Preparation method of high-purity pyromellitic dianhydride

Abstract

The application provides a preparation method of high-purity pyromellitic dianhydride, which is prepared from 2,3-dichloromaleic anhydride as raw material, and sequentially subjected to Diels-Alder cycloaddition reaction, elimination reaction and oxidation reaction to obtain high-purity pyromellitic acid, and then subjected to dehydration treatment to obtain high-purity pyromellitic dianhydride. In the preparation method, pyromellitic acid is used as a dehydration raw material to prepare anhydride, so that intermediate acid by-products are not easy to be generated, and the product purity is improved; the dehydration reaction is promoted by adopting azeotropic distillation and separation coupling, so that the reaction yield is improved; the preparation steps are few, the reaction process is simple, the reaction condition is mild, the raw materials are economically inexpensive and easy to obtain; the catalyst or the aid used in each step does not generate harmful substances, and is green and environment-friendly.

Description

Technical Field

[0001] The invention belongs to the technical field of compound preparation, and particularly relates to a method for preparing high-purity pyromellitic dianhydride. Background Art

[0002] Pyromellitic dianhydride (PMDA), also known as pyromellitic anhydride, is chemically called 1,2,4,5-benzoic dianhydride. It appears as a white powder or needle-shaped crystals, with a melting point of 284-286°C and a boiling point of 397-400°C. Pyromellitic dianhydride is a common pharmaceutical and chemical intermediate and one of the most important dianhydride monomers for the synthesis of high-temperature resistant insulating materials such as polyimide. It is also a curing agent for epoxy resins and polyester resins, an auxiliary agent for powder coatings, and a major raw material for high-performance plasticizers. Currently, the synthesis methods of pyromellitic dianhydride mainly include: (1) duromellitic anhydride oxidation method; (2) trimethylol alkylation-oxidation method; (3) trimethylol carbonylation-oxidation method; and (4) xylene chloromethylation-oxidation method. Among them, the most mature process in industry is the duromellitic anhydride oxidation method.

[0003] Existing durene oxidation processes are divided into two categories: room-temperature gas-phase oxidation using durene as the raw material and liquid-phase oxidation using 2,4,5-trimethylbenzaldehyde. The gas-phase air oxidation process uses oxygen as the oxidant to oxidize durene to produce durene oxidation products, which are then hydrolyzed to produce pyromellitic dianhydride. This involves gasification, oxidation, capture, hydrolysis and decolorization, filtration, and refining. This method is prone to producing byproducts, primarily phthalic anhydride and some intermediate acids, such as 4,5-dimethyldiacid, 5-methyltrimellitic acid, 2,5-dimethyltricarboxylic acid, and 4,6-dimethyltricarboxylic acid. Consequently, it suffers from poor selectivity, low recycling conversion rates, high product impurities, and low product quality yield. The liquid-phase air oxidation process of durenyl includes processes such as batching, oxidation, crystallization centrifugation, dehydration to anhydride, refining, and acetic acid recovery. Although the yield of synthesized durenyl anhydride is high, the metal catalytic system causes heavy metal residues in the product. In addition, the process route is long, the reaction temperature is harsh, and the equipment requirements are high, which limits its application. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defects in the prior art and provide a method for preparing high-purity pyromellitic dianhydride.

[0005] The present invention uses 2,3-dichloromaleic anhydride as a raw material, sequentially undergoes a Diels-Alder cycloaddition reaction, an elimination reaction, and an oxidation reaction to obtain high-purity pyromellitic acid, which is then dehydrated to obtain high-purity pyromellitic dianhydride. The preparation route is as follows:

[0006]

[0007] Step 1: 2,3-dichloromaleic anhydride and 2,3-dimethyl-1,3-butadiene are heated under reflux in a water bath at 40-50° C. for 4-10 hours in an anhydrous organic solvent 1; after the reaction is completed, the solid product is rotary evaporated and dried to obtain 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride;

[0008] Step 2: 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride is heated in a water bath with sodium hydroxide ethanol solution and refluxed for 2 to 6 hours; after the reaction, cooling and crystallization are performed, filtration, extraction, and column chromatography purification are performed to obtain 4,5-dimethyl-1,2-benzenedicarboxylic acid;

