2, 6-dimethyl naphthalate and preparation method thereof

Abstract

The invention provides dimethyl 2, 6-naphthalate and a preparation method thereof, and the preparation method comprises the following steps: catalyzing an esterification reaction of 2, 6-naphthalic acid and methanol by using an acidic catalyst, adding a recrystallization solvent into a crude product of the dimethyl 2, 6-naphthalate, heating the formed mixed solution to 40-120 DEG C, and after the crude product of the dimethyl 2, 6-naphthalate is not dissolved any more, filtering to obtain the dimethyl 2, 6-naphthalate. Carrying out suction filtration on the mixed solution, carrying out cooling crystallization on the obtained filtrate, and washing and drying the separated solid to obtain high-purity dimethyl 2, 6-naphthalate; wherein the acid catalyst is selected from sulfuric acid, phosphoric acid, ferrous sulfate or molybdenum trioxide, and the recrystallization solvent is selected from any one or two of acetonitrile, o-xylene and methanol; the acid catalyst used in the invention can accelerate the reaction rate, improve the reaction proceeding degree, and improve the yield and purity of a crude product; the recrystallization solvent screened by the method has the advantages of good purification effect, recyclability, low industrial application cost and the like.

Description

Technical Field

[0001] The invention relates to the technical field of fine chemicals, and in particular to dimethyl 2,6-naphthalenedicarboxylate and a preparation method thereof. Background Art

[0002] 2,6-Dimethyl naphthalate (2,6-NDC) is an important raw material for the preparation of polyethylene naphthalate (PEN). PEN masterbatch is usually obtained by transesterification, prepolymerization and final polycondensation of 2,6-dimethyl naphthalate and ethylene glycol. The structure of polyethylene naphthalate is similar to that of polyethylene terephthalate, and the polymerization unit changes from benzene ring to naphthalene ring. This structural change makes the mechanical strength, thermal stability, gas barrier properties and other properties of PEN stronger than those of polyethylene terephthalate. These characteristics make it an ideal polyester material for advanced applications such as electronic equipment, optical components, food packaging and aerospace. Its excellent performance and huge market potential have attracted widespread attention in the polyester industry.

[0003] The key to producing high-performance PEN masterbatch is to ensure the high purity of the raw materials, that is, the purity of 2,6-naphthalene dicarboxylic acid dimethyl ester. The preparation of 2,6-naphthalene dicarboxylic acid dimethyl ester is generally carried out by esterification of 2,6-naphthalene dicarboxylic acid (2,6-NDA) and methanol. The reaction impurities mainly include unreacted raw material 2,6-NDA and monoester produced by poor esterification.

[0004] How to optimize the reaction conditions, accelerate the reaction rate, and improve the yield and purity of dimethyl 2,6-naphthalenedicarboxylate is a problem that has attracted much attention at present. Summary of the invention

[0005] In view of the above problems existing in the prior art, the present invention provides a 2,6-naphthalene dicarboxylic acid dimethyl ester and a preparation method thereof, and obtains high-purity 2,6-naphthalene dicarboxylic acid dimethyl ester by optimizing the reaction and purification conditions.

[0006] The specific content of the invention is as follows: In a first aspect, the present invention provides a method for preparing dimethyl 2,6-naphthalene dicarboxylate, the preparation method comprising: S1. In an inert gas environment, an acidic catalyst is used to catalyze an esterification reaction between 2,6-naphthalenedicarboxylic acid and methanol. The obtained reaction product is cooled and crystallized. The precipitated solid is washed and dried to obtain a crude product of dimethyl 2,6-naphthalenedicarboxylate; S2, adding a recrystallization solvent to the crude product of dimethyl 2,6-naphthalene dicarboxylate, heating the formed mixed solution to 40-120° C., and filtering the mixed solution after the crude product of dimethyl 2,6-naphthalene dicarboxylate no longer dissolves to obtain a filtrate; S3, cooling and crystallizing the filtrate, washing and drying the precipitated solid to obtain high-purity dimethyl 2,6-naphthalene dicarboxylate; Wherein, the acidic catalyst is selected from sulfuric acid, phosphoric acid, ferrous sulfate or molybdenum trioxide, and the recrystallization solvent is selected from any one or two of acetonitrile, o-xylene and methanol.

[0007] Optionally, the acidic catalyst is selected from molybdenum trioxide, and the recrystallization solvent is selected from o-xylene.

[0008] Optionally, in step S1, the mass ratio of the 2,6-naphthalene dicarboxylic acid, the methanol and the acid catalyst is 30-50:60-250:0.5-5.

