JPH066574B2 - Indian manufacturing method - Google Patents

Description

The present invention relates to a method for producing an indole from aniline and ethylene glycol. More specifically, the present invention relates to a method for preventing a decrease in catalyst activity due to circulation of recovered aniline in producing indole by reacting aniline and ethylene glycol in a gas phase in the presence of a catalyst.

[Problems to be Solved by Prior Art and Invention]

Indole is widely used as a raw material for chemical industries such as fragrances and dyes, and has recently been attracting attention as a raw material for synthesizing amino acids. Conventionally, indole has been manufactured using expensive raw materials or through a long and complicated process. However, recently, a gas-phase catalytic reaction between aniline and ethylene glycol has been found as a method for producing indole by using an inexpensive raw material and a simple process.

Various solid acid catalysts and metal catalysts have been proposed as catalysts for the reaction. These catalysts show a decrease in activity when they are reacted over a long period of time, although the degree varies depending on the type of catalyst and the reaction conditions, and they require a regeneration operation by heating and firing in an oxygen atmosphere on a regular basis. To do. The regeneration operation is complicated, and if the frequency is too high, the economical efficiency of the process may be impaired. The details of the cause of this decrease in activity are not clear, but since regeneration can be activated by a conventional method, the main cause is that organic substances become carbonaceous matter under the reaction conditions and are deposited on the catalyst surface. It is thought that this is to cover the active points of. Although the reduction in activity is reduced by carrying out the reaction in a hydrogen atmosphere or by adding water to the reaction system, this is not always sufficient.

On the other hand, it is known that a large excess of aniline must be present in the reaction system in order to obtain indole in good yield. Therefore, in the case of industrially producing indole by this method, it is indispensable to separate and collect a large amount of aniline contained in the obtained reaction mixture and use it again as a raw material in the reaction. However, in this case, there is a problem that the reduction in the activity of the catalyst is significantly promoted as compared with the case where the recovered aniline is not used. That is, when the reaction is carried out without recycling the recovered aniline, when the recovered aniline is supplied while the other conditions remain the same, the catalyst can maintain a sufficient reaction activity for several hundred hours. The activity is significantly reduced, and the frequency of regeneration operation is significantly increased. Therefore, it is presumed that the recovered aniline contains impurities harmful to the catalyst.

In the reaction liquid obtained by condensing the gaseous reaction mixture discharged from the reactor, not only the indole and water formed, unreacted ethylene glycol, excess aniline but also small amounts of various side reaction products Exists. Although some of them are separated and fixed, the chemical structures, physicochemical properties, etc. of many by-products have not been clarified. Among these side reaction products, when recycled to the reactor without being separated or harmless from the recovered aniline, they become carbonaceous and deposit on the catalyst surface, which causes a decrease in their activity. Is considered to be included.

Moreover, since the aniline recovered by distillation according to the usual method adversely affects the catalytic activity, the side reaction product causing the activity decrease is difficult to separate from aniline by the distillation operation due to the gas-liquid equilibrium relationship with aniline. Is considered to be.

Therefore, an object of the present invention is to effectively detoxify a side reaction product that is harmful to such a catalyst and is difficult to separate by distillation from aniline, and even if the recovered aniline is used as a reaction raw material, the activity of the catalyst is reduced. It is intended to provide a method for producing indole which can avoid the adverse effect on.

[Means for solving problems]

As a result of various studies to achieve the above-mentioned object of the present invention, the present inventors separated a aniline by distillation, recovered it, and supplied it again to the reaction apparatus. It is extremely effective to make the side reaction product harmless to the catalyst by bringing it into contact with an acidic substance and then subjecting it to distillation or by bringing the recovered aniline into contact with an acidic substance before feeding to the reactor. I found that. The method for producing an indole of the present invention comprises subjecting ethylene glycol and a stoichiometric excess of aniline to a gas phase reaction in the presence of a catalyst to obtain an indole, a side reaction product and unreacted aniline. In the method for producing an indole by separating and recovering aniline from the reaction liquid containing the aniline and recycling the recovered aniline in the gas phase reaction,
It is characterized in that the reaction solution or the recovered aniline is brought into contact with an acidic substance.

