JPH08333299A - Purification of naphthalenedicarboxylic acid - Google Patents

Abstract

PURPOSE: To obtain purified naphthalenedicarboxylic acid which is useful as a feedstock for poly(ethylene naphthalate) resin in high recovery by using pyridines and aliphatic amines as a solvent. CONSTITUTION: (A) Crude naphthalenedicarboxylic acid obtained by oxidation of dialkylnaphthalene is dissolved in (B) a mixed solvent of (i) pyridines such as pyridine, methylpyridine or the like and (ii) aliphatic amines such as ethylamine and (C) the amine salt of naphthalenedicarboxylic acid is crystallized out and recovered. The recovered amine salt (C) is heated at, for example, 100-250 deg.C to decompose the amine salt (C). In a preferred embodiment, the dicarboxylic acid is dissolved in the component (B) and the solution is treated with hydrogen in the presence of a catalyst containing at least one metal selected from Pt, Pd, Rh, Ni and Co, then the dicarboxylic acid is crystallized out, where the weight ratio of the component (i) to the component (ii) is 2-5 in the solvent.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for purifying crude naphthalene dicarboxylic acid obtained by oxidizing dialkylnaphthalene. Naphthalenedicarboxylic acid is useful as a raw material for polyethylene naphthalate (PEN) resin, which has excellent performance.

[0002]

BACKGROUND ART Polyester obtained by polymerizing naphthalene dicarboxylic acid and diols such as ethylene glycol has excellent tensile strength and heat resistance, and is industrially used as a material for films, polyester fibers, plastic bottles and the like. Has important uses. In particular, polyethylene naphthalate (PEN) obtained by polymerizing 2,6-naphthalenedicarboxylic acid and ethylene glycol is expected to grow in demand in the near future as an excellent industrial resin that replaces polyethylene terephthalate (PET).

Naphthalenedicarboxylic acid is obtained by reacting dialkylnaphthalene with acetic acid in the presence of heavy metals such as Co and Mn and a bromine compound at high temperature and high pressure with molecular oxygen. However, the naphthalenedicarboxylic acid thus obtained includes, in addition to Co and Mn, which are catalytic metals, formylnaphthoic acid and methylnaphthoic acid, which are intermediate products of the oxidation reaction, trimellitic acid generated by decomposition, and naphthalenedicarboxylic acid added with bromine. Impurities such as naphthoic acid and naphthalenetricarboxylic acid derived from impurities of acid bromide and raw material dialkylnaphthalene are included as impurities. When naphthalene dicarboxylic acid containing these impurities is used as a raw material for the polymerization with diols, the obtained polyester has lower heat resistance and softening point, and deterioration in quality such as coloring. In particular, when formylnaphthoic acid is contained in a certain amount or more, the degree of polymerization does not increase, and gelation and coloring occur, so it is important to control the amount thereof. Therefore, high-purity naphthalenedicarboxylic acid with a purity of 99% or higher is required to obtain high-quality polyester.

Generally, for purification of organic matter, distillation, recrystallization,
Although methods such as sublimation are industrially used, naphthalene dicarboxylic acid cannot be distilled because it decomposes by heating at high temperature, and is usually insoluble in a general solvent, so a simple purification method by recrystallization. Is difficult to apply. Therefore, an industrial method for obtaining high-purity naphthalenedicarboxylic acid has not been established yet, and crude naphthalenedicarboxylic acid is reacted with alcohols such as methanol to give a naphthalenedicarboxylic acid ester, which is purified by an operation such as distillation or recrystallization. The method of doing is done.

However, as dimethyl terephthalate has been replaced by high-purity terephthalic acid, and it has become possible to produce polyethylene terephthalate by the economically advantageous direct polymerization method, the raw material of polyethylene naphthalate is not naphthalene dicarboxylic acid ester but high Purity naphthalene dicarboxylic acid is desirable, and establishment of a purification method therefor is urgently needed.

