Preparation method of high-purity pyromellitic dianhydride and high-purity pyromellitic dianhydride obtained thereby

Abstract

The application discloses a preparation method of high-purity pyromellitic dianhydride and high-purity pyromellitic dianhydride obtained by the method. The preparation method comprises the following steps: taking crude pyromellitic acid as raw material, and sequentially performing dehydration and melt crystallization treatment to obtain high-purity pyromellitic dianhydride. In the method, the melt crystallization technology is used to purify the pyromellitic dianhydride, and the dehydration reaction of the pyromellitic acid is coupled with the melt crystallization technology. The method not only utilizes the outstanding advantages of the melt crystallization, such as small environmental pollution and low energy consumption, but also integrates the dehydration reaction and the crystallization process in an integrated kettle type reaction device. The coupling operation is more suitable for the requirements of green production of the modern chemical industry.

Description

Technical Field

[0001] This invention belongs to the field of purification or preparation, and particularly relates to a method for preparing high-purity pyromellitic dianhydride and the obtained high-purity pyromellitic dianhydride. Background Technology

[0002] Pyromellitic dianhydride (PMDA) has a melting point of 284–287°C and is a white powder at room temperature. It is an important industrial raw material in the materials and chemical industries. Fully aromatic polyimides (PI) synthesized from pyromellitic dianhydride possess high heat resistance, strong hydrolysis resistance, high flexibility, and strong mechanical properties, and have been widely used in aerospace, microelectronics, nanotechnology, liquid crystals, separation membranes, lasers, and other fields.

[0003] PMDA is currently mainly prepared using a gas-phase oxidation process with mesitylene. The crude product is then indirectly collected to obtain crude anhydride. The crude anhydride obtained from a single collection process has a higher purity, typically 92%–98%; the crude anhydride obtained from a second collection process has a relatively lower purity, generally between 70% and 80%; and the crude anhydride obtained from multiple collection processes has even lower purity. The main impurities in PMDA are phthalic anhydride, trimellitic anhydride, pyromellitic acid, pyromellitic acid monoanhydride, 5-methyltrimeric anhydride, mechanical impurities, and ultrafine foreign matter. There are various methods for purifying PMDA crude anhydride; currently, the mainstream process in industrial production is the hydrolysis-dehydration-sublimation process. The process involves first hydrolyzing crude anhydride into pyromellitic acid, followed by decolorization and recrystallization to obtain purified pyromellitic acid, which is then dehydrated to obtain secondary crude anhydride. Next, a certain amount of silica gel is added to the surface of the secondary crude anhydride, and it is sublimated and recrystallized under high vacuum in a sublimation reactor to obtain high-purity PMDA (see CN103435628A, 2013). The disadvantages of this process are the need to transfer the secondary crude anhydride from the dehydration reactor to the sublimation reactor, resulting in complex procedures, high energy consumption, and significant dust pollution.

