A method for purifying pyromellitic dianhydride

Abstract

This invention provides a method for purifying pyromellitic dianhydride. The purification method includes: dissolving crude anhydride in a mixed solvent, filtering, crystallizing the filtrate, separating the solid and liquid, and drying the separated solid to obtain purified pyromellitic dianhydride; the mixed solvent includes acetone and carbonate; the crystallization includes: first performing evaporation crystallization, and then cooling the resulting solid-liquid mixture for crystallization. The purification method for pyromellitic dianhydride provided by this invention has simple steps, is easy to operate, has a high product yield, and the obtained purified pyromellitic dianhydride has the characteristics of low impurity content and uniform particle size.

Description

Technical Field

[0001] This invention belongs to the field of chemical separation technology, specifically relating to a purification method for pyromellitic dianhydride. Background Technology

[0002] Pyromellitic dianhydride, abbreviated as PMDA, is an important organic chemical raw material. It is crucial for the production of polyimide resins and films, matting agents, plasticizers, epoxy resin curing agents, polyester resin crosslinking agents, and phenolic resin stabilizers. Polyimide, in particular, is a novel material exhibiting high-temperature resistance, low-temperature resistance, radiation resistance, impact resistance, and excellent electrical and mechanical properties, making it irreplaceable in the aerospace and electromechanical industries. With the continuous expansion of the polyimide market, the demand for PMDA, as a primary raw material for its synthesis, is also increasing daily.

[0003] The main routes for synthesizing pyromellitic dianhydride from mesitylene are the gas-phase oxidation of mesitylene in air and the liquid-phase oxidation of mesitylene in air. The gas-phase method has been industrialized, while the liquid-phase method has not yet been industrialized due to the high impurity content in the product. The purity of pyromellitic dianhydride produced by the gas-phase method after collection is generally 92%–98%, which is insufficient to meet the requirements of subsequent polymerization and requires purification.

[0004] Furthermore, the particle size uniformity of the product significantly affects the performance of pyromellitic dianhydride in polymerization reactions. This is because uniformly sized pyromellitic dianhydride not only results in more regular polymer chains but also reduces side reactions during polymerization. Therefore, uniformly sized pyromellitic dianhydride is beneficial for improving polymerization efficiency and polymer product quality. However, most existing methods for purifying pyromellitic dianhydride focus primarily on purity, rarely considering improving particle size uniformity.

[0005] CN114853775B discloses a method for purifying pyromellitic dianhydride using a mixed solvent. The steps include dissolution, filtration, oxidation, distillation, crystallization, filtration, and drying to obtain the target product. During dissolution, acetone and acetonitrile are mixed in a weight ratio of (2-4):1 to form a mixed solvent, and the weight ratio of crude dianhydride to the mixed solvent is 1:(4-12). During oxidation, ozone-containing air is introduced into the filtrate for oxidation. This method yields pyromellitic dianhydride with low impurity content, improving product quality. Simultaneously, the purification process is energy-efficient, and the refined dianhydride product exhibits high purity, high melting point, high yield, and uniform particle size. However, this method requires ozone oxidation, which is not only cumbersome to operate but also increases the difficulty of post-processing. Summary of the Invention

[0006] Based on the above analysis, the present invention aims to provide a purification method for pyromellitic dianhydride, in order to solve the technical problems of low purity and uneven particle size distribution of the purified pyromellitic dianhydride products in the prior art.

[0007] The objective of this invention is mainly achieved through the following technical solutions.

[0008] This invention provides a method for purifying pyromellitic dianhydride, comprising: dissolving crude anhydride in a mixed solvent, filtering and crystallizing the filtrate, separating the solid and liquid, and drying the separated solid to obtain purified pyromellitic dianhydride;

[0009] The mixed solvent includes acetone and carbonate;

[0010] The crystallization process includes: first, evaporation crystallization, and then cooling and crystallizing the solid-liquid mixture obtained after evaporation crystallization.

[0011] In this invention, the use of a mixed solvent to dissolve crude anhydride can dissolve pyromellitic dianhydride and soluble impurities in the crude anhydride in the mixed solvent; the filtration step can filter out insoluble mechanical impurities; crystallizing the filtrate can precipitate pyromellitic dianhydride in the filtrate; solid-liquid separation can obtain purified pyromellitic dianhydride (solid), while leaving soluble impurities in the solvent (liquid).

