Kinetic study of the zeolite catalyzed isomerization of aniline

The kinetics of the rearrangement of aniline to a-picoline, catalyzed by zeolite ZSM-5 in the presence of ammonia as the co-catalyst and at elevated temperature, has been studied. The ammonia partial pressure has a pronounced influence on the yield of the reaction, optimum ammonia pressures being ca. 20-25 bar at an aniline pressure between 1.5 and 3.1 bar. A classical Langmuir-Hinshelwood kinetic model, in which the two reactants are assumed to absorb on separate acid centers of the catalyst, cannot account for the observed results: first, because a proton transfer occurs on adsorption and the two reactants lose their nucleophilicity, and, second, because even at high Si/Al ratios of the zeolite, when the active centers are very far from each other, the aniline rearrangement still takes place. A kinetic model which includes the formation of a co-adsorption complex of aniline and ammonia as the reactive surface species can explain the kinetic results. Comparison of the experimental data with the simulated reaction data suggests that, at high ammonia pressures, the formation of ammonia clusters consisting of four NH3 molecules adsorbing on a single acid center is very likely. The catalyst surface is saturated under the applied reaction conditions, and aniline adsorbs much stronger on the zeolite than ammonia because of additional van der Waals interactions with the pore walls. (C) 1998 Elsevier Science B.V. All rights reserved.