A method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation

Abstract

The present invention belongs to the technical field of chemical synthesis, and provides a method for catalytic hydrogenation to prepare 3-methyl-4-aminobenzoic acid, which comprises the following steps: mixing 3-methyl-4-nitrobenzoic acid with a solvent, adjusting the pH value to alkaline while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution; pouring the 3-methyl-4-nitrobenzoic acid solution into a reaction kettle, adding a modified palladium catalyst and an auxiliary agent, displacing with nitrogen and hydrogen respectively for 2-3 times in sequence, then introducing hydrogen, controlling the pressure and temperature, and carrying out a hydrogenation reaction to obtain a reaction product; waiting for the reaction kettle to cool, filtering the reaction product, adjusting the filtrate to a weakly acidic pH value, crystallizing and drying to obtain 3-methyl-4-aminobenzoic acid. The preparation method of the present invention has simple steps, and the required product can be generated by a one-step method. At the same time, the used modified palladium catalyst can be recycled, with less generation of three wastes, realizing the coordinated development of economic benefits and environmental benefits. Moreover, the yield and purity of 3-methyl-4-aminobenzoic acid are high, and it has good application value.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical synthesis, and particularly to a method for catalytic hydrogenation to prepare 3-methyl-4-aminobenzoic acid. Background Art

[0002] 3-Methyl-4-aminobenzoic acid is an important organic intermediate, which is widely used in the pharmaceutical, pesticide and dye industries. At present, the synthesis of 3-methyl-4-aminobenzoic acid mainly includes two methods: iron powder reduction and catalytic hydrogenation reduction. Iron powder reduction produces a large amount of solid waste and wastewater, causing a burden on the environment, and is also accompanied by problems such as low yield and high energy consumption. Catalytic hydrogenation technology has attracted attention due to its potential in improving reaction efficiency, reducing energy consumption and reducing the generation of by-products. However, the existing catalytic hydrogenation technology still faces some challenges in the synthesis of 3-methyl-4-aminobenzoic acid, including insufficient selectivity and stability of the catalyst, and strict requirements for reaction conditions. Although some progress has been made in the development of catalysts and the optimization of reaction conditions in recent years, there is still a lack of an efficient, economical and environmentally friendly catalytic hydrogenation method for synthesizing 3-methyl-4-aminobenzoic acid in the market.

[0003] Chinese Patent CN 106831460A discloses a method for preparing 3-methyl-4-aminobenzoic acid. After adding 3-methyl-4-nitrobenzoic acid and a quaternary ammonium salt type phase transfer catalyst into a reaction device, a solvent is added and stirred evenly, and then reduced iron powder and a protonic acid are added to react to obtain a reaction solution; then the reaction device is cooled, sodium carbonate and activated carbon are added for decolorization, the pH value is adjusted to be alkaline, the iron mud is filtered, washed twice with a sodium carbonate solution, and the filtered mother liquor and the washing solution are mixed to obtain a light yellow solution; the light yellow solution is added with an acid to adjust the pH value to be slightly acidic, a white-like precipitate is precipitated, and after filtration, washing with water and drying, 3-methyl-4-aminobenzoic acid is obtained. The preparation method of this invention is simple and low-cost. However, more toxic iron mud and wastewater will be generated during the preparation process, and the product yield is only up to 90.1% at most.

[0004] Chinese Patent CN 107501106A discloses a method for catalytic hydrogenation to prepare 3-methyl-4-aminobenzoic acid. 3-Methyl-4-nitrobenzoic acid and a solvent are added into a reaction vessel to obtain a 3-methyl-4-nitrobenzoic acid solution, and the pH value of the solution is adjusted to be alkaline; the obtained alkaline aqueous solution is poured into an autoclave, a catalyst is added, and nitrogen and hydrogen are each replaced three times, hydrogen is charged for 2 minutes, and then the pressure is increased to 3.45 - 4.50 MPa with hydrogen. The reaction is carried out at a pressure of 2.50 - 4.50 MPa and a temperature of 100 - 160 °C for 1 - 8 h to obtain a reaction solution; the reaction is post-treated to obtain 3-methyl-4-aminobenzoic acid. This method has easily available raw materials, less generation of three wastes, and is environmentally friendly. However, its filtrate still needs to be added with sodium carbonate and activated carbon for decolorization, the steps are complicated, and the molar yield is only up to 90% at most.

[0005] Therefore, there is an urgent need for a new catalytic hydrogenation method to achieve the synthesis of 3-methyl-4-aminobenzoic acid with high selectivity and high yield. Summary of the Invention

[0006] Aiming at the existing technical problems, the purpose of the present invention is to provide a method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation. The preparation method of the present invention has simple steps, and the desired product can be generated by a one-step method. At the same time, the modified palladium catalyst used can be recycled, with less generation of three wastes, realizing the coordinated development of economic benefits and environmental benefits, and the 3-methyl-4-aminobenzoic acid has a high yield and high purity, and has good application value.

[0007] In order to achieve the above purpose, the technical scheme adopted by the present invention is as follows:

[0008] The present invention provides a method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation, comprising the following steps:

[0009] A1. Mix 3-methyl-4-nitrobenzoic acid with a solvent, and adjust the pH value to alkaline while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution;

[0010] A2. Pour the 3-methyl-4-nitrobenzoic acid solution obtained in step A1 into a reaction kettle, add a modified palladium catalyst and an auxiliary agent, displace with nitrogen and hydrogen 2-3 times in sequence, and then introduce hydrogen, control the pressure and temperature, and carry out a hydrogenation reaction to obtain a reactant;

[0011] A3. Wait for the reaction kettle to cool, filter the reactant obtained in step A2, adjust the filtrate to a weakly acidic pH value, crystallize and dry to obtain 3-methyl-4-aminobenzoic acid.

[0012] The reaction mechanism and function of the present invention are as follows:

[0013] 1. The present invention synthesizes a new type of modified palladium catalyst, which has good catalytic stability and high selectivity, effectively reduces the occurrence of side reactions, and the modified palladium catalyst is easy to recycle and reuse, and has a broad application prospect.

[0014] First of all, the carrier synthesized by the present invention using 1,3,5-tris(4-aminophenyl)benzene and 2,5-diethyl-1,4-phenylenedialdehyde as organic ligands has a highly ordered pore structure. Metal nanoparticles can enter the pores to improve the yield and purity of 3-methyl-4-aminobenzoic acid. At the same time, the metal fixed in the pores has high dispersion, and the catalyst can exhibit excellent stability and catalytic performance under low metal loading and mild reaction conditions.

[0015] Secondly, the metal precursor has a great influence on the adsorption performance of metal ions and the catalytic activity of the modified palladium catalyst. In this application, a specific metal precursor is selected to prepare the metal precursor solution. Chloropalladic acid and iridium chloride have good adsorption effects and excellent dispersibility compared with other metal precursors, which can improve the reaction process of nitrobenzene hydrogenation reduction.

[0016] In addition, the applicant synthesizes a bimetallic catalyst, which can improve the catalytic activity and selectivity. This may be because the iridium sites mainly strengthen the activation of 3-methyl-4-nitrobenzoic acid and the adsorption of intermediates, while the palladium sites are beneficial to hydrogen dissociation. The hydrogen and electrons on palladium can be quickly transferred to iridium, enabling the hydrogenation process to proceed to the greatest extent and achieving the preparation of 3-methyl-4-aminobenzoic acid with high activity and high selectivity.

[0017] 2. The present invention adds an amine substance as an auxiliary agent, which can effectively achieve decolorization without affecting the yield and purity of 3-methyl-4-aminobenzoic acid, simplifies the production process, and ensures the color and cleanliness of the product.

[0018] 3. The present invention strictly controls the reaction pressure and temperature to avoid a decrease in yield caused by a slow reaction rate. Further, the applicant controls the addition amounts of the modified palladium catalyst and the auxiliary agent to improve the reaction yield and purity.

[0019] 4. The present invention synthesizes 3-methyl-4-aminobenzoic acid by a one-step method, reduces the generation of intermediate products, saves energy consumption, simplifies the process, and reduces pollutant emissions.

[0020] In some embodiments, the solvent in step A1 is methanol.

[0021] In some embodiments, the mass fraction of the 3-methyl-4-nitrobenzoic acid solution in step A1 is 10-25%.