[0012] In the purification of pyromellitic dianhydride, the solvent used to dissolve the crude anhydride is crucial. By selecting a suitable solvent, the difference in solubility of pyromellitic dianhydride and impurities in the solvent can be utilized to purify the pyromellitic dianhydride. However, obtaining pyromellitic dianhydride with both high purity and good particle size uniformity is quite difficult.

[0013] The inventors of this application, through extensive research, discovered that by using a mixed solvent including acetone and carbonate to dissolve the crude anhydride, and employing a two-step crystallization method of evaporation crystallization followed by cooling crystallization in the crystallization process of the filtered filtrate, not only can purified pyromellitic dianhydride with both high purity and good particle size uniformity be obtained, but the yield of purified pyromellitic dianhydride can also be improved, and the energy consumption of the purification process can be effectively reduced. This is likely because carbonate is essentially insoluble in pyromellitic dianhydride at room temperature, but it is quite soluble in impurities contained in the crude anhydride, such as alkylbenzene carboxylic acids and coloring components, thus carbonate has good selectivity as a solvent. Using carbonates in combination with acetone offers several advantages. First, it facilitates the precipitation of pyromellitic dianhydride while minimizing impurities, reducing losses during purification and improving the purity and particle size uniformity of the purified pyromellitic dianhydride. Second, the significant difference in boiling points between carbonates and acetone, coupled with the absence of azeotropy, allows for raising the dissolution temperature of the crude anhydride. Only acetone evaporates and refluxes, while carbonates remain stationary, effectively reducing energy consumption. Furthermore, combining these properties of the mixed solvent with a two-step crystallization method—evaporation followed by cooling—further enhances the purity, particle size uniformity, and yield of purified pyromellitic dianhydride while simultaneously reducing separation energy consumption.

[0014] According to some embodiments of the present invention, the carbonate includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, preferably at least one of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0015] According to some embodiments of the present invention, the mass ratio of acetone to carbonate is (1-10):1, preferably (4-9):1.

[0016] According to some embodiments of the present invention, the mass ratio of crude anhydride to mixed solvent is 1:(1 to 10), preferably 1:(6 to 8).

[0017] According to some embodiments of the present invention, the temperature at which the crude anhydride is dissolved is 25–80°C, preferably 40–70°C.

[0018] According to some embodiments of the present invention, activated carbon is added to the mixed solvent.

[0019] In this invention, activated carbon primarily functions as a decolorizing agent, and various common activated carbons with decolorizing effects can be used. The amount of activated carbon added to the mixed solvent is also appropriate to achieve the desired decolorizing effect.

[0020] In this invention, by using carbonate and acetone together as a mixed solvent, the amount of activated carbon used for decolorization can be reduced.

[0021] According to some embodiments of the present invention, the mass ratio of activated carbon to crude anhydride is 1:(5-20), preferably 1:(10-15).

[0022] According to some embodiments of the present invention, the evaporation crystallization includes microwave treatment; preferably, the frequency of the microwave is 300MHz to 300GHz.

[0023] According to some embodiments of the present invention, the solvent evaporated during the evaporation and crystallization can be reused to dissolve crude anhydride.

[0024] In this invention, the evaporation and crystallization temperature is mainly determined by the evaporation temperature of the solvent itself. For specific solvents, those skilled in the art can easily obtain their evaporation temperatures, which will not be elaborated upon here.

[0025] According to some embodiments of the present invention, the solvent evaporated during the evaporation crystallization accounts for 1 / 5 to 4 / 5 of the mixed solvent.

[0026] In this invention, if too much solvent is evaporated during the evaporation and crystallization process, the concentration of pyromellitic dianhydride in the solution can be further increased, making it easier for it to precipitate in the subsequent cooling and crystallization process, thereby reducing energy consumption; however, if the amount of solvent in the solid-liquid mixture decreases, impurities may also precipitate, which will lead to a decrease in the purity of the precipitated pyromellitic dianhydride.

[0027] According to some embodiments of the present invention, the crystallization temperature of the cooling crystallization is 5 to 15°C, and the cooling rate is 0.1 to 1°C / min.

[0028] According to some embodiments of the present invention, the stirring rate for the cooling crystallization is 15 to 30 rpm.

