Process for producing naphthalenedicarboxylic acid together with trimellitic acid

Disclosed herein is a process for producing 2,6-naphthalenedicarboxylic acid together with trimellitic acid comprising oxidizing 2,6-diisopropylnaphthalene with molecular oxygen in the presence of a catalyst comprising a heavy metal(s) and bromine.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for producing 
2,6-naphthalenedicarboxylic acid together with trimellitic acid by 
oxidation of 2,6-diisopropylnaphthalene (hereinafter referred to as 
"2,6-DIPN"). 
2,6-naphthalenedicarboxylic acid is useful as a starting material for 
polyethylene naphthalate, polyester, polyamide, etc. which are useful as a 
material for films and synthetic fibers having excellent heat resistance 
and mechanical properties, while anhydrous trimellitic acid is useful as a 
starting material for heat-resistant plasticizing ester, polyimide resins, 
curing agents for epoxy resin, coating materials and stabilizers. 
Conventionally, trimellitic acid was synthesized by oxidation of 
pseudocumene with chromic acid or oxidation of rosin with nitric acid, but 
at present it is more industrially produced by air-oxidation of 
pseudocumene. 
As a process for producing 2,6-naphthalenedicarboxylic acid (hereinunder 
referred to as "2,6-NDCA"), a method of oxidizing 2,6-dialkylnaphthalene 
such as 2,6-dimethylnaphthalene and 2,6-diisopropylnaphthalene in a 
solvent of acetic acid with molecular oxygen in the presence of a catalyst 
comprising cobalt and/or manganese, and bromine has been proposed (e.g., 
Japanese Patent Publication No. 48-27318 (1973), Japanese Patent 
Application Laying-Open (KOKAI) No. 60-89445 (1985) and Japanese Patent 
Application Laying-Open (KOKAI) No. 60-89446 (1985)). 
According to such a method, since a large amount of bromine is used as 
compared with that of a heavy metal(s) in a mixed catalyst in order to 
obtain 2,6-NDCA with high yield, not only the alkyl group of 
2,6-dialkylnaphthalene is oxidized but also the naphthalene ring is 
brominated, thereby producing various kinds of 
bromonaphthalenedicarboxylic acids as by-products as well as 2,6-NDCA. 
Since the physical and chemical nature of these 
bromonaphthalenedicarboxylic acids resemble those of 2,6-NDCA, it is very 
difficult to separate those bromonaphthalenedicarboxylic acids from 
2,6-NDCA for purification. Therefore, removal of by-product bromides from 
crude 2,6-NDCA is the most important in producing 2,6-NDCA, as described 
in detail, for example, in Japanese Patent Publication No. 56-3858 (1981). 
Resins such as polyethylene naphthalate obtained from 2,6-NDCA 
contaminated by remaining bromides have a low softening point, and it is 
difficult to produce a film, fiber, etc. having high heat resistance and 
good mechanical properties out of such a contaminated 2,6-NDCA. In the 
above-described methods for producing 2,6-NDCA, aldehydes and colored 
substances having unknown structures are undesirably produced as 
by-products as well as bromonaphthalenedicarboxylic acids. These 
by-products are also difficult to separate and remove from 2,6-NDCA. 
Therefore, a conventional process for producing 2,6-NDCA requires many 
steps for separation and purification of 2,6-NDCA. 
For example, in the method for producing 2,6-NDCA disclosed in Japanese 
Patent Publication No. 56-3858 (1981), 2,6-NDCA is obtained by oxidizing 
2,6-dimethylnaphthalene with molecular oxygen in the presence of a 
catalyst comprising bromine, cobalt and manganese. Since a large amount of 
bromine is used as compared with that of cobalt and manganese (the atomic 
ratio of bromine to cobalt and manganese is 1.7), 1,000 to 2,000 ppm of 
bromine is contained in the crude 2,6-NDCA which is separated from the 
reaction mixture and 10 to 40 ppm bromine is still contained even after 
purification. 
As described above, in the conventional processes for producing 2,6-NDCA 
using a large amount of bromine, a large amount of 
bromonaphthalenedicarboxylic acids, aldehydes, colored substances having 
unknown structure are produced as by-products. Therefore, many complicated 
steps of purification are required for producing high-purity 2,6-NDCA to 
remove these by-products from the crude 2,6-NDCA, so that the conventional 
processes are unsatisfactory. Particularly, removal of bromides which have 
a fatal influence on resin products obtained from 2,6-NDCA is difficult. 