[0009] Step 3: In an organic solvent 2-water mixed solution, 4,5-dimethyl-1,2-benzenedicarboxylic acid, potassium monopersulfate complex salt (Oxone), and KBr are irradiated with visible light at room temperature, and the reaction progress is monitored by thin layer chromatography; after the reaction is completed, saturated Na2SO3 is added for quenching, separation extraction, activated carbon is used for impurity removal, and filtration is performed. The filtrate is washed with brine, dried over anhydrous Na2SO4, concentrated under reduced pressure, and purified by column chromatography to obtain high-purity pyromellitic acid; Step 4: Pyromellitic acid and a dehydrating agent are azeotropically distilled in an anhydrous organic solvent 3 at 95-120°C, and the reaction progress is monitored by thin layer chromatography; after the reaction is completed, atmospheric distillation and reduced pressure distillation are performed in sequence, cooled for crystallization, filtered, and washed to obtain a crude product; heated with acetone to dissolve and activated carbon is added, filtered while hot, the filtrate is recrystallized with petroleum ether, filtered, and infrared dried to obtain high-purity pyromellitic dianhydride crystals.

[0010] Preferably, in step 1, the molar ratio of 2,3-dimethyl-1,3-butadiene to 2,3-dichloromaleic anhydride is 1.05-1.3, and the water bath heating temperature is 50-60° C.; the anhydrous organic solvent 1 is at least one of tetrahydrofuran, acetone, chloroform, and ethyl acetate.

[0011] Preferably, in step 2, the molar ratio of 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride to sodium hydroxide is 1:(2-3); the concentration of the sodium hydroxide ethanol solution is 15wt% to 30wt%, and the reflux temperature is controlled at 70-75°C; the adsorbent used for the column chromatography is alumina, and the eluent is chloroform / methanol with a v:v ratio of (3-5):1.

[0012] It should be noted that the elimination product obtained in step 2 is 4,5-dimethyl-1,2-benzenedicarboxylic acid rather than 4,5-dimethyl-phthalic anhydride. This is because the aqueous medium and heating conditions cause the 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride raw material to be hydrolyzed, but the hydrolysis process of the anhydride has no obvious effect on the elimination reaction.

[0013] Preferably, in step 3, the molar ratio of 4,5-dimethyl-1,2-benzenedicarboxylic acid to potassium monopersulfate complex salt and KBr is 1:(4-4.5):(4-4.5), and the volume ratio of organic solvent 2 to water is 5-9; the organic solvent 2 is at least one of tetrahydrofuran, dichloroethane, and dichloromethane; the visible light irradiation intensity is 300-550 mW / cm 2 , the irradiation time is 9 to 24 hours; the adsorbent used for column chromatography is aluminum oxide, and the eluent is hexane / ethyl acetate with v:v = (19 to 15): (1 to 5).

[0014] Potassium monopersulfate (Oxone) is an inorganic peroxide that exists in the form of a triple salt formed by potassium monopersulfate, potassium bisulfate, and potassium sulfate. Its molecular formula is 2KHSO5·KHSO4·K2SO4, and its main active ingredient is KHSO5. Under visible light irradiation, potassium persulfate first decomposes to produce sulfate radicals (·SO 4- ) and other active oxygen species, Br - Can react with potassium persulfate to produce SO 4- Reacts to generate bromine radicals (Br·), which can trigger the subsequent reaction; the excited 4,5-dimethyl-phthalic acid or its ground state molecules react with Br· or SO 4- The reaction occurs, resulting in the oxidation of the benzyl methyl group, ultimately producing pyromellitic acid.

[0015] Preferably, in step 4, the molar ratio of pyromellitic acid to the dehydrating agent is 1:(4-5); the dehydrating agent is acetic anhydride or trifluoroacetic anhydride, and the dehydrating agent needs to be dried before use.

