Preparation method and application of 2-methyl-6-alkylnaphthalene

Abstract

The invention provides a preparation method and application of 2-methyl-6-alkyl naphthalene, and the preparation method of 2-methyl-6-alkyl naphthalene comprises the following steps: carrying out first reflux reaction on 2-methyl-6-acyl naphthalene, hydrazine hydrate and strong base in an organic solvent to obtain a first product; carrying out first distillation on the first product, and then carrying out second reflux reaction to obtain a second product; extracting and washing the second product, removing the organic solvent, and carrying out second distillation to obtain a 2-methyl-6-alkylnaphthalene crude product; and carrying out melt crystallization on the 2-methyl-6-alkyl naphthalene crude product, so as to obtain the 2-methyl-6-alkyl naphthalene. According to the preparation method of the 2-methyl-6-alkylnaphthalene, the 2-methyl-6-alkylnaphthalene with high selectivity and high yield can be obtained, and meanwhile, the preparation method is wide in raw material source, simple in process operation, mild in reaction condition and suitable for large-scale popularization.

Description

Technical Field

[0001] The present application relates to the technical field of chemical synthesis, and in particular to a preparation method and application of 2-methyl-6-alkylnaphthalene. Background Art

[0002] 2,6-Naphthalene dicarboxylic acid (2,6-NDA) is an important intermediate for manufacturing high-strength polyester fibers and F-class insulation materials with excellent dyeing properties. It is also widely used in the fields of dyes and medicine. The molecular structure of 2,6-naphthalene dicarboxylic acid is highly symmetrical. Polyethylene 2,6-naphthalene dicarboxylate (PEN) obtained by reacting with ethylene glycol has the characteristics of a linear polymer and is a high-performance material with good rigidity, high strength and excellent thermal processing properties.

[0003] Both 2-methyl-6-acylanthanaphthalene and 2-methyl-6-alkylnaphthalene are important chemical substances, and they are the precursors of 2,6-naphthalenedicarboxylic acid. 2-methylnaphthalene is widely available and cheap (coal tar, ethylene tar, etc. are rich in a considerable amount of 2-methylnaphthalene), and can be used as a source material to synthesize 2-methyl-6-acylanthanaphthalene and 2-methyl-6-alkylnaphthalene through acylation and alkylation reactions of 2-methylnaphthalene, which greatly reduces the production cost.

[0004] At present, the synthesis of 2-methyl-6-alkylnaphthalene mostly uses 2-methylnaphthalene to react with methanol, ethylene, and isopropanol for alkylation to synthesize 2,6-dimethylnaphthalene (2,6-DMN), 2-methyl-6-ethylnaphthalene, and 2-methyl-6-isopropylnaphthalene, respectively. The current research hotspot of researchers is to prepare 2,6-dialkylnaphthalene by methylnaphthalene or naphthalene under the catalysis of molecular sieves. For example, Fraenkel et al. found that compared with HY and HM, HZSM-5 has more obvious selectivity in the process of alkylation of naphthalene and methylnaphthalene methanol. Under the same conditions, when HY and HM are used as molecular sieves, the conversion rate of β-methylnaphthalene is 30%, but the selectivity of 2,6- / 2,7-DMN is only 15%; but when HZSM-5 is used as a catalyst, the selectivity of 2,6- / 2,7-DMN is 61%, although the conversion rate of β-methylnaphthalene is only 19%. However, this method shortens the path, but the boiling point difference between the 2,6- / 2,7-DMN generated in the reaction is only 0.3℃, which is difficult to separate. The molecular sieve catalytic synthesis method not only has many steps, but also has a low conversion rate of 2-methylnaphthalene, and the types of alkyl naphthalene impurities are relatively complex (dialkylnaphthalene, trialkylnaphthalene, polyalkylnaphthalene, etc.), the separation of alkyl naphthalene isomers is difficult, and the overall yield is low.

[0005] Similar studies also include:

[0006] JPS63112526A (1986-10-27) discloses the production of 2-methyl-6-ethylnaphthalene, and it reports that 2-methylnaphthalene and ethylene are in anhydrous AlCl3 catalyst and there is a method for synthesizing a mixture of methyl-ethylnaphthalene, and provides a method for separating and recovering high-purity 2-methyl-6-ethylnaphthalene from a mixture rich in methyl-monoethylnaphthalene. Specifically adopt complex crystallization method, ethylnaphthalene mixture and m-nitrobenzoic acid react in a solvent that dissolves methyl-ethylnaphthalene components but is not easy to dissolve m-nitrobenzoic acid, and then cool to m-nitrobenzoic acid. The complex mainly composed of p-nitrobenzoic acid and 2,6-isomer is precipitated and separated from the solvent, water and extraction agent are added to the complex, and the complex is thermally decomposed to the boiling point of the solvent, the solvent is removed, and the residue is recrystallized through cyclohexane to obtain high-purity 2-methyl-6-ethylnaphthalene. However, due to alkylation reaction, 2-methyl-6-ethylnaphthalene has a low selectivity, which causes the separation method to be complicated to operate, and the extraction agent used is more toxic.

[0007] In the production of 2,6-dialkylnaphthalene disclosed in JPH0585962A (1991-09-24), a specific faujasite zeolite is used as a catalyst to efficiently isomerize mixed dialkylnaphthalene, using dialkylnaphthalene as a raw material (generally 2-methyl-6-alkylnaphthalene, 2-methyl-7-alkylnaphthalene, 2-methyl alkylnaphthalene mixture as a raw material), heating and stirring the reaction in an autoclave at a certain temperature, and isomerizing in the presence of a catalyst, 2,6-diethylnaphthalene, 2-methyl-6-isopropylnaphthalene, 2-methyl-6-ethylnaphthalene or 2-ethyl-6-isopropylnaphthalene can be obtained in good yields. Since the selectivity of the isomerization reaction is low, the raw material used is a specific proportion of dialkylnaphthalene, and the raw material is not easy to obtain. Although this method can synthesize a variety of 2-methyl-6-alkylnaphthalene, the cost is very high.

