A method for preparing 6-hydroxy-2-naphthoic acid

Abstract

The present invention provides a method for preparing 6-hydroxy-2-naphthoic acid, comprising: S1, 2-naphthol reacts with an alkaline potassium compound to generate 2-naphthol salt, an organic solvent having a boiling point higher than the dehydration temperature is added to prevent the organic solvent from being evaporated together during the dehydration process; S2, heating and dehydrating to obtain anhydrous 2-naphthol salt; S3, making 2-naphthol salt and carbon dioxide undergo carboxylation reaction under the action of a catalyst; S4, adding water to dissolve and then separating, neutralizing the aqueous phase with acid, and obtaining a crude 2,6 acid product after acidification; S5, after separation and purification, obtaining a 2,6 acid finished product. The present invention introduces a nickel-based catalyst during the carboxylation reaction, and the nickel-based catalyst promotes C(sp 2 )-H bond with CO 2 The carboxylation reaction plays an important role in controlling the reaction sites and can improve the conversion rate of 2,6-acid. At the same time, the catalyst can reduce the temperature at which the carboxylation reaction occurs, which on the one hand reduces the cost of heating, and on the other hand prevents 2-naphthol from being oxidized to naphthoquinone due to high temperature, thereby avoiding coloration and improving product quality.

Description

Technical Field

[0001] The invention relates to the field of organic synthesis, in particular to a method for preparing 6-hydroxy-2-naphthoic acid. Background Art

[0002] 6-Hydroxy-2-naphthoic acid, also known as 2,6-acid, abbreviated as 2,6-HNA, has good heat resistance and processing properties, and is an important organic intermediate in the fields of engineering plastics, organic pigments, liquid crystal materials, and medicine. With the rapid development of technologies such as liquid crystals, higher and higher requirements are placed on the purity of 2,6-acid, which is one of the raw materials for preparing liquid crystal materials.

[0003] Patent CN87105505A discloses a method for producing 2-hydroxynaphthalene-6-carboxylic acid, which specifically comprises reacting 2-naphthol with an alkaline potassium compound in an organic solvent to generate 2-naphthol potassium salt, dehydrating the solution in the solvent to obtain anhydrous 2-naphthol potassium salt, using a suspension method to allow the 2-naphthol potassium salt to undergo a carboxylation reaction with carbon dioxide to generate 2-naphthol and 2,6-acid salt, dissolving the salt in water and separating the salt from the solvent, neutralizing the aqueous phase with sulfuric acid, acidifying the salt to obtain 2,6-acid, and filtering the salt by suction, centrifuging, and drying the salt to obtain the finished product.

[0004] However, this production method has the following defects in practical application: (1) the organic solvent will be evaporated during the dehydration process, and it is necessary to replenish the organic solvent or return the evaporated solvent after dehydration, which adds additional operation steps; (2) the preparation process of 2,6 acid contains a main reaction and multiple side reactions, and the by-products introduced are not only 2,3 acid, but also 2,1 acid, the isomer of 2,6 acid. The selectivity in the embodiment is too one-sided, and the selectivity of the 2,6 acid product actually prepared is relatively lower; (3) the temperature of the carboxylation reaction needs to be controlled at above 250°C, which easily causes the generated 2-naphthol to be oxidized to naphthoquinone at high temperature, coloring the product and reducing the product quality; (4) the yield of 2,6 acid in the embodiment is only 30%~56%, the yield is not high, and the yield is not stable.

[0005] In this regard, the present invention improves the existing production process, restrains the occurrence of side reactions, ensures the high efficiency and stability of the 2,6 acid selection ratio, and improves the yield of 2,6 acid. Summary of the invention

[0006] In view of the shortcomings of the prior art described above, the object of the present invention is to provide a method for preparing 6-hydroxy-2-naphthoic acid, which is used to solve the problem that the preparation process of 6-hydroxy-2-naphthoic acid in the prior art has many by-products, resulting in low selectivity and yield of 6-hydroxy-2-naphthoic acid.