In the method of the present invention, aniline and ethylene glycol are reacted in the gas phase in the presence of a solid acid catalyst or a metal catalyst.

Among the catalysts used in this method, the following may be mentioned as the solid acid catalyst.

(1) Si, Al, B, Sb, Bi, Sn, Pb, Ga, Ti, In, Sr, Ca, Zr, Be, Mg, Y, C
Oxides or hydroxides of at least one element selected from u, Ag, Zn, Cd and lanthanide elements (hereinafter referred to as catalyst substance
(Referred to as (1)), for example, CdO, ZnO, PbO ₂ ,
Al ₂ O ₃ -B ₂ O ₃ , SiO ₂ -ZnO, SiO ₂ -CaO, SiO ₂ -In ₂ O ₃ , SiO ₂ -SrO, S
iO ₂ -CdO, SiO ₂ -Al ₂ O ₃ , SiO ₂ -MgO, TiO ₂ -SnO ₂ , TiO ₂ -ZrO ₂ CdO
₂ -Bi ₂ O ₃ , SiO ₂ -Y ₂ O ₃ , SiO ₂ , Bi ₂ O ₃ -BeO, SiO ₂ -Ga ₂ O ₃ , SiO ₂ -L
a ₂ O ₃ , SiO ₂ -Ce ₂ O ₃ , SiO ₂ -ZnO-AgO, SiO ₂ -MgO-CuO, etc.

(2) Containing a sulfide or selenide of at least one element selected from Pd, Pt, Cr, Fe, Ni, Co, Zn, Mo, Cd and W (hereinafter referred to as catalyst substance (2)) Catalysts such as PdS, Pt
S, CrS, FeS, NiS, CoS, ZnS, MoS ₂ , Cds, WS ₂ , ZnSe, CdSe, etc.

(3) Fe, Tl, Ca, Mn, Bi, St, Y, Al, Zn, Cd, Ni, Mg, In, Be, Co, Ga
And an inorganic salt of at least one element selected from lanthanide elements, that is, a halide, a carbonate, a nitrate, a sulfate, a phosphate, a pyrophosphate, a phosphomolybdate, a silicotungstate (hereinafter referred to as a catalyst substance ( Referred to as 3)), for example, ferric sulfate, thallium sulfate, calcium sulfate, manganese sulfate, bismuth sulfate, strontium sulfate, yttrium sulfate, cadmium bromide, aluminum sulfate, zinc sulfate, nickel sulfate, cadmium chloride. , Magnesium sulfate, indium sulfate, beryllium sulfate, cadmium nitrate, cobalt sulfate, aluminum zinc sulfate, magnesium chloride, cadmium sulfate, cadmium phosphate, etc.

Further, as a metal catalyst, Cu, Ag, Pt, Pd, Ni, Co, Fe <Ir, Os, R
A catalyst containing at least one element selected from u and Rh (hereinafter referred to as catalyst substance (4)) can be mentioned.

Among the above groups of catalyst substances, as the most preferably used, in the group of catalyst substances (1), SiO ₂ -ZnO-AgO, in the group of catalyst substances (2), cadmium sulfide is a catalyst substance ( In the group of 3), cadmium sulfate is included, and in the group of the catalytic substance (4), Ag supported on a carrier having a large specific surface area is included.

These catalysts can be produced by any known method. That is, the catalyst substance (1) is a method of hydrolyzing a water-soluble salt of the catalyst-constituting element into a hydroxide, drying the resulting gel, calcining, or thermally decomposing the easily decomposable salt in air. Can be manufactured by.

The catalyst substance (2) is produced by a method of adding sodium sulfate or potassium selenide to a water-soluble salt of a catalyst constituent element, or a method of contacting the catalyst constituent element or a salt thereof with hydrogen sulfide gas or hydrogen selenide gas. can do.

