Selective three-phase hydrogenation of aromatic nitro-compounds over β-molybdenum nitride

A tetragonal molybdenum nitride (ß-Mo₂N) has been prepared by temperature programmed treatment of MoO₃ in flowing N₂ + H₂ and for the first time shown to catalyze the liquid phase selective hydrogenation (T = 423 K; P_H2 = 11 bar) of a series of para-substituted (-H, -OH, -O-CH₃, -CH₃, -Cl, -I and -NO₂) nitrobenzenes to give the corresponding aromatic amine. Reaction over Pd/Al₂O₃, as a benchmark catalyst (Pd particle size ca. 18 nm), resulted in a composite hydrodechlorination/hydrogenation of p-chloronitrobenzene (as a representative nitroarene) to generate nitrobenzene and aniline. ß-Mo₂N has been characterized in terms of temperature-programmed reduction (TPR), H₂ chemisorption/temperature programmed desorption (TPD), BET surface area/pore volume, elemental analysis, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning (SEM) and transmission (TEM) electronic microscopy. Elemental analysis, XRD, SEM and TEM have confirmed the formation of tetragonal ß-Mo₂N, characterized by an agglomeration of flake-like crystallites. Post-synthesis, the nitride was passivated by contact with 1% (v/v) O₂/He at ambient temperature and XPS analysis has demