GB2182928A - Process for producing 2,6-naphthalenedicarboxylic acid - Google Patents

Abstract

2,6-Naphthalenedicarboxylic acid is prepared by oxidizing 2,6-diisopropylnaphthalene or an oxidation intermediate thereof in a solvent comprising an aliphatic monocarboxylic acid of 1 to 3 carbon atoms by molecular oxygen in the presence of a catalyst comprising (i) bromine and (ii) cobalt and/or manganese wherein the atomic ratio of bromine (i) to cobalt and/or manganese (ii) is from 1 x 10<-4> to below 1 x 10<-2>:1.

Description

SPECIFICATION Process for producing 2,6-naphthalenedicarboxylic acid The present invention relates to a process for producing 2,6-naphthalenedicarboxylic acid, which is utilized in the production of poly(ethylene naphthalate) which is useful as a raw material for producing films and fiber products and the other polyesters and polyamides.

Hitherto, as the process for producing 2,6-naphthalenedicarboxylic acid, a process for oxidizing 2,6dimethylnaphthalene, for instance by molecular oxygen in acetic acid and in the presence of a catalystcomprising cobalt, manganese and bromine has been known. Since the above-mentioned oxidation is carried out relatively easily, it is possible to obtain 2,6-naphthalenedicarboxylic acid at a relatively high purity and in a high yield.

However, the process for producing 2,6-dimethyl-naphthalene used as the starting substance in the above process is complicated and accordingly, it has been difficult to obtain 2,6-dimethylnaphthalene in a large amount and at a low cost. As the process for synthesizing 2,6-dimethyl-naphthalene, methylation of naphthalene, isomerization ofthe dimethylnaphthalene isomers other than 2,6-isomer, disproportionation of monomethylnaphthalene, trans-alkylation of monomethylnaphthalene, etc. have been known, however, by any one ofthe above-mentioned processes, it is impossibleto avoid the formation of the isomer(s) otherthan 2,6-dimethyl-naphthalene, particularly 2,7-dimethylnaphthalene.Further, since the 2,7-isomer closely resembles 2,6-dimethylnaphthalene in the melting point, boiling point and solubility, it has been extremely difficult to isolate 2,6-dimethylnaphthalene from the 2,7-dimethylnaphthalenes obtained by the above synthetic methods for 2,6-dimethylnaphthalene.

On the other hand, as compared to 2,6-dimethyl-naphthalene mentioned above, 2,6diisopropylnaphthalene can be easily synthesized by reacting naphthalene with propylene, and there is an advantage that alkylation, disproportionation, isomerization and trans-alkylation of 2,6-diisopropylnaphthalene can be carried out relatively easily. In addition, the melting point of 2,7-diisopropylnaphthalene and that of 2,6-diisopropylnaphthalene are remarkably different from each other and accordingly, 2,6diisopropylnaphthalene can be easily isolated from the diisopropylnaphthalenes.

However, according to the present inventors' studies, although 2,6-diisopropylnaphthalene can be easily available as shown above, in the case of oxidizing 2,6-diisopropylnaphthalene by applying the known reaction conditions suitable for oxidizing p-xylene and 2,6-dimethylnaphthalene, a large amount of by-products such as ring-brominatedproducts (bromonaphthalenedicarboxylic acids, etc.) is formed and accordingly, the yield and the purity of 2,6-naphthalenedicarboxylic acid are low. Consequently, the above known reaction conditions are not practical for producing 2,6-naphthalenedicarboxylic acid.

In this connection, in the case of using the catalyst used for oxidizing p-xylene and dimethylnaphthalene, namely, in the case where the catalyst comprising cobalt, manganese and bromine wherein the atomic ratio of the bromine to cobalt and manganese is 0.01 to 10 is used in the oxidation of 2,6-diisopropylnaphthalene, side reactions are promoted, and the production of ring-brominated products could not be controlled low even by reducing the amount of supply of 2,6-diisopropyi-naphthaleneto the reaction system for improving the dispersion of 2,6-diisopropylnaphthalene in the reaction system and reducing the concentration of 2,6diisopropylnaphthalene in the reaction system.

