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The production of glyoxalic acid via nitric acid oxidation of glyoxal is a mature technology

Synthesis of glyoxalic acid 1. Synthesis methods. The synthesis of glyoxalic acid can be broadly divided into two categories: chemical synthesis and electrochemical synthesis. Among these, the methods that are used in industrial production include the nitric acid oxidation of glyoxal in chemical synthesis, the ozonolysis of cyclohexanone oxide, as well as the electrolytic reduction of oxalic acid in electrochemical synthesis. 1.1 Chemical synthesis methods 1.1.1 Oxidation of glyoxal with nitric acid The oxidation of glyoxal using nitric acid involves mild reaction conditions, a simple and well-established process, and low equipment costs; as a result, it is used by most manufacturers both domestically and internationally. However, this method presents difficulties in separating the waste acid, results in low product purity, and yields poor-quality products. Equipment used for nitric acid corrosion is prone to boiling over at high concentrations. Unreacted nitric acid causes glyoxalic acid to oxidize and decompose slowly; the nitrogen monoxide gas produced as a result pollutes the environment. Unconverted glyoxal interferes with the condensation reaction of glyoxalic acid, limiting its range of applications. Wang Yinhua and others from Hebei Hongyuan Chemical Co., Ltd. improved the process conditions to address the aforementioned shortcomings, developing a unique composite catalyst A that enhanced the selectivity of the glyoxal oxidation reaction as well as the yield of glycolic acid. At the same time, the original low-pressure oxidation was replaced by a combined oxidation using atmospheric air and nitric acid, which not only resolved the safety issues associated with the operation but also reduced nitrogen oxide pollution. The process involves adding catalyst A to a 25–30% aqueous solution of glyoxal, introducing air into the mixture, and slowly adding nitric acid at 40–60°C, ensuring that the conversion rate of glyoxal exceeds 98%. The reaction mixture is then evaporated, cooled to 0°C, crystallized, and the by-product oxalic acid is separated by centrifugation, thereby yielding an aqueous solution of 4O-glyoxalic acid. Although the glyoxal nitric acid oxidation method involves only one oxidation step, it requires stringent conditions for the oxidation process and precise process control, with the selection of catalyst being crucial. Li Jiansheng from the School of Biology and Environmental Engineering at Tianjin Vocational University, and Song Haiyan from the Fine Chemicals Research Institute at Tianjin Chemical Research and Design Institute, used different catalysts to produce glycolic acid through the oxidation of glyoxal with nitric acid. They explored the mechanism of action of these catalysts and investigated the effect of various catalysts on the yield of glycolic acid. The situation is as follows: (1) Sodium nitrite as a catalyst – In current industrial processes for the production of glyoxalic acid, sodium nitrite is generally used as a catalyst, with a dosage of 0.1 to 0.2% of the weight of the reaction mixture. A control experiment was conducted using a laboratory setup with sodium nitrite as a catalyst, yielding an acetaldehyde acid productivity of 73.9%. The role of sodium nitrite is to react with nitric acid in the reaction mixture to produce nitrous acid (HNO), while the actual catalyst is HNO2. In the experiment, without adding any catalyst, the oxidation of glyoxal by nitric acid cannot be initiated at 40–45°C. Methods such as introducing air, raising the temperature, and increasing the nitric acid concentration in the reaction solution can indeed trigger oxidation reactions randomly, but these randomly initiated oxidation reactions are difficult to control, often leading to material loss; as a result, the content of glyoxalic acid in the reaction products is very low, with oxalic acid and carbon dioxide being the main products formed. In the reaction of nitric acid oxidation of glyoxal to glycolic acid, nitrous acid plays a crucial role. As a donor of oxygen atoms, HNO is reduced to NO, and in solution, NO is oxidized by nitric acid or oxygen to regenerate HNO. (2) Inorganic acids as auxiliary catalysts: The increased solubility of NO in inorganic acid solutions facilitates the formation and increase in the concentration of HNO3; theoretically, they can act as auxiliary catalysts to accelerate the nitric acid oxidation reaction and improve yield. Experimental studies were conducted on the process of producing glycolic