[0016] Preferably, the amount of the anhydrous organic solvent 3 in step 4 is equivalent to 5 to 20 times the mass of pyromellitic acid, and the anhydrous organic solvent 3 is selected from at least one of toluene, xylene, chlorobenzene, pyridine, anisole, diisobutyl ether, and low alkyl carboxylic acid esters; the low alkyl carboxylic acid ester is selected from at least one of butyl acetate, n-butyl formate, pentyl formate, butyl acetate, and pentyl acetate.

[0017] In step 4, azeotropic distillation is used to lower the dehydration temperature of pyromellitic acid; during the azeotropic distillation, a water layer is collected using a water separator to promote the dehydration reaction to move towards the formation of pyromellitic dianhydride, accelerate the reaction rate, shorten the reaction time, and improve the yield; and the purity of the product is further increased through purification steps of atmospheric distillation, reduced pressure distillation, and recrystallization.

[0018] Preferably, the purity of the pyromellitic acid is greater than 95% by weight.

[0019] Preferably, the purity of the pyromellitic dianhydride is greater than 98% by weight, preferably 99% by weight.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] In the preparation method provided by the present invention, pyromellitic acid is used as a dehydration raw material to prepare anhydride, thereby reducing the generation of phthalic anhydride and some intermediate acid by-products, and the product purity is high; the dehydration process adopts the method of azeotropic distillation and separation coupling with an organic solvent to promote the reaction to the direction of generating anhydride, and the reaction yield is high.

[0022] The invention provides a method for preparing high-purity pyromellitic dianhydride, which has few preparation steps, a simple reaction process, mild reaction conditions, and involves economical, cheap and easily available raw materials; the catalysts or auxiliary agents used in each step do not produce harmful substances, and are green and environmentally friendly. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is further described in detail below with reference to several preferred embodiments. These embodiments are implemented based on the technical solution of the invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments. Matters not described in detail in this specification of the present invention are well known to those skilled in the art.

[0024] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0025] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials, reagents, etc. used in the examples are all commercially available unless otherwise specified.

[0026] In the present application, since pyromellitic acid and pyromellitic dianhydride have relatively high boiling points, their purity was determined using a K2025 high performance liquid chromatograph.

[0027] A C18 reverse-phase column (4.60 mm × 250 mm × 5.0 μm) was used at a column temperature of 35°C. The injection volume was 10 μL. The mobile phase consisted of methanol and water (80:20) at a flow rate of 1.0 mL / min. The analysis time was 24 minutes, and the detection wavelength was 250 nm. Quantitative analysis was performed using the peak area normalization method. Samples were filtered through a 0.22 μm filter prior to testing.

[0028] Color of pyromellitic dianhydride: Expressed as a methanol solution. Measure the absorbance of a 5.0000 g sample of pyromellitic dianhydride in 100 mL of methanol (AR) at 431 nm. The average absorbance of five measurements (100 times) represents the color of the methanol solution.

[0029] Example 1

[0030] Pyromellitic dianhydride. The preparation process is as follows:

[0031] Step one: 0.10 mol 2,3-dichloromaleic anhydride and 40 mL tetrahydrofuran were added to a round-bottom flask, and 0.13 mol 2,3-dimethyl-1,3-butadiene was added dropwise to the solution under the condition of 50°C water bath. After the addition was completed, a reflux condenser was installed, and constant temperature reflux was carried out for 6 h. After the reaction was completed, the solid product was dried at 60°C for 6 h by rotary evaporation, and 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride was obtained with a yield of 88.5%.

[0032] Step two: 0.16 mol sodium hydroxide and 40 mL 95% ethanol aqueous solution were mixed in a round-bottom flask, and then 78 mmol 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride was added. The reaction system was kept stirring, and the reaction was carried out under reflux at 75°C water bath. Thin layer chromatography was used to determine the completion of the reaction. After the solution was slightly cooled, 0.5 mol / L dilute hydrochloric acid was added dropwise until the pH was about 4. Chloroform was used for extraction for 3 times, and the combined organic phase was concentrated and placed in an ice water bath for crystallization. Filtration was carried out, and the solid was washed with cold chloroform, dried with anhydrous Na2SO4, and column chromatography was carried out on an alumina column (chloroform:methanol=3:1). After vacuum distillation, 4,5-dimethyl-1,2-benzenedicarboxylic acid was obtained with a yield of 82.5%.

