glyoxylic acid yield and the economic efficiency of the production process. Experimental results show that in the Fenton oxidation of glyoxal to glyoxylic acid at -5°C to 5°C, when the glyoxal conversion is 70%-80%, the glyoxylic acid selectivity can reach 90%-95%. A significant decrease in glyoxylic acid selectivity occurs primarily when the glyoxal conversion exceeds 85%.

[0031] The present invention adopts a photo-Fenton oxidation method. When the glyoxal conversion rate is below 70%-80%, hydroxyl radicals are generated mainly by the Fenton method and supplemented by photocatalysis, that is, they are mainly generated by the action of H2O2 on the photocatalyst, supplemented by the action of light on the photocatalyst. The reaction temperature is controlled at 10-20°C to prevent the large-scale decomposition of the raw material glyoxal and the product glyoxylic acid, while ensuring that the oxidation reaction speed is fast. When the glyoxal conversion rate is higher than 85%, hydroxyl radicals are mainly generated by the photocatalysis method and supplemented by the Fenton method, that is, they are mainly generated by the action of sunlight on the photo-Fenton catalyst, supplemented by the action of H2O2 on the photo-Fenton catalyst. The reaction temperature is controlled at 20-30°C to ensure that the oxidation reaction speed is fast even at a low concentration of hydroxyl radicals, thereby improving the selectivity of glyoxal oxidation to glyoxylic acid.

[0032] In the Fenton process of oxidizing glyoxal to produce glyoxylic acid, the molar ratio of glyoxal, H2O2, and divalent iron is an important control condition. However, in the photo-Fenton process of oxidizing glyoxal to produce glyoxylic acid, the photo-Fenton catalyst is insoluble, the reaction process of generating hydroxyl radicals is slow, and the oxidation reaction process is stable. Excessive addition of the photo-Fenton catalyst does not have much impact. The insufficient amount of H2O2 oxidant added can be supplemented by absorbing sunlight in a photocatalytic manner. There are no strict requirements for controlling the oxidation process conditions, but more sunlight exposure time is required. The two-stage photo-Fenton oxidation with different oxidation process conditions can be arranged in different reactors. In the photo-Fenton method of the present invention, photocatalytic oxidation and Fenton oxidation have a synergistic oxidation effect. The photo-Fenton oxidation method can reduce the consumption of H2O2 oxidant by 30%-50%, which has outstanding substantive characteristics and significant technological progress.

[0033] In order to improve the solar light absorption effect of the oxidation reaction liquid, you can choose to use focusing, internal and external light irradiation, liquid film, waterfall or fountain reactors.

[0034] In the present invention, a lower oxidation reaction liquid concentration is selected to make the temperature change of the reaction liquid stable, inhibit the spontaneous decomposition of H2O2, thereby improving the selectivity of the glyoxal oxidation reaction and appropriately increasing the subsequent concentration multiple of the reaction liquid to compensate.

[0035] In the present invention, oxalic acid is added to the initial reaction solution to reduce the degree of change in the acidity of the reaction solution and minimize the chance of the product glyoxylic acid being further oxidized to oxalic acid and CO2.

[0036] In the present invention, the content of glyoxylic acid is determined by the sodium bisulfite addition method; the content of glyoxalic acid is determined by first neutralizing the reaction solution and then disproportionating it with an excess sodium hydroxide solution; the content of oxalic acid is determined by first forming calcium oxalate precipitation, separating it, and then determining it with the potassium permanganate method.

[0037] The H2O2 concentration in the oxidation reaction liquid of the present invention is determined by spectrophotometry, with titanium salt as a color developer. Under strong acidic conditions, H2O2 and titanium ions can form a yellow complex, and the measurement wavelength is 415nm.

[0038] The raw materials used in the present invention, such as glyoxal, titanyl sulfate, oxalic acid, ammonium sulfate, ferrosoferric oxide, and ammonia water, are all commercially available raw materials.

[0039] (3) Beneficial effects

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] (1) The photo-Fenton method is used to oxidize glyoxal aqueous solution to glyoxylic acid aqueous solution. No pollutants are generated during the process, which is safe and environmentally friendly, providing a new route for the green synthesis of glyoxylic acid.

[0042] (2) Photo-Fenton catalyst SO of the present invention 2- 4 / The photocatalytic efficiency of TiO2-Fe3O4 is several times that of TiO2 or Fe3O4 photocatalysts. It is simple to prepare, can be recovered by magnetic adsorption, and has low cost.

[0043] (3) H2O2, glyoxylic acid and oxalic acid in the oxidation reaction solution can act as complexing agents, which can complex and dissolve titanium ions and iron ions in the photocatalyst to form photosensitizing compounds, which promote the photo-Fenton reaction.

