A process for the preparation of 4-hydroxyisophthalic acid

Abstract

The application belongs to the technical field of organic synthesis, and discloses a preparation method of 4-hydroxy isophthalic acid. The target product is prepared through three steps from phenol as raw material: firstly, phenol is neutralized into salt with alkali, and is dehydrated under a vacuum degree greater than or equal to 0.09 MPa and at 140-160 DEG C to obtain phenol salt; then, the phenol salt is subjected to carboxylation reaction under the action of carbonate assistant and formate catalyst at 250-280 DEG C and under a carbon monoxide pressure of 3-5 MPa to obtain 4-hydroxy isophthalic acid salt; finally, the product is obtained through water dissolving, strong acid acidification to pH=1-2, and filtration and drying. The process is simple, the conditions are mild, no expensive equipment and reagents are needed, the raw material is cheap and easy to obtain, and the process is green and environment-friendly; the product purity is greater than or equal to 99%, and the total yield is greater than or equal to 70%, which effectively solves the defects of low yield, harsh conditions, high cost and high equipment requirement in the prior art, and is suitable for industrial large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of organic synthesis technology, specifically relating to a method for preparing 4-hydroxyisophthalic acid, which is particularly suitable for the large-scale industrial production of high-purity 4-hydroxyisophthalic acid. Background Technology

[0002] 4-Hydroxyisophthalic acid (HOH) is an important fine organic chemical product with the molecular formula C8H6O5 and a molecular weight of 182.13. This compound contains both a hydroxyl group and two carboxyl groups in its molecular structure, exhibiting excellent reactivity and coordination ability, and is widely used in pharmaceuticals, polymer materials, and functional chemicals. In the pharmaceutical industry, it is a key intermediate for drugs such as aspirin and diaryl oxalate, and some derivatives have shown significant in vitro inhibitory activity against tumor cells such as breast cancer and gastric cancer. In the materials science field, it can be used as a monomer to synthesize high-performance polyesters, polyamides, and other polymer materials, and can also be used to prepare special functional materials, showing broad market application prospects.

[0003] Currently, the main drawbacks of existing methods for synthesizing 4-hydroxyisophthalic acid are as follows: First, the Blanc chloromethylation-oxidation-demethylation method, while yielding approximately 78%, requires highly corrosive catalysts such as aluminum trichloride, placing extremely high demands on the materials used in production equipment, resulting in high investment and maintenance costs. Second, the Kolbe-Schmitt reaction byproduct extraction method, with a yield below 23%, requires stringent reaction conditions, and is difficult to separate and purify, making large-scale production challenging. Third, the hydroxylation method using halogenated aromatic hydrocarbons requires expensive additives, leading to high production costs and poor economic efficiency. These existing technologies generally suffer from low yields, stringent conditions, high equipment requirements, and high costs, severely hindering the industrial production and widespread application of 4-hydroxyisophthalic acid. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing 4-hydroxyisophthalic acid preparation processes, such as low yield, harsh reaction conditions, high equipment requirements, expensive additives, and poor environmental performance. This invention provides a simple, mild, inexpensive and readily available raw material method for preparing 4-hydroxyisophthalic acid, which produces high-purity products with excellent yield and is also environmentally friendly, thereby achieving energy conservation, emission reduction, and low-cost large-scale production.

[0005] To address the aforementioned technical problems, this invention provides a method for preparing 4-hydroxyisophthalic acid, using phenol as a starting material, and preparing the target product through a three-step reaction involving salt formation, high-pressure carboxylation, and acid precipitation, comprising the following steps:

[0006] (1) Neutralize phenol with alkali, then remove water under vacuum to obtain solid phenol salt;

[0007] (2) Add solid phenol salt, auxiliaries and catalyst together to a high-pressure reactor, introduce carbon monoxide and maintain the pressure inside the reactor to carry out the carboxylation reaction to obtain 4-hydroxyisophthalate;

[0008] (3) Dissolve 4-hydroxyisophthalate in water to prepare a salt solution, then acidify with a strong acid to precipitate, filter and dry to obtain 4-hydroxyisophthalic acid.

[0009] Furthermore, the alkali mentioned in step (1) is one or both of potassium hydroxide and sodium hydroxide; the molar ratio of phenol to alkali is 1:(1-2).

[0010] Furthermore, the neutralization reaction in step (1) is carried out at a temperature of 100–120°C for 1–2 hours.

