Process for the purification of 2,6-dimethylnaphthalene and its use in the production of 2,6-naphthalenedicarboxylic acid

Abstract: The present invention relates to processes for the production of highly pure 2,6-dimethylnaphthalene (2,6-DMN) by crystallisation of a mixture of DMN isomers; it also relates to a process for producing a 2,6-naphthalenedicarboxylic acid and an ester thereof from the 2,6-DMN obtained by the above process. In a first aspect, crystallisation is carried out in the presence of a solvent, preferably an aliphatic or alicyclic saturated hydrocarbon. In a second aspect, suppression of the adhesion of the crystals to the inside wall of the crystalliser is achieved by a number of methods. In a third aspect, means are provided to suppress the agglomeration of the crystals. In a fourth aspect, the growth of the crystals is increased. In a fifth aspect, a filtration apparatus, preferably a rotatory vacuum filter, is used, the filtered cake is peeled off from a filter cloth, and said cloth is cleaned with a solvent. Finally, the purified 2,6-DMN is oxidised in the liquid phase in order to produce 2,6-naphthalenedicarboxylic acid, which can be esterified.

Description: 1. Field of the Invention The present invention relates to a process for producing 2,6-dimethylnaphthalene and a process for producing a 2,6-naphthalenedicarboxylic acid. More particularly, it pertains to a process for producing highly pure 2,6-dimethylnaphthalene which is useful as a starting raw material for a 2,6-naphthalenedicarboxylic acid and the like in a high recovery rate in an industrially advantageous manner by means of an crystallization treatment of the mixture of dimethylnaphthalene isomers, and a process for producing a 2,6-naphthalenedicarboxylic acid from the 2,6-dimethylnaphthalene obtained by the aforesaid process which acid is useful as a starting raw material for a high performance polyester and the like. 2. Description of the Related Arts It has heretofore been known that a 2,6-naphthalenedicarboxylic acid and an ester thereof are each a compound of industrial importance as a starting raw material for a high performance polyester which is employed for the production of polyethylene naphthalate in the form of fiber, film and the like that is excellent in tensile strength and heat resistance. The 2,6-naphthalenedicarboxylic acid and an ester thereof that are used for such a purpose are required to be highly pure and besides 2,6-dimethylnaphthalene (hereinafter dimethylnaphthalene is sometimes abbreviated to "DMN") which is used as a starting raw material thereof is required to be also highly pure because of the reasons described hereunder. Specifically, 2,6-DMN, when being low in purity, causes the impurities contained therein to be oxidized or esterified, and eventually lowers the purity of a 2,6-naphthalenedicarboxylic acid and an ester thereof when being produced therefrom. A part of the impurities that are formed during the steps of oxidation and esterification, originating from the impurities in 2,6-DMN is extremely difficult to remove, thereby making it also extremely difficult to obtain a 2,6-naphthalenedicarboxylic acid or a dimethyl 2,6-naphthalenedicarboxylate each having high purity. In addition, such impurities, when being present in 2,6-DMN, deteriorates not only the purity of the above-mentioned acid and ester, but also the yield thereof based on 2,6-DMN to a remarkable extent. It is therefore indispensable that highly pure 2,6-DMN be obtained in order to produce a 2,6-naphthalenedicarboxylic acid and a dimethyl 2,6-naphthalenedicarboxylate under industrially advantageous conditions. DMN has 10 isomers according to the positions of two methyl groups. Accordingly, 2,6-DMN as a starting raw material for a 2,6-naphthalenedicarboxylic acid is required to be a highly pure product substantially free from any of the isomers other than 2,6-DMN. It being so, 2,6-DMN as a starting raw material for oxidation is desired to be highly pure as much as possible. In order to enhance the purity of 2,6-DMN, however, the burden imposed on its purification is increased. Hence, the purity of 2,6-DMN as a starting raw material for oxidation should be determined taking into consideration such factors as the burden on its purification, relation between the DMN purity and the result of the oxidation reaction, influence of its purity on the purity of the oxidation product and, in the case of purifying as a dimethyl 2,6-naphthalenedicarboxylate, relation between the DMN purity and the result of the methylesterification reaction and the burden on purifying the methyl ester in addition thereto. It is usually necessary to set the purity of 2,6-DMN on a high level of at least 98.0%, comprehensively taking these factors into consideration. As the process for producing 2,6-DMN, there are available for example, a process in which 2,6-DMN is isolated from a tar fraction or a petroleum fraction, a process in which naphthalene or methylnaphthalene is methylated, succeedingly isomerized and separated and the like processes. Since the fractions and isomerization reaction products contain almost all of the 10 kinds of isomers, 2,6-DMN needs to be isolated from the mixture of a lot of isomers. On the other hand, Japanese Patent Application Laid-Open Nos. 134634/1974, 8935/1975, 76852/1973 and 129534/1975 disclose a process for producing o-tolylpentene-2 in high yield from o-xylene and butadiene; a process for producing 1,5-dimethyltetralin by cyclizing o-tolylpentene-2; a process for producing 1,5-DMN in high yield and in high selectivity by dehydrogenating 1,5-dimethyltetralin; and a process for producing a mixture of isomers consisting essentially of 1,5-, 1,6- and 2,6-DMNs by isomerizing 1,5-DMN. Accordingly, by combining the above-mentioned processes it is made possible to produce a mixture of isomers consisting essentially of 1,5-, 1,6- and 2,6-DMNs from o-xylene and butadiene. Thus, there is made available a process for producing 2,6-DMN by isolating 2,6-DMN from the aforesaid mixture. As described hereinbefore, any of the processes for producing 2,6-DMN that have heretofore been available makes it necessary to isolate 2,6-DMN from the mixture of isomers to recover the same. However, it is extremely difficult to purify 2,6-DMN by means of distillation which is frequently applied to the separation and purification of ordinary organic compounds, since 10 kinds of isomers have each a boiling point very close to one another as shown hereunder together with the melting point. Boiling point (°C) Melting point (°C) 1,5-DMN 269 82 1,6-DMN 266 -16 2,6-DMN 262 112 1,7-DMN 263 -14 1,8-DMN 270 65 2,7-DMN 262 98 1,3-DMN 265 -4.2 1,4-DMN 265 6 2,3-DMN 269 104 1,2-DMN 271 -3.5 As is clear from the table, 2,6-DMN has the highest melting point of all the DMN isomers. On the other hand, it is known that 2,6-DMN forms a eutectic together with at least one of 1,5-DMN, 2,7-DMN and 2,3-DMN. It is therefore, necessary that the ratio by amount of 2,6-DMN in the isomer mixture to the isomers be more than the compositional ratio of the eutectic in order to precipitate 2,6-DMN as a crystal by means of crystallization from the mixture of the isomers. That is to say, the condition under which 2,6-DMN is at first precipitated as a crystal by cooling is that the molar ratios of 1,5-DMN, 2,7-DMN and 2,3-DMN each in the mixture of the isomers to 2,6-DMN in the same are not more than 1.9, 1.4 and 1.1, respectively. As a method for isolating 2,6-DMN from the mixture of the isomers, there are proposed a crystallization method, an adsorption method, a method in which 2,6-DMN is caused to form a complex by the use of a certain