Kinetics of the reduction of aromatic nitro compounds with hydrazine hydrate in the presence of an iron oxide hydroxide catalyst

A kinetic model has been developed for the reduction of 4-nitrotoluene to 4-toluidine and 4-nitroaniline to p-phenylenediamine by hydrazine hydrate in the presence of an iron oxide hydrogen-transfer catalyst. While the initial concentrations of 4-nitrotoluene, 4-nitroaniline and hydrazine hydrate were varied the conversion was measured with gas chromatography and UV/Vis spectroscopy. The rate constants and adsorption constants of 4-nitrotoluene, 4-nitroaniline and hydrazine hydrate were determined by least square fitting of the model equations to the experimental data. Nitro arenes with electron-donating substituents show a smaller rate constant as well as a smaller adsorption constant than nitro compounds with electron-attracting substituents. Their high adsorption constants led to zero order kinetics already at moderate concentrations of the nitro compounds, while the small adsorption constant of hydrazine hydrate leads to square root kinetic dependence in hydrazine hydrate. A competitive adsorption experiment between 4-nitrotoluene and 4-nitroaniline showed that the kinetic parameters were different from those obtained from the experiments with single compounds. In order to get a more accurate model, the interaction between the different nitro compounds must be taken into account. (C) 1999 Elsevier Science B.V. All rights reserved.