Preparation method of high-purity pyromellitic dianhydride

Abstract

The invention provides a preparation method of high-purity pyromellitic dianhydride, which comprises the following steps: by taking 2, 3-dichloromaleic anhydride as a raw material, sequentially carrying out Diels-Alder cycloaddition reaction, elimination reaction and oxidation reaction to obtain high-purity pyromellitic acid, and then carrying out dehydration treatment to obtain the high-purity pyromellitic dianhydride. According to the preparation method provided by the invention, pyromellitic acid is used as a dehydration raw material to prepare anhydride, so that intermediate acid byproducts are not easy to generate, and the product purity is improved; the dehydration reaction is promoted by adopting azeotropic distillation and separation coupling, so that the reaction yield is increased; preparation steps are few, the reaction process is simple, reaction conditions are mild, and related raw materials are economical, cheap and easy to obtain; the catalyst or auxiliary agent 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 anhydride (PMDA), also known as 1,2,4,5-benzoic anhydride, has the chemical name of 1,2,4,5-benzoic anhydride. It is a white powder or needle-shaped crystal, with a melting point of 284-286°C and a boiling point of 397-400°C. Pyromellitic anhydride is a common pharmaceutical and chemical intermediate. It is one of the most important dianhydride monomers for synthesizing 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 main raw material for high-performance plasticizers. At present, the synthesis methods of pyromellitic anhydride mainly include: (1) duromere oxidation method; (2) trimethylol alkylation-oxidation method; (3) trimethylol carbonylation-oxidation method; (4) xylene chloromethylation-oxidation method. Among them, the more mature process in industry is duromere oxidation method.

[0003] The existing durene oxidation process is divided into two categories: room temperature gas phase oxidation using durene as raw material and liquid phase oxidation using 2,4,5-trimethylbenzaldehyde. The durene gas phase air oxidation process uses oxygen as an oxidant to oxidize durene to generate durene oxidation products, which are then hydrolyzed to obtain pyromellitic anhydride; it includes processes such as gasification, oxidation, capture, hydrolysis decolorization, filtration, and refining; this method is prone to produce by-products, mainly phthalic anhydride and some intermediate acids, such as 4,5-dimethyl diacid, 5-methyl trimellitic acid, 2,5-dimethyl triacid, 4,6-dimethyl triacid, etc., so there are problems such as poor selectivity, low circulation conversion rate, many product impurities, and low product quality yield. The liquid-phase air oxidation process of tetramethylbenzene includes the processes of batching, oxidation, crystallization centrifugation, dehydration to anhydride, refining, and acetic acid recovery. Although the yield of synthetic anhydride is relatively high, the metal catalytic system causes heavy metal residues in the product. In addition, the process route is relatively long, the reaction temperature is harsh, and the equipment requirements are relatively 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 Diels-Alder cycloaddition reaction, elimination reaction and oxidation reaction to obtain high-purity pyromellitic acid, and then performs dehydration treatment 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 in a water bath under reflux at 40-50° C. for 4-10 hours in an anhydrous organic solvent 1; after the reaction is completed, rotary evaporation and the solid product is 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 refluxed in a sodium hydroxide ethanol solution under water bath heating for 2 to 6 hours; after the reaction, cooling, crystallization, filtration, extraction, and column chromatography purification are performed to obtain 4,5-dimethyl-1,2-phthalic 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, liquid separation extraction, activated carbon impurity removal, filtration, 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 at 95-120° C. in an anhydrous organic solvent 3, and the reaction progress is monitored by thin layer chromatography; after the reaction is completed, atmospheric distillation and reduced pressure distillation are carried out 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 anhydride 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%-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 v:v=(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), 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 alumina, and the eluent is hexane / ethyl acetate with v:v=(19 to 15):(1 to 5).

