VAPOR-PHASE AROMATIC NITRATION WITH DINITROGEN TETROXIDE OVER SOLID ACIDS - KINETICS AND MECHANISM

The mechanism of the vapour-phase mononitration of aromatics with dini trogen tetroxide has been investigated over silica-alumina and the lar ge-pore zeolite beta (BEA). A Rideal-Eley rate law is observed in both cases, showing that the reaction occurs between the chemisorbed aroma tic and the gaseous nitrating agent. The effects of substituents are b etter correlated by a relation between the activity and the ionisation potential of the aromatic than by the Hammett equation. Both results conflict with the ''classical'' electrophilic aromatic substitution (S EAr), but agree with the formation of an aromatic radical cation inter mediate at the surface of the catalyst and its reaction with a nitroge n dioxide radical in the rate determining step. The generation of elec tron-acceptor sites on the catalyst surface by nitrogen dioxide radica ls is suggested. Addition of water in the feed inhibits the reaction b y competitive adsorption over silica-alumina, but improves the efficie ncy of BEA, certainly by ''steam cleaning'' of the surface of this hyd rophobic catalyst. (C) 1994 Academic Press, Inc.