Mg–Fe Hydrotalcite as a Catalyst for the Reduction of Aromatic Nitro Compounds with Hydrazine Hydrate

Catalysts consisting of mixed oxides of Fe3+ and Mg2+ were prepared by decarbonation of Mg–Fe hydrotalcite-like precursors. They show high activity and selectivity for the selective reduction of aromatic nitro compounds under mild reaction conditions. They are regenerable and can be recycled without any loss of activity. The solids were characterized by XRD, XPS, and Mössbauer spectroscopy. From Mössbauer spectroscopy the solid appears as a well-dispersed ferrihydrite phase supported by a MgFeO matrix, and only Fe3+ ions in ferrihydrite can be reversibly reduced by hydrazine and reoxidized by the nitro compound. This observation and the value of the slope of the Hammett plot suggest that the mechanism is similar to that reported for iron oxides. The activity is proportional to the fractional surface of iron oxide, and at equal surface area, MgFe oxides are more active than iron oxides and form negligible amounts of hydroxylamine intermediates.