Method for preparing 3-methyl-4-aminobenzoic acid through catalytic hydrogenation

Abstract

The invention belongs to the technical field of chemical synthesis, and provides a method for preparing 3-methyl-4-aminobenzoic acid through catalytic hydrogenation, which comprises the following steps: mixing 3-methyl-4-nitrobenzoic acid and a solvent, and adjusting the pH value to be alkaline while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution; the method comprises the following steps: pouring a 3-methyl-4-nitrobenzoic acid solution into a reaction kettle, adding a modified palladium catalyst and an auxiliary agent, sequentially replacing with nitrogen and hydrogen for 2-3 times, introducing hydrogen, controlling pressure and temperature, and carrying out hydrogenation reaction to obtain a reactant; and after the reaction kettle is cooled, filtering reactants, adjusting the pH value of filtrate to be weakly acidic, crystallizing and drying to obtain the 3-methyl-4-aminobenzoic acid. The preparation method disclosed by the invention is simple in steps, a required product can be generated by a one-step method, meanwhile, the used modified palladium catalyst can be recycled, the generation amount of three wastes is small, coordinated development of economic benefits and environmental benefits is realized, and the 3-methyl-4-aminobenzoic acid is high in yield and purity and has a relatively good application value.

Description

Technical Field

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

[0002] 3-Methyl-4-aminobenzoic acid is an important organic intermediate widely used in medicine, pesticide and dye industries. At present, the synthesis of 3-methyl-4-aminobenzoic acid is mainly carried out by iron powder reduction and catalytic hydrogenation reduction. Iron powder reduction produces a large amount of solid waste and wastewater, which burdens the environment and is accompanied by 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 formation 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 on reaction conditions. Although some progress has been made in the development of catalysts and 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 on 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 phase transfer catalyst into a reaction device, a solvent is added and stirred evenly, and reduced iron powder and 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, iron mud is filtered, and the solution is washed twice with sodium carbonate solution, and the mother liquor and the washing solution are mixed to obtain a light yellow solution; acid is added to the light yellow solution to adjust the pH value to be slightly acidic, and an off-white precipitate is precipitated, which is filtered, washed with water, and dried to obtain 3-methyl-4-aminobenzoic acid. The preparation method of this invention is simple and low in cost, but a large amount of toxic iron mud and wastewater will be generated during the preparation process, and the maximum product yield is only 90.1%.

[0004] Chinese patent CN 107501106A discloses a method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation, which comprises the following steps: adding 3-methyl-4-nitrobenzoic acid and a solvent into a reaction vessel to obtain a 3-methyl-4-nitrobenzoic acid solution, and adjusting the pH value of the solution to be alkaline; pouring the obtained alkaline aqueous solution into an autoclave, adding a catalyst, replacing the solution with nitrogen and hydrogen three times respectively, filling the autoclave with hydrogen for 2 minutes, and then pressurizing the solution with hydrogen to 3.45-4.50 MPa, reacting the solution at a pressure of 2.50-4.50 MPa and a temperature of 100-160° C. for 1-8 hours to obtain a reaction solution; and performing post-treatment on the reaction to obtain 3-methyl-4-aminobenzoic acid. The method comprises the following steps: the raw materials are readily available, the three wastes are generated in small amounts, and the method is environmentally friendly; however, the filtrate thereof needs to be decolorized by adding sodium carbonate and activated carbon, and the steps are complicated, and the maximum molar yield is only 90%.

[0005] Therefore, a new catalytic hydrogenation method is urgently needed to achieve high selectivity and high yield synthesis of 3-methyl-4-aminobenzoic acid. Summary of the invention

[0006] In view of the existing technical problems, the object 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, can generate the desired product in one step, and the modified palladium catalyst used can be recycled and reused, the amount of three wastes generated is small, and the coordinated development of economic benefits and environmental benefits is achieved, and the yield and purity of 3-methyl-4-aminobenzoic acid are high, and the method has good application value.

[0007] In order to achieve the above object, the technical solution 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 and a solvent, and adjust the pH value to alkaline while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution;

[0010] 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;

[0011] A3. After the reactor is cooled, the reactant obtained in step A2 is filtered, the filtrate is adjusted to a weakly acidic pH value, and then crystallized and dried to obtain 3-methyl-4-aminobenzoic acid.

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

[0013] 1. The present invention synthesizes a novel 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 relatively broad application prospect.

[0014] Firstly, the carrier synthesized by the present invention using 1,3,5-tri(4-aminophenyl)benzene and 2,5-diethyl-1,4-terephthalaldehyde as organic ligands has a highly ordered pore structure, and 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 dispersibility, and the catalyst can show 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 modified palladium catalysts. This application selects specific metal precursors to prepare metal precursor solutions. Chloropaldic acid and iridium chloride have better adsorption effects and better dispersibility than other metal precursors, which can improve the reaction process of nitrobenzene hydrogenation reduction.