[0004] In response to the aforementioned problems in the PMDA production process, other PMDA purification methods have been extensively studied. These mainly include the following methods: (1) Hot gas flow carrying method, which uses high-temperature gas to sublimate the anhydride and then cools the mixed gas to condense the product (see US3328428A, 1967; EP0612748A, 1994; CN103435928A, 2001, etc.). The large-scale application of this process requires heating and cooling a large amount of gas, which places high demands on the production equipment. (2) Vacuum distillation method, which purifies crude anhydride by vacuum distillation under high vacuum and high temperature conditions (see US4014755, 1977). However, the pipeline is prone to blockage during the production process, the process is difficult to operate, and the resulting product is dark in color. (3) Solution washing purification method: This method uses anhydrous solvents such as benzonitrile, 1,4-dioxane, diethyl ether, or acetic anhydride. First, soluble impurities in the crude anhydride with low purity are removed by washing. Then, the washed solid material is decolorized and recrystallized to obtain a high-purity PMDA product (see JPS61215352A, 1985; CN101580509B, 2009, etc.). This method improves the purity of the low-purity dioxin through washing pretreatment, followed by decolorization and recrystallization. (4) Solution decolorization and recrystallization purification method: High-purity crude anhydride product (purity > 90%) is used. It is dissolved in acetic anhydride or a mixture of acetic anhydride and other organic solvents under heating. Activated carbon, kaolin, alumina and other adsorbents are added for decolorization. After hot filtration and cooling crystallization, high-purity PMDA product can be obtained. The solvent used can be recovered by distillation (see US2002049339A1, 2002; CN100506778C, 2004; CN102336762A, 2010; CN104892621B, 2015, etc.). Although the above-mentioned solution washing purification method (3) and solution decolorization and recrystallization purification method (4) can obtain high-purity PMDA, obtaining high-purity products requires repeated recrystallization and multiple mother liquor recovery, making the production process relatively complex. The management, use and post-treatment of a large amount of acetic anhydride during the production process also increases the production cost. Therefore, the large-scale production of PMDA in China still adopts the mainstream process method mentioned above, namely the crude anhydride hydrolysis-dehydration-sublimation process. However, this production process is not only energy-intensive, but also cumbersome to operate and difficult to manage in an orderly manner. Summary of the Invention

[0005] To overcome the problems existing in the prior art, this invention provides a method for preparing high-purity pyromellitic dianhydride and the resulting high-purity pyromellitic dianhydride. The purpose of this invention is to overcome the shortcomings of existing pyromellitic dianhydride sublimation purification processes and provide a simple purification method that does not introduce additional solvents, as well as a new method for preparing high-purity pyromellitic dianhydride that integrates dehydration reaction and melt crystallization purification technology. Specifically, the use of melt crystallization technology to purify pyromellitic dianhydride, and the coupling of the dehydration reaction of pyromellitic dianhydride with melt crystallization technology, not only utilizes the significant advantages of melt crystallization—low environmental pollution and low energy consumption—but also conducts the dehydration reaction and crystallization process in an integrated batch reactor. This coupled operation is more suitable for the requirements of green production in modern chemical industries.

[0006] One of the objectives of this invention is to provide a method for preparing high-purity pyromellitic dianhydride, comprising: using crude pyromellitic acid as raw material, sequentially performing dehydration and melt crystallization treatments to obtain high-purity pyromellitic dianhydride.

[0007] This invention proposes for the first time to couple dehydration with melt crystallization to directly prepare high-purity pyromellitic dianhydride from pyromellitic acid.

[0008] In a preferred embodiment, the melt crystallization process includes melting, crystallization, and sweating.

[0009] The crude pyromellitic acid is dehydrated to obtain crude pyromellitic dianhydride; the crude pyromellitic dianhydride is then subjected to melt crystallization treatment: first, the crude pyromellitic dianhydride is melted to obtain a molten liquid, then the molten liquid is crystallized to obtain crude pyromellitic dianhydride crystals, and finally, the crude pyromellitic dianhydride crystals are subjected to perspiration to obtain high-purity pyromellitic dianhydride crystals.

[0010] In a preferred embodiment, the dehydration conditions include: raising the temperature to 200-300°C, maintaining a vacuum degree below 0.1 MPa, and maintaining the time for 2-10 hours.

[0011] In a further preferred embodiment, the dehydration conditions include: a temperature of 230–280°C, a vacuum level maintained below 0.095 MPa, and a time of 4–7 hours.

[0012] For example, the dehydration conditions include: the temperature rising to 230°C, 240°C, 250°C, 260°C, 270°C or 280°C, the vacuum degree being maintained below 0.095 MPa, and the time being maintained at 4h, 5h, 6h or 7h.

[0013] In a preferred embodiment, the dehydration is carried out in a melt crystallizer.

