A method for refining terephthalaldehyde

Abstract

This invention relates to a method for purifying terephthalaldehyde. More specifically, this invention relates to a method in which crude terephthalaldehyde is subjected to centrifugal chemical extraction to react impurities such as organic carboxylic acids into carboxylate salts, which then enter the aqueous phase. The separated oil phase is then subjected to distillation and melt crystallization to remove trace impurities such as benzaldehyde and p-methylbenzaldehyde, thereby obtaining high-purity terephthalaldehyde. This invention combines centrifugal chemical extraction and vacuum distillation coupled with melt crystallization, which can efficiently obtain high-purity terephthalaldehyde products. Because the purification process does not use organic solvents, it reduces wastewater pollution to the environment and has good prospects for industrial application.

Description

Technical Field

[0001] This invention relates to a method for purifying terephthalaldehyde. More specifically, this invention relates to a method in which crude terephthalaldehyde is subjected to centrifugal chemical extraction to react impurity organic carboxylic acids into carboxylate salts which enter the aqueous phase. The separated oil phase is then subjected to distillation and melt crystallization to remove trace impurities such as benzaldehyde and p-methylbenzaldehyde to obtain high-purity terephthalaldehyde. Background Technology

[0002] Terephthalaldehyde is an important downstream fine chemical product of p-xylene, mainly used in the synthesis of fluorescent whitening agents, pharmaceutical products, dyes, fragrances, and UV sunscreens. The traditional production method of terephthalaldehyde involves chlorinating p-xylene followed by hydrolysis in the presence of nitric acid or metal oxides. This process consumes large quantities of toxic chlorine, nitric acid, and sodium hydroxide, producing gases such as hydrogen chloride, NOx, and chloroalkylbenzenes, which easily cause environmental pollution and equipment corrosion. Material leaks can also harm human health. Besides the traditional chlorination method, other methods for synthesizing terephthalaldehyde include the hydrogenation of terephthalic acid (esters), the hydrogenation of terephthaloyl chloride, and the direct oxidation of p-xylene. In the production process of terephthalaldehyde, various byproducts such as benzoic acid, p-methylbenzoic acid, aldehyde benzoic acid, and terephthalic acid are generally produced. There are few reports on how to remove impurities from terephthalaldehyde products, especially since benzoic acid has a similar boiling point and melting point to terephthalaldehyde.

[0003] US Patent 2888488 discloses a method for preparing terephthalaldehyde, wherein the product is purified by solvent extraction, drying, and sublimation. However, this method requires a complex process and uses environmentally unfriendly compounds such as chloroform and / or sulfonates as solvents. Russian Patent RU2568439 discloses a method for obtaining high-purity terephthalaldehyde by extraction with isooctane as a solvent.

[0004] Chinese patent ZL200580028008.8 discloses a method for preparing high-purity terephthalaldehyde from impurity-containing terephthalaldehyde crystals by recrystallization using alcohol as solvent and water as antisolvent. This method yields terephthalaldehyde with a purity of up to 99.9% and a yield of up to 79.2%. Because terephthalaldehyde readily undergoes an aldol condensation reaction with methanol, Chinese invention patent ZL200780024375.X reports a method for obtaining high-purity terephthalaldehyde by dissolving crude terephthalaldehyde in a dimethyl sulfoxide (DMSO) system followed by recrystallization in an aqueous system. In the examples, the purity of terephthalaldehyde obtained by gas chromatography can reach over 99%. However, this method requires the use of DMSO solvent, increasing solvent recovery costs, and this solvent is harmful to both the environment and human health. Chinese invention patent ZL200680006347.0 discloses a method for separating aromatic dialdehydes from a mixture generated by the gas-phase oxidation of xylene. The specific process involves cooling the gas-phase reaction mixture to 5–70°C to condense the aromatic dialdehydes; and then separating the condensed aromatic dialdehydes from the residual reaction mixture. This process does not describe a method for further purifying crude terephthalaldehyde.