[0016] In addition, the applicant synthesized a bimetallic catalyst that can improve catalytic activity and selectivity. This may be because the iridium site mainly enhances the activation of 3-methyl-4-nitrobenzoic acid and the adsorption of intermediates, while the palladium site is conducive to hydrogen dissociation. The hydrogen and electrons on the palladium are quickly transferred to the iridium, which can maximize the hydrogenation process and achieve high activity and high selectivity for the preparation of 3-methyl-4-aminobenzoic acid.

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

[0018] 3. The present invention strictly controls the pressure and temperature of the reaction to avoid a slow reaction rate leading to a decrease in yield. The applicant further increases the yield and purity of the reaction by controlling the amount of modified palladium catalyst and auxiliary agent added.

[0019] 4. The present invention uses a one-step method to synthesize 3-methyl-4-aminobenzoic acid, which reduces the generation of intermediate products, saves energy consumption, simplifies the process, and reduces the emission of pollutants.

[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 method for preparing the modified palladium catalyst in step A1 comprises the following steps:

[0023] S1. Add 1,3,5-tris(4-aminophenyl)benzene and 2,5-diethyl-1,4-terephthalaldehyde 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, deionized water and glacial acetic acid are sequentially added dropwise to the mixed solution of step S1, and after the addition is completed, ultrasonication is performed for 3-5 minutes, heated to 65-80° C., reacted for 2-3 days, cooled, filtered, washed, dried, and ground to obtain a powder;

[0025] S3. The powder obtained in step S2 is dispersed in methanol, and ultrasonically treated for 20-40 min to obtain a suspension for standby use; the composite metal precursor is dispersed in methanol, stirred, and a metal precursor solution is obtained for standby use;

[0026] S4. The suspension obtained in step S3 and the metal precursor solution are mixed, a methanol solution of sodium borohydride is added, the mixture is stirred for 10-12 hours, filtered, washed, and vacuum dried 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-terephthalaldehyde 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-terephthalaldehyde.

[0030] In some embodiments, the composite organic solvent in step S1 is 1,4-dioxane and xylene, 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-terephthalaldehyde.

[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 amount of the modified palladium catalyst added 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 added 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 pressure and temperature conditions in step A2 are specifically: pressure 1.0-3.5 MPa, temperature 80-140°C.

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

[0037] In some embodiments, the yield of the 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 present invention has the following beneficial effects:

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

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

[0041] 3. The present invention adds amine substances as auxiliary agents, which can effectively achieve decolorization without affecting the yield and purity of 3-methyl-4-aminobenzoic acid, thereby simplifying the production process. DETAILED DESCRIPTION

[0042] The present invention will be described below in conjunction with specific embodiments. It should be noted that the following embodiments are examples of the present invention and are only used to illustrate the present invention, rather than to limit the present invention. Other combinations and various modifications within the concept of the present invention may be performed without departing from the spirit or scope of the present invention.

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

[0044] In order to facilitate those skilled in the art to implement the present invention, the sources of some raw materials in the embodiments and comparative examples are described as follows: raw materials are not specially described and 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. 0.5 mmol of 1,3,5-tris(4-aminophenyl)benzene and 0.55 mmol of 2,5-diethyl-1,4-terephthalaldehyde were added to a reaction kettle, stirred for 15 min, 19.6 ml of 1,4-dioxane and 4.8 ml of xylene were added, heated to 75° C., stirred until dissolved, and cooled 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 sequentially added dropwise to the mixed solution of step S1. After the addition was completed, ultrasonic was performed for 3 min, heated to 75 ° C, reacted for 3 d, cooled to room temperature, filtered, and the solid was washed with toluene 3 times, 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 ultrasonically treated for 30 min to obtain a suspension for standby use; 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 use;

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

[0051] Preparation Example 2

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

[0053] Preparation Example 3

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

[0055] The powder obtained in step S2 was dispersed in 45 ml of methanol and subjected to ultrasonic treatment for 30 min to obtain a suspension for use; 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 use.

[0056] Preparation Example 4

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

[0058] The powder obtained in step S2 was dispersed in 45 ml of methanol and subjected to ultrasonic treatment for 30 min to obtain a suspension for use; 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 use.

[0059] Example 1

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

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

[0062] A2. The 3-methyl-4-nitrobenzoic acid solution obtained in step A1 was poured into a reaction vessel, 3.15 g of modified palladium catalyst A and 4.95 g of triethylamine were added, replaced with nitrogen and hydrogen three times respectively, and then hydrogen was introduced, and the hydrogenation reaction was carried out at a pressure of 2 MPa and a temperature of 110 ° C for 5 h to obtain a reactant;

[0063] A3. After the reactor is cooled to below 25°C, the reactant obtained in step A2 is filtered, and the filtrate is adjusted to a weakly acidic pH value with a 20wt% aqueous hydrochloric acid solution, crystallized at room temperature, and vacuum dried 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. 90 g of 3-methyl-4-nitrobenzoic acid and 900 ml of methanol were mixed and the pH value was adjusted to 8 with 15 wt % aqueous sodium hydroxide solution while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution;

[0067] A2. The 3-methyl-4-nitrobenzoic acid solution obtained in step A1 was poured into a reaction vessel, 1.8 g of modified palladium catalyst A and 2.7 g of triethylamine were added, replaced with nitrogen and hydrogen twice each, and then hydrogen was introduced, and the hydrogenation reaction was carried out at a pressure of 1 MPa and a temperature of 80 ° C for 7 h to obtain a reactant;

[0068] A3. After the reactor is cooled to below 25°C, the reactant obtained in step A2 is filtered, and the filtrate is adjusted to a weakly acidic pH value with a 20wt% aqueous hydrochloric acid solution, crystallized at room temperature, and vacuum dried 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. 90 g of 3-methyl-4-nitrobenzoic acid and 360 ml of methanol were mixed and the pH value was adjusted to 8 with 15 wt % aqueous sodium hydroxide solution while stirring to obtain a 3-methyl-4-nitrobenzoic acid solution;

[0072] A2. The 3-methyl-4-nitrobenzoic acid solution obtained in step A1 was poured into a reaction vessel, 4.5 g of modified palladium catalyst A and 7.2 g of triethylamine were added, replaced with nitrogen and hydrogen three times respectively, and then hydrogen was introduced, and the hydrogenation reaction was carried out at a pressure of 3.5 MPa and a temperature of 140 ° C for 3 h to obtain a reactant;

[0073] A3. After the reactor is cooled to 25°C, the reactant obtained in step A2 is filtered, and the filtrate is adjusted to a weakly acidic pH value with a 20wt% aqueous hydrochloric acid solution, crystallized at room temperature, and vacuum dried at 70°C to constant weight to obtain 3-methyl-4-aminobenzoic acid.

[0074] Example 4

[0075] 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 modified palladium catalyst B is used to replace modified palladium catalyst A.

[0076] Example 5

[0077] 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 modified palladium catalyst C is used to replace modified palladium catalyst A.

[0078] Example 6

[0079] 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 modified palladium catalyst D is used to replace modified palladium catalyst A.

[0080] Comparative Example 1

[0081] A method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation comprises the following steps:

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

[0083] A2. The 3-methyl-4-nitrobenzoic acid solution obtained in step A1 was poured into a reaction vessel, 3.15 g of modified palladium catalyst A was added, and the mixture was replaced three times with nitrogen and hydrogen respectively, and then hydrogen was introduced, and the hydrogenation reaction was carried out at a pressure of 2 MPa and a temperature of 110 ° C for 5 h to obtain a reactant;

[0084] A3. After the reactor is cooled to 25°C, the reactant obtained in step A2 is filtered, and the filtrate is adjusted to a weakly acidic pH value with a 20wt% aqueous hydrochloric acid solution, crystallized at room temperature, and vacuum dried at 70°C to constant weight 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] The 3-methyl-4-aminobenzoic acid prepared in the above Examples 1-6 and Comparative Examples 1-2 was tested. The product was measured and the purity was calculated using a high performance liquid chromatograph, and the color of the product was 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 prepared and synthesized in Example 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, when preparing the modified palladium catalyst, Example 4-6 changes the molar ratio of 1,3,5-tri(4-aminophenyl)benzene and 2,5-diethyl-1,4-terephthalaldehyde, which affects the pore structure of the carrier, weakens the adsorption of metal nanoparticles on the carrier, affects the catalytic activity, and thus reduces the yield and purity of the product; Example 5 changes the metal precursor, using only metallic palladium, the synergistic effect of palladium and iridium is lost, the catalytic activity and selectivity are weakened, and Example 6 changes the precursor of the composite metal, the adsorption performance of the metal ions is reduced, 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, Comparative Example 1 does not add amine substances. Although it has little effect on the yield and purity of 3-methyl-4-aminobenzoic acid, it will make the color of the product abnormal and slightly red.

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

[0095] The above is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present application. Although the present application is disclosed as above in the preferred embodiment, it is not intended to limit the present application. Any technician familiar with the profession, without departing from the scope of the technical solution of the present application, using the technical content disclosed above to make slight changes or modifications are equivalent to equivalent implementation cases. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still 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 is cooled, the reactant obtained in step A2 is filtered, the filtrate is adjusted to a weakly acidic pH value, and then crystallized and dried to obtain 3-methyl-4-aminobenzoic acid.

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 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 to 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, and after the addition is completed, ultrasonication is performed for 3-5 minutes, 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 20-40 min to obtain a suspension for standby use; the composite metal precursor is dispersed in methanol, stirred, and a metal precursor solution is obtained for standby use; S4. The suspension obtained in step S3 and the metal precursor solution are mixed, a methanol solution of sodium borohydride is added, the mixture is stirred for 10-12 hours, filtered, washed, and vacuum dried to obtain a modified palladium catalyst.

4. The method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation according to claim 3, characterized in that: 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).

5. The method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation according to claim 3, characterized in that: 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.

6. 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.

7. The method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation according to claim 1, characterized in that: 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.

8. The method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation according to claim 7, characterized in that: The amine substance is triethylamine and / or N,N-dimethylformamide.

9. 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.

10. A method for preparing 3-methyl-4-aminobenzoic acid by catalytic hydrogenation according to any one of claims 1 to 9, 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%.