Dissolve naphthalenedicarboxylic acid in a solvent,
The following methods have been proposed as purification methods.
In US Pat. No. 5,256,817, purification is carried out by dissolving in water or an acetic acid aqueous solution at a high temperature of 300 ° C. or higher, hydrogenating and crystallization. In this method, high temperature is required to obtain high solubility, and therefore a side reaction is likely to occur, for example, naphthoic acid by decarboxylation reaction, tetralindicarboxylic acid is produced by nuclear hydrogenation reaction, and formylnaphthoic acid is hydrogenated. There is a problem that it is insufficient and remains in the crystal after crystallization.

Further, JP-A-62-230747 discloses a purification method by dissolving crude 2,6-naphthalenedicarboxylic acid in a solvent such as dimethylsulfamide, dimethylacetamide, dimethylformamide and crystallization. However, the solubility of 2,6-naphthalenedicarboxylic acid in these solvents is low, and when the solution is hydrogenated, the solvent is also hydrogenated, so that the hydrogenation treatment cannot be carried out, and particularly during the polymerization. It has the drawback that complete removal of the problematic formylnaphthoic acid is difficult. Japanese Unexamined Patent Publication (Kokai) No. 5-32586 discloses a purification method by dissolving in pyridine and crystallization. However, the solubility of 2,6-naphthalenedicarboxylic acid has a low temperature dependency, so that the recovery rate is low.

Therefore, a method has been proposed in which naphthalenedicarboxylic acid is dissolved in an alkali to improve the solubility as an alkali salt and then purified. For example, in Japanese Examined Patent Publication No. 52-20993 and Japanese Examined Patent Publication No. 48-68554, crude naphthalene dicarboxylic acid is added to KOH.
Dissolved in an alkaline aqueous solution such as NaOH or NaOH, and precipitated as a monoalkali salt by acid precipitation with carbon dioxide gas or sulfur dioxide gas,
2,6-naphthalenedicarboxylic acid is liberated by bringing the monoalkali salt into contact with water to disproportionate it. However, in these methods, when the monoalkali salt is precipitated, salts of other impurities such as 2,6-formylnaphthoic acid are also precipitated at the same time, and it is necessary to treat and recover a large amount of alkali and acid. There are drawbacks.

Further, in Japanese Examined Patent Publication No. 52-20994 and Japanese Patent Laid-Open No. 48-68555, crude 2,6-naphthalenedicarboxylic acid is dissolved in an aqueous alkaline solution such as KOH or NaOH and cooled or concentrated to crystallize with a dialkali salt. And a further disproportionation to obtain a purified 2,6-naphthalenedicarboxylic acid have been proposed. However, in this method, the temperature dependence of the solubility of the alkali salt is low, the recovery rate is low because the solubility of the dialkali salt in water is very high even at low temperatures, and there is a problem that it is difficult to remove a trace amount of alkali in the crystal. is there. In JP-A-50-135062, crude 2,6-naphthalenedicarboxylic acid is dissolved in an aqueous solution of an aliphatic amine having 6 or less carbon atoms, crystallized as a diamine salt by cooling or concentration, and purified by thermal decomposition. -A method for obtaining naphthalenedicarboxylic acid is shown. However, in this method, the solubility of the amine salt in water is very high even at a low temperature, and the recovery rate is low, which is an industrial problem. Further, in JP-A-5-294892, naphthalenedicarboxylic acid is crystallized after being dissolved in a mixed solvent of amines and alcohols, and the precipitated naphthalenedicarboxylic acid amine salt is thermally decomposed at a temperature of the boiling point of the amine or higher, and purified. A method for obtaining the prepared naphthalenedicarboxylic acid is shown. This method also has a drawback that the recovery rate is low because the solubility of the amine salt of naphthalenedicarboxylic acid in the lower alcohol is very high.