[0011] Furthermore, the vacuum degree of the vacuum drainage in step (1) is ≥0.09 MPa, and the temperature is 140~160℃.

[0012] Furthermore, the auxiliary agent in step (2) is one or more of potassium carbonate, sodium carbonate, and cesium carbonate.

[0013] Furthermore, the catalyst in step (2) is one or more of sodium acetate, potassium acetate, potassium formate, and sodium formate.

[0014] Furthermore, in step (2), the molar ratio of phenol salt: additive: catalyst is 1:(1~1.5):(1~1.5).

[0015] Furthermore, the carboxylation reaction in step (2) is carried out at a temperature of 250–280°C, a pressure of 3–5 MPa, and a reaction time of 3–4 h.

[0016] Furthermore, the concentration of the salt solution in step (3) is 20-60%.

[0017] Furthermore, in step (3), the concentration of the strong acid is 10-30%, the acidification temperature is 60-90℃, and the pH of the acid precipitation is 1-2.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. The process is simple, the reaction conditions are mild, no demanding equipment or expensive reagents are required, and the raw materials are all basic chemical products that are readily available and inexpensive, making them suitable for large-scale industrial production.

[0020] 2. The reaction is green and environmentally friendly. Unreacted phenol can be recycled, by-product inorganic salts can be purified and sold, and reaction water can be recycled, achieving energy conservation and emission reduction and reducing production costs.

[0021] 3. The product has excellent quality, with a purity of ≥99%, a total yield of ≥70%, and extremely low by-product content, meeting the requirements for use in pharmaceuticals and high-end materials;

[0022] 4. The entire process involves no highly corrosive reagents, requires minimal equipment, significantly reduces investment and maintenance costs, and greatly improves production safety and stability. Detailed Implementation

[0023] The present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0024] Unless otherwise specified, all raw materials used in this invention are commercially available conventional chemical products.

[0025] Example 1

[0026] This embodiment provides a method for preparing 4-hydroxyisophthalic acid, the specific steps of which are as follows:

[0027] (1) 1 mol of phenol and 1.01 mol of potassium hydroxide were added to the reaction vessel, the vessel was sealed under vacuum, stirred and heated to 100°C, and reacted at 100°C for 1 h. After the reaction was completed, the gas was vented, and the vacuum was evacuated to a vacuum degree ≥0.09 MPa for vacuum dehydration. The temperature was continuously raised to 140°C until no water was distilled out, and then the vacuum was stopped and the temperature was lowered to obtain solid potassium phenolate.

[0028] (2) Add the potassium phenolate obtained in step (1), 1 mol of potassium carbonate, and 1 mol of potassium formate to a high-pressure reactor, stir and heat to 250°C, introduce carbon monoxide and maintain the pressure inside the reactor at 3 MPa, keep the temperature constant for 3 h for carboxylation reaction, and cool down after the reaction to obtain potassium 4-hydroxyisophthalate.

[0029] (3) Dissolve the potassium 4-hydroxyisophthalate obtained in step (2) in water to prepare a 40% (w / w) salt solution. Heat the solution to 80°C and slowly add a 20% (w / w) dilute sulfuric acid solution for acidification. Adjust the pH of the salt solution to 1.0 and stir at a constant temperature for 0.5 h to crystallize. After filtration and drying, solid 4-hydroxyisophthalic acid product is obtained. The purity of the product is 99.17% and the total yield is 70.13% as determined by high performance liquid chromatography.

[0030] Example 2

[0031] This embodiment provides a method for preparing 4-hydroxyisophthalic acid, the specific steps of which are as follows:

[0032] (1) 1 mol of phenol and 1.01 mol of sodium hydroxide were added to the reaction vessel, the vessel was sealed under vacuum, stirred and heated to 100°C, and kept at 100°C for 1 h. After the reaction was completed, the gas was vented, and the vacuum was evacuated to a vacuum degree ≥0.09 MPa for vacuum dehydration. The temperature was continuously raised to 140°C until no water was distilled out of the system. The vacuum was then stopped and the temperature was lowered to obtain solid sodium phenolate.

[0033] (2) Add the sodium phenolate, 1 mol sodium carbonate, and 1 mol sodium formate obtained in step (1) into a high-pressure reactor, stir and heat to 250°C, introduce carbon monoxide and maintain the pressure inside the reactor at 3 MPa, keep warm for 3 h for carboxylation reaction, and cool down after the reaction to obtain sodium 4-hydroxyisophthalate.