kind of an organic compound, the resultant complex is separated and then it is decomposed to recover 2,6-DMN, and the like methods. Of these methods, the crystallization method is most simple, convenient and suitable as an industrial isolation method. In particular, in the case of producing a mixture of isomers consisting essentially of 1,5-, 1,6- and 2,6-DMN from o-xylene and butadiene as starting materials and isolating 2,6-DMN therefrom, a crystallization method is effective because of the comparatively small number of isomers in the starting raw material to be purified. In the case of methylating naphthalenes, isomerizing the reaction product and isolating 2,6-DMN or in the case of isolating the same from a tar fraction or a petroleum fraction, the combination of an adsorption method and a crystallization method is usually employed, since 2,6-DMN needs to be isolated from the mixture of a large number of isomers. It is well known that in the case of isomerizing DMN, isomerization between adjacent β-positions and isomerization of methyl-migration from one ring to another are unlikely to take place as compared with that between α-position and β-position. Specifically, the above-mentioned 10 DMN isomers are classified into four groups, namely A to D groups as undermentioned with regard to isomerization, and isomerization among different groups is unlikely to take place as compared with that in the same group. Group A --- 1,5-DMN, 1,6-DMN, 2,6-DMN Group B --- 1,8-DMN, 1,7-DMN, 2,7-DMN Group C --- 1,4-DMN, 1,3-DMN, 2,3-DMN Group D --- 1,2-DMN As a method for isolating 2,6-DMN from a mixture of isomers by means of crystallization, there is disclosed in Japanese Patent Publication No. 22553/1975, a method in which the mixture is cooled to -10 to 20°C and the precipitated crystal is treated with methanol. The treatment with methanol is supposedly aimed at the improvement on the filterability of the crystal, but necessitates two-stage procedures. Japanese Patent Application Laid-Open No. 5767/1973 discloses a method in which a mixture of DMN isomers is washed or recrystallized by the use of an aromatic hydrocarbon. In this case, the mother liquor, which is formed after the separation of the crystal and contains the aromatic hydrocarbon, is usually isomerized to enhance the 2,6-DMN concentratiod and thereafter returned to the crystallization step. The aromatic hydrocarbon, when being contained in the isomerization procedure, which is usually carried out under a heating condition by using an acidic catalyst such as silica-alumina, zeolite and HF, causes partial disproportionation reaction to take place between DMN and the hydrocarbon, thus lowering the yield of the objective 2,6-DMN. There is disclosed in the aforesaid Japanese Patent Publication No. 22553/1975 and Japanese Patent Application Laid-Open No. 5767/1973, that there is produced the objective 2,6-DMN having purity of 97.5 to 99.3%. However, any of the methods in the disclosures necessitates troublesome procedures and equipment therefor and requires a solid-liquid separation for the slurry obtained by crystallization in order to recover highly pure 2,6-DMN. In the case of industrial practice, the crystallization and solid-liquid separation need to be steadily carried out for a long period of time. Nevertheless, no description is so far found with regard to the change in the properties or purity of 2,6-DMN which is obtained in the case of continuously carrying out the crystallization and solid-liquid separation. A process for the purification of 2,6-dimethylnaphthalene by crystallization is furthermore described in US-A-3,541,175 which document deals with the advantage of avoiding the adhesion of the formed crystals to the inside wall of the crystallizer. In addition, no detailed description is given of the means for separating the precipitated crystal from the mother liquor in any of the above-mentioned disclosures. In the case of separating the crystal from the slurry formed by crystallization for the sake of industrial practice, there are required simplified convenient equipment and procedures, stabilized running and a high separation efficiency. However, any description made from such industrial point of view is so far not found as to the means for isolating 2,6-DMN from the 2,6-DMN crystal-containing slurry obtained by crystallizing a mixture of DMN isomers. SUMMARY OF THE INVENTION The present inventors synthesized 1,5-DMN from o-xylene and butadiene as the starting raw materials and isomerized the resultant 1,5-DMN to obtain the mixture of isomers consisting essentially of 1,5-, 1,6- and 2,6-DMNs, which was cooled as it is to precipitate 2,6-DMN crystal in the form of scale. The crystal thus obtained was poor in the efficiency of separation from the mother liquor at the time of filtration, whereby it was difficult to obtain highly pure 2,6-DMN. As is the case with the foregoing, 2,6-DMN crystal in the form of scale was obtained from the mixture of isomers containing 2,7-, 1,7-DMN, etc. other than 1,5-, 1,6- and 2,6-DMN as the starting raw material for crystallization, but it was extremely poor in filtration efficiency. It is judged from the above-mentioned phenomena that the method in which a mixture of DMN isomers is cooled as it is to precipitate 2,6-DMN in the form of crystal is not suited for industrial operation because of the inferior properties of the 2,6-DMN crystals to be precipitated. It is the first object of the present invention to provide, under such circumstances, a process for isolating and recovering highly pure 2,6-DMN in an industrially advantageous manner by precipitating 2,6-DMN from a mixture of DMN isomers through crystallization to produce highly pure 2,6-DMN in high yield, wherein 2,6-DMN crystal having satisfactory filterability as well as favorable properties is made to precipitate. A cooling-system tank-type crystallizer equipped with a stirrer and a jacket was continuously charged with the mixture of the isomers to precipitate 2,6-DMN crystal, while passing a coolant through the jacket. As a result, the precipitated crystal adhered closely to the inside wall of the crystallizer, whereby the heat transfer through the walls of the crystallizer was soon made insufficient, thus making it impossible to continue the procedure of precipitating the crystal. It is thought that the decrease in heat transfer performance has been caused by the crystal which precipitated on the heat transfer surface by locally lowered temperature of the slurry in the vicinity of the heat transfer surface as compared with the temperature of the other portion, and which adhered closely to the aforesaid surface. As a method for preventing the trouble due to the adhesion of crystal to the heat transfer surface, there is disclosed in Japanese Patent Publication No. 25402/1986, a method in which the crystal precipitated on the heat transfer surface is scraped off. The disclosure insists that the heat transfer surface is always kept clean and the prescribed heat transfer performance is maintained, since the crystal precipitated on the heat transfer surface is physically removed. However, the above-mentioned method is accompanied with various troubles because of the sliding part installed inside the crystallizer, which troubles are exemplified by frequent occurrence of sedimentation-solidification of the crystal in part in the crystallizer and of the abrasion or damage to scraping impellers that makes it impossible to continue crystal precipitation procedures. Specifically it is a further object of the present invention to provide a process for producing highly pure 2,6-DMN in an industrially advantageous manner