[0009] Optionally, in step S1, the inert gas is nitrogen, argon or helium.

[0010] Optionally, in step S1, the reaction pressure of the esterification reaction is 5-8 MPa, the reaction temperature is 140-180° C., and the reaction time is 2-3 h.

[0011] Optionally, in step S2, the mass ratio of the crude dimethyl 2,6-naphthalene dicarboxylate to the recrystallization solvent is 30-50:300-1000.

[0012] Optionally, in step S2, the temperature of the cooling crystallization is 0-20°C; The washing is performed with methanol solution for 1-3 times; The drying temperature is 40-80°C.

[0013] Optionally, in step S3, the temperature of the cooling crystallization is 0-20°C; The washing is performed with methanol solution for 1-3 times; The drying temperature is 40-80°C.

[0014] Optionally, the yield of the crude dimethyl 2,6-naphthalene dicarboxylate is 94.5-97.8%; and / or The purity of the dimethyl 2,6-naphthalene dicarboxylate is 98.4-99.5%.

[0015] In a second aspect, the present invention provides dimethyl 2,6-naphthalene dicarboxylate, wherein the dimethyl 2,6-naphthalene dicarboxylate is obtained according to the preparation method described in the first aspect.

[0016] Compared with the prior art, the present invention has the following advantages: The invention provides a preparation method of dimethyl 2,6-naphthalene dicarboxylate, comprising: S1, in an inert gas environment, catalyzing esterification reaction of 2,6-naphthalene dicarboxylic acid and methanol with an acidic catalyst, cooling and crystallizing the obtained reaction product, washing and drying the precipitated solid, and obtaining a crude product of dimethyl 2,6-naphthalene dicarboxylate; S2, adding a recrystallization solvent to the crude product of dimethyl 2,6-naphthalene dicarboxylate, heating the formed mixed solution to 40-120°C, and filtering the mixed solution until the crude product of dimethyl 2,6-naphthalene dicarboxylate no longer dissolves, and obtaining a filtrate; S3, cooling and crystallizing the filtrate, washing and drying the precipitated solid, and obtaining high-purity dimethyl 2,6-naphthalene dicarboxylate; wherein the acidic catalyst is selected from sulfuric acid, phosphoric acid, ferrous sulfate or molybdenum trioxide, and the recrystallization solvent is selected from any one or two of acetonitrile, o-xylene and methanol.

[0017] The acidic catalyst used in the present invention can accelerate the reaction rate and improve the degree of reaction, thereby improving the yield and purity of the crude product; the reaction time of the traditional method of converting 2,6-naphthalenedicarboxylic acid to dimethyl 2,6-naphthalenedicarboxylic acid is generally 3-5 hours. The reaction time of the present invention is 2-3 hours, which greatly shortens the reaction time, improves the product capacity, saves energy consumption in the reaction process, and has great economic benefits and commercial value.

[0018] The purity of dimethyl 2,6-naphthalene dicarboxylate prepared by the method provided by the present invention is 98.4-99.5%, which is greatly improved compared with the purity of 94-96% in the current technology, and effectively solves the problem that the current purification technology of dimethyl 2,6-naphthalene dicarboxylate is imperfect and the purity after purification is not high, and the recrystallization operation is simple, and the solvent can be recycled, which is conducive to the industrial application of the method and the promotion of high-performance polyester materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1 The flowchart of the preparation method of dimethyl 2,6-naphthalene dicarboxylate provided in the embodiment of the present invention is shown; Figure 2 The NMR image of dimethyl 2,6-naphthalene dicarboxylate provided in the embodiment of the present invention is shown; Figure 3 The liquid chromatogram of dimethyl 2,6-naphthalene dicarboxylate provided in the embodiment of the present invention is shown. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means a limitation on the present invention and its application or use. Based on the embodiments of the present invention, any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts falls within the scope of protection of the present invention. In addition, all other embodiments obtained by ordinary technicians in this field without carrying out creative work belong to the scope of protection of the present invention.

[0022] In the embodiment, no specific experimental steps or conditions are indicated, and the operation or conditions of the conventional experimental steps described in the prior art in this area can be carried out. The reagents used and other instruments that do not indicate the manufacturer are all conventional reagent products that can be obtained commercially. In addition, the accompanying drawings are only schematic diagrams of the embodiments of the present invention and are not necessarily drawn to scale. The same reference numerals in the figures represent the same or similar parts, and thus their repeated description will be omitted. Some block diagrams shown in the accompanying drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0023] Technologies, methods, and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but where appropriate, the technologies, methods, and equipment should be considered as part of the description of the present invention.