[0007] To achieve the above object and other related objects, the present invention provides a method for preparing 6-hydroxy-2-naphthoic acid, comprising the following steps:

[0008] S1, 2-naphthol reacts with an alkaline potassium compound to generate a 2-naphthol salt, and an organic solvent is added, wherein the boiling point of the organic solvent is higher than the dehydration temperature of step S2;

[0009] S2, heating to remove water from the organic solvent to obtain anhydrous 2-naphthol salt;

[0010] S3, using a suspension method to make 2-naphthol salt and carbon dioxide undergo carboxylation reaction under the action of a catalyst to generate 2-naphthol and 2,6 acid salt;

[0011] S4, adding water to dissolve and separating from the organic solvent, neutralizing the aqueous phase with acid, and acidifying to obtain a crude product of 6-hydroxy-2-naphthoic acid;

[0012] S5. After separation and purification of the crude 6-hydroxy-2-naphthoic acid product, the finished 6-hydroxy-2-naphthoic acid product is obtained.

[0013] In step S1, the present invention selects an organic solvent with a boiling point higher than the dehydration temperature to prevent the organic solvent from being evaporated during the dehydration process. Therefore, there is no need to add additional organic solvent or return the evaporated organic solvent after dehydration, thereby avoiding unnecessary operating steps and simplifying the process flow.

[0014] Furthermore, the organic solvent is selected from alkanes with carbon chain lengths of C12-C18, which are liquid at room temperature and easier to mix evenly, and have boiling points greater than 200° C. Furthermore, the organic solvent is selected from hexadecane.

[0015] Further, the amount of the organic solvent relative to the 2-naphthol salt needs to be controlled within a suitable range. If the amount of the organic solvent is too high or too low, the contact reaction between the 2-naphthol salt and CO2 will be affected, the occurrence of side reactions will be promoted, and the selectivity and yield of 2,6 acid will be reduced. Therefore, the molar ratio of the 2-naphthol salt to the organic solvent is controlled to 1:5 to 1:15, preferably 1:9 to 1:11, and more preferably 1:10.

[0016] Furthermore, in step S1, the alkaline potassium compound includes potassium hydroxide, potassium carbonate or potassium bicarbonate, preferably potassium hydroxide. 2-Naphthol usually reacts with the alkaline potassium compound to generate 2-naphthol potassium, and the molar ratio of 2-naphthol to potassium hydroxide is set at 0.95-1.05:0.95-1.05; preferably 1:1.

[0017] In step S2, the dehydration reaction includes continuously introducing an inert gas into the autoclave, wherein the inert gas is preferably nitrogen. The dehydration temperature is controlled at 160°C to 200°C, preferably about 180°C; the dehydration process is continuously heated for 30 to 60 minutes; and the dehydration is carried out under normal pressure.

[0018] In step S3, the catalyst is a nickel-based catalyst, and the nickel-based catalyst includes ethylene glycol dimethyl ether nickel bromide, nickel bromide, nickel chloride or nickel (II) trifluoromethanesulfonate, preferably ethylene glycol dimethyl ether nickel bromide.

[0019] First, the nickel-based catalyst promotes the C(sp 2 )-H bond with CO2, which plays an important role in controlling the reaction site in the carboxylation reaction, thereby effectively improving the conversion rate of 2,6 acid; secondly, the nickel-based catalyst can accelerate the carboxylation reaction of the target site with CO2, reduce the reaction activation energy, and thus reduce the temperature, pressure and reaction time of the carboxylation reaction; furthermore, when the nickel-based catalyst lowers the reaction temperature, it can prevent 2-naphthol from being oxidized to naphthoquinone due to high temperature, thereby avoiding product coloration and improving product quality.

[0020] Furthermore, the molar ratio of the catalyst to the 2-naphthol salt is controlled at 0.01-0.2:1; preferably 0.05-0.15:1.

[0021] Furthermore, the temperature of the carboxylation reaction is controlled between 140°C and 200°C, preferably between 160°C and 180°C.

[0022] Furthermore, the carboxylation reaction can occur at normal pressure, and slightly increasing the pressure of carbon dioxide can accelerate the reaction rate and improve the reaction efficiency. The pressure of carbon dioxide is controlled at 1-4 atm, preferably 2-3 atm.

[0023] Furthermore, the carboxylation reaction time is controlled within 1 to 3 hours, preferably 2 hours.

[0024] Furthermore, during the carboxylation reaction, a certain amount of potassium source can be added, and the potassium source can convert the 2-naphthol generated by the carboxylation reaction into potassium 2-naphthol, which can continue to participate in the reaction. The potassium source includes potassium carbonate, potassium bicarbonate, potassium sulfate, etc., preferably potassium carbonate. The molar ratio of potassium carbonate to potassium 2-naphthol is controlled at 0.1-1:1, preferably 0.5:1.

[0025] Furthermore, the dehydration reaction in step S2 and the carboxylation reaction in step S3 are both carried out in an autoclave.

[0026] In step S4, after the carboxylation reaction, water is added to the mixture to separate the 2,6 acid salt from the organic solvent, and the pH is adjusted to 6.5-8 to obtain a crude 2,6 acid product. Preferably, the pH is adjusted to 7.

[0027] Furthermore, step S5 is to separate and purify the crude 2,6 acid to remove impurities. Specifically, in step S5, a certain amount of organic solvent is added to the aqueous phase, heated and stirred, the aqueous phase is extracted after suction filtration, the pH is adjusted, adsorbed by a resin column, and then distilled, washed, and dried to obtain a 2,6 acid finished product. The purity of the purified 2,6 acid finished product is greater than 99.6%, wherein the contents of 2,1 acid and 2,3 acid are both less than 0.05%. In addition, all existing methods for purifying 2,6 acid can be adopted.

[0028] As described above, a method for preparing 6-hydroxy-2-naphthoic acid of the present invention has the following beneficial effects:

[0029] 1. The present invention selects an organic solvent with a boiling point higher than the dehydration temperature to prevent the organic solvent from being evaporated during the dehydration process. Therefore, there is no need to add additional organic solvent or return the evaporated organic solvent after dehydration, thereby avoiding unnecessary operating steps, simplifying the process flow, effectively improving production efficiency and reducing costs.

[0030] 2. The present invention introduces a nickel-based catalyst during the carboxylation reaction. First, the nickel-based catalyst promotes the C(sp 2 )-H bond with CO2 plays an important role in controlling the reaction site in the carboxylation reaction, improving the conversion rate of 2,6 acid, thereby effectively improving the selectivity and yield of 2,6 acid; secondly, the catalytic effect of the nickel-based catalyst enables the carboxylation reaction to be carried out at a lower temperature, reducing the cost caused by heating; thirdly, lowering the temperature of the carboxylation reaction can avoid the high-temperature oxidation of the by-product 2-naphthol to generate naphthoquinone, and no subsequent decolorization treatment is required, which can effectively reduce production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a process flow chart of the preparation method of the present invention.

[0032] Figure 2 This is a graph showing the effect of reaction temperature on 2,6 acid yield and selectivity.

[0033] Figure 3 This is the HNMR analysis result of the finished 2,6 acid.

[0034] Figure 4 This is the CNMR analysis result of the finished 2,6 acid.

[0035] Figure 5 This is the mass spectrometry test result of the finished 2,6-acid product. DETAILED DESCRIPTION

[0036] The following is a description of the implementation of the present invention by means of specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification.

[0037] Example 1

[0038] Effect of the amount of organic solvent

[0039] 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h; the mixture was mixed with 10 ml (34 mmol), 14 ml (47.8 mmol), 18 ml (61.5 mmol), 20 ml (68 mmol), 22 ml (75 mmol), 26 ml (88 mmol), 30 ml (102.5 mmol) of hexadecane in a 100 ml autoclave; heated to 180 ° C for 40 min under N2 slow blowing, cooled to 160 ° C and kept warm, then 0.68 mmol ethylene glycol dimethyl ether nickel bromide and 3.4 mmol potassium carbonate were added, and 2 atm CO2 was continuously charged for 2 h. After the carboxylation reaction was completed, water was added to dissolve the product, and sulfuric acid was used to acidify it, and the contents of 2,1 acid, 2,3 acid and 2,6 acid were detected respectively. The test results are shown in Table 1.

[0040] Table 1. Effect of potassium 2-naphthol concentration

[0041]

[0042] According to the test results, when the molar ratio of potassium 2-naphthol to hexadecane is 1:10, the selectivity and yield of 2,6 acid are the highest, which shows that the catalytic effect of this concentration is better. As the amount of organic solvent decreases, the total yield of HNA (2,1 acid + 2,3 acid + 2,6 acid) will increase slightly, but the selectivity of its by-products will increase significantly, reducing the selectivity and yield of 2,6 acid. As the amount of organic solvent increases, the total yield will decrease, and the selectivity of 2,3 acid will increase significantly, so it is necessary to control the amount of organic solvent within an appropriate range.