[0005] Chinese patent ZL200780041280.9 utilizes the different solubilities of terephthalaldehyde and impurities in aromatic hydrocarbons and co-solvents. It describes a method for obtaining terephthalaldehyde by fully contacting crude terephthalaldehyde with p-xylene, dimethyl sulfoxide, and terephthalaldehyde under high temperature conditions, followed by cooling and recrystallization. However, due to the influence of impurities on recrystallization during this process, the purity of the obtained terephthalaldehyde is difficult to reach above 99%.

[0006] Chinese invention patent 202010540003.X reports a method for synthesizing terephthalaldehyde in a microchannel reactor. The main steps involve using a chlorinated derivative of p-xylene as a reactant, reacting it under the catalysis of hexamethylenetetramine, and then removing impurities through filtration and washing. This separation process is complex, including cooling crystallization, pressure filtration I, washing with sodium carbonate solution, pressure filtration II, secondary washing, pressure filtration, centrifugation, and drying. Furthermore, the purity of the product is not disclosed in this method. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a refined combined process that utilizes centrifugal chemical extraction and vacuum distillation coupled with melt crystallization to obtain high-purity terephthalaldehyde products, thereby meeting the demand for high-quality terephthalaldehyde. Furthermore, the refining process does not use organic solvents, reducing wastewater pollution to the environment.

[0008] The purification method for terephthalaldehyde according to the present invention includes the following steps in sequence: (1) Dissolve the alkali in water and mix it with the crude molten terephthalaldehyde in a centrifugal extractor. Then chemically extract the mixture at 116~130℃ for 0.01-0.5 hours to obtain a material with separate oil and water phases. (2) Separate the oil phase and water phase of the material obtained in step (1) respectively, wherein terephthalaldehyde is enriched in the oil phase and impurities are enriched in the water phase; (3) Cool the aqueous phase obtained in step (2) to crystallize and obtain a small amount of solid terephthalaldehyde; (4) The solid terephthalaldehyde obtained in step (3) and the oil phase in step (2) are mixed and purified by distillation to obtain liquid terephthalaldehyde; (5) Melt and crystallize the terephthalaldehyde liquid in step (4) to obtain high-purity terephthalaldehyde liquid; (6) Cool and granulate the liquid terephthalaldehyde from step (5) to obtain solid terephthalaldehyde product.

[0009] The purity of the above-mentioned crude terephthalaldehyde is 70%~95%, preferably 80%~92%.

[0010] In step (1) above, the mass ratio of crude terephthalaldehyde, alkali and water is 1:(0.05~0.3):(0.1~2), preferably 1:(0.08~0.2):(0.2~1).

[0011] The base in step (1) above includes one or more combinations of inorganic bases. The inorganic bases include one or more combinations of Group IA and Group IIA metal oxides, metal hydroxides, metal carbonates, metal bicarbonates, metal phosphates, and metal hydrogen phosphates.

[0012] In step (4) above, the distillation conditions are: 5-150 plates in the distillation column, 100-250℃ in the reboiler, 1-20:1 reflux ratio, and 50Pa-10000Pa operating pressure. Preferred distillation conditions are: 10-100 plates in the distillation column, 140-200℃ in the reboiler, 2-15:1 reflux ratio, and 100Pa-5000Pa operating pressure.

[0013] Compared with existing technologies, this invention first utilizes centrifugal chemical extraction to rapidly and fully react impurity organic carboxylic acids into carboxylate salts that enter the aqueous phase, and then efficiently separates them through centrifugation and oil phase separation. The separated oil phase is then subjected to distillation and melt crystallization to remove trace impurities such as benzaldehyde and p-methylbenzaldehyde to obtain high-purity terephthalaldehyde. The terephthalaldehyde liquid is then cooled and crystallized, and granulated to obtain a high-purity terephthalaldehyde product with a particle size of 1-4 mm and a purity of over 99.92%, meeting the requirements for pharmaceutical-grade applications.

[0014] In this invention, the purity of terephthalaldehyde is determined by gas chromatography with an internal standard method. Specifically, terephthalaldehyde is dissolved in acetone, and 1,3,5-trichlorobenzene is added as an internal standard. The gas chromatography analysis conditions are a column oven temperature of 200°C and an Agilent DB-5 column.

[0015] This invention combines centrifugal chemical extraction and vacuum distillation coupled with melt crystallization to efficiently obtain high-purity terephthalaldehyde. Since the refining process does not use organic solvents, it reduces wastewater pollution and has good prospects for industrial application. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to the embodiments, but the present invention is not limited to these embodiments. Example 1

[0017] 200g of crude terephthalaldehyde (85% purity) was mixed with sodium hydroxide and water at a mass ratio of 1:0.05:2. The mixture was then centrifuged at 130℃ for 0.2 hours to obtain a separated oil and aqueous phase. The aqueous phase was cooled and crystallized to obtain a small amount of solid terephthalaldehyde. This solid was mixed with the oil phase and subjected to vacuum distillation in a distillation column under the following conditions: 10 trays, reboiler temperature of 190℃, reflux ratio of 8, and operating pressure of 2500 Pa. Liquid terephthalaldehyde was collected at the top of the column and further purified by melt crystallization. The melt crystallization process is as follows: Terephthalaldehyde is fed into a melt crystallizer at 130°C, and cooled for crystallization at a rate of 8°C / h until the temperature drops to 110°C. The mother liquor is then discharged. The crystals remaining in the melt crystallizer are then subjected to crystal sweating, and the high-purity terephthalaldehyde is collected. The crystal sweating process includes the following steps: a heating rate of 8°C / h, sweating temperature raised to 118°C, discharge of the sweating liquid, further heating to melt the crystals, and collection of the high-purity terephthalaldehyde. Further cooling and crystallization, followed by pulverization, yields 132.14 g of fine-particle terephthalaldehyde product with a purity of 99.92% and a separation yield of 77.67%. Example 2

[0018] 200g of crude terephthalaldehyde (85% purity) was mixed with sodium hydroxide and water at a mass ratio of 1:0.2:1. The mixture was then centrifuged at 130℃ for 0.1 hours to obtain a separated oil and aqueous phase. The aqueous phase was cooled and crystallized to obtain a small amount of solid terephthalaldehyde. This solid was mixed with the oil phase and subjected to vacuum distillation in a distillation column under the following conditions: 20 trays, reboiler temperature of 180℃, reflux ratio of 10, and operating pressure of 2000 Pa. Liquid terephthalaldehyde was collected at the top of the column and further purified by melt crystallization. The melt crystallization process is as follows: Terephthalaldehyde is fed into a melt crystallizer at 125°C, and cooled for crystallization at a rate of 5°C / h until the temperature drops to 108°C. The mother liquor is then discharged. The crystals remaining in the melt crystallizer are then subjected to crystal sweating, and the high-purity terephthalaldehyde is collected. The crystal sweating process includes the following steps: a heating rate of 5°C / h, sweating temperature raised to 118°C, discharge of the sweating liquid, further heating to melt the crystals, and collection of the high-purity terephthalaldehyde. Further cooling and crystallization, followed by pulverization, yields 133.34 g of fine-particle terephthalaldehyde product with a purity of 99.94% and a separation yield of 78.39%. Example 3

[0019] 200g of crude terephthalaldehyde (85% purity) was mixed with sodium hydroxide and water at a mass ratio of 1:0.2:1. The mixture was then centrifuged and chemically extracted at 130℃ for 0.06 hours, resulting in a separated oil and aqueous phase. The aqueous phase was cooled and crystallized to obtain a small amount of solid terephthalaldehyde. This solid was mixed with the oil phase and subjected to vacuum distillation in a distillation column under the following conditions: 30 trays, reboiler temperature of 170℃, reflux ratio of 12, and operating pressure of 1500 Pa. Liquid terephthalaldehyde was collected at the top of the column and further purified by melt crystallization. The melt crystallization process is as follows: Terephthalaldehyde is fed into a melt crystallizer at 125°C, and cooled for crystallization at a rate of 3°C / h until the temperature drops to 110°C. The mother liquor is then discharged. The crystals remaining in the melt crystallizer are then subjected to crystal sweating, and the high-purity terephthalaldehyde is collected. The crystal sweating process includes the following steps: a heating rate of 3°C / h, sweating temperature raised to 118°C, discharge of the sweating liquid, further heating to melt the crystals, and collection of the high-purity terephthalaldehyde. Further cooling and crystallization, followed by pulverization, yields 136.36 g of fine-particle terephthalaldehyde product with a purity of 99.97% and a separation yield of 80.19%. Example 4

[0020] 200g of crude terephthalaldehyde (85% purity) was mixed with sodium hydroxide and water at a mass ratio of 1:0.07:1.5. The mixture was then centrifuged at 125℃ for 0.2 hours to obtain a separated oil and aqueous phase. The aqueous phase was cooled and crystallized to obtain a small amount of solid terephthalaldehyde. This solid was mixed with the oil phase and subjected to vacuum distillation in a distillation column under the following conditions: 40 trays, reboiler temperature of 160℃, reflux ratio of 4, and operating pressure of 1000 Pa. Liquid terephthalaldehyde was collected at the top of the column and further purified by melt crystallization. The melt crystallization process is as follows: Terephthalaldehyde is fed into a melt crystallizer at 122°C, and cooled for crystallization at a rate of 4°C / h until the temperature drops to 110°C. The mother liquor is then discharged. The crystals remaining in the melt crystallizer are then subjected to crystal sweating, and the high-purity terephthalaldehyde is collected. The crystal sweating process includes the following steps: a heating rate of 4°C / h, sweating temperature raised to 117°C, sweating liquid discharged, further heating to melt the crystals, and then collecting the high-purity terephthalaldehyde. Further cooling and crystallization, followed by pulverization, yields 139.8 g of fine-particle terephthalaldehyde product with a purity of 99.92% and a separation yield of 82.17%. Example 5

[0021] 200g of crude terephthalaldehyde (85% purity) was mixed with sodium hydroxide and water at a mass ratio of 1:0.2:1. The mixture was then centrifuged and chemically extracted at 125℃ for 0.1 hours, resulting in a separated oil and aqueous phase. The aqueous phase was cooled and crystallized to obtain a small amount of solid terephthalaldehyde. This solid was mixed with the oil phase and subjected to vacuum distillation in a distillation column under the following conditions: 50 trays, reboiler temperature of 150℃, reflux ratio of 6, and operating pressure of 500 Pa. Liquid terephthalaldehyde was collected at the top of the column and further purified by melt crystallization. The melt crystallization process is as follows: Terephthalaldehyde is fed into a melt crystallizer at 120°C, and cooled for crystallization at a rate of 2°C / h until the temperature drops to 110°C. The mother liquor is then discharged. The crystals remaining in the melt crystallizer are then subjected to crystal sweating, and the high-purity terephthalaldehyde is collected. The crystal sweating process includes the following steps: a heating rate of 2°C / h, sweating temperature raised to 117°C, discharge of the sweating liquid, further heating to melt the crystals, and collection of the high-purity terephthalaldehyde. Further cooling and crystallization, followed by pulverization, yields 141.14 g of fine-particle terephthalaldehyde product with a purity of 99.95% and a separation yield of 82.98%. Example 6

[0022] 200g of crude terephthalaldehyde (85% purity) was mixed with sodium hydroxide and water at a mass ratio of 1:0.2:1. The mixture was then centrifuged and chemically extracted at 125℃ for 0.06 hours, resulting in a separated oil and aqueous phase. The aqueous phase was cooled and crystallized to obtain a small amount of solid terephthalaldehyde. This solid was mixed with the oil phase and subjected to vacuum distillation in a distillation column under the following conditions: 60 trays, reboiler temperature of 190℃, reflux ratio of 8, and operating pressure of 2500 Pa. Liquid terephthalaldehyde was collected at the top of the column and further purified by melt crystallization. The melt crystallization process is as follows: Terephthalaldehyde is fed into a melt crystallizer at 125°C, and cooled for crystallization at a rate of 2°C / h until the temperature drops to 110°C. The mother liquor is then discharged. The crystals remaining in the melt crystallizer are then subjected to crystal sweating, and the high-purity terephthalaldehyde is collected. The crystal sweating process includes the following steps: a heating rate of 2°C / h, sweating temperature raised to 119°C, sweating liquid discharged, further heating to melt the crystals, and then collecting the high-purity terephthalaldehyde. Further cooling and crystallization, followed by pulverization, yields 135.29 g of fine-particle terephthalaldehyde product with a purity of 99.97% and a separation yield of 79.56%. Example 7

[0023] 200g of crude terephthalaldehyde (85% purity) was mixed with sodium hydroxide and water at a mass ratio of 1:0.1:0.8. The mixture was then centrifuged at 120℃ for 0.2 hours to obtain a separated oil and aqueous phase. The aqueous phase was cooled and crystallized to obtain a small amount of solid terephthalaldehyde. This solid was mixed with the oil phase and subjected to vacuum distillation in a distillation column under the following conditions: 30 trays, reboiler temperature of 180℃, reflux ratio of 10, and operating pressure of 2000 Pa. Liquid terephthalaldehyde was collected at the top of the column and further purified by melt crystallization. The melt crystallization process is as follows: Terephthalaldehyde is fed into a melt crystallizer at 123°C, and cooled for crystallization at a rate of 6°C / h until the temperature drops to 110°C. The mother liquor is then discharged. The crystals remaining in the melt crystallizer are then subjected to crystal sweating, and the high-purity terephthalaldehyde is collected. The crystal sweating process includes the following steps: a heating rate of 6°C / h, sweating temperature raised to 119°C, sweating liquid discharged, further heating to melt the crystals, and then collecting the high-purity terephthalaldehyde. Further cooling and crystallization, followed by pulverization, yields 131.69 g of fine-particle terephthalaldehyde product with a purity of 99.95% and a separation yield of 77.4%. Example 8

[0024] 200g of crude terephthalaldehyde (85% purity) was mixed with sodium hydroxide and water at a mass ratio of 1:0.1:0.5. The mixture was then centrifuged at 120℃ for 0.1 hours to obtain a two-phase mixture consisting of an oil phase and an aqueous phase. The aqueous phase was cooled and crystallized to obtain a small amount of solid terephthalaldehyde. This solid was mixed with the oil phase and subjected to vacuum distillation in a distillation column under the following conditions: 20 trays, reboiler temperature of 170℃, reflux ratio of 12, and operating pressure of 1500 Pa. Liquid terephthalaldehyde was collected at the top of the column and further purified by melt crystallization. The melt crystallization process is as follows: Terephthalaldehyde is fed into a melt crystallizer at 122°C, and cooled for crystallization at a rate of 5°C / h until the temperature drops to 108°C. The mother liquor is then discharged. The crystals remaining in the melt crystallizer are then subjected to crystal sweating, and the high-purity terephthalaldehyde is collected. The crystal sweating process includes the following steps: a heating rate of 5°C / h, sweating temperature raised to 119°C, discharge of the sweating liquid, further heating to melt the crystals, and collection of the high-purity terephthalaldehyde. Further cooling and crystallization, followed by pulverization, yields 138.53 g of fine-particle terephthalaldehyde product with a purity of 99.95% and a separation yield of 81.45%. Example 9

[0025] 200g of crude terephthalaldehyde (85% purity) was mixed with sodium hydroxide and water at a mass ratio of 1:0.1:0.8. The mixture was then centrifuged at 120℃ for 0.06 hours to obtain a two-phase mixture consisting of an oil phase and an aqueous phase. The aqueous phase was cooled and crystallized to obtain a small amount of solid terephthalaldehyde. This solid was mixed with the oil phase and subjected to vacuum distillation in a distillation column under the following conditions: 30 trays, reboiler temperature of 160℃, reflux ratio of 4, and operating pressure of 1000 Pa. Liquid terephthalaldehyde was collected at the top of the column and further purified by melt crystallization. The melt crystallization process is as follows: Terephthalaldehyde is fed into a melt crystallizer at 123°C, and cooled for crystallization at a rate of 2°C / h until the temperature drops to 107°C. The mother liquor is then discharged. The crystals remaining in the melt crystallizer are then subjected to crystal sweating, and the high-purity terephthalaldehyde is collected. The crystal sweating process includes the following steps: a heating rate of 2°C / h, sweating temperature raised to 116°C, discharge of the sweating liquid, further heating to melt the crystals, and collection of the high-purity terephthalaldehyde. Further cooling and crystallization, followed by pulverization, yields 140.34 g of fine-particle terephthalaldehyde product with a purity of 99.97% and a separation yield of 82.53%.

[0026] While the specific embodiments of the present invention have been described in detail above, it should be noted that the scope of protection of the present invention is not limited to these specific embodiments, but is determined by the claims. Those skilled in the art can make appropriate modifications to these embodiments without departing from the technical concept and spirit of the present invention, and these modified embodiments are obviously also included within the scope of protection of the present invention.

Claims

1. A method for purifying terephthalaldehyde, characterized in that, Including the following steps in sequence: (1) Dissolve the alkali in water and mix it with the crude molten terephthalaldehyde in a centrifugal extractor. Then chemically extract the mixture at 116~130℃ for 0.01-0.5 hours to obtain a material with separate oil and water phases. (2) Separate the oil phase and water phase of the material obtained in step (1) respectively, wherein terephthalaldehyde is enriched in the oil phase and impurities are enriched in the water phase; (3) Cool the aqueous phase obtained in step (2) to crystallize and obtain solid terephthalaldehyde; (4) The solid terephthalaldehyde obtained in step (3) and the oil phase in step (2) are mixed and purified by distillation to obtain liquid terephthalaldehyde; (5) Melt and crystallize the terephthalaldehyde liquid in step (4) to obtain high-purity terephthalaldehyde liquid; (6) Cool and granulate the liquid terephthalaldehyde from step (5) to obtain solid terephthalaldehyde product.

2. The method for purifying terephthalaldehyde according to claim 1, characterized in that... The purity of the crude terephthalaldehyde is 70%~95%.

3. The method for purifying terephthalaldehyde according to claim 2, characterized in that... The purity of the crude terephthalaldehyde is 80%~92%.

4. The method for purifying terephthalaldehyde according to claim 1, characterized in that... The mass ratio of crude terephthalaldehyde, alkali, and water is 1:(0.05~0.3):(0.1~2).

5. The method for purifying terephthalaldehyde according to claim 4, characterized in that... The mass ratio of crude terephthalaldehyde, alkali, and water is 1:(0.08~0.2):(0.2~1).

6. The method for purifying terephthalaldehyde according to claim 1, characterized in that... The alkali mentioned includes one or more combinations of inorganic alkalis.

7. The method for purifying terephthalaldehyde according to claim 6, characterized in that... The inorganic bases mentioned include one or more combinations of metal oxides, metal hydroxides, metal carbonates, metal bicarbonates, metal phosphates, and metal hydrogen phosphates from Group IA and Group IIA.

8. The purification method of terephthalaldehyde according to claim 1, wherein the distillation in step (4) can be carried out continuously or intermittently, and the distillation conditions are as follows: the number of distillation column plates is 5-150, the bottom temperature is 100-250℃, the top reflux ratio is 1-20:1, and the column operating pressure is 50Pa-10000Pa.

9. The method for purifying terephthalaldehyde according to claim 8, characterized in that... The distillation conditions are as follows: the number of distillation column trays is 10-100, the reboiler temperature is 140-200℃, the reflux ratio at the top of the column is 2-15:1, and the operating pressure of the column is 100Pa-5000Pa.

10. The method for purifying terephthalaldehyde according to claim 1, characterized in that... The melting crystallization in step (5) includes the following steps: terephthalaldehyde liquid is fed into a melting crystallizer at a temperature of 115~130℃, cooled and crystallized at a cooling rate of 2~8℃ / h, and the temperature drops to 100~115℃, and the mother liquor is discharged; then the crystals remaining in the melting crystallizer are subjected to crystal sweating, and the high-purity terephthalaldehyde is collected; the crystal sweating includes the following steps: the heating rate is 2~8℃ / h, the sweating temperature is raised to 115~120℃, the sweating liquid is discharged, the crystals are melted by heating, and the high-purity terephthalaldehyde is collected.