Process for the production of high-purity dimethyl 2,6-naphthalenedicarboxylate and naphthalenedicarboxylic acid

Abstract: A process for the production of dimethyl 2,6-naphthalenedicarboxylate, which process comprises esterifying with methanol 2,6-naphthalenedicarboxylic acid obtained by liquid phase oxidation of 2,6-dialkylnaphthalene with molecular oxygen in the presence of an oxidation catalyst, and then purifying the thus obtained crude ester in the presence of an aromatic hydrocarbon solvent, in which process impurities contained in a reaction product can be effectively removed with simple procedures, and a process for the production of naphthalenedicarboxylic acid, which process comprises oxidizing dialkylnaphthalene and/or an oxide thereof with an oxygen-containing gas in a solvent containing a lower aliphatic carboxylic acid and in the presence of an oxidation catalyst comprising at least cobalt, manganese and bromine, the oxidation being carried out using, as a catalyst component, a solid naphthalenedicarboxylic acid metal salt.

Description: The present invention relates to a process for the production of dimethyl 2,6-naphthalenedicarboxylate. More specifically, it relates to a process for the production of high-purity dimethyl 2,6-naphthalenedicarboxylate (to be sometimes referred to as "2,6-NDCM" hereinafter), in which 2,6-naphthalenedicarboxylic acid (to be sometimes referred to as "2,6-NDCA" hereinafter) obtained by liquid phase oxidation is esterified with methanol and then resultant ester is purified (to be referred tows "present invention" hereinafter). 2,6-NDCA and 2,6-NDCM in particular, are useful materials as materials for a high-function polyester. Related art of the present invention will be explained below. 2,6-NDCA is obtained by a method in which 2,6-dialkylnaphthalene is oxidized in solvent containing a lower aliphatic carboxylic acid in the presence of an oxidation catalyst containing cobalt, manganese and bromine (JP-B-34-2666 and JP-B-56-3337). 2,6-NDCA is also obtained by a method in which liquid phase 2,6-dimethylnapthalene is oxidised in the presence of a cobalt/manganese/bromine catalyst (US-A-4 886 901). 2,6-NDCA obtained by the above method contains organic impurities such as trimellitic acid, 6-formyl-2-naphtoic acid, etc., and heavy metals such as cobalt, manganese, etc., from the catalyst. 2,6-NDCA can be suitably used as a material for a high-function polymer only after the above impurities, etc., are removed. Since, however, 2,6-NDCA has a low solubility in a solvent and is decomposed at a melting point, it is difficult to purify 2,6-NDCA while it remains intact. There have been therefore proposed many methods in which 2,6-NDCA is esterified with methanol to form 2,6-NDCM and then 2,6-NDCM is distilled and/or recrystallized. In US A 5 262 560, molten dimethyl 2,6-naphthalene dicarboxylate is present in the esterification media. For obtaining particularly high-quality 2,6-NDCM, it is effective to employ a method in which the purification is carried out by a combination of distillation and recrystallization (JP-A-50-116461, JP-B-57-35697, EP-A-0 721 931 and JP-B-46-9697). Further, the above heavy metals such as cobalt, manganese, etc., contained in 2,6-NDCA are expensive, and it is industrially preferred to recover and recycle them. The method of recovering oxidation catalyst metals contained in 2,6-NDCA is largely classified to a method in which a 2,6-NDCA crystal is washed with a solvent such as a mineral acid aqueous solution or an alcohol containing an acid component to elute a metal content and the metal content is recovered (JP-A-62-212345 and JP-A-5-253496) and a method in which 2,6-NDCA is esterified and the heavy metals are recovered from the reaction mixture (JP-A-3-223233 and JP-A-4-364152). When the former method in which a 2,6-NDCA crystal is washed with a solvent to elute catalyst metals is used as a method for recovering the oxidation catalyst contained in 2,6-NDCA, there is required an apparatus for separating a wash liquid and the 2,6-NDCA crystal. There is another method for recovering a metal content from a wash liquid, in which a carbonic acid compound is added to form an insoluble carbonate. However, there is required a solid-liquid separation apparatus for separating a precipitate of the above insoluble carbonate, and the apparatus is complicated. Further, the ratio of removal of the metal content by the washing is not so high, so that the washed 2,6-NDCA still contains a large amount of metals, and it is further required to remove the metal content by some other purification method. On the other hand, in the method of JP-A-3-223233 which uses means of recovering the catalyst metals from a crude ester obtained by esterification of 2,6-NDCA, the oxidation catalyst metals are dissolved in a solvent by using, as a catalyst, a mineral acid such as sulfuric acid for the esterification. Then, a compound which generates carbonate ion is added to mother liquor from which the esterification product is separated, and the catalyst metals are recovered in the form of an insoluble carbonate. The above method has problems that reactor materials are limited due to the corrosiveness of the mineral acid such as sulfuric acid and that it is required to treat waste acid by neutralization. The above method is therefore not suitable for use in a large-scale industrial plant. In the method disclosed in EP-A-0 450 621, the oxidation catalyst metals are precipitated as carbonates. These are dissolved in water and then recycled to the oxidation reactor. In the method disclosed in JP-A-4-364152, insoluble oxidation catalyst metals are separated from the esterification product in a molten state or a solution thereof in methanol by filtration or centrifugalization. For maintaining the esterification product in a molten state, however, it is required to heat the esterification product to a temperature equivalent to, or higher than, the melting point (about 190°C) of 2,6-NDCA, and it is technically very difficult to separate and recover the insoluble catalyst metals from the molten liquid by filtration and centrifugalization on an industrial scale. In the method of separating the insoluble oxidation catalyst metals from the methanol solution, the solubility of 2,6-NDCM is very low at the boiling point of methanol as shown in Examples to be described later, and it is therefore required to heat the methanol solution to a temperature higher than the boiling point of methanol under elevated pressure in order to dissolve the esterification product containing 2,6-NDCM as a main component in an industrially practical amount of a solvent. Further, it is also required to separate the catalyst metals under the above elevated pressure, and the cost for facilities required therefor increases. Further, the present inventor's studies have revealed the following. A