[0033] Step three: 60 mmol 4,5-dimethyl-1,2-benzenedicarboxylic acid, 0.26 mol Oxone, and 0.26 mol KBr were added to a 360 mL mixed solution of dichloromethane-water (v:v=8:1), and stirring was carried out at room temperature under visible light irradiation (550 mW / cm 2 , PLS-SXE 300 xenon lamp light source, Beijing Phoelex Technology Co., Ltd.) for 12 h. After the reaction was completed, 50 mL saturated Na2SO3 was used for quenching, and ethyl acetate was used for extraction for 3 times. Activated carbon was added to the collected organic phase and stirred for 1 h. Hot filtration was carried out, and the filtrate was washed with brine, dried with anhydrous Na2SO4, concentrated under reduced pressure, and purified by column chromatography on an alumina column (ethyl acetate:hexane=1:8). Pyromellitic dianhydride was obtained with a yield of 93.1% and a purity of 96.4%.

[0034] Step 4: In a three-necked flask equipped with a reflux condenser and a water separator, add 54 mmol of pyromellitic acid, 0.27 mol of acetic anhydride, 76 mL of toluene, and 24 mL of chlorobenzene. Slowly heat in an oil bath to dissolve completely. Continue heating to 120°C for azeotropic distillation, leaving the aqueous layer at the bottom of the water separator and the organic phase refluxed into the flask. Monitor the reaction progress by thin-layer chromatography. After the reaction is complete, cool the solution to room temperature and transfer it to a distillation apparatus. Recover the acetic anhydride by atmospheric distillation at 60°C. Remove impurities such as toluene and chlorobenzene by reduced pressure distillation. Cool the residue in an ice bath for crystallization and filter to obtain a crude product. Wash the crude product with methyl tert-butyl ether, dissolve it in acetone again by heating, then add activated carbon, stir for 1 hour, and filter it while hot. Add petroleum ether to the filtrate and dissolve it by heating. Let it cool and crystallize by cooling. Filter and squeeze the liquid from the solid with a glass stopper. Dry it with infrared light to obtain pyromellitic dianhydride as white crystals with a yield of 94.3%, a purity of 99.0%, and a color of 2.

[0035] Example 2

[0036] Pyromellitic dianhydride. The preparation process is as follows:

[0037] Step 1: Add 0.15 mol of 2,3-dichloromaleic anhydride and 45 mL of tetrahydrofuran to a round-bottom flask. Add 0.18 mol of 2,3-dimethyl-1,3-butadiene dropwise to the solution in a 55°C water bath. After the addition is complete, install a reflux condenser and reflux at constant temperature for 8 hours. After the reaction is complete, rotary evaporate and vacuum dry the solid product at 60°C for 6 hours to obtain 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride; yield 86.4%;

[0038] Step 2: In a round-bottom flask, add 0.22 mol of sodium hydroxide and 40 mL of 95% aqueous ethanol, followed by the addition of 0.10 mol of 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride. The reaction system was stirred and refluxed in a 72°C water bath. Completion of the reaction was confirmed by thin-layer chromatography. The solution was cooled slightly, and 0.5 mol / L dilute hydrochloric acid was added dropwise with stirring until the pH reached ~4. Extraction was performed three times with chloroform. The combined organic phases were concentrated and crystallized in an ice-water bath. Filter the solid, wash it with cold chloroform, dry it with anhydrous Na2SO4, and chromatograph it on an alumina column (chloroform:methanol = 4:1). The product was then distilled under reduced pressure to obtain 4,5-dimethyl-1,2-benzenedicarboxylic acid in an 81.7% yield.

[0039] Step 3: In 360 mL of a mixed solution of dichloromethane and water (v:v = 9:1), 72 mmol of 4,5-dimethyl-1,2-benzenedicarboxylic acid, 0.30 mol of Oxone, and 0.30 mol of KBr were irradiated with visible light (500 mW / cm 2PLS-SXE 300 xenon lamp light source, Beijing Pofei Technology Co., Ltd.) under room temperature for 14 h; after reaction, the solution was quenched with 50 mL saturated Na2SO3, extracted with ethyl acetate for 3 times, the organic phase was collected and stirred with activated carbon for 1 h, then filtered hot, the filtrate was washed with brine, dried with anhydrous Na2SO4, concentrated under reduced pressure, and purified by column chromatography (ethyl acetate: hexane = 1:3) to obtain pyromellitic acid; the yield was 92.8%, and the purity was 96.1%.

[0040] Step four: a three-necked flask with a reflux condenser and a water separator was charged with 60 mmol of pyromellitic acid, 0.29 mol of acetic anhydride, and 110 mL of toluene, and the solution was slowly heated to complete dissolution under an oil bath; the temperature was continuously increased to 105°C for azeotropic distillation, and the water layer was left at the bottom of the water separator, and the organic phase was refluxed into the flask, and the reaction progress was monitored by thin layer chromatography. After the reaction was completed, the solution was cooled to room temperature, transferred to a distillation device, and impurities such as p-xylene, acetic anhydride, butyl acetate and acetic acid were removed by distillation under reduced pressure; the distillation residue was cooled in an ice bath to crystallize, and the crude product was filtered; the crude product was washed with methyl tert-butyl ether, redissolved in acetone, then activated carbon was added and stirred for 1 h, and the filtrate was filtered hot, and the filtrate was further dissolved in petroleum ether, cooled and crystallized, filtered, and the liquid in the solid was squeezed out with a glass plug, and dried in an infrared oven to obtain white pyromellitic dianhydride crystals, with a yield of 93.6%, a purity of 99.3%, and a color of 1.

[0041] Example 3

[0042] Pyromellitic dianhydride. The preparation process is as follows:

[0043] Step one: 96 mmol of 2,3-dichloromaleic anhydride and 40 mL of tetrahydrofuran were added to a round-bottom flask, and 0.11 mol of 2,3-dimethyl-1,3-butadiene was added dropwise to the solution under the condition of a 50°C water bath; after the addition was completed, a reflux condenser was installed, and the solution was refluxed for 5.5 h; after the reaction was completed, the solid product was dried at 60°C under vacuum for 6 h to obtain 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride, with a yield of 85.1%.

[0044] Step 2: In a round-bottom flask, add 0.18 mol of sodium hydroxide and 40 mL of 95% aqueous ethanol, followed by the addition of 80 mmol of 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride. The reaction system was stirred and refluxed in a 71°C water bath. Completion of the reaction was confirmed by thin-layer chromatography. The solution was cooled slightly, and 0.5 mol / L dilute hydrochloric acid was added dropwise with stirring until the pH reached ~4. Extraction was performed three times with chloroform. The combined organic phases were concentrated and crystallized in an ice-water bath. The solid product was filtered, washed with cold chloroform, dried over anhydrous Na2SO4, and chromatographed on an alumina column (chloroform:methanol = 5:1). The product was then distilled under reduced pressure to obtain 4,5-dimethyl-1,2-benzenedicarboxylic acid in a yield of 79.6%.

[0045] Step 3: In 300 mL of a mixed solution of dichloromethane and water (v:v = 9:1), 55 mmol of 4,5-dimethyl-1,2-benzenedicarboxylic acid, 0.23 mol of Oxone, and 0.23 mol of KBr were irradiated with visible light (400 mW / cm 2 , PLS-SXE 300 xenon lamp light source, Beijing Perfect Light Technology Co., Ltd.) was stirred at room temperature for 18 h; after the reaction was completed, it was quenched with 50 mL of saturated Na2SO3, extracted with ethyl acetate three times, the organic phase was collected, activated carbon was added, and stirred for 1 h, hot filtered, and the filtrate was washed with brine, dried over Na2SO4, concentrated under reduced pressure, and purified by alumina column chromatography (ethyl acetate:hexane=1:8) to obtain pyromellitic acid with a yield of 92.0% and a purity of 95.9%.