Thus, it is extremely difficult to produce 2,6-NDCA of a satisfactorily 
high purity by a conventional process. 
As a result of researches by the present inventors on a process for 
producing high-purity 2,6-NDCA, in particular, 2,6-NDCA substantially free 
from bromine, it has been found that if the atomic ratio of bromine to 
cobalt and/or manganese in the catalyst is reduced much more than in a 
conventional process, in a process for producing 2,6-NDCA by oxidizing 
2,6-DIPN with molecular oxygen in the presence of a catalyst comprising 
cobalt, manganese or a mixture thereof and bromine, production of 
by-products, in particular, production of bromides such as a 
bromonaphthalenedicarboxylic acid is suppressed, thereby producing 
2,6-NDCA with high purity together with high-purity trimellitic acid and, 
in addition, the recovery and regeneration of a catalyst of a heavy 
metal(s) is facilitated. The present invention has been achieved on the 
basis of this finding. 
SUMMARY OF THE INVENTION 
In an aspect of the present invention, there is provided a process for 
producing 2,6-naphthalenedicarboxylic acid together with trimellitic acid 
comprising the steps of: 
oxidizing 2,6-diisopropylnaphthalene in a solvent of a lower aliphatic 
monocarboxylic acid containing not more than 30 wt % water with molecular 
oxygen in the presence of a catalyst comprising cobalt, manganese or a 
mixture thereof and bromine of not less than 0.0001 and less than 0.01 in 
an atomic ratio to the heavy metals; 
cooling the reaction mixture and separating 2,6-naphthalenedicarboxylic 
acid and the heavy metal salt(s) of trimellitic acid as a solid; 
adding the separated mixture to an aqueous solution of a mineral acid so 
that the pH of the solution is not higher than 3 to dissolve the heavy 
metal salts of trimellitic acid, and filtering out insoluble 
2,6-naphthalenedicarboxylic acid; 
concentrating the filtrate while maintaining the pH of the filtrate at not 
higher than 3, cooling the filtrate, and filtering out trimellitic acid; 
and 
adding an alkali carbonate, an alkali dicarbonate, or the mixture thereof 
to the filtrate to recover the heavy metal(s) as an alkali carbonate(s), a 
basic alkali carbonate(s) or a mixture thereof. 
DETAILED DESCRIPTION OF THE INVENTION 
A process for producing 2,6-naphthalenedicarboxylic acid together with 
trimellitic acid according to the present invention comprises the steps of 
oxidizing 2,6-DIPN in a solvent of a lower aliphatic monocarboxylic acid 
containing not more than 30 wt % water with molecular oxygen in the 
presence of a catalyst comprising cobalt, manganese or a mixture thereof 
and bromine of not less than 0.0001 and less than 0.01 in an atomic ratio 
to the heavy metal or metals; 
cooling the reaction mixture and separating 2,6-NDCA and the heavy metal 
salt(s) of trimellitic acid as a solid; 
adding the separated 2,6-NDCA and heavy metal salt(s) of trimellitic acid 
to an aqueous solution of a mineral acid so that the pH of the solution is 
not higher than 3 to dissolve only the heavy metal salt(s) of trimellitic 
acid, and separating 2,6-NDCA which is insoluble in the aqueous solution 
of mineral acid by filtration; 
concentrating the filtrate while maintaining the pH of the filtrate at not 
higher than 3, cooling the filtrate and separating trimellitic acid by 
filtration; and 
adding an alkali carbonate, an alkali hydrogen carbonate, or the mixture 
thereof into the filtrate to recover the heavy metal(s) as a precipitate 
of an alkali carbonate(s), a basic alkali carbonate(s) or a mixture 
thereof. 
The solvent used in the present invention is a lower aliphatic 
monocarboxylic acid containing not more than 30 wt %, preferably not more 
than 15 wt % water. If the water content is over 30 wt %, not only is the 
rate of oxidation greatly reduced, but also the heavy metal salts of 
trimellitic acid are dissolved in the solvent, thereby reducing the 
recovery of the heavy metal salt(s) of trimellitic acid in separating as 
insoluble(s). In addition, the purity of trimellitic acid is 
disadvantageously lowered. As the lower aliphatic monocarboxylic acid, for 
example, formic acid, acetic acid, propionic acid and butyric acid, which 
have not more than 4 carbon atoms, may be used. Among these, acetic acid 
is the most preferable. 
The cobalt and manganese components of the catalyst used in the present 
invention are exemplified by inorganic compounds such as oxides, 
hydroxides, carbonates and halides of cobalt or manganese, and cobalt 
salts or manganese salts of fatty acids such as formic acid, acetic acid 
and propionic acid and organic acids such as naphthenic acid and aromatic 
carboxylic acid. Among these, salts of fatty acids and, in particular, 
cobaltous acetate and manganese acetate are preferable. 
As the bromine component of the catalyst used in the present invention, any 
of bromine element, inorganic and organic compounds of bromine is usable 
so long as it is dissolved in a solvent for oxidation reaction and 
generates bromide ions. For example, molecular bromine, inorganic bromine 
compounds such as hydrogen bromide and inorganic bromides, alkyl bromides 
such as methyl bromide and ethyl bromide, and brominated fatty acids such 
as bromoacetic acid may be used. Among these, hydrogen bromide, sodium 
bromide, potassium bromide, ammonium bromide, cobaltous bromide, manganese 
bromide, etc. are preferable. 
As the molecular oxygen used in oxidation, oxygen gas or a mixed gas of 
oxygen and inert gas is usable, but compressed air is industrially 
preferable. The higher the partial pressure of oxygen in the reaction 
system is, the more quickly oxidation proceeds, but in practical use, the 
partial pressure of oxygen of not less than 0.1 kg/cm.sup.2 (absolute 
pressure; the same shall apply hereinafter), preferably 0.2 to 8 
kg/cm.sup.2 is sufficient, and little industrial merit is found in 
adopting a partial pressure exceeding 8 kg/cm.sup.2. When a mixed gas is 
used, the total pressure is not specifically limited, but a pressure of 7 
to 30 kg/cm.sup.2 is sufficient for practical use. 
The oxidation temperature is 140.degree. to 210.degree. C., preferably 
160.degree. to 200.degree. C. If the temperature is lower than 140.degree. 
C., the reaction rate is greatly lowered, while an oxidation temperature 
exceeding 200.degree. C. causes disadvantageous increase of the loss of 
the solvent by burning. 
The amount of cobalt, manganese or a mixture thereof used in the present 
invention is preferably 0.005 to 0.2 mol, more preferably 0.01 to 0.12 
mol, to 100 g of the solvent used for reaction. If a large amount of 
cobalt and manganese or a mixture thereof is used beyond 0.2 mol to 100 g 
of the solvent, the amount of by products is increased. On the other hand, 
if the amount thereof is less than 0.005 mol to 100 g of the solvent, the 
yields of 2,6-NDCA and trimellitic acid are disadvantageously reduced. 
Cobalt and manganese may be used singly or as a mixture thereof, but a 
mixture is preferable, because it exhibits a higher activity. In this 
case, the mixing ratio of cobalt and manganese is not specially restricted 
because it depends on other reaction conditions such as the reaction 
temperature, the amount of bromine and the partial pressure of oxygen, but 
the atomic ratio of cobalt to manganese is preferably 5:95 to 70:30. 
The amount of bromine is not less than 0.0001 and less than 0.01, 
preferably 0.0005 to 0.008 in an atomic ratio to the total amount of 
cobalt and manganese. 
Japanese Patent Application Laying-Open (KOKAI) No. 60-89445 (1985) 
discloses that although the amount of bromine depends upon not only the 
concentration of a heavy metal(s) but also the reaction conditions such as 
the reaction temperature, the concentration of a starting material and the 
amount of a solvent, but it is preferably 0.01 to 2 in an atomic ratio to 
the amount of the heavy metal. However, according to the findings of the 
present inventors, use of bromine of not less than 0.01 in an atomic ratio 
to a heavy metal produced a considerable amount of 
bromonaphthalenedicarboxylic acid as a by-product which was difficult to 
remove, and it was extremely difficult to obtain high-purity 2,6-NDCA and 
trimellitic acid. Even if bromine of less than 0.01 in an atomic ratio to 
cobalt and manganese is used, a bromide is inevitably produced as a 
by-product, but the amount of by-product is very small, and it is easy to 
reduce the bromine content in 2,6-NDCA and trimellitic acid to not more 
than 1 ppm by the purification method described, for example, in Japanese 
Patent Publication No. 56-3858 (1981). Since use of an extremely small 
amount of bromine reduces the oxidation rate economically 
disadvantageously, it is preferable to use bromine of not less than 0.0001 
in an atomic ratio to cobalt and manganese. 
Incidentally, when the catalyst used in the present invention which 
contains a trace amount of bromine is used in oxidation of aromatic 
hydrocarbon having a methyl group such as p-xylene and 
2,6-dimethylnaphthalene, the activity of the catalyst is too low for 
practical use. In contrast, when the catalyst is used in oxidation of 
2,6-DIPN, since the hydrogen on the tertiary carbon of the isopropyl group 
is more active than the hydrogen of the methyl group, oxidation proceeds 
in the presence of such a catalyst in which the bromine content is 
extremely small and, in addition, it is possible to suppress the 
production of a by-product(s) such as bromonaphthalenedicarboxylic acid, 
as described above. 
In the present invention, the amount of the starting material 2,6-DIPN 
supplied into the reaction system is preferably not more than 20 parts by 
weight to 100 parts by weight of the reaction solvent. If more than 20 
parts by weight of 2,6-DIPN to 100 parts by weight of the solvent exists 
in the reaction system, the concentration of unstable oxidation 
intermediates increases, while the amount of molecular oxygen dissolved in 
the solvent relatively decreases. As a result, the amount of by-products 
such as aldehydes, oxidative-polymerization products and colored 
substances increase, thereby disadvantageously reducing the yields of 
2,6-NDCA and trimellitic acid. The amount of 2,6-DIPN in the reaction 
system is preferably not more than 0.4, more preferably 0.05 to 0.3 in a 
mol ratio to the total amount of cobalt and manganese in the oxidation 
catalyst in order to suppress the side reaction. 
When 2,6-DIPN is oxidized under the above-described conditions, 2,6-NDCA 
and trimellitic acid can be produced advantageously. 
2,6-NDCA, which has a very small solubility in the reaction solvent, 
separates out with the proceeding of the reaction and about 98% of the 
produced 2,6-NDCA separates out as a free acid by the end of the reaction. 
On the other hand, trimellitic acid is highly soluble in the reaction 
system, but when substantially all the 2,6-DIPN in the reaction system is 
consumed and the concentration of the oxidation intermediate, 
6-isopropyl-2-naphthoic acid is reduced down to not more than 1%, 
trimellitic acid forms the insoluble 1:1 salt(s) with cobalt, manganese, 
or a mixture thereof, and about 97% of the produced trimellitic acid 
separates out. 
After the end of the reaction, the reaction mixture is cooled to 90.degree. 
C. or lower and filtered to obtain a mixture of 2,6-NDCA and the heavy 
metal salt(s) of trimellitic acid. It is possible to use the filtrate 
repeatedly as the solvent for reaction after adjusting the water content 
thereof, if necessary. 
When the mixture of 2,6-NDCA and the heavy metal salt(s) of trimellitic 
acid obtained is added to an aqueous solution of a mineral acid, such as 
hydrochloric acid, nitric acid or sulfuric acid and the solution is 
adjusted to pH of not higher than 3, preferably 1 to 2, the heavy metal 
salt(s) of trimellitic acid alone are dissolved. When the insoluble 
2,6-NDCA is filtered out and washed with water, crude 2,6-NDCA having a 
purity of about 98% and containing not more than 100 ppm bromine is 
substantially quantatively recovered. 
The filtrate is next concentrated while maintaining the pH thereof at not 
higher than 3, preferably 1 to 2 and cooled to separate trimellitic acid, 
which is washed with water to recover trimellitic acid having a purity of 
98% and containing not more than 50 ppm of bromine. Preferably the steps 
of concentration and separation are repeated several times in order to 
facilitate the filtration and prevent trimellitic acid being accompanied 
by the heavy metals. If the pH of the filtrate is higher than 3, 
trimellitic acid is contaminated by the heavy metals and it is difficult 
to obtain high-purity trimellitic acid, so the adjustment of pH is 
important. 