[0034] (3) Dissolve the sodium 4-hydroxyisophthalate obtained in step (2) in water to prepare a 40% (w / w) salt solution. Heat the solution to 80°C and slowly add a 20% (w / w) dilute sulfuric acid solution for acidification. Adjust the pH of the salt solution to 1.0 and stir at a constant temperature for 0.5 h to crystallize. After filtration and drying, solid 4-hydroxyisophthalic acid product is obtained. The purity of the product is 99.24% and the total yield is 70.15% as determined by high performance liquid chromatography.

[0035] The experimental comparison data of Example 1 and Example 2 are summarized in Table 1.

[0036] As can be seen from the experimental data in Table 1, after replacing potassium hydroxide with sodium hydroxide and potassium carbonate with sodium carbonate in the system, the content of reaction byproducts, product purity and total yield are basically the same as in Example 1. This indicates that the type of alkali metal has no significant effect on the carboxylation reaction process of this invention. Both potassium hydroxide / sodium hydroxide and potassium carbonate / sodium carbonate can achieve equally excellent reaction effects. The process is highly versatile and the raw materials can be flexibly selected according to actual production needs.

[0037] Table 1. Comparison data of experiments in Example 1 and Example 2

[0038]

[0039] Example 3

[0040] The only difference between this embodiment and embodiment 1 is that the neutralization reaction time at 100°C in step (1) is adjusted to 0.5 h, while all other process parameters and operation steps are completely consistent with embodiment 1.

[0041] Example 4

[0042] The only difference between this embodiment and embodiment 1 is that the neutralization reaction time at 100°C in step (1) is adjusted to 2 hours. All other process parameters and operation steps are completely consistent with those in embodiment 1.

[0043] The experimental data comparing the neutralization reaction times of Examples 1, 3, and 4 are summarized in Table 2.

[0044] Table 2 shows that when the neutralization reaction time is 0.5 h, the phenol raw material reaction is incomplete. A small amount of phenol is recovered with steam during the 140℃ vacuum dehydration process, and the amount of water removed is low, indicating that the reaction did not achieve complete salt formation. When the neutralization reaction time is extended to 1 h, the phenol reacts completely, no phenol is recovered during the vacuum dehydration process, and the amount of water removed is close to the theoretical dehydration value, indicating the best reaction effect. Further extending the neutralization reaction time to 2 h does not significantly change the amount of water removed, nor does it further improve the completeness of the reaction; it only increases energy consumption. Therefore, under the neutralization reaction conditions of 100℃, a holding time of 1 h is the optimal process duration.

[0045] Table 2 Comparison of Neutralization Reaction Time Experimental Data

[0046]

[0047] Example 5

[0048] The only difference between this embodiment and embodiment 1 is that the neutralization reaction temperature in step (1) is 80°C, while all other process parameters and operating steps are completely consistent with those in embodiment 1.

[0049] Example 6

[0050] The only difference between this embodiment and Example 1 is that the neutralization reaction temperature in step (1) is 120°C, while all other process parameters and operating steps are completely consistent with Example 1.

[0051] The neutralization reaction temperature control experimental data of Examples 1, 5, and 6 are summarized in Table 3.

[0052] As shown in Table 3, when the neutralization reaction temperature is 80℃, the phenol raw material reacts incompletely. In the subsequent vacuum dehydration process, a small amount of phenol is recovered with the steam distillation, and the actual amount of water removed from the system is low, resulting in poor reaction efficiency. When the reaction temperature is increased to 120℃, the amount of water removed and the amount of phenol residue recovered are basically the same as under the 100℃ condition, and the raw phenol can react completely, but the energy consumption is higher than that at 100℃. Considering the overall reaction efficiency, energy cost, and raw material utilization rate, 100℃ is the optimal reaction temperature for the neutralization reaction in this invention.

[0053] Table 3. Comparison of neutralization reaction temperatures - experimental data

[0054]

[0055] Example 7

[0056] The only difference between this embodiment and embodiment 1 is that the stopping temperature for vacuum dehydration in step (1) is 100°C, while the other process parameters and operating steps are completely consistent with those in embodiment 1.

[0057] Example 8

[0058] The only difference between this embodiment and embodiment 1 is that the stopping temperature for vacuum dehydration in step (1) is 120°C, while the other process parameters and operating steps are completely consistent with those in embodiment 1.

[0059] Example 9

[0060] The only difference between this embodiment and embodiment 1 is that the stopping temperature for vacuum dehydration in step (1) is 160°C, while the other process parameters and operating steps are completely consistent with those in embodiment 1.