[0044] (4) The photo-Fenton method solves the problems of low oxidation efficiency of the photocatalytic method and excessive H2O2 consumption in Fenton oxidation, and has prospects for industrial application. DETAILED DESCRIPTION

[0045] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0046] Example 1

[0047] 16g (0.1 mol) of titanium oxysulfate (TiOSO4) was dissolved in 100mL of 0.2M aqueous sulfuric acid solution, then neutralized with 6M ammonia to a pH of 7-9, forming a white colloidal precipitate of TiO2, primarily composed of TiO(OH)2. The TiO2 colloidal precipitate was separated and washed with deionized water until free of sulfate ions. The white precipitate was dispersed in 100mL of aqueous solution containing 9g (0.1 mol) of oxalic acid and heated in a water bath at 60-80°C for 0.5h to form a transparent nano-TiO2 hydrosol. 23.1g (0.1 mol) of 300-mesh magnetic Fe3O4 powder was added to the nano-TiO2 hydrosol and mixed thoroughly to form a TiO2-Fe3O4 composite material. The TiO2-Fe3O4 composite material was dried in a forced air oven at 250-300°C to decompose the oxalic acid organic matter, yielding 31.1g of TiO2-Fe3O4 composite powder.

[0048] The TiO2-Fe3O4 composite powder was immersed in 200ml of a 0.5mol / L ammonium sulfate aqueous solution for 2-4 hours, filtered to separate the solid powder, and dried in an oven at 110-150°C for 2-4 hours. The dried ammonium sulfate-adsorbed TiO2-Fe3O4 solid powder was transferred to an alumina crucible and calcined in a 550°C furnace for 3 hours. The crucible and sample were cooled to room temperature. The ammonium sulfate combined with the TiO2-Fe3O4 to produce 31.6g of a photo-Fenton catalyst with a mass composition of 25.3% TiO2, 73.1% Fe3O4, and 1.6% S.

[0049] Example 2

[0050] An oxidation reactor is charged with 72.5 g (0.5 mol) of 40% glyoxal, 12.6 g (0.1 mol) of oxalic acid crystals, and 15 g of a photo-Fenton catalyst. The mixture is diluted to 1000 mL with deionized water. Compressed air is introduced through a gas distributor for stirring, and 113 g (0.25 mol) of 30% H2O2 is added. After 0.5 hour of sunlight irradiation, the H2O2 and the photo-Fenton catalyst generate hydroxyl radicals, initiating the oxidation of glyoxal to glyoxylic acid. The reaction temperature slowly rises, generating O2 and CO2 gases. The reaction temperature is controlled at 10-20°C using cooling water. Over 2-4 hours, 113 g (0.25 mol) of a 30% H2O2 aqueous solution is continuously added, with a molar ratio of H2O2 to glyoxal of 1:1. The conversion rate of glyoxal in the oxidation reaction solution reaches 87%. The reaction was continued under sunlight for 4 hours, with the reaction temperature controlled at 20-30°C. The oxidation process was terminated when the glyoxal conversion in the reaction solution reached 99%. The reaction solution was measured to contain 0.41 mol of glyoxylic acid and 0.15 mol of oxalic acid, resulting in an estimated molar selectivity of 82% for glyoxylic acid and 10% for oxalic acid.

[0051] The photo-Fenton catalyst is filtered and separated for recycling, and mixed anion and cation exchange resins are used to adsorb the metal ion complexes dissolved in the oxidation solution. Since trace iron ions form anion complexes with oxalic acid, mixed anion exchange resins are required for adsorption and removal. The oxidation reaction solution is vacuum concentrated at 50-60°C to a glyoxylic acid concentration of 30%-40%. The concentrated mother liquor is cooled with water to 10-20°C in a crystallizer, and the precipitated oxalic acid crystals are separated by crystallization. The crystallization mother liquor is further vacuum concentrated at 50-60°C to a glyoxylic acid concentration of 49%-50%, then cooled and crystallized at -2 to 2°C for 1-2 hours. The precipitated oxalic acid crystals are separated from the mother liquor to obtain a glyoxylic acid product with a mass concentration of 50%-51%, a glyoxal mass percentage of less than 0.5%, and an oxalic acid mass percentage of less than 0.5%.

[0052] Comparative Example 1

[0053] Adding mass concentration of 40% glyoxal 145g (0.5mol) and ferrous sulfate catalyst 13.9g (0.05mol) into oxidation reactor, being diluted to 1000mL with deionized water, passing compressed air through gas distributor and stirring, controlling the reaction temperature with cooling water is 10-30 ℃, adding H2O2 226g (0.5mol) with mass concentration of 30% for 4-6h, continuing reaction for 2 hours after adding, to H2O2 concentration no longer significantly changes. Recording the glyoxal conversion rate in the reaction solution is 95%, and the oxidation process is terminated. Recording the reaction solution contains glyoxylic acid 0.32mol, contains oxalic acid 0.07mol, and deduces that the molar selectivity of glyoxylic acid is 64%, and the molar selectivity of oxalic acid is 14%.

[0054] Meanwhile, the contents not described in detail in this specification belong to the prior art known to those skilled in the art.