crystal of 2,6-NDCM obtained by recrystallization from the methanol solution has the form of scales and has a small bulk density, and it brings a large amount of mother liquor into a cake when the solid-liquid separation is carried out by a method of filtration or centrifugalization, so that no sufficient effect of removing impurities can be obtained. It is an object of the present invention to provide a process for the production of high-purity 2,6-NDCM, in which impurities contained in a reaction product obtained by the esterification of 2,6-NDCA prepared by the liquid phase oxidation of 2,6-dialkylnaphthalene are effectively removed with simple procedures. It is further another object of the present invention to provide a process for industrially advantageously producing 2,6-NDCM by recovering oxidation catalyst metals in the above process. According to the present invention, there is provided a process for the production of 2,6-NDCM, which comprises esterifying with methanol 2,6-naphthalenedicarboxylic acid obtained by liquid phase oxidation of 2,6-dialkylnaphthalene with molecular oxygen in the presence of an oxidation catalyst, the esterification being carried out by mixing 2,6-naphthalenedicarboxylic acid in molten 2,6-NDCM and continuously supplying vaporized methanol to the resultant mixure and then purifying the thus obtained crude ester, by mixing the crude ester formed by the esterification in the above process with an aromatic hydrocarbon solvent, heating the mixture to dissolve the crude ester in the aromatic hydrocarbon solvent, removing an insoluble contained in the solution of the crude ester by filtering or centrifugalizing, cooling a solution of the crude ester after the removal of the insoluble, to recrystallize and separate dimethyl 2,6-naphthalenedicarboxylate, and distilling the separated crystal of dimethyl 2,6-naphthalenedicarboxylate under reduced pressure to remove contents having high-boiling points, the contents having high boiling points being recycled to the esterification step. According to the present invention, there is also provided a process for the production of high-purity 2,6-NDCM, which comprises distilling the crude ester formed by the esterification in the above process, in the presence of an aromatic hydrocarbon solvent by distilling the crude ester under reduced pressure to remove contents having high boiling points and recrystallizing distillate 2,6-NDCM in an aromatic hydrocarbon solvent to purify the 2,6-NDCM, as defined in claim 2. According to the present invention there is also provided a process for the production of high-purity dimethyl 2,6-naphthalenedicarboxylate, which comprises mixing the contents having high boiling points separated in the above process with an aromatic hydrocarbon solvent, heating the mixture to dissolve the contents in the aromatic hydrocarbon solvent, removing an insoluble in the solution by filtering or centrifugalizing, cooling the resultant solution free of the insoluble to precipitate a crystal of dimethyl 2,6-naphthalenedicarboxylate and recovering the dimethyl 2,6-naphthalenedicarboxylate. According to the present invention further, there is provided a process for the production of high-purity dimethyl 2,6-naphthalenedicarboxylate, which comprises separating and recovering the oxidation catalyst contained in the crude ester in the above process, as a substance insoluble in an aromatic hydrocarbon solvent, and recycling the recovered insoluble substance as a catalyst source for the oxidation. Fig. 1 shows a flow chart of a case where a crude ester from which insoluble catalyst metals are removed is purified and then purified by distillation according to a method (A) in the present invention. Fig. 2 shows a flow chart of a case where a crude ester is distilled and then purified by recrystallization according to a method (B) in the present invention. The present inventors have made diligent studies of methods of purifying 2,6-NDCA by the esterification of the 2,6-NDCA with methanol and, as a result, have found the following. When 2,6-NDCA is purified by recrystallization in the presence of an aromatic hydrocarbon as a solvent, 2,6-NDCA having excellent qualities can be obtained. When an insoluble oxidation catalyst content contained in an esterification product from 2,6-NDCA is recovered, an oxidation catalyst metal salt can be easily recovered at a high yield by dissolving the esterification product or contents having high boiling points obtained by distilling the esterification product in an aromatic hydrocarbon as a solvent and treating the resultant solution by filtering or centrifugal precipitation. The present invention has been thus arrived at. That is, the present invention is directed to a process for the production of high-purity dimethyl 2,6-naphthalenedicarboxylate by oxidizing 2,6-dialkylnaphthalene with molecular oxygen in a liquid phase oxidation, to obtain 2,6-naphthalenedicarboxylic acid, esterifying the 2,6-naphthalenedicarboxylic acid with methanol to obtain a crude ester and purifying the crude ester, wherein the crude ester is purified in the presence of an aromatic hydrocarbon as a solvent. Specifically, in the present invention, the process for the production of high-purity 2,6-NDCM comprises oxidizing 2,6-dialkylnaphthalene with a gas containing molecular oxygen in a solvent containing a low aliphatic carboxylic acid in the presence of a catalyst composed of a heavy metal compound and a bromine compound, to prepare 2,6-NDCA, esterifying the 2,6-NDCA with methanol to obtain a crude ester, and purifying the crude ester by recrystallization and distillation, and the present invention 1 has a characteristic feature in that an aromatic hydrocarbon is used as a solvent for the separation of an insoluble contained in the crude ester and the recrystallization of the crude ester. The process for the production of high-purity 2,6-NDCM according to the present invention comprises 1 ○ the step of oxidizing 2,6-dialkylnaphthalene to obtain 2,6-NDCA, 2 ○ the step of esterifying the 2,6-NDCA to obtain a crude ester, 3 ○ the step of dissolving the crude ester, 4 ○ the steps of separating an insoluble, 5 ○ the step of recrystallization and 6 ○ the step of distillation. Each step will be explained in detail hereinafter. (Oxidation step) The dialkylnaphthalene used as a raw material for the oxidation in the present invention includes dimethylnaphthalene, diethylnaphthalene and diisopropylnaphthalene. The oxide derivative thereof includes oxidation intermediates of the above dialkylnapthalenes such as formylnaphthoic acid and acetylnaphthoic acid, alkylacylnaphthalenes such as methyl acetylnaphthalene and methylbutylylnaphthalene, and diacylnaphthalene. Of the above dialkylnaphthalenes and oxide derivatives thereof, 2,6-substituted compounds are industrially useful. The 2,6-dialkylnaphthalene