[0014] In a preferred embodiment, the melting is carried out as follows: first, the temperature is raised to 285-350°C until the material (crude pyromellitic dianhydride) is completely melted, and then the temperature is controlled at 285-320°C for 1-50 minutes to obtain a molten liquid (pyromellitic dianhydride).

[0015] In a further preferred embodiment, the melting is carried out as follows: first, the temperature is raised to 290-320°C until the material (crude pyromellitic dianhydride) is completely melted, and then the temperature is controlled at 285-300°C for 2-30 minutes to obtain a molten liquid (pyromellitic dianhydride).

[0016] For example, the melting process is carried out as follows: first, the temperature is raised to 290°C, 300°C, 310°C or 320°C until the material (crude pyromellitic dianhydride) is completely melted, and then the temperature is controlled at 285°C, 290°C, 295°C or 300°C for 2 min, 5 min, 10 min, 15 min, 20 min, 25 min or 30 min to obtain a molten liquid (pyromellitic dianhydride).

[0017] In a further preferred embodiment, the melting is carried out as follows: first, the temperature is raised to 295-305°C until the crude pyromellitic dianhydride is completely melted, and then the temperature is controlled at 288-295°C for 2-30 minutes to obtain a molten liquid of pyromellitic dianhydride.

[0018] In a preferred embodiment, after the melting treatment, a molten liquid is obtained, and the molten liquid is crystallized, wherein the crystallization is carried out as follows: the temperature is lowered to 276-284°C at a rate of 0.1-1.0°C / min and maintained for 10-60 min to allow pyromellitic dianhydride to fully crystallize. After the mother liquor is discharged, crude pyromellitic dianhydride crystals are obtained.

[0019] For example, the crystallization is carried out as follows: the temperature is lowered to 276℃, 277℃, 278℃, 279℃, 280℃, 281℃, 282℃, 283℃, or 284℃ at a rate of 0.1℃ / min, 0.2℃ / min, 0.3℃ / min, 0.4℃ / min, 0.5℃ / min, 0.6℃ / min, 0.7℃ / min, 0.8℃ / min, 0.9℃ / min, or 1.0℃ / min, and maintained for 10 min, 20 min, 30 min, 40 min, 50 min, or 60 min, to allow the pyromellitic dianhydride to fully crystallize. After draining the mother liquor, crude pyromellitic dianhydride crystals are obtained.

[0020] In a further preferred embodiment, during the crystallization, the cooling rate is 0.1–0.3 °C / min, and the crystallization time is maintained at 40–60 min.

[0021] In a preferred embodiment, after crystallization, crude crystals are obtained. The crude crystals (pyromellitic dianhydride crude crystals) are then subjected to sweating, wherein the sweating process is as follows: the crystals are heated to 280-320°C at a heating rate of 0.05-2.0°C / min (to induce sweating), and maintained for 5-80 minutes (to expel sweat), thereby obtaining refined pyromellitic dianhydride crystals.

[0022] In a further preferred embodiment, the sweating process is as follows: the crystals (crude pyromellitic dianhydride crystals) are heated to 280-300°C at a heating rate of 0.1-1.0°C / min (to induce sweating), and maintained for 10-50 minutes (to expel sweat), thereby obtaining refined pyromellitic dianhydride crystals.

[0023] For example, the sweating process is as follows: the crystals (crude pyromellitic dianhydride crystals) are heated to 280°C, 285°C, 290°C, 295°C, or 300°C at a heating rate of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1.0°C / min (to induce sweating), and maintained for 10 min, 20 min, 30 min, 40 min, or 50 min (to expel sweat), to obtain refined pyromellitic dianhydride crystals.

[0024] In a further preferred embodiment, the sweating process is as follows: the crude crystals (pyromellitic dianhydride crude crystals) are heated to 282-290°C at a heating rate of 0.1-0.3°C / min (to induce sweating), and maintained for 10-50 minutes (to expel sweat) to obtain high-purity pyromellitic dianhydride crystals.

