The zeolite-catalysed isomerization of aniline to α-picoline Original Research Article

The rearrangement of aniline to α-picoline, catalysed by zeolite ZSM-5 in the presence of ammonia at elevated temperature, was studied. Reactions were carried out in a continuous flow reactor at 673 K and a constant total pressure of 75 bar with the NH3 pressure varying from 5 to 60 bar. The yield of α-picoline mainly depends on the ammonia pressure. Optimum ammonia pressures are around 20–25 bar, because at low pressure there is not enough ammonia present to form the reactive surface species, whereas the catalyst is poisoned by ammonia when the pressure is too high. Selectivity shows a similar behaviour. Low ammonia pressure favours the condensation of two aniline molecules to diphenylamine, and high pressure favours the decomposition to acetonitrile and subsequently the formation of methylanilines. The aniline to α-picoline reaction depends on the number of active sites as well as on the nature of the T-atom in the zeolite framework. Ga-MFI shows a higher activity than ZSM-5, but the acidity of B-MFI is too weak, and Fe-MFI is not stable in the presence of ammonia.