JPS6150942B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing indole. Specifically, the present invention relates to a method for producing highly pure indole by separating and purifying indole from a coal tar fraction. Indole is a white crystal with a boiling point of 253°C and a melting point of 53°C, and has been known for a long time. This indole is a useful compound that can be used as a raw material for the synthesis of tryptophan, an essential amino acid, or as a flavoring agent. Coal tar is generally first separated into light oil, carbol oil, naphthalene oil, washing oil, anthracene oil, pitch, etc. by distillation, and then necessary components are separated from each fraction to obtain useful products. Although it varies depending on the cutting temperature during distillation of coal tar, there are some fractions of α- and β-methylnaphthalene in fractions around 250°C, such as methylnaphthalene fractions containing high concentrations of indole. Contains biphenyl, dimethylnaphthalenes, diphenylene oxide, acenaphthene, indole, methylindoles, etc. Among these, the content of indole is 2 to 10%, and the content of methyl indoles is 1 to 5%. Indole can be obtained by further separating the indole-containing fraction by distillation, etc.
The indole content in the above-mentioned fractions is low;
Moreover, since it contains many components with boiling points close to this, it is extremely difficult to obtain highly pure indole by distillation alone. In particular, it is difficult to obtain indole for use in perfumery, which requires extremely high purity. As an advantageous method for separating indole in a coal tar fraction, a method is known in which indole is reacted with a caustic alkali to be separated as an alkali metal salt of indole. Furthermore, the present inventors have already proposed a method for separating indoles by reacting indoles in the methylnaphthalene fraction of coal tar with caustic potash (Japanese Patent Laid-Open No. 113727/1983). That is, the methylnaphthalene fraction and a caustic potash aqueous solution are mixed and heated to evaporate the water, thereby reacting the indoles in the methylnaphthalene fraction with the caustic potash to produce indole potash (including potassium salts of methylindoles).
shall be. This indole potash is solid at room temperature, but when heated, it becomes a liquid with a higher specific gravity than the methylnaphthalene fraction, for example 150 to 200.
It can be separated into liquid-liquid from methylnaphthalene at a temperature of °C. When water is added to this indole potash and heated to 50 to 100°C, it returns to indoles and a caustic potash aqueous solution, so indoles can be recovered by such a method. However, indoles (hereinafter referred to as crude indoles) contain, in addition to indole and methylindole isomers, neutral oils having boiling points close to indole, such as methylnaphthalene and dimethylnaphthalenes. Therefore, the indole concentration in crude indole is usually only 50-75%. Therefore,
It is difficult to recover highly pure indole from this crude indole by distillation alone. This is because when crude indole is distilled, methyl indoles can be separated relatively easily, but it is difficult to separate the neutral oil component whose boiling point is close to that of indole. On the other hand, when separation by distillation is difficult, it is generally known to separate the target product by recrystallization. However, according to the research of the present inventors,
When indole is obtained by recrystallization, methyl indoles are substances that inhibit the improvement of purity, while neutral oil components that are difficult to separate by distillation are not inhibited, and the present invention was completed. That is, the method for producing indole according to the present invention involves distilling crude indole separated from a coal tar fraction to obtain an indole fraction having an indole concentration of 75 to 98% by weight and a methyl indole concentration of 4% by weight or less, This is then carried out by recrystallizing the fraction with a solvent to recover high purity indole. For distillation of crude indole, an ordinary packed column, perforated plate column, bubble bell column, etc. are used. The number of theoretical plates in the distillation column cannot be determined uniquely in relation to operating conditions such as the reflux ratio, but it is sufficient as long as the number of plates is sufficient to separate indole and methylindoles. The indole concentration in crude indoles is 50 to 70% by weight, but it is distilled to reduce the indole concentration.
The content is 75-98% by weight, preferably 80-95% by weight.
There is no specific regulation value for the content of neutral oil in the distillate obtained in this way, but unless the content of methyl indoles is reduced to 4% by weight or less, high purity indole cannot be obtained by recrystallization. The amount is preferably 0.3 to 3% by weight. The higher the indole concentration in this fraction, the higher the yield during recrystallization, but the lower the distillation yield, so if the cut is made to achieve the indole concentration that maximizes the overall yield of distillation and recrystallization. Economical. The indole fraction thus obtained is further purified by recrystallization, and the recrystallization solvents used in the method of the present invention include hexane,
Paraffinic solvents such as heptane and octane, and naphthenic solvents such as cyclohexane, methylcyclopentane and methylcyclohexane may be used alone or in mixtures. It goes without saying that these materials have a large difference in solubility depending on temperature, but those having particularly good ability to separate methyl indole, which has similar properties, are used. The boiling point of the solvent used is 50-150
℃ is preferable, and 50 to 120℃ is particularly desirable. After recrystallization, it is necessary to evaporate the solvent adhering to the separately obtained indole, but the melting point of indole is relatively low at 53°C, so evaporating the solvent at a high temperature will melt the indole, which is not preferable. Therefore, it is preferable to evaporate at a low temperature (50° C. or lower). On the other hand, if the boiling point of the solvent used is low, handling becomes difficult. Even if aromatic hydrocarbons such as benzene and toluene, alcohols such as methanol and ethanol, and ketones such as acetone and methyl ethyl ketone can be used as recrystallization solvents, satisfactory effects cannot be obtained because of the high solubility of indole. do not have. On the other hand, paraffinic or naphthenic solvents such as heptane and cyclohexane have moderate solubility, so the solubility is appropriate, and the use of these solvents can remove tar odor.
Highly pure indole that even passes the grade of indole for fragrances can be obtained. However, even if such a solvent is used, if methyl indoles are present in an amount exceeding 4% by weight in the indole fraction,
Indole purity is 99% by weight even after recrystallization
It cannot be more than that. The ratio of the indole fraction to the solvent varies depending on the type of solvent and the concentration of indole, but when using a solvent with medium solubility such as n-heptane, it is 1 to 5 times the weight of the indole fraction. , preferably 1.5 to 3 times the weight. If the amount of solvent used is excessive, the yield of indole will decrease. Also,
If the amount of solvent used is insufficient, not only will the purification effect deteriorate, but the slurry concentration will be too high, making it difficult to handle. The crude indole used in the present invention is a coal tar fraction that can be distilled to give an indole fraction with an indole concentration of 80 to 98% by weight and a methylindole concentration of 4% by weight or less. Either can be used, but preferably
It is obtained by hydrolyzing a reaction product obtained by heat-treating a coal tar fraction containing a fraction at around 253° C., more preferably a methylnaphthalene fraction obtained from coal tar, with caustic alkali. Therefore, this methylnaphthalene fraction is usually mainly a fraction with a temperature of 230 to 260°C. In addition, examples of caustic alkali include caustic soda,
Although there are caustic potash and the like, caustic potash is preferable, and the amount of caustic alkali added is 1 to 10% by weight, preferably 3 to 6% by weight based on the methylnaphthalene fraction.
It is. The higher the heat treatment temperature, the shorter the treatment time and the higher the treatment effect.
~260℃. This heat treatment is carried out for 1 to 6 hours by adding caustic potash to methylnaphthalene and distilling the produced water out of the system. After heat treatment, if left to stand, it will separate into an oil layer and a pitch-like substance, so after separating the oil layer, water is added to the pitch-like substance and heated to perform hydrolysis. The amount of water is such that the concentration of caustic alkali produced by hydrolysis is 40 to 50% by weight.
An appropriate amount is . If a large amount is added, not only will it be difficult to separate the layers of the hydrolyzed solution, but also it will be difficult to reuse the caustic alkali, and if a small amount is added, the freezing point will become high. The appropriate heating temperature is above the melting point and below the boiling point, usually 50-150℃, preferably 70-150℃.
The temperature is 110℃. The reaction time depends on the reaction temperature, but at a temperature of around 100°C, a suitable time is about 5 minutes to 5 hours. It is not preferable to take more time than necessary because polymerization of indole will occur. After hydrolysis is complete, if left to stand still, it will separate into an oil layer and a water layer.
Crude indole is obtained by removing the aqueous layer. Next, the present invention will be explained in detail by giving examples. In addition, "%" and "part" in the following examples are based on weight unless otherwise specified. The raw materials and products were analyzed by gas chromatography. Example 1 Boiling point 230~ obtained by fractional distillation of coal tar
Methylnaphthalene fraction (α- and β-
Total amount of methylnaphthalene 65.2%, indole
4.7%, 3-methylindole 0.3%, 2-methylindole 0.2% and 5-methylindole 0.1
% content) was used as the raw material. 800 parts of this methylnaphthalene fraction and 62 parts of a 50% caustic potassium aqueous solution were mixed, heated at a temperature of 245°C to distill off 26 parts of produced water, and reacted for 1 hour to obtain 79 parts of indole potash. 32 parts of water was added to this indole potash and heated at 90°C for 3 hours to hydrolyze the indole potash. The produced crude indole and a caustic potassium aqueous solution were separated into layers to obtain 54 parts of crude indole. The composition of the obtained crude indole was as follows.