[0061] The comparative experimental data of vacuum stopping temperature for Examples 1, 7, 8, and 9 are summarized in Table 4.

[0062] As shown in Table 4, when the vacuum dehydration stop temperature is 100℃, there is still a trace amount of water remaining in the system, and the dehydration is incomplete, which will affect the subsequent carboxylation reaction. The amount of water dehydrated at 160℃ is basically close to that at 140℃, and the dehydration is complete, but the energy consumption is higher than that at 140℃. When the vacuum dehydration stop temperature is 140℃, the amount of water dehydrated is closest to the theoretical value, the dehydration effect is the best, and the energy consumption is reasonable. This is the optimal stop temperature for vacuum dehydration in this invention.

[0063] Table 4 Comparison of vacuum stopping temperature experimental data

[0064]

[0065] Example 10

[0066] The only difference between this embodiment and Example 1 is that the carboxylation reaction temperature in step (2) is 180°C, while the other process parameters and operating steps are completely consistent with Example 1.

[0067] Example 11

[0068] The only difference between this embodiment and Example 1 is that the carboxylation reaction temperature in step (2) is 200°C, while the other process parameters and operating steps are completely consistent with Example 1.

[0069] Example 12

[0070] The only difference between this embodiment and Example 1 is that the carboxylation reaction temperature in step (2) is 230°C, while the other process parameters and operating steps are completely consistent with Example 1.

[0071] Example 13

[0072] The only difference between this embodiment and Example 1 is that the carboxylation reaction temperature in step (2) is 280°C, while the other process parameters and operating steps are completely consistent with Example 1.

[0073] Example 14

[0074] The only difference between this embodiment and Example 1 is that the carboxylation reaction temperature in step (2) is 300°C, while the other process parameters and operating steps are completely consistent with Example 1.

[0075] The comparative experimental data of carboxylation reaction temperatures for Examples 1, 10, 11, 12, 13, and 14 are summarized in Table 5.

[0076] Table 5 Comparison of Carboxylation Reaction Temperatures

[0077]

[0078] As shown in Table 5, the experimental data indicate that two main byproducts exist during the carboxylation reaction of this invention. The content of byproduct 1 (p-hydroxybenzoic acid) gradually decreases with increasing carboxylation reaction temperature, while the content of byproduct 2 (other impurities produced by over-carboxylation or high-temperature decomposition) gradually increases with increasing carboxylation reaction temperature. When the carboxylation reaction temperature is 250℃, the total amount of both byproducts is the lowest, and the product purity and total yield are both optimal, which is the optimal temperature for the carboxylation reaction of this invention.

[0079] Example 15

[0080] The only difference between this embodiment and Example 1 is that the carboxylation reaction pressure in step (2) is 1 MPa, while the other process parameters and operating steps are completely consistent with Example 1.

[0081] Example 16

[0082] The only difference between this embodiment and Example 1 is that the carboxylation reaction pressure in step (2) is 2 MPa, while the other process parameters and operating steps are completely consistent with Example 1.

[0083] Example 17

[0084] The only difference between this embodiment and Example 1 is that the carboxylation reaction pressure in step (2) is 5 MPa, while the other process parameters and operating steps are completely consistent with Example 1.

[0085] Example 18

[0086] The only difference between this embodiment and Example 1 is that the carboxylation reaction pressure in step (2) is 7 MPa, while the other process parameters and operating steps are completely consistent with Example 1.

[0087] Example 19

[0088] The only difference between this embodiment and Example 1 is that the carboxylation reaction pressure in step (2) is 9 MPa, while the other process parameters and operating steps are completely consistent with Example 1.

[0089] The comparative experimental data of carboxylation reaction pressure in Examples 1, 15, 16, 17, 18, and 19 are summarized in Table 6.

[0090] Table 6. Comparison of Carboxylation Reaction Pressure Experimental Data

[0091]

[0092] As shown in Table 6, within the carboxylation reaction pressure range of 1–9 MPa, the content of byproduct 1 gradually decreases while the content of byproduct 2 gradually increases with increasing pressure. Within the carboxylation reaction pressure range of 3–9 MPa, the product maintains high purity, and the total yield fluctuates slightly with pressure changes. When the carboxylation reaction pressure is 3 MPa, the total amount of byproducts is lowest, the product purity and total yield are optimal, and the reaction pressure is moderate with lower equipment requirements, making it the most suitable carboxylation reaction pressure for this invention.

