JPS6052691B2 - Purification method of benzoic acid - Google Patents

Description

[Detailed description of the invention] The present invention relates to a method for purifying benzoic acid.

Specifically, by treating at high temperature under reduced pressure,
This invention relates to a method for purifying benzoic acid obtained by liquid-phase oxidation of toluene with a molecular oxygen-containing gas. Benzoic acid is widely used in large quantities as a preservative in food additives, aniline dyes, pharmaceuticals, fragrances, paints, and as a mordant for printing.

As a method for producing benzoic acid, (1) toluene or
oxidation of ethylbenzene, (2) hydrolysis of nitrile,
(Hydrolysis of 3E chloride, (4) decarboxylation of phthalic anhydride, etc.), but currently, in general, toluene is converted into a liquid phase at high temperature and pressure using a molecular oxygen-containing gas using metal salts as a catalyst. Obtained by oxidation [by Kirkoosmar,
encyclopedia of chemicals and technology
(Encyclopedia of Chemical Te
chn()logy) 2nd edition, Volume 3, page 423, etc.].

Benzoic acid obtained by the latter liquid-phase oxidation method of toluene contains diphenyl, methyldiphenyls, and other reaction intermediate products such as benzaldehyde and benzyl alcohol because the oxidation is carried out at high temperature and high pressure. , contains a wide variety of by-products. Unreacted toluene, reaction intermediate products such as benzaldehyde and benzyl alcohol can be easily removed by distillation or rectification, but diphenyl, methyldiphenyl and other unknown by-product impurities are removed. Not only is it extremely difficult to do so, but the odor present in the product is severe and extremely unpleasant. Conventionally, the method for purifying oily by-products such as diphenyl, methyldiphenyl, etc. and other unknown unpleasant odor-producing substances has been to recrystallize benzoic acid obtained by liquid phase oxidation in a solvent such as toluene. A method in which insoluble oil is removed as a benzoate salt using an alkaline aqueous solution, and then neutralized with sulfuric acid etc. for purification. A method in which crude benzoic acid obtained by recovering unreacted toluene at high temperature is heat treated with sulfuric acid to remove it. There are methods such as the method according to US Pat.

However, the recrystallization method using a solvent not only requires a large amount of solvent, but is also unsuitable for mass production due to equipment considerations.

Furthermore, the method of treatment using an aqueous alkali solution similarly requires a large amount of alkali and also poses pollution problems such as waste liquid treatment, which is not preferable. In the method of heat treatment with sulfuric acid at high temperature, oily substances such as diphenyl are very stable substances even if heat treated with sulfuric acid at high temperature.
A large amount of sulfuric acid is required, so when benzoic acid is obtained by rectification after heat treatment with sulfuric acid, not only is there a large amount of residue in the pot, but it is difficult to extract the residue, and a large amount of sulfuric acid is used. Therefore, problems also arise regarding the material of the device. The method of US Pat. No. 3,235,588 requires a large amount of water like the above-mentioned solvent recrystallization method, and also requires treatment of wastewater, and therefore is not an industrially suitable treatment method.

Furthermore, it is stated that odorless benzoic acid can be obtained according to the method of JP-A No. 51-8893, but since it is treated with a large amount of active gas, it is necessary to treat the exhaust gas from the viewpoint of preventing air pollution. It is said that Therefore, the cost of exhaust gas treatment is added to the production cost of expensive inert gas.
This cannot be said to be an industrially advantageous treatment method. As a result of intensive research in an attempt to improve the various drawbacks of the above-mentioned conventional methods, the present inventors distilled benzoic acid obtained by liquid phase oxidation of toluene under high temperature and pressure using a molecular oxygen-containing gas to remove unreacted toluene. After recovery, the crude product thus obtained is heat treated with a small amount of sulfuric acid, followed by rectification, and the benzoic acid obtained is treated at a temperature of 50 to 100°C under reduced pressure of less than 200 wrInHg. It was discovered that unpleasant odor components can be completely removed by this method, and the method of the present invention has been completed.

Benzoic acid purified by the method of the present invention is an oxidized product obtained by oxidizing toluene with a molecular oxygen-containing gas, such as air, under high temperature and high pressure in a lower aliphatic carboxylic acid solvent in the presence of metal salts in a conventional manner. This is crude benzoic acid obtained by distilling the product to separate and remove unreacted toluene.

The sulfuric acid used in the sulfuric acid heat treatment of crude benzoic acid is:
A concentration of 90% by weight or more is used, and the amount used is 0.1 to 3 parts by weight, preferably 0.5 to 2 parts by weight, per 10 parts by weight of crude benzoic acid.