Further, the catalyst substance (4) which is a metal catalyst can be produced by a method of reducing a salt, hydroxide or oxide of a catalyst constituent element with a reducing agent such as hydrogen, formalin, formic acid, phosphorous acid and hydrazine. it can.

These solid acid catalysts or metal catalysts are the above-mentioned catalyst substances (1),
Each of (2), (3) and (4) may be used alone, or two or more of them may be mixed, or these may be supported on a carrier.
Any of the commonly used carriers can be used, but diatomaceous earth, pumice stone, titania, silica-alumina, alumina, magnesia, silica gel, activated clay, asbestos and the like are usually used. The catalyst substance is supported on these carriers by a conventional method to prepare a supported catalyst.

The amount of the catalyst substance supported on the carrier is not particularly limited,
Usually, an appropriate amount of the catalyst substance, for example, 1 to 50%, is supported according to the carrier.

In the method for producing indole of the present invention, the reaction between aniline and ethylene glycol is carried out in the gas phase in the presence of the above-mentioned catalyst, and any of a fixed bed, a fluidized bed or a moving reaction apparatus is used as a reaction apparatus. obtain.

The aniline and ethylene glycol introduced into the reactor are
The range of 0.01 to 1 mol of ethylene glycol, and especially the range of 0.05 to 0.5 mol is preferable for 1 mol of aniline.

The introduction amount of aniline and ethylene glycol, which are the raw materials, is in the range of 0.01 to 10 hr ^{−1 in} terms of liquid hourly space velocity (LHSV), which is vaporized in the evaporator in advance and then introduced into the reactor. At that time, water vapor, hydrogen, carbon monoxide, carbon dioxide, methane, nitrogen, neon, argon or the like may be included as a carrier gas. Particularly, steam, hydrogen and carbon monoxide are preferable because they increase the catalyst life.

The reaction temperature is preferably 200 to 600 ° C, particularly preferably 250 to 500 ° C.

The reaction pressure can be selected from the standard atmospheric pressure or higher, and any pressure other than the pressure at which the components contained in the raw material or the reaction mixture at the reaction temperature form the condensed phase, but practically 1.1 × 10 ⁵ to 1.0 × 10 ⁷ Pa range is preferable, 2.0 × 1.0
The range of ^{5 to} 5.0 × 10 ⁶ Pa is more preferable.

In the method of the present invention, the reaction liquid obtained by condensing the gaseous reaction mixture discharged from the reactor for producing indole is used as it is, or after the organic phase is separated into an aqueous phase and an organic phase, an acidic substance is added to the organic phase. After contacting with, collect aniline,
This recovered aniline or aniline is recovered from this reaction solution, and after contacting this recovered aniline with an acidic substance, it is recycled and reused in the reactor.

The acidic substance in the present invention is an acidic aqueous solution or a substance exhibiting solid acidity. As an acidic aqueous solution, (1) HClO ₃ , HClO ₂ , HClO, HBrO ₄ , HBrO ₃ , HBrO ₂ HBrO, H ₅ IO ₆ , HIO ₄ , HIO ₃ , HIO, HMnO ₄ , H ₂ MnO ₄ HTcO ₄ , H ₂ TcO ₄ ,, HReO ₄ , H ₂ ReO ₄ , H ₂ SO ₄ , H ₂ S ₂ O ₇ H ₂ SO ₅ , H ₂ S ₂ O ₈ , H ₂ S ₂ O ₃ , H ₂ S ₂ O ₆ , H ₂ SO ₃ , H ₂ S ₂ O ₅ , H ₂ S ₂ O ₂ , H ₂ S
₂ O ₄ , H ₂ SO ₂ , H ₂ SxO ₆ (X = 3,4, ...), H ₂ SeO ₄ , H ₂ SeO ₃ , H ₆ TeO ₆ , H ₂
CrO ₄ , H ₂ CrO ₇ HNO ₃ , HNO ₄ , HNO ₂ , HNO, H ₂ NO ₂ , H ₂ N ₂ O ₂ , H ₃ PO ₄ , H ₄ P ₂ O ₇ , (HP
O ₃ ) n, H ₃ PO ₅ , H ₄ P ₂ O ₈ , H ₂ PHO ₃ , H ₂ P ₂ H ₂ O ₅ , HPH ₂ O ₂ , H ₃ AsO ₄ , H ₃
AsO ₃ HSb (OH) ₆ ,, H ₂ CO ₃ ,, HOCN, HNCO, HCNO, H ₄ SiO ₄ (H ₂ SiO ₃ ) n, H ₃ BO ₃ , (HBO) n, H ₄ B ₂ O ₄ etc.Oxo Acid, (2) HCl, HF, HBr, HI, HCN, H ₂ S and other hydrogen acids, (3) CH ₃ COOH, HCOOH, (COOH) ₂ , CH ₂ (COOH) ₂ , organic acids such as
Or _{(4) CuSO 4, FeSO 4} , CdSO 4, Al 2 (SO 4) 3, KHSO 4, MgSO 4, MgSO 4, K
An aqueous solution containing one or more selected from strong acid salts such as H ₂ PO ₄ can be exemplified.

