Method for preparing maleic acid through catalytic oxidation of 5-hydroxymethylfurfural

Abstract

The invention provides a method for preparing maleic acid by catalytic oxidation of 5-hydroxymethylfurfural, which adopts a carbon nitride nonmetal catalyst, oxygen or air as an oxidant and water as a solvent to realize efficient catalytic oxidation of 5-hydroxymethylfurfural to prepare maleic acid. The method has the advantages of low catalyst cost, simple synthesis, green and environment-friendly reaction system and the like.

Description

Technical Field

[0001] The present invention relates to a method for catalytic oxidation of 5-hydroxymethylfurfural to prepare maleic acid. Background Art

[0002] Maleic acid (MAc) and maleic anhydride (MA), also known as malic acid and maleic anhydride, have important applications in industry and are versatile raw materials for synthesizing products such as unsaturated polyester resins, pharmaceuticals, agrochemicals, lubricant additives, and polymers. Currently, the industrial production of MAc or MA mainly uses benzene, butene, and butane as raw materials, and these raw materials are mainly produced in coal chemical industry and petrochemical industry. In order to slow down the exploitation and consumption of fossil fuels, the research on using green biomass resources to prepare MAc or MA has received extensive attention. 5-Hydroxymethylfurfural (HMF) is a platform molecule for the value-added conversion of biomass raw materials. Due to the presence of a furan ring, an aldehyde group, and a hydroxyl group, it can undergo oxidation, hydrogenation, ring-opening, and polymerization reactions to further prepare a variety of fine chemicals and high molecular polymers, including MAc and MA. For example, CN104119305A discloses a method for catalytic selective oxidation of HMF to prepare MA using a bismuth compound as a catalyst and molecular oxygen as an oxidant, as follows: 0.32 g of HMF, 2 mL of acetic acid, and bismuth phosphate are added to a high-temperature and high-pressure reaction kettle, and 0.8 MPa of oxygen is introduced into the kettle. The reaction is carried out at 80 °C for 6 h, and the yield of MA is 50.7%. CN112645908A also discloses a method for synthesizing MA using HMF. 0.1 mmol of HMF and 10 mg of CuO are dispersed in 1 mL of acetonitrile, sealed after being fully replaced with oxygen, and stirred and reacted under illumination at 9 W and 455 nm at room temperature for 12 h. The conversion rate of HMF is 60%, and the yield of MA is 45%. However, organic solvents are added in these catalytic reactions, making these reaction systems fail to meet the requirements of green environmental protection. Therefore, the development of a new catalytic system is beneficial to solve the current problems such as low catalytic efficiency and environmental unfriendliness. For this reason, the present invention designs and develops a method for catalytic oxidation of 5-hydroxymethylfurfural to prepare maleic acid. This method uses a carbon nitride non-metal catalyst, with oxygen or air as an oxidant and water as a solvent, realizing the efficient catalytic oxidation of 5-hydroxymethylfurfural to prepare maleic acid, not only reducing the catalyst cost, but also making the reaction system green and environmentally friendly. Summary of the Invention

[0003] The purpose of the present invention is to provide a green, environmentally friendly, and efficient method for catalytic oxidation of 5-hydroxymethylfurfural to prepare maleic acid.

[0004] The present invention provides a method for catalytically oxidizing 5-hydroxymethylfurfural to prepare maleic acid, and the preparation conditions and steps of the catalyst are as follows: (1) Weigh 9 g of urea as a raw material, grind it and place it in a graphite crucible, put it into a tubular furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tubular furnace from room temperature to 450-550 °C at a heating rate of 5 °C / min, and maintain it for 1-5 hours. Then, the tubular furnace is naturally cooled, and the solid product in the crucible is taken out and ground to obtain a 1-g-CN-X-Y (X - calcination temperature, unit: °C; Y - calcination time, unit: hour) catalyst; (2) Weigh 9 g of cyanuric acid as a raw material, grind it and place it in a graphite crucible, put it into a tubular furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tubular furnace from room temperature to 475-525 °C at a heating rate of 5 °C / min, and maintain it for 1-4 hours. Then, the tubular furnace is naturally cooled, and the solid product in the crucible is taken out and ground to obtain a 2-g-CN-X-Y (X - calcination temperature, Y - calcination time) catalyst.

