Process for the preparation of indoles

A process for preventing a decrease in the yield of indole in an indole-forming process comprising:

• (a) reacting a stoichiometric excess of an aniline of formula (III):
• ##STR5## wherein R.sub.1 is a hydrogen atom, a halogen atom, a lower alkyl group, a hydroxyl group, a lower alkoxy group or a nitro group, with a diol of formula (IV):
• ##STR6## wherein R.sub.2 and R.sub.3 are each a hydrogen atom or a lower alkyl group which may be substituted by a hydroxyl group, in the presence of a catalyst containing at least one element selected from Group Ib of the periodic table in the gas phase;
• (b) condensing the resultant gaseous reaction mixture to form a liquid mixture containing the indole, the excess aniline and compounds of formulas (I) and (II):
• ##STR7## wherein R.sub.1, R.sub.2 and R.sub.3 are defined as above;
• (c) separating the excess aniline from the liquid mixture;
• (d) subjecting the liquid mixture or the aniline separated to a heat-treatment to convert the compound of formula (II) into non-volatile substances;
• (e) subjecting the liquid mixture or aniline thus heat-treated to distillation to obtain the aniline containing not more than 2 percent by weight of the compound of formula (I) and not more than 0.2 percent by weight of the compound of formula (II);
• and
• (f) recycling the aniline from step (e) and using the aniline from step (e) in step (a);
• whereby reaction (a) is conducted without the reduction of activity of the catalyst.