Further, as a substance showing solid acidity, (1) acid clay, clarit, pentonite, kaolin,
Clay minerals that exist naturally or can be chemically synthesized, such as Fras Earth, montmorillonite, phlorizin, and zeolite, silica gel, alumina, sulfuric acid, phosphoric acid,
Such as those to which malonic acid is attached, phosphoric acid using quartz sand as a carrier, solidified acids such as calcined compounds of diatomaceous earth and phosphoric acid, (2) cation exchange resins, cation exchange membranes, cation exchange fibers, etc. Resins with cation exchange ability, (3) ZnO, As ₂ O ₃ , V ₂ O ₅ , Sb ₂ O ₅ , CaSO ₄ , COSO ₄ , CoSO ₄ , SrSO ₄ ,
_{MgSO 4, KHSO 4, K 2} SO 4, (NH 4) 2 SO 4, Al 2 (SO 4) 3, Fe (SO 4) 3, Cr 2
(SO ₄ ) ₃ , Ca (NO ₃ ) ₂・ 4H ₂ O, Bi (NO ₃ ) ₃ , 5H ₂ O, Zn (NO ₃ ) ₂・ 6H ₂ O, F
_{e (NO 3) 3 · 9H} 2 O, CaCO 3, Zr phosphate, Ti phosphate, AlPO _4, P
Inorganic compounds such as bCl ₂ , CaCl ₂ , AgCl, H ₂ WO ₄ , AgClO ₄ , CaS, Mg (ClO ₄ ) ₂ , (4) Al ₂ O ₃ , TiO ₂ , CeO ₂ , SiO ₂ , MnSO ₄ , NiSO ₄ , CuSO ₄ , CdSO ₄ , Z
nSO ₄ , FeSO ₄ , BaSO ₄ , HgCl ₂ , CuCl ₂ , AlCl ₃ , SnCl ₂ , ZnS, etc. which have been heat treated, or (5) TiO ₂ -Al ₂ O ₃ , TiO ₂ -Si ₂ , TiO ₂ -ZrO ₂ , TiO ₂ -MgO, ZnO-Al ₂ O
_{3, ZnO-SiO 2, ZnO} -ZrO 2, Al 2 O 3 -ZrO 2, SiO 2 -Al 2 O 3, SiO 2 -WO 3
Examples thereof include composite oxides such as SiO ₂ —V ₂ O ₅ and SiO ₂ —MgO.

In the method of the present invention, the method of bringing the reaction liquid containing the side reaction product and aniline or the recovered aniline into contact with the acidic substance may take various methods depending on the acidic substance used. For example, there are a method of directly adding to the reaction solution to dissolve or suspend, a method of adding an aqueous solution and stirring, or a method of passing the reaction solution through a column filled with an acidic substance in the case of an aqueous solution. However, the method is not limited to these, and various methods can be applied, although the reaction solution and the acidic substance can be brought into contact with each other. The form of these methods may be a batch type, a semi-batch type, or a continuous type.