includes 2,6-dimethylnaphthalene, 2,6-dietnylnaphthalene and 2,6-diisopropylnaphthalene. The solvent for the oxidation is preferably selected from lower aliphatic carboxylic acids such as formic acid, acetic acid, propionic acid and butyric acid, and acetic acid is the most preferred. While the solvent may contain water, the content thereof is 30 % by weight or less. The amount of the solvent based on the weight of the 2,6-dialkylnaphthalene as a raw material for the oxidation, is 1 to 20 times, preferably 3 to 10 times. As a heavy metal oxidation catalyst used for the oxidation, a cobalt compound, a manganese compound and a bromine compound are used. Compounds of heavy metals such as iron, cerium and nickel may be added as required. The above compounds of cobalt, manganese, iron, cerium and nickel include organic acid salts, hydroxides, halides and carbonates. An acetic acid salt and a bromide are particularly preferred. The bromine compound may be any compound so long as it is dissolved in a reaction system to generate bromide ion. The bromine compound includes inorganic bromides such as hydrogen bromide, sodium bromide, cobalt bromide and manganese bromide and organic bromides such as tetrabromoethane. Hydorgen bromide, cobalt bromide and manganese bromide are particularly preferred. The metal catalyst is used in such an amount that the molar ratio of the total amount of heavy metal components including cobalt and manganese to the amount of the 2,6-dialkylnaphthalene as an oxidation material is 0.02 to-0.5, preferably 0.03 to 0.3, more preferably 0.04 to 0.2. Further, the bromine is used in such an amount that the molar ratio of bromine to the oxidation material is 0.01 to 0.3, preferably 0.015 to 0.15, more preferably 0.02 to 0.1. The molecular-oxygen-containing gas used for the oxidation includes oxygen gas and a mixture prepared by mixing oxygen gas with an inert gas such as nitrogen or argon. Air is the most generally used. The temperature for the oxidation is 170 to 250°C, preferably 180 to 240°C, more preferably 190 to 230°C. When the oxidation temperature is low, the amount of an intermediate increases, and the purity of NDCA decreases. When the temperature is too high, the combustion loss of the lower aliphatic carboxylic acid as a solvent increases, and the yield of NDCA decreases. The pressure for the oxidation is 5 to 40 kg/cm 2 G, preferably 10 to 30 kg/cm 2 G. The oxidation can be carried out by any one of a batch method, a semi-batch method and a continuous method. A reactor for the oxidation is provided with a reflux condenser for condensing a large amount of the solvent which is brought together with a discharged gas and water formed by the oxidation. While the condensed solvent and the water are generally recycled to the reactor, part of them is withdrawn from the reaction system for adjusting the water concentration in the reactor. The oxygen concentration of the discharged gas from the reactor is 0.1 to 8 % by volume, preferably 1 to 5 % by volume. A 2,6-NDCA crystal formed by the oxidation is separated from the solvent with a solid-liquid separator. The separator includes a centrifugal precipitator, a centrifugal filter and a vacuum filter. The minimum size of a crystal that can be separated with the above separator is generally 5 µm or larger when a decanter-type centrifugal precipitator is used, and it is 10 µm to 20 µm or more when a centrifugal filter or a vacuum filter is used. (Esterification step) In the present invention, the esterification of 2,6-NDCA with methanol can be carried out by any known method. For example, preferred is a method in which 2,6-NDCM in a molten state is used as a solvent and vaporized methanol is supplied to a slurry prepared by dispersing a crystal of 2,6-NDCA in the 2,6-NDCM. The esterification is preferably carried out by a semi-continuous method or a continuous method. In the semi-continuous method, 2,6-NDCA and 2,6-NDCM are charged into a reactor and heated to a predetermined temperature to form a liquid phase, vaporized methanol is continuously supplied to the liquid phase and excessive vaporized methanol is continuously withdrawn from a gaseous phase portion of the reactor together with water formed by the reaction so as to maintain a constant pressure. In the continuous method, 2,6-NDCA is dispersed in 2,6-NDCM in a molten state to form a slurry, the slurry is continuously supplied, and a reaction product is continuously withdrawn. Methanol is supplied to a liquid phase similarly to the semi-continuous method, and excessive methanol is withdrawn from a gaseous phase together with water formed by the reaction. For obtaining 2,6-NDCM at high reaction yields, preferably, at least two vessel-shaped reactors are used and 2,6-NDCA and methanol are brought into contact countercurrently, or a plate column or a bubble cup column of which the inside is divided in multi-stages is used and a liquid and a gas is brought into contact countercurrently. The temperature for the esterification is 190 to 320°C, preferably 230 to 300°C, and the pressure for the esterification is 2 to 40 kg/cm 2 G, preferably 10 to 30 kg/cm 2 G. As a catalyst for the esterification, it is preferred to use a molybdenum compound such as molybdenum trioxide. However, when the reaction is carried out at a high temperature of 240°C or higher, the esterification can be carried out in the absence of a catalyst. The use of a mineral acid such as sulfuric acid is not preferred, since it makes the oxidation metal catalyst soluble. As a solvent for the esterification, 2,6-NDCM is used, and part of the crude ester obtained by the esterification is recycled. The amount of the crude ester to be recycled is properly 2 to 6 times the weight of 2,6-NDCA. For improving the yield of 2,6-NDCM, it is preferred to use methanol in an amount greater than its stoichiometric amount, and the molar ratio of methanol to supplied 2,6-NDCM is 5 to 40, preferably 10 to 25. In the semi-continuous method, excessive methanol contained in a reaction product is removed by evaporation by reducing the pressure of the reactor to atmospheric pressure, and in the continuous method, excessive methanol contained in a reaction product is removed by evaporation by withdrawing the reaction product into a vessel held under atmospheric pressure conditions, whereby a crude ester in a molten state can be obtained. According to the above reaction methods, 2,6-NDCM can be obtained at high yields under relatively low reaction pressure by the use of a small amount of methanol. Further, since methanol contained in the reaction product can be easily removed, a crude ester suitable for use in the subsequent purification step can be obtained. (Step of dissolving crude ester and step of separating insoluble) The characteristic feature of the present invention is that a crude ester formed by the esterification