ZEOLITE-CATALYZED ISOMERIZATION OF AROMATIC-AMINES TO METHYL-AZA-AROMATICS

The scope and mechanism of the isomerization of arylamines to methyl-s ubstituted aromatic heterocycles have been studied. Aniline, toluidine s, naphthylamines and m-phenylenediamine all reacted to the correspond ing ortho-methyl-substituted aza-aromatics when exposed to high NH3 pr essure and elevated temperature in the presence of acid catalysts. Zeo lites with a three-dimensional pore structure, especially H-ZSM-5, sho wed the best performance. Optimum reaction conditions are around 600 K and 10 MPa. Two mechanisms which had been proposed earlier for this a pparent N-ortho C exchange reaction proved untenable. Neither incorpor ation of the N atom into the aromatic ring nor a mechanism based on an intramolecular Ritter reaction could explain the required high NH3 pr essure or the product distribution. Two new mechanisms are proposed wh ich can explain all observations. In both mechanisms, reaction starts with addition of NH3 to the arylamine, followed by ring opening. In on e mechanism an alkyno-imine intermediate is formed; in the other mecha nism an enamino-imine intermediate is formed through a reverse aldol r eaction. In both cases ring closure and NH3 elimination lead to the re quired aromatic heterocycles. The high NH3 pressure is explained by th e need to add NH3 to the aromatic ring, and the high temperature by th e need to desorb NH3 from the acid sites. (C) 1995 Academic Press, Inc .