[0008] The paper Direct Benzylic Alkylation via Ni-Catalyzed Selective Benzylicsp3 C-O Activation (J.Am.Chem.Soc.2008,130,3268-3269) reported the Ni-catalyzed activation of benzyl sp3C-O bonds to achieve efficient and highly chemically selective cross-coupling reactions of benzyl ethers with Grignard reagents. Through this method, sp2C-OMe (naphthol methoxy) and benzyl sp3C-OMe (methoxy) in the same molecule can be distinguished by ligand-regulated methylation, which provides a new tool for selective methylation assembly of complex molecules. However, the raw materials (2-methoxy-6-(methoxymethyl) naphthalene) required for the synthesis of 2-methyl-6-ethylnaphthalene by this method are difficult to obtain, and the catalyst used is relatively expensive, resulting in extremely high synthesis costs.

[0009] CN114195609B (2020-09-02) discloses a method for synthesizing 2-methyl-6-ethylnaphthalene, which uses 5-methyl furanaldehyde, acetone and ethylene as raw materials to prepare 2-methyl-6-ethylnaphthalene. After three major synthesis steps of Claisen-Schmidt reaction, Diels-Alder reaction and serial coupling dehydrogenation-hydrodeoxygenation reaction, 2-methyl-6-ethylnaphthalene is finally obtained, and the selectivity of 2-methyl-6-ethylnaphthalene is >90%, and the yield is >68.0%. However, due to the large number of steps and the complex reaction, the catalyst used is mainly a solid-supported catalyst, and the preparation process is relatively cumbersome.

[0010] In addition, 2-methylnaphthalene is used as raw material, acyl chloride is used as acylating agent, aluminum chloride is used as Lewis acid catalyst, and nitrobenzene is used as solvent. Friedel-Crafts acylation reaction is carried out at a certain temperature and normal pressure to synthesize 2-methyl-acylnaphthalene, and the conversion rate and yield are high. Among them, the selectivity of 2-methyl-6-acylnaphthalene is high, which can reach more than 85%. However, the boiling points between the acylnaphthalene isomers are close (330° C.), and the melting point difference is not large. Further purification requires the distillation and solvent crystallization process with a high plate number, which leads to a reduction in yield. 2-methyl-acylnaphthalene has a higher boiling point than dialkylnaphthalene (the boiling point of dimethylnaphthalene isomers is 262-265° C., the boiling point of methylethylnaphthalene isomers is about 270° C., and the boiling point of methylisopropylnaphthalene isomers is about 284° C.), and the energy consumption is large during separation. The melting point difference between 2-methyl-acylnaphthalene and dialkylnaphthalene is smaller (the melting point difference between 2-methyl-6-propionyl and 2-methyl-7-propionylnaphthalene is 12 to 14°C, the melting point difference between 2,6-diisopropylnaphthalene and 2,7-diisopropylnaphthalene is 70 to 73°C, and the melting point difference between 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene is 15 to 18°C), which is not conducive to separation by melt crystallization.

[0011] In summary, the above-mentioned synthesis method of related 2-methyl-6-alkylnaphthalene has the following defects:

[0012] 1. The alkylation method of 2-methylnaphthalene is mainly used. Due to the low selectivity of the alkylation reaction, the mono-alkylation and poly-alkylation products generated are complex, resulting in great difficulty in separation, and the yield of the obtained 2-methyl-6-alkylnaphthalene is not high enough, especially for 2-methyl-6-ethylnaphthalene, 2-methyl-6-propylnaphthalene, etc.

[0013] 2. The cost of the raw materials used is relatively high (2-methoxy-6-(methoxymethyl)naphthalene) or the catalyst preparation is complicated (5-methylfuranaldehyde synthesis method), the synthesis steps are relatively complicated, and it is difficult to scale up the production.

[0014] Therefore, there is an urgent need to develop a method for synthesizing 2-methyl-6-alkylnaphthalene with a wide range of raw material sources, simple process operation, mild reaction conditions, and high product yield and selectivity. Summary of the invention

[0015] In view of this, an object of the present application is to provide a method for preparing 2-methyl-6-alkylnaphthalene, which uses 2-methyl-6-acylnaphthalene, hydrazine hydrate and a strong base as raw materials, undergoes two reflux reactions and melt crystallization, and can obtain 2-methyl-6-alkylnaphthalene with high selectivity and high yield. At the same time, the preparation method has a wide source of raw materials, simple process operation, mild reaction conditions, and is suitable for large-scale promotion.

[0016] Another object of the present application is to provide an application of 2-methyl-6-alkylnaphthalene prepared by a method for preparing 2-methyl-6-alkylnaphthalene.

[0017] To achieve the above object, the first aspect of the present application proposes a method for preparing 2-methyl-6-alkylnaphthalene, comprising:

[0018] Conducting a first reflux reaction of 2-methyl-6-acylnaphthalene, hydrazine hydrate and a strong base in an organic solvent to obtain a first product;

[0019] The first product is subjected to a first distillation, followed by a second reflux reaction to obtain a second product;

[0020] Extracting, washing, removing the organic solvent and performing a second distillation on the second product to obtain a crude 2-methyl-6-alkylnaphthalene product;

[0021] The crude 2-methyl-6-alkylnaphthalene product is melt-crystallized to obtain the 2-methyl-6-alkylnaphthalene.

[0022] In some embodiments, the 2-methyl-6-acylnaphthalene includes at least one of 2-methyl-6-acetylnaphthalene and 2-methyl-6-propionylnaphthalene.

[0023] In some embodiments, the molar ratio of the 2-methyl-6-acylnaphthalene, hydrazine hydrate and strong base is (1.6-2.4):(4-6):(4-6).

[0024] In some embodiments, the strong base includes at least one of potassium hydroxide and sodium hydroxide.

[0025] In some embodiments, the organic solvent includes at least one of diethylene glycol, dipropylene glycol, and glycerol.

[0026] In some embodiments, the purity of the 2-methyl-6-acylnaphthalene is above 93.0 wt %.

[0027] In some embodiments, the reaction temperature of the first reflux reaction is 120-140° C., and the reaction time of the first reflux reaction is 2-4 h.

[0028] In some embodiments, the reaction temperature of the second reflux reaction is 210-220° C., and the reaction time of the second reflux reaction is 4-8 h.

[0029] In some embodiments, the first distillation is atmospheric distillation, the temperature of the first distillation is 120-140° C., and the time of the first distillation is 20-40 min.

[0030] In some embodiments, the second distillation is a reduced pressure high vacuum distillation, the pressure of the second distillation is -0.08 MPa to -0.09 MPa, the temperature of the second distillation is 180-200° C., and the time of the second distillation is 1-2 h.