[0043] Example 2

[0044] Effect of catalyst type and concentration

[0045] (1) 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h; the mixture was mixed with 20 ml (68 mmol) hexadecane in a 100 ml autoclave; heated to 180 °C for 40 min under a slow blowing of N2, then cooled to 160 °C and kept warm, and then 0.068 mmol, 0.34 mmol, 0.68 mmol, 1.02 mmol, 1.36 mmol ethylene glycol dimethyl ether nickel bromide and 3.4 mmol potassium carbonate were added, and 2 atm CO2 was continuously introduced for 2 h. After the carboxylation reaction was completed, water was added to dissolve the product, and sulfuric acid was used to acidify it. The contents of 2,1 acid, 2,3 acid and 2,6 acid were detected respectively. The test results are shown in Table 2;

[0046] (2) 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h; the mixture was mixed with 20 ml (68 mmol) hexadecane in a 100 ml autoclave; heated to 180 °C for 40 min under a slow blowing of N2, then cooled to 160 °C and kept warm, and then 0.34 mmol, 0.68 mmol, 1.02 mmol nickel bromide and 3.4 mmol potassium carbonate were added respectively, and 2 atm CO2 was continuously introduced for 2 h. After the carboxylation reaction was completed, water was added to dissolve the product, and sulfuric acid was used to acidify it, and the contents of 2,1 acid, 2,3 acid and 2,6 acid were detected respectively. The test results are shown in Table 2;

[0047] (3) 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h; the mixture was mixed with 20 ml (68 mmol) hexadecane in a 100 ml autoclave; heated to 180 °C for 40 min under a slow blowing of N2, then cooled to 160 °C and kept warm, and then 0.34 mmol, 0.68 mmol, 1.02 mmol nickel chloride and 3.4 mmol potassium carbonate were added respectively, and 2 atm CO2 was continuously charged for 2 h. After the carboxylation reaction was completed, water was added to dissolve the product, and sulfuric acid was used to acidify it, and the contents of 2,1 acid, 2,3 acid and 2,6 acid were detected respectively. The test results are shown in Table 2;

[0048] (4) 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h. The mixture was mixed with 20 ml (68 mmol) hexadecane in a 100 ml autoclave. The mixture was heated to 180 °C for 40 min under a slow blowing of N2, then cooled to 160 °C and kept warm. Then 0.34 mmol, 0.68 mmol, and 1.02 mmol nickel (II) trifluoromethanesulfonate and 3.4 mmol potassium carbonate were added, respectively, and 2 atm CO2 was continuously introduced for 2 h. After the carboxylation reaction was completed, water was added to dissolve the product, and sulfuric acid was used to acidify it. The contents of 2,1 acid, 2,3 acid and 2,6 acid were detected, respectively. The test results are shown in Table 2.

[0049] Table 2. Effect of different catalyst addition amounts

[0050]

[0051] According to the test results, when no catalyst is introduced, the yield and selectivity of 2,6 acid are extremely low due to the low temperature of the carboxylation reaction. However, the present invention introduces a nickel-based catalyst. First, the nickel-based catalyst significantly inhibits the generation of 2,1 acid and 2,3 acid, controls the CO2 and C(sp) far from -OK. 2 )-H bond carboxylation reaction, thereby improving the selectivity and yield of 2,6 acid; secondly, the catalytic effect of the nickel-based catalyst enables the carboxylation reaction to be carried out at a lower temperature, reducing the production cost caused by the temperature increase. In addition, the present invention controls the amount of nickel-based catalyst added to make the selectivity of 2,6 acid higher than 90%, and the yield higher than 58%. By controlling the selection and dosage of the catalyst, the yield can reach up to 74%; when the molar ratio of the catalyst to 2-naphthol potassium is between 0.05 and 0.15, the yield and selectivity are the highest, which has great economic benefits.

[0052] Example 3

[0053] Effects of different carboxylation methods on product quality

[0054] Catalytic carboxylation method: 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h; the mixture was mixed with 20 ml (68 mmol) hexadecane in a 100 ml autoclave; heated to 180 °C under slow N2 blowing and maintained for 40 min, then cooled to 160 °C, 0.68 mmol ethylene glycol dimethyl ether nickel bromide and 3.4 mmol potassium carbonate were added, and 2 atm CO2 was charged for carboxylation reaction for 2 h; after the carboxylation reaction was completed, water was added to dissolve the product, and it was acidified with sulfuric acid, and then filtered by suction, centrifuged and dried to obtain the product.

