Preparation method of polymer-grade p-hydroxybenzoic acid

Abstract

The invention belongs to the technical field of preparation of organic compounds, and particularly discloses a preparation method of polymer-grade p-hydroxybenzoic acid. Comprising the following steps: preparing a potassium phenolate and metal composite salt, performing carboxylation reaction, performing decolorization treatment, performing acidification purification and the like: performing dehydration treatment on phenol, potassium hydroxide, potassium carbonate and formate under the action of a metal catalyst to obtain the potassium phenolate and metal composite salt; continuously introducing carbon monoxide or mixed gas of carbon monoxide and carbon dioxide to carry out carboxylation reaction, and adding water to dissolve to obtain a p-hydroxybenzoic acid dipotassium salt solution; and after decolorizing with activated carbon, separating potassium sulfate and a target product through two times of acidification to obtain the polymer-grade p-hydroxybenzoic acid. Through combination of carbon monoxide or carbon monoxide and carbon dioxide and screening of the catalyst, the one-time carboxylation conversion rate is increased to 90% or above, the product yield reaches up to 85%, the purity is larger than or equal to 99.96%, the chromaticity is stabilized at 8 Hazen, purification is simple, the emission of three wastes is remarkably reduced, and large-scale production of the polymer-grade p-hydroxybenzoic acid is achieved.

Description

Technical Field

[0001] This invention belongs to the field of organic compound preparation technology, specifically relating to a method for preparing methylparaben using a novel and efficient co-catalytic system. Background Technology

[0002] p-Hydroxybenzoic acid (p-hydroxybenzoic acid) has a wide range of important applications in industry and scientific research, mainly in the following aspects: In the field of chemical synthesis, p-hydroxybenzoic acid, as a starting material or intermediate for organic synthesis, can be used to synthesize various organic compounds such as pharmaceuticals, dyes, and fragrances due to the good reactivity and selectivity of its hydroxyl and carboxyl functional groups; In the polymer industry, p-hydroxybenzoic acid can be used as a monomer or functional additive for polymers, for example, playing a key role in regulating the properties of polymers such as polyester resins and polyamides; In the field of preservation, due to its good antibacterial properties, it is often used as a preservative in food and cosmetics, effectively extending the shelf life of products; In the pharmaceutical field, p-hydroxybenzoic acid and its derivatives are also widely used in drug synthesis, acting as intermediates or pharmacophores for certain drugs. In summary, p-hydroxybenzoic acid occupies an important position in many industrial and scientific fields due to its diverse chemical properties and reactivity, and has extremely high application value.

[0003] Currently, the industrial production of p-hydroxybenzoic acid mainly adopts the Kolbe–Schmitt reaction (potassium phenolate and carbon dioxide carboxylation method). This method has obvious technical defects: only potassium phenolate can react with carbon dioxide to produce p-hydroxybenzoic acid, and the carboxylation reaction of two molecules of potassium phenolate can only produce one molecule of p-hydroxybenzoic acid and one molecule of phenol. In order to improve the product yield, multiple phenol distillation and carboxylation operations are required. This not only greatly increases energy consumption and reaction time, but also results in the total yield always being less than 50%. At the same time, there are also problems such as large emissions of waste gas, wastewater, and solid waste, and high difficulty in purifying polymer-grade products, which seriously restrict the large-scale production and application expansion of p-hydroxybenzoic acid. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of the traditional Kolbe-Schmitt carboxylation process, such as low yield from multiple carboxylation processes, high emissions of waste, and difficulty in purifying polymer-grade p-hydroxybenzoic acid. This invention provides a method for preparing polymer-grade p-hydroxybenzoic acid that not only enables large-scale, green production of polymer-grade p-hydroxybenzoic acid, but also features a simple process, high yield of the target product from a single carboxylation step, easy separation, no need for recrystallization, and low emissions of waste.

[0005] To address the aforementioned technical problems, this invention provides a method for preparing polymer-grade p-hydroxybenzoic acid. Under the action of a metal catalyst, a synergistic reaction system is constructed by introducing carbon monoxide or a mixture of carbon monoxide and carbon dioxide, formic acid or its potassium salt. The method optimizes the raw material ratio, reaction temperature, pressure, and dehydration-acid precipitation process parameters to achieve efficient carboxylation reaction and product purification. Specifically, the method includes the following steps: (1) Phenol, potassium hydroxide, potassium carbonate, formate and water are added to the reaction vessel, stirred to dissolve and mix evenly, then metal catalyst is added, and while stirring, the temperature is raised to 80-90°C. First, the free water in the raw material is removed by depressurization, while avoiding the volatilization of the raw material. Then the temperature is raised to 240-270°C for acid-base neutralization reaction. After the reaction is completed, the water generated by the reaction is removed by depressurization again to obtain molten potassium phenolate and metal composite salt. (2) Continuously introduce carbon monoxide or a mixture of carbon monoxide and carbon dioxide into the reaction vessel to maintain a constant pressure inside the vessel and carry out the carboxylation reaction. After the reaction is completed, release the pressure and add water to dissolve the carboxylated dipotassium salt solution of p-hydroxybenzoate. (3) Add dilute sulfuric acid to the dipotassium salt solution of p-hydroxybenzoic acid to adjust the pH to 6-7, cool to 40-50℃, centrifuge to filter out the generated potassium sulfate, continue to add dilute sulfuric acid to the centrifuged liquid to adjust the pH to 2.5-3, cool to 20-30℃ and centrifuge to separate, and the solid is purified (e.g., washed with water) and dried to obtain polymer-grade p-hydroxybenzoic acid.

[0006] As a further description of the above technical solution: the metal catalyst in step (1) is a nanoscale bimetallic catalyst or a nanoscale single metal compound, and the amount of metal catalyst used is 5% to 10% of the mass of phenol.

[0007] As a further description of the above technical solution: the nanoscale bimetallic catalyst is any one of nanoscale copper-magnesium alloy, nanoscale copper-nickel alloy, or nanoscale nickel-magnesium alloy; the nanoscale single metal compound is any one of nanoscale copper oxide, nanoscale magnesium oxide, nanoscale cuprous chloride, nanoscale copper chloride, or nanoscale nickel chloride.

[0008] As a further description of the above technical solution: in step (1), the molar ratio of phenol, potassium hydroxide, potassium carbonate and formic acid is 1:(1.1~1.5):(0.15~1.0):(1.1~1.2).

[0009] As a further description of the above technical solution: the temperature for depressurization to remove water from the raw material in step (1) is 80-90℃ and the pressure is 0.09-0.11 MPa.

[0010] As a further description of the above technical solution: the volume ratio of carbon monoxide to carbon dioxide in the mixed gas in step (2) is 1:1 to 2:1.

[0011] As a further description of the above technical solution: the reaction pressure of the carboxylation reaction in step (2) is 0.5 to 2.5 MPa, and the reaction time is 3 to 6 h.

[0012] As a further description of the above technical solution: the mass fraction of the dilute sulfuric acid in step (3) is 25% to 30%.

[0013] As a further description of the above technical solution: the drying temperature in step (3) is 95-105℃.

[0014] As a further description of the above technical solution: the dipotassium hydroxybenzoate solution obtained in step (2) is decolorized by activated carbon before the operation in step (3).

[0015] Compared with the prior art, the beneficial effects of the present invention are: This invention constructs a multi-component synergistic reaction system of "metal catalyst-multi-base synergy-novel carboxylation gas" and precisely optimizes key process parameters to achieve efficient and green preparation of polymer-grade p-hydroxybenzoic acid: the metal catalyst (nanoscale bimetallic alloy or single-metal compound) serves as the core active component, which not only significantly reduces the activation energy of the carboxylation reaction but also specifically enhances the adsorption and activation efficiency of carbon monoxide (or its mixture with carbon dioxide) in the potassium phenolate complex salt system. The bimetallic catalyst enhances the gas synergistic effect through its multiple active sites, while the single-metal compound combines cost advantages with directional catalytic ability. Both can break through the inherent bottleneck of "limited reaction between potassium phenolate and carbon dioxide" in traditional processes, effectively suppressing side reactions such as tar formation and ortho-carboxylation, and significantly improving reaction selectivity and primary carboxylation conversion rate. Formate (preferably potassium formate) and potassium carbonate form a highly efficient synergistic effect. Formate, as a dedicated carboxyl group donor, can directly provide active carboxyl groups for the carboxylation reaction, assisting in the enhancement of the reaction efficiency. The process increases the reaction rate and, together with potassium carbonate, regulates the alkalinity of the system, stabilizing the structure and reactivity of the potassium phenolate complex salt. This prevents localized over-alkaliness from causing raw material decomposition or exacerbating side reactions. Potassium carbonate can also react with the water produced in the reaction, continuously driving the dehydration process and further promoting acid-base neutralization and the forward carboxylation reaction. Compared to traditional single carbon dioxide carboxylation, the introduction of carbon monoxide or its mixture with carbon dioxide not only broadens the carboxylation reaction pathway, but also allows carbon monoxide and carbon dioxide in the mixed gas to synergistically participate in the carboxylation process, increasing the reaction rate and depth. Single carbon monoxide exhibits better adsorption and activation efficiency with nano-metal catalysts, enabling precise and targeted promotion of para-carboxylation, optimizing product color, and reducing post-processing load. Simultaneously, continuous aeration maintains stable system pressure, avoiding pressure fluctuations caused by gas consumption and ensuring reaction uniformity. Combined with subsequent acid precipitation and activated carbon decolorization processes, efficient separation of the product from salt impurities and colored impurities can be achieved without complex purification operations such as recrystallization.

[0016] This invention increases the carboxylation conversion rate to over 90% in a single step, far exceeding the conversion efficiency of the traditional Kolbe-Schmitt reaction. The product yield can reach up to 85%, with a stable purity of over 99.96% and a color of 8 Hazen, all meeting polymerization-grade requirements. Furthermore, this invention eliminates the need for multiple carboxylation, phenol removal, and recrystallization operations found in traditional processes, significantly simplifying the production process and shortening the production cycle by over 60%. It effectively reduces the generation of byproducts such as tar, significantly lowering emissions and raw material consumption. This aligns with green chemical production principles and offers significant cost advantages. Moreover, the flexible selection of the catalytic system and carboxylation gas allows for adaptation to different industrial scenarios, fundamentally solving the technical pain points of low yield, excessive waste, and difficult purification in traditional processes. It is easily scalable for industrial production and possesses extremely high industrial application prospects and commercial value. Detailed Implementation

[0017] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0018] Example 1 This embodiment provides a method for preparing polymer-grade p-hydroxybenzoic acid, including the following steps: (1) Add 310g of phenol to a 2L high-pressure vessel with a stirring device, then add 260g of potassium hydroxide, 85g of potassium carbonate, 330g of potassium formate powder and 100 mL of water (to facilitate the dissolution of raw materials). After stirring and dissolving and mixing evenly, add 15.5g of nano-sized copper-nickel alloy with an average particle size of 100 nm (copper-nickel molar ratio 1:1). While stirring, raise the temperature to 80°C and remove the free water in the raw materials (including the crystal water in the raw materials, the added water and the water generated in the early stage of the reaction) under reduced pressure of 0.1MPa. Then raise the temperature to 270°C and keep the reaction at this temperature for 60 minutes. After the reaction is completed, remove the water generated in the reaction under reduced pressure of 0.1MPa again to obtain molten potassium phenolate and metal composite salt. (2) Using high-purity carbon monoxide and carbon dioxide cylinders, continuously introduce a mixture of carbon monoxide and carbon dioxide gas with a volume ratio of 1:1 into the autoclave, maintain the pressure inside the autoclave at 1.5 MPa, keep the temperature inside the autoclave at 270℃, and carry out the carboxylation reaction. After the reaction has been carried out for 3 hours, close the cylinder valve and release the pressure to stop the reaction. When the temperature inside the autoclave drops to about 200℃, remove a small amount of phenol by vacuum distillation at 0.1 MPa, add 1L of distilled water to dissolve, and obtain a reddish-brown dipotassium salt aqueous solution of p-hydroxybenzoic acid. (3) Add 50g of activated carbon to the aqueous solution of dipotassium p-hydroxybenzoate, stir and decolorize for 60 minutes, then filter out the activated carbon to obtain the decolorized aqueous solution of dipotassium p-hydroxybenzoate. (4) Add 30% dilute sulfuric acid to the decolorized dipotassium salt of p-hydroxybenzoic acid to adjust the pH to 6.5, cool to 45°C, and solid potassium sulfate will precipitate. After centrifugation to remove potassium sulfate, 1078g of centrifuged liquid is obtained. Continue to add dilute sulfuric acid to the centrifuged liquid to adjust the pH to 2.5, and white p-hydroxybenzoic acid solid will precipitate. Cool to about 30°C. After the solid has completely precipitated, centrifuge and filter to separate it. Wash the filter cake twice with 50 mL of distilled water to remove potassium sulfate impurities adsorbed on the surface of the filter cake. Finally, dry at 100°C for 8 h to obtain 317.8g of white solid with a purity of 99.961%, a yield of 68.8%, and a color of 8 Hazen, which is the polymer grade p-hydroxybenzoic acid.