The filtrate from which trimellitic acid has been separated contains cobalt 
and manganese dissolved therein, and it is economically important to 
recover the heavy metals. In order to recover the heavy metals, an alkali 
carbonate, an alkali hydrogen carbonate or a mixture thereof is added to 
the filtrate so that the pH of the solution is not less than 7, preferably 
9 to 10. Then, cobalt and/or manganese are precipitated as a 
water-insoluble carbonate(s) and/or a basic carbonate(s), thereby 
facilitating recovery of the heavy metal(s). In this case, it is important 
to use the above-described alkali carbonate and/or alkali hydrogen 
carbonate to adjust the pH. For example, if an alkali hydroxide is used, 
the heavy metal salt(s) is precipitated in the form of such a fine 
precipitate(s) that it is difficult to filter it out and to remove other 
alkali metal salts which have adhered to the precipitated heavy metal 
salt(s) by washing with water. If the heavy metal(s) containing such 
alkali metal salts as impurities are used again as an oxidation 
catalyst(s), the alkali metals are stored in the reaction system and 
exercise a deleterious effect on oxidation. 
According to the present invention, it is easy to control the ratio of the 
amounts of 2,6-NDCA and trimellitic acid to be produced in the range in 
which the yield of trimellitic acid does not exceed 60% by controlling, 
for example, the amount of the catalyst used in oxidation, namely the 
amount of cobalt and/or manganese and bromine. For example, when 0.017 mol 
of 2,6-DIPN is oxidized in 100 g of acetic acid with compressed air of 20 
kg/cm.sup.2 at 180.degree. C., if the catalyst is used in an amount of 0.1 
mol as a heavy metal(s) to 100 g of acetic acid, the yield of 2,6-NDCA is 
80 to 85%, and the yield of trimellitic acid is 10 to 15%. On the other 
hand, if the catalyst is used in an amount of 0.01 mol as a heavy metal to 
100 g of acetic acid, the yield of 2,6-NDCA is 43 to 55%, and the yield of 
trimellitic acid is 40 to 52%. In this way, it is possible to control the 
ratio of the amounts of 2,6-NDCA and trimellitic acid to be produced in 
accordance with the purpose of production. 
In this case, it is also important that the atomic ratio of the amount of 
bromine to the total amount of cobalt and manganese is not less than 
0.0001 and less than 0.01. 
As described above, a process for producing 2,6-NDCA together with 
trimellitic acid according to the present invention is advantageous in 
that since the production of by-products is very small, 2,6-NDCA is 
produced with high purity, in that many purification steps which are 
required in a conventional process for producing 2,6-NDCA are dispensed 
with, and in that trimellitic acid is also obtained with high purity. 
Thus, this process is capable of industrially producing 2,6-NDCA together 
with trimellitic acid. 
The present invention will be explained further in detail while referring 
to the following non-limitative examples. 
The purity of 2,6-NDCA and trimellitic acid was measured by a high 
performance liquid chromatography (A) and the bromine content was measured 
by X-ray fluorometry (B). 
(A) Model 510 type HPLC manufactured by Waters Co. 
Column: coupled column of Lichrosorb (RP-8, 5 .mu.m, manufactured by Merck 
& Co.) and radial pack cartridge C-8 (manufactured by Waters Co.) 
Mobile phase: water of pH 3/acetonitrile=45/55 (by volume) 
Flow rate: 0.6 cc/min. 
Internal standard: 2-naphthoic acid 
Detection Wavelength: 260 nm 
(B) RIGAKUDENKI X-ray fluorometer (3080E 2 type) 
X-ray tube: rhodium, 50 kV, 50 mA 
Detector: PC detector 
Crystals: germanium 
Detection limit: 3 ppm 
10 g of a sample as a tablet of 30 mm in diameter was subject to the 
measurement.