[0005] The present invention provides a method for catalytically oxidizing 5-hydroxymethylfurfural to prepare maleic acid, and the catalytic reaction conditions and steps are as follows: Weigh a certain amount of catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 0.5-1.5 MPa of oxygen or air. Start stirring, heat the reaction kettle to 80-90 °C and keep it stable, and react for 1-5 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate to determine the conversion rate of 5-hydroxymethylfurfural and the yield of maleic acid.

[0006] The beneficial effects of the present invention: The raw materials of this catalyst are widely sourced and inexpensive. When used in the reaction of catalytically oxidizing 5-hydroxymethylfurfural to prepare maleic acid, the reaction conditions are mild, green and environmentally friendly, and the yield is relatively high.

[0007] Specific embodiments.

[0008] Example 1: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible, put it into a tubular furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tubular furnace from room temperature to 550 °C at a heating rate of 5 °C / min, and hold for 4 hours. Subsequently, the tubular furnace cools naturally, take out the solid product in the crucible and grind it to obtain the 1-g-CN-550-4 catalyst. Weigh 0.0234 g of the 1-g-CN-550-4 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the reaction liquid and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 97.53%, and the yield of maleic acid is 27.95%.

[0009] Example 2: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible, put it into a tubular furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tubular furnace from room temperature to 525 °C at a heating rate of 5 °C / min, and hold for 4 hours. Subsequently, the tubular furnace cools naturally, take out the solid product in the crucible and grind it to obtain the 1-g-CN-525-4 catalyst. Weigh 0.0234 g of the 1-g-CN-525-4 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the reaction liquid and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 99.64%, and the yield of maleic acid is 39.88%.

[0010] Example 3: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 500 °C at a heating rate of 5 °C / min, and maintain for 4 hours. Then, let the tube furnace cool naturally, take out the solid product in the crucible and grind it to obtain the 1-g-CN-500-4 catalyst. Weigh 0.0234 g of the 1-g-CN-500-4 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After closing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is over, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the reaction liquid and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 99.12%, and the yield of maleic acid is 36.07%.

[0011] Example 4: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 475 °C at a heating rate of 5 °C / min, and maintain for 4 hours. Then, let the tube furnace cool naturally, take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-4 catalyst. Weigh 0.0234 g of the 1-g-CN-475-4 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After closing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is over, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the reaction liquid and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 96.20%, and the yield of maleic acid is 48.67%.

[0012] Example 5: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. At a heating rate of 5 °C / min, heat the tube furnace from room temperature to 450 °C and hold for 4 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-450-4 catalyst. Weigh 0.0234 g of the 1-g-CN-450-4 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Then add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve the pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 99.42%, and the yield of maleic acid is 25.40%.

[0013] Example 6: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. At a heating rate of 5 °C / min, heat the tube furnace from room temperature to 475 °C and hold for 1 hour. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Then add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve the pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 98.40%, and the yield of maleic acid is 54.68%.

[0014] Example 7: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and then continuously introduce nitrogen. Heat the tube furnace from room temperature to 475 °C at a heating rate of 5 °C / min and hold for 2 hours. Subsequently, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-2 catalyst. Weigh 0.0234 g of the 1-g-CN-475-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Then add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen and then fill it with 1 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 95.77%, and the yield of maleic acid is 48.33%.

[0015] Example 8: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and then continuously introduce nitrogen. Heat the tube furnace from room temperature to 475 °C at a heating rate of 5 °C / min and hold for 3 hours. Subsequently, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-3 catalyst. Weigh 0.0234 g of the 1-g-CN-475-3 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Then add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen and then fill it with 1 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 97.13%, and the yield of maleic acid is 49.21%.