by crystallizing 2,6-DMN to purify the same, wherein the trouble due to the aforesaid adhesion of the crystal is avoided to smoothly proceed with the procedure of precipitating the crystals. In the case of the continuous crystallization procedure by the present inventors, the average retention time of the liquid to be crystallized in the crystallizer was 8 hours. As a result, separation of at least 99% pure crystal was possible during a short period after the start of the operation, but the purity of the crystal decreased with the lapse of time, thus making it impossible to maintain the prescribed purity of the crystal. The purity of the crystal as mentioned herein is the purity of the crystal to be fed to the next step of oxidation reaction, excluding a solvent and rinsing liquid that can easily be removed by distillation or the like. In addition, it is net a further aspect of the present invention to provide a process for producing 2,6-naphthalenedicarboxylic acid in an industrially advantageous manner preferably from o-xylene and butadiene as the starting raw materials based on the understanding of the above-mentioned process for producing 2,6-DMN. Under such circumstances, intensive research and investigation were accumulated by the present inventors in order to attain the foregoing objects. As a result it has been found that highly pure 2,6-DMN can be produced from a mixture of DMN isomers in high yield in an industrially advantageous manner by putting the following processes into practice. The present invention has been accomplished on the basis of such finding and information. With regard to the first object of the invention, that is, the process for improving the properties and filterability of the 2,6-DMN crystal precipitated by cooling a mixture of DMN isomers, it has been found that by making the crystallization system coexist with a solvent selected from the group consisting of aliphatic saturated hydrocarbon and alicyclic saturated hydrocarbons, the properties of the precipitated crystal of 2,6-DMN change to a great extent and also the filterability thereof is markedly improved. Specifically the present invention provides, a process for producing 2,6-DMN which comprises crystallizing 2,6-DMN from a mixture of DMN isomers in the presence of an aliphatic saturated hydrocarbon or an alicyclic saturated hydrocarbon. With reference to the aspect of avoiding the trouble due to the adhesion of the crystal, it has been found that the procedure of precipitating the crystal can smoothly be effected by cooling the mixture of DMN isomers, while suppressing the adhesion of the crystal to the inside wall of the crystallizer. Specifically the inventive process for producing 2,6-DMN comprises crystallizing 2,6-DMN from a mixture of DMN isomers or a solution containing a mixture of DMN isomers by the use of a tank-type crystallizer, while suppressing the adhesion of the crystal to the inside wall of the crystallizer by using a method in which a crystallizer equipped with a heat transfer surface is used, a coolant is passed in contact with the heat transfer surface and the difference in temperature between the coolant and a mixture of DMN isomers or a solution containing a mixture of DMN isomers is maintained at 40°C or lower. Furthermore, the present invention provides a process for producing a 2,6-naphthalenedicarboxylic acid which comprises subjecting the 2,6-DMN which is produced by the process as set forth previously to liquid-phase oxidation; and a process as set forth in the preceding, wherein the 2,6-DMN is produced by crystallizing the isomerization reaction product of 1,5-DMN which is formed from o-xylene and butadiene each as a starting raw material. DESCRIPTION OF PREFERRED EMBODIMENT The invention relates to a process for producing 2,6-DMN by crystallizing 2,6-DMN from a mixture of DMN isomers in the presence of a solvent. As the mixture of DMN isomers to be used as the starting raw material in the present invention, there are available a material separated from tar fraction or petroleum fraction, and a material produced by methylating naphthalene or methylnaphthalene, isomerizing the methylation product and isolating the resultant product. Particularly suitable materials are the isomerization reaction product of 1,5-DMN produced from o-xylene and butadiene as starting raw materials, the isomerization reaction product of the mother liquor which is formed by separating 2,6-DMN crystal from aforesaid isomerization reaction product of 1,5-DMN and the mixture of these isomerization reaction products. In order to precipitate 2,6-DMN crystal in the process according to the present invention, the mixture of DMN isomers needs to have the following compositional ratio in view of the compositional ratio of the eutectic as mentioned hereinbefore. 1,5-DMN/2,6-DMN < 1.9 2,7-DMN/2,6-DMN < 1.4 2,3-DMN/2,6-DMN < 1.1 Although depending on the catalyst to be used, the isomerization reaction product of 1,5-DMN synthesized from o-xylene and butadiene as starting raw material usually contains 5 to 20% of 1,5-DMN, 35 to 50% of 1,6-DMN, 35 to 50% of 2,6-DMN and at most 5% of the other DMN isomers, which satisfy the above-prescribed compositional ratio, and thus can be used for producing 2,6-DMN by cooling itself. The mother liquor which is formed by separating 2,6-DMN crystal from the above-mentioned isomerization reaction product principally contains 1,5-DMN and 1,6-DMN and can be recycled as a raw material for isomerization. During isomerization reaction, however, side reaction is liable to take place, including the formation of methylnaphthalene (MMN) and trimethylnaphthalene (TMN) due to disproportionation and the formation of isomers belonging to the 2,7-group due to the isomerization beyond the 1,5-group. MMN and TMN can be removed by distillation, but the isomers outside the 1,5-group are difficult to remove by distillation, and are accumulated in the system if the mother liquor is circulated through the isomerization step. Detailed investigation was made by the present inventors on the relation between the concentration of the impurities in the raw material for crystallization and the concentration of impurities remaining in the objective crystal. As a result, there was observed a phenomenon that in the case where 2,7-DMN, MMN and/or TMN are present to a certain or more extent in the raw material for crystallization, 2,7-DMN, 2-MMN and/or part of TMN isomers remain in the objective crystal in amounts larger than the amount derived from eutetic theory, thereby making it difficult to obtain highly pure crystal. As a result of further investigation, it has been found that the components such as 1,5-DMN, 1,6-DMN, 1-MMN and the isomer other than the above-remaining isomer in TMN that are contained in DMN isomerization reaction product to be used for raw material for crystallization can easily be removed by rinsing 2,6-DMN crystal after precipitation, and the concentration of such impurities steadily decreases with an increase in the amount of a rising liquid, but that 2,7-DMN, 2-MMN and part of TMN isomers hardly change in concentration even after rinsing the crystal and they are inevitably drawn and incorporated inside the crystal at the time of precipitation of 2,6-DMN as crystal instead of being attached, as the components of the mother liquor, onto the surface of the crystal. The amounts of 2,7-DMN, 2-MMN and part of TMN isomers that are inevitably contained in the crystal at the time when the 2,6-DMN crystal is precipitated are determined approximately by the ratio by amount of each component to the dimethylnaphthalene series in the raw material for crystallization. When 