[0046] Step 4: In a three-necked flask with a reflux condenser and a water separator, add 48 mmol of pyromellitic acid, 0.22 mol of trifluoroacetic anhydride, 75 mL of pyridine and 15 mL of p-xylene, and slowly heat in an oil bath to completely dissolve; continue to heat to 120°C for azeotropic distillation, with the aqueous layer remaining at the bottom of the water separator and the organic phase refluxed into the flask, and monitor the reaction progress by thin-layer chromatography. After the reaction, the solution was cooled to room temperature and transferred to a distillation apparatus. Low-boiling-point substances such as trifluoroacetic anhydride were recovered by atmospheric distillation at 45°C. Then, p-xylene and pyridine impurities were removed by reduced pressure distillation. The distillation residue was cooled in an ice bath for crystallization and filtered to obtain a crude product. The crude product was washed with methyl tert-butyl ether, heated and dissolved again in acetone, and activated carbon was added and stirred for 1 hour. The product was filtered while hot, and petroleum ether was added to the filtrate and heated and dissolved. The product was allowed to stand for cooling and crystallization was carried out by cooling. The product was filtered, and the liquid in the solid was squeezed out with a glass stopper. The product was dried by infrared filtration to obtain white crystals of pyromellitic dianhydride with a yield of 93.1%, a purity of 99.6%, and a chromaticity of 2.

[0047] Example 4

[0048] Pyromellitic dianhydride. The preparation process is as follows:

[0049] Step 1: Add 90 mmol of 2,3-dichloromaleic anhydride and 36 mL of tetrahydrofuran to a round-bottom flask. Add 94.5 mmol of 2,3-dimethyl-1,3-butadiene dropwise to the solution in a 55°C water bath. After the addition is complete, install a reflux condenser and reflux at a constant temperature for 5 hours. After the reaction is completed, rotary evaporate and vacuum dry the solid product at 60°C for 6 hours to obtain 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride with a yield of 81.5%.

[0050] Step 2: In a round-bottom flask, add 0.16 mol of sodium hydroxide and 30 mL of 95% aqueous ethanol, followed by the addition of 68 mmol of 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride. The reaction system was stirred and refluxed in a 71°C water bath. Completion of the reaction was confirmed by thin-layer chromatography. The solution was cooled slightly, and 0.5 mol / L dilute hydrochloric acid was added dropwise with stirring until the pH reached ~4. Extraction was performed three times with chloroform. The combined organic phases were concentrated and crystallized in an ice-water bath. The solid product was filtered, washed with cold chloroform, dried over anhydrous Na2SO4, and chromatographed on an alumina column (chloroform:methanol = 5:1). The product was then distilled under reduced pressure to obtain 4,5-dimethyl-1,2-benzenedicarboxylic acid in an 80.4% yield.

[0051] Step 3: In a 300 mL dichloromethane-water (v:v=8:1) mixed solution, 40 mmol of 4,5-dimethyl-1,2-benzenedicarboxylic acid, 0.164 mol of Oxone, and 0.164 mol of KBr were irradiated with visible light (350 mW / cm 2 , PLS-SXE 300 xenon lamp light source, Beijing Perfect Light Technology Co., Ltd.) and stirred at room temperature for 20 h; after the reaction was completed, quenched with 20 mL of saturated Na2SO3 and extracted with ethyl acetate three times; the organic phase was collected, activated carbon was added and stirred for 1 h, and hot filtered. The filtrate was washed with brine, dried over Na2SO4, concentrated under reduced pressure, and purified by alumina column chromatography (ethyl acetate:hexane=1:3) to obtain pyromellitic acid with a yield of 91.3% and a purity of 95.4%.

