Ferrihydrite Supported on Steel Slags as Catalyst for the Hydrogenation of Nitroarenes: A Virtuous Cycle of Wastes

This study deals with the reduction reaction of nitroarenes using hydrazine monohydrate as the reducing agent and iron-supported steel slag as a novel green heterogeneous catalyst. Steel slag is a byproduct of the steel industry, which, due to its alkalinity, can act as a reactive support that can trigger the formation of catalytically active iron oxides/hydroxides. A systematic study is conducted to evaluate the catalytic activity of steel slags modified with the following salts (or mixtures): FeSO₄·7H₂O, FeCl₃·6H₂O, and FeCl₂·4H₂O. The modified steel slags are characterized by X-ray powder diffraction, Mössbauer spectroscopy, scanning electron microscopy, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, nitrogen sorption analysis, and X-ray photoelectron spectroscopy. All iron-supporting steel slags demonstrate active behavior in the hydrogenation of nitrobenzene at 80 °C with the best results, in terms of activity, selectivity, and recyclability achieved with the catalyst prepared from FeCl₃·6H₂O (Fe3). The scalability of the reaction is confirmed by carrying out a test on 12.5 mmol of substrate. The superiority of Fe3 compared with the other studied materials is ascribed to its morphology and the remarkably high surficial area. The iron species active in the Fe3 catalyst are noncrystalline oxo–hydroxo species of Fe(III) (2L-ferrihydrite).