is dissolved in an aromatic hydrocarbon as a solvent to separate an insoluble. The above insoluble is separated by a method (A) in which a crude ester formed by the esterification is mixed with an aromatic hydrocarbon, the mixture is heated to dissolve the total amount of 2,6-NDCM contained in the crude ester in the aromatic hydrocarbon, and then the insoluble contained in the crude ester is separated by filtration or centrifugal precipitation, or by a method (B) in which a crude ester formed by the esterification is distilled under reduced pressure to separate it into a fraction of 2,6-NDCM having an increased concentration and concentrated contents having high boiling points such as catalyst metals, monomethyl 2,6-naphthalenedicarboxylate and organic impurities having a high boiling point, then, the contents having high boiling points are dissolved in the above aromatic hydrocarbon solvent and an insoluble containing the catalyst metals are separated and recovered by the same method as that described above. In the latter method (B), the metal contents are concentrated and the amount of the solvent is decreased, so that the catalyst metals can be recovered with a smaller separator device. The fraction from the distillation is dissolved in an aromatic hydrocarbon solvent and used in the recrystallization step to be described later. The aromatic hydrocarbon used as the solvent is preferably selected from hydrocarbons having a boiling point in the range of from 100 to 170°C, such as toluene, xylenes and trimethylbenzenes. Toluene and xylenes are particularly preferred. The amount of the solvent is required to be sufficient for dissolving the total amount of 2,6-NDCM contained in the crude ester and for obtaining a sufficient purification effect in the subsequent purification step. However, when it is used excessively, a large-capacity apparatus is required. The amount of the solvent is therefore 2 to 15 times, preferably 3 to 10 times, as large as the weight of the crude ester. When the aromatic hydrocarbon is used as a solvent, the crude ester can be dissolved, and an insoluble can be separated, at a temperature lower than the boiling point of the solvent, and the dissolving and separation procedures can be carried out with a simple apparatus as compared with a case where methanol is used which requires the dissolving and separation procedures under elevated pressure. The insoluble recovered by the above method contains almost all of oxidation catalyst metals which have been contained in NDCA used as a raw material for the esterification. In the above insoluble, a major part other than the catalyst metals used for the oxidation and the esterification is composed of NDCA, and it contains almost no trimellitic acid which works as an inhibiting substance against a reaction in the oxidation. The above insoluble can be therefore suitably used as a catalyst source for the oxidation. The above insoluble is used as a catalyst source for the oxidation by a method in which the insoluble is directly supplied to an oxidation reactor while it is in the form of a solid or a method in which the insoluble is mixed with water or a lower aliphatic carboxylic acid containing water (preferably acetic acid containing at least 1 % by weight of water) to elute oxidation catalyst metals, the mixture is separated into a solid and a liquid, a crystal is removed and a mother liquor is supplied to the oxidation. (Recrystallization step) When the above method (A) is employed, the purification of NDCM by recrystallization is carried out as follows. 2,6-NDCM is purified by cooling the aromatic hydrocarbon solution containing the crude ester from which insoluble catalyst metal contents have been removed, to precipitate a crystal of 2,6-NDCM and separating the solution into the crystal and a mother liquor by filtration or centrifugal precipitation. When the above method (B) is employed, the recrystallization is carried out in the following two procedures. In one procedure, the crude ester formed by the esterification is distilled under reduced pressure to remove contents having high boiling points and 2,6-NDCM as a distillate is purified by recrystallization from an aroamtic hydrocarbon solvent, and in the other procedure, contents having high boiling points are mixed with an aromatic hydrocarbon solvent, the mixture is heated to dissolve the contents having high boiling points in the aromatic hydrocarbon solvent, the resultant aromatic hydrocarbon solution is filtered or centrifugalized to remove an insoluble, and the remaining solution is cooled to precipitate a crystal. The temperature for the above recrystallization is preferably in the range of from 20 to 70°C. The recrystallization can be carried out by any one of a batch method and a continuous method. In a batch method, the rate of cooling the solution is preferably 20°C/minute or smaller, and in the continuous method, at least two crystallization vessels are preferably used to precipitate a crystal stepwise. A separated crystal may be re-dissolved in a solvent as required and recrystallized to obtain a crystal having a higher purity. In this case, a separated mother liquor obtained by recrystallization in the above second step is used as a solvent in the step of dissolving the product formed by the above esterification, whereby the purification by recrystallization in two steps can be carried out without increasing the amount of the solvent. In the method (B), the mother liquor separated in the recrystallization step can be used as a solvent in the step of dissolving contents having high boiling points obtained by the distillation, and a crystal obtained in the step of recovering a crystal from the contents having high boiling points can be recycled to the esterification. (Distillation step) While the distillation column used for purification by distillation includes a packed column, a plate column and a perforated plate column, it is preferred to use a distillation column having a structure in which a pressure difference between a column top and a column bottom is as small as possible. The distillation is preferably carried out under a reduced pressure of 1 to 50 mmHg at a temperature between 210 and 280°C. When the temperature for the distillation is lower than 210°C, the distillation is difficult since the vapor pressure of 2,6-NDCM is low. When the above temperature is higher than 280°C, undesirably, 2,6-NDCM is decomposed. The contents having high boiling point, separated by distillation in the above method (A), contain monomethyl 2,6-naphthalenedicarboxylate and 2,6-NDCA, and the yield in the process can be improved by recycling the above contents having high boiling points to the esterification. Further, the contents having high boiling points, separated by distillation in the above method (B), further contain catalyst components used for the