[0052] Step 4: In a three-necked flask equipped with a reflux condenser and a water separator, add 36 mmol of pyromellitic acid, 0.15 mol of acetic anhydride, 60 mL of butyl acetate, and 12 mL of toluene. Slowly heat the flask to 60°C in an oil bath to dissolve the mixture completely. Continue heating the mixture to 120°C for azeotropic distillation, leaving the aqueous layer at the bottom of the water separator while the organic phase refluxes into the flask. Monitor the reaction progress by thin-layer chromatography. After the reaction is complete, allow the solution to cool to room temperature and transfer it to a distillation apparatus. Reduced pressure distillation is performed to remove impurities such as toluene, acetic acid, and acetic anhydride. The residue is cooled in an ice bath for crystallization and filtered to obtain a crude product. The crude product is washed with methyl tert-butyl ether and reheated and dissolved in acetone. Activated carbon is then added and stirred for 1 hour. The mixture is filtered while hot. Petroleum ether is added to the filtrate and heated to dissolve the mixture. Allow the mixture to cool, crystallize, filter, and squeeze the liquid from the solids using a glass stopper. Finally, infrared drying is performed to obtain white crystals of pyromellitic dianhydride with a yield of 92.7%, a purity of 99.8%, and a color of 1.

[0053] Comparative Example 1

[0054] The difference between the embodiment 1 and the embodiment 2 is that the visible light irradiation intensity in step 3 is 250 mW / cm 2 , the irradiation time is 24h.

[0055] The yield of the obtained pyromellitic acid is only 78.6%, and the purity is 82.7%; the yield of the pyromellitic dianhydride obtained in step 4 is 93.5%, the purity is 86.4%, and the chroma is 6.

[0056] Comparative Example 2

[0057] The difference between the pyromellitic dianhydride and Example 1 is that the molar amount of Oxone and KBr in step 3 is 3 times that of 4,5-dimethyl-1,2-benzenedicarboxylic acid.

[0058] The yield of the obtained pyromellitic acid is only 66%, and the purity is 78.2%. The yield of the pyromellitic dianhydride obtained in step 4 is 92.7%, the purity is 83%, and the chroma is 7.

[0059] Comparative Example 3

[0060] Pyromellitic dianhydride differs from Example 1 in that, in step 4, azeotropic evaporation is not employed. Instead, pyromellitic acid and acetic anhydride are heated to 120°C in xylene solvent for anhydrification for 24 hours. The product is isolated and purified as in Example 1. The resulting pyromellitic dianhydride has a yield of 83.3%, a purity of 92.8%, and a color of 4.

[0061] Comparative Example 4

[0062] The difference between the pyromellitic dianhydride and Example 1 is that the amount of acetic anhydride used in step 4 is 6 times the molar amount of pyromellitic acid. The yield of the obtained pyromellitic dianhydride is 94.8%, the purity is 95.2%, and the color is 3.

[0063] Combined with the above results, the yield and purity of the intermediate product pyromellitic acid and the final pyromellitic dianhydride in Examples 1-4 both exceeded those of Comparative Examples 1-4. In Comparative Example 1, the visible light irradiation intensity was insufficient. Even with extended reaction time, the reaction system was still insufficient for strong electronic transitions to occur, resulting in fewer excited-state molecules and less reactivity with free radicals, which reduced the yield of pyromellitic acid. In Comparative Example 2, due to insufficient amounts of oxidant and catalyst, some 4,5-dimethyl-1,2-benzenedicarboxylic acid remained in the intermediate stage of oxidation of the benzylmethyl group to the aldehyde group, resulting in an increase in byproducts. A comparison of Example 1 and Comparative Example 3 demonstrates that azeotropic distillation and separation coupling improve the efficiency of the dehydration reaction and promote the formation of the anhydride. The difference in product purity between Example 1 and Comparative Example 4 demonstrates that the purity of pyromellitic dianhydride is related to the amount of dehydrating agent used. Increasing the amount of dehydrating agent ensures sufficient reaction of the pyromellitic acid, thereby improving the yield. However, excessive amounts of dehydrating agent increase the acetic acid byproduct and make purification of the anhydride more difficult, resulting in a decrease in purity.

[0064] In addition, the inventors of this case also conducted experiments with other raw materials, process operations, and process conditions described in this specification with reference to the aforementioned embodiments, and obtained relatively ideal results.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing pyromellitic dianhydride, characterized in that: Using 2,3-dichloromaleic anhydride as the raw material, a Diels-Alder cycloaddition reaction, an elimination reaction, and an oxidation reaction are sequentially performed to obtain pyromellitic acid with a purity of ≥95% by weight, which is then dehydrated to obtain pyromellitic dianhydride with a purity of ≥99% by weight. The preparation route is as follows: 。 2. The preparation method according to claim 1, wherein Here are the steps: Step 1: 2,3-dichloromaleic anhydride and 2,3-dimethyl-1,3-butadiene are refluxed in an anhydrous organic solvent 1 at 40-50°C for 4-10 hours; after the reaction is completed, the solid product is rotary evaporated and dried to obtain 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride; Step 2: 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride is heated in a water bath with sodium hydroxide ethanol solution and refluxed for 2-6 hours; after the reaction, cooling and crystallization are performed, filtration, extraction, and column chromatography purification are performed to obtain 4,5-dimethyl-1,2-benzenedicarboxylic acid; Step 3: In an organic solvent 2-water mixed solution, 4,5-dimethyl-1,2-benzenedicarboxylic acid, potassium monopersulfate complex salt, and KBr are irradiated with visible light at room temperature, and the reaction progress is monitored by thin layer chromatography; after the reaction is completed, saturated Na2SO3 is added to quench, and the liquid is separated and extracted, impurities are removed by activated carbon, and filtered. The filtrate is washed with brine, dried over anhydrous Na2SO4, concentrated under reduced pressure, and purified by column chromatography to obtain pyromellitic acid; Step 4: Azeotropically distilling pyromellitic acid and a dehydrating agent in an anhydrous organic solvent 3 at 95-120°C, and monitoring the reaction progress by thin-layer chromatography; after the reaction, performing atmospheric distillation and vacuum distillation in sequence, cooling and crystallizing, filtering, and washing to obtain a crude product; reheating and dissolving with acetone and adding activated carbon, filtering while hot, and recrystallizing the filtrate with petroleum ether, filtering, and infrared drying to obtain pyromellitic dianhydride crystals.

3. The preparation method according to claim 2, wherein: In step 1, the molar ratio of 2,3-dimethyl-1,3-butadiene to 2,3-dichloromaleic anhydride is 1.05-1.3; the anhydrous organic solvent 1 is at least one of tetrahydrofuran, acetone, chloroform, and ethyl acetate.

4. The preparation method according to claim 2, wherein: In step 2, the molar ratio of 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride to sodium hydroxide is 1:(2-3); the concentration of the sodium hydroxide ethanol solution is 15wt%-30wt%, and the reflux temperature is 70-75°C; the adsorbent used for column chromatography is alumina, and the eluent used is chloroform / methanol with a v:v ratio of (3-5):

1.

5. The preparation method according to claim 2, wherein: In step 3, the molar ratio of 4,5-dimethyl-1,2-benzenedicarboxylic acid to potassium monopersulfate and KBr is 1:(4-4.5):(4-4.5), the volume ratio of organic solvent 2 to water is 5-9; the organic solvent 2 is at least one of tetrahydrofuran, dichloroethane, and dichloromethane; the visible light irradiation intensity is 300-550 mW / cm 2 The irradiation time was 9~24h; the adsorbent used for column chromatography was alumina, and the eluent was hexane / ethyl acetate with v:v=(19~15):(1~5).

6. The preparation method according to claim 2, wherein: In step 4, the molar ratio of pyromellitic acid to the dehydrating agent is 1:(4-5); the dehydrating agent is acetic anhydride or trifluoroacetic anhydride, and the dehydrating agent needs to be dried before use.

7. The preparation method according to claim 2, wherein: In step 4, the amount of the anhydrous organic solvent 3 is equivalent to 5 to 20 times the mass of pyromellitic acid, and the anhydrous organic solvent 3 is selected from at least one of toluene, xylene, chlorobenzene, pyridine, anisole, diisobutyl ether, and a low alkyl carboxylate; the low alkyl carboxylate is selected from at least one of butyl acetate, n-butyl formate, pentyl formate, butyl acetate, and pentyl acetate.

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