10 parts by weight of 2,7-DMN is contained in 100 parts by weight of dimethylnaphthalene series, the amount of 2,7-DMN contained in 2,6-DMN crystal accompanied therewith is about 0.7 to 1.4% by weight. When 5 parts by weight of 2,7-DMN is contained in 100 parts by weight of dimethylnaphthalene series, the amount of 2,7-DMN contained in 2,6-DMN crystal accompanied therewith is about 0.4 to 0.8% by weight. On the other hand, when 5 parts by weight of MMN is contained in 100 parts by weight of dimethylnaphthalene series, the amount of 2-MMN contained in 2,6-DMN crystal accompanied therewith is about 0.7 to 1.3% by weight; and when 5 parts by weight of TMN is contained in 100 parts by weight of dimethylnaphthalene series, the amount of a part of TMN isomers contained in 2,6-DMN crystal accompanied therewith is about 0.2 to 0.6% by weight. It is therefore desirable, in order to obtain 2,6-DMN having high purity of 98% or higher, that the concentration of 2,7-DMN based on dimethylnaphthalene series in the raw material for crystallization be set on at most 10% by weight when the raw material is almost free from MMN and TMN. In the case where a considerable amount of MMN or TMN is contained in the raw material for crystallization, it is desirable, in order to restrict the concentrations of 2,7-DMN, MMN and TMN remaining in 2,6-DMN to at most 0.5% by weight, respectively, that the concentrations of 2,7-DMN, MMN and TMN based on dimethylnaphthalene series in the raw material for crystallization be set on at most 5%, 3% and 10% by weight, respectively. The solvent to be used in the invention, which is in the form of liquid under working temperatures, is selected from the aliphatic saturated hydrocarbons and alicyclic saturated hydrocarbons each having preferably 5 to 14, more preferably 6 to 12 carbon atoms. Specific examples thereof include pentane, hexane, heptane, octane, nonane, decane, undecane, dodecane, cyclopentane, cyclohexane, methylcyclopentane, methylcyclohexane, cyclooctane, methylcyclooctane, decalin, methyldecalin and dimethyldecalin. The solvent may be used alone or in combination with at least one other. The amount of the solvent to be used is determined taking into consideration the solubility of 2,6-DMN therein, 2,6-DMN concentration in the mixture of DMN isomers, operating temperature, slurry concentration and the like. Preferably the amount thereof is such that the crystallizing operation temperature is 10 to 60°C, especially around room temperature, and it is determined in the aforesaid manner. A crystallizing operation temperature lower than room temperature leads to the necessity for cooling energy, whereas that being unreasonably high leads to the necessity for heating energy. The amount of precipitated crystal at the time of crystallization is preferably large because of higher crystal yield per unit operation. However, excessively large amount of crystal brings about difficulty in handling such as transportation and stirring. The slurry concentration is usually preferably 10 to 40% by weight. For example, when a mixture of DMN isomers containing 1,5-DMN, 1,6-DMN and 2,6-DMN in amounts of 8%, 46% and 46% by weight, respectively and n-heptane in one-half the amount of the mixture are mixed with heating to form a liquid and then cooled to 25°C, the slurry concentration becomes about 20% by weight. In this way, the amount of the solvent to be used varies depending on a variety of factors, and is usually in the range of 0.01 to 20 times the weight of the mixture of DMN isomers. An amount thereof less than 0.01% by weight results in less effect on the improvement in the properties of 2,6-DMN crystal, thus forming crystal hard to filter; whereas that more than 20% by weight lowers economical efficiency, since the effect on the improvement in the crystal properties is not proportional to the amount used and the crystallizer becomes excessively large. From the viewpoint of the balance between filterability and economical efficiency the amount thereof is preferably in the range of 0.1 to 10 times the weight of the mixture of DMN isomers. The crystallizer to be used for precipitating 2,6-DMN crystal from the mixture of DMN isomers in the presence of a solvent is not specifically limited, but is available from those used for usual recrystallizing operation without any modification. A preferable crystallizer is a tank-type crystallizer. An apparatus for separating the crystal from the mother liquor after precipitating the crystal is not specifically limited as well, but is available from those used for usual solid-liquid separation such as a centrifugal separator and a filter without any modification. The crystal is usually separated with a solid-liquid separator, and then rinsed preferably with the solvent same as that used in crystallization. As mentioned hereinbefore, it is known that in purification by crystallization, the operability for filtration/separation and the purity of the objective crystal are greatly influenced by the properties of the crystal formed. Specifically, 2,6-DMN crystal precipitated from the mixture of DMN isomers, which is in the form of scale, has extremely poor filterability as mentioned hereinbefore. However, 1,5-DMN crystal precipitated therefrom, which is in the form of large plate, has excellent filterability. In this way, the crystal properties greatly change with change only in the position of a substituent group, and originate from the properties inherent in the substance itself forming the crystal. By carrying out the crystallization in the presence of a solvent such as an aliphatic or alicyclic saturated hydrocarbon according to the invention, the properties of 2,6-DMN crystal are improved, thus facilitating industrial crystallizing separation. The properties of 2,6-DMN crystal can be improved by the use of an aromatic hydrocarbon as a solvent, but are improved to a greater extent by using an aliphatic or alicyclic saturated hydrocarbon. In order to enhance the yield of 2,6-DMN, there is adopted a system wherein the mother liquor formed by separating the crystal is usually employed for isomerization and the resultant isomerization liquid product is again crystallized. In this case, when an aromatic hydrocarbon as a solvent is used in the crysatllization, the mother liquor after the separation of the crystal contains the aromatic hydrocarbon. From the industrial standpoint, the aforesaid aromatic hydrocarbon is unsuitable as a coexistent material, since it is apt to bring about disproportionation reaction along with DMN in the isomerization step, thereby causing a fear of deteriorating the yield of the objective DMN. 2,6-DMN is somewhat soluble in any of the aforesaid hydrocarbons. Thus in order to obtain the same yield of 2,6-DMN crystal by using any of the hydrocarbons, the crystallization temperature needs to be lowered. The necessary lowering in the crystallization temperature varies depending upon the kind, use amount, etc. of the hydrocarbon. In the case of using a solvent of an aliphatic or alicyclic saturated hydrocarbon such as n-heptane in the same amount as the mixed liquid of DMN, the temperature at which 2,6-DMN begins to precipitate is about 25°C lower than the temperature in a solventless case. It follows that lowering in the crystallization temperature increases the energy required for cooling, accordingly, the coexistence of an aliphatic or alicyclic saturated hydrocarbon is usually. industrially unfavorable. At first glance, the coexistent substance in the process of the invention seems to be industrially unfavorable. Nevertheless the remarkable improvement in the properties of 2,6-DMN can exhibit a working