[0019] The p-hydroxybenzoic acid prepared in this embodiment was tested, and all its indicators met the quality standards for polymer grade, as shown in Table 1. It was a white crystalline powder, odorless and with clear, transparent solubility. Its core indicators were superior to the polymer grade standards: content reached 99.961% (≥99.6%), melting point 216℃ (within the range of 214~217℃), loss on drying 0.12% (≤0.20%), and color 8 Hazen (≤10). Meanwhile, the contents of impurities such as ash, sulfate, and chloride, as well as byproducts such as phenol and salicylic acid, were all below the limit values. Residual metal ions such as potassium, sodium, and iron were strictly controlled within 5 ppm. Indicators such as 4-hydroxyisophthalic acid and methyl insoluble matter also met the requirements. The overall quality fully met the application standards for polymer grade p-hydroxybenzoic acid.

[0020] Table 1. Results of quality indicators for p-hydroxybenzoic acid. ; Example 2 This embodiment provides a method for preparing polymer-grade p-hydroxybenzoic acid, including the following steps: (1) Add 310g of phenol to a 2L high-pressure vessel with a stirring device, then add 205g of potassium hydroxide, 425g of potassium carbonate, 330g of potassium formate powder and 100 mL of water (to facilitate the dissolution of raw materials). After stirring and dissolving and mixing evenly, add 31g of nano-sized nickel chloride with an average particle size of 100 nm. While stirring, raise the temperature to 85°C and remove the free water in the raw materials (including the crystal water in the raw materials, the added water and the water generated in the early stage of the reaction) under reduced pressure of 0.11MPa. Then raise the temperature to 250°C and keep the reaction at this temperature for 60 minutes. After the reaction is completed, remove the water generated in the reaction under reduced pressure of 0.11MPa again to obtain molten potassium phenolate and metal composite salt. (2) Using high-purity carbon monoxide and carbon dioxide cylinders, continuously introduce a mixture of carbon monoxide and carbon dioxide gas with a volume ratio of 2:1 into the autoclave, maintain the pressure inside the autoclave at 2.5 MPa, keep the temperature inside the autoclave at 250℃, and carry out the carboxylation reaction. After 5 h of reaction, close the cylinder valve and release the pressure to stop the reaction. When the temperature inside the autoclave drops to about 200℃, remove a small amount of phenol by vacuum distillation at 0.11 MPa, add 1 L of distilled water to dissolve, and obtain a reddish-brown dipotassium salt aqueous solution of p-hydroxybenzoic acid. (3) Add 80g of activated carbon to the aqueous solution of dipotassium p-hydroxybenzoate, stir and decolorize for 60 minutes, then filter out the activated carbon to obtain the decolorized aqueous solution of dipotassium p-hydroxybenzoate. (4) Add 25% dilute sulfuric acid to the decolorized dipotassium salt aqueous solution of p-hydroxybenzoic acid to adjust the pH to 6.8, cool to 50°C, and solid potassium sulfate will precipitate. After centrifugation to remove potassium sulfate, 1103g of centrifuged liquid is obtained. Continue to add dilute sulfuric acid to the centrifuged liquid to adjust the pH to 3.0, and white p-hydroxybenzoic acid solid will precipitate. Cool to about 25°C. After the solid has completely precipitated, centrifuge and filter to separate it. Wash the filter cake twice with 50 mL of distilled water to remove potassium sulfate impurities adsorbed on the surface of the filter cake. Finally, dry at 95°C for 10 h to obtain 298.9g of white solid with a purity of 99.976%, a yield of 65.5%, and a color of 8 Hazen, which is the polymer grade p-hydroxybenzoic acid.