EXAMPLE 1 
2070 g of glacial acetic acid, 130 g of cobalt acetate tetrahydrate, 400 g 
of manganese acetate tetrahydrate, 1.2 g of ammonium bromide and 75 g of 
2,6-diisopropylnaphthalene were put into a 5-l autoclave of stainless 
steel lined with titanium and equipped with a reflux condenser, a gas 
inlet tube, a temperature measuring tube and a stirrer. The mixture was 
heated at 180.degree. to 190.degree. C. and vigorously stirred for 5 hours 
while blowing compressed air at a rate of 300 an hour under the pressure 
of 20 kg/cm.sup.2. After the reaction, the reaction mixture was cooled to 
80.degree. C., and the precipitate was filtered out, thoroughly washed 
with hot acetic acid and was dried to obtain 77 g of crude crystals (A). 
6% dilute sulfuric acid was added to the crude crystals (A) under stirring 
to adjust the pH of the mixture to 1.5. After stirring the mixture at 
80.degree. to 90.degree. C. for 60 minutes, the mixture was filtered while 
hot, to obtain pale yellow powder. This powder was thoroughly washed with 
water and dried to obtain 64.2 g of powder. As a result of the purity test 
and the bromine determination of the powder obtained, the purity proved to 
be 97.3% and the bromine content was 75 ppm. No trimellitic acid was 
contained in the powder. The yield of 2,6-NDCA based on the raw material 
was 81.8%. 
The filtrate and the washing were combined and adjusted to pH 1.5 with 6% 
dilute sulfuric acid and was thereafter concentrated to about 50 g. The 
concentrate was cooled to 25.degree. C. and precipitated pale yellow 
crystals of crude trimellitic acid were filtered out and washed to obtain 
crude crystals. After the filtrate and the washing were adjusted again to 
pH 1.5, a similar process was repeated to obtain pale yellow crude 
crystals again. The total amount of the crude crystals after drying was 
7.9 g. As a result of the purity test and the bromine determination of the 
crude crystals, the purity proved to be 98.9% and the bromine content was 
45 ppm. The yield of trimellitic acid based on the raw material was 10.5%. 
When an aqueous solution of 25% sodium carbonate was added to the filtrate 
and the washing after the separation and the recovery of trimellitic acid 
while stirring the mixture at 30.degree. C. to adjust the pH to 9.5, 
precipitate was generated. The precipitate was filtered out and dried to 
obtain 10.5 g of basic carbonates of cobalt and manganese. Analysis by 
atomic absorption spectroscopy proved that the recoveries of cobalt and 
manganese were 98.6% and 99.0%, respectively, based on the cobalt and 
manganese contained in the crude crystals (A). When the recovered basic 
carbonates of cobalt and manganese were added to 250 g of acetic acid and 
the mixture was stirred at 80.degree. to 90.degree. C. for 4 hours, the 
basic carbonates were completely dissolved in the acetic acid while 
generating carbon dioxide gas. The filtrate and the washing after the 
above-described oxidation were added to the acetic acid solution and the 
mixture was concentrated to 2.62 kg. Oxidation was carried out by adding 
75 g of 2,6-diisopropylnaphthalene to the concentrate under the same 
conditions as the above. The oxidation proceeded normally and trimellitic 
acid and 2,6-NDCA were produced with similar yields. 
EXAMPLE 2 
2070 g of glacial acetic acid, 13 g of cobaltous acetate tetrahydrate, 40 g 
of manganese acetate tetrahydrate, 0.12 g of ammonium bromide and 75 g of 
2,6-diisopropylnaphthalene were put into the same autoclave as used in 
Example 1 and the mixture was heated at 180.degree. to 190.degree. C. and 
vigorously stirred for 5 hours while blowing compressed air at a rate of 
300 l an hour under the pressure of 20 kg/cm.sup.2. The reaction mixture 
was subjected to the same treatment to that of Example 1 to obtain 82.5 g 
of a mixture (B) of cobalt and manganese salts of trimellitic acid and 
2,6-NDCA. The mixture (B) was further subjected to the same treatment as 
in Example 1 to obtain 39.1 g of 2,6-NDCA and 32.5 g of trimellitic acid. 
The yields of 2,6-NDCA and trimellitic acid based on the raw material were 
50.0% and 43.2%, respectively. The purity of 2,6-NDCA was 97.6% and the 
bromine content was 63 ppm. The purity of trimellitic acid was 98.1% and 
the bromine content was 35 ppm. 
When an aqueous solution of 25% sodium carbonate was added to the filtrate 
after the separation and recovery of trimellitic acid and the mixture was 
subjected to the same treatment as in Example 1, 98.5% and 99.2% of the 
cobalt and the manganese, respectively, contained in the mixture (B) were 
recovered. 