esterification, and part of the contents having high boiling points can be recycled to the esterification. As explained, the process for the production of high-purity 2,6-NDCM includes a method (A) in which the crude ester from which insoluble catalyst metals are removed is purified by recrystallization and then purified by distillation and a method (B) in which the crude ester is distilled and then purified by recrystallization. The method (A) comprises 1 ○ the step of oxidizing 2,6-dialkylnaphthalene to obtain 2,6-NDCA, 2 ○ the step of esterifying the 2,6-NDCA with methanol to obtain a crude ester, 3 ○ the dissolving step of mixing the crude ester with an aromatic hydrocarbon solvent and heating the mixture to dissolve 2,6-NDCM of the crude ester in the aromatic hydrocarbon solvent, 4 ○ the step of treating a solution of the crude ester in the aromatic hydrocarbon solvent by filtration or precipitation to separate and remove an insoluble of the crude ester, 5 ○ the recrystallization step of cooling the resultant insoluble-free solution to precipitate a crystal of 2,6-NDCM and separating the solution into a solid and a liquid to obtain recrystallized 2,6-NDCM and 6 ○ the distillation step of distilling the recrystallized 2,6-NDCM under reduced pressure to remove contents having high boiling points and obtain high-purity 2,6-NDCM. The method (B) comprises 1 ○ the step of oxidizing 2,6-dialkylnaphthalene to obtain 2,6-NDCA, 2 ○ the step of esterifying the 2,6-NDCA with methanol to form a crude ester, 3 ○ the step of distilling the crude ester under reduced pressure to remove contents having high boiling points, 4 ○ the step of recrystallizing 2,6-NDCM as a distillate from an aromatic hydrocarbon solvent to obtain high-purity 2,6-NDCM, 5 ○ the dissolving step of mixing the contents having high boiling points, separated by the distillation, with an aromatic hydrocarbon solvent and heating the mixture to dissolve the contents having high boiling points in the aromatic hydrocarbon solvent, 6 ○ the step of treating a solution of the contents having high boiling points in the aromatic hydrocarbon by filtration or precipitation to separate and remove an insoluble, and 7 ○ the crystal recovery step of cooling the resultant insoluble-free solution to precipitate a crystal and separating the crystal from the solution. In any of the above two methods, there can be obtained 2,6-NDCM having a low inorganic impurity content and a low acid value and having an excellent color value and excellent qualities, and effective components of the oxidation catalyst can be effectively recovered. Fig. 1 shows an example of flow chart of a case according to the method (A), where a crude ester from which insoluble catalyst metals are removed is purified by recrystallization and then a crystal is purified by distillation. In Fig. 1, 2,6-dialkylnaphthalene is introduced into an oxidation reactor 1 through a line 11, and a solvent of the oxidation and a catalyst are introduced into the oxidation reactor 1 through a line 12. In the oxidation reactor 1, an oxygen-containing gas (air) is introduced through a line 13, the 2,6-dialkylnaphthalene is oxidized, and an off-gas is discharged through a line 15. 2,6-NDCA formed by the oxidation is introduced into a solid-liquid separator 2 through a line 14, and separated 2,6-NDCA is dried with dryer 3 and introduced into an esterification reactor 4 through a line 18. A liquid separated in the solid-liquid separator 2 is recycled to the oxidation reactor 1 through a line 17. In the esterification reactor 4, 2,6-NDCA is esterified with methanol introduced through a line 19, and unreacted methanol and formed water are discharged through a line 20. A crude ester from the esterification reactor 4 is introduced into an ester-dissolving vessel 5 through a line 21, and dissolved in an aromatic hydrocarbon solvent introduced from a line 22. The resultant crude ester solution is introduced into a solid-liquid separator 6 through a line 23, and an insoluble containing catalyst components is recycled to the oxidation reactor 1 through a line 25. A liquid separated in the solid-liquid separator 6 is introduced into a solid-liquid separator 8 through a line 24 and a crystallization vessel 7 to be separated into a recrystallization separated cake which is to be introduced into a dryer 9 through a line 26 and a separated liquid from a line 27. The recrystallization separated cake is dried in the dryer 9 and introduced into a distillation column 10, and high-purity 2,6-NDCM as a product is obtained as a distillate through a line 28. Contents having high boiling points, separated in the distillation column 10, are recycled to the esterification reactor 4 through a line 29. Fig. 2 shows an example of flow chart of a case according to the method (B), where a crude ester is distilled and the purification is carried out by recrystallization. In Fig. 2, 2,6-dialkylnaphthalene as a raw material is introduced into an oxidation reactor 1 through a line 11, and a solvent for the oxidation and a catalyst are introduced into the oxidation reactor 1 through a line 12. In the oxidation reactor 1, an oxygen-containing gas (air) is introduced through a line 13, the 2,6-dialkylnaphthalene is oxidized, and an off-gas is discharged through a line 15. 2,6-NDCA formed by the oxidation is introduced into a solid-liquid separator 2 through a line 14, and separated 2,6-NDCA is dried in a dryer 3 and introduced into an esterification reactor 4 through a line 18. A liquid separated in the solid-liquid separator 2 is recycled to the oxidation reactor 1 through a line 17. In the esterification reactor 4, 2,6-NDCA is esterified with methanol introduced through a line 19, and unreacted methanol and formed water are discharged through a line 20. A crude ester formed in the esterification reactor 4 is introduced into an ester distillation column 10 through a line 21. A distillate from the ester distillation column 10 is introduced into an ester-dissolving vessel 5 through a line 34, and dissolved in an aromatic hydrocarbon solvent introduced through a line 22. The resultant solution is introduced into a solid-liquid separator 8 through a line 23 and a crystallization vessel 7. In the solid-liquid separator 8, the solution is separated to a recrystallization-separated cake to be sent through a line 26 and a separated liquid to be sent through a line 27. The recrystallization-separated cake is dried in a dryer 9, and high-purity 2,6-NDCM as a product is obtained through a line 28. Contents having high boiling points, i.e., residue, in the ester distillation column 10 are introduced into an ester 31 through a line 29 and dissolved in an aromatic hydrocarbon solvent from a line 35 (separated liquid from the line 27), and the resultant solution is introduced into a solid-liquid separator 6 through a line 36. In the solid-liquid