[0016] Example 9: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and then continue to continuously introduce nitrogen. With a heating rate of 5 °C / min, heat the tube furnace from room temperature to 475 °C and hold for 5 hours. Subsequently, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-5 catalyst. Weigh 0.0234 g of the 1-g-CN-475-5 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Then add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring, and heat the reaction kettle to 85 °C and keep it stable for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 96.00%, and the yield of maleic acid is 42.10%.

[0017] Example 10: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and then continue to continuously introduce nitrogen. With a heating rate of 5 °C / min, heat the tube furnace from room temperature to 475 °C and hold for 1 hour. Subsequently, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Then add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 0.5 MPa of oxygen. Start stirring, and heat the reaction kettle to 85 °C and keep it stable for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 99.61%, and the yield of maleic acid is 46.45%.

[0018] Example 11: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible, put it into a tubular furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tubular furnace from room temperature to 475 °C at a heating rate of 5 °C / min, and hold for 1 hour. Then, let the tubular furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1.5 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the reaction liquid and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 94.85%, and the yield of maleic acid is 37.75%.

[0019] Example 12: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible, put it into a tubular furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tubular furnace from room temperature to 475 °C at a heating rate of 5 °C / min, and hold for 1 hour. Then, let the tubular furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, fill it with 0.5 MPa of air. Start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the reaction liquid and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 99.71%, and the yield of maleic acid is 38.78%.

[0020] Example 13: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. Heat the tube furnace from room temperature to 475 °C at a heating rate of 5 °C / min and hold for 1 hour. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Add 10 mL of water. After sealing the reaction kettle, fill it with 1 MPa of air, turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve the pressure. Filter and separate the liquid after the reaction and the catalyst solid. Perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 99.67%, and the yield of maleic acid is 40.88%.

[0021] Example 14: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. Heat the tube furnace from room temperature to 475 °C at a heating rate of 5 °C / min and hold for 1 hour. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Add 10 mL of water. After sealing the reaction kettle, fill it with 1.5 MPa of air, turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve the pressure. Filter and separate the liquid after the reaction and the catalyst solid. Perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 98.03%, and the yield of maleic acid is 41.35%.

[0022] Example 15: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 475 °C at a heating rate of 5 °C / min and hold for 1 hour. Subsequently, the tube furnace cools naturally, take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring, heat the reaction kettle to 80 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and depressurize, filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 93.92%, and the yield of maleic acid is 40.89%.

[0023] Example 16: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 475 °C at a heating rate of 5 °C / min and hold for 1 hour. Subsequently, the tube furnace cools naturally, take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring, heat the reaction kettle to 90 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and depressurize, filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 96.42%, and the yield of maleic acid is 57.34%.

[0024] Example 17: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible and put it into a tubular furnace. Replace the air in the tube with nitrogen for 10 min, and then continuously introduce nitrogen. At a heating rate of 5 °C / min, heat the tubular furnace from room temperature to 475 °C and keep it for 1 hour. Then, let the tubular furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring. After heating the reaction kettle to 85 °C and stabilizing, react for 1 hour. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the liquid after the reaction and the catalyst solid. Perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 81.77%, and the yield of maleic acid is 21.23%.

[0025] Example 18: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible and put it into a tubular furnace. Replace the air in the tube with nitrogen for 10 min, and then continuously introduce nitrogen. At a heating rate of 5 °C / min, heat the tubular furnace from room temperature to 475 °C and keep it for 1 hour. Then, let the tubular furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring. After heating the reaction kettle to 85 °C and stabilizing, react for 2 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the liquid after the reaction and the catalyst solid. Perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 92.29%, and the yield of maleic acid is 36.05%.

[0026] Example 19: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 475 °C at a heating rate of 5 °C / min and hold for 1 hour. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, then fill it with 1 MPa of oxygen, turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 3 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve the pressure, filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 95.96%, and the yield of maleic acid is 42.21%.