[0022] In some embodiments, the preparation method of the modified palladium catalyst in step A1 includes the following steps:

[0023] S1. Add 1,3,5-tris(4-aminophenyl)benzene and 2,5-diethyl-1,4-p-phthalaldehyde to a reaction kettle, stir, add a composite organic solvent, heat to 65-80 °C, stir, and cool to obtain a mixed solution;

[0024] S2. Under ultrasonic conditions, sequentially dropwise add deionized water and glacial acetic acid to the mixed solution obtained in step S1. After the addition is completed, ultrasonicate for 3-5 min, heat to 65-80 °C, react for 2-3 d, cool, filter, wash, dry, and grind to obtain a powder;

[0025] S3. Disperse the powder obtained in step S2 in methanol and ultrasonically treat it for 20 - 40 min to obtain a suspension for standby; disperse the composite metal precursor in methanol, stir to obtain a metal precursor solution for standby;

[0026] S4. Mix the suspension and the metal precursor solution obtained in step S3, add a methanol solution of sodium borohydride, stir for 10 - 12 h, filter, wash, and vacuum dry to obtain a modified palladium catalyst.

[0027] In some embodiments, the molar ratio of 1,3,5 - tris(4 - aminophenyl)benzene to 2,5 - diethyl - 1,4 - p - benzenedicarboxaldehyde in step S1 is 1:(1.05 - 1.2).

[0028] In some embodiments, the composite metal precursor in step S3 is palladium chloride and iridium chloride, and the molar ratio of the two is (1.2 - 1.6):1.

[0029] Preferably, the total mass of the composite metal precursor in step S3 is 2.8 - 6% of the total mass of 1,3,5 - tris(4 - aminophenyl)benzene and 2,5 - diethyl - 1,4 - p - benzenedicarboxaldehyde.

[0030] In some embodiments, the composite organic solvent in step S1 is 1,4 - dioxane and xylene, and the volume ratio of the two is (3 - 5):1, and the composite organic solvent accounts for 8 - 9% of the total mass of 1,3,5 - tris(4 - aminophenyl)benzene and 2,5 - diethyl - 1,4 - p - benzenedicarboxaldehyde.

[0031] In some embodiments, the volume ratio of deionized water to glacial acetic acid in step S2 is 1:1.5.

[0032] In some embodiments, the addition amount of the modified palladium catalyst in step A2 is 2 - 5% of the mass of 3 - methyl - 4 - nitrobenzoic acid.

[0033] In some embodiments, the auxiliary agent in step A2 is an amine substance, and the addition amount of the amine substance is 3 - 8% of the mass of 3 - methyl - 4 - nitrobenzoic acid.

[0034] In some embodiments, the amine substance is triethylamine and / or N,N - dimethylformamide.

[0035] In some embodiments, the conditions of pressure and temperature in step A2 are specifically: pressure 1.0 - 3.5 MPa, temperature 80 - 140 °C.

[0036] Preferably, the conditions of pressure and temperature in step A2 are specifically: pressure 1.5 - 3 MPa, temperature 90 - 125 °C.

[0037] In some embodiments, the yield of 3-methyl-4-aminobenzoic acid is not less than 97%, and the purity is not less than 99%.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0039] 1. The preparation method of the present invention has simple steps, and the desired product can be generated by a one-step method, reducing the generation of intermediate products. At the same time, the modified palladium catalyst used can be recycled and reused, with less generation of three wastes, achieving the coordinated development of economic benefits and environmental benefits. Moreover, the yield of 3-methyl-4-aminobenzoic acid is not less than 97%, and the purity is not less than 99%, having good application value.

[0040] 2. The present invention synthesizes a new type of modified palladium catalyst, which has good catalytic stability and high selectivity, effectively reducing the occurrence of side reactions.

[0041] 3. The present invention adds an amine substance as an auxiliary agent, which can effectively achieve decolorization without affecting the yield and purity of 3-methyl-4-aminobenzoic acid, simplifying the production process. Specific Embodiments

[0042] The present invention will be described below in conjunction with specific implementation examples. It should be noted that the following examples are examples of the present invention, only for explaining the present invention, rather than limiting the present invention. Without departing from the gist or scope of the present invention, other combinations and various improvements within the concept of the present invention can be made.

[0043] According to the raw material ratios and production methods specified in the following examples and comparative examples, 3-methyl-4-aminobenzoic acid was synthesized.

[0044] For the convenience of those skilled in the art to implement the present invention, the sources of some raw materials in the examples and comparative examples are described as follows: Unless otherwise specified, the raw materials can be purchased from the market.

[0045] Preparation Example 1

[0046] The preparation method of modified palladium catalyst A comprises the following steps:

[0047] S1. Add 0.5 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.55 mmol of 2,5-diethyl-1,4-p-benzenedicarboxaldehyde to a reaction kettle, stir for 15 min, add 19.6 ml of 1,4-dioxane and 4.8 ml of xylene, heat to 75 °C, stir until dissolved, and cool to room temperature to obtain a mixed solution;

[0048] S2. Under ultrasonic conditions, 4.8 ml of deionized water and 7.2 ml of glacial acetic acid were successively added dropwise to the mixed solution in step S1. After the addition was completed, ultrasonic treatment was carried out for 3 min, then heated to 75 °C, reacted for 3 d, cooled to room temperature, filtered, the solid was washed 3 times with toluene, vacuum dried at 75 °C for 12 h, and ground to obtain a powder;

[0049] S3. The powder obtained in step S2 was dispersed in 45 ml of methanol and ultrasonic treated for 30 min to obtain a suspension for standby; 0.03 mmol of palladium chloride and 0.02 mmol of iridium chloride were dispersed in 15 ml of methanol and stirred for 5 min to obtain a metal precursor solution for standby;

[0050] S4. The suspension and the metal precursor solution obtained in step S3 were mixed, 3 ml of a methanol solution of 0.5 mol / L sodium borohydride was added, stirred for 12 h, filtered, the solid was washed 3 times with methanol, and vacuum dried at 75 °C for 12 h to obtain the modified palladium catalyst A.

[0051] Preparation Example 2

[0052] The preparation method of the modified palladium catalyst B is the same as that of Preparation Example 1, except that the addition amount of 2,5 - diethyl - 1,4 - p - benzenedicarboxaldehyde in step S1 is 0.4 mmol.

[0053] Preparation Example 3

[0054] The preparation method of the modified palladium catalyst C is the same as that of Preparation Example 1, except for step S3:

[0055] The powder obtained in step S2 was dispersed in 45 ml of methanol and ultrasonic treated for 30 min to obtain a suspension for standby; 0.05 mmol of palladium chloride was dispersed in 15 ml of methanol and stirred for 5 min to obtain a metal precursor solution for standby.

[0056] Preparation Example 4

[0057] The preparation method of the modified palladium catalyst D is the same as that of Preparation Example 1, except for step S3:

[0058] The powder obtained in step S2 was dispersed in 45 ml of methanol and ultrasonic treated for 30 min to obtain a suspension for standby; 0.03 mmol of palladium acetate and 0.02 mmol of iridium acetate were dispersed in 15 ml of methanol and stirred for 5 min to obtain a metal precursor solution for standby.

[0059] Example 1

[0060] A method for catalytic hydrogenation to prepare 3 - methyl - 4 - aminobenzoic acid comprises the following steps:

[0061] A1. Mix 90 g of 3-methyl-4-nitrobenzoic acid with 500 ml of methanol, and adjust the pH value to 8 with 15 wt% aqueous sodium hydroxide solution while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution;

[0062] A2. Pour the 3-methyl-4-nitrobenzoic acid solution obtained in step A1 into a reaction kettle, add 3.15 g of modified palladium catalyst A and 4.95 g of triethylamine, displace with nitrogen and hydrogen three times each in sequence, then introduce hydrogen, and carry out a hydrogenation reaction at a pressure of 2 MPa and a temperature of 110 °C for 5 h to obtain a reaction product;

[0063] A3. Wait for the reaction kettle to cool to below 25 °C, filter the reaction product obtained in step A2, adjust the filtrate to a weakly acidic pH value with 20 wt% aqueous hydrochloric acid solution, crystallize at room temperature, and vacuum dry at 70 °C to constant weight to obtain 3-methyl-4-aminobenzoic acid.

[0064] Example 2

[0065] A method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation comprises the following steps:

[0066] A1. Mix 90 g of 3-methyl-4-nitrobenzoic acid with 900 ml of methanol, and adjust the pH value to 8 with 15 wt% aqueous sodium hydroxide solution while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution;

[0067] A2. Pour the 3-methyl-4-nitrobenzoic acid solution obtained in step A1 into a reaction kettle, add 1.8 g of modified palladium catalyst A and 2.7 g of triethylamine, displace with nitrogen and hydrogen two times each in sequence, then introduce hydrogen, and carry out a hydrogenation reaction at a pressure of 1 MPa and a temperature of 80 °C for 7 h to obtain a reaction product;

[0068] A3. Wait for the reaction kettle to cool to below 25 °C, filter the reaction product obtained in step A2, adjust the filtrate to a weakly acidic pH value with 20 wt% aqueous hydrochloric acid solution, crystallize at room temperature, and vacuum dry at 70 °C to constant weight to obtain 3-methyl-4-aminobenzoic acid.

[0069] Example 3

[0070] A method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation comprises the following steps:

[0071] A1. Mix 90 g of 3-methyl-4-nitrobenzoic acid with 360 ml of methanol, and adjust the pH value to 8 with 15 wt% aqueous sodium hydroxide solution while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution;

[0072] A2. Pour the 3-methyl-4-nitrobenzoic acid solution obtained in step A1 into a reaction kettle, add 4.5 g of modified palladium catalyst A and 7.2 g of triethylamine, displace with nitrogen and hydrogen three times each in sequence, then introduce hydrogen, and carry out a hydrogenation reaction at a pressure of 3.5 MPa and a temperature of 140 °C for 3 h to obtain a reaction product;

[0073] A3. Wait for the reaction kettle to cool to 25 °C, filter the reaction product obtained in step A2, adjust the filtrate to a weakly acidic pH value with a 20 wt% hydrochloric acid aqueous solution, crystallize at room temperature, and dry to constant weight under vacuum at 70 °C to obtain 3-methyl-4-aminobenzoic acid.

[0074] Example 4

[0075] A method for catalytic hydrogenation to prepare 3-methyl-4-aminobenzoic acid, the specific implementation manner is the same as that of Example 1, except that an equal amount of modified palladium catalyst B is used to replace modified palladium catalyst A.

[0076] Example 5

[0077] A method for catalytic hydrogenation to prepare 3-methyl-4-aminobenzoic acid, the specific implementation manner is the same as that of Example 1, except that an equal amount of modified palladium catalyst C is used to replace modified palladium catalyst A.

[0078] Example 6

[0079] A method for catalytic hydrogenation to prepare 3-methyl-4-aminobenzoic acid, the specific implementation manner is the same as that of Example 1, except that an equal amount of modified palladium catalyst D is used to replace modified palladium catalyst A.

[0080] Comparative Example 1

[0081] A method for catalytic hydrogenation to prepare 3-methyl-4-aminobenzoic acid, comprising the following steps:

[0082] A1. Mix 90 g of 3-methyl-4-nitrobenzoic acid and 500 ml of methanol, and adjust the pH value to 8 with a 15 wt% sodium hydroxide aqueous solution while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution;

[0083] A2. Pour the 3-methyl-4-nitrobenzoic acid solution obtained in step A1 into a reaction kettle, add 3.15 g of modified palladium catalyst A, displace with nitrogen and hydrogen three times each in sequence, then introduce hydrogen, and carry out a hydrogenation reaction at a pressure of 2 MPa and a temperature of 110 °C for 5 h to obtain a reaction product;

[0084] A3. After the reaction kettle is cooled to 25 °C, filter the reactants obtained in step A2, adjust the filtrate to weakly acidic pH value with 20 wt% hydrochloric acid aqueous solution, crystallize at room temperature, and dry to constant weight under vacuum at 70 °C to obtain 3-methyl-4-aminobenzoic acid.

[0085] Comparative Example 2

[0086] A method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation, the specific implementation method is the same as that of Example 1, except that an equal amount of commercially available palladium sulfate is used to replace the modified palladium catalyst A.

[0087] Effect evaluation:

[0088] Test the 3-methyl-4-aminobenzoic acid prepared in the above Examples 1-6 and Comparative Examples 1-2. The product is measured by high performance liquid chromatography and the purity is calculated, and the color of the product is observed. The specific results are shown in Table 1.

[0089] Table 1

[0090] Serial Number Yield / % Purity / % Color Example 1 98.1 99.9 Pure light yellow Example 2 97.2 99.5 Pure light yellow Example 3 97.6 99.7 Pure light yellow Example 4 95.9 97.4 Pure light yellow Example 5 91.2 95.7 Pure light yellow Example 6 92.7 97.0 Pure light yellow Comparative Example 1 97.6 99.5 Impure light yellow Comparative Example 2 90.0 94.4 Pure light yellow

[0091] From the results in Table 1, it can be seen that the yield of 3-methyl-4-aminobenzoic acid synthesized in Examples 1-3 is not less than 97%, and the purity is not less than 99%, which has good application value.

[0092] Compared with Example 1, in Examples 4-6, when preparing the modified palladium catalyst, in Example 4, the molar ratio of 1,3,5-tris(4-aminophenyl)benzene and 2,5-diethyl-1,4-p-benzenedicarboxaldehyde was changed, which affected the pore structure of the carrier, weakened the adsorption of metal nanoparticles on the carrier, affected the catalytic activity, and further reduced the yield and purity of the product; in Example 5, the metal precursor was changed, only using palladium metal, the synergistic effect of palladium and iridium was missing, and the catalytic activity and selectivity were weakened; in Example 6, the precursor of the composite metal was changed, and the adsorption performance of metal ions decreased, affecting the catalytic activity. All of the above will further reduce the yield and purity of 3-methyl-4-aminobenzoic acid.

[0093] Compared with Example 1, in Comparative Example 1, no amine substance was added. Although it had little effect on the yield and purity of 3-methyl-4-aminobenzoic acid, it would make the color of the product incorrect and slightly red.

[0094] Compared with Example 1, in Comparative Example 2, an equal amount of commercially available palladium sulfate was used to replace the modified palladium catalyst A, and the catalytic activity decreased, affecting the yield and purity of 3-methyl-4-aminobenzoic acid.

[0095] As described above, it is only the preferred embodiment of the present invention and does not impose any form of limitation on this application. Although this application is disclosed with the preferred embodiment as above, it is not intended to limit this application. Any person skilled in the art, without departing from the scope of the technical solution of this application, making some changes or modifications using the disclosed technical content is equivalent to equivalent implementation cases. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution.

Claims

1. A method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation, characterized in that: The following steps are included: A1. Mix 3-methyl-4-nitrobenzoic acid and a solvent, and adjust the pH value to alkaline while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution; A2. The 3-methyl-4-nitrobenzoic acid solution obtained in step A1 is poured into a reaction vessel, a modified palladium catalyst and an additive are added, and the mixture is replaced 2-3 times with nitrogen and hydrogen respectively, and then hydrogen is introduced, and the pressure and temperature are controlled to carry out a hydrogenation reaction to obtain a reactant; A3. After the reactor was cooled, the reaction product obtained in step A2 was filtered, the filtrate was adjusted to a weakly acidic pH, and crystallized and dried to obtain 3-methyl-4-aminobenzoic acid; The preparation method of the modified palladium catalyst in step A1 comprises the following steps: S1. Add 1,3,5-tris(4-aminophenyl)benzene and 2,5-diethyl-1,4-terephthalaldehyde into a reaction kettle, stir, add a composite organic solvent, heat to 65-80°C, stir, and cool to obtain a mixed solution; S2. Under ultrasonic conditions, deionized water and glacial acetic acid are sequentially added dropwise to the mixed solution of step S1. After the addition is complete, ultrasonic treatment is performed for 3-5 minutes, the mixture is heated to 65-80°C, reacted for 2-3 days, cooled, filtered, washed, dried, and ground to obtain a powder; S3. The powder obtained in step S2 is dispersed in methanol and ultrasonically treated for 20-40min to obtain a suspension for later use; the composite metal precursor is dispersed in methanol and stirred to obtain a metal precursor solution for later use; S4. The suspension obtained in step S3 and the metal precursor solution are mixed, a methanol solution of sodium borohydride is added, stirred for 10-12h, filtered, washed, and dried in vacuo to obtain a modified palladium catalyst; The molar ratio of 1,3,5-tris(4-aminophenyl)benzene to 2,5-diethyl-1,4-terephthalaldehyde in step S1 is 1:(1.05-1.2); The composite metal precursor in step S3 is palladium chloride and iridium chloride, and the molar ratio of the two is (1.2-1.6): 1; The auxiliary agent in step A2 is an amine substance, and the added amount of the amine substance is 3-8% of the mass of 3-methyl-4-nitrobenzoic acid; the amine substance is triethylamine and / or N,N-dimethylformamide.

2. The method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation according to claim 1, characterized in that: The solvent in step A1 is methanol.

3. The method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation according to claim 1, characterized in that: The amount of the modified palladium catalyst added in step A2 is 2-5% of the mass of 3-methyl-4-nitrobenzoic acid.

4. The method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation according to claim 1, characterized in that: The pressure and temperature conditions in step A2 are specifically: pressure 1.0-3.5 MPa, temperature 80-140°C.

5. A method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation according to any one of claims 1 to 4, characterized in that: The yield of the 3-methyl-4-aminobenzoic acid is not less than 97%, and the purity is not less than 99%.