[0029] According to some embodiments of the present invention, the cooling crystallization includes ultrasonic treatment, preferably, the ultrasonic frequency is 20-40 kHz.

[0030] According to some embodiments of the present invention, the solid-liquid separation is performed by centrifugal separation.

[0031] According to some embodiments of the present invention, the liquid obtained from solid-liquid separation can be reused to dissolve crude anhydride.

[0032] According to some embodiments of the present invention, the drying temperature is 180-200°C and the pressure is atmospheric pressure.

[0033] Compared with the prior art, the present invention can achieve at least the following beneficial effects:

[0034] The purification method for pyromellitic dianhydride provided by this invention is simple, easy to operate, and has a high product yield. The purified pyromellitic dianhydride has the characteristics of low impurity content and uniform particle size. Attached Figure Description

[0035] Figure 1 This is a process flow diagram of the purification method of pyromellitic dianhydride used in Example 1. Detailed Implementation

[0036] To make the technical problem to be solved, the technical solution, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely for illustrating this patent and do not limit the scope of protection of this invention in any way.

[0037] Unless otherwise defined, the technical terms used in the following embodiments have the same meaning as commonly understood by those skilled in the art. Unless otherwise specified, the reagents used in the following embodiments are conventional biochemical reagents; the raw materials, instruments, and equipment used in the following embodiments can all be obtained commercially or by existing methods; unless otherwise specified, the reagent dosages are those used in routine experimental operations; unless otherwise specified, the experimental methods are conventional methods.

[0038] Example 1

[0039] Acetone and dimethyl carbonate were mixed in a 6:1 mass ratio to prepare a mixed solvent. Crude anhydride (pyromellitic dianhydride purity 98%) in a 1:7 mass ratio and the mixed solvent were added to a dissolving vessel for dissolution. Activated carbon was then added to the dissolving vessel at a mass ratio of 1:10 to the crude anhydride. The temperature of the dissolving vessel was controlled at 57°C, the pressure at atmospheric pressure, and the stirring speed at 60 rpm. After all the crude anhydride was dissolved, the mixture was held for 30 minutes. The dissolved crude anhydride solution was filtered hot using a precision filter. The filtrate was then subjected to evaporation crystallization in an evaporator crystallizer equipped with a microwave generator at a frequency of 2450 MHz. Three-fifths of the solvent was evaporated from the filtrate and returned to the dissolving vessel for reuse. The remaining solid-liquid mixture was then subjected to cooling crystallization in a jacketed stirred crystallizer at a stirring speed of 20 rpm and a cooling rate of 0.5°C / min. The final crystallization temperature was controlled at 10°C and held for 30 minutes. The jacketed stirred crystallizer was equipped with an ultrasonic generator at a frequency of 25 kHz. The solid-liquid mixture after cooling and crystallization is centrifuged and separated. The mother liquor is returned to the dissolution vessel for reuse. The wet solid after centrifugation is dried at 190°C and normal pressure to finally obtain purified pyromellitic dianhydride.

[0040] Example 2

[0041] Acetone and dimethyl carbonate were mixed in a 7:1 mass ratio to prepare a mixed solvent. Crude anhydride (pyromellitic dianhydride purity 98%) in a 1:8 mass ratio and the mixed solvent were added to a dissolving vessel for dissolution. Activated carbon was then added to the dissolving vessel at a mass ratio of 1:15 to the crude anhydride. The temperature of the dissolving vessel was controlled at 50°C, the pressure at atmospheric pressure, and the stirring speed at 60 rpm. After all the crude anhydride was dissolved, the mixture was held for 30 minutes. The dissolved crude anhydride solution was filtered hot using a precision filter. The filtrate was then subjected to evaporation crystallization in an evaporator crystallizer equipped with a microwave generator at a frequency of 915 MHz. Half of the solvent was evaporated from the filtrate and returned to the dissolving vessel for reuse. The remaining solid-liquid mixture was then subjected to cooling crystallization in a jacketed stirred crystallizer at a stirring speed of 15 rpm and a cooling rate of 1°C / min. The final crystallization temperature was controlled at 15°C and held for 60 minutes. The jacketed stirred crystallizer was equipped with an ultrasonic generator at a frequency of 28 kHz. The solid-liquid mixture after cooling and crystallization is centrifuged and separated. The mother liquor is returned to the dissolution vessel for reuse. The wet solid after centrifugation is dried at 180°C and normal pressure to finally obtain purified pyromellitic dianhydride.