[0010]

SUMMARY OF THE INVENTION The object of the present invention is to improve the problems of the prior art as described above, and to obtain a highly pure naphthalenedicarboxylic acid from a crude naphthalenedicarboxylic acid with a high recovery rate and industrially. It is to provide a method that can be easily manufactured.

[0011]

Means for Solving the Problems As a result of intensive studies conducted by the present inventors on a method for purifying crude naphthalenedicarboxylic acid, naphthalenedicarboxylic acid reacts with an aliphatic amine to form a naphthalenedicarboxylic acid amine salt, The amine salt exhibits a large temperature dependence of solubility in pyridines and exhibits low solubility at low temperatures, and naphthalenedicarboxylic acid is dissolved in the solvent and subjected to hydrogenation treatment to formylnaphthoic acid or naphthalenedicarboxylic acid bromide. The amount of the naphthalenedicarboxylic acid amine salt crystals can be obtained at a high recovery rate by crystallization under appropriate conditions, and the naphthalenedicarboxylic acid amine salt is decomposed by heating to obtain a purified naphthalenedicarboxylic acid. , Easily from crude naphthalene dicarboxylic acid in high yield,
It was found that high-purity naphthalene dicarboxylic acid was obtained,
The present invention has been reached.

That is, according to the present invention, crude naphthalenedicarboxylic acid obtained by the oxidation reaction of dialkylnaphthalene is dissolved in a mixed solvent consisting of pyridines and aliphatic amines, and the naphthalenedicarboxylic acid amine salt is recovered by crystallization and recovered. A method for purifying naphthalenedicarboxylic acid characterized by thermally decomposing the amine salt, and after dissolving the crude naphthalenedicarboxylic acid in a mixed solvent, Pt, Pd, Rh, Ru,
In the presence of a catalyst containing at least one metal selected from Ni and Co
It is a method for purifying the naphthalenedicarboxylic acid, which comprises performing crystallization at a temperature of 250 ° C. or lower.

The crude naphthalenedicarboxylic acid used as a raw material in the present invention may be any one obtained by the oxidation reaction of dialkylnaphthalene, and the kind of dialkylnaphthalene used in the oxidation reaction and the oxidation conditions are not particularly limited. There is no. Examples of the dialkylnaphthalene include dimethylnaphthalene, diethylnaphthalene, dipropylnaphthalene, diisopropylnaphthalene and the like, and there are 10 isomers depending on the position of the alkyl group. Among them, as a raw material of polyester, 2,6-substituted products and 2,7-substituted products are useful, and 2,6-naphthalenedicarboxylic acid is particularly useful. These dialkylnaphthalenes are usually oxidized with molecular oxygen in the presence of an oxidation catalyst mainly containing heavy metals and bromine to obtain naphthalenedicarboxylic acid.

Examples of pyridines used as the mixed solvent in the method of the present invention include pyridine, methylpyridine, dimethylpyridine, ethylpyridine, diethylpyridine, ethylmethylpyridine, propylpyridine, isopropylpyridine, N-methylpyridine and N-. Examples include ethyl pyridine, N-propyl pyridine, N-isopropyl pyridine, collidine and the like. Of these, pyridine and methylpyridine are particularly preferably used.

The aliphatic amines used in the mixed solvent include, for example, methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, propylamine, isopropylamine, dipropylamine, diisopropylamine, butylamine, isobutylamine, sec-Butylamine, tert-butylamine, dibutylamine, diisobutylamine, tributylamine, pentylamine, dipentylamine, tripentylamine, 2-ethylhexylamine and other alkylamines, piperidine, N-methylpiperidine, pyrrolidine, ethyleneimine, hexa Examples thereof include alicyclic amines such as methyleneimine. Of these, ethylamines are preferred because of their ease of handling and availability, and triethylamine, which has a low decomposition temperature when forming an amine with naphthalenedicarboxylic acid, is particularly preferred.

In the method of the present invention, a mixed solvent containing at least one of the above aliphatic amines and pyridines is used. The mixing ratio of the solvent significantly affects the solubility of naphthalenedicarboxylic acid, and thus needs to be carefully controlled. The mixing ratio varies depending on the solvent species used, but the weight ratio of the pyridines to the aliphatic amines is usually 1 to 10 times, preferably 2 to 5 times.
The weight ratio of the mixed solvent of the pyridines and the aliphatic amines to the naphthalene dicarboxylic acid used is not particularly limited, but when carrying out the hydrotreatment, the amount sufficient to dissolve the crude naphthalene dicarboxylic acid at the reaction temperature is the minimum. is necessary. Since this amount varies depending on the type of solvent used and the mixing ratio, it is appropriately determined for each mixed solvent. The weight ratio of the mixed solvent to the crude naphthalene dicarboxylic acid is generally 1 to 100 times, preferably 2 to 20 times.

In the method of the present invention, it is desirable that the crude naphthalenedicarboxylic acid is dissolved in the mixed solvent and then hydrogenated. Hydrotreating is carried out by using Pt, Pd, R as a catalyst on a large surface area carrier such as activated carbon, silica, or alumina.
One in which at least one metal selected from h, Ru, Ni, and Co is dispersed is used. A catalyst in which Pd or Pt is dispersed on activated carbon is preferable. In the presence of these catalysts, a solution of crude naphthalenedicarboxylic acid dissolved in the solvent is hydrogenated. The reaction system may be a batch system or a continuous flow system, but industrially the continuous flow system is preferred. The reaction conditions for the hydrotreatment differ depending on the catalyst species used, the amount of catalyst used, and the residence time, but the reaction temperature is usually 100 to 250 ° C, and it is desirable that the temperature is the same as the temperature at which the crude naphthalenedicarboxylic acid is dissolved in the mixed solvent. . The hydrogen partial pressure is 0.01-30 kg / cm ² ,
It is preferably 0.01 to 10 kg / cm ² . Under severe reaction conditions at temperatures higher than 250 ° C, as a side reaction, formation of tetralindicarboxylic acid (TDCA) by nuclear hydrogenation of the naphthalene ring, or formation of naphthoic acid by decarboxylation and decarbonylation may occur. When naphthalene dicarboxylic acid is dissolved in a mixed solvent system of pyridine and amine, it can be dissolved at a relatively low temperature, and amine has an effect of suppressing the nuclear hydrogenation reaction, so that these side reactions were suppressed. Above, the target reaction can be performed highly selectively.

In the method of the present invention, a high-purity naphthalenedicarboxylic acid is obtained by recovering the amine salt of naphthalenedicarboxylic acid by crystallization and thermally decomposing the recovered amine salt. The crystallization method may be either a batch method or a continuous method, but the continuous method is superior when a large amount of industrial process is processed. Usually, crystallization is performed by the following method. First, naphthalenedicarboxylic acid is dissolved in the above-mentioned mixed solvent of pyridines and aliphatic amines. The temperature at that time is
The temperature is 80 to 300 ° C, preferably 100 to 250 ° C. The pressure in the system depends on the type of solvent used and the temperature and is not particularly limited. Subsequently, if necessary, hydrogenation treatment or decolorization treatment with activated carbon is performed, and insoluble matter is removed by filtration. Next, crystals are precipitated by cooling or evaporation of the solvent, and solid-liquid separation operation is performed by filtration or centrifugation. The crystallization temperature at this time is −
The temperature is 50 to 150 ° C, preferably -20 to 80 ° C. At the time of filtration, the weight ratio of the liquid to the crystals in the slurry to be filtered is
It is 1 to 100 times, preferably 3 to 10 times. If the weight ratio of the liquid is small, the recovery rate increases but the degree of purification decreases, and the slurry concentration is high, making the filtration operation difficult. On the contrary, when the weight ratio of the liquid is large, the degree of purification is increased, but the recovery rate is low and the circulation amount of the mother liquid is large, which is uneconomical. Therefore, the weight ratio of the liquid to the crystals in the slurry to be filtered is determined in consideration of the degree of purification and economy. The precipitated crystal is an amine salt of naphthalenedicarboxylic acid and contains impurities due to the inclusion of the mother liquor. Therefore, the crystal is washed with a solvent having a low solubility for the target substance. Usually, the aliphatic amine or pyridine used as the washing solvent may be used alone. The mother liquor and the washing liquid are usually recycled as they are as crystallization raw materials, but if necessary, they are reused after extracting impurities.