[0024] In the description of the present invention, it should be understood that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

[0025] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0026] Although the research on the esterification reaction of 2,6-naphthalene dicarboxylic acid and methanol is relatively mature, there has always been a problem of low yield and purity, which is not conducive to industrial production and application. And the existing research is more focused on how to esterify to obtain 2,6-naphthalene dicarboxylic acid dimethyl ester with high yield and purity. For example, the patent CN 112441918 A mentions that 2,6-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid dimethyl ester, methanol and metal alkoxide catalyst are added together into an autoclave for reaction, and the yield of the prepared product can reach 99.6%, but the purity is unknown. The patent CN 112239405 B mentions that the conversion rate of 2,6-NDA can reach more than 94% by using imidazole sulfonic acid functionalized ionic liquid as a catalyst, but there is still room for improvement in purity.

[0027] The present invention uses an acidic catalyst and some organic solvents for recrystallization and purification, which can shorten the reaction time and improve the yield and purity of the product, greatly improve production efficiency, reduce production costs, and promote the development of high-performance polyester materials. The specific implementation content is as follows: In a first aspect, the present invention provides a method for preparing dimethyl 2,6-naphthalene dicarboxylate. Figure 1 The flow chart of the preparation method of dimethyl 2,6-naphthalene dicarboxylate provided in the embodiment of the present invention is shown, as Figure 1 As shown, the preparation method comprises: S1. In an inert gas environment, an acidic catalyst is used to catalyze an esterification reaction between 2,6-naphthalenedicarboxylic acid and methanol. The obtained reaction product is cooled and crystallized. The precipitated solid is washed and dried to obtain a crude product of dimethyl 2,6-naphthalenedicarboxylate; When this step is implemented, the acidic catalyst is selected from sulfuric acid, phosphoric acid, ferrous sulfate or molybdenum trioxide; the above catalysts show high catalytic efficiency in the esterification reaction, can accelerate the reaction process and shorten the reaction time; they have good selectivity for the formation of dimethyl 2,6-naphthalene dicarboxylate, reduce side reactions and improve product purity. In addition, the above catalysts also show good chemical stability in the esterification reaction, are not easy to deactivate, and can be reused; compared with strong acid catalysts, ferrous sulfate and molybdenum trioxide have less environmental impact, and waste liquid treatment is relatively easy, so they are better choices.

[0028] When this step is implemented specifically, 30-50 parts of 2,6-naphthalenedicarboxylic acid, 60-250 parts of methanol, and 0.5-5 parts of an acidic catalyst are taken by weight, added to a reactor, and the air in the reactor is replaced with an inert gas. After the replacement is completed, the reactor is pressurized to 5.0-8.0 MPa with an inert gas, and the temperature in the reactor is controlled to be 140-180° C., and the materials in the reactor are reacted for 2-3 h under stirring (stirring rate is 200-400 rpm). After the reaction is completed, the mixed material containing 2,6-naphthalenedicarboxylic acid dimethyl ester crystals in the reactor is cooled and crystallized at 0-20° C., and the precipitated solid is centrifuged and filtered, washed with methanol 1-3 times, and dried at 40-80° C. to obtain a crude product of 2,6-naphthalenedicarboxylic acid dimethyl ester.

[0029] When this step is specifically implemented, the yield of the obtained crude product of dimethyl 2,6-naphthalene dicarboxylate is 94.5-97.8%.

[0030] When this step is specifically implemented, the inert gas environment is nitrogen, argon or helium.

[0031] S2, adding a recrystallization solvent to the crude product of dimethyl 2,6-naphthalene dicarboxylate, heating the formed mixed solution to 40-120° C., and filtering the mixed solution after the crude product of dimethyl 2,6-naphthalene dicarboxylate no longer dissolves to obtain a filtrate; S3, cooling and crystallizing the filtrate, washing and drying the precipitated solid to obtain high-purity dimethyl 2,6-naphthalene dicarboxylate; In specific implementation, the recrystallization solvent is selected from any one or two of acetonitrile, o-xylene and methanol. Among them, acetonitrile has a low boiling point and has good solubility in dimethyl 2,6-naphthalene dicarboxylate, which is easy to remove by evaporation, simplifying solvent recovery; acetonitrile can effectively remove impurities in dimethyl 2,6-naphthalene dicarboxylate and improve product purity; o-xylene has high solubility in dimethyl 2,6-naphthalene dicarboxylate and is easy to precipitate crystals after cooling. It can effectively separate the target product from impurities and improve purity; methanol has good solubility in dimethyl 2,6-naphthalene dicarboxylate, which is easy to remove by evaporation, simplifying solvent recovery. The preferred recrystallization solvent is selected from o-xylene.

[0032] In the specific implementation, 30-50 parts of crude 2,6-naphthalenedicarboxylic acid dimethyl ester are added to a three-necked flask by mass, 300-1000 parts of recrystallization solvent are added, the stirring rate is set to 150 rpm, the temperature is gradually raised to 40-140 ° C, and after most of it is dissolved, it is filtered while hot. The filtered mother liquor is cooled at 0-20 ° C for crystallization. After the crystallization is completed, it is centrifugally filtered and washed with deionized water 1-3 times, and dried at 40-80 ° C to obtain high-purity 2,6-naphthalenedicarboxylic acid dimethyl ester.

[0033] The purity of dimethyl 2,6-naphthalene dicarboxylate prepared by the method provided by the present invention is 98.4-99.5%, which is greatly improved compared with the purity of 94-96% in the current technology, and effectively solves the problem that the current purification technology of dimethyl 2,6-naphthalene dicarboxylate is imperfect and the purity after purification is not high, and the recrystallization operation is simple, and the solvent can be recycled, which is conducive to the industrial application of the method and the promotion of high-performance polyester materials.

[0034] In a second aspect, the present invention provides dimethyl 2,6-naphthalene dicarboxylate, wherein the dimethyl 2,6-naphthalene dicarboxylate is obtained according to the preparation method described in the first aspect.

[0035] In order to enable those skilled in the art to more clearly understand the present invention, the 2,6-naphthalene dicarboxylic acid dimethyl ester and the preparation method thereof according to the present invention are now described in detail through the following examples.

[0036] Example 1 (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid (2,6-NDA), 3 g of sulfuric acid, and 60 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 5.0 MPa, then close the nitrogen valve, control the reactor to heat up to 140°C, set the stirring rate to 200 rpm, control the reactor temperature to 140±1°C, and the pressure to 5.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure and open the reactor to obtain the mixed material.

[0037] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol, filtered, and placed in a vacuum oven at 40 °C for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate (crude 2,6-NDC).

[0038] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 600 g of acetonitrile, gradually heat to 60 °C, and remove the three-necked flask after most of it is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals (refined 2,6-NDC).

[0039] Analytical testing: (1) Qualitative analysis using 400 MHz NMR: Weigh a portion of refined 2,6-naphthalene dicarboxylic acid dimethyl ester and dissolve it in deuterated dimethyl sulfoxide.

[0040] Figure 2 The NMR of dimethyl 2,6-naphthalene dicarboxylate provided in the embodiment of the present invention is shown. Figure 2 As shown, the abscissa represents the peak positions of different groups, and the ordinate represents the peak intensity. The peak positions in the figure are basically consistent with those of pure 2,6-naphthalene dimethyl dicarboxylate, with few impurity peaks and low intensity, indicating that high-purity 2,6-naphthalene dimethyl dicarboxylate was successfully synthesized.

[0041] (2) Calculation method for the yield of crude 2,6-naphthalene dimethyl ester (crude 2,6-NDC): After obtaining the crude 2,6-NDC crystals, weigh them and record the weight as .

[0042] The yield calculation formula of crude 2,6-NDC is:

[0043] (3) Calculation of the yield of high-purity dimethyl 2,6-naphthalene dicarboxylate (refined 2,6-NDC): After obtaining the refined 2,6-NDC crystals, weigh them and record the weight as .

[0044] The yield calculation formula of refined 2,6-NDC is:

[0045] (4) Quantitative analysis and detection using high performance liquid chromatography: Weigh a portion of refined 2,6-naphthalene dicarboxylic acid dimethyl ester and dissolve it in chromatography grade acetonitrile. The chromatographic conditions are: external standard method, mobile phase is methanol, 0.1% (volume fraction) phosphoric acid aqueous solution and acetonitrile, mobile phase volume is 1 mL / min, column temperature is 30°C, and elution gradient is shown in the following table: Table 1 Mobile phase elution gradient for high performance chromatography analysis

[0046] The crystal yield and purity of dimethyl 2,6-naphthalene dicarboxylate are shown in the following table: Table 2 Yield and purity of 2,6-NDC crystals obtained in Example 1

[0047] Example 2 (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 5 g of sulfuric acid, and 60 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 5.0 MPa, then close the nitrogen valve, control the reactor to heat up to 140°C, set the stirring rate to 200 rpm, control the reactor temperature to 140±1°C, and the pressure to 5.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure and open the reactor to obtain the mixed material.