[0031] In some embodiments, the extraction agent includes at least one of petroleum ether, dichloromethane, and n-hexane at 60-90°C.

[0032] In some embodiments, the washing detergent includes at least one of dilute sulfuric acid, water, and a 10.0 wt % sodium chloride aqueous solution.

[0033] In some embodiments, the pH of the second product after the washing is neutral.

[0034] In some embodiments, the method of removing the organic solvent comprises distillation.

[0035] In some embodiments, the melt crystallization comprises: sequentially subjecting the crude 2-methyl-6-alkylnaphthalene product to crystallization, first filtration, sweating, second filtration, and heating to melt.

[0036] In some embodiments, the crystallization method comprises: cooling to 50-70° C. at a rate of 1-3° C. / h, and then keeping the temperature for 0.5-1.5 h.

[0037] In some embodiments, the first filtration and the second filtration both include at least one of vacuum filtration and centrifugal separation.

[0038] In some embodiments, the sweating method comprises: heating the solid phase after the first filtration to 65-85°C at a rate of 3-5°C / h, and then keeping the temperature for 1-2 hours.

[0039] In some embodiments, the method of heating and melting includes: heating the temperature of the solid phase after the second filtration to 80-85° C. until the crystal layer is completely melted.

[0040] In some embodiments, the purity of the 2-methyl-6-alkylnaphthalene is above 99.9 wt %.

[0041] The second aspect of the present application also relates to the use of 2-methyl-6-alkylnaphthalene prepared by the preparation method of 2-methyl-6-alkylnaphthalene described in the present application in the production of 2,6-naphthalenedicarboxylic acid.

[0042] The preparation method of 2-methyl-6-alkylnaphthalene described in the present application can at least bring the following beneficial effects:

[0043] 2-methyl-6-acylnaphthalene, hydrazine hydrate and a strong base are used as raw materials, and subjected to two reflux reactions and melt crystallization to obtain 2-methyl-6-alkylnaphthalene with high selectivity and high yield (for example, the yield of high-purity 2-methyl-6-ethylnaphthalene is greater than 70.0%, and the yield of high-purity 2-methyl-6-propylnaphthalene is greater than 75.0%). At the same time, the preparation method has a wide source of raw materials, simple process operation, mild reaction conditions, and is suitable for large-scale promotion.

[0044] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings.

[0046] in:

[0047] Figure 1 The present invention is a flow chart of a method for preparing 2-methyl-6-alkylnaphthalene according to an exemplary embodiment of the present invention.

[0048] Figure 2 This is the H-NMR spectrum of 2-methyl-6-ethylnaphthalene synthesized in Example 2. DETAILED DESCRIPTION

[0049] The embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, but cannot be understood as limiting the present application.

[0050] In this application, the disclosure of numerical ranges includes all values ​​within the entire range and the disclosure of further subdivided ranges, including the endpoints and sub-ranges given in these ranges.

[0051] In this application, the raw materials, equipment, etc. involved, unless otherwise specified, are all raw materials and equipment that can be made through commercial channels or known methods; the methods involved, unless otherwise specified, are all conventional methods.

[0052] The inventive concept of the embodiments of the present application is:

[0053] 2-Methylnaphthalene is acylated to obtain 2-methyl-6-acylnaphthalene, which is initially purified and reduced to obtain 2-methyl-6-alkylnaphthalene, which is then purified by melt crystallization to obtain high-purity 2-methyl-6-alkylnaphthalene, and finally oxidized by air to obtain high-purity 2,6-naphthalenedicarboxylic acid.

[0054] An exemplary reaction scheme is as follows:

[0055] .

[0057] A method for preparing 2-methyl-6-alkylnaphthalene according to an embodiment of the present application is described below with reference to the accompanying drawings.

[0058] Figure 1 The present invention is a flow chart of a method for preparing 2-methyl-6-alkylnaphthalene according to an exemplary embodiment of the present invention.

[0059] like Figure 1 As shown, the preparation method of 2-methyl-6-alkylnaphthalene of the embodiment of the present application comprises the following steps:

[0060] S101. Perform a first reflux reaction on 2-methyl-6-acylnaphthalene, hydrazine hydrate and a strong base in an organic solvent to obtain a first product.

[0061] In the embodiments of the present application, 2-methyl-6-acylnaphthalene can be a commercially available product or can be homemade.

[0062] As an optional example, when 2-methyl-6-acylnaphthalene needs to be made by oneself, the synthesis method of 2-methyl-6-acylnaphthalene comprises the following steps:

[0063] (1) Preparation of acylation solution: Dissolve the catalyst anhydrous aluminum chloride in nitrobenzene, and slowly add the acyl chloride after the system is cooled to about 5°C and continue stirring for a period of time to obtain an acylation solution containing the catalyst.

[0064] (2) Preparation of raw material solution: Nitrobenzene and 2-methylnaphthalene are added into a reactor equipped with a stirring device (e.g., a stirrer) to prepare a raw material solution.

[0065] (3) Acylation reaction: The raw material liquid prepared in step (2) is placed in an ice-oil bath and cooled to about -5°C, and then the catalyst-containing acylation liquid prepared in step (1) is slowly added dropwise with continuous stirring. After the addition is complete, the reaction temperature is raised to about 45°C. After a period of reaction, a dark brown homogeneous solution is obtained.

[0066] (4) Hydrolysis: After the reaction is completed, the dark brown oil phase homogeneous solution obtained in step (3) is poured into a beaker containing an ice-water mixture for hydrolysis (oil phase volume: ice-water mixture volume = 1:10), the hydrolysis temperature is controlled not to exceed 5°C (e.g., 0-5°C), and the solution is washed with deionized water for multiple times until the pH value of the aqueous phase is close to neutral, and the oil phase is obtained for analysis.

[0067] (5) Purification by reduced pressure distillation: Nitrobenzene is recovered by reduced pressure distillation to obtain a crude acyl naphthalene product at the bottom of the kettle. The acyl naphthalene at the bottom of the kettle is then subjected to reduced pressure distillation to remove heavy colored substances to obtain white 2-methyl-6-acyl naphthalene.

[0068] In the embodiments of the present application, the preparation method of 2-methyl-6-acylanthanaphthalene can refer to the method in the patent application previously applied by the applicant with application number 202111093577.8 and invention name "A method for continuous and synchronous hydrolysis of an acylation reaction solution".