[0025] In a preferred embodiment, the melt crystallization process is repeated 0 to 5 times, preferably 0 to 3 times, and more preferably 0 to 2 times.

[0026] In a preferred embodiment, the discharged crystallization mother liquor and sweat are collected and mixed, and reserved as raw materials for the next melting and crystallization.

[0027] In this invention, the purity of the final purified product obtained by the method, as determined by liquid chromatography, is ≥99.5%.

[0028] Specifically, this method involves adding pyromellitic dianhydride into a melt crystallizer, first performing a dehydration reaction under vacuum to obtain crude pyromellitic dianhydride, then continuing heating under normal pressure until the pyromellitic dianhydride is completely melted, and then achieving the preparation of high-purity pyromellitic dianhydride crystals with a purity of over 99.5% through a series of operations such as programmed cooling and sweating.

[0029] The coupling of pyromellitic dianhydride dehydration and melt crystallization has two advantages: 1. It is compatible with the existing oxidative dehydration process for producing pyromellitic dianhydride; 2. The fusion of the two unit operations greatly simplifies the experimental setup.

[0030] A second objective of this invention is to provide high-purity or refined pyromellitic dianhydride obtained by the method described in one objective of this invention, preferably with a purity ≥ 99.5%.

[0031] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values; these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In the following, various technical solutions can, in principle, be combined with each other to obtain new technical solutions, which should also be considered as specifically disclosed herein.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) Compared with the existing pyromellitic dianhydride sublimation purification process, no solvent is required, resulting in less pollution. In particular, compared with the existing sublimation purification process (which mainly solves the problems of high energy consumption and complicated process of the existing sublimation purification process), the integrated batch reactor avoids the series of intermediate steps such as transferring from the dehydration reactor to the sublimation reactor in the original process, making the operation simpler, and the equipment cost and operating cost are relatively low, which is conducive to industrial promotion.

[0034] (2) Compared with the prior art, the present invention couples the dehydration reaction with the melt crystallization technology, and the resulting pyromellitic dianhydride product has high purity, simple operation process, green and environmentally friendly process route, and the crystallization mother liquor and sweating liquid can be centrally recycled and reused, which better meets the requirements of large-scale industrial production.

[0035] (3) This invention couples dehydration reaction with melt crystallization technology, which solves the problems of high energy consumption, cumbersome operation and difficult orderly management of production environment in the existing solution crystallization + sublimation refining production process. Attached Figure Description

[0036] Figure 1 The image shows the appearance of the pyromellitic dianhydride prepared according to the present invention.

[0037] Figure 2 The liquid chromatogram of the hydrolysis of pyromellitic dianhydride to pyromellitic acid prepared according to the present invention is shown.

[0038] Figure 3The NMR spectrum of the hydrolysis of pyromellitic dianhydride to pyromellitic acid prepared according to the present invention is shown. Detailed Implementation

[0039] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0040] It should also be noted that the various specific technical features described in the following embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the various possible combinations will not be described separately in this invention.

[0041] Furthermore, various embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention. The resulting technical solutions are part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0042] Unless otherwise specified, the raw materials used in the embodiments are all publicly available in the prior art, such as those that can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0043] Example 1

[0044] Weigh 600.0g of crude pyromellitic acid raw material and add it to the melting crystallizer. Seal the lid and raise the temperature to 200℃ with a vacuum of 0.010Mpa. Maintain this temperature for 10 hours until no more water vapor is emitted. Then, turn off the vacuum system.

[0045] The temperature was further increased to 285℃ under normal pressure to completely melt the crude pyromellitic dianhydride. The melt was then maintained at 285℃ for 1 minute, followed by a cooling rate of 0.3℃ / min to 276℃. After maintaining this temperature for 50 minutes, the mother liquor was discharged, yielding crude pyromellitic dianhydride crystals. These crystals were then subjected to a heating and sweating process at a rate of 0.1℃ / min, reaching 280℃ and maintaining this temperature for 15 minutes to expel the sweat, resulting in primary purified pyromellitic dianhydride crystals.