[0021] Example 3 This embodiment provides a method for preparing polymer-grade p-hydroxybenzoic acid, including the following steps: (1) Add 310g of phenol to a 2L high-pressure vessel with a stirring device, then add 270g of potassium hydroxide, 85g of potassium carbonate, 310g of potassium formate powder and 80 mL of water (to facilitate the dissolution of raw materials). After stirring and dissolving and mixing evenly, add 15.5g of nano-sized copper chloride with an average particle size of 100 nm. While stirring, raise the temperature to 90°C and remove the free water in the raw materials (including the crystal water in the raw materials, the added water and the water generated in the early stage of the reaction) under reduced pressure of 0.09MPa. Then raise the temperature to 240°C and keep the reaction at this temperature for 100 minutes. After the reaction is completed, remove the water generated in the reaction under reduced pressure of 0.09MPa again to obtain molten potassium phenolate and metal composite salt. (2) Using high-purity carbon monoxide and carbon dioxide cylinders, continuously introduce a mixture of carbon monoxide and carbon dioxide gas with a volume ratio of 2:1 into the autoclave, maintain the pressure inside the autoclave at 0.5 MPa, and keep the temperature inside the autoclave at 240℃ to carry out the carboxylation reaction. After 6 h of reaction, close the cylinder valve and release the pressure to stop the reaction. When the temperature inside the autoclave drops to about 200℃, remove a small amount of phenol by vacuum distillation at 0.09 MPa, add 1 L of distilled water to dissolve, and obtain a reddish-brown dipotassium salt aqueous solution of p-hydroxybenzoic acid. (3) Add 100g of activated carbon to the aqueous solution of dipotassium p-hydroxybenzoate, stir and decolorize for 60 minutes, then filter out the activated carbon to obtain the decolorized aqueous solution of dipotassium p-hydroxybenzoate. (4) Add 30% dilute sulfuric acid to the decolorized dipotassium salt aqueous solution of p-hydroxybenzoic acid to adjust the pH to 7.0, cool to 40°C, and solid potassium sulfate will precipitate. After centrifugation to remove potassium sulfate, 1131g of centrifuged liquid is obtained. Continue to add dilute sulfuric acid to the centrifuged liquid to adjust the pH to 3.0, and white p-hydroxybenzoic acid solid will precipitate. Cool to about 20°C. After the solid has completely precipitated, centrifuge and filter to separate it. Wash the filter cake twice with 50 mL of distilled water to remove potassium sulfate impurities adsorbed on the surface of the filter cake. Finally, dry at 105°C for 4 h to obtain 382.6g of white solid with a purity of 99.967%, a yield of 84.2%, and a color of 8 Hazen, which is the polymer grade p-hydroxybenzoic acid.