[0048] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0049] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 300 g of methanol, gradually raise the temperature to 40 °C, and remove the three-necked flask after most of it is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0050] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 3 Yield and purity of 2,6-NDC crystals obtained in Example 2

[0051] Example 3 (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 5 g of sulfuric acid, and 60 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 5.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 5.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure and open the reactor to obtain the mixed material.

[0052] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0053] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 600 g of methanol, gradually heat to 40 °C, and remove the three-necked flask after most of it is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0054] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 4 Yield and purity of 2,6-NDC crystals obtained in Example 3

[0055] Example 4 This embodiment provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 5 g of sulfuric acid, and 150 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 5.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 5.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure and open the reactor to obtain the mixed material.

[0056] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0057] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 600 g of methanol, gradually heat to 40 °C, and remove the three-necked flask after most of it is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0058] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 5 Yield and purity of 2,6-NDC crystals obtained in Example 4

[0059] Example 5 This embodiment provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 50 g of 2,6-naphthalene dicarboxylic acid, 5 g of sulfuric acid, and 250 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure and open the reactor to obtain the mixed material.

[0060] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0061] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 800 g of o-xylene, gradually raise the temperature to 120 °C, and remove the three-necked flask after most of the ester is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0062] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 6 Yield and purity of 2,6-NDC crystals obtained in Example 5

[0063] Figure 3 The liquid chromatogram of dimethyl 2,6-naphthalene dicarboxylate provided in the embodiment of the present invention is shown. Figure 3 As shown in the liquid chromatogram, the product peak appears around 6.5 minutes, the impurity peak appears around 5.5 minutes, and the peak before 4 minutes is the solvent peak. Analysis of the peak area shows that the product purity is 99.5% and the impurity peak content is 0.5%.

[0064] Example 6 This embodiment provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 50 g of 2,6-naphthalene dicarboxylic acid, 5 g of sulfuric acid, and 250 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 180°C, set the stirring rate to 200 rpm, control the reactor temperature to 180±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure and open the reactor to obtain the mixed material.

[0065] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0066] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 1000 g of o-xylene, gradually raise the temperature to 120 °C, and remove the three-necked flask after most of it is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0067] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 7 Yield and purity of 2,6-NDC crystals obtained in Example 6

[0068] Example 7 This embodiment provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 50 g of 2,6-naphthalene dicarboxylic acid, 3 g of ferrous sulfate, and 250 g of methanol and add them to the reactor. After closing the lid of the reactor, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0069] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0070] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 600 g of o-xylene, gradually raise the temperature to 120 °C, and remove the three-necked flask after most of the ester is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0071] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 8 Yield and purity of 2,6-NDC crystals obtained in Example 7

[0072] Example 8 This embodiment provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 0.5 g of ferrous sulfate, and 250 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0073] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0074] (3) Weigh 50 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 1000 g of o-xylene, gradually raise the temperature to 120 °C, and remove the three-necked flask after most of the ester is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0075] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 9 Yield and purity of 2,6-NDC crystals obtained in Example 8

[0076] Example 9 This embodiment provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 0.5 g of molybdenum trioxide, and 250 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, open the reactor to obtain the mixed material.

[0077] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0078] (3) Weigh 50 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 1000 g of o-xylene, gradually raise the temperature to 120 °C, and remove the three-necked flask after most of the ester is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0079] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 10 Yield and purity of 2,6-NDC crystals obtained in Example 9

[0080] Example 10 This embodiment provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 50 g of 2,6-naphthalene dicarboxylic acid, 3 g of molybdenum trioxide, and 250 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0081] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0082] (3) Weigh 50 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 1000 g of o-xylene, gradually raise the temperature to 120 °C, and remove the three-necked flask after most of the ester is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0083] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 11 Yield and purity of 2,6-NDC crystals obtained in Example 10

[0084] Embodiment 11 This embodiment provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 3 g of phosphoric acid, and 250 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0085] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0086] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 1000 g of o-xylene, gradually raise the temperature to 120 °C, and remove the three-necked flask after most of it is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0087] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 12 Yield and purity of 2,6-NDC crystals obtained in Example 11

[0088] Example 12 This embodiment provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 5 g of phosphoric acid, and 250 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0089] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0090] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 1000 g of o-xylene, gradually raise the temperature to 120 °C, and remove the three-necked flask after most of it is dissolved. Transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and wash with deionized water three times, then filter and dry to obtain high-purity 2,6-naphthalene dimethyl ester crystals.