[0055] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0056] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for synthesizing glyoxylic acid by photo-Fenton oxidation of glyoxal, characterized in that: Including photo-Fenton catalyst SO 2- 4 / Preparation of TiO2-Fe3O4 and implementation of oxidation process conditions Part II, the photo-Fenton catalyst SO 2- The preparation method of 4 / TiO2-Fe3O4 specifically comprises the following steps: S1. Hydrolyze titanyl sulfate to form a white precipitate of TiO(OH)2, wash with deionized water until there is no sulfate ion in the precipitate, and filter and separate; S2. Disperse the white precipitate in an oxalic acid aqueous solution and heat in a water bath at 60-80°C for 0.5-2h to form a transparent nano-TiO2 hydrosol; S3, adding magnetic Fe3O4 powder with a particle size of 300 mesh to the nano-TiO2 aqueous sol, controlling the molar ratio of TiO2 to Fe3O4 to be 1:0.3-3, and mixing them evenly to form a TiO2-Fe3O4 composite material to effectively inhibit the dissolution of Fe3O4 in the oxidation reaction solution; S4, drying the TiO2-Fe3O4 composite material in a blast oven at 250-300° C. to decompose the oxalic acid organic matter therein to obtain TiO2-Fe3O4 composite material powder; S5, immersing the TiO2-Fe3O4 composite material powder in an aqueous solution of ammonium sulfate with a concentration of 0.25-0.5 mol / L for 2-4 hours, filtering and separating the solid powder, and drying it in an oven at 110-150° C. for 2-4 hours; S6. The dried TiO2-Fe3O4 solid powder adsorbed with ammonium sulfate was transferred into an alumina crucible and calcined in a high-temperature furnace at 500-600°C for 3-6 hours. The crucible and the sample were cooled to room temperature along with the furnace. Ammonium sulfate combined with TiO2-Fe3O4 to obtain the photo-Fenton catalyst SO 2- 4 / TiO2-Fe3O4, whose mass composition is: TiO210.2-52.6%, Fe3O445.4%-88.5%, S 1.2%-2.1%; The specific steps of implementing the oxidation process conditions are: T1. Add 40% by mass concentration of glyoxal, oxalic acid, deionized water, and a photo-Fenton catalyst to a transparent oxidation reactor, so that the initial molar concentration of glyoxal in the reaction solution is 0.25-0.5 mol / L, the initial molar concentration of oxalic acid is 0.01-0.1 mol / L, and the photo-Fenton catalyst is 0.1-0.5 mol / L (calculated as Fe). Compressed air is introduced through a gas distributor with stirring, and a 30% by mass concentration of H2O2 aqueous solution is continuously added to control the initial molar concentration of H2O2 to 0.1-0.5 mol / L. T2. After 0.1-0.5h of sunlight irradiation, H2O2 and photo-Fenton catalyst generate hydroxyl radicals, and the reaction of oxidizing glyoxal to generate glyoxylic acid is initiated. The reaction temperature slowly rises and O2 and CO2 gases are generated. The reaction temperature is controlled at 10-20℃ with cooling water. A 30% mass concentration of H2O2 aqueous solution is continuously added within 2-4h. The molar ratio of added H2O2 to glyoxal is controlled to be 0.5-1.0:

1. The hydroxyl radical oxidant lacking in glyoxal is completely oxidized by photocatalysis. T3. When the conversion rate of glyoxal in the oxidation reaction liquid reaches 85%-90%, no more H2O2 aqueous solution is added, and glyoxal is mainly oxidized by hydroxyl radicals generated by light irradiation, and the reaction temperature is controlled at 20-30°C; when the conversion rate of glyoxal in the reaction liquid reaches 99%, the oxidation process is terminated, and the molar selectivity of glyoxylic acid in the reaction liquid is 78%-82%, the molar selectivity of oxalic acid is 8%-12%, and the molar selectivity of CO2 is 8%-12%; T4. Separate the photo-Fenton catalyst for recycling, and use a mixed anion and cation exchange resin to adsorb the metal ion complex dissolved in the oxidation solution; vacuum concentrate the oxidation reaction solution at 50-60°C to a glyoxylic acid mass concentration of 30%-40%, cool the concentrated mother liquor to 10-20°C with water in a crystallizer, and separate the precipitated oxalic acid crystals; T5. Further concentrating the crystallization mother liquor under vacuum at 50-60° C. to a glyoxylic acid mass concentration of 49%-50%, followed by cooling and crystallization at -2 to 2° C. for 1-2 hours, separating the precipitated oxalic acid crystals from the mother liquor, and obtaining a glyoxylic acid product with a mass concentration of 50%-51%, wherein the mass percentages of glyoxal and oxalic acid are less than 0.5%, and the mass percentages of glyoxal and oxalic acid are less than 0.5%.

2. The method for synthesizing glyoxylic acid by photo-Fenton oxidation of glyoxal according to claim 1, wherein: In the step S3, magnetic Fe3O4 powder with a particle size of 300 mesh is added to the nano-TiO2 aqueous sol, and the molar ratio of TiO2 to Fe3O4 is controlled to be 1:0.5-1.

3. The method for synthesizing glyoxylic acid by photo-Fenton oxidation of glyoxal according to claim 1, wherein: In step T1, a 30% H2O2 aqueous solution is continuously added to control the initial molar concentration of H2O2 to be 0.15-0.25 mol / L.