[0046] Repeat the above melting, crystallization, and sweating process: continue heating at atmospheric pressure to 290℃ to completely melt the first-refined pyromellitic dianhydride. Then, cool the melt to 288℃ and maintain it for 3 minutes. Next, cool it to 276℃ at a rate of 1.0℃ / min and maintain it for 10 minutes before draining the crystallization mother liquor. The resulting crystals are then subjected to a heating and sweating process at a rate of 1.0℃ / min, heating to 280℃ and maintaining it for 15 minutes to remove the sweat. The resulting crystals are then purified in a melt crystallizer to obtain second-refined pyromellitic dianhydride crystals. Figure 1 The product shown appears as colorless needle-like crystals, as analyzed by high-performance liquid chromatography (HPLC). Figure 2 As shown in the figure, the purity is greater than 99.5%, and the product yield is approximately 58.6%.

[0047] Example 2

[0048] Weigh 800.0g of crude pyromellitic acid raw material and add it to the melting crystallizer. Seal the lid of the vessel, raise the temperature to 200℃, and set the vacuum degree to 0.095Mpa. Maintain this temperature for 7 hours until no more water vapor is emitted. Then, turn off the vacuum system.

[0049] The temperature was further increased to 350℃ under normal pressure to completely melt the crude pyromellitic dianhydride. The melt was then cooled to 320℃ and held for 50 minutes. Next, the temperature was lowered to 284℃ at a rate of 0.5℃ / min and held for 10 minutes before the mother liquor was discharged, yielding crude pyromellitic dianhydride crystals. These crystals were then subjected to a heating and sweating process at a rate of 0.1℃ / min, reaching 290℃ and holding for 10 minutes to expel the sweat, resulting in primary purified pyromellitic dianhydride crystals. The product appeared as colorless needle-like crystals, and high-performance liquid chromatography analysis showed a purity greater than 99.5% and a yield of approximately 52.3%.

[0050] Example 3

[0051] Weigh 700.0g of crude pyromellitic acid raw material and add it to the melting crystallizer. Seal the lid and raise the temperature to 300℃ with a vacuum of 0.083Mpa. Maintain this temperature for 2 hours until no more water vapor is emitted. Then, turn off the vacuum system.

[0052] The temperature was further increased to 300℃ under normal pressure to completely melt the crude pyromellitic dianhydride. The melt was then cooled to 288℃ and held for 3 minutes. Next, the temperature was lowered to 276℃ at a rate of 0.1℃ / min and held for 60 minutes before the mother liquor was discharged, yielding crude pyromellitic dianhydride crystals. These crystals were then subjected to a heating and sweating process at a rate of 1.0℃ / min, reaching 282℃ and held for 30 minutes to expel the sweat, resulting in primary purified pyromellitic dianhydride crystals.

[0053] The temperature was further increased to 290℃ under normal pressure to melt all the primary purified pyromellitic dianhydride crystals. The melt was then cooled to 288℃ and held for 3 minutes. Next, it was cooled to 276℃ at a rate of 1.0℃ / min and held for 10 minutes before the mother liquor was discharged. The resulting crude crystals were then subjected to a heating-to-sweating process at a rate of 0.05℃ / min, reaching 280℃ and holding for 80 minutes to expel the sweat. The resulting secondary purified pyromellitic dianhydride crystals were obtained in a melt crystallizer. The product appeared as colorless needle-like crystals. High-performance liquid chromatography analysis showed a purity greater than 99.5% and a yield of approximately 61.7%.

[0054] Example 4

[0055] Weigh 650.0g of crude pyromellitic acid raw material and add it to the melting crystallizer. Seal the lid and raise the temperature to 280℃ with a vacuum of 0.087Mpa. Maintain this temperature for 6 hours until no more water vapor is emitted. Then, turn off the vacuum system.