[0069] In some embodiments, 2-methyl-6-acylnaphthalene includes, but is not limited to, at least one of 2-methyl-6-acetylnaphthalene, 2-methyl-6-propionylnaphthalene, and the like.

[0070] In some embodiments, the molar ratio of 2-methyl-6-acylnaphthalene, hydrazine hydrate and strong base is (1.6-2.4):(4-6):(4-6), including but not limited to 1.6:5:5, 2.4:5:5, 2:4:4, 2:5:6 or 2:5:5, etc.

[0071] In some embodiments, the strong base includes but is not limited to at least one of potassium hydroxide, sodium hydroxide, and the like.

[0072] In some embodiments, the organic solvent includes, but is not limited to, at least one of diethylene glycol, dipropylene glycol, glycerol, and the like.

[0073] In some embodiments, the mass ratio of the organic solvent to 2-methyl-6-acylnaphthalene is (6.5-6.8):1, including but not limited to 6.7:1, 6.6:1, etc.

[0074] In some embodiments, the purity of 2-methyl-6-acylnaphthalene is above 93.0 wt %, for example, above 93.5%, above 94.5%, above 95.5, etc.

[0075] In some embodiments, the reaction temperature of the first reflux reaction is 120-140° C., including but not limited to 120° C., 125° C., 130° C., 135° C. or 140° C., etc. When the reaction temperature of the first reflux reaction is within the above range, the reaction of forming hydrazone can be carried out moderately; when the reaction temperature is less than 120° C., the reaction is not sufficient; when the reaction temperature is greater than 140° C., the reaction is too intense in the early stage and more side reactions occur.

[0076] In some embodiments, the reaction time of the first reflux reaction is 2-4 h, including but not limited to 2 h, 2.5 h, 3 h, 3.5 h or 4 h.

[0077] In some embodiments, the first reflux reaction is performed under stirring conditions.

[0078] It should be noted that the embodiments of the present application do not limit the stirring speed of the first reflux reaction.

[0079] As an optional example, 2-methyl-6-acylnaphthalene, hydrazine hydrate and a strong base are subjected to a first reflux reaction in an organic solvent, comprising:

[0080] 2-Methyl-6-acylnaphthalene, an organic solvent, hydrazine hydrate and a strong base are sequentially added into a three-necked flask equipped with a mechanical stirrer, a reflux condenser and a thermometer, the temperature is raised to the reaction temperature of the first reflux reaction, and the first reflux reaction is carried out by keeping the temperature under reflux.

[0081] S102, subjecting the first product obtained in step S101 to a first distillation, and then to a second reflux reaction to obtain a second product.

[0082] In some embodiments, the first distillation is an atmospheric distillation.

[0083] In some embodiments, the temperature of the first distillation is 120-140°C, including but not limited to 120°C, 125°C, 130, 135°C, or 140°C, etc.

[0084] In some embodiments, the first distillation time is 20 min-40 min, including but not limited to 25 min, 30 min or 35 min, etc.

[0085] In some embodiments, the reaction temperature of the second reflux reaction is 210-220°C, including but not limited to 210°C, 205°C or 220°C.

[0086] In some embodiments, the reaction time of the second reflux reaction is 4-8 hours, including but not limited to 4 hours, 5 hours, 6 hours, 7 hours or 8 hours.

[0087] S103, extracting, washing, removing the organic solvent, and performing a second distillation on the second product obtained in step S102 to obtain a crude 2-methyl-6-alkylnaphthalene product.

[0088] In some embodiments, the method for preparing 2-methyl-6-alkylnaphthalene of the embodiment of the present application further includes a step of cooling the second product obtained in step S102 before extraction.

[0089] Exemplarily, the second product obtained in step S102 is cooled to 20-40° C., such as 30° C.

[0090] In some embodiments, the extraction agent includes, but is not limited to, at least one of petroleum ether, dichloromethane, n-hexane, etc. at 60-90°C.

[0091] Illustratively, petroleum ether at 60-90°C includes, but is not limited to, petroleum ether at 60°C, petroleum ether at 70°C, petroleum ether at 80°C, petroleum ether at 90°C, and the like.

[0092] In some embodiments, the washing detergent includes but is not limited to at least one of dilute sulfuric acid, water, a 10.0 wt % sodium chloride aqueous solution, and the like.

[0093] As an optional example, the washing detergent includes 30 wt % of dilute sulfuric acid and water.

[0094] In some embodiments, the pH of the second product after washing is neutral.

[0095] In the examples of the present application, the pH is neutral, that is, pH 7.

[0096] In some embodiments, the method of removing the organic solvent includes distillation. Exemplarily, the distillation includes but is not limited to at least one of atmospheric distillation, reduced pressure distillation, molecular distillation, and the like.

[0097] In some embodiments, the second distillation is a reduced pressure high vacuum distillation.

[0098] In some embodiments, the pressure of the second distillation is -0.08 MPa to -0.09 MPa, such as -0.085 MPa.

[0099] In some embodiments, the temperature of the second distillation is 180-200°C, such as 190°C.

[0100] In some embodiments, the second distillation time is 1.0 h-2.0 h, such as 1.5 h.

[0101] As an optional example, the method of extracting, washing, removing the organic solvent, and performing a second distillation on the second product obtained in step S102 includes:

[0102] The second product was extracted three times with petroleum ether at 60-90°C, then washed twice with 30wt% dilute sulfuric acid, then washed with water until neutral, and finally the organic solvent was evaporated, and the product was distilled under reduced pressure and high vacuum, and the front fraction was removed to collect the crude product of 2-methyl-6-alkylnaphthalene.

[0103] In the embodiments of the present application, the crude product of 2-methyl-6-alkylnaphthalene contains 2-methyl-6-alkylnaphthalene, 2-methyl-7-propylnaphthalene and 5 other isomers (there are 7 substitution positions on 2-methylnaphthalene, which will generate 7 acylnaphthalene isomers, except for the 6- and 7-positions, there are 5 more isomers).

[0104] S104, melt-crystallizing the crude 2-methyl-6-alkylnaphthalene to obtain 2-methyl-6-alkylnaphthalene.

[0105] In some embodiments, melt crystallization comprises: sequentially subjecting the crude 2-methyl-6-alkylnaphthalene product to crystallization, first filtration, sweating, second filtration, and heating to melt.