[0042] Example 3

[0043] The purification method for pyromellitic dianhydride is the same as in Example 1, except that dimethyl carbonate is replaced with an equal mass of diethyl carbonate.

[0044] Example 4

[0045] The purification method for pyromellitic dianhydride is the same as in Example 1, except that dimethyl carbonate is replaced with an equal mass of ethylene carbonate.

[0046] Example 5

[0047] The purification method for pyromellitic dianhydride is the same as in Example 1, except that dimethyl carbonate is replaced with an equal mass of propylene carbonate.

[0048] Example 6

[0049] The purification method for pyromellitic dianhydride is the same as in Example 1, except that a microwave generator is not installed in the evaporator crystallizer.

[0050] Example 7

[0051] The purification method for pyromellitic dianhydride is the same as in Example 1, except that an ultrasonic generator is not installed in the jacketed stirred crystallizer.

[0052] Example 8

[0053] The purification method for pyromellitic dianhydride is the same as in Example 1, except that the mass ratio of acetone to dimethyl carbonate in the mixed solvent is 1:1.

[0054] Example 9

[0055] The purification method for pyromellitic dianhydride is the same as in Example 1, except that the mass ratio of acetone to dimethyl carbonate in the mixed solvent is 1:2.

[0056] Comparative Example 1

[0057] The purification method for pyromellitic dianhydride is the same as in Example 1, except that the mixed solvent is replaced with an equal mass of acetone.

[0058] Comparative Example 2

[0059] The purification method for pyromellitic dianhydride is the same as in Example 1, except that the mixed solvent is replaced with an equal mass of dimethyl carbonate.

[0060] Comparative Example 3

[0061] The purification method for pyromellitic dianhydride is the same as in Example 1, except that dimethyl carbonate in the mixed solvent is replaced with an equal mass of acetonitrile.

[0062] Comparative Example 4

[0063] The purification method for pyromellitic dianhydride is the same as in Example 1, except that dimethyl carbonate in the mixed solvent is replaced with an equal mass of ethyl acetate.

[0064] Comparative Example 5

[0065] Acetone and dimethyl carbonate were mixed in a 6:1 mass ratio to prepare a mixed solvent. Crude pyromellitic dianhydride (98% purity) in a 1:7 mass ratio was added to a dissolving vessel for dissolution. Activated carbon was then added to the dissolving vessel at a 1:10 mass ratio to the crude dianhydride. The temperature of the dissolving vessel was controlled at 57°C, the pressure at atmospheric pressure, and the stirring speed at 60 rpm. After the crude dianhydride was completely dissolved, the mixture was kept for 30 minutes. The dissolved crude dianhydride solution was filtered hot using a precision filter. The filtrate was then subjected to evaporation crystallization in an evaporator equipped with a microwave generator at a frequency of 2450 MHz. The solid-liquid mixture after evaporating 3 / 5 of the solvent was centrifuged. The mother liquor was returned to the dissolving vessel for reuse. The wet solid after centrifugation was dried at 190°C and atmospheric pressure to obtain purified pyromellitic dianhydride.

[0066] Comparative Example 6

[0067] Acetone and dimethyl carbonate were mixed in a 6:1 mass ratio to prepare a solvent. Crude pyromellitic dianhydride (98% purity) in a 1:7 mass ratio was added to a dissolving vessel for dissolution. Activated carbon was then added to the dissolving vessel at a 1:10 mass ratio to the crude dianhydride. The temperature of the dissolving vessel was controlled at 57°C, the pressure at atmospheric pressure, and the stirring speed at 60 rpm. After the crude dianhydride was completely dissolved, the mixture was held for 30 minutes. The dissolved crude dianhydride solution was filtered hot using a precision filter. The filtrate was then cooled and crystallized in a jacketed stirred crystallizer at a stirring rate of 20 rpm and a cooling rate of 0.5°C / min, maintaining a final crystallization temperature of 10°C for 30 minutes. The crystallizer was equipped with an ultrasonic generator with an ultrasonic frequency of 25 kHz. The solid-liquid mixture after cooling and crystallization was centrifuged. The mother liquor was returned to the dissolving vessel for reuse. The wet solid after centrifugation was dried at 190°C and atmospheric pressure to obtain purified pyromellitic dianhydride.