[0022] Example 4 This embodiment provides a method for preparing polymer-grade p-hydroxybenzoic acid, including the following steps: (1) Add 310g of phenol to a 2L high-pressure vessel with a stirring device, then add 205g of potassium hydroxide, 425g of potassium carbonate, 330g of potassium formate powder and 100 mL of water (to facilitate the dissolution of raw materials). After stirring and dissolving and mixing evenly, add 15.5g of nano-sized nickel-magnesium alloy with an average particle size of 100 nm (nickel-magnesium molar ratio 1.5:1). Continue stirring and raise the temperature to 80°C. Remove the free water in the raw materials (including the crystal water in the raw materials, the added water and the water generated in the early stage of the reaction) under reduced pressure of 0.1MPa. Then raise the temperature to 260°C and keep the reaction at this temperature for 80 minutes. After the reaction is completed, remove the water generated in the reaction under reduced pressure of 0.1MPa again to obtain molten potassium phenolate and metal composite salt. (2) Using high-purity carbon monoxide and carbon dioxide cylinders, continuously introduce a mixture of carbon monoxide and carbon dioxide gas with a volume ratio of 1:1 into the autoclave, maintain the pressure inside the autoclave at 2.0 MPa, and keep the temperature inside the autoclave at 260℃ to carry out the carboxylation reaction. After the reaction has been going on for 4 hours, close the cylinder valve and release the pressure to stop the reaction. When the temperature inside the autoclave drops to about 200℃, remove a small amount of phenol by vacuum distillation at 0.1 MPa, add 1L of distilled water to dissolve it, and obtain a reddish-brown dipotassium salt aqueous solution of p-hydroxybenzoic acid. (3) Add 100g of activated carbon to the aqueous solution of dipotassium p-hydroxybenzoate, stir and decolorize for 60 minutes, then filter out the activated carbon to obtain the decolorized aqueous solution of dipotassium p-hydroxybenzoate. (4) Add 30% dilute sulfuric acid to the decolorized dipotassium salt aqueous solution of p-hydroxybenzoic acid to adjust the pH to 6.0, cool to 45°C, and solid potassium sulfate will precipitate. After centrifugation to remove potassium sulfate, 1126g of centrifuged liquid is obtained. Continue to add dilute sulfuric acid to the centrifuged liquid to adjust the pH to 3.0, and white p-hydroxybenzoic acid solid will precipitate. Cool to about 30°C. After the solid has completely precipitated, centrifuge and filter to separate it. Wash the filter cake twice with 50 mL of distilled water to remove potassium sulfate impurities adsorbed on the surface of the filter cake. Finally, dry at 100°C for 6 h to obtain 390.9g of white solid with a purity of 99.973%, a yield of 85%, and a color of 8 Hazen, which is the polymer grade p-hydroxybenzoic acid.

[0023] Example 5 This embodiment provides a method for preparing polymer-grade p-hydroxybenzoic acid, including the following steps: (1) Add 310g of phenol to a 2L high-pressure vessel with a stirring device, then add 260g of potassium hydroxide, 85g of potassium carbonate, 330g of potassium formate powder and 100 mL of water (to facilitate the dissolution of raw materials). After stirring and dissolving and mixing evenly, add 15.5g of nano-sized nickel-magnesium alloy with an average particle size of 100 nm (nickel-magnesium molar ratio 1:1). Continue stirring and raise the temperature to 80°C. Remove the free water in the raw materials (including the crystal water in the raw materials, the added water and the water generated in the early stage of the reaction) under reduced pressure of 0.1MPa. Then raise the temperature to 270°C and keep the reaction at this temperature for 60 minutes. After the reaction is completed, remove the water generated in the reaction under reduced pressure of 0.1MPa again to obtain molten potassium phenolate and metal composite salt. (2) Using a high-purity (purity ≥99.99%) carbon monoxide cylinder, continuously introduce carbon monoxide gas into the autoclave to maintain the pressure inside the autoclave at 1.5 MPa and the temperature inside the autoclave at 270℃, and carry out the carboxylation reaction. After the reaction has been carried out for 3 hours, close the cylinder valve and release the pressure to stop the reaction. When the temperature inside the autoclave drops to about 200℃, remove a small amount of phenol by vacuum distillation at 0.1 MPa, add 1L of distilled water to dissolve it, and obtain a reddish-brown dipotassium salt aqueous solution of p-hydroxybenzoic acid. (3) Add 50g of activated carbon to the aqueous solution of dipotassium p-hydroxybenzoate, stir and decolorize for 60 minutes, then filter out the activated carbon to obtain the decolorized aqueous solution of dipotassium p-hydroxybenzoate. (4) Add 30% dilute sulfuric acid to the decolorized dipotassium hydroxybenzoate aqueous solution to adjust the pH to 6.5, cool to 45°C, and solid potassium sulfate precipitates. After centrifugation to remove the potassium sulfate, 1012g of centrifuged liquid is obtained. Continue to add dilute sulfuric acid to the centrifuged liquid to adjust the pH to 2.5, and white solid p-hydroxybenzoic acid precipitates. Cool to about 30°C, and after the solid has completely precipitated, separate by centrifugation and filtration. Wash the filter cake twice with 50 mL of distilled water to remove potassium sulfate impurities adsorbed on the surface of the filter cake. Finally, dry at 100°C for 8 h to obtain 262.8g of white solid with a purity of 99.966%, a yield of 56.9%, and a color of 8 Hazen, which is the polymer grade p-hydroxybenzoic acid. The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions will not cause the essence of the corresponding technical solutions to deviate from the protection scope of the present invention.