acid by nitric acid oxidation of glyoxal using hydrochloric acid, nitric acid, sulfuric acid, etc., respectively. Experimental results show that in the process of producing glycolic acid by nitric acid oxidation of glyoxal, using inorganic acids as auxiliary catalysts can increase the yield of glycolic acid by 7.3–10.7%, with the order of effectiveness being HCl > HNO3. >HSO. After the oxidation reaction is complete, the excess inorganic acid co-catalyst can be separated and removed by electrodialysis, a method that holds industrial application potential. For the production of products such as p-hydroxybenzene hydrazine, the excess inorganic acid catalysts do not need to be separated and can be used directly in subsequent reactions. Inorganic acids act as co-catalysts to increase the yield of glyoxylic acid because NO generated by the oxidation reaction has a high solubility in inorganic acids. In solution, NO can be oxidized by oxygen to produce HNO3, and it can also be oxidized by nitric acid to produce HNO3 as well; this increases the concentration of HNO3 in the solution, accelerates the rate of the main reaction, and reduces the intensity of side reactions. In HC1 solution, HNO is converted into NOCl molecules with higher solubility, which act as donors of oxygen atoms; therefore, its effectiveness is the greatest. In the HSO solution, the concentration of HNO is the lowest, so its effect strength is the smallest. (3) Solid acids as auxiliary catalysts: Solid acids can catalyze the air oxidation of NO to produce HNO3; theoretically, they can be used as auxiliary catalysts to improve oxidation selectivity. The process of producing glycolic acid via nitric acid oxidation of glyoxal was studied using solid acids such as vanadium pentoxide (VOs) and cerium dioxide (CeO). Experimental results show that in the process of producing glyoxalic acid by nitric acid oxidation of glyoxal, vanadium pentoxide can increase the yield of glyoxalic acid by 5.3%; however, due to its solubility in the reaction mixture, its separation and recovery are rather complicated. Cerium dioxide can increase the glyoxalic acid yield by 6.3; since it is insoluble in the reaction mixture, it can be separated by filtration after the reaction is complete, making it suitable for industrial application. Solid acids used as auxiliary catalysts can **improve oxidation selectivity. The proposed reaction mechanism is that they catalyze the reaction of NO in the gas and liquid phases with nitric acid and oxygen to form HNO3, thereby increasing the concentration of nitrous acid HNO2 in the reaction mixture and accelerating the rate of the main reaction. (4) Metal salts complexing NO as catalysts: Some metal salts have the ability to complex NO, which increases its solubility in solution and facilitates the formation of HNO3. Theoretically, they can be used as auxiliary catalysts to enhance oxidation selectivity. Ferrous sulfate (Fe(SO)z) and copper nitrate (Cu(NO. were used respectively. )) and magnesium nitrate (Mg(NO. )) and other metal salts were investigated as auxiliary catalysts in the nitric acid oxidation of glyoxal to produce glycolic acid. Experimental results show that using metal salts as auxiliary catalysts in the nitric acid oxidation of glyoxal to produce glycolic acid can increase the oxidation selectivity by 1.3–4.3, with the order of effectiveness being Cu(NO3)2 > Fe(SO4)2 > Mg(NO3)2. The possible mechanism by which metal salts act as auxiliary catalysts to improve selectivity is that they can complex with NO to form unstable metal salts, thereby increasing the solubility of NO in the solution to some extent and facilitating the formation of HNO3 donors at higher concentrations. The order of strength of action is determined by the stability of the complexes; copper ions form the most stable complexes, while magnesium ions form the least stable complexes. Due to the large amount of oxalic acid generated in the later stages of the oxidation reaction, metal ions are converted into oxalate precipitates, the complex is destroyed, and the yield of glyoxalic acid does not increase significantly; further research is needed to improve this situation. (5) Conclusion The comprehensive results show that the yield of glyoxylic acid is 73.9 when sodium nitrite is used as a catalyst ； Inorganic acid-assisted catalysts can **increase the solubility of NO, thereby raising the concentration of HNO3 in the solution; under the presence of hydrochloric acid, the yield of glyoxalic acid is 84.6% ； Solid acid-assisted catalysts can accelerate the oxidation of NO and increase the HNO concentration; in the presence of cerium dioxide, the yield of glyoxalic acid reaches 80.2% ； Catalytic metal complexes used as auxiliary catalysts can increase the solubility of N2O in solution to a certain extent, which facilitates the formation of higher concentrations of HNO3; in the presence of copper nitrate, the yield of glyoxalic acid was 78.2%. 