[0056] Under normal pressure, the temperature was further increased to 295℃ to completely melt the crude pyromellitic dianhydride. The melt was then cooled to 288℃ and held for 30 minutes. The temperature was then decreased to 284℃ at a rate of 0.7℃ / min and held for 60 minutes before the mother liquor was discharged. The resulting crude pyromellitic dianhydride crystals were then subjected to a heating and sweating process at a rate of 0.3℃ / min. The temperature was increased to 300℃ and held for 50 minutes to expel the sweat, resulting in a first-stage purified pyromellitic dianhydride crystal.

[0057] The temperature was raised again to 302℃ under normal pressure to melt all the primary refined pyromellitic dianhydride crystals. The melt was then cooled to 295℃ and held for 30 minutes. The temperature was then lowered to 284℃ at a rate of 0.2℃ / min and held for 60 minutes before the mother liquor was discharged. The resulting crude pyromellitic dianhydride crystals were then heated to 290℃ and held for 15 minutes to induce sweating, thus obtaining secondary refined pyromellitic dianhydride crystals.

[0058] Then, under normal pressure, the temperature was further increased to 302℃ to completely melt the second-refined pyromellitic dianhydride crystals. The melt was then cooled to 295℃ and held for 30 minutes. Next, the temperature was lowered to 284℃ at a rate of 0.5℃ / min and held for 60 minutes before the mother liquor was discharged. The resulting crude crystals were then subjected to a heating-to-sweating process at a rate of 2℃ / min, reaching 320℃ and holding for 5 minutes to expel the sweat, thus obtaining the third-refined pyromellitic dianhydride crystals. The product appeared as colorless needle-like crystals. High-performance liquid chromatography analysis showed a purity greater than 99.5% and a yield of approximately 50.8%.

[0059] Example 5

[0060] Weigh 680.0g of crude pyromellitic acid raw material and add it to the melting crystallizer. Seal the lid and raise the temperature to 265℃ with a vacuum of 0.092Mpa. Maintain this temperature for 5 hours until no more water vapor is emitted. Then, turn off the vacuum system.

[0061] The temperature was further increased to 295℃ under normal pressure to completely melt the crude pyromellitic dianhydride. The melt was then cooled to 294℃ and held for 20 minutes. The temperature was then decreased to 276℃ at a rate of 1.0℃ / min and held for 50 minutes before the mother liquor was discharged. The resulting crude crystals were heated to 280℃ at a rate of 1.0℃ / min to induce sweating. The temperature was then increased to 280℃ and held for 40 minutes to induce sweating, resulting in a first-stage purified pyromellitic dianhydride crystal.

[0062] The temperature was then raised again to 295℃ under normal pressure to completely melt the first-refined pyromellitic dianhydride crystals. The melt was then cooled to 294℃ and held for 20 minutes. Next, it was cooled to 276℃ at a rate of 1.0℃ / min and held for 10 minutes before the mother liquor was discharged. The resulting crude pyromellitic dianhydride crystals were then subjected to a heating-to-sweating process at a rate of 0.3℃ / min, reaching 280℃ and holding for 30 minutes to expel the sweat. The resulting crystals were then purified in a melt crystallizer to obtain second-refined pyromellitic dianhydride crystals. The product appeared as colorless needle-like crystals. High-performance liquid chromatography analysis showed a purity greater than 99.5% and a yield of approximately 59.6%.

[0063] Example 6

[0064] Weigh 590.0g of crude pyromellitic acid raw material and add it to the melting crystallizer. Seal the lid and raise the temperature to 300℃ with a vacuum of 0.090Mpa. Maintain this temperature for 2 hours until no more water vapor is emitted. Then, turn off the vacuum system.

