We show for the first time that the NH4+ exchanged form of ZSM-5 zeolite is as effective as the H+ form of the catalyst for the regioselective conversion of toluene to mononitrotoluene (MNT) using 90% nitric acid as the sole nitrating agent. The auto-ionization of the acid in the absence of protons on the zeolite surface is sufficient for the formation of reactive nitronium ions. We also find that the regioselectivity for the formation of p-MNT is substantially increased over both forms of the zeolite catalyst by first intercalating the acid in the zeolite micropores prior to the introduction of toluene. For instance, the p/o isomer ratio is increased from values in the range 1.3-2.3 under conventional batch reaction conditions to values of 1.8-8.9 when the acid is sequestered in the zeolite. The sequestration of nitric acid in both ion exchanged forms of the zeolite confines more of the nitration reaction to the regioselective environment of the micropores and reduces the extent of reaction in homogeneous toluene solution. However, the ammonium form of the zeolite is preferred over the protonated form when the Si/Al ratio of the zeolite allows for the presence of at least one ammonium ion per unit cell (Si/Al <= 40), because under these conditions far less benzaldehyde and other undesired reaction products are formed in comparison to the protonated form of the zeolite at the same SUM ratios. The ability of NH4+-ZSM-5 derivatives to limit the formation of unwanted toluene oxidation products is not well understood, but the observed selectivity may be related to the replacement of ammonium ions by protons, the formation of ammonium nitrate, and the buffering of nitric acid within the micropores of the zeolite. (c) 2007 Elsevier Inc. All rights reserved.