1.1.2 Ozone oxidation method (1) Maleic anhydride ozone oxidation method: In this method, maleic anhydride is dissolved in methanol or formic acid, oxygen is introduced to dilute the ozone, and the ozone oxidation reaction takes place at temperatures below 10°C. Odorants are decomposed using catalytic hydrogenation or zinc powder reduction, the hydrogenation catalyst or zinc residue is separated, hydrolysis is carried out, and the excess solvent is distilled off to obtain the final product, glyoxylic acid. In the 1990s, the Austrian company Linz built a plant capable of producing glycolic acid using the maleic anhydride ozone oxidation method, with a yield of over 909/6; the by-products were carbon dioxide and a small amount of oxalic acid. (2) Direct ozone oxidation of maleic acid aqueous solution: Adipic anhydride readily deliquesces upon contact with water to form maleic acid. In 1903, Harries first synthesized glyoxalic acid by directly oxidizing it with ozone in an aqueous maleic acid solution; in this reaction, the utilization rate of maleic acid was only 50%. Harries did not measure the amount of ozone absorbed in the reaction, making it impossible to determine whether the reaction proceeded completely; as a result, the yield was very low, and the product could only be obtained in the form of benzohydrazide. In 1966, Black et al. passed oxygen with a volume fraction of 4 through an aqueous maleic acid solution with a mass fraction of about 40 at 15–25°C. /O ozonation is carried out until the reaction is complete; thereafter, reduced pressure distillation is performed at 50°C to remove the formic acid produced as a by-product and most of the solvent water. The residue is then crystallized at 20°C to yield the monohydrate of glyoxalic acid. By measuring the aldehyde content and performing elemental analysis, it was found that the glyoxalic acid yield is close to 50% (calculated based on 2 mol of glyoxalic acid being produced per 1 mol of maleic acid), with a product purity of over 97%. The product quality of this route is good, with a yield as high as 93% and a conversion rate of maleic acid of 100%. However, the reaction conditions are too stringent; it requires a reactor operating at low temperatures of 40–45°C under high pressure. The by-product formic acid corrodes the equipment, resulting in high investment costs and high electricity consumption. In addition, the generation and treatment of ozone present difficulties. (3) Ozonolysis in organic solvents followed by reagent reduction: In 1964, Thompson et al. used butyl maleate at -40°C. After ozonation in C, it is rapidly reduced at low temperature using hypophosphite to yield butyl glyoxylate, with a yield of 78%. In 1966, Pappas et al. used methanol as a solvent at -50°C. Malic acid is ozonized in C, then reduced with dimethyl sulfide at 2°C; the yield of glyoxalic acid was 91% (determined by 2,4-dinitrophenylhydrazine). Furthermore, they also attempted to use diethyl maleate and dimethyl maleate as starting materials to obtain the corresponding glycolic acid esters, which were then hydrolyzed to yield glycolic acid, but there was no significant increase in yield. Since dimethyl sulfide and phosphites are both expensive reducing agents and difficult to separate from glyoxalic acid, they cannot be used for large-scale production. Subsequently, Pryde was mixed with ethyl acetate in a volume ratio of 1:4, and a small amount of pyridine was added; the latter, together with an equal amount of a mixture of fatty alcohols and fatty acids, further increased the yield of glyoxalic acid. However, product separation and solvent recovery remain very difficult. In 1972, Callghan et al. prepared solutions of maleic acid along with propyl or ethyl acetate at 5. O4 is introduced on both the left and right sides of C. /Ozeotrope to complete the reaction, then pass O to wash away excess O. Then, keep the temperature below 30°C and pass SO to reduce the peroxides. After the reaction is complete, a quantitative amount of CaO is added; the resulting CaSO4 precipitate is separated, after which a small amount of water is added at 50°C. The solvent was removed by vacuum distillation under reduced pressure to yield glyoxylic acid monohydrate in a yield of over 95%. This method is simple and practical, but SO and SO. It causes