[0106] In some embodiments, the crystallization method comprises: cooling to 50-70° C. at a rate of 1-3° C. / h, and then maintaining the temperature for 0.5-1.5 h.

[0107] Illustratively, in the above crystallization process, the cooling rate includes but is not limited to 1°C / h, 1.5°C / h, 2°C / h, 2.5°C / h or 3°C / h, etc.

[0108] Illustratively, during the crystallization process, the temperature is lowered to, including but not limited to, 50°C, 55°C, 60°C, 65 or 70°C.

[0109] Illustratively, in the above crystallization process, the holding time includes but is not limited to 0.5 h, 1 h or 1.5 h, etc.

[0110] In some embodiments, the first filtration and the second filtration methods both include but are not limited to at least one of vacuum filtration, centrifugal separation, and the like.

[0111] As an optional example, the first filtration and the second filtration are both performed by vacuum filtration.

[0112] In some embodiments, the sweating method comprises: heating the solid phase after the first filtration to 65-85° C. at a rate of 3-5° C. / h, and then keeping the temperature for 1-2 hours.

[0113] Illustratively, during the sweating process, the heating rate includes but is not limited to 3°C / h, 3.5°C / h, 4°C / h, 4.5°C / h or 5°C / h, etc.

[0114] Illustratively, during the sweating process, the temperature is raised to, including but not limited to, 65°C, 70°C, 75°C, 80 or 85°C, etc.

[0115] Illustratively, during the sweating process, the heat preservation time includes but is not limited to 1 hour, 1.5 hours or 2 hours.

[0116] In some embodiments, the method of melting by heating includes: raising the temperature of the solid phase after the second filtration to 80-85° C. until the crystal layer is completely melted.

[0117] In the embodiments of the present application, by converting 2-methyl-acylnaphthalene into 2-methyl-alkylnaphthalene and then subjecting it to melt crystallization purification, it is possible to remove light component isomers without distillation purification, and a high-purity 6-substituted 2-methyl-6-alkylnaphthalene product can be obtained by primary melt crystallization, which can be further oxidized to obtain high-purity 2,6-naphthalene dicarboxylic acid (2,6-NDA), providing a new path for the synthesis of high-purity 2,6-NDA.

[0118] In the examples of the present application, the crude product of 2-methyl-6-acylnaphthalene to 2-methyl-6-alkylnaphthalene undergoes a first reflux reaction and a second reflux reaction, and these two reactions are essentially reduction reactions.

[0119] In some embodiments, the purity of 2-methyl-6-alkylnaphthalene is above 99.9 wt %, such as 99.91 wt %, 99.95 wt %, 99.991 wt % or 99.999 wt %.

[0120] The preparation method of 2-methyl-6-alkylnaphthalene of the embodiment of the present application, using 2-methyl-6-acylnaphthalene, hydrazine hydrate and strong base as raw materials, through two reflux reactions and melt crystallization, can obtain 2-methyl-6-alkylnaphthalene with high selectivity and high yield (for example, the yield of high-purity 2-methyl-6-ethylnaphthalene is greater than 70.0%, and the yield of high-purity 2-methyl-6-propylnaphthalene is greater than 75.0%), and the preparation method has a wide source of raw materials, simple process operation, mild reaction conditions, and is suitable for large-scale promotion. In addition, the 2-methyl-6-alkylnaphthalene prepared by the preparation method of 2-methyl-6-alkylnaphthalene of the embodiment of the present application can be further oxidized to obtain high-purity (2,6-NDA), providing a new path for synthesizing high-purity 2,6-NDA.

[0121] The 2-methyl-6-alkylnaphthalene prepared by the preparation method of 2-methyl-6-alkylnaphthalene according to the embodiment of the present application can be widely used in the production of 2,6-naphthalene dicarboxylic acid.

[0122] Certain features of the present technology are further illustrated in the following non-limiting examples.

[0123] Preparation Example 1

[0124] This preparation example is the synthesis of 2-methyl-6-propionylnaphthalene, and its synthesis reaction equation is shown in Formula 1:

[0125]

[0126] The synthetic method of 2-methyl-6-propionylnaphthalene comprises the following steps:

[0127] (1) Preparation of acylation solution: 200.0 g of anhydrous aluminum chloride (1.5 mol) as a catalyst was dissolved in 1230 g (10.0 mol) of nitrobenzene. After the system was cooled to about 5°C, 120 g of propionyl chloride (1.3 mol) was slowly added and stirred for 1 hour to obtain an acylation solution containing the catalyst.

[0128] (2) Preparation of raw material solution: 615.0 g (5 mol) of nitrobenzene and 142.0 g (1.0 mol) of 2-methylnaphthalene were added to a reactor equipped with a stirring device (eg, a stirrer) to prepare a raw material solution.

[0129] (3) Acylation reaction: Acylation reaction is carried out according to the ratio of n(methylnaphthalene):n(propionyl chloride):n(catalyst) of 1.0:1.3:1.5. The raw material solution prepared in step (2) is placed in an ice-oil bath and cooled to -5°C, and then the acylation solution containing the catalyst prepared in step (1) is slowly added dropwise with continuous stirring. After the addition is completed, the reaction temperature is increased to 45°C and the reaction is carried out for 5 hours to obtain a dark brown homogeneous solution.

[0130] (4) Hydrolysis: After the reaction is completed, the dark brown homogeneous solution obtained in step (3) is poured into a beaker filled with 1 mol / L ice-water mixture for hydrolysis (oil phase volume: ice-water mixture volume = 1:10, and the hydrolysis temperature is controlled at 3° C.), and washed with deionized water for 3 times until the pH value of the aqueous phase of the solution is close to neutral. The oil phase is obtained for analysis, and the conversion rate of 2-methylnaphthalene is 99.0%, and the selectivity of 2-methyl-6-propionylnaphthalene is 93.0%.

[0131] (5) Purification by reduced pressure distillation: Nitrobenzene is recovered by reduced pressure distillation to obtain a crude acyl naphthalene product at the bottom of the kettle. The acyl naphthalene at the bottom of the kettle is then subjected to reduced pressure distillation to remove heavy colored substances to obtain 182.1 g of white 2-methyl-6-propionyl naphthalene. The overall yield of 2-methyl-6-propionyl naphthalene is 92.0% and the purity is 93.0 wt%.