When an acidic aqueous solution is used, the pH of the aqueous solution should be less than pH 7.

In the case of the batch method, the amount of these acidic substances used is about 0.001 to 100 parts by volume per 1 part by volume of the reaction solution, and the treatment time is about 1 minute to 40 hours. In the case of the continuous system, for example, in the method of the tank type, the amount of the acidic substance used is approximately the same as the batch system in the volume ratio of the reaction liquid in the tank to the acidic substance, and the treatment time is the same as that in the tank of the reaction liquid. The average residence time is about 1 minute to 40 hours. However, the amount of the acidic substance used and the treatment time are not limited to this range, and the amount and the time required to achieve the object of the present invention depending on the amount of the side reaction product contained in the reaction solution. Can be selected.

Although the treatment temperature in the method of the present invention varies depending on the acidic substance used, it is preferably 150 ° C. or lower because of problems such as evaporation of the acidic aqueous solution and alteration of the ion exchange resin. The treatment pressure may be normal pressure, elevated pressure or reduced pressure within a range in which the reaction liquid or acidic aqueous solution to be treated remains liquid.

A treatment for bringing the reaction liquid and the acidic substance into contact with each other,
The recovered aniline obtained by the combination with the subsequent distillation or by the distillation and the treatment by contacting with an acidic substance is reused in the reaction as it is or as a mixture with fresh aniline.

[Operation and effect of the invention]

According to the method of the present invention, ethylene glycol and a stoichiometric excess of aniline with respect to ethylene glycol are subjected to a gas phase reaction, and the excess aniline is separated and recovered from a reaction liquid obtained by condensing the reaction mixture, and is again recovered. When used as a raw material, the reaction liquid is brought into contact with an acidic substance and then aniline is separated and recovered, or the aniline separated and recovered from the reaction liquid is brought into contact with an acidic substance, so that a side reaction harmful to the catalyst is generated. Things can be made harmless. Therefore, even if the recovered aniline is used, the activity of the catalyst can be maintained for a long time and the indole can be produced in good yield.

〔Example〕

Hereinafter, the method and effects of the present invention will be specifically described with reference to Reference Examples, Comparative Examples and Examples.

Reference Example 1 A reaction tube made of stainless steel having an inner diameter of 25 mm was filled with 500 ml of a catalyst in which powdery cadmium sulfide was compression-molded to have a particle size of 3 to 4 mm and charged. ² hydrogen gas under pressure of 7kg / cm 2
The temperature of the catalyst layer was gradually raised from room temperature to 350 ° C. and maintained at 350 ° C. Then, aniline and 33% ethylene glycol aqueous solution were added to each of 234
After mixing at g / hr and 48 g / hr and vaporizing, they were supplied to a reaction tube to carry out a reaction. Immediately after the start of the reaction, the indole yield based on ethylene glycol was 74.8%. This yield gradually decreased with the lapse of time and reached 62.0% 200 hours after the start of the reaction. The aniline used in the reaction is a commercial industrial aniline.

Comparative Example 1 In Reference Example 1, the reaction mixture exiting the reaction tube was cooled to room temperature, the resulting condensate was separated into two phases, an aqueous phase and an organic phase, and aniline was distilled from the latter as follows. Separated and collected. That is, a distillation pot with an internal volume of 30, an inner diameter of 80 mm,
25 kg of the above organic phase in a packed bed of 2000 mm height 1/4 inch Raschig ring packing and a glass distillation device that sounds from the condenser
Was charged, and batch distillation was performed at a pressure of 10 Torr and a reflux ratio of 0.2. An initial fraction of 0.4 kg and then 20.3 kg of an aniline fraction were distilled. Similar batch distillation was repeated to obtain about 80 kg of recovered aniline. As a result of analysis, the purity of recovered aniline is 99%.
That was all.