severe corrosion of equipment and also leads to environmental pollution. Literature reports on the relationship between yield and ozonation temperature: reducing the temperature of the ozonation reaction increases the yield, but with the reagent reduction method, maintaining the temperature at -10 to 5°C is sufficient to meet production requirements. (4) Ozonolysis-catalytic hydrogenation method: In 1972, Callighan and others first used the ozonolysis-catalytic hydrogenation method to synthesize glyoxalic acid; they used methanol as a solvent at -50°C. In C, maleic acid is oxidized with ozone, the residual ozone is blown out using nitrogen, and then the temperature is maintained below 15°C, using 5% (by mass, the same hereafter) Pd/Al. O. Used as a catalyst, it catalyzes the hydrogenation of ozonation products at atmospheric pressure; the catalyst is then separated by filtration, and semi-acetal of glyoxalic acid is obtained through vacuum distillation, with a yield as high as 95%. After hydrolyzing the hemiacetal and distilling off methanol, an aqueous solution of glyoxylic acid is obtained. In this process, the yield is highly dependent on the amount of catalyst used; 0.5 g of catalyst is required for 1 mol of maleic acid. If the amount of catalyst is reduced, the yield drops sharply, and no method for recycling the catalyst is proposed in this process. In 1988, Arashiba and others improved this process by adding certain antioxidants during the catalytic hydrogenation step, thereby significantly enhancing the selectivity and yield of the reaction. These antioxidants include certain quinones, phenols, and aromatic amines, as well as some organic sulfur and organic phosphine compounds. In 1991, Alexander and others made further improvements to the process: using cation exchange resin as a catalyst, maleic anhydride was first esterified with ethanol, n-butanol, or n-decanol, followed by ozonation in the presence of excess methanol. After removing the residual ozone with nitrogen, a PL or Pd/C catalyst was added for catalytic hydrogenation reduction, yielding glyoxylate hemiacetals in high yield; these were then hydrolyzed and distilled to obtain glyoxylic acid solution. By esterifying the maleic acid first, the ozonation temperature was increased to 0–4°C and the reduction temperature to 30–40°C, which extended the catalyst’s lifespan and significantly improved the operating conditions, while the yield remained above 95%. The company in Linz, Austria, uses this process. (5) Problems in the synthesis of glyoxylic acid by ozone oxidation: ① Control of reaction temperature. The ozone oxidation step requires strict low temperatures, which poses difficulties for production. If maleic acid is first esterified with medium- to long-chain primary alcohols, the reaction temperature can be raised above 0°C, but this adds complexity to the subsequent processing. ②Selection of solvent. The selected solvent must not only be able to stabilize the ozonation intermediates but also must have no toxic effect on the catalyst during the catalytic reduction process. At the same time, since glyoxalic acid is prone to decomposition, the solvent must be able to be effectively separated during post-treatment. ⑧Selection of the reducing agent. Choosing a good reducing agent is the key to the entire experiment. After comprehensive comparison, a catalytic hydrogenation reducing agent is a better choice, as hydrogen is inexpensive and readily available, does not contaminate the product, and simplifies downstream processing. However, such catalysts are expensive and prone to poisoning; therefore, it is necessary to further develop new catalysts and additives to improve their resistance to poisoning and selectivity. ④Methods for the separation and purification of products. The separation of glyoxalic acid from by-products is a common challenge in all methods for producing glyoxalic acid. At present, there are no effective methods for this purpose, either domestically or internationally; therefore, it is highly necessary to conduct research on new processes for separation and purification. 