[0091] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 13 Yield and purity of 2,6-NDC crystals obtained in Example 12

[0092] The yield of the crude product of 2,6-naphthalenedicarboxylic acid dimethyl ester prepared in the above-mentioned embodiments 1-12 is 94.6-97.8%, and the purity of high-purity 2,6-naphthalenedicarboxylic acid dimethyl ester is 98.4-99.5%. The present invention uses acidic catalysts such as sulfuric acid and phosphoric acid as esterification process catalysts to obtain 2,6-naphthalenedicarboxylic acid dimethyl ester with higher yield and purity. Recrystallization and purification using organic solvents such as acetonitrile and o-xylene can effectively improve the purity of the product 2,6-naphthalenedicarboxylic acid dimethyl ester. In addition, the recrystallization solvent can be recycled in the method, and has a wide range of industrial application prospects and commercial value. Relative to other catalysts, the catalytic effect of ferrous sulfate and molybdenum trioxide is good, and the environmental impact is small, and waste liquid treatment is relatively easy, which is a better choice.

[0093] Comparative Example 1 This comparative example provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 6 g of sulfuric acid, and 250 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0094] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0095] (3) Weigh 30 g of crude dimethyl 2,6-naphthalene dicarboxylate into a three-necked flask, add 1000 g of o-xylene, and dissolve it at room temperature. The solubility is very poor and recrystallization is impossible.

[0096] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 14 Yield and purity of 2,6-NDC crystals obtained in Comparative Example 1

[0097] Comparative Example 2 This comparative example provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 6 g of molybdenum trioxide, and 250 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0098] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0099] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 1000 g of acetonitrile, and after most of it is dissolved at room temperature, remove the three-necked flask, transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and filter, wash with deionized water three times, and then filter and dry to obtain purified 2,6-naphthalene dimethyl ester crystals.

[0100] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 15 Yield and purity of 2,6-NDC crystals obtained in Comparative Example 2

[0101] Comparative Example 3 This comparative example provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 5 g of sulfuric acid, and 250 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 200°C, set the stirring rate to 200 rpm, control the reactor temperature to 200±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0102] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0103] (3) Weigh 30 g of crude dimethyl 2,6-naphthalene dicarboxylate into a three-necked flask, add 600 g of methanol, and dissolve at room temperature. Remove the three-necked flask. The solubility is poor and recrystallization cannot be performed.

[0104] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 16 Yield and purity of 2,6-NDC crystals obtained in Comparative Example 3

[0105] Comparative Example 4 This comparative example provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 5 g of sulfuric acid, and 30 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0106] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0107] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 200 g of o-xylene, heat to 120 °C to dissolve most of it, remove the three-necked flask, transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0 °C. After the crystallization is completed, centrifuge and filter, wash with deionized water three times, and then filter and dry to obtain purified 2,6-naphthalene dimethyl ester crystals.

[0108] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 17 Yield and purity of 2,6-NDC crystals obtained in Comparative Example 4

[0109] Comparative Example 5 This comparative example provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 5 g of sulfuric acid, and 60 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 120°C, set the stirring rate to 200 rpm, control the reactor temperature to 120±1°C, and the pressure to 10.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0110] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0111] (3) Weigh 30 g of crude 2,6-naphthalene dimethyl ester into a three-necked flask, add 200 g of acetonitrile, heat to 60°C to dissolve, remove the three-necked flask, transfer the solid-liquid mixture to a vacuum filtration device for filtration, and recover the hot mother liquor into the three-necked flask for cooling and crystallization at 0°C. After the crystallization is completed, centrifuge and filter, wash with deionized water three times, and then filter and dry to obtain purified 2,6-naphthalene dimethyl ester crystals.