A high-purity naphthalenedicarboxylic acid can be obtained by heating the naphthalenedicarboxylic acid amine salt obtained by crystallization as it is or in a solvent. The heating temperature is 50 to 300 above the boiling point of the amine used.
C., preferably in the range of 100 to 250.degree. Heating above the required temperature causes decomposition or coloration of naphthalenedicarboxylic acid, and if it is too low, the decomposition rate is slow, which is economically disadvantageous. When the naphthalene dicarboxylic acid amine salt is heated as it is, it is desirable to reduce the pressure in the system or to pass an inert gas in order to avoid coloring. As the inert gas, for example, nitrogen, helium, argon or the like is used. By heating, the amine salt of naphthalenedicarboxylic acid is decomposed, and the amine generated as a gas can be recovered in almost the entire amount by cooling and collecting. The solvent used when the naphthalenedicarboxylic acid amine salt is heated in the solvent is not particularly limited as long as it has no reactivity with the naphthalenedicarboxylic acid or the used aliphatic amine under heating conditions. For example, durene, mesitylene, aromatic hydrocarbons such as toluene, saturated hydrocarbons such as liquid paraffin,
It is selected from esters, ethers and the like. The purified naphthalenedicarboxylic acid produced by heating can be recovered by an operation such as filtration or centrifugation. Also, the generated amine is continuously distilled off from the solvent during heating, or
A method of collecting the solvent from the solvent after filtering while hot is used.
Since the recovered amine is reused, it is desirable that the solvent used for heating does not generate an azeotrope with the amine.

In the method of the present invention, the pyridines used as the solvent are recovered as a filtrate during crystallization, the aliphatic amine is recovered by the method as described above, and if necessary, easily purified by a method such as distillation. Can be used in circulation. If a large amount of amines or pyridines remains in the purified naphthalene dicarboxylic acid, the quality of the polyester may be adversely affected, such as coloring of the polyester. Therefore, it is necessary to sufficiently wash the crystals.

[0021]

The present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples. In the following examples and comparative examples,
As for the purity and properties of the raw materials and the crystals of purified naphthalene dicarboxylic acid, organic substances were gas-esterified after methyl esterification, fluorescent X-ray analysis was used for inorganic substances, total nitrogen analysis for traces of residual nitrogen, and laser diffraction particle size distribution was used for particle size measurement. It analyzed by the measuring method. For the evaluation of hue, use a 0.5g sample.
It was dissolved in 10 g of dimethylacetamide, the insoluble material was removed by filtration, and the absorbance at a wavelength of 500 nm was measured using a 10 mm quartz cell. The abbreviations used in each table are as follows. 2,6-NDCA 2,6-naphthalenedicarboxylic acid 2-NA 2-naphthoic acid 2,6-MNA 2,6-methylnaphthoic acid 2,6-FNA 2,6-formylnaphthoic acid TMA trimellitic acid NTCA naphthalene tricarboxylic acid Acid Br-2,6-NDCA 2,6-Naphthalenedicarboxylic acid bromide TDCA Tetralindicarboxylic acid LE Low boiling HE High boiling TEA Triethylamine

Reference Example 1 Mixing ratio of triethylamine to pyridine was 0, 10,
80 ° C in a solvent that is 15, 20, 30, 40, 50, 100 wt%
The solubility data for 2,6-naphthalenedicarboxylic acid at 150 ° C is shown in Table 1 (for 20wt% solution, the data at 30 ° C and 0 ° C are also shown). When only pyridine is used as solvent (0 wt%)
Has a small temperature dependence of solubility, and hardly dissolves naphthalene dicarboxylic acid with TEA alone (100 wt%), but shows large temperature dependence and solubility in a solvent with an appropriate mixing ratio, and low solubility at low temperatures. .