[Table] Crude indole was distilled using a distillation column with 30 theoretical plates. The distillation pressure is normal pressure, and the reflux ratio is
It was set as 10. By selecting the cutting temperature, an indole fraction having the composition shown in Table 1 was obtained. This indole fraction was recrystallized from n-heptane. First, 107 parts of indole fraction and n
- 267 parts of heptane and dissolved at 40°C, cooled to 20°C, precipitated crystals were centrifuged, and dried overnight at normal temperature and pressure to obtain purified indole. The results are shown in Table 2. Example 2 In the method of Example 1, the cut temperature during crude indole distillation was changed to obtain an indole fraction having the composition shown in Table 1. 107 parts of this indole fraction and n
- 267 parts of heptane and dissolved at 40°C, cooled to 20°C, precipitated crystals were centrifuged, and dried overnight at normal temperature and pressure to obtain purified indole. The results are shown in Table 2. Comparative Example In the method of Example 1, the cut temperature during crude indole distillation was changed to obtain an indole fraction having the composition shown in Table 1. 107 parts of this indole fraction and n
- 267 parts of heptane and dissolved at 50°C, cooled to 20°C, precipitated crystals were centrifuged, and dried overnight at normal temperature and pressure to obtain purified indole. The results are shown in Table 2.

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Claims (1)

[Claims] 1 The crude indole obtained by hydrolyzing the reaction product obtained by heating the methylnaphthalene fraction separated from the coal tar fraction with caustic alkali is distilled to an indole concentration of 75 to 98% by weight and methyl An indole fraction with a concentration of indoles of 4% by weight or less is taken out, and then the fraction has a boiling point of 50 to
A method for producing indole, which comprises recovering high-purity indole by recrystallizing it in a paraffinic or naphthenic hydrocarbon solvent at 150°C.