[0065] The temperature was further increased to 302℃ under normal pressure to completely melt the crude pyromellitic dianhydride. The melt was then cooled to 295℃ and held for 30 minutes. Next, the temperature was lowered to 284℃ at a rate of 0.5℃ / min and held for 60 minutes before the mother liquor was discharged. The resulting crude pyromellitic dianhydride crystals were then heated to 290℃ at a rate of 0.1℃ / min to induce sweating, and the temperature was maintained for 30 minutes to expel the sweat, yielding first-stage purified pyromellitic dianhydride crystals. The product appeared as colorless needle-like crystals. High-performance liquid chromatography analysis showed a purity greater than 99.5% and a yield of approximately 53.7%.

[0066] Example 7

[0067] Weigh 630.0g of crude pyromellitic acid raw material and add it to the melting crystallizer. Seal the lid and raise the temperature to 245℃ with a vacuum of 0.075Mpa. Maintain this temperature for 7 hours until no more water vapor is emitted. Then, turn off the vacuum system.

[0068] The temperature was further increased to 299℃ under normal pressure to completely melt the crude pyromellitic dianhydride. The melt was then cooled to 291℃ and held for 15 minutes. Next, the temperature was lowered to 280℃ at a rate of 0.8℃ / min and held for 30 minutes before the mother liquor was discharged, yielding crude pyromellitic dianhydride crystals. These crystals were then subjected to a sweating process at a rate of 0.2℃ / min, reaching 283℃ and held for 30 minutes to expel the sweat, resulting in a first-stage purified pyromellitic dianhydride crystal. The product appeared as colorless needle-like crystals. High-performance liquid chromatography (HPLC) analysis showed a purity greater than 99.5% and a yield of approximately 66.7%.

[0069] Example 8

[0070] Weigh 660.0g of crude pyromellitic acid raw material and add it to the melting crystallizer. Seal the lid and raise the temperature to 278℃ with a vacuum of 0.085Mpa. Maintain this temperature for 4 hours until no more water vapor is emitted. Then, turn off the vacuum system.

[0071] Under normal pressure, the temperature was further increased to 301℃ to completely melt the crude pyromellitic dianhydride. The resulting melt was then cooled to 292℃ and maintained for 25 minutes. Subsequently, the temperature was lowered to 282℃ at a rate of 0.6℃ / min and maintained for 60 minutes before the mother liquor for crystallization was discharged. The resulting crude pyromellitic dianhydride crystals were then subjected to a heating and sweating process at a rate of 0.4℃ / min, and the temperature was increased to 285℃ and maintained for 10 minutes to expel the sweat, thus obtaining a first-stage purified pyromellitic dianhydride crystal.

[0072] The temperature was further increased to 301℃ under normal pressure to completely melt the first-refined pyromellitic dianhydride crystals. The resulting melt was cooled to 292℃ and held for 25 minutes. Then, it was cooled to 282℃ at a rate of 0.6℃ / min and held for 40 minutes before the mother liquor was discharged. The resulting crude pyromellitic dianhydride crystals were then subjected to a heating and sweating process at a rate of 0.4℃ / min, reaching 285℃ and holding for 10 minutes to expel the sweat, thus obtaining the second-refined pyromellitic dianhydride crystals. The product appeared as colorless needle-like crystals. High-performance liquid chromatography analysis showed a purity greater than 99.5% and a yield of approximately 59.9%.

[0073] Comparative Example 1

[0074] Referring to the method in CN103435628A, crude pyromellitic dianhydride was placed on a sublimation boat, and a small amount of silica gel was spread on the crude product. The sublimation boat was placed in a heating chamber, and the heating temperature was controlled at 210℃. The vacuum pump was controlled to evacuate the gas to a vacuum degree of 0.05 kPa. The pale yellow powdery solid collected in the vacuum cooling tube was pyromellitic dianhydride. The purity of the product was 98.9% according to chromatographic analysis. According to the power meter analysis, its energy consumption per unit mass was 180% of that of the melt crystallization method. The excess energy loss mainly came from maintaining the high vacuum system.