[0023]

[Table 1]

Reference Example 2 Cobalt acetate 4 was added to a 5 L titanium autoclave equipped with a reflux condenser, a gas blowing tube, a stirrer and a temperature measuring tube.
Hydrochloride 3.8g, manganese acetate tetrahydrate 32.0g, hydrobromic acid
5.5 g, glacial acetic acid 1800 g and 99.5% pure 2,6-dimethylnaphthalene 180 g were charged, and the mixture was stirred while maintaining the reaction temperature at 200 to 210 ° C. and the reaction system internal pressure at 20 kg / cm ² . Then of the same purity
131 g of 2,6-dimethylnaphthalene was charged and 786 g of a catalyst solution having the same composition was fed for 2 hours, and at the same time, exhaust gas
It was extracted at a rate of 3.1 L / min, and compressed air was supplied while maintaining a constant pressure. After the completion of feeding, the mixture was kept for 30 minutes, the reactor was cooled to room temperature, the reaction mixture was recovered, washed with water and acetic acid, and then dried. As a result, the crude composition 2 shown in Table 2,
6-Naphthalenedicarboxylic acid was obtained with a yield of 85.6%.

[0025]

[Table 2]

Example 1 The crude 2,6-prepared in Reference Example 2 was placed in a 300 ml autoclave.
50 g of naphthalenedicarboxylic acid, 120 g of pyridine, 80 g of triethylamine, and 2.5 g of 1% Pd / C powder were charged, and after substituting with nitrogen, hydrogen was charged at 5 kg / cm ² and reacted at 150 ° C. for 2 hours. Table 3 shows the organic composition of the raw materials and the reaction product liquid. 2,6-
Hydrogenation of FNA and hydrogenolysis of Br-2,6-NDCA occurred, and side reactions such as slightly low boiling materials and high boiling materials also occurred.

Comparative Example 1 A reaction temperature of 280 ° C. was applied using the same equipment and charging conditions as in Example 1.
And reacted for 2 hours. Table 3 shows the composition of the reaction product liquid. Since the reaction conditions are harsh, the formation of 2-naphthoic acid and 2,6-methylnaphthoic acid and the increase of low-boiling substances and high-boiling substances are remarkable.

[0028]

[Table 3]

Example 2 Crude 2,6-naphthalenedicarboxylic acid 25.0 obtained in Reference Example 2
g, pyridine 80.1g, triethylamine 20.0g 150
It melt | dissolved at ℃, and the insoluble matter was filtered with the filter of 1 micrometer.
The filtrate was equipped with a stirrer, a filter and an outlet on the top.
The mixture was charged into a 300 ml SUS autoclave, purged with nitrogen, and then heated at 150 ° C for 2 hours. The mixture was cooled to 20 ° C. over 8 hours, the precipitated crystals were separated by filtration, and washed with 25 g of triethylamine. The crystals were heated at 150 ° C. for 3 hours with stirring under vacuum. 22.9 g of pure white purified 2,6-naphthalenedicarboxylic acid crystals were obtained (recovery rate 92.6%). Table 4 shows the results of the composition analysis of the raw material and the crystal together with the implementation conditions.