[0055] High temperature carboxylation method: 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h; the mixture was mixed with 20 ml (68 mmol) hexadecane in a 100 ml autoclave; heated to 180 °C under slow N2 blowing and maintained for 40 min, then heated to 260 °C and added with 3.4 mmol potassium carbonate, and filled with 2 atm CO2 for carboxylation reaction, and the reaction was continued for 6 h; after the carboxylation reaction was completed, water was added to dissolve the product, and it was acidified with sulfuric acid, and then filtered by suction, centrifuged and dried to obtain the product.

[0056] The products prepared by the two methods were prepared into 10% w / v methanol solution and added into a 10 mm cuvette. The chromaticity and transmittance at wavelengths of 425 nm and 500 nm were detected respectively. Three groups were tested in parallel. The test results are shown in Table 3.

[0057] Table 3. Color and transmittance of products under different methods

[0058]

[0059] Under naked eye observation, the product prepared by catalytic carboxylation of the present invention is white, while the product prepared under traditional high temperature carboxylation is light yellow. The difference in color between the two can also be reflected from Table 3. The chromaticity of the product of high temperature carboxylation is significantly higher, and the transmittance is significantly reduced. This is because high temperature can cause the 2-naphthol in the carboxylation reaction to be oxidized and converted into naphthoquinone, which colors the product and reduces the product quality. In addition, the product needs to be decolorized after coloring, which increases production costs.

[0060] Example 4

[0061] Effect of reaction temperature

[0062] 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h; the mixture was mixed with 20 ml (68 mmol) hexadecane in a 100 ml autoclave; heated to 180°C for 40 min under slow blowing of N2, and the temperature was adjusted to 140°C, 160°C, 180°C, 200°C, and 220°C respectively and kept warm, and then 0.68 mmol ethylene glycol dimethyl ether nickel bromide and 3.4 mmol potassium carbonate were added, and 2 atm CO2 was charged to carry out carboxylation reaction for 2 h; after the carboxylation reaction was completed, water was added to dissolve the product, and sulfuric acid was used to acidify it, and the contents of 2,1 acid, 2,3 acid and 2,6 acid were detected respectively. The test results are as follows: Figure 2 shown.

[0063] By screening the reaction temperature, the optimal reaction conditions were obtained between 160°C and 180°C. These results are related to the thermal stability of HNA isomers, and the order of thermal stability is 2,6 acid > 2,3 acid > 2,1 acid, which is consistent with the good product distribution.

[0064] Example 5

[0065] The impact of reaction time

[0066] 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h; the mixture was mixed with 20 ml (68 mmol) hexadecane in a 100 ml autoclave; heated to 180 ° C for 40 min under slow blowing of N2, cooled to 160 ° C and kept warm, then 0.68 mmol ethylene glycol dimethyl ether nickel bromide and 3.4 mmol potassium carbonate were added, and 2 atm CO2 was continuously charged, and the reaction time was controlled at 1h, 2h, 3h, and 4h, respectively. After the carboxylation reaction was completed, water was added to dissolve the product, and sulfuric acid was used to acidify it, and the contents of 2,1 acid, 2,3 acid and 2,6 acid were detected respectively, where the total yield = (actual yield of 2,1 acid + 2,3 acid + 2,6 acid) / (theoretical yield of 2,6 acid), 2,6 acid yield = (actual yield of 2,6 acid) / (theoretical yield of 2,6 acid), and the test results are shown in Table 4.

[0067] Table 4. Effect of reaction time

[0068]

[0069] By screening the reaction time, the optimal reaction conditions were obtained when the reaction time was 2 hours. It can be seen that in a short reaction time of 1 hour, the total yield was found to be the highest, but the formation selectivity of 2,3 acid and 2,1 acid exceeded 11.8%; after 1 hour, the ratio of 2,3 acid and 2,1 acid began to decrease, while the selectivity of 2,6 acid increased. At 2 hours, the yield of 2,6 acid was the highest, reaching 74%; after the reaction exceeded 3 hours, the yield and selectivity of 2,6 acid began to decrease. This shows that all HNA isomers are generated within 1 hour, and then 2,1 acid and 2,3 acid tend to undergo thermal decarboxylation to generate naphthol or rearrange to generate 2,6 acid, and finally 2,6 acid decarboxylation; therefore, it is necessary to control the appropriate reaction time to maximize the selectivity and yield of 2,6 acid.