separator 6, an insoluble containing catalyst components is recycled to the oxidation reactor 1 through a line 25. A separated liquid from the solid-liquid separator 6 introduced into a crystallization vessel 32 and a solid-liquid separator 33 through a line 37, and a recovered crystal is recycled to the esterification reactor 4 through a line 38. According to the present invention, there is provided a process which enables the production of remarkably high-purity dimethyl 2,6-naphthalenedicarboxylate from a crude ester obtained by the esterification of 2,6-naphthalenedicarboxylic acid with methanol. According to the production process provided by the present invention, further, catalyst metals used for the oxidation are recovered at high yields and can be recycled to the oxidation. Therefore, dimethyl 2,6-naphthalenedicarboxylate can be remarkably industrially advantageously produced according to the present invention, and the present invention is industrially highly significant. Examples The present invention will be more specifically explained with reference to Examples hereinafter. However, the present invention shall not be limited by these Examples. Table 1 shows measurement values of solubility of 2,6-NDCM in various solvents. It is shown that as compared with the solubility of 2,6-NDCM in aromatic hydrocarbons, the solubility of 2,6-NDCM in methanol at the boiling point of methanol is very low. Solvent Normal boiling point Solubility of 2,6-NDCM at a boiling point Methanol 65°C 0.67 g/100 g solvent Toluene 111°C 17.5 g/100 g solvent m-Xylene 139°C 40 g/100 g solvent o-Xylene 144°C 52 g/100 g solvent Example 1 (Method (A)) (1) Oxidation Cobalt acetate tetrahydrate, manganese acetate tetrahydrate, a 47 wt% hydrogen bromide aqueous solution and water were mixed with, and dissolved in 250 kg of glacial acetic acid to obtain a catalyst solution having a cobalt concentration of 0.1 % by weight, a manganese concentration of 0.6 % by weight, a bromine concentration of 0.5 % by weight and a water concentration of 3 % by weight. A 60-liter reactor made of titanium and equipped with a stirrer and a reflux condenser was charged with 32 kg of the above catalyst solution with a pump. Further, a vessel other than the reactor was charged with the catalyst solution of 2,6-dimethylnaphthalene having a purity of 99.7 % by weight, and 2,6-dimethylnaphthalene was melted under heat at a temperature of 120°C or higher. The pressure inside the reactor was adjusted to 20 kg/cm 2 G with nitrogen, and the catalyst solution was heated to 210°C with stirring. After the temperature and the pressure were stabilized, 2,6-dimethylnaphthalene was supplied to the reactor at a rate of 6 kg/hour, and at the same time, compressed air was supplied to the reactor at a rate of about 17 Nm3/hour to initiate an oxidation. When 2,6-dimethylnaphthalene was supplied up to an amount of 8 kg (80 minutes after the initiation of the reaction), the supply of the above catalyst solution was initiated at a rate of 24 kg/hour, and a reaction product was withdrawn to a receiving vessel under atmospheric pressure such that the liquid level in the reactor was constant. The reaction was continued for about 8 hours, the supply of 2,6-dimethylnaphthalene, the catalyst solution and air was terminated to finish the reaction. A product remaining in the reactor was also withdrawn to the receiving vessel, to give 295 kg of a reaction product. The above reaction product was separated into a cake and a mother liquor with a decanter type centrifugal separator. The separated cake was dried to give 74.8 g of a crude 2,6-NDCA crystal. The liquid content of the separated cake, calculated on the basis of a drying loss, was 39 % by weight (based on wet weight). Table 2 shows composition of the crude 2,6-NDCA. The yield of the 2,6-NDCA on the basis of the supplied 2,6-dimethylnaphthalene was 92.3 mol%. Further, 48 % by weight of the catalyst metals (total of cobalt and manganese) used for the reaction remained in the formed crude 2,6-NDCA crystal. composition of crude 2,6-NDCA wt% 2, 6-NDCA 94.8 2-formyl-6-naphthoic acid 0.29 Trimellitic acid 2.4 Cobalt 0.098 Manganese 0.93 (2) Esterification A 6-liter autoclave made of stainless steel and equipped with a stirrer was charged with 1.2 kg of the crude 2,6-NDCA obtained by the oxidation, 3.6 kg of a crude ester obtained by esterifying the above crude 2,6-NDCA in advance, and 1.2 g of molybdenum trioxide, and after nitrogen substitution, the contents in the autoclave were heated to 270°C with stirring. Methanol was supplied from a nozzle positioned in a bottom portion of the reactor at a rate of 1.06 kg/hour, to initiate an esterification. When the pressure reached 15 kg/cm 2 G, excess methanol and formed water were withdrawn through a vapor-withdrawing line provided in an upper portion of the reactor, and the reaction was continued for about 3 hours while maintaining the pressure at a constant level. After completion of the reaction, a crude ester was withdrawn. Further, an ester content withdrawn out of the reactor together with methanol vapor was recovered in the form of a crystal by evaporating the methanol. The total amount of the crystal recovered from the methanol and the crude ester in a molten state was 4.93 kg. Table 3 shows the composition of the so-obained crude ester. The yield of the 2,6-NDCM was 92.8 mol%. Composition of crude ester wt% 2, 6-NDCM 89.3 Monomethyl 2,6-naphthalenedicarboxylate 1.9 2, 6-NDCA 3.8 Methyl 2-formyl-6-naphthoate 0.13 Trimethyl trimellitate 2.3 Cobalt 0.088 Manganese 0.83 Molybdenum 0.057 (3) Dissolving of crude ester and separation of insoluble A 700-mesh filter made of a metal was set in a liquid-withdrawing tubing of an autoclave equipped with a stirrer, to use the autoclave as a pressure filter device. The above autoclave was charged with 800 g of the crude ester obtained by the above esterification and 4,000 g of o-xylene as a solvent. The mixture in the autoclave was heated to about 120°C with stirring and maintained for about 30 minutes, and then the pressure inside the autoclave was elevated to about 1 kg/cm 2 G with nitrogen. The valve of the liquid-withdrawing tubing was opened, and an ester solution was filtered. After completion of the filtering, an insoluble collected with the filter was washed with 400 g of o-xylene which was heated to 120°C. The wash liquid was mixed with the ester solution which had been filtered. The insoluble collected with the filter was dried to give 41.8 g of a crystal. Table 4 shows the composition of the obtained insoluble. The insoluble contained cobalt and manganese in an amount of at least 98 % by weight based on the cobalt and manganese which had been contained in the crude 2,6-NDCA used as a raw material for the esterification. Composition of insoluble wt% Cobalt 1.68 Manganese 15.8 Molybdenum 1.0 2,6-NDCA 71.8 