1.1.3 Other oxidation methods (1) Oxygen oxidation of glyoxal: This method is simple and rapid, representing a new approach for the synthesis of glycolic acid. Compared with the nitric acid method widely used both domestically and internationally, oxygen offers advantages such as simplicity and speed, high selectivity, high yield and quality, ease of post-treatment, reduced costs, minimal pollution, mild reaction conditions, low energy consumption, stable reactions, and excellent product quality. This reaction is a gas-liquid two-phase reaction, and low-pressure oxygen can be used to control the degree of oxidation of glyoxal, thereby increasing the yield of glyoxylic acid. Inorganic acids, which do not require separation, can be used in the synthesis of allantoin; it is a good new method worthy of widespread application. (2) Oxidation of glyoxal with hydrogen peroxide: This method overcomes the problems associated with nitric acid and chlorination oxidation methods, such as difficulty in control, severe corrosion, and high pollution. The key issue that needs to be addressed at present is the catalyst; the optimal amount of catalyst required is as much as 50% of the amount of glyoxal, which hinders the industrialization of this process. (3) Oxidation of glyoxal by chlorine gas: This reaction exhibits high selectivity and conversion rates; the selectivity is typically at least 80% and the conversion rate is at least 90%. However, for every 1 mol of glyoxalic acid produced, 2 moles of hydrogen chloride are generated as a by-product. This hydrogen chloride affects the reaction rate, and as its concentration in the solution increases, the reaction rate drops sharply. Moreover, the product contains a high level of hydrochloric acid, which is corrosive to equipment; since it is a gas-liquid reaction, pressure vessels are required, and the reaction time is long. (4) Catalytic oxidation by glycolate oxidase: This process developed by the American company DuPont involves the reaction of a 10% glycolate solution with air or pure oxygen at 1.5 MPa, at 515°C, in the presence of glycolate oxidase, hydrogen peroxide enzyme, flavin mononucleotide catalyst, and ethylenediamine, to produce glyoxalic acid. This process is simple; the product can be separated easily, it is pollution-free, the product purity is high, the yield is good, and the quality is excellent. However, it requires high-pressure equipment, and the reaction time for a 2L solution is as long as 77 hours. This method is one of the most actively researched synthesis methods in recent years, but its drawback is the excessively long reaction time. (5) Dichloroacetate: The reaction conditions are mild and easy to control; when the amount of organic acid used is twice that of dichloroacetate, the yield can reach 90%. Disadvantages: Since the raw materials are often mixtures of monohalides and dihalides and are difficult to purify, this severely affects the yield and quality of the product. Moreover, the reaction produces halogenated acids, making their separation and purification challenging. However, this method yields high product yields, but it features a complex process route, long reaction times, numerous operational steps, high production costs, and inconsistent product quality. (6) Sodium hypochlorite oxidation: An 849,6 NaClO solution was added to a mixture containing 7.59,6 glyoxal and 9.5% hydrochloric acid; after a reaction time of 3 hours, the conversion rate of glyoxal was 70%, with a selectivity for glycolic acid of 44%. This method involves a large volume of reaction solution, substantial amounts of wastewater to be treated, unstable sodium hypochlorite, a very low yield of product, and the salts present in the product affect its quality. (7) Metal-catalyzed air oxidation: In recent years, Japan, Germany, and France have studied the process of producing glyoxalic acid through air oxidation using metals as catalysts. Mitsui East Asia Chemical Company in Japan used Pt as the main catalyst; in a 1-glyoxal solution, the conversion rate was 95.4, and the selectivity was 82.19, 5%. Nippon Synthetic Chemical Industry Company used Pt, Pd, or their compounds as catalysts (with a content of 20.4–50.4%) to oxidize 59.6% glyoxal solution; the conversion rate of glyoxal was 80%, the selectivity for glycolic acid was 65%, and the selectivity for oxalic acid was 17%. The French company Hoechst converted the substance at slightly above room temperature, using a Pt/C catalyst, achieving a conversion rate of 85.5% and a yield of 70%. The German company Degussa used a composite catalyst consisting of 2.5% Pt, 59.6% Mo, and C, achieving a conversion rate of 90% and a selectivity of 68%. The metal-catalyzed air oxidation of glyoxal proceeds under mild conditions, has a short process flow, is easy to operate, and causes no pollution. However, an expensive catalyst is required. By selecting appropriate primary and secondary catalysts, carriers, and reaction conditions, it is possible to further improve the reaction’s conversion rate and selectivity; this method is worth further investigation. (8) Acrylic acid pyrolysis oxidation: The acrylic acid pyrolysis oxidation