[0027] Example 20: Weigh 9 g of urea as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 475 °C at a heating rate of 5 °C / min and hold for 1 hour. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 1-g-CN-475-1 catalyst. Weigh 0.0234 g of the 1-g-CN-475-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, then fill it with 1 MPa of oxygen, turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 5 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve the pressure, filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 98.63%, and the yield of maleic acid is 54.69%.

[0028] Example 21: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 475 °C at a heating rate of 5 °C / min, and hold for 4 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-475-4 catalyst. Weigh 0.0234 g of the 2-g-CN-475-4 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the reaction liquid and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 97.94%, and the yield of maleic acid is 42.09%.

[0029] Example 22: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 500 °C at a heating rate of 5 °C / min, and hold for 4 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-500-4 catalyst. Weigh 0.0234 g of the 2-g-CN-500-4 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the reaction liquid and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 94.97%, and the yield of maleic acid is 46.30%.

[0030] Example 23: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 525 °C at a heating rate of 5 °C / min and hold for 4 hours. Subsequently, the tube furnace cools naturally, take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-4 catalyst. Weigh 0.0234 g of the 2-g-CN-525-4 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After closing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 98.04%, and the yield of maleic acid is 48.90%.

[0031] Example 24: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 525 °C at a heating rate of 5 °C / min and hold for 1 hour. Subsequently, the tube furnace cools naturally, take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-1 catalyst. Weigh 0.0234 g of the 2-g-CN-525-1 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After closing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 94.07%, and the yield of maleic acid is 55.30%.

[0032] Example 25: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. At a heating rate of 5 °C / min, heat the tube furnace from room temperature to 525 °C and hold for 2 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve the pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 93.81%, and the yield of maleic acid is 73.06%.

[0033] Example 26: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. At a heating rate of 5 °C / min, heat the tube furnace from room temperature to 525 °C and hold for 3 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-3 catalyst. Weigh 0.0234 g of the 2-g-CN-525-3 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve the pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 97.78%, and the yield of maleic acid is 53.46%.

[0034] Example 27: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 525 °C at a heating rate of 5 °C / min and hold for 2 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 0.5 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the reaction liquid and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 98.10%, and the yield of maleic acid is 36.69%.

[0035] Example 28: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 525 °C at a heating rate of 5 °C / min and hold for 2 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, displace the air in the kettle with oxygen, and then fill it with 1.5 MPa of oxygen. Start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the reaction liquid and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 93.89%, and the yield of maleic acid is 71.40%.

[0036] Example 29: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 525 °C at a heating rate of 5 °C / min and hold for 2 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, fill it with 0.5 MPa of air, start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 99.68%, and the yield of maleic acid is 32.71%.

[0037] Example 30: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 525 °C at a heating rate of 5 °C / min and hold for 2 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, fill it with 1 MPa of air, start stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure, filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 99.25%, and the yield of maleic acid is 49.01%.

[0038] Example 31: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. At a heating rate of 5 °C / min, heat the tube furnace from room temperature to 525 °C and hold for 2 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Then add 10 mL of water. After sealing the reaction kettle, fill it with air at 1.5 MPa, start stirring, and heat the reaction kettle to 85 °C and keep it stable for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the reaction liquid and the catalyst solid. Perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 99.18%, and the yield of maleic acid is 49.92%.

[0039] Example 32: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. At a heating rate of 5 °C / min, heat the tube furnace from room temperature to 525 °C and hold for 2 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Then add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with oxygen at 1 MPa. Start stirring, and heat the reaction kettle to 80 °C and keep it stable for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the reaction liquid and the catalyst solid. Perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 93.31%, and the yield of maleic acid is 63.19%.

[0040] Example 33: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and then continuously pass nitrogen. At a heating rate of 5 °C / min, heat the tube furnace from room temperature to 525 °C and hold for 2 hours. Subsequently, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Then add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Turn on the stirring, heat the reaction kettle to 90 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 95.34%, and the yield of maleic acid is 75.17%.