Monomethyl 2,6-naphthalenedicarboxylate 1.7 2,6-NDCM 1.4 Trimellitic acid 0.16 (4) Recrystallization The crude ester solution from which the insoluble had been filtered off in the above (3) was cooled to 40°C with stirring, to precipitate a crystal. A precipitated crystal was separated from a mothor liquor by filtration with a glass filter by means of suction. A resultant cake was washed with about 800 g of o-xylene and then dried. The dry weight of the crystal was 697 g, and the liquid content of the cake, calculated on the basis of a dry loss, was 11 % by weight based on the weight of a wet cake. (5) Distillation The ester crystal obtained by the recrystallization was purified by distillation according to a batch method. The distillation was carried out with a 10-stage distillation column under a condition where the column top pressure was set at 16 mmHg, to give purified 2,6-NDCM at a distillate amount ratio of 89 % by weight based on the weight of a charged crystal. Concerning the qualities of the obtained purified 2,6-NDCM, it had an organic impurity content of 0.028 % by weight and an acid value of 0.004 mg-KOH/g. Example 2 Purified 2,6-NDCM was obtained from the same crude ester as that obtained by the esterification in Example 1 by carrying out the dissolving of the crude ester, the separation of an insoluble, crystallization and distillation in the same manner as in Example 1 except that the o-xylene used as a solvent in the dissolving step and as a wash liquid for washing a separated cake in the recrystallization step was replaced with toluene. Table 5 shows the liquid content of a cake obtained by the recrystallization and the product qualities of the purified 2,6-NDCM obtained by the distillation. Comparative Example 1 Purified 2,6-NDCM was obtained from the same crude ester as that obtained by the esterification in Example 1 in the same manner as in Example 1 except that the o-xylene used as a solvent in the dissolving step and as a wash liquid for washing a separated cake in the recrystallization step was replaced with methanol. In addition, the heating temperature for dissolving the entire amount of 2,6-NDCM in the step of dissolving the crude ester was required to be 130°C, and the pressure in this case was about 7 kg/cm 2 G. Table 5 shows the liquid content of a cake obtained by the recrystallization and the product qualities of the purified 2,6-NDCM obtained by the distillation. As a characteristic impurity, there was found anhydrous monomethyl trimellitate which it is considered had been formed from trimethyl trimellitate which had not been removed by the recrystallization and had been decomposed during the distillation. Example 3 (Method (B)) 800 Grams of the same crude ester as that obtained by the esterification in Example 1 was distilled under a reduced pressure of 16 mmHg by a batch method using the same distillation column as that used in Example 1, to give 664 g of a distillate. The distillate was dissolved in 4,000 g of m-xylene and then recrystallized in the same manner as in Example 1, to give purified 2,6-NDCM. Table 5 shows the liquid content of a cake in the recrystallization and the product qualities of the purified 2,6-NDCM. Further, the residue from the above distillation was dissolved, under heat, in 2,000 g of a separated mother liquor obtained by the recrystallization of the distillate together with an ester remaining in the distillation column, and a residue solution was taken out of the distillation column. The residue solution was charged into the same pressure filter as that used in Example 1, and an insoluble was separated in the same manner as in Example 1. The insoluble had a dry weight of 43.3 g, and its composition was as shown in Table 6. In the above method, at least 98 % by weight, based on the cobalt and manganese contained in the crude 2,6-NDCA used as a raw material for the esterification, of cobalt and manganese were contained in the insoluble. Ex. 2 CEx. 1 Ex. 3 Liquid content of cake separated by recrystallization (based on wet product) 13 wt% 28 wt% 10 wt% Product qualities of purified NDCM Content of organic impurity 0.033 0.087 0.035 Acid value (mg-KOH/g) 0.005 0.048 0.005 Ex. = Example, CEx. = Comparative Example Composition of insoluble wt% Cobalt 1.61 Manganese 15.3 Molybdenum 1.0 2, 6-NDCA 71.6 Monomethyl 2,6-naphthalenedicarboxylate 2.7 2, 6-NDCM 1.3 Trimellitic acid 0.0 Comparative Example 2 Cobalt acetate tetrahydrate, manganese acetate tetrahydrate, a 47 wt% hydrogen bromide aqueous solution and water were mixed with, and dissolved in, 287 g of glacial acetic acid, to obtain 300 g of a catalyst solution having a cobalt concentration of 0.05 % by weight, a manganese concentration of 0.3 % by weight, a bromine concentration of 0.25 % by weight and a water contentration of 3 % by weight. A 500-ml autoclave made of titanium and equipped with a stirrer, a reflux condenser and a raw material feeding pump was charged with 112 g of the above catalyst solution. The remaining catalyst solution in an amount of 188 g was mixed with 37.5 g of 2,6-dimethylnaphthalene, and the mixture was charged into a raw material supply vessel and heated to dissolve the dimethynaphthalene, whereby a raw material solution was obtained. The pressure inside the autoclave was adjusted to 18 kg/cm 2 G with nitrogen, and the catalyst solution was heated to 200°C with stirring. After the temperature and the pressure were stabilized, 2,6-dimethylnaphthalene, the catalyst solution and compressed air were supplied to the autoclave to initiate an oxidation. While the flow rate of supplied air was adjusted such that discharged gas had an oxygen concentration of 2 % by volume, the raw material solution was continuously supplied over 1 hour. After completion of the supply of the raw material solution, the supply of air was continued until discharge gas had an oxygen concentration of 10 % by volume. After completion of the reaction, the autoclave was cooled to room temperature, and a reaction production was withdrawn and filtered through a glass filter by means of suction to separate a crystal. The separated cake was measured for a weight and dried with a dryer to give 50.7 g of a crude 2,6-NDCA crystal. Table 7 shows the composition of the crude 2,6-NDCA and the yield of 2,6-NDCA based on the supplied 2,6-dimethylnaphthalene. Example 4 (Recovery and recycle of oxidation catalyst metals) 2,6-Dimethylnaphthalene was oxidized under the same conditions as those in Comparative Example 2 with the same procedures as those in Comparative Example 2 except that the same insoluble containing oxidation catalyst metals as that separated and recovered from the solution of the crude ester in o-xylene in Example 1 was used as part of cobalt and manganese sources. The amount of the insoluble used for the preparation of a catalyst solution was 3.0 g, and cobalt acetate tetrahydrate