[0075] Comparative Example 2

[0076] Weigh 630.0g of crude pyromellitic acid raw material and add it to the melt crystallizer. Seal the lid of the vessel, raise the temperature to 190℃, and set the vacuum degree to 0.130Mpa. Maintain this temperature for 7 hours and then turn off the vacuum system.

[0077] The temperature was further increased to 284℃ under normal pressure to completely melt the crude pyromellitic dianhydride. The melt was then cooled to 330℃ and held for 60 minutes. Next, the temperature was lowered to 280℃ at a rate of 2.0℃ / min and held for 30 minutes before the mother liquor was discharged, yielding crude pyromellitic dianhydride crystals. These crystals were then subjected to a sweating process at a rate of 0.2℃ / min, reaching 322℃ and held for 3 minutes to expel the sweat, resulting in a first-stage purified pyromellitic dianhydride crystal. The product appeared as colorless needle-like crystals. High-performance liquid chromatography (HPLC) analysis showed a purity of 98.9% and a yield of approximately 16.7%.

[0078] Experimental Example 1: Liquid Chromatography Analysis and Nuclear Magnetic Resonance Characterization

[0079] Acid anhydrides are reactive substances that may undergo partial transformation in NMR solvents, making it impossible to accurately measure their purity. Therefore, the conventional method for analyzing the purity of acid anhydrides is to first hydrolyze them to produce acids, followed by liquid chromatography and NMR characterization.

[0080] Specifically, the pyromellitic dianhydride prepared in Example 1 was heated under reflux in pure water for 8 hours. After cooling, the solid was separated and dried to obtain pyromellitic tetracarboxylic acid, which was then analyzed by liquid chromatography and characterized by nuclear magnetic resonance. Figure 2 The image shows the liquid chromatogram of the pyromellitic acid produced by hydrolysis. Figure 3 This is the NMR spectrum of pyromellitic tetracarboxylic acid. Analysis shows the product purity is greater than 99.5%.

[0081] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing high-purity pyromellitic dianhydride, comprising: Using crude pyromellitic acid as raw material, high-purity pyromellitic dianhydride is obtained by sequentially performing dehydration and melt crystallization treatments. The dehydration is carried out in a melt crystallizer, and the melt crystallization treatment includes melting, crystallization, and sweating. The melting is carried out as follows: first, the temperature is raised to 285~350℃ until the material is completely melted, and then the temperature is controlled at 285~320℃ and maintained for 1~50 minutes to obtain a molten liquid. The crystallization is carried out as follows: the temperature is lowered to 276~284℃ at a rate of 0.1~1.0℃ / min and maintained for 10~60 minutes to allow the pyromellitic dianhydride to fully crystallize. After the mother liquor is discharged, crude pyromellitic dianhydride crystals are obtained. The sweating is carried out as follows: the crystals are heated to 280~320℃ at a heating rate of 0.05~2.0℃ / min and maintained for 5~80 minutes to obtain refined pyromellitic dianhydride crystals.

2. The preparation method according to claim 1, characterized in that, The dehydration conditions include: temperature raised to 200~300℃, vacuum degree maintained below 0.1Mpa, and time maintained for 2~10h.

3. The preparation method according to claim 1, characterized in that, The melt is obtained after the melting treatment, and the melt is then crystallized.

4. The preparation method according to claim 1, characterized in that, The crystallization process yields coarse crystals, which are then subjected to the sweating process.

5. The preparation method according to any one of claims 1 to 4, characterized in that, Repeat the melt crystallization process 0 to 5 times.

6. The preparation method according to claim 5, characterized in that, Repeat the melt crystallization process 0 to 3 times.

7. The preparation method according to claim 5, characterized in that, The purity of the final purified product obtained by the method, as determined by liquid chromatography, is ≥99.5%.

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