EXAMPLE 3 
2.8 kg of glacial acetic acid, 180 g of cobaltous acetate tetrahydrate, 530 
g of manganese acetate tetrahydrate and 2 g of ammonium bromide were put 
into a 5-l autoclave of stainless steel lined with titanium and equipped 
with a reflux condenser, a gas inlet tube, a gas outlet tube, an overflow 
slurry outlet connected to a stainless steel vessel of inner volume of 30 
l, and a stirrer. Compressed air was blown into the mixture at the rate of 
600 l an hour under a pressure of 20 kg/cm.sup.2 while stirring the 
mixture at 170.degree. to 190.degree. C. Simultaneously, a solution of 
1.67 kg of 2,6-DIPN, 16.7 kg of glacial acetic acid, 1.06 kg of cobaltous 
acetate tetrahydrate, 3.15 kg of manganese acetate tetrahydrate and 11.7 g 
of ammonium bromide was continuously supplied to the mixture at the rate 
of 994 g an hour. After 16-hour reaction, the reaction mixture remaining 
in the autoclave and overflown into the vessel were cooled to 90.degree. 
C. The precipitate was filtered out, and after being thoroughly washed 
with hot acetic acid the precipitate was dried to obtain 1210 g of a 
mixture (C) of 2,6-NDCA and the cobalt and manganese salts of trimellitic 
acid. 
About 3 kg of water and 10% dilute hydrochloric acid was added to the 
mixture(s) under stirring to adjust the pH of the mixture to 1.5. After 
the mixture was stirred at 70.degree. to 80.degree. C. for one hour, pale 
yellow precipitate was filtered out while hot, thoroughly washed with 
water and dried to obtain 1.03 kg of crude 2,6-NDCA. The purity of the 
crude 2,6-NDCA was 97.8% and the bromine content was 97 ppm. The content 
of trimellitic acid therein was 0.04%, and the yield of 2,6-NDCA based on 
the raw material was 84.3%. 
The filtrate and the washing after the separation of 2,6-NDCA were adjusted 
to pH 1.5 with hydrochloric acid and was thereafter concentrated to about 
840 g. The concentrate was cooled to 25.degree. C. to precipitate pale 
yellow crystals. The precipitate was filtered out and washed to obtain 
trimellitic acid. The filtrate and the washing were subjected to the same 
treatment as in Example 1 to obtain trimellitic acid again. The total 
amount of trimellitic acid after drying was 134 g. The purity was 98.6% 
and the bromine content was 44 ppm. The yield of trimellitic acid based on 
the raw material was 11.4%. 
An aqueous solution of 10% sodium carbonate and 10% sodium dicarbonate was 
added to the filtrate and the washing after the separation and the 
recovery of trimellitic acid to adjust the pH to 9.5, and cobalt and 
manganese were recovered in the same manner as in Example 1. The 
recoveries of cobalt and manganese were 98.1% and 98.8%, respectively, 
based on the cobalt and manganese contained in the mixture (C). 
The crude 2,6-NDCA was purified in accordance with Example 4 disclosed in 
Japanese Patent Publication No. 56-3858 (1981). That is, 100 g of 2,6-NDCA 
was dissolved in 800 g of an aqueous solution of 5% NaOH under heating. 10 
g of active carbon powder was added thereto and after being stirred at 
20.degree. C. for 30 minutes, the mixture was filtered. 6N dilute 
hydrochloric acid was added to the filtrate under stirring, while 
maintaining the filtrate at 60.degree. C. When the pH of the filtrate was 
lowered to 7.0, the filtrate was cooled to 20.degree. C. and was stirred 
for 30 minutes, thereby separating the crystals of monosodium salt of 
2,6-NDCA. The crystals were filtered out and dissolved in 2 kg of water. 
When 6N dilute hydrochloric acid was added thereto under stirring at 
90.degree. C. to adjust the pH to 2, 2,6-NDCA was separated. The separated 
2,6-NDCA was filtered out while hot, washed with water and dried to obtain 
81.4 g of purified 2,6-NDCA. The purity was 99.1% and no bromine was 
detected (detection limit: 3 ppm).