process developed in Japan involves adding a HO solution to acrylic acid and CH. In the CN and chromium-substituted silicate mixture, stirring was carried out at 40°C for 8 hours. The acrylate conversion rate is 98, the selectivity for glyoxalic acid reaches 49, and the catalysts include Ag, etc. The reaction conditions are mild, and the product quality is high. 1.2 Electrochemical synthesis methods 1.2.1 Electroreductive reduction of oxalic acid This method is typically carried out in an electrolytic cell, where, under the action of direct current, a saturated solution of oxalic acid in the cathode chamber is electroreduced to yield glyoxalic acid and water. The product is discharged when the mass fraction of glyoxalic acid reaches 4 or higher; after evaporation, concentration, freezing, and filtration, a glyoxalic acid product with a mass fraction of 20–50% is obtained. The electrolytic reduction of oxalic acid has attracted extensive research at home and abroad due to its advantages such as simple process, inexpensive raw materials, and alignment with the trends toward green chemistry in organic electrochemistry. Wang Shuo and Wu Sufang from Zhejiang University, in collaboration with the Institute of Chemical Reaction Engineering, studied the process and techniques for the continuous electrolytic reduction of oxalic acid to produce glyoxalic acid. Continuous electrolysis experiments were conducted using an ion-exchange membrane electrolyzer with one membrane and two chambers and an electrolysis area of 10 cm×10 cm. The electrode materials and the cation exchange membrane materials were compared, and the effect of electrolyte flow rate on current efficiency, as well as the effects of current density, electrolysis temperature, and electrolysis time on the electrolysis process, were studied. The results show ； (1) The effect of plate materials on electrolysis was experimentally studied, comparing the current efficiency and yield when lead plates and stainless steel plates were used as cathode plates, respectively, and when lead plates and tantalum-iridium electrodes were used as anode plates; it was found that using tantalum-iridium as the anode plate and lead as the cathode plate yielded better results. (2) The experiment compared the effects of different ion exchange membranes on electrolysis, concluding that the fluorosulfonic acid cation exchange membrane NF-1 yielded better electrolytic results than the polyvinylidene fluoride homogeneous membrane HF-101. (3) On the preferred electrodes and ion exchange membranes, the effects of flow rate, current density, and electrolysis time on the electrolysis process were studied. It was found that due to the adsorption of reactants and products on the cathode surface as well as the competitive nature of surface primary and secondary reactions, there is an optimal value for the flow rate during electrolysis. The optimal electrolysis conditions were determined to be: current density of 1000 A•m⁻², electrolysis temperature of 25°C, electrolysis time of 4.5 hours, and flow rate of 2 L•min⁻¹. In the past, due to the fact that the degree of electrolysis during the electrolytic process was influenced by the concentration of the electrolyte, the concentration of glycolic acid produced was not high, remaining below 8. Liu Shisong from the School of Chemical Engineering at Shijiazhuang University studied the electrolytic reaction of oxidizing oxalic acid into glycolic acid using SPE electrodes at room temperature, and also conducted research on the preparation of such electrodes as well as their reusability. Experiments show that electrolysis using SPE electrodes can achieve high yields; further purification is possible through heating and cooling for crystallization. This approach effectively addresses the problems associated with conventional electrodes. Moreover, in SPE electrodes, the H+ ions produced during anodic electrolysis combine directly with oxalic acid without interacting much with the electrolyte, thereby saving a significant amount of electrical energy. Hence, the use of SPE electrodes for the electrolytic conversion of oxalic acid into glyoxalic acid holds industrial application value. 