[0093] Example 20

[0094] The only difference between this embodiment and Example 1 is that the carboxylation reaction time in step (2) is 2 h, while the other process parameters and operation steps are completely consistent with Example 1.

[0095] Example 21

[0096] The only difference between this embodiment and Example 1 is that the carboxylation reaction time in step (2) is 4 h, while the other process parameters and operation steps are completely consistent with Example 1.

[0097] Example 22

[0098] The only difference between this embodiment and Example 1 is that the carboxylation reaction time in step (2) is 5 h, while the other process parameters and operation steps are completely consistent with Example 1.

[0099] Example 23

[0100] The only difference between this embodiment and Example 1 is that the carboxylation reaction time in step (2) is 7 h, while the other process parameters and operation steps are completely consistent with Example 1.

[0101] The comparative experimental data of carboxylation reaction time for Examples 1, 20, 21, 22, and 23 are summarized in Table 7.

[0102] As shown in Table 7, when the carboxylation reaction time is too short (2 h), the raw material conversion is incomplete, the content of byproduct 1 increases significantly, and the product purity and total yield are low. As the reaction time is extended, the content of byproduct 1 gradually decreases, but the content of byproduct 2 increases significantly, and the product purity and total yield show a downward trend. When the reaction time is 3 h, the total amount of byproducts is the lowest, and the product purity and total yield are the best, which is the optimal carboxylation reaction time of this invention.

[0103] Table 7 Comparison of carboxylation reaction time experimental data

[0104]

[0105] Example 24

[0106] The only difference between this embodiment and Example 1 is that the molar ratio of potassium phenolate to potassium carbonate in step (2) is 1:0.5. All other process parameters and operating steps are completely consistent with Example 1.

[0107] Example 25

[0108] The only difference between this embodiment and Example 1 is that the molar ratio of potassium phenolate to potassium carbonate in step (2) is 1:1.5. All other process parameters and operating steps are completely consistent with Example 1.

[0109] Example 26

[0110] The only difference between this embodiment and embodiment 1 is that the molar ratio of potassium phenolate to potassium carbonate in step (2) is 1:2, while the other process parameters and operating steps are completely consistent with those in embodiment 1.

[0111] Example 27

[0112] The only difference between this embodiment and embodiment 1 is that the molar ratio of potassium phenolate to potassium carbonate in step (2) is 1:3. All other process parameters and operating steps are completely consistent with those in embodiment 1.

[0113] The experimental data comparing the molar ratios of potassium phenolate and additives in Examples 1, 24, 25, 26, and 27 are summarized in Table 8.

[0114] Table 8 shows that when the amount of potassium carbonate additive is insufficient (the molar ratio of potassium phenolate to potassium carbonate is 1:0.5), the carboxylation reaction is incomplete, the content of byproduct 1 increases significantly, and the product purity is low. As the amount of potassium carbonate increases, the content of byproduct 1 gradually decreases, but the content of byproduct 2 increases significantly, and the product purity and total yield gradually decrease. When the molar ratio of potassium phenolate to potassium carbonate is 1:1, the total amount of byproducts is the lowest, and the product purity and total yield are optimal, representing the best ratio of potassium phenolate to additive.

[0115] Table 8 Comparison of molar ratios of potassium phenolate and auxiliaries

[0116]

[0117] Example 28

[0118] The only difference between this embodiment and Example 1 is that the molar ratio of potassium phenolate to potassium formate catalyst in step (2) is 1:0.1. All other process parameters and operating steps are completely consistent with Example 1.

[0119] Example 29

[0120] The only difference between this embodiment and Example 1 is that the molar ratio of potassium phenolate to potassium formate catalyst in step (2) is 1:0.5. All other process parameters and operating steps are completely consistent with Example 1.

[0121] Example 30

[0122] The only difference between this embodiment and Example 1 is that the molar ratio of potassium phenolate to potassium formate catalyst in step (2) is 1:1.5. All other process parameters and operating steps are completely consistent with Example 1.

[0123] Example 31

[0124] The only difference between this embodiment and Example 1 is that the molar ratio of potassium phenolate to potassium formate catalyst in step (2) is 1:2. All other process parameters and operating steps are completely consistent with Example 1.

[0125] The experimental data comparing the molar ratios of potassium phenolate and catalyst in Examples 1, 28, 29, 30, and 31 are summarized in Table 9.