effect which far outweighs the disadvantage. The use of an aliphatic or alicyclic saturated hydrocarbon, in which DMN is less soluble than in an aromatic hydrocarbon is more advantageous than the use of an aromatic hydrocarbon, since it can mitigate the disadvantage of increased energy requirement as compared with the use of an aromatic hydrocarbon and since the use of the former decreases the viscosity of the crystallization mother liquor to a greater extent than the use of the latter. The cause for the improvement in the properties of the precipitated 2,6-DMN crystal due to the coexistence of an aliphatic or alicyclic saturated hydrocarbon as a solvent is still unclear. It is known that a high crystal concentration in a slurry is apt to produce small crystal, but the aforesaid cause is not due to the effect on lowering the crystal concentration by the use of a solvent. Specifically, the crystal obtained by mixing an aliphatic or alicyclic saturated hydrocarbon with a mixture of DMN isomers, and then cooling the resultant mixture to precipitate 2,6-DMN crystal and form a slurry with a crystal concentration of 25% by weight, is favorable in properties and easy to filtrate as compared with the slurry obtained by cooling the mixture of DMN isomers alone to precipitate 2,6-DMN crystal and form a slurry with a crystal concentration of 25% by weight, which is hard to filtrate. It is advantageous at the time of crystallizing 2,6-DMN from a mixture of DMN isomers or a solution containing a mixture of DMN isomers through cooling or evaporation by the use of a tank-type crystallizer, to adopt for the purpose of suppressing the adhesion of the crystal to the inside wall of the crystallizer a method in which a crystallizer equipped with a heat transfer surface is used, a coolant is passed in contact with the heat transfer surface and the difference in temperature between the coolant and a mixture of DMN isomers or a solution containing a mixture of DMN isomers is maintained at 40°C or lower. At first, regarding the cooling method by indirect cooling by using a tank-type crystallizer equipped with a heat transfer surface, there are available a cooling method by using a jacketed tank-type crystallizer and passing a coolant through the jacket; a cooling method by installing a cooling coil inside a tank-type crystallizer and passing a coolant through the cooling coil; a cooling method by installing a heat exchanger outside a tank-type crystallizer and circulating the liquid to be crystallized through the heat exchanger with a pump or the like; and a cooling method in which the aforesaid methods are combined. In such indirect cooling, the liquid to be crystallized is cooled from the wall surface through which heat is transferred, and if there is little difference in temperature between the liquid and a coolant, precipitation rate of the crystal is low, requiring a long retention time, whereas much temperature difference therebetween is liable to cause crystal adhesion onto the wall surface because of a high degree of supersaturation. The liability to adhesion onto the wall surface depends upon the properties inherent to the crystal in question, the kind of the crystal and the precipitation conditions at the time of crystal precipitation. As a result of experimental investigation made by the present inventors on a method capable of controlling the degree of supersaturation, preventing the adhesion of the precipitated crystal onto the wall surface and precipitating the crystal at a practically sufficient rate, it has been found that in the case of precipitating 2,6-DMN crystal by cooling a mixture of DMN isomers, the adhesion of the crystal onto the wall surface is almost negligible provided that the difference in temperature between the liquid to be crystallized and the coolant is maintained at 40°C or lower, and even if the adhesion is not negligible, it does not cause difficulty in continuing the crystallizing operation. In carrying out the crystallizing operation, when the above-mentioned difference in temperature is kept at 40°C or lower, the crystal adhesion thereonto is almost negligible even if the mixture of DMN isomers is cooled as it is. However, 2,6-DMN crystallized under a solventless condition has unfavorable crystal properties due to its scaly state and is hard to filter, whereby highly pure 2,6-DMN is made difficult to obtain after filtration separation. The use of a hydrocarbon as a solvent, especially an aliphatic or alicyclic saturated hydrocarbon in precipitating the crystal improves the crystal properties, thus enabling highly pure 2,6-DMN to be formed after filtration separation. For the purpose of preventing the crystal from adhering onto the wall surface, a solvent is not always necessary, but in order to obtain highly pure 2,6-DMN crystal it is indispensable to employ a solvent. The amount of the solvent to be used varies depending upon various factors. In the case of indirect cooling for crystallization by the reason same as that described hereinbefore, the amount is 0.01 to 20 times, preferably 0.1 to 10 times the weight of the mixture of DMN isomers. The crystallizer for carrying out the present invention is not specifically limited, but is selected according to each of the methods. Likewise, an apparatus for separating the crystal from the mother liquor after precipitating the crystal is not specifically limited, but may be selected for use without modification from the apparatus usually used for solid-liquid separation such as a centrifugal separator and a filter. The crystal is usually separated in a solid-liquid separator and then rinsed to afford highly pure 2,6-DMN. In the case where further highly pure 2,6-DMN is required, such 2,6-DMN is obtained, as is described hereinafter in the working examples, by dispersing again the 2,6-DMN crystal obtained by any of the foregoing methods in a solvent and thereafter separating the crystal. Moreover the invention relates to a process for producing a 2,6-naphthalenedicarboxylic acid by subjecting the 2,6-DMN produced according to the invention to liquid-phase oxidation. Particulary preferable 2,6-DMN to be used in this aspect of the invention includes 2,6-DMN which is obtained by isomerizing 1,5-DMN produced from o-xylene and butadiene as starting raw materials, and subjecting the resultant isomerization reaction product to a crystallizing treatment according to the first aspect of the invention; 2,6-DMN which is obtained by filtering treatment according to the second aspect of the invention; and 2,6-DMN which is obtained according to the combination of the first and second aspects of the invention. The isomerization reaction product from 1,5-DMN is produced by cyclizing o-tolylpentene-2 formed from o-xylene and butadiene to produce 1,5-demethyltetralin, dehydrogenating the same to produce 1,5-DMN and isomerizing the 1,5-DMN, and is a mixture of isomers consisting essentially of 1,5-, 1,6- and 2,6-DMN. The liquid-phase oxidation process for 2,6-DMN can be selected for use from the conventional well-known process without specific limitation. In general, mention may be made of a process in which 2,6-DMN is subjected to liquid-phase oxidation by molecular oxygen in a solvent such as acetic acid in the presence of a catalyst of Co-Mn-Br system, and more specifically, of a process in which 2,6-DMN is subjected to liquid-phase oxidation by air in a solvent comprising acetic acid or water-containing acetic acid containing a small amount of water in the presence of a catalyst composed of a cobalt compound, a manganese compound and a bromine compound under reactional conditions including a pressure of 5 to 40 kg/cm 2 