Using this recovered aniline, an indole synthesis reaction was carried out under exactly the same conditions as in Reference Example 1. The indole yield based on ethylene glycol immediately after the start of the reaction was 75.2%, which was almost the same as in Reference Example 1, but 43.6% 200 hours after the start of the reaction.
Fell to.

Reference Example 2 A stainless steel reaction tube having an inner diameter of 20 mm was filled with 400 ml of a pellet-shaped catalyst in which 13% of silver was loaded on commercially available tablet-molded SiO ₂ having a diameter of 3 mm and a height of 2.5 mm. The reaction tube was kept at 350 ° C., and a pre-vaporized raw material mixture of aniline, ethylene glycol and water at a molar ratio of 12: 1: 30 was supplied at 300 g / hr, and at the same time, hydrogen gas was supplied to the reaction tube at 60 STP / hr, The indole synthesis reaction was carried out at atmospheric pressure.

The indole yield based on ethylene glycol after 1 hour of reaction time was 70.5%. This yield gradually decreased with the passage of time, and was 51.0% 50 hours after the start of the reaction. The aniline used in the reaction was a commercially available industrial aniline similar to Reference Example 1.

Comparative Example 2 The reaction mixture leaving the reaction tube in Reference Example 2 was cooled to room temperature, and the resulting condensate was separated into two phases, an aqueous phase and an organic phase,
The latter was separated and recovered in the same manner as in Comparative Example 1. About 20 kg of the collected aniline was analyzed and found to have a purity of 99% or higher.

Using this recovered aniline, an indole synthesis reaction was carried out under the same conditions as in Reference Example 2. The indole yield based on ethylene glycol 1 hour after the start of the reaction was 70.1% which was almost the same as that of Reference Example 2, but it was 4 hours after the start of the reaction.
It fell to 3.4%.

Reference Example 3 The indole synthesis reaction was performed in the same manner as in Reference Example 2 except that the catalyst was changed. The catalyst used was the same as that used in Reference Example 2.
This is a SiO ₂ carrier carrying 2.5% of copper. The indole yield based on ethylene glycol one hour after the start of the reaction was 67.0%, and the indole yield after 50 hours was 41.8%.

Comparative Example 3 The reaction mixture leaving the reaction tube in Reference Example 3 was cooled to room temperature, the obtained condensate was separated into two phases, an aqueous phase and an organic phase, and aniline was obtained from the latter by distillation as in Comparative Example 1. Separated and collected. As a result of analysis, about 20 kg of the collected aniline had a purity of 99% or more.

Using this recovered aniline, an indole synthesis reaction was carried out under the same conditions as in Reference Example 3. The indole yield based on ethylene glycol one hour after the start of the reaction was 66.7%, which was almost the same as in Reference Example 3, but it decreased to 33.3% 50 hours after the start of the reaction.

Example 1 About 100 kg of an organic phase obtained by carrying out an indole synthesis reaction under the same conditions as in Reference Example 1 was charged with a cation exchange resin (trade name: Levatit SP112, manufactured by Bayer AG, West Germany) 5 in a packed column at 2.5 kg / hr. Flowed at a flow rate. The temperature of this treatment was room temperature and the pressure was room temperature.

This treated solution was subjected to batch distillation in the same manner as in Comparative Example 1 to recover about 80 kg of aniline and subjected to the same indole synthesis reaction as in Reference Example 1. The indole yield based on ethylene glycol immediately after the start of the reaction was almost the same as in Reference Example 1.
74.6%, the yield after 200 hours was 60.5%, and the decrease in yield was less than that in Comparative Example 1.

Example 2 The recovered aniline obtained in Comparative Example 1 was treated with a cation exchange resin in the same manner as in Example 1 and subjected to the same indole synthesis reaction as in Reference Example 1. Immediately after the start of the reaction, the indole yield based on ethylene glycol was 74.4%, 2
The indole yield after 00 hours was 58.2%.

Example 3 In the indole synthesis reaction under the same conditions as in Reference Example 1,
About 100 kg of the obtained organic phase was placed in a stirring tank and subjected to the following treatment at 70 ° C. and normal pressure.

B. Add 20 kg of pH 4 HCl aqueous solution and stir for 3 hours.

( ₂₎ Add 20 kg of pH 5.5 KH ₂ PO ₄ aqueous solution and stir for 3 hours.

(C) Add 50 kg of KH ₂ PO ₄ aqueous solution at pH 6.5 and stir for 20 hours.

After removing the aqueous phase by liquid-liquid separation, these liquids were batch-distilled under the same distillation equipment and operating conditions as in the comparative example, yielding about 80 kg.
Aniline was recovered. When these recovered anilines were subjected to the same indole synthesis reaction as in Reference Example 1, the respective indole yields based on ethylene glycol were
It was as shown in Table-1.

Example 4 About 25 kg of the organic phase obtained in the indole synthesis reaction under the same conditions as in Reference Example 2 was treated with a cation exchange resin under the same conditions as in Example 1. From this treatment liquid, Comparative Example 1
Approximately 20 by batch distillation using the same distillation equipment and operating conditions
kg of aniline was recovered, and the indole synthesis reaction was carried out under the same conditions as in Reference Example 2. The indole yield based on ethylene glycol 1 hour after the start of the reaction was 69.9%, which was almost the same as in Reference Example 2, and the indole yield 50 hours after the start of the reaction was 51.5%. There was little decline.

Example 5 About 25 kg of the organic phase obtained in the indole synthesis reaction under the same conditions as in Reference Example 3 was treated with a cation exchange resin under the same conditions as in Example 1, and then batchwise as in Comparative Example 1. Distilled to recover about 20 kg of aniline. Using this recovered aniline, an indole synthesis reaction was performed under the same conditions as in Reference Example 3. The indole yield based on ethylene glycol 1 hour after the start of the reaction was 67.2%, and the indole yield 50 hours after the start of the reaction was 42.3%, showing a smaller decrease in yield than in Comparative Example 3.

Example 6 To about 25 kg of the organic phase obtained in the indole synthesis reaction under the same conditions as in Reference Example 2, 5 kg of 0.1 N dilute sulfuric acid aqueous solution was added, and the mixture was stirred at 50 ° C. and normal pressure for 3 hours.

Then, the mixture was allowed to stand still, the aqueous phase was removed by liquid-liquid separation, and then about 20 kg of aniline was recovered by palindromic distillation under the same distillation apparatus and operating conditions as in Comparative Example 1, under the same conditions as in Reference Example 2. An indole synthesis reaction was performed. The yield of indole 1 hour after the start of the reaction was 69.8%, and the yield after 50 hours was 54.2%.

Example 7 To about 25 kg of the organic phase obtained in the indole synthesis reaction under the same conditions as in Reference Example 2 was added 5 kg of an aqueous acetic acid solution having a pH of 5.0, and
The mixture was stirred at 0 ° C. and normal pressure for 3 hours. Then, the mixture was allowed to stand still, the aqueous phase was removed by liquid-liquid separation, and then about 20 kg of aniline was recovered by batch distillation using exactly the same distillation apparatus and operating conditions as in Comparative Example 1, and the indole was prepared under the same conditions as in Reference Example 2. A synthetic reaction was carried out. The indole yield 1 hour after the start of the reaction was 70.3%, which was almost the same as in Reference Example 2, and the yield after 50 hours was 52.3%. The decrease in yield was less than that in Comparative Example 2.

Example 8 The same operation as in Example 4 was carried out except that the cation exchange resin was used instead of the cation exchange resin as the packing material, and the results in Table 2 were obtained. In all cases, the decrease in yield was less than that in Comparative Example 2.

Claims (1)

[Claims] 1. A reaction containing a gas phase reaction of ethylene glycol with stoichiometric excess of aniline in the presence of a catalyst to obtain an indole, a side reaction product and unreacted aniline. Aniline is separated and collected from the liquid,
A method for producing indole by recycling the recovered aniline in the gas phase reaction, wherein the reaction solution or the recovered aniline is contacted with an acidic substance.