[0132] Preparation Example 2

[0133] The synthetic method of 2-methyl-6-acetylnaphthalene comprises the following steps:

[0134] (1) Preparation of acylation solution: 200.0 g of anhydrous aluminum chloride (1.5 mol) as a catalyst was dissolved in 1230 g (10.0 mol) of nitrobenzene. After the system was cooled to about 5°C, 120 g of acetyl chloride (1.3 mol) was slowly added and stirred for 1 hour to obtain an acylation solution containing the catalyst.

[0135] (2) Preparation of raw material solution: 615.0 g (5 mol) of nitrobenzene and 142.0 g (1.0 mol) of 2-methylnaphthalene were added to a reactor equipped with a stirring device (eg, a stirrer) to prepare a raw material solution.

[0136] (3) Acylation reaction: Acylation reaction is carried out according to the ratio of n(methylnaphthalene):n(acetyl chloride):n(catalyst) of 1.0:1.3:1.5. The raw material solution prepared in step (2) is placed in an ice-oil bath and cooled to -5°C, and then the acylation solution containing the catalyst prepared in step (1) is slowly added dropwise with continuous stirring. After the addition is completed, the reaction temperature is increased to 45°C and the reaction is carried out for 5 hours to obtain a dark brown homogeneous solution.

[0137] (4) Hydrolysis: After the reaction is completed, the dark brown homogeneous solution obtained in step (3) is poured into a beaker filled with 1 mol / L ice-water mixture for hydrolysis (oil phase volume: ice-water mixture volume = 1:10, and the hydrolysis temperature is controlled at 3° C.), and washed with deionized water three times until the pH value of the aqueous phase of the solution is close to neutral. The oil phase is obtained for analysis, and the conversion rate of 2-methylnaphthalene is 99.0%, and the selectivity of 2-methyl-6-acetylnaphthalene is 85%.

[0138] (5) Purification by reduced pressure distillation: Nitrobenzene is recovered by reduced pressure distillation to obtain a crude acyl naphthalene product at the bottom of the kettle. The acyl naphthalene at the bottom of the kettle is then subjected to reduced pressure distillation to remove heavy colored substances to obtain 162.9 g of white 2-methyl-6-acetyl naphthalene. The overall yield of 2-methyl-6-acetyl naphthalene is 92.0% and the purity is 88.0 wt%.

[0139] Example 1

[0140] This embodiment is the preparation of 2-methyl-6-propylnaphthalene, and its preparation reaction equation is shown in Formula 2:

[0141]

[0142] The preparation method of 2-methyl-6-propylnaphthalene comprises the following steps:

[0143] 1) Synthesis of 2-methyl-6-propylnaphthalene

[0144] In a three-necked flask equipped with mechanical stirring (e.g., a stirrer), a reflux condenser and a thermometer, 39.6 g (0.20 mol) of 2-methyl-6-propionyl naphthalene with a purity of 93.0 wt% synthesized in Preparation Example 1, 220 ml of diethylene glycol, 25.0 g (0.5 mol) of hydrazine hydrate, and 28.0 g (0.5 mol) of KOH were added in sequence, the mixture was heated to 130 ° C, kept at reflux for 3 h, and refluxed under atmospheric pressure (the atmospheric distillation temperature was 130 ° C for 30 min), and low-boiling-point fraction water, excess hydrazine, etc. were evaporated; then the temperature was slowly raised, and when the temperature reached 210-220 ° C, the mixture was kept at reflux for 6 h, the reaction was terminated, the temperature was cooled to 30 ° C, and the organic phase was extracted three times with 400 ml of 70 ° C petroleum ether (400 ml was used for each extraction, and 1.2 L of petroleum ether was used for three extractions). 70 ℃ petroleum ether), then washed twice with 30wt% dilute sulfuric acid, washed with water until neutral, evaporated the solvent (diethylene glycol was evaporated at normal pressure, boiling point is 245 ℃), reduced pressure high vacuum distillation (reduced pressure high altitude distillation pressure is -0.085MPa, temperature is 190 ℃, time is 1.5h), card removed the front fraction, collected 2-methyl-6-propylnaphthalene crude product 32.6g, yield 88.6%, by gas chromatography (GC) analysis, purity is 97.0wt% (containing 2.8wt% of 2-methyl-7-propylnaphthalene and 0.2wt% of other 5 isomers).

[0145] 2) Purification of 2-methyl-6-propylnaphthalene

[0146] The crude product of 97.0wt% purity 2-methyl-6-propylnaphthalene (containing 2.8wt% 2-methyl-7-propionylnaphthalene) obtained in step 1) is melt crystallized. 200g of the crude product (temperature is 77°C) is placed in a tubular crystallizer for crystallization, the cooling rate is 2°C / h, the final temperature of the cooling is 63°C, after the final temperature is kept for 1h, the discharge valve at the bottom of the crystallizer is opened, the vacuum filtration is opened, and the mother liquor is separated. The sweating heating rate is 4°C / h, the final temperature of the sweating is 78°C, after the final temperature is kept for 1h, the vacuum filtration is opened, and the sweat is separated. Close the discharge valve, raise the temperature to 85°C, the crystal layer is completely melted, and the discharge valve is opened to obtain a 2-methyl-6-propylnaphthalene product (purity 99.9wt%, yield 92.5%).

[0147] Example 2

[0148] This embodiment is the preparation of 2-methyl-6-ethylnaphthalene, and its preparation reaction equation is shown in Formula 3:

[0149]

[0150] The preparation method of 2-methyl-6-ethylnaphthalene comprises the following steps:

[0151] 1) Synthesis of 2-methyl-6-ethylnaphthalene

[0152] In a three-necked flask equipped with mechanical stirring (e.g., a stirrer), a reflux condenser and a thermometer, 36.84 g (0.20 mol) of 2-methyl-6-acetylnaphthalene with a purity of 88.0 wt% synthesized in Preparation Example 2, 220 ml of diethylene glycol, 25.0 g (0.5 mol) of hydrazine hydrate, and 28.0 g (0.5 mol) of KOH were added in sequence, the temperature was raised to 130 ° C, and the mixture was kept at reflux for 3 h. The reflux was changed to atmospheric distillation (the atmospheric distillation temperature was 130 ° C, and the time was 30 min), and low-boiling-point fraction water, excess hydrazine, etc. were evaporated; then the temperature was slowly raised, and when the temperature rose to 210-220 ° C, the mixture was kept at reflux for 6 h, the reaction was terminated, and the temperature was cooled to 30 ° C, and the organic phase was extracted three times with 400 ml of 70 ° C petroleum ether (400 ml was used for each extraction, and 1.2 L of petroleum ether was used for three extractions in total). 70 ℃ petroleum ether), then washed twice with 30wt% dilute sulfuric acid, washed with water until neutral, evaporated the solvent (diethylene glycol was evaporated at normal pressure, boiling point is 245 ℃), reduced pressure high vacuum distillation (reduced pressure high altitude distillation pressure is -0.085MPa, temperature is 190 ℃, time is 1.5h), card removed the front fraction, collected 2-methyl-6-ethylnaphthalene crude product 30.0g, yield 88.3%, by gas chromatography (GC) analysis, purity is 94.2wt% (containing 5.0wt% of 2-methyl-7-ethylnaphthalene and 0.8wt% of other 5 isomers).

[0153] 2) Purification of 2-methyl-6-ethylnaphthalene

[0154] The crude product of 94.2wt% purity of 2-methyl-6-ethylnaphthalene (containing 5.0wt% of 2-methyl-7-acetylnaphthalene) obtained in step 1) is melt crystallized. 200g of the crude product (temperature is 74°C) is placed in a tubular crystallizer for crystallization, the cooling rate is 2°C / h, the final temperature of the cooling is 58°C, after the final temperature is kept for 1h, the discharge valve at the bottom of the crystallizer is opened, the vacuum filtration is opened, and the mother liquor is separated. The sweating heating rate is 4°C / h, the final temperature of the sweating is 75°C, after the final temperature is kept for 1.5h, the vacuum filtration is opened, and the sweat is separated. Close the discharge valve, raise the temperature to 81°C, the crystal layer is completely melted, and the discharge valve is opened to obtain a 2-methyl-6-ethylnaphthalene product (purity 99.9wt%, yield 88.1%).

[0155] Example 3

[0156] This embodiment is basically the same as Embodiment 1, except that:

[0157] In step 1), 39.6 g (0.2 mol) of 2-methyl-6-propionylnaphthalene with a purity of 93.0 wt% synthesized in Preparation Example 1, 220 ml of diethylene glycol, 37.5 g (0.75 mol) of hydrazine hydrate, and 42 g (0.75 mol) of KOH were added in sequence. 31.27 g of crude product of 2-methyl-6-propylnaphthalene was collected with a yield of 85.0%. The purity was 96.0 wt% (containing 2.8 wt% of 2-methyl-7-propylnaphthalene and 1.2 wt% of the other five isomers) by gas chromatography (GC).

[0158] In step 2), the feed valve is opened to obtain 2-methyl-6-propylnaphthalene product (purity 99.0wt%, yield 90.0%).

[0159] Example 4

[0160] This embodiment is basically the same as Embodiment 1, except that:

[0161] In step 1), 39.6 g (0.2 mol) of 2-methyl-6-propionylnaphthalene with a purity of 93.0 wt% synthesized in Preparation Example 1, 220 ml of diethylene glycol, 16.67 g (0.33 mol) of hydrazine hydrate, and 18.67 g (0.33 mol) of KOH were added in sequence. 29.43 g of crude product of 2-methyl-6-propylnaphthalene was collected with a yield of 80.0%. The purity was 96.5 wt% (containing 2.8 wt% of 2-methyl-7-propylnaphthalene and 0.7 wt% of the other five isomers) by gas chromatography (GC).

[0162] In step 2), the feed valve is opened to obtain 2-methyl-6-propylnaphthalene product (purity 99.5wt%, yield 96.5%).

[0163] Example 5

[0164] This embodiment is basically the same as Embodiment 1, except that:

[0165] In step 1), 39.6 g (0.20 mol) of 2-methyl-6-propionylnaphthalene with a purity of 93.0 wt% synthesized in Preparation Example 1, 220 ml of diethylene glycol, 25.0 g (0.5 mol) of hydrazine hydrate, and 28.0 g (0.5 mol) of KOH were added in sequence, and the temperature was raised to 120° C. 29.25 g of crude product of 2-methyl-6-propylnaphthalene was collected with a yield of 79.5%. The purity was 95.0 wt% (containing 2.8 wt% of 2-methyl-7-propylnaphthalene and 2.2 wt% of the other five isomers) by gas chromatography (GC).

[0166] In step 2), the feed valve is opened to obtain 2-methyl-6-propylnaphthalene product (purity 99.3wt%, yield 96.0%).

[0167] Example 6

[0168] This embodiment is basically the same as Embodiment 1, except that:

[0169] In step 1), 39.6 g (0.20 mol) of 2-methyl-6-propionylnaphthalene with a purity of 93.0 wt% synthesized in Preparation Example 1, 220 ml of diethylene glycol, 25.0 g (0.5 mol) of hydrazine hydrate, and 28.0 g (0.5 mol) of KOH were added in sequence, and the temperature was raised to 140° C. 30.35 g of crude product of 2-methyl-6-propylnaphthalene was collected with a yield of 82.5%. The purity was 95.5 wt% (containing 2.8 wt% of 2-methyl-7-propylnaphthalene and 1.7 wt% of the other five isomers) by gas chromatography (GC).

[0170] In step 2), the feed valve is opened to obtain 2-methyl-6-propylnaphthalene product (purity 99.2wt%, yield 96.4%).

[0171] Application Example 1

[0172] 37.22 g of 2-methyl-6-propylnaphthalene with a purity of 99.9 wt % obtained in Example 1 was added to 600 mL of acetic acid solvent, and then 12.33 g of catalyst (cobalt acetate-manganese acetate-potassium bromide, the mass ratio of cobalt acetate, manganese acetate and potassium bromide was 2.5:5.3:4.5) was added, and the air-liquid phase oxidation reaction was carried out at 210° C. and 2.1 MPa for 1 h with an air flow rate of 2 L / min. The reaction was stopped and cooled to room temperature. The reaction solution was subjected to solid-liquid separation by centrifugation, washed with glacial acetic acid and hot distilled water-methanol, and then dried to obtain 39.1 g of white solid powder. After liquid phase analysis, the purity of 2,6-naphthalenedicarboxylic acid was 99.5 wt %.