G, approximately and a temperature of 180 to 250°C, approximately, which process enables the production of 2,6-naphthalenedicarboxylic acid at a high yield of 80% or more. The catalyst is used in amounts of usually 0.002 to 0.1 mol as cobalt atom, 0.01 to 0.2 mol as manganese atom and 0.01 to 0.2 mol as bromine atom each based on one mol of 2,6-DMN as the starting raw material. According to the process of the present invention for producing 2,6-DMN, the precipitated 2,6-DMN crystal is remarkably improved in filterability with great change in its properties, and highly pure 2,6-DMN is obtained in high yield in an industrially advantageous manner by making an aliphatic or alicyclic saturated hydrocarbon as a solvent coexist in the crystallizing treatment of the mixture of DMN isomers. It is particularly effective to use, as the mixture of DMN isomers, the isomerization reaction product from 1,5-DMN which is formed from o-xylene and butadiene as the starting raw materials. By subjecting the 2,6-DMN thus obtained to liquid-phase oxidation, a 2,6-naphthalenedicarboxylic acid is extremely advantageously produced which is useful as a starting raw material for high performance polyester, and the like. In addition, by esterifying the 2,6-naphthalenedicarboxylic acid thus obtained, a corresponding ester, for example, a methyl ester is efficiently obtained. In the following, the present invention will be described in more detail with reference to working examples. The concentration of each of the components in the following working examples, comparative examples and reference examples was obtained by gas chromatography. Reference Example 1 DMN comprising as a principal component, 1,5-DMN which had been synthesized from o-xylene and butadiene as starting raw materials was isomerized in the presence of HF as the catalyst to afford a DMN isomerization reaction product having chemical composition as given in Table 1. Reference Example 2 DMN comprising as a principal component, 1,5-DMN which had been synthesized from o-xylene and butadiene as starting raw materials was isomerized in the presence of mordenite (produced by Tosoh Corporation) as the catalyst to afford a DMN isomerization reaction product having chemical composition as given in Table 1 Composition(wt%) Reference Example 1 Reference Example 2 1,5-DMN 8.54 15.73 1,6-DMN 41.12 45.87 2,6-DMN 48.73 35.10 Other DMN isomers 0.14 0.39 Low boiling substances 0.60 1.80 High boiling substances 0.87 1.11 [First aspect of the invention] Example 1 100 g of the DMN isomerization reaction product produced in Reference Example 1 and 100 g of n-heptane were placed in a 400 milliliter (mL) glass container equipped with a stirrer under stirring and heating to 75°C to form homogeneous liquid as a whole. It was cooled under stirring to 50°C, then further cooled to 20°C over a period of 2 hours, and maintained thereat for 30 min. to precipitate crystal. The precipitated crystal was in the form of board. The content in the glass container was filtered by suction with a G2 glass filter (standard maximum diameter of opening of 100 to 150 µm), and the crystal thus obtained was analyzed. As a result, the purity of the 2,6-DMN crystal was 91.3%. After the filtration by suction, the crystal was rinsed with 30 g of n-heptane at 20°C to afford 32.6 g of DMN crystal having chemical composition as given in Table 2. The percentage of 2,6-DMN in the resultant crystal based on 2,6-DMN in the starting raw material (hereinafter referred to as "crystal yield") was 66.4%. Example 2 The procedure in Example 1 was repeated to precipitate crystal except that 60 g of cyclohexane was used in place of 100 g of n-heptane. The precipitated crystal was in the form of board. The content in the glass container was filtered by suction with a G2 glass filter, and the crystal thus obtained was analyzed. As a result, the purity of the 2,6-DMN crystal was 88.7%. After the filtration by suction, the crystal was rinsed with 30 g of n-heptane at 20°C to afford 34.0 g of DMN crystal having chemical composition as given in Table 2 at a crystal yield of 69.1%. Comparative Example 1 The procedure in Example 1 was repeated to precipitate crystal except that the DMN isomerization reaction product was used in an amount of 200 g and that the use of n-heptane was intentionally omitted. The precipitated crystal was in the form of fine scale. The content in the glass container was filtered by suction with G2 glass filter and the crystal thus obtained was analyzed. As a result, the purity of the 2,6-DMN crystal was 74.1%. After the filtration by suction, the crystal was rinsed with 100g of n-heptane at 20°C to afford 88.2 g of DMN crystal having 2,6-DMN purity of 83.0% at a crystal yield of 75.1%. Comparative Example 2 200 g of the DMN isomerization reaction product produced in Reference Example 1 was placed in a 400 milliliter (mL) glass container which had been used in Example 1 under heating to 75°C to melt into liquid as a whole. It was cooled to 45°C over a period of 2 hours, and maintained thereat for 30 min. to precipitate crystal. The precipitated crystal was in the form of fine scale. The content in the glass container was filtered by suction with a G2 glass filter and the crystal thus obtained was analyzed. As a result, the purity of the 2,6-DMN crystal was 78.2%. After the filtration by suction, the crystal was rinsed with 100 g of n-heptane at 20°C to afford 74.6 g of DMN crystal having 2,6-DMN purity of 84.7% at a crystal yield of 64.8%. Although the crystal yield is comparable to that in Example 1, 2,6-DMN purity is remarkable low as compared with that in Example 1 in spite of the amount of n-heptane used for rinsing much larger than the amount in Example 1. Comparative Example 3 The procedure in Example 1 was repeated to carry out crystallization, filtration and rinsing except that toluene was used in place of n-heptane. As a result, there was obtained 29,6 g of DMN crystal having chemical composition as given in Table 2 at a crystal yield of 57.4%. The precipitated crystal was larger than that obtained by cooling the DMN isomerization reaction product as such under a solventless condition, but was smaller than that obtained by using n-heptane. The purity of the resultant crystal was higher than that obtained by cooling the DMN isomerization reaction product as such under the solventless condition, but was lower than that obtained by using n-heptane. Example 3 The procedure in Example 1 was repeated to precipitate crystal except that 100 g of the DMN isomerization reaction product produced in Reference Example 2 and 100 g of isooctane were used in place of the product from Reference Example 1 and n-heptane, and that the homogeneous liquid was cooled from 50°C to 0°C over a period of 3 hours. The precipitated crystal was in the form of board. Then the content in the glass container was filtered by suction with a G2 glass filter, and the crystal thus obtained was analysed. As a result, the purity of the 2,6-DMN was 86.3%. Thereafter, the crystal was rinsed with 30 g of isooctane at 0°C to afford 24.8 g of DMN crystal having chemical composition as given in Table 2 at a crystal yield of 70.1%. Composition(wt%) Example 1 Example 2 Comparative Example 2 Example 3 1,5-DMN 0.12 0.15 0.91 0.21 1,6-DMN 0.56 0.73 4.39 0.61 2, 6-DMN 99.31 99.10 94.54 99.15 Other DMN isomers <0.01 <0.01 <0.01 <0.01 Low boiling substances <0.01 0.01 0.06 0.02 High boiling substances 0.01 0.01 0.09 0.01 Comparative Example 4 200 g of the DMN isomerization reaction product produced in Reference Example 2 was placed in a 400 milliliter (mL) glass container which had been used in Example 1 under heating to 75°C