[0126] As shown in Table 9, when the amount of potassium formate catalyst is too low, the catalytic efficiency of the carboxylation reaction is insufficient, the raw material conversion is incomplete, the content of byproduct 1 increases significantly, and the product purity and total yield are low. As the amount of catalyst increases, the content of byproduct 1 gradually decreases, but the content of byproduct 2 gradually increases, and the product purity and total yield show a downward trend. When the molar ratio of potassium phenolate to potassium formate is 1:1, the total amount of byproducts is the lowest, and the product purity and total yield are the best, which is the optimal catalyst ratio.

[0127] Table 9 Comparison of Molar Ratios of Potassium Phenol and Catalyst

[0128]

[0129] From Examples 1 to 31 and the comparative experiments of each group, it can be seen that the optimal process parameters and core conclusions for the preparation of 4-hydroxyisophthalic acid in this invention are as follows: the optimal reaction temperature for the neutralization reaction is 100℃, the optimal reaction time is 1h, and the optimal stopping temperature for vacuum dehydration is 140℃; the optimal reaction temperature for the carboxylation reaction is 250℃, the optimal reaction pressure is 3 MPa, and the optimal reaction time is 3h; the optimal molar ratio of potassium phenolate to auxiliaries (potassium carbonate / sodium carbonate) in the carboxylation reaction is 1:1, and the optimal molar ratio of phenolate to catalyst (potassium formate / sodium formate) is 1:1; the type of alkali metal has no significant effect on the carboxylation reaction, potassium hydroxide and sodium hydroxide, potassium carbonate and sodium carbonate can be used interchangeably, and the content of reaction byproducts, product purity and total yield are basically the same, the process is highly versatile and the raw material selection is flexible.

[0130] In summary, the basic raw material used in this invention is phenol, the auxiliary agent is carbonate, and the catalyst is formate, all of which are basic chemical raw materials that are inexpensive and readily available. The reaction conditions are mild and not harsh, requiring no expensive specialized equipment, resulting in low equipment investment and suitability for large-scale industrial production. Under the above-mentioned optimal process parameters, the 4-hydroxyisophthalic acid product prepared has a purity of ≥99%, produces only a small amount of byproducts (mainly p-hydroxybenzoic acid), and has a total yield of ≥70%. The product quality is stable and the economic benefits are significant.

[0131] 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 4-hydroxyisophthalic acid, characterized in that, Includes the following steps, (1) Neutralize phenol with alkali, then remove water under vacuum to obtain solid phenol salt; (2) Add solid phenol salt, auxiliaries and catalyst together to a high-pressure reactor, introduce carbon monoxide and maintain the pressure inside the reactor to carry out the carboxylation reaction to obtain 4-hydroxyisophthalate; (3) Dissolve 4-hydroxyisophthalate in water to prepare a salt solution, then acidify with a strong acid to precipitate, filter and dry to obtain 4-hydroxyisophthalic acid.

2. The preparation method according to claim 1, characterized in that: The alkali mentioned in step (1) is one or both of potassium hydroxide and sodium hydroxide; the molar ratio of phenol to alkali is 1:(1-2).

3. The preparation method according to claim 1, characterized in that: The neutralization reaction in step (1) is carried out at a temperature of 100-120°C for 1-2 hours.

4. The preparation method according to claim 1, characterized in that: In step (1), the vacuum degree of the vacuum pumping and drainage is ≥0.09 MPa and the temperature is 140~160℃.

5. The preparation method according to claim 1, characterized in that: The auxiliary agent mentioned in step (2) is one or more of potassium carbonate, sodium carbonate, and cesium carbonate.

6. The preparation method according to claim 1, characterized in that: The catalyst in step (2) is one or more of sodium acetate, potassium acetate, potassium formate, and sodium formate.

7. The preparation method according to claim 1, 5, or 6, characterized in that: The molar ratio of phenol salt, auxiliary agent and catalyst in step (2) is 1:(1~1.5):(1~1.5).

8. The preparation method according to claim 1, characterized in that: The carboxylation reaction in step (2) is carried out at a temperature of 250–280°C, a pressure of 3–5 MPa, and a time of 3–4 h.

9. The preparation method according to claim 1, characterized in that: The concentration of the salt solution in step (3) is 20-60%.

10. The preparation method according to claim 1, characterized in that: The concentration of the strong acid in step (3) is 10-30%, the acidification temperature is 60-90℃, and the pH of the acid precipitation is 1-2.

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