and manganese acetate tetrahydrate were used as supplements for deficient metal contents for adjusting the catalyst concentration to a predetermined value. The weight of a dry crystal of the obtained crude 2,6-NDCA was 53.2 g. Table 7 shows the composition of the crude 2,6-NDCA and the yield of the 2,6-NDCA based on the supplied 2,6-dimethylnaphthalene. In the calculation of the yield, the content of 2,6-NDCA in the insoluble was deducted from the amount of 2,6-NDCA contained in the reaction product. Example 5 3 Grams of the insoluble containing oxidation catalyst metals separated and recovered from the o-xylene solution of the crude ester in Example 1 was mixed with 60 g of acetic acid containing 30 % by weight of water, and the mixture was stirred at 80°C for 20 minutes and then fractionated to a crystal and a filtrate with a glass filter. Concerning the transfer of metal contents into the filtrate, the transfer ratio of cobalt was 98.1 %, the transfer ratio of manganese was 95.4 %, and the transfer ratio of molybdenum was 11.5 %. The mother liquor from which the metal contents had been extracted was heated to evaporate the solvent and concentrate it. Then, 2,6-dimethylnaphthalene was oxidized under the same conditions as those in Comparative Example 2 and with the same procedures as those in Comparative Example 2 except that the above obtained concentrated solution was used as part of cobalt and manganese sources for the preparation of a catalyst solution. In addition, cobalt acetate tetrahydrate and manganese acetate tetrahydrate were used as supplements for deficient metal contents for adjusting the catalyst concentration to a predetermined value. The weight of a cry crystal of the obtained crude 2,6-NDCA was 50.8 g. Table 7 shows the composition of the crude 2,6-NDCA and the yield of the 2,6-NDCA based on the supplied 2,6-dimethylnaphthalene. Composition (wt%) CEx. 2 Ex. 4 Ex. 5 2,6-NDCA 95.1 94.0 94.8 2-formyl-6-naphthoic acid 0.24 0.21 0.25 Trimellitic acid 2.2 2.8 2.3 Yield of 2,6-NDCA (mol%) 93.3 92.5 93.1

Claims: A process for the production of dimethyl 2,6-naphthalenedicarboxylate, which process comprises esterifying with methanol 2,6-naphthalenedicarboxylic acid obtained by liquid phase oxidation of 2,6-dialkylnaphthalene with molecular oxygen in the presence of an oxidation catalyst, the esterification being carried out by mixing the 2,6-naphthalenedicarboxylic acid in molten dimethyl 2,6-naphthalenedicarboxylate and continuously supplying vaporized methanol to the resultant mixture, and then purifying the thus obtained crude ester by a process comprising (a) mixing the crude ester with an aromatic hydrocarbon solvent; (b) heating the thus obtained mixture to dissolve the crude ester in the aromatic hydrocarbon solvent; (c) removing impurities insoluble in the aromatic hydrocarbon solvent by filtering or centrifuging; (d) cooling the thus obtained solution to recrystallize and separate dimethyl 2,6-naphthalenedicarboxylate; and (e) distilling the thus obtained dimethyl 2,6-naphthalenedicarboxylate under reduced pressure to remove impurities having a high-boiling point, the impurities removed in step (e) being recycled to the esterification. A process for the production of dimethyl 2,6-naphthalenedicarboxylate, which process comprises esterifying with methanol 2,6-naphthalenedicarboxylic acid obtained by liquid phase oxidation of 2,6-dialkylnaphthalene with molecular oxygen in the presence of an oxidation catalyst, the esterification being carried out by mixing the 2,6-naphthalenedicarboxylic acid in molten dimethyl 2,6-naphthalenedicarboxylate and continuously supplying vaporized methanol to the resultant mixture, and then purifying the thus obtained crude ester by a process comprising (i) distilling the crude ester under reduced pressure to remove impurities having a high boiling point; and (ii) recrystallising the distillate dimethyl 2,6-naphthalenedicarboxylate thereby obtained in an aromatic hydrocarbon solvent, the impurities having a high boiling point being separated from the residue obtained in step (i) by filtering or centrifuging and mixing with an aromatic hydrocarbon solvent, heating the resultant mixture to dissolve the impurities in the aromatic hydrocarbon solvent, removing compounds insoluble in the thus obtained solution by filtering or centrifuging, and cooling the thus obtained solution free of the insoluble compounds to precipitate crystals, the precipitated crystals being recycled to the esterification. A process according to claim 2, wherein at least a part of the mother liquor from the recrystallization step (ii) is used as the aromatic hydrocarbon solvent for dissolving the impurities having a high boiling point. A process according to any one of the preceding claims, wherein the 2,6-dialkylnaphthalene is 2,6-dimethylnaphthalene. A process according to any one of the preceding claims, wherein the aromatic hydrocarbon has a boiling point of from 100 °C to 170 °C. A process according to claim 9, wherein the aromatic hydrocarbon is at least one of toluene, xylene and trimethylbenzene. A process according to any one of the preceding claims, wherein the esterification is carried out at a temperature of from 190 to 320 °C and at a gauge pressure of from 2 to 40 x 9.81 x 10 4 Pa (2 to 40 kg/cm 2 G). A process according to any one of the preceding claims, wherein the oxidation catalyst comprises at least cobalt, manganese and bromine. A process according to any one of the preceding claims, wherein compounds insoluble in the aromatic hydrocarbon are separated and recovered from the crude ester and the recovered insoluble compounds are recycled as a catalyst source for the oxidation. A process according to claim 9, wherein the insoluble compounds are recycled to the oxidation in the form of a solid. A process according to claim 9, wherein the insoluble compounds are mixed with water or formic acid, acetic acid, propionic acid or butyric acid solvent containing water, the resultant mixture is stirred to form a solution, components insoluble in the thus obtained solution are removed and the remaining solution is sued as a catalyst source for the oxidation. A process for preparing a polyester, which process comprises: (a) preparing dimethyl 2,6-naphthalenedicarboxylate by a process according to any one of claims 1 to 11; and (b) using the thus obtained compound to prepare a polyester.

Inventors: MACHIDA HIROSHI, NAKAYA KENGI, TANAKA KAZUO, ZAIMA FUMIYA, MACHIDA, HIROSHI, NAKAYA, KENGI, TANAKA, KAZUO, ZAIMA, FUMIYA, Machida, Hiroshi, Mitsubishi Gas Chem. Co. Inc., Nakaya, Kengi, Mitsubishi Gas Chem. Co. Inc., Tanaka, Kazuo, Mitsubishi Gas Chem. Co. Inc., Zaima, Fumiya, Mitsubishi Gas Chem. Co. Inc.
Applicants: MITSUBISHI GAS CHEMICAL CO, MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1998
Main classification: C07C67
IPC codes: C07C51, C07C67