[0070] Example 6

[0071] Effect of reaction pressure

[0072] 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h. The mixture was mixed with 20 ml (68 mmol) hexadecane in a 100 ml autoclave. The mixture was heated to 180°C for 40 min under a slow blowing of N2, then cooled to 160°C and kept warm. Then 0.68 mmol ethylene glycol dimethyl ether nickel bromide and 3.4 mmol potassium carbonate were added, and 1 atm, 2 atm, 3 atm and 4 atm of CO2 were respectively charged for carboxylation reaction. The reaction time was 2 h. After the carboxylation reaction was completed, water was added to dissolve the product, and sulfuric acid was used for acidification. The contents of 2,1 acid, 2,3 acid and 2,6 acid were detected respectively. The detection results are shown in Table 5.

[0073] Table 5. Effect of reaction pressure

[0074]

[0075] By screening the reaction pressure, the optimal reaction conditions were obtained at 2~3atm. This is because too high or too low CO2 pressure may inhibit the interaction between K-2-naphthyl oxides, resulting in low 2,6-HNA yield.

[0076] Example 7

[0077] Effect of potassium source addition

[0078] 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water were mixed and stirred at room temperature for 1 h. The mixture was mixed with 20 ml (68 mmol) hexadecane in a 100 ml autoclave. The mixture was heated to 180°C for 40 min under a slow blowing of N2, then cooled to 160°C and kept warm. Then 0.68 mmol ethylene glycol dimethyl ether nickel bromide and 0 mmol, 0.68 mmol, 3.4 mmol and 6.8 mmol K2CO3 were added, and 2 atm CO2 was charged to carry out carboxylation reaction for 2 h. After the carboxylation reaction was completed, water was added to dissolve the product, and sulfuric acid was used to acidify it. The contents of 2,1 acid, 2,3 acid and 2,6 acid were detected respectively. The detection results are shown in Table 6.

[0079] Table 6. Effect of potassium source addition

[0080]

[0081] Compared with the case without adding K2CO3, when the ratio of K2CO3 / 2-naphthol potassium was 0.1, the introduction of K2CO3 increased the HNA yield from 62.4% to 75.4%, the yield of 2,6 acid from 60% to 73.2%, and the selectivity of 2,6 acid was significantly improved. When the ratio of K2CO3 / K-2-naphthol exceeded 0.5, the increase in the amount of K2CO3 added had little effect on the product yield and selectivity. This indicates that K2CO3 can convert the 2-naphthol generated by the carboxylation reaction into 2-naphthol potassium, which can continue to participate in the reaction; and K2CO3 plays a key catalytic role in promoting the carboxylation of 2-naphthol potassium and inhibiting the decarboxylation of unstable HNA isomers (such as 2,1 acid and 2,3 acid).

[0082] Example 8

[0083] A method for preparing 6-hydroxy-2-naphthoic acid comprises the following steps:

[0084] S1. Mix 6.8 mmol 2-naphthol (SigmaAldrich, 98%), 6.8 mmol potassium hydroxide and 5 ml water, and stir at room temperature for 1 h to generate 2-naphthol potassium; mix the mixed solution with 20 ml (68 mmol) hexadecane in a 100 ml autoclave;

[0085] S2, heating to 180°C under slow blowing of N2, maintaining for 40 min, to obtain anhydrous 2-naphthol potassium;

[0086] S3, add 0.68mmol ethylene glycol dimethyl ether nickel bromide and 3.4mmol K2CO3, charge with 2atm CO2, maintain at 180℃ for 2h, generate 2,6 acid salt;

[0087] S4, after the reaction is completed, the pressure is released, the temperature is lowered to 60°C, pure water at 60°C is added, and then the pH is adjusted to about 7, the mixture is stirred evenly, the mixture is allowed to stand for stratification, and the aqueous phase is extracted;

[0088] S5. Add a certain amount of naphthalene to the aqueous phase, stir evenly at 60°C, extract the aqueous phase after suction filtration, cool the aqueous phase to room temperature and adjust the pH to about 4, adsorb it through resin, and then distill, wash and dry to obtain the finished 2,6-acid.