[0041] Example 34: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and then continuously pass nitrogen. At a heating rate of 5 °C / min, heat the tube furnace from room temperature to 525 °C and hold for 2 hours. Subsequently, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Then add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 1 hour. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 83.26%, and the yield of maleic acid is 27.96%.

[0042] Example 35: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. At a heating rate of 5 °C / min, heat the tube furnace from room temperature to 525 °C and keep it for 2 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring. After heating the reaction kettle to 85 °C and stabilizing it, react for 2 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the liquid after the reaction and the catalyst solid. Perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 89.15%, and the yield of maleic acid is 41.83%.

[0043] Example 36: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. At a heating rate of 5 °C / min, heat the tube furnace from room temperature to 525 °C and keep it for 2 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle. Add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Start stirring. After heating the reaction kettle to 85 °C and stabilizing it, react for 3 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve pressure. Filter and separate the liquid after the reaction and the catalyst solid. Perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 92.87%, and the yield of maleic acid is 52.32%.

[0044] Example 37: Weigh 9 g of cyanuric acid as the raw material. After grinding, place it in a graphite crucible and put it into a tube furnace. Replace the air in the tube with nitrogen for 10 min, and continue to continuously introduce nitrogen. Heat the tube furnace from room temperature to 525 °C at a heating rate of 5 °C / min and hold for 2 hours. Then, let the tube furnace cool naturally. Take out the solid product in the crucible and grind it to obtain the 2-g-CN-525-2 catalyst. Weigh 0.0234 g of the 2-g-CN-525-2 catalyst, 0.0952 g of NaOH, and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 5 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve the pressure. Filter and separate the liquid after the reaction and the catalyst solid, and perform high-performance liquid chromatography analysis on the filtrate. The conversion rate of 5-hydroxymethylfurfural is 97.87%, and the yield of maleic acid is 23.32%.

[0045] Example 38: Weigh 0.0952 g of NaOH and 0.1 g of 5-hydroxymethylfurfural and place them in a reaction kettle, and add 10 mL of water. After sealing the reaction kettle, replace the air in the kettle with oxygen, and then fill it with 1 MPa of oxygen. Turn on the stirring, heat the reaction kettle to 85 °C and keep it stable, and react for 4 hours. After the reaction is completed, place the reaction kettle in an ice-water bath to cool and relieve the pressure, and perform high-performance liquid chromatography analysis on the liquid. The conversion rate of 5-hydroxymethylfurfural is 94.92%, and the yield of maleic acid is 0.00%.

Claims

1. A method for catalytically oxidizing 5-hydroxymethylfurfural to prepare maleic acid, characterized in that: In an aqueous NaOH solution, using 5-hydroxymethylfurfural as the raw material and oxygen or air as the oxidant, 5-hydroxymethylfurfural is catalytically oxidized to maleic acid by using a carbon nitride catalyst. The pressure of the oxygen or air is 0.5 - 1.5 MPa, the reaction temperature is 80 - 90 °C, and the reaction time is 1 - 5 hours.

2. The method for catalytic oxidation of 5-hydroxymethylfurfural to prepare maleic acid according to claim 1, characterized in that The preparation method of the carbon nitride catalyst is as follows: Weigh a certain mass of urea as the raw material, grind it and place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 450 - 550 °C at a heating rate of 5 °C / min and keep it for 1 - 5 h. Then, the tube furnace is naturally cooled, and the solid product in the crucible is taken out and ground to obtain the catalyst.

3. The method for preparing maleic acid by catalytic oxidation of 5-hydroxymethylfurfural according to claim 1, characterized in that The preparation method of the carbon nitride catalyst is as follows: Weigh a certain mass of cyanuric acid as the raw material, grind it and place it in a graphite crucible, put it into a tube furnace, displace the air in the tube with nitrogen for 10 min, continue to continuously introduce nitrogen, and heat the tube furnace from room temperature to 475 - 525 °C at a heating rate of 5 °C / min and keep it for 1 - 4 h. Then, the tube furnace is naturally cooled, and the solid product in the crucible is taken out and ground to obtain the catalyst.