[0014] Potassium monopersulfate complex salt (Oxone) is an inorganic peroxide, which exists in the form of a triple salt of potassium monopersulfate, potassium hydrogen sulfate 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 free radicals (·SO 4- ) and other active oxygen species, Br - SO produced by potassium persulfate 4- Reacts to generate bromine radicals (Br·), which can initiate the subsequent reaction; the excited 4,5-dimethylphthalic 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 forming 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 reduce the dehydration temperature of pyromellitic acid; a water layer is collected by a water separator during the azeotropic distillation process to promote the dehydration reaction to move in the direction of generating pyromellitic dianhydride, accelerate the reaction rate, shorten the reaction time, and improve the yield; and the purity of the product is further increased through the 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 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 in 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 economical, cheap and easily available raw materials; the catalyst or auxiliary agent used in each step does not produce harmful substances, and is green and environmentally friendly. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is further described in detail below in conjunction with several preferred embodiments. This embodiment is implemented on the premise of the technical solution of the invention, and a detailed implementation method and a specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments. The contents not described in detail in the specification of the present invention belong to the known technology of professional and technical personnel in this field.

[0024] Unless otherwise defined, all professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional 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 all conventional methods, and the materials, reagents, etc. used in the examples are all available from commercial sources unless otherwise specified.

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

[0027] A C18 reverse phase column (4.60 mm × 250 mm × 5.0 μm) was used, the column temperature was 35°C, the injection volume was 10 μL; the mobile phase was methanol-water (80:20), the flow rate was 1.0 mL / min; the test time was 24 min, and the detection wavelength was 250 nm. The peak area normalization method was used for quantitative analysis, and the sample must be filtered through a 0.22 μm filter before testing.

[0028] Chroma of pyromellitic dianhydride: Expressed as the chroma of methanol solution. Measure the absorbance of 5.0000g of pyromellitic dianhydride sample in 100mL methanol (AR) at 431nm. 100 times the average absorbance of five measurements is the chroma of the methanol solution.

[0029] Example 1

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

[0031] Step 1: Add 0.10 mol 2,3-dichloromaleic anhydride and 40 mL tetrahydrofuran into a round-bottom flask, and add 0.13 mol 2,3-dimethyl-1,3-butadiene dropwise into the solution under 50°C water bath conditions; after the addition is completed, install a reflux condenser and reflux at a constant temperature for 6 hours; after the reaction is completed, rotary evaporate and 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 88.5%.

[0032] Step 2: Add 0.16mol sodium hydroxide and 40mL 95% ethanol aqueous solution to a round-bottom flask, and then add 78mmol 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride; keep the reaction system in a stirring state, reflux reaction in a 75°C water bath, and use thin layer chromatography to determine the reaction is complete. The solution is slightly cooled, and 0.5mol / L dilute hydrochloric acid is added dropwise and stirred until pH~4; chloroform extraction is performed 3 times, the organic phases are combined and concentrated, and placed in an ice water bath for crystallization; filter, wash the solid with cold chloroform, dry with anhydrous Na2SO4, chromatograph on an alumina column (chloroform: methanol = 3:1), and distill under reduced pressure to obtain 4,5-dimethyl-1,2-benzenedicarboxylic acid with a yield of 82.5%.

[0033] Step 3: In a 360 mL dichloromethane-water (v:v=8:1) mixed solution, 60 mmol 4,5-dimethyl-1,2-benzenedicarboxylic acid, 0.26 mol Oxone, and 0.26 mol KBr were irradiated with visible light (550 mW / cm 2 , PLS-SXE 300 xenon lamp light source, Beijing Bofeilai Technology Co., Ltd.) was stirred at room temperature for 12 hours; after the reaction was completed, it was quenched with 50 mL of saturated Na2SO3, extracted with ethyl acetate three times, activated carbon was added to the collected organic phase, stirred for 1 hour, hot filtered, the filtrate was washed with brine, dried over anhydrous Na2SO4, concentrated under reduced pressure, and purified by alumina column chromatography (ethyl acetate: hexane = 1:8) to obtain 2-hydroxy-1,4-dimethylbenzene tetracarboxylic acid with a yield of 93.1% and a purity of 96.4%.