to melt into liquid as a whole. It was cooled to 45°C over a period of 2 hours, and maintained thereat for 30 min. to precipitate crystal. The precipitated crystal was in the form of fine scale. The content in the glass container was filtered by suction with a G2 glass filter and the crystal thus obtained was analyzed. As a result, the purity of the 2,6-DMN crystal was 68.5%. After the filtration by suction, the crystal was rinsed with 100 g of n-heptane at 20°C to afford 30.5 g of DMN crystal having 2,6-DMN purity of 77.3% at a crystal yield of 33.6%. Although the crystal concentration at the time of crystallization was almost the same as that in Example 4, 2,6-DMN purity is remarkably low because of the poor properties of the precipitated crystal in spite of the amount of the rinsing liquid much larger than the amount in Example 3. Example 4 A mixture of DMN isomers comprising 1,5-DMN as a principal component and a mixture of DMN isomers comprising 1,7-DMN as a principal component were mixed and isomerized in the presence of mordenite type H as the catalyst to afford a DMN isomerization reaction product (starting raw material for crystallization) having chemical composition as given in Table 3. 100 g of the DMN isomerization reaction product produced in such a manner and 100 g of n-heptane ware placed in a 400 milliliter (mL) glass container equipped with a stirrer under stirring and heating to 75°C to form homogenerous liquid as a whole. It was cooled under stirring to 50°C, then further cooled to 10°C over a period of 3 hours, and maintained thereat for 30 min. to precipitate crystal. The precipitated crystal was in the form of board. The content in the glass container was filtered by suction with a G2 glass filter, and the crystal thus obtained was analyzed. As a result, the purity of the 2,6-DMN crystal was 90.7%. After the filtration by suction, the crystal was rinsed with 30 g of n-heptane at 10°C to afford 24.1 g of DMN crystal having chemical composition as given in the Table 3 at a crystal yield of 60.6%. Example 5 The procedure in Example 4 was repeated to carry out crystallization, filtration and rinsing except that decalin was used in place of n-heptane. As a result, there was obtained DMN-crystal having chemical composition as given in Table 3 in an amount of 23.6 g at a crystal yield of 59.4%. Composition(wt%) Starting raw material for crystallization Example 4 Example 5 1,5-DMN 11.12 0.11 0.10 1,6-DMN 35.09 0.36 0.31 2,6-DMN 39.44 99.13 99.25 1,7-DMN 5.43 0.03 0.04 2,7-DMN 4.71 0.31 0.25 Other DMN isomers 2.03 0.03 0.02 Low boiling substances 1.36 0.02 0.02 High boiling substances 0.82 0.01 0.01 [Suppression of crystal adhesion] Example 6 Crystallizing operation was carried out by the use of a tank-type crystallizer comprising a vertical cylindrical vessel which had an inside diameter of 1000 mm and a cylinder length of 2000 mm and was equipped with a stirrer; a cooling jacket with a heat transfer area of 5.1 m 2 ; 4 baffle plates inside; a thermometer inserted at the bottom portion thereof for measuring the temperature of the content in the vessel; and an overflow pipe at the top portion thereof for maintaining the volume of the liquid (slurry) in the vessel at 1.3 m 3 by withdrawing the content therethrough as an overflow. The mixture of 1,5-DMN and n-heptane as the solvent was isomerized to afford an isomerization reaction product having chemical composition as given in Table 4. This product as the starting raw material for crystallization was heated to 80°C to form homogeneous liquid as a whole, preliminarily fed in the crystallizer at 80°C up to the overflow level and cooled with stirring to 60°C. When the temperature inside the crystallizer reached 60°C, the raw material at 80°C was continuously fed therein from the bottom at a constant rate of 200 kg/hr, which was the starting time of crystallizing operation. Simultaneously with the starting time, cooling water at 40°C was passed through the jacket to cool the content in the crystallizer, the temperature of which was maintained at 80°C by regulating the flow rate of the cooling water at 40°C to be passed through the jacket. The content therein was mixed at an agitation power of 1.8 kW/m 3 , and the slurry containing precipitated crystal was continuously taken out from the top portion as the overflow. The aforesaid operation was continued for 24 hours, during which time the cooling performance of the jacket and the cooling water flow rate of 0.17 m 3 /hr in average were stabilized, enabling continuous operation without any trouble. The concentration of the crystal in the slurry which was continuously taken out by overflowing was also stable at about 14% by weight. After the lapse of 24 hours, the feeding of the raw material and the cooling water was stopped, and the slurry and cooling water were entirely withdrawn through the bottom of the crystallizer. Then, the inside of the vessel was observed. As a result, no crystal adhesion was observed even at the jacket fitted portion. The crystal was separated from the resultant slurry and recovered as 2,6-DMN having purity of 99.2%. The above-mentioned procedure was repeated except that the cooling water temperature was set at 10°C. The flow rate of the cooling water was 0.07 m 3 /hr at the start of the crystallizing operation and was comparatively stable until 3 hours after the start. However, after the lapse of 3 hours from the start thereof, the flow rate of the cooling water began to increase because of lowered heat transfer performance, and after the lapse of 8 hours therefrom, the flow rate reached its upper limit of 5.0 m 3 /hr. Thereafter the temperature inside the crystallizer began to rise because of the insufficient cooling capacity. Thus after the lapse of 8.5 hours therefrom, feeding of the raw material and the cooling water was stopped. Then the slurry and cooling water were entirely withdrawn through the bottom of the crystallizer, and the inside of the vessel was observed. As a result, there was observed crystal scale in a thickness of about 5 mm stuck to the jacket fitted portion. Composition of starting raw material for crystallization (wt%) Example 6 Example 7 n-Heptane 10.0 - Toluene - 10. 0 2,6-DMN 43.9 43.6 1,6-DMA 37.0 37.1 1,5-DMN 7.7 7.9 Other DMN 0.1 0.1 Low boiling substances 0.5 0.5 High boiling substances 0.8 0.8 Example 7 By the use of the same apparatus as in Example 6, crystallizing operation was carried out by using a mixture having chemical composition as given in Table 4 which mixture was obtained by mixing toluene with the isomerization reaction product obtained by isomerizing 1,5-DMN produced from o-xylene and butadiene. This mixture as the starting raw material for crystallization was fed in the crystallizer at 80°C up to the overflow level and cooled with stirring to 60°C. When the temperature inside the crystallizer reached 60°C, the raw material at 80°C was continuously fed therein from the bottom at a constant rate of 200 kg/hr, which was the starting time of crystallizing operation. Simultaneously with the starting time, cooling water at 40°C was passed through the jacket to cool the content in the crystallizer, the temperature of which was maintained at 60°C by regulating the flow rate of the cooling water at 40°C to be passed through the jacket. The content therein was mixed at an agitation power of 1.8 kW/m 3 , and the slurry containing precipitated crystal was continuously taken out from the top portion as the overflow. The aforesaid operation was continued for 24 hours, during which time the cooling performance of the jacket and the cooling water flow rate of 0.15 m 3 /hr in average were stabilized, enabling