Claims

1. A method for preparing polymer-grade p-hydroxybenzoic acid, characterized in that, Includes the following steps: (1) Phenol, potassium hydroxide, potassium carbonate, formate and water are added to the reaction vessel, stirred to dissolve and mix evenly, then metal catalyst is added, and while stirring, the temperature is raised to 80-90°C. First, the free water in the raw material is removed by depressurization, and then the temperature is raised to 240-270°C for acid-base neutralization reaction. After the reaction is completed, the water generated by the reaction is removed by depressurization again to obtain molten potassium phenolate and metal composite salt. (2) Continuously introduce carbon monoxide or a mixture of carbon monoxide and carbon dioxide into the reaction vessel to maintain a constant pressure inside the vessel and carry out the carboxylation reaction. After the reaction is completed, release the pressure and add water to dissolve the carboxylated dipotassium salt solution of p-hydroxybenzoate. (3) Add dilute sulfuric acid to the dipotassium salt solution of p-hydroxybenzoic acid to adjust the pH to 6-7, cool to 40-50℃, centrifuge to filter out the generated potassium sulfate, continue to add dilute sulfuric acid to the centrifuged liquid to adjust the pH to 2.5-3, cool to 20-30℃ and centrifuge to separate, and the solid is purified and dried to obtain polymer-grade p-hydroxybenzoic acid.

2. The preparation method according to claim 1, characterized in that: The metal catalyst mentioned in step (1) is a nanoscale bimetallic catalyst or a nanoscale single metal compound, and the amount of metal catalyst used is 5% to 10% of the mass of phenol.

3. The preparation method according to claim 2, characterized in that: The nanoscale bimetallic catalyst is any one of nanoscale copper-magnesium alloy, nanoscale copper-nickel alloy, or nanoscale nickel-magnesium alloy; the nanoscale single-metal compound is any one of nanoscale copper oxide, nanoscale magnesium oxide, nanoscale cuprous chloride, nanoscale copper chloride, or nanoscale nickel chloride.

4. The preparation method according to claim 1, characterized in that: In step (1), the molar ratio of phenol, potassium hydroxide, potassium carbonate and formate is 1:(1.1~1.5):(0.15~1.0):(1.1~1.2).

5. The preparation method according to claim 1, characterized in that: The pressure for removing free water from the raw material by depressurization in step (1) is 0.09 to 0.11 MPa.

6. The preparation method according to claim 1, characterized in that: The volume ratio of carbon monoxide to carbon dioxide in the mixed gas in step (2) is 1:1 to 2:

1.

7. The preparation method according to claim 6, characterized in that: The reaction pressure of the carboxylation reaction in step (2) is 0.5 to 2.5 MPa, and the reaction time is 3 to 6 h.

8. The preparation method according to claim 1, characterized in that: The mass fraction of the dilute sulfuric acid in step (3) is 25% to 30%.

9. The preparation method according to claim 1, characterized in that: The drying temperature in step (3) is 95-105℃.

10. The preparation method according to any one of claims 1 to 9, characterized in that: The dipotassium hydroxybenzoate solution obtained in step (2) is decolorized with activated carbon before proceeding to step (3).