Example 3 Crude 2,6-naphthalenedicarboxylic acid 25.0 obtained in Reference Example 2
g, pyridine 80.0g, triethylamine 20.4g 150
The mixture was dissolved at ℃, filtered through a 1 μm filter, and the filtrate was charged into the 300 ml SUS autoclave used in Example 2, purged with nitrogen, and then stirred at 150 ℃ for 2 hours. Up to 20 ℃ 8
After cooling over time, the precipitated crystals were filtered and washed with 25 g of triethylamine. Add 100 g of mesitylene,
While distilling the gas phase from the top at a rate of 50 g / hr at 200 ° C,
The operation of adding the same amount of mesitylene was continuously performed for 5 hours. Filtration was performed at 200 ° C. while hot to obtain 23.2 g of pure white purified 2,6-naphthalenedicarboxylic acid (recovery rate 93.5%). The composition of the crystals is shown in Table 4.

Example 4 Crude 2,6-naphthalenedicarboxylic acid 25.0 obtained in Reference Example 2
g, γ-pyridine 80.0g, triethylamine 20.0g
Dissolve by heating at 120 ℃, filter with a 1μm filter, and transfer the filtrate to a 300ml glass 4-neck flask at atmospheric pressure at 120 ℃.
It was stirred for 2 hr. Cooling the filtrate to 0 ° C. over 8 hours,
The precipitated crystals were separated by filtration and washed with 25 g of triethylamine. The crystals were heated under vacuum for 3 hours at 150 ° C. with stirring, and 24.1 g of pure white purified 2,6-naphthalenedicarboxylic acid crystals were obtained (recovery rate 97.4%). The composition of the crystals is shown in Table 4.

[0032]

[Table 4]

Comparative Example 2 A 300 ml glass four-necked flask was charged with the same amount of pure water as 50.0 g of the crude 2,6-naphthalenedicarboxylic acid obtained in Reference Example 2 and 56 g (1.2 equivalents) of triethylamine, and the mixture was heated to 90 ° C. It melted by heating. The mixture was cooled to 0 ° C. over 8 hours, the precipitated crystals were filtered off, and washed with 50 g of triethylamine. 100 crystals
Transfer to a ml eggplant flask and stir under vacuum for 150
Heated at ℃ for 3 hr. 22.4 g of pure white purified 2,6-naphthalenedicarboxylic acid was obtained (recovery rate 45.3%). The composition of the crystals is shown in Table 5. The obtained crystals have high purity but low yield.

Comparative Example 3 10 g of crude 2,6-naphthalenedicarboxylic acid obtained in Reference Example 2
150g of pyridine and pyridine were heated and dissolved at 150 ℃, filtered through a 1μm filter, and the filtrate was transferred to a 300ml autoclave equipped with a stirrer, a filter and an outlet at the top.
The mixture was stirred at 0 ° C for 2 hr. Pyridine from the upper outlet
50 g was distilled off, the mixture was further cooled to 20 ° C., and the precipitated crystals were separated by filtration. 6.8 g of slightly colored crystals were obtained (recovery rate 68.7
%). The composition of the crystals is shown in Table 5. The crystals are colored,
The yield is low and a large amount of solvent is required.

[0035]

[Table 5]

[0036]

As is apparent from the examples, by purifying the crude naphthalenedicarboxylic acid obtained by oxidizing the dialkylnaphthalene by the method of the present invention, highly pure naphthalenedicarboxylic acid can be easily recovered with extremely high recovery rate. Obtainable. Since the pyridines and the aliphatic amines used as the solvent can be easily recycled, the method of the present invention is an industrially advantageous method, and its industrial significance is great.

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Claims (2)

[Claims] 1. A crude naphthalenedicarboxylic acid obtained by an oxidation reaction of a dialkylnaphthalene is dissolved in a mixed solvent of pyridines and aliphatic amines, and a naphthalenedicarboxylic acid amine salt is recovered by crystallization, and the recovered amine is recovered. Method for purifying naphthalenedicarboxylic acid characterized in that salt is decomposed by heating 2. Hydrogenation at a temperature of 250 ° C. or lower in the presence of a catalyst containing at least one metal selected from Pt, Pd, Rh, Ru, Ni and Co after dissolving crude naphthalene dicarboxylic acid in a mixed solvent. The method according to claim 1, wherein the treatment and crystallization are performed.