[0173] Application Example 2

[0174] 34.38 g of 2-methyl-6-ethylnaphthalene with a purity of 99.9 wt % obtained in Example 2 was added to 600 mL of acetic acid solvent, and then 12.33 g of catalyst (cobalt acetate-manganese acetate-potassium bromide, the mass ratio of cobalt acetate, manganese acetate and potassium bromide was 2.5:5.3:4.5) was added, and the air-liquid phase oxidation reaction was carried out at 200° C. and 1.9 MPa for 1 h with an air flow rate of 2 L / min. The reaction was stopped and cooled to room temperature. The reaction solution was subjected to solid-liquid separation by centrifugation, washed with glacial acetic acid and hot distilled water-methanol, and then dried to obtain 35.3 g of white solid powder. After liquid phase analysis, the purity of 2,6-naphthalene dicarboxylic acid was 99.5 wt %.

[0175] Comparative Example 1

[0176] 37.22 g of commercially available 2-methyl-6-propylnaphthalene with a purity of 98.00 wt% was added to 600 mL of acetic acid solvent, and 12.33 g of catalyst (cobalt acetate-manganese acetate-potassium bromide, the mass ratio of cobalt acetate, manganese acetate and potassium bromide is 2.5:5.3:4.5) was added, and the air-liquid phase oxidation reaction was carried out at 200°C and 1.9 MPa for 1 h with an air flow rate of 2 L / min. The reaction was stopped and cooled to room temperature. The reaction solution was subjected to solid-liquid separation by centrifugation, washed with glacial acetic acid and hot distilled water-methanol, and then dried to obtain 36.9 g of white solid powder. After liquid phase analysis, the purity of 2,6-naphthalene dicarboxylic acid was 96.5 wt%.

[0177] Comparative Example 2

[0178] 34.38 g of commercially available 2-methyl-6-ethylnaphthalene with a purity of 98.00 wt% was added to 600 mL of acetic acid solvent, and 12.33 g of catalyst (cobalt acetate-manganese acetate-potassium bromide, the mass ratio of cobalt acetate, manganese acetate and potassium bromide was 2.5:5.3:4.5) was added, and the air-liquid phase oxidation reaction was carried out at 200°C and 1.9 MPa for 1 h with an air flow rate of 2 L / min. The reaction was stopped and cooled to room temperature. The reaction solution was subjected to solid-liquid separation by centrifugation, washed with glacial acetic acid and hot distilled water-methanol, and then dried to obtain 34.1 g of white solid powder. After liquid phase analysis, the purity of 2,6-naphthalene dicarboxylic acid was 96.5 wt%.

[0179] In the present application, the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. mean 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 application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics 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 and the features of the different embodiments or examples, without contradiction.

[0180] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of this application, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0181] Although the embodiments of the present application have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be understood as limitations on the present application. Ordinary technicians in this field can change, modify, replace and modify the above embodiments within the scope of the present application.

Claims

1. A method for preparing 2-methyl-6-alkylnaphthalene, characterized in that: include: Conducting a first reflux reaction of 2-methyl-6-acylnaphthalene, hydrazine hydrate and a strong base in an organic solvent to obtain a first product; The first product is subjected to a first distillation, followed by a second reflux reaction to obtain a second product; Extracting, washing, removing the organic solvent and performing a second distillation on the second product to obtain a crude 2-methyl-6-alkylnaphthalene product; The crude 2-methyl-6-alkylnaphthalene product is melt-crystallized to obtain the 2-methyl-6-alkylnaphthalene.

2. The preparation method according to claim 1, characterized in that: The 2-methyl-6-acylnaphthalene includes at least one of 2-methyl-6-acetylnaphthalene and 2-methyl-6-propionylnaphthalene.

3. The preparation method according to claim 1, characterized in that: The molar ratio of the 2-methyl-6-acylnaphthalene, hydrazine hydrate and strong base is (1.6-2.4):(4-6):(4-6).

4. The preparation method according to claim 1, characterized in that: The strong base includes at least one of potassium hydroxide and sodium hydroxide; and / or, The organic solvent includes at least one of diethylene glycol, dipropylene glycol, and glycerol; And / or, the purity of the 2-methyl-6-acylnaphthalene is above 93.0wt%.

5. The preparation method according to claim 1, characterized in that: The reaction temperature of the first reflux reaction is 120-140° C., and the reaction time of the first reflux reaction is 2-4 h; and / or, The reaction temperature of the second reflux reaction is 210-220° C., and the reaction time of the second reflux reaction is 4-8 h.

6. The preparation method according to claim 1, characterized in that: The first distillation is atmospheric distillation, the temperature of the first distillation is 120-140° C., and the time of the first distillation is 20-40 min; and / or, The second distillation is a reduced pressure high vacuum distillation, the pressure of the second distillation is -0.08 MPa to -0.09 MPa, the temperature of the second distillation is 180-200° C., and the time of the second distillation is 1-2 h.

7. The preparation method according to claim 1, characterized in that: The extraction agent comprises at least one of petroleum ether, dichloromethane and n-hexane at 60-90°C; and / or, The washing detergent comprises at least one of dilute sulfuric acid, water, and a 10.0 wt % sodium chloride aqueous solution; and / or, The pH of the second product after the washing is neutral; and / or, The method of removing the organic solvent includes distillation.

8. The preparation method according to claim 1, characterized in that: The melt crystallization comprises: The crude 2-methyl-6-alkylnaphthalene product is sequentially subjected to crystallization, first filtration, sweating, second filtration, and heating and melting; And / or, the purity of the 2-methyl-6-alkylnaphthalene is above 99.9wt%.

9. The preparation method according to claim 8, characterized in that: The crystallization method comprises: cooling to 50-70° C. at a rate of 1-3° C. / h, and then keeping the temperature for 0.5-1.5 h; and / or, The first filtration and the second filtration both include at least one of vacuum filtration and centrifugal separation; and / or, The sweating method comprises: heating the solid phase after the first filtration to 65-85°C at a rate of 3-5°C / h, and then keeping the temperature for 1-2h; and / or, The heating and melting method comprises: heating the temperature of the solid phase after the second filtration to 80-85° C. until the crystal layer is completely melted.

10. Use of 2-methyl-6-alkylnaphthalene prepared by the preparation method according to any one of claims 1 to 9 in the production of 2,6-naphthalenedicarboxylic acid.

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