[0034] Step 4: In a three-necked flask with a reflux condenser and a water separator, add 54mmol of pyromellitic acid, 0.27mol of acetic anhydride, 76mL of toluene and 24mL of chlorobenzene, and slowly heat up in an oil bath to dissolve completely; continue to heat up to 120℃ for azeotropic distillation, the water layer remains at the bottom of the water separator, the organic phase refluxes into the flask, and the reaction progress is monitored by thin layer chromatography. After the reaction is completed, wait for the solution to cool to room temperature and transfer it to a distillation apparatus; 60℃ atmospheric distillation to recover acetic anhydride; reduced pressure distillation to remove impurities such as toluene and chlorobenzene; the distillation residue is cooled and crystallized in an ice bath, and the crude product is filtered; the crude product is washed with methyl tert-butyl ether, heated and dissolved in acetone again, and then activated carbon is added and stirred for 1h and filtered while hot, petroleum ether is added to the filtrate and heated and dissolved, and the filtrate is cooled and crystallized, and the liquid in the solid is squeezed out with a glass stopper, and infrared drying is performed to obtain white crystals of pyromellitic anhydride, with a yield of 94.3%, a purity of 99.0%, and a chromaticity 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 into a round-bottom flask, and dropwise add 0.18 mol of 2,3-dimethyl-1,3-butadiene into the solution under a 55°C water bath condition; after the addition is completed, install a reflux condenser and reflux at a constant temperature for 8 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; the yield is 86.4%;

[0038] Step 2: Add 0.22mol sodium hydroxide and 40mL 95% ethanol aqueous solution to a round-bottom flask and mix, then add 0.10mol 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride; keep the reaction system in a stirring state, reflux reaction in a 72°C water bath, and use thin layer chromatography to determine that the reaction is complete. The solution is slightly cooled, and 0.5mol / L dilute hydrochloric acid is added dropwise and stirred until pH~4; chloroform extraction is performed 3 times, the organic phases are combined and concentrated, and crystallization is performed in an ice water bath; filter, wash the solid with cold chloroform, dry with anhydrous Na2SO4, chromatograph on an alumina column (chloroform: methanol = 4:1), and distill under reduced pressure to obtain 4,5-dimethyl-1,2-benzenedicarboxylic acid; the yield is 81.7%.

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

[0040] Step 4: In a three-necked flask with a reflux condenser and a water separator, add 60mmol of pyromellitic acid, 0.29mol of acetic anhydride, and 110mL of toluene, and slowly heat up in an oil bath to dissolve completely; continue to heat to 105°C for azeotropic distillation, the water layer remains at the bottom of the water separator, the organic phase refluxes into the flask, and the reaction progress is monitored by thin layer chromatography. After the reaction is completed, wait for the solution to cool to room temperature, transfer it to a distillation apparatus, and remove impurities such as p-xylene, acetic anhydride, butyl acetate, and acetic acid by reduced pressure distillation; the distillation residue is cooled and crystallized in an ice bath, and the crude product is filtered; the crude product is washed with methyl tert-butyl ether, heated and dissolved in acetone again, then activated carbon is added and stirred for 1h, filtered while hot, petroleum ether is continued to be added to the filtrate to heat and dissolve, stand and cool, cool and crystallize, filter, squeeze out the liquid in the solid with a glass stopper, and infrared drying to obtain white crystals of pyromellitic anhydride, with a yield of 93.6%, a purity of 99.3%, and a chromaticity of 1.

[0041] Example 3

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

[0043] Step 1: Add 96mmol 2,3-dichloromaleic anhydride and 40mL tetrahydrofuran into a round-bottom flask, and add 0.11mol 2,3-dimethyl-1,3-butadiene dropwise into the solution under 50°C water bath condition; after the addition is completed, install a reflux condenser and reflux at a constant temperature for 5.5h; after the reaction is completed, rotary evaporate and vacuum dry the solid product at 60°C for 6h to obtain 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride with a yield of 85.1%.

[0044] Step 2: Add 0.18 mol of sodium hydroxide and 40 mL of 95% ethanol aqueous solution to a round-bottom flask, and then add 80 mmol of 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride; keep the reaction system in a stirring state, reflux reaction in a 71°C water bath, and use thin layer chromatography to determine that the reaction is complete. The solution is slightly cooled, and 0.5 mol / L dilute hydrochloric acid is added dropwise and stirred until pH ~ 4; chloroform extraction is performed 3 times, the organic phases are combined and concentrated, and placed in an ice water bath for crystallization; filter, wash the solid product with cold chloroform, dry with anhydrous Na2SO4, chromatograph on an alumina column (chloroform: methanol = 5:1), and distill under reduced pressure to obtain 4,5-dimethyl-1,2-benzenedicarboxylic acid with a yield of 79.6%.

[0045] Step 3: In a 300 mL dichloromethane-water (v:v=9:1) mixed solution, 55 mmol 4,5-dimethyl-1,2-benzenedicarboxylic acid, 0.23 mol Oxone, and 0.23 mol KBr were irradiated with visible light (400 mW / cm 2 , PLS-SXE 300 xenon lamp light source, Beijing Bofeilai Technology Co., Ltd.) was stirred at room temperature for 18 hours; after the reaction was completed, it was quenched with 50 mL of saturated Na2SO3, extracted with ethyl acetate three times, activated carbon was added to the collected organic phase, stirred for 1 hour, hot filtered, the filtrate was washed with brine, dried with Na2SO4, concentrated under reduced pressure, and purified by alumina column chromatography (ethyl acetate: hexane = 1:8) to obtain 2-methyl-1-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 isophthalic acid, 0.22 mol of trifluoroacetic anhydride, 75 mL of pyridine and 15 mL of p-xylene, and slowly heat up in an oil bath to completely dissolve; continue to heat up to 120°C for azeotropic distillation, with the water 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 is completed, the solution is cooled to room temperature and transferred to a distillation device, and low-boiling point substances such as trifluoroacetic anhydride are recovered by atmospheric distillation at 45°C; then, p-xylene and pyridine impurities are removed by reduced pressure distillation; the distillation residue is cooled in an ice bath for crystallization, and a crude product is obtained by filtration; the crude product is washed with methyl tert-butyl ether, heated and dissolved in acetone again, and then activated carbon is added and stirred for 1 hour, filtered while hot, petroleum ether is added to the filtrate and heated and dissolved, and the filtrate is allowed to stand for cooling, crystallized by cooling, filtered, and the liquid in the solid is squeezed out with a glass stopper, and infrared drying is performed to obtain white crystals of pyromellitic anhydride 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 2,3-dichloromaleic anhydride and 36 mL tetrahydrofuran into a round-bottom flask, and add 94.5 mmol 2,3-dimethyl-1,3-butadiene dropwise into the solution under 55°C water bath condition; after the addition is completed, 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: Add 0.16mol sodium hydroxide and 30mL 95% ethanol aqueous solution to a round-bottom flask, and then add 68mmol 1,2-dichloro-4,5-dimethyl-1,2,3,6-tetrahydrophthalic anhydride; keep the reaction system in a stirring state, reflux reaction in a 71°C water bath, and use thin layer chromatography to determine the reaction is complete. The solution is slightly cooled, and 0.5mol / L dilute hydrochloric acid is added dropwise and stirred until pH~4; chloroform extraction is performed 3 times, the organic phases are combined and concentrated, and placed in an ice water bath for crystallization; filter, wash the solid product with cold chloroform, dry with anhydrous Na2SO4, chromatograph on an alumina column (chloroform: methanol = 5:1), and distill under reduced pressure to obtain 4,5-dimethyl-1,2-benzenedicarboxylic acid with a yield of 80.4%.

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

[0052] Step 4: In a three-necked flask with a reflux condenser and a water separator, add 36mmol of pyromellitic acid, 0.15mol of acetic anhydride, 60mL of butyl acetate and 12mL of toluene, and slowly heat the flask to 60°C in an oil bath to dissolve completely; continue to heat to 120°C for azeotropic distillation, the water layer remains at the bottom of the water separator, the organic phase refluxes into the flask, and the reaction progress is monitored by thin layer chromatography. After the reaction is completed, wait for the solution to cool to room temperature and transfer it to a distillation apparatus; remove impurities such as toluene, acetic acid and acetic anhydride by vacuum distillation; cool the distillation residue in an ice bath for crystallization, and filter to obtain a crude product; wash the crude product with methyl tert-butyl ether, heat and dissolve it in acetone again, then add activated carbon and stir for 1h, filter it while hot, add petroleum ether to the filtrate and heat and dissolve it, let it stand and cool, crystallize by cooling, filter, squeeze out the liquid in the solid with a glass stopper, and finally dry it with infrared to obtain white crystals of pyromellitic anhydride with a yield of 92.7%, a purity of 99.8%, and a chromaticity 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 pyromellitic dianhydride obtained in step 4 is 93.5%, the purity is 86.4%, and the chromaticity is 6.

[0056] Comparative Example 2

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

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

[0059] Comparative Example 3

[0060] The difference between the pyromellitic acid dianhydride and the embodiment 1 is that the step 4 does not adopt azeotropic evaporation, the pyromellitic acid and acetic anhydride are heated to 120°C in a xylene solvent for heating anhydride, and the reaction time is 24 hours; the separation and purification process of the product is the same as that of the embodiment 1. The yield of the obtained pyromellitic acid dianhydride is 83.3%, the purity is 92.8%, and the chromaticity is 4.

[0061] Comparative Example 4

[0062] The difference between the pyromellitic acid dianhydride and the embodiment 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 acid dianhydride is 94.8%, the purity is 95.2%, and the chromaticity is 3.

[0063] Combined with the above results, the intermediate product of pyromellitic acid and the final pyromellitic anhydride of Examples 1 to 4 have higher yields and purities than Comparative Examples 1 to 4. Among them, the visible light irradiation intensity of Comparative Example 1 is insufficient. Even if the time is extended, the reaction system is still insufficient for strong electronic transitions to occur, and there are fewer excited state molecules, which are not easy to react with free radicals, so that the yield of pyromellitic acid is reduced; while in Comparative Example 2, due to insufficient amounts of oxidant and catalyst, part of 4,5-dimethyl-1,2-benzenedicarboxylic acid stays in the intermediate stage of benzylmethyl oxidation to aldehyde group, resulting in an increase in by-products. The comparison between Example 1 and Comparative Example 3 shows that azeotropic distillation and separation coupling improve the efficiency of dehydration reaction and promote the formation of anhydride. The difference in product purity between Example 1 and Comparative Example 4 shows that the purity of pyromellitic anhydride is related to the amount of dehydrating agent. Increasing the amount of dehydrating agent can ensure the full reaction of pyromellitic acid, thereby improving the yield; but if the dehydrating agent is too much, the acetic acid by-product increases and the purification difficulty of anhydride increases, 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 aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned 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 high-purity pyromellitic dianhydride, characterized in that: Using 2,3-dichloromaleic anhydride as a raw material, a Diels-Alder cycloaddition reaction, an elimination reaction and an oxidation reaction are sequentially performed to obtain high-purity pyromellitic acid, and then a dehydration treatment is performed to obtain high-purity pyromellitic dianhydride; the preparation route is as follows:

2. The preparation method according to claim 1, characterized in that 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, rotary evaporation and the solid product is 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 refluxed in a sodium hydroxide ethanol solution under water bath heating for 2 to 6 hours; after the reaction, cooling, crystallization, 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 for quenching, liquid separation extraction is performed, activated carbon is used for impurity removal, and filtering is performed, and 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: azeotropically distilling pyromellitic acid and a dehydrating agent in an anhydrous organic solvent 3 at 95-120° C., and monitoring the progress of the reaction by thin layer chromatography; after the reaction is completed, performing atmospheric distillation and reduced pressure 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 high-purity pyromellitic anhydride crystals.

3. The preparation method according to claim 2, characterized in that: 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.

4. The preparation method according to claim 2, characterized in that: 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 aluminum oxide, and the eluent used is chloroform / methanol with v:v=(3-5):

1.

5. The preparation method according to claim 2, characterized in that: 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), 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 alumina, and the eluent is hexane / ethyl acetate with v:v=(19 to 15):(1 to 5).

6. The preparation method according to claim 2, characterized in that: 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, characterized in that: 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 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.

8. The preparation method according to any one of claims 1 to 7, characterized in that: The purity of the pyromellitic acid is greater than 95% by weight.

9. The preparation method according to any one of claim 8, characterized in that: The purity of the pyromellitic dianhydride is greater than 98% by weight, preferably 99% by weight.

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