[0089] The obtained 2,6 acid product was subjected to HNMR analysis, CNMR analysis and mass spectrometry analysis. The analysis results refer to Figures 3-5 After testing, the purity of the finished 2,6-acid was greater than 99.6%, and the contents of 2-naphthol, 2,1-acid and 2,3-acid were all less than 0.03%.

[0090] In summary, the present invention selects an organic solvent with a boiling point higher than the dehydration temperature to prevent the organic solvent from being evaporated together during the dehydration process; in addition, the present invention introduces a nickel-based catalyst during the carboxylation reaction. First, the nickel-based catalyst promotes the carboxylation reaction of the C(sp2)-H bond away from the functional group (-OK) and CO2, plays an important role in controlling the reaction site in the carboxylation reaction, improves the conversion rate of 2,6 acid, and thus can effectively improve the selectivity and yield of 2,6 acid; second, the catalytic effect of the nickel-based catalyst enables the carboxylation reaction to be carried out at a lower temperature, reducing the cost caused by heating; third, reducing the temperature of the carboxylation reaction can prevent the by-product 2-naphthol from being oxidized at high temperature to generate naphthoquinone, and no subsequent decolorization treatment is required, which can effectively reduce the production cost. Therefore, the present invention effectively overcomes the various shortcomings in the prior art and has a high industrial utilization value.

[0091] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A method for preparing 6-hydroxy-2-naphthoic acid, characterized in that: The following steps are involved: S1, 2-naphthol reacts with an alkaline potassium compound to generate a 2-naphthol salt, and an organic solvent is added, wherein the boiling point of the organic solvent is higher than the dehydration temperature of step S2; S2, heating to remove water from the organic solvent to obtain anhydrous 2-naphthol salt; S3, using a suspension method to make 2-naphthol salt and carbon dioxide undergo carboxylation reaction under the action of a catalyst to generate 2-naphthol and 2,6 acid salt; the catalyst is a nickel-based catalyst, and the nickel-based catalyst includes ethylene glycol dimethyl ether nickel bromide, nickel bromide, nickel chloride or nickel (II) trifluoromethanesulfonate; S4, adding water to dissolve and separating from the organic solvent, neutralizing the aqueous phase with acid, and acidifying to obtain a crude product of 6-hydroxy-2-naphthoic acid; S5. After separation and purification of the crude 6-hydroxy-2-naphthoic acid product, the finished 6-hydroxy-2-naphthoic acid product is obtained.

2. A method for preparing 6-hydroxy-2-naphthoic acid according to claim 1, characterized in that, The molar ratio of the catalyst to the 2-naphthol salt is 0.01-0.2:

1.

3. A method for preparing 6-hydroxy-2-naphthoic acid according to claim 1, characterized in that, The organic solvent is selected from alkanes with a carbon chain length of C12-C18.

4. A method for preparing 6-hydroxy-2-naphthoic acid according to claim 3, characterized in that, The molar ratio of 2-naphthol salt to the organic solvent is 1:9-1:

11.

5. A method for preparing 6-hydroxy-2-naphthoic acid according to claim 1, characterized in that, In step S2, the dehydration temperature is controlled between 160°C and 200°C.

6. A method for preparing 6-hydroxy-2-naphthoic acid according to claim 1, characterized in that, In step S3, the temperature of the carboxylation reaction is controlled between 140°C and 200°C.

7. A method for preparing 6-hydroxy-2-naphthoic acid according to claim 1, characterized in that, In step S3, the pressure of carbon dioxide in the carboxylation reaction is controlled at 1-4 atm.

8. A method for preparing 6-hydroxy-2-naphthoic acid according to claim 1, characterized in that, In step S3, the carboxylation reaction time is controlled within 1 to 3 hours.

9. A method for preparing 6-hydroxy-2-naphthoic acid according to claim 1, characterized in that, In step S3, during the carboxylation reaction, a certain amount of potassium source is added, wherein the potassium source includes potassium carbonate, potassium bicarbonate or potassium sulfate, and the molar ratio of the potassium source to the 2-naphthol salt is controlled at 0.1-1:1.

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