[0112] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 18 Yield and purity of 2,6-NDC crystals obtained in Comparative Example 5

[0113] Comparative Example 6 This comparative example provides a method for preparing high-purity dimethyl 2,6-naphthalene dicarboxylate, comprising the following steps: (1) Accurately weigh 30 g of 2,6-naphthalene dicarboxylic acid, 1 g of sulfuric acid, and 300 g of methanol. Cover the reactor and add 2,6-naphthalene dicarboxylic acid, sulfuric acid, and methanol into the reactor. After closing the reactor cover, open the nitrogen valve of the reactor, replace the air in the reactor with nitrogen and pressurize it to 8.0 MPa, then close the nitrogen valve, control the reactor to heat up to 160°C, set the stirring rate to 200 rpm, control the reactor temperature to 160±1°C, and the pressure to 8.0 MPa. Maintain this condition for 3 hours and then terminate the reaction. After the temperature drops to room temperature, release the pressure, and open the reactor to obtain the mixed material.

[0114] (2) The obtained mixture was transferred to a three-necked flask and the temperature was continued to drop for crystallization. After crystallization at 0 °C for 1 h, the mixture was centrifuged and filtered. The obtained solid was washed three times with methanol. After filtration, the solid was placed in a vacuum oven at 40 °C and dried for two hours to obtain crude dimethyl 2,6-naphthalene dicarboxylate.

[0115] (3) Weigh 30 g of crude dimethyl 2,6-naphthalene dicarboxylate into a three-necked flask, add 100 g of methanol, and heat to 40 °C. It is found that the dissolution is very poor and the methanol ratio is too low to allow recrystallization.

[0116] The yield and purity of the product 2,6-NDC were determined by the method shown in Example 1. The results are shown in the following table: Table 19 Yield and purity of 2,6-NDC crystals obtained in Comparative Example 6

[0117] In Comparative Example 1, the amount of sulfuric acid used as the catalyst is higher than the range value, and the excess catalyst causes more side reactions and reduces the yield. In addition, during the recrystallization process, o-xylene has a poor effect of dissolving the product at room temperature, and recrystallization purification cannot be performed.

[0118] In Comparative Example 2, the amount of molybdenum trioxide catalyst used was higher than the range value, resulting in more side reactions and lower yields. In addition, the dissolution effect of acetonitrile at room temperature during the recrystallization process was acceptable, but it was unable to effectively separate impurities and 2,6-NDC, resulting in a low purity improvement.

[0119] In Comparative Example 3, the esterification reaction temperature of 200°C is higher than the optimal range, resulting in serious methanol volatilization, poor esterification effect, and low purity. In addition, during the recrystallization process, the methanol dissolves the product poorly at room temperature, and recrystallization purification cannot be performed.

[0120] In Comparative Example 4, the amount of methanol used was less than the range value, the esterification reaction was not complete, and the purity of the product was low. In the recrystallization process, the amount of o-xylene used was less than the range value, the impurities and the product could not be effectively analyzed, and the purification effect was poor.

[0121] In Comparative Example 5, the reaction temperature was lower than the range value, the reaction rate was slow, and the esterification process was difficult to carry out. The purity of the product is low. The amount of acetonitrile used in the recrystallization process is less than the range value, impurities and products cannot be effectively analyzed, and the purification effect is poor.

[0122] In Comparative Example 6, during the recrystallization process, the proportion of the recrystallization solvent (methanol) was too low, resulting in failure of recrystallization.

[0123] It can be seen that in the present invention, when sulfuric acid is selected as the acidic catalyst, the preferred mass ratio of 2,6-naphthalene dicarboxylic acid to sulfuric acid is 30-50:3-5; when ferrous sulfate is selected as the acidic catalyst, the preferred mass ratio of 2,6-naphthalene dicarboxylic acid to ferrous sulfate is 30-50:0.5-3; when phosphoric acid is selected as the acidic catalyst, the preferred mass ratio of 2,6-naphthalene dicarboxylic acid to phosphoric acid is 30-50:3-5; when molybdenum trioxide is selected as the acidic catalyst, the preferred mass ratio of 2,6-naphthalene dicarboxylic acid to phosphoric acid is 30-50:3-5. The mass ratio of molybdenum trioxide is 30-50:0.5-3; when the recrystallization solvent is methanol, the preferred mass ratio of dimethyl 2,6-naphthalene dicarboxylate to methanol is 30-50:300-1000, when the recrystallization solvent is acetonitrile, the preferred mass ratio of dimethyl 2,6-naphthalene dicarboxylate to acetonitrile is 30-50:300-800, when the recrystallization solvent is o-xylene, the preferred mass ratio of dimethyl 2,6-naphthalene dicarboxylate to o-xylene is 30-50:600-1000.

