1. Extraction from washing oil fractions In high-temperature coal tar, it contains about 0.10%-0.16% of indole. Generally, it can be extracted from coal tar and washing oil fractions. The washing oil fraction is washed with alkali and acid to obtain the methylnaphthalene fraction, which is then rectified in a high-efficiency tower with 60 theoretical plates, and the fraction at 225-256°C is cut out and melted by adding potassium hydroxide. The reaction was carried out at 170-240°C for 2-4h, and stirred until bubbling stopped. Let it stand still, release the lower layer of indole potassium to cool, break it up and wash it with benzene to remove oil at low temperature. Then hydrolyze at 50-70°C to obtain crude indole oil, which is distilled in a distillation column with 20 theoretical plates to obtain a reflux ratio of 8-10:1, cut off the fraction with a temperature of 170-256°C at the top of the tower, cool and crystallize , Centrifugal filtration, that is, refined indole. After pressing, the oil content is below 3%, and recrystallized with ethanol to obtain refined Indole Sigma with a purity of 99%.
2. It can be obtained by catalytic dehydrogenation of o-aminoethylbenzene. O-aminoethylbenzene is dehydrocyclized at 550°C in the presence of aluminum nitrate (or aluminum oxide) in a nitrogen stream, and dihydroindoline is obtained by distillation under reduced pressure. Then dehydrogenate at 640°C to obtain Indole Sigma. Other production methods include the reaction of o-nitrotoluene and oxalate to generate o-nitrophenylpyruvate and then make α-indole carboxylic acid, and finally dry distillation with lime to obtain the product; aniline and acetylene in Synthesize indole by heating at 600-650°C; synthesize Indole Sigma from o-carboxyphenylglycine through 3-hydroxy-2-indole carboxylic acid and indole acid; oxidize cyanine with concentrated nitric acid or chromic acid to obtain indole quinone, and Indole can be obtained by distillation with zinc powder. Indole can also be obtained by mixing nitrocinnamic acid and 10 parts of potassium hydroxide powder, adding iron filings and heating to melt the mixture. In the synthesis of indole and its homologues, the Fischer synthesis method is the most common. Aromatic hydrazones of ketones or aldehydes act under acidic conditions to undergo a rearrangement reaction similar to benzidine to generate Indole Sigma.