The heat treatment temperature is 180 to 240°C, preferably 200 to 220°C. Regarding the heat treatment time, after reaching a predetermined temperature and dropping the sulfuric acid, the treatment can be completed for 2 hours or more, but preferably 4 to 8 hours. Under the above conditions, if the amount of sulfuric acid is less than 0.1 parts by weight per 10 parts by weight of crude benzoic acid, the effect will be weak; on the other hand, if the amount exceeds 3 parts by weight, Although it is very effective in removing oily substances, it not only leaves a large amount of distillation residue after treatment, but also solidifies the polymer, making it difficult to discharge, and it is also unfavorable due to the material of the equipment. . As for the treatment temperature, after recovering unreacted toluene, a high temperature is desirable in order to remove intermediate products as much as possible, but it is desirable to treat at a temperature below the boiling point of benzoic acid, and from the viewpoint of preventing the decarboxylation reaction of benzoic acid, the treatment temperature is 180 °C.
Preferably, the treatment is carried out at a temperature of -240°C. Furthermore, regarding the treatment time, it is usually sufficient to treat for 2 hours or more after the predetermined temperature is reached and sulfuric acid is added while stirring, but it is desirable to treat for 4 to 8 hours for operational and economical reasons. Benzoic acid treated with sulfuric acid in this way is
40~2007177! Hgll is subjected to vacuum rectification at a temperature of 62 to 206°C to remove non-volatile components.

Regarding the method of treating rectified benzoic acid at a high temperature under reduced pressure, it is necessary to treat the temperature at a temperature below the melting point of benzoic acid, but a temperature in the range of 50 to 100°C is sufficient and preferably. is 70-90°C.

The degree of reduced pressure is usually 200TrrlnHg or less, preferably 3 to 100TrfmHg, and most preferably 5 to 20wrInHg. Furthermore, small amounts of inert gas or air can also be introduced if it is possible to process on-device. Regarding the processing time, 4 to 2 hours is sufficient, but 6 to 1 hour is preferable from the operational and economical advantages. The particle size of the benzoic acid used in the method of the present invention may normally be in the form of flakes or powder, but it is desirable that the particle size be as small as possible in order to enhance the treatment effect.

Regarding the treatment method, it may be carried out in a cylindrical container equipped with a jacket and a stirrer, or it may be carried out continuously in a moving bed or a fluidized bed. The most desirable method is to simultaneously treat benzoic acid in a fluidized bed when granulating it to a particle size of . Next, the method of the present invention will be explained in more detail with reference to Examples.

In addition, all parts and percentages in the following examples are based on weight unless otherwise specified. Example 1 A temperature of 150° C. in an acetic acid solvent in the presence of cobalt acetate,
The reaction product obtained by liquid-phase oxidation of toluene with air under a pressure of 10 kgIc1i was oxidized at normal pressure and liquid temperature from 110 to 200.
℃ distillation to remove and recover unreacted toluene to obtain crude benzoic acid.

To this crude benzoic acid, add 1 part of 98% sulfuric acid per 1 (1) part of the crude benzoic acid.
The mixture was added dropwise at a ratio of 1.5 parts, and stirring was continued for 6 hours. After heat treatment with sulfuric acid, vacuum rectification was performed at 186° C. and 100 TfrmHg to obtain benzoic acid. The content of oil such as diphenyl in this benzoic acid was 0.12%. When this benzoic acid powder was treated for 8 hours with stirring at a reduced pressure of 10w0nHg and a temperature of 85°C, odorless benzoic acid was obtained, and its oil content such as diphenyl was 0.01%.
It was below. The method for measuring oil content such as diphenyl is to add 20 g of a 10% aqueous sodium hydroxide solution per 4 parts of benzoic acid, shake it, and extract the oil content in the upper layer with n-hexane. , by separating n-hexane.

Examples 2 to 4 To the crude benzoic acid obtained in the same manner as in Example 1, 0.5 parts, 2 parts, and 3 parts of 98% sulfuric acid were added dropwise per 1 (1) part of the crude benzoic acid under a pressure of 10T1r1nHg. Then, heat treatment was performed at 200° C. for 6 hours in the same manner as in Example 1, and the results shown in Table 1 were obtained.

Examples 5 to 8 Examples were prepared by adding dropwise 1 part of 98% sulfuric acid per 1 (1) part of crude benzoic acid obtained in the same manner as in Example 1, and varying the degree of vacuum, temperature, and treatment time. When heat treatment was carried out in the same manner as in 1, the results shown in Table 2 were obtained.

Comparative Examples 1 to 4 Crude benzoic acid obtained in the same manner as in Example 1 was heat-treated for 1 hour at different degrees of vacuum and temperature without being heat-treated with sulfuric acid, and the results shown in Table 3 were obtained. It was raised.

Claims (1)

[Claims] 1. Benzoic acid obtained by liquid phase oxidation of toluene under high temperature and pressure using a molecular oxygen-containing gas is distilled to recover unreacted toluene, and the crude product thus obtained is oxidized with sulfuric acid. 1. A method for purifying benzoic acid, which is characterized in that the benzoic acid obtained is subjected to heat treatment at a temperature of 50 to 100° C. under a reduced pressure of 200 mmHg or less, followed by rectification. 2 0.3 to 3 parts of sulfuric acid per 100 parts by weight of crude benzoic acid
2. A method according to claim 1, wherein parts by weight are used. 3 Heat treatment of crude benzoic acid with sulfuric acid at 18-240℃
A method according to claim 1 or 2, which is carried out at a temperature of .