[0068] Performance evaluation of refined pyromellitic dianhydride

[0069] The yields of purified pyromellitic dianhydride obtained in each example and comparative example were calculated, and its purity, particle size, and CV value were tested. The results are shown in Table 1.

[0070] in,

[0071] (1) Purity: High performance liquid chromatography was used, referring to GB / T 26792-2019.

[0072] (2) Particle size: Measured using a laser particle size analyzer. D50 represents the particle size corresponding to a cumulative particle size distribution percentage of 50%; D10 represents the particle size corresponding to a cumulative particle size distribution percentage of 10%; D90 represents the particle size corresponding to a cumulative particle size distribution percentage of 90%.

[0073] (3) Coefficient of Variation (CV): This value represents the degree of dispersion of particle size distribution and is an important indicator for evaluating the uniformity and consistency of particulate materials.

[0074] CV=(D90-D10) / (2×D50)×100%.

[0075] The smaller the CV value, the more uniform the particle size distribution and the smaller the particle size difference; the larger the CV value, the more uneven the particle size distribution and the larger the particle size difference.

[0076] Table 1

[0077] Group Yield (%) purity(%) D50(μm) CV value (%) Example 1 95.4 99.99 121 32.6 Example 2 94.9 99.96 126 35.7 Example 3 95.1 99.97 122 34.8 Example 4 94.9 99.95 116 39.7 Example 5 95 99.94 123 41.5 Example 6 95.3 99.98 126 51.6 Example 7 94.9 99.97 130 43.1 Example 8 95.2 99.93 126 38.5 Example 9 94.6 99.78 118 42.4 Comparative Example 1 94.1 99.90 125 41.2 Comparative Example 2 75.1 99.84 115 40.9 Comparative Example 3 93.2 99.89 131 44.7 Comparative Example 4 93.4 99.87 124 46.0 Comparative Example 5 56.4 99.52 82 54.9 Comparative Example 6 72.1 99.45 111 61.7

[0078] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A method for purifying pyromellitic dianhydride, characterized in that, include: The crude anhydride was dissolved in a mixed solvent, filtered, and the filtrate was crystallized. The solid and liquid were separated, and the separated solid was dried to obtain purified pyromellitic dianhydride. The mixed solvent includes acetone and carbonate; The crystallization process includes: first, evaporation crystallization, and then cooling and crystallizing the solid-liquid mixture obtained after evaporation crystallization.

2. The refining method according to claim 1, characterized in that, The carbonate includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, preferably at least one of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

3. The refining method according to claim 1 or 2, characterized in that, The mass ratio of acetone to carbonate is (1-10):1, preferably (4-9):

1.

4. The refining method according to any one of claims 1-3, characterized in that, The mass ratio of crude anhydride to mixed solvent is 1:(1-10), preferably 1:(6-8).

5. The refining method according to any one of claims 1-4, characterized in that, The temperature at which the crude anhydride is dissolved is 25–80°C, preferably 40–70°C.

6. The refining method according to any one of claims 1-5, characterized in that, Activated carbon is added to the mixed solvent; preferably, the mass ratio of activated carbon to crude anhydride is 1:(5-20), more preferably 1:(10-15).

7. The refining method according to any one of claims 1-6, characterized in that, The evaporation crystallization includes microwave treatment; preferably, the frequency of the microwave is 300MHz to 300GHz. And / or, the solvent evaporated during the evaporation crystallization accounts for 1 / 5 to 4 / 5 of the mixed solvent.

8. The refining method according to any one of claims 1-7, characterized in that, The crystallization temperature for the cooling crystallization is 5–15°C, and the cooling rate is 0.1–1°C / min. And / or, the stirring rate for the cooling crystallization is 15 to 30 rpm; And / or, the cooling crystallization includes ultrasonic treatment, preferably, the ultrasonic frequency is 20-40 kHz.

9. The refining method according to any one of claims 1-8, characterized in that, The solid-liquid separation is performed by centrifugation.

10. The refining method according to any one of claims 1-9, characterized in that, The drying temperature is 180–200°C.

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