continuous operation without any trouble. The concentration of the crystal in the slurry which was continuously taken out by overflowing was also stable at about 12% by weight. After the lapse of 24 hours, the feeding of the raw material and the cooling water was stopped, and the slurry and cooling water were entirely withdrawn through the bottom of the crystallizer. Then, the inside of the vessel was observed. As a result, no crystal adhesion was observed even at the jacket fitted portion. The crystal was separated from the resultant slurry and recovered as 2,6-DMN having purity of 97.2%. The above-mentioned procedure was repeated except that the cooling water temperature was set at 10°C. The Flow rate of the cooling water was 0.07 m 3 /hr at the start of the crystallizing operation and was comparatively stable until 3 hours after the start. However, after the lapse of 3 hours from the start thereof, the flow rate of the cooling water began to increase because of lowered heat transfer performance, and after the lapse of 8 hours therefrom, the flow rate reached its upper limit of 5.0 m 3 /hr. Thereafter the temperature inside the crystallizer began to rise because of the insufficient cooling capacity. Thus after the lapse of 8.5 hours therefrom, feeding of the raw material and the cooling water was stopped. Then the slurry and cooling water were entirely withdrawn through the bottom of the crystallizer, and the inside of the vessel was observed. As a result, there was observed crystal scale in a thickness of about 5 mm stuck to the jacket fitted portion. Example 8 Acetic acid in an amount of 288.9 g was mixed with 3.2 g of water, 0.64 g of cobalt acetate quadrihydrate, 5.35 g of manganese acetate quadrihydrate, and 1.94 g of hydrogen bromide (47% aqueous solution) to prepare a catalyst solution. Then, 120.0 g of the above-prepared catalyst solution was fed in a 0.5 liter titanium-made autoclave (reactor) equipped with a stirrer, a reflux condenser and a raw material transfer pump. The balance 180.0 g of the catalyst solution was mixed with 30.0 g of the 2.6-dimethylnaphthalene (DMN) as obtained in Example 1. The resultant mixture was fed in a raw material feed tank and was heated to dissolve 2,6-DMN and prepare a raw material solution. The reaction system was pressurized with nitrogen gas to a regulated pressure of 18 kg/cm 2 G, and the content in the reactor was heated to 200°C under stirrring. When the temperature and the pressure in the reactor were stabilized, the raw material solution and compressed air were fed in the reactor to start oxidation reaction. The raw material solution was continuously fed in the reactor over a period of one hour, while the flow rate of the feed air was regulated so that the concentration of oxygen in the exhaust gas was made to be 2% by volume. During the above period, the partial pressure of oxygen in the reactor was 0.12 kg/cm 2 (absolute pressure). After the feed of the raw material solution was finished, air was continuously fed for 9 minutes. After the completion of the oxidation reaction, the reactor was cooled to room temperature, and the resultant reaction product was taken out from the reactor and filtered by suction with a glass filter to separate the filter cake in the form of crystal. Thereafter the cake thus separated was cleaned by rinsing with 80.0 g of acetic acid containing 20% by weight of water. The cake was weighed and subsequently dried with a dryer to afford 40.82 g of crude naphthalenedicarboxylic acid (NDCA) in the form of crystal. Thus the NDCA in the from of dried crystal was obtained in a purity of 96.8% by weight at a yield of 95.2 mol% based on the 2,6-DMN fed in the reaction system. Comparative Example 5 DMN comprising as a principal component, 1,5-DMN which had been synthesized from o-xylene and butadiene as starting raw materials was incorporated with 2,7-DMN, MMN and TMN to form a DMN mixture, which was then isomerized in the presence of mordenite (produced by Tosoh Corporation) as the catalyst to afford a DMN isomerization reaction product. Subsequently 200 g of the DMN isomerization reaction product thus produced and 100 g of n-heptane were placed in a 400 mL glass container equipped with a stirrer under stirring and heating to 80°C to form a solution, which was gradually cooled to 20°C to precipitate crystal. The content in the glass container was filtered by suction with a G2 glass filter (standard maximum diameter of opening of 100 to 150 µm), and. the crystal thus obtained was rinsed three times each with n-heptane at 0°C. In Table 6 are given the chemical compositions of the DMN isomerization reaction product (starting raw material for cyrstallization), of the crystal after filtration and of the crystal after every rinsing. It can be seen from the table that 1,6-DMN, 1,5-DMN and 1,7-DMN each contained in the impurities are steadily removed by every rinsing, whereas 2,7-DMN, MMN and TMN are difficult to remove by means of rinsing.

Claims: A process for producing 2,6-dimethylnaphthalene, which comprises crystallizing 2,6-dimethylnaphthalene from a mixture of dimethylnaphthalene isomers with at least one solvent selected from the group consisting of aliphatic saturated hydrocarbons and alicyclic saturated hydrocarbons, while suppressing the adhesion of the crystals to the inside of a crystallizer by using a method in which a crystallizer equipped with a heat transfer surface is used and a coolant is passed in contact with the heat transfer surface while maintaining the difference in temperature between the coolant and the mixture of dimethylnaphthalene isomers with the solvent at 40 °C or lower. The process according to Claim 1, wherein the mixture of dimethylnaphthalene isomers consists of the isomerization reaction product of 1,5-dimethylnaphthalene which is formed from o-xylene and butadiene each as a starting raw material, or of the isomerization reaction product of the mother liquor which is formed by separating 2,6-dimethylnaphthalene crystal from said isomerization reaction product of 1,5-dimethylnaphthalene, or of both the isomerization reaction products. The process according to Claim 1 or 2, wherein the mixture of dimethylnaphthalene isomers contains at most 10% by weight of 2,7-dimethylnaphthalene based on the whole amount of the dimethylnaphthalene series. The process according to Claim 3, wherein the mixure of dimethylnaphthalene isomers contains at most 5% by weight of 2,7-dimethylnaphthalene, at most 3% by weight of methylnaphthalene and at most 10% by weight of trimethylnaphthalene each based on the whole amount of the dimethylnaphthalene series. A process for producing a 2,6-naphthalenedicarboxylic acid which comprises preparing 2,6-dimethylnaphthalene by a process according to any one of claims 1 to 4, and subjecting it to liquid-phase oxidation. The process according to Claim 5, wherein the 2,6-dimethylnaphthalene is obtained by crystallizing the isomerization reaction product of 1,5-dimethylnaphthalene produced from o-xylene and butadiene as starting raw materials. A process for producing a 2,6-naphthaienedicarboxylic acid ester which comprises preparing 2,6-naphtalenedicarboxylic acid by a process according to claims 5 or 6, and esterifying said 2,6-naphthalenedicarboxylic acid.

Inventors: AGEISHI KUNIAKI, KEDO KO, SIGEMATSU RYUSUKE, TAKAGAWA MAKOTO, AGEISHI, KUNIAKI, KEDO, KO, SIGEMATSU, RYUSUKE, TAKAGAWA, MAKOTO, Ageishi, Kuniaki, Mitsubishi Gas Chem. Co. Inc., Kedo, Ko, c/o Mitsubishi Gas Chem. Co. Inc.
Applicants: MITSUBISHI GAS CHEMICAL CO, MITSUBISHI GAS CHEMICAL COMPANY, INC.
Application year: 1997
Main classification: C07C7
IPC codes: C07C15, C07C7, C07C51