In order to solve these problems, for instance, a process for producing naphthoic acid and/or naphthalenedicarboxylic acid by oxidizing mono- and/or dialkylnaphthalene having at least one ethyl group and/ or isopropyl group by molecular oxygen in the presence of a catalyst containing cobalt, manganese and bromine in the following ratio has been proposed (Japanese Patent Publication No. 48-27318 (1983)): X+Y+Z2.0 Y=0.15/X 0.2 < 10 =X+Y - wherein X, Y and Z respectively represent the weight ratio of cobalt atom, manganese atom and bromine atom per one part by weight of the alkylnaphthalene used in the reaction.

However, according to the above proposed method, due to the formation of a large amount of by-products, the yield of the object product is only about 50%, and the proposed process cannot be said to be an industrially advantageous process.

Further, a process for producing 2,6-naphthalenedicarboxylic acid comprising oxidizing 2,6 diisopropylnaphthalene or an oxidation intermediatethereof in a solventcontaining at least 50% byweight of an aliphatic mono-carboxylic acid of 1 to 3 carbon atoms by molecular oxygen in the presence of a catalyst comprising (i) heavy metal of cobalt and/or manganese and (ii) bromine at the ratio of at least 0.2 mol ofthe heavy metal as a constituting componentofthe catalyst per mol of 2,6-diisopropylnaphthalene or the oxidation intermediate (Japanese Patent Application Laying-Open (KOKAI) No. 60-89445 (1985)), and a process for producing 2.6-naphthalenedicarboxylic acid, comprising oxidizing 2,6-diisopropylnaphthalene or the oxidation intermediate in a solvent containing at least 50% by weight of an aliphatic monocarboxylic acid by molecular oxygen in the presence of a catalyst comprising (i) heavy metal of cobalt and/or manganese and (ii) bromine in an amount of at least 1 % by weight ofthe heavy metal as a constituting component ofthe catalyst to the amount ofthe aliphatic carboxylic acid of 1 to 3 carbon atoms (Japanese Patent Application Laying Open (KOKAI) No.60-89446(1985)) have been proposed.

In both of the above PatentApplications (KOKAI), it was described that the amount of bromine used in the oxidation is preferably from 0.01 to 2 in the atomic ratio to the total amount of cobalt and/or manganese and that a part of the used bromine is converted to the ring-brominated products.

Although the ring-brominated product of 2,6-diisopropylnaphthalene and that ofthe oxidation intermediate thereof are converted to carboxylic acid by the oxidation of the side chain in both the processes, since the physical properties ofthese carboxylic acids ofthe ring-brominated products closely resemble the physical properties of 2,6-naphthalenedicarboxylic acid, it has been extremely difficult to isolate and purify 2,6-naphthalenedicarboxylic acid from the reaction products.

Concerning the ring-brominated products, for instance, Japanese Patent Publication No.56-3858(1981) discloses that the removal ofthe bromine derivative(s) from the reaction product is most important in the purification of 2,6-naphthalenedicarboxylic acid and that in the case where such a bromine derivative is contained in 2,6-naphthalenedicarboxylic acid, the melting point ofthe resin obtained by using such 2,6naphthalenedicarboxylicacid is lowered resulting inthefatal defect.

In addition, the above-mentioned Patent Publication discloses in its example that the crude 2,6naphthalenedicarboxylicacid contains from 1000to 2000 ppm of bromine and lOto 40 ppm of bromine remains in the purified product even after carrying the purification by various methods. Namely, a process for producing 2,6-naphthalenedicarboxylic acid, being capable of controlling the formation of the ringbrominated products has been desired.

As a result of the present inventors' detailed studies on the oxidation of 2,6-diisopropylnaphthalene,the present inventors have fond that the contribution of bromine to the oxidation of the first one of the isopropyl groups (first step) is differentfrom thatto the subsequent oxidation of the second one of the isopropyl groups (second step) in the oxidation reaction of 2,6-diisopropylnaphthalene with molecular oxygen in the presence ofcobaltand/or manganese and bromine.

Namely, in thefirststep, bromine is not necessaryorthe presenceofan extremely small amountofbromine may be sufficient. In this case, the presence of a large amount of bromine raisesthe largeamountof formation ofthe ring-brominated products as well as the formation of by-products such as the polycondensates which hinderthe second step. Further, it has been found by the present inventors that in the second step, the presence of a minute amount of bromine is unexpectedly effective for the oxidation ofthe second isopropyl group in the case where the amount ofthe by-products formed in the first step is small.In addition, in the second step, the presence of a large amount of bromine raises a large amountoftheformation of the ring-brominated products and other by-products.

On the basis ofthe findings, the present inventors have further found that 2,6-naphthalenedicarboxylic acid can be advantageously availablewith a remarkably small amount ofthe by-products such as the ringbrominated products by oxidizing 2,6-diisopropylnaphthalene in the presence of a catalyst comprising cobalt and/or manganese and an extremely small amount of bromine, and on the basis ofthisfindings,the present inventors have attained the present invention.

In an aspect of the present invention, there is provided a process for producing 2,6naphthalenedicarboxylic acid, which process comprises the step of oxidizing 2,6-diisopropylnaphthalene or an oxidation intermediate thereof in a solvent comprising an aliphatic monocarboxylic acid of 1 to 3 carbon atoms in the presence of a catalyst comprising bromine and cobalt, manganese or a mixture of cobalt and manganese wherein the atomic ratio of the amount of bromine to the amount of cobalt, manganese orthe mixture of cobalt and manganese is not less than 1 x 1 and a n d below 1 x 10-2.

The oxidation intermediate of 2,6-diisopropylnaphthalene herein mentioned arethe compounds represen- ted by the formula(I):

wherein R1 represents

and R2 represents

As cobalt and manganese which are respectively the components ofthe catalyst of the present invention, oxide, hydroxide, inorganic salts such as carbonate and halides, salts of fatty acid-such at formic acid, acetic acid, propionic acid, salts of aromatic carboxylic acid such as naphthenic acid may be mentioned, and of these compounds, salts of fatty acid, particularly the salt of acetic acid is preferable.

As bromine, eitherthe organic compound thereoforthe inorganic compound thereof may be used so long as the compound dissolves in the oxidation reaction system to form bromide ion. Molecular bromine, inorganic bromine compounds such as hydrogen bromide, bromides, etc., alkyl bromides such as methyl bromide and ethyl bromide and brominated fatty acids such as bromoacetic acid may be mentioned. Ofthese substances, hydrogen bromide, sodium bromide, potassium bromide, ammonium bromide, cobalt bromide and manganese bromide are preferable.

In the present invention, the bromine compound is used in such an amount that the atomic ratio of bromine to cobalt, manganese orthe mixture of bromine and cobalt is in the range of not less than 1 x 10-4 and below 1 x 10-2, preferably from 5x 10into 8x 10-3.

In the case where the atomic ratio of bromine is lowerthan 1 x 10-4, the oxidation of 2,6diisopropylnaphthalene into 2,6-naphthalenedicarboxylic acid is carried out slowly and such a slow oxidation cannot be said to be industrially advantageous, and on the other hand, in the case where the atomic ratio of bromine is not less than 1 x 0-2, the side reactions are promoted to form a large amount of the ringbrominated products, and accordingly, it is not favorable to use bromine of an amount which is out ofthe range of not less than 1 x 10-4 and below 1 x 1 10-2.

In this connection, in the case where the oxidation catalyst containing such an extremely small amount of bromine as in the present invention is applied to the oxidation of an aromatic hydrocarbon having methyl groups as the side chain thereof such as p-xylene and 2,6-dimethylnaphthalene,the catalytic effect is too low to be applied in practical use. On the other hand, since the side chain of 2,6-diisopropylnaphthalene and the oxidation intermediates thereof is highly active as compared to methyl group, bromine can effectively cata lyre the oxidation of 2,6-diisopropylnaphthalene and the oxidation intermediates thereof in such a small amount as in the present invention, and in addition, it is possible to suppress the formation ofthe ringbrominated products caused by bromine because of a small amount of bromine.