1.2.2 Glyoxal anodization method: The electro-oxidation using glyoxal offers the advantage of electrolytic synthesis; electricity is used in place of oxidizing and reducing agents, which reduces pollution and lowers energy and raw material consumption. However, glyoxal is prone to further oxidation into oxalic acid and carbon dioxide, resulting in a low yield. The unreacted glyoxal is difficult to handle, which severely affects the quality of downstream products. Moreover, glyoxal is an expensive raw material; it is generally used in combination with cathodic electrochemical reduction of oxalic acid to facilitate bipolar chamber paired electrolysis. By comparing the advantages and disadvantages of the oxalic acid electrolytic reduction method and the glyoxal nitric acid oxidation method in terms of cost and other factors, it is concluded that the oxalic acid electrolytic reduction method uses inexpensive raw materials and causes no pollution, but it is only suitable for small-scale production. 1.2.3 Diaphragm-free oxalic acid electrolytic reduction method using soluble anodes: In the diaphragm-free oxalic acid electrolytic reduction method that uses metal zinc and aluminum as soluble anodes, glyoxalic acid is formed at the anode, while zinc oxalate is generated through the dissolution of the anode. The disadvantage of this process is the excessive production of zinc oxalate as a by-product, low utilization efficiency of raw materials, and high consumption of oxalic acid. For every 1 ton of glyoxalic acid (in 40% aqueous solution) produced, 1.4 tons of zinc oxalate are generated as a by-product; since zinc oxalate has limited industrial applications, it needs to be converted and utilized in order to reduce the production costs of glyoxalic acid. 1.2.4 Electroreductive reduction of oxalic acid using an insoluble anode without a diaphragm: In this method, acetaldehyde acid is formed at the cathode, while oxygen is primarily released at the anode, with some of the oxalic acid being decomposed. The current efficiency is 70%, the chemical yield of glyoxalic acid is 80%, and power consumption is reduced by 25%. The production process and equipment are simplified, but controlling the side reaction of anode decomposition into oxalic acid remains a major issue, making it difficult to meet the requirements for large-scale production. 1.2.5 Fixed-bed electrolyzers and variable-current electrolysis for glyoxal: The current efficiency of fixed-bed electrolyzers and variable-current electrolysis methods for glyoxal is low, and the current density is small (i.e., the production capacity of the electrolyzer is limited). To date, there have been no reports on the simultaneous use of fixed-bed electrolyzers and variable-current electrolysis in the process of electro-oxidizing glyoxal to produce glycolic acid. Electrochemical synthesis of glyoxalic acid is considered the most promising and competitive production route for glyoxalic acid, attracting significant attention from researchers both domestically and internationally. In particular, research on the paired electrolytic synthesis of glyoxalic acid and the membraneless electrolytic synthesis of glyoxalic acid from oxalic acid, as well as the development of catalyst electrodes, is ongoing and advancing continuously. 1.3 Comparison of synthesis methods The glyoxal nitric acid oxidation method, cyclohexanone oxide ozone oxidation method, and oxalic acid electrolytic reduction method are the three methods available for industrial application; a comparison of these three methods is shown in Table 1. Table 1 Comparison of industrial production methods for glyoxalic acid: Oxidation of glyoxal with nitric acid, Oxidation of maleic anhydride with ozone, Electrochemical reduction of oxalic acid. Advantages: Mature process, mild conditions, low investment required; raw materials are inexpensive and readily available, environmental pollution is minimal, product quality is good, solid products can be obtained, and large-scale production is feasible. Abundant and cheap raw materials, simple operation, mild reaction conditions, environmentally friendly with no pollution, and few by-products. Disadvantages: Complex operation, high solvent loss, severe equipment corrosion, significant environmental pollution, and low product purity. High technical requirements, huge equipment investment, high electricity consumption, and high production costs. High electricity consumption, low product purity, unstable quality, as well as high investment and production costs. Scope of application: Suitable for large-scale production in areas with abundant electrical resources. Current status: It is a widely used process route, but due to its disadvantages, it is gradually being phased out. Although it is the most advanced technology, due to its high technical requirements and substantial investment costs, as well as considering China’s overall situation, its application in the country is not easy; it represents a green growth path with great potential for development

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