[0124] In summary, the present invention discloses a method for preparing high-purity 2,6-naphthalene dicarboxylic acid dimethyl ester, which uses 2,6-naphthalene dicarboxylic acid as a raw material and an appropriate amount of an acidic catalyst such as sulfuric acid and phosphoric acid as a catalyst, and the obtained crude 2,6-naphthalene dicarboxylic acid dimethyl ester is recrystallized and purified by using an appropriate amount of a solvent such as acetonitrile and methanol to obtain high-purity 2,6-naphthalene dicarboxylic acid dimethyl ester. The method is simple and reliable, and the solvent can be recycled, which is conducive to the industrial application of the process and lays a foundation for the development of high-performance polyester materials.

[0125] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification.

[0126] For the method embodiments, for the sake of simplicity, they are all described as a series of action combinations, but those skilled in the art should know that the present invention is not limited by the order of the actions described, because according to the present invention, some steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also know that the embodiments described in the specification are all preferred embodiments, and the actions and components involved are not necessarily required by the present invention.

[0127] The above is a detailed introduction to a kind of dimethyl 2,6-naphthalene dicarboxylate and its preparation method provided by the present invention. The principle and implementation mode of the present invention are explained in this article by using specific examples. The description of the above embodiments is only used to help understand the method of the present invention and its core idea. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation mode and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for preparing dimethyl 2,6-naphthalene dicarboxylate, characterized in that: The preparation method comprises: S1. In an inert gas environment, an acidic catalyst is used to catalyze an esterification reaction between 2,6-naphthalenedicarboxylic acid and methanol. The obtained reaction product is cooled and crystallized. The precipitated solid is washed and dried to obtain a crude product of dimethyl 2,6-naphthalenedicarboxylate; S2, adding a recrystallization solvent to the crude product of dimethyl 2,6-naphthalene dicarboxylate, heating the formed mixed solution to 40-120° C., and filtering the mixed solution after the crude product of dimethyl 2,6-naphthalene dicarboxylate no longer dissolves to obtain a filtrate; S3, cooling and crystallizing the filtrate, washing and drying the precipitated solid to obtain high-purity dimethyl 2,6-naphthalene dicarboxylate; Wherein, the acidic catalyst is selected from sulfuric acid, phosphoric acid, ferrous sulfate or molybdenum trioxide, and the recrystallization solvent is selected from acetonitrile, o-xylene or methanol.

2. The method for preparing dimethyl 2,6-naphthalene dicarboxylate according to claim 1, characterized in that: The acidic catalyst is selected from molybdenum trioxide, and the recrystallization solvent is selected from o-xylene.

3. The preparation method of dimethyl 2,6-naphthalene dicarboxylate according to claim 1, characterized in that: In step S1, the mass ratio of the 2,6-naphthalene dicarboxylic acid, the methanol and the acid catalyst is 30-50:60-250:0.5-5.

4. The method for preparing dimethyl 2,6-naphthalenedicarboxylate according to claim 1, characterized in that: In step S1, the inert gas is nitrogen, argon or helium.

5. The method for preparing dimethyl 2,6-naphthalene dicarboxylate according to claim 1, characterized in that: In step S1, the reaction pressure of the esterification reaction is 5-8 MPa, the reaction temperature is 140-180° C., and the reaction time is 2-3 h.

6. The method for preparing dimethyl 2,6-naphthalene dicarboxylate according to claim 1, characterized in that: In step S2, the mass ratio of the crude dimethyl 2,6-naphthalene dicarboxylate to the recrystallization solvent is 30-50:300-1000.

7. The method for preparing dimethyl 2,6-naphthalene dicarboxylate according to claim 1, characterized in that: In step S2, the temperature of the cooling crystallization is 0-20°C; The washing is performed with methanol solution for 1-3 times; The drying temperature is 40-80°C.

8. The method for preparing dimethyl 2,6-naphthalene dicarboxylate according to claim 1, characterized in that: In step S3, the temperature of the cooling crystallization is 0-20°C; The washing is performed with methanol solution for 1-3 times; The drying temperature is 40-80°C.

9. The method for preparing dimethyl 2,6-naphthalene dicarboxylate according to claim 1, characterized in that: The yield of the crude dimethyl 2,6-naphthalene dicarboxylate is 94.5-97.8%; and / or The purity of the dimethyl 2,6-naphthalene dicarboxylate is 98.4-99.5%.

10. A dimethyl 2,6-naphthalene dicarboxylate, characterized in that: The dimethyl 2,6-naphthalene dicarboxylate is obtained according to the preparation method described in any one of claims 1 to 9.

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