Namely, in the case where 2,6-naphthalenedicarboxylic acid is produced by oxidizing 2,6diisopropylnaphthalene and the oxidation intermediates thereof, it is possible to suppress the formation of the ring-brominated products which exert a bad influence upon the physical properties of poly-(ethylene naphthalate), etc. which are made from 2,6-naphthalenedicarboxylic acid and to obtain 2,6naphthalenedicarboxylic acid advantageously by using the catalyst of the present invention containing an extremely small amount of bromine.

On the other hand, the amount of cobalt and/or manganese which are/is the heavy metal component ofthe catalyst used in the present invention is in the range offrom 0.03 to 0.15 mol, preferably from 0.04 to 0.12 mol as metal to 100 g of the solvent used in the reaction.

In addition, although cobalt and manganese can be used individually or as a mixture, the mixture shows a higher catalytic activity, and in the case of using the mixture, although the ratio of cobaitto manganese is not restricted, it is preferable that the atomic ratio of cobalt to manganese is in the range of from 5:95 to 70:30.

Besides, as the solvent for oxidation reaction, any aliphatic monocarboxylic acid of 1 to 3 carbon atoms may be used and formic acid, acetic acid, propionic acid, etc. may be exemplified, however, acetic acid is most preferable.

In the present invention, 2,6-diisopropylnaphthalene and the oxidation intermediates thereof asthestarting substance may be supplied at a concentration of not more than 20 parts by weight to 100 parts by weight ofthe solvent, and it is not favorable to supply the starting substance at a higher concentration than that because the solubility of oxygen into the solvent is reduced.

The molecular oxygen used in the oxidation is used as pure molecular oxygen or a gaseous mixture of oxygen and an inert gas, however, air may be satisfactorily uSed for practical use.

Although the oxidation reaction in the present invention proceeds more rapidly as the partial pressure of oxygen in the system is higher, the partial pressure of oxygen of higher than 0.1 kg/cm2G, preferably from 0.2 to 8 kg/cm2G is practically sufficient.

In the case of using a gaseous mixture of molecular oxygen and an inert gas, although the total pressure of the gaseous mixture is not particularly limited,the reaction proceeds rapidly usually at the total pressure of from 4to 30 kg/cm2G.

The oxidation is usually carried out at the reaction temperature of from 140 to 21 OOC, preferably from 160to 200"C for 1 to 10 hours, preferably from 2 to 7 hours. At the temperature below 140"C, the reaction proceeds slowly, and since the combustion loss of the solvent in the system is raised at the temperature over 21 00C, such a high temperature is not favorable.

In carrying outthe process for producing 2,6-naphthalenedicarboxylic acid according to the present invention, the oxidation reaction may be carried out by mixing 2,6-diisopropylnaphthalene or the oxidation intermediate thereof with an aliphatic monocarboxylic acid of not more than 3 carbon atoms and a catalyst comprising bromine and cobalt, manganese or a mixture of cobalt and manganese wherein the atomic ratio of bromineto cobalt, manganese our a mixture of cobalt and manganese is not less than 1 x 10-4 and below 1 x 10-2 and by heating thethusformed mixturewhile introducing pure molecular oxygen ora mixture of molecular oxygen and an inertgas into the mixture.

However, in order to suppress the side reactions and to obtain highly pure 2,6-naphthalenedicarboxylic acid in a high yield, it is more preferable to carry outthe oxidation reaction by continuously and separately supplying a mixture of the aliphatic monocarboxylic acid and the catalyst and 2,6-diisopropylnaphthalene or the oxidation intermediate thereof while introducing pure molecular oxygen or a mixture of molecular oxygen and an inert gas into the reaction system under a pressure, because it is possible to maintain a low concentration of 2,6-diisopropylnaphthalene or the oxidation intermediate thereof in the reaction system.

The present invention and the effectthereofwill be more precisely explained while referring tothefollowing nonlimitative Examples.

Example 1: Into a 5-litre titanium autoclave provided with a reflux condenser, a gas inlet tube, a gas outlettube and a stirrer, a mixture of 2070 g of glacial acetic acid, 132 g of cobalt acetate tetrahydrate, 391 g of manganese acetatetetrahydrate and 1 g of ammonium bromide was introduced at a rate of 752 g per hour and while maintaining the content of the autoclave at 180"C under a pressure of 10 kg/cm2G, compressed airwas introduced into the autoclave with vigorous stirring at such a rate that oxygen was supplied at a rate of 171 g/hour.

The atomic ratio of bromine to cobalt and manganese in the system [Br/(Co + Mn)l was 5 x10-3.

In the next place, 2,6-diisopropylnaphthalene (hereinafter referred to as DIPN) was supplied into the reaction system at a rate of 60 g/hourto carry outthe reaction.

After 20 hours, a precipitate mainly comprising 2,6-naphthalenedicarboxylic acid (hereinafter referred to as NDCA) was collected from the thus obtained reaction mixture by filtration and washed with hotacetaic acid to obtain crude NDCA.

The yield ofthe crude NDCA based on the starting substance, DlPN, was 85.3% and the content of bromine in crude NDCA was 105 ppm.

The thus obtained crude NDCA was subjected to purification according to the procedures disclosed in Japanese Patent Publication No.56-3858(1981) as follows.

In the first place, 9 of crude NDCA were added to 80 g of aqueous 5% solution of sodium hydroxide and after removing a small amountofalkali-insoluble matters from thethusformed mixture byfiltration, the pH ofthefiltrate was reduced to 7 with 6N hydrochloric acid to precipitate monosodium salt of NDCA.

In the next place, 9 of monosodium saltofNDCAwere added to 200 g ofwaterandthe pH ofthe mixture was reduced to 2with 6N hydrochloric acid atabout90 Cto precipitate NDCA,therebyobtaining purified NDCA. The content of bromine in the thus obtained, purified NDCAwas less than 1 ppm.

Example2: In a similar autoclave to that in Example la a mixture of 2070 g ofglacial acetic acid, 132 g of cobalt acetate tetrahydrate, 391 g of manganese acetate tetrahydrate and 0.1 g of ammonium bromide was introduced at a rate of 1000 g/hour. The atomic ratio ofbromineto cobalt and manganese [Br/(Co + Mn)] was 5 x10-4.

In the next place, while vigorously stirring the mixture in the autoclave at 1 80"C under a pressure of 20 kg/cm2G, compressed air was introduced into the mixture at such a rate that oxygen was supplied at a rate of 180 g/hour and at the same time, DlPN was supplied into the mixture at a rate of 80 g/hourto carry outthe reaction.

After 20 hours, crude NDCAwas collected by filtration in the same manner as in Example 1. The yield ofthe crude NDCA based on the starting substance, DIPN, was 73.3% and the content of bromine in crude NDCA was 40 ppm.

Comparative Example?: In the same manner as in Example 1 except for using a mixture containing 14.9 g of ammonium bromide (the atomic ratio of bromine to cobalt and manganese is 7.5 x 10-2) instead of 1 g of ammonium bromide, the reaction was carried out.

As the results, the yield of the crude NDCA based on the starting substance, DlPN, was 64.9% and the content of bromine in the crude NDCAwas as large as 3765 ppm.

On subjecting the crude NDCAto purification in the same manners in Example 1 ,the content of bromine in the purified NDCAwas reduced only to 83 ppm.

Example 3: Into a similar autoclave to that in Example 1, a mixture of 2410 g ofglacial acetic acid, 97 g of cobalt acetate tetrahydrate, 193 g of manganese acetate tetrahydrate and 0.12 g of ammonium bromide was introduced at a rateof870 g/hour. The atomic ratio of bromine to cobalt and manganese [Br/(Co + Mn)] was 1.1 x 10-3.

In the next place, DIPN was introduced into the autoclave atthe rate of 80 g/hourwhile vigorously stirring the mixture at 170 C under a pressure of 30 kg/cm2G and introducing compressed air into the mixture at such a rate that oxygen was supplied at the rate of 174 g/hour.

The yield ofthe crude NDCAwas 72.3% based on the starting substance DIPN, and the content of bromine in crude NDCAwas 53 ppm.

Example 4: Into a similarautoclave to that in Example 1, a mixture of 2070 g of glacial acetic acid, 350 g ofcobaltacetate tetrahydrate, 170 g of manganese acetatetetrahydrate and 1.5 g of ammonium bromide was introduced ata rateof 1296 g/hour. The atomic ratio of bromineto cobaltand manganese [Br/(Co + Mn)] in the mixturewas 7.4x 10-3.

In the next place, while keeping the thus introduced mixture at 185"C under a pressure of 20 kg/cm2G and under a vigorous stirring, compressed air was introduced into the mixture at such a ratethat oxygen was supplied at a rate of 240 g/hourand at the same time, DIPN was introduced into the autoclave at a rate of 140 g/hour to carry out the reaction.

The yield ofthe crude NDCA based on the starting material, DIPN, was 80.7% and the content of bromine in crudeNDCAwas212ppm.

On purifying the crude NDCA in the same manner as in Example 1 ,the content of bromine in the purified NDCA was reduced to 2 ppm.

Comparative Examples 2 to 5: Into a 200 ml titanium autoclave provided with a reflux condenser, a gas inlet tube, a gas outlet tu be and a stirrer, 100 g of glacial acetic acid, each of the catalysts shown in Table 1 and 10g of DIPN (Comparative Example 2) or 10g of 2,6-dimethylnaphthalene (referred to as DMN, Comparative Examples 3 to 5) was introduced.

The oxidation was carried out for 3 hours by introducing air into the mixture in the autoclave at such a rate that oxygen was supplied at a rate of 8 g/hour, at 180"C under a pressure of 10 kg/cm2G.

The results are shown also in Table 1.

TABLE 1 No. of Comparative Example 2 3 4 5 Starting substance DIPN DMN DMN DMN Cobaltacetate-4H2O 0.59 0.5g 0.5g 6.4g Catalyst Manganeseacetate'4H2O 1.0g 1.0g 1.0g 18.9g Ammonium bromide lOg g lOg 9 0.1 g 0.05g Unreacted starting substance 0% 1.5% 47.0% 78.3% Product YieldofcrudeNDCA 32% 87.2% 45.6% 18.5% Bromine content in crude NDCA 4010 ppm 1480 ppm 580 ppm 275 ppm

Claims (8)

1. A process for producing 2,6-naphthaienedicarboxylic acid, which process comprises oxidizing 2,6 diisopropylnaphthalene or an oxidation intermediate thereof in a solvent comprising an aliphatic monocarboxylic acid of 1 to 3 carbon atoms by molecular oxygen in the presence of a catalyst comprising (i) bromine and (ii) cobalt and/or manganese wherein the atomic ratio of bromine (i)to cobaltand/ormanganese (ii) is from 1 x 10-4to below 1 x10-2:1

2. A process according to ciaim 1,wherein 1 wherein said oxidation is carried out at a temperature of from 140 to 210 C under a pressure offrom 4to 30 kg/cm2G.

3. A process according to claim 1 or 2, wherein the amount of said cobalt and/or manganese is from 0.03 to 0.15 mol per 100 g of said aliphatic monocarboxylic acid.

4. A process according to any one ofthe preceding claims, wherein the atomic ratio of cobalt to man ganeseisfrom5:95to70:30.

5. A process according to any one of the preceding claims, wherein said aliphatic monocarboxylic acid is acetic acid.

6. A process according to any one of the preceding claims, wherein the atomic ratio of bromine (i)to cobalt and/or manganese (ii) is from 5x 10-4to 8 x 0: 1.

7. A processforthe preparation of 2,6-naphthalenedicarboxylic acid, said process being substantially as hereinbefore described in any one of Examples 1 to 4.

8. Poly(ethylene naphthalate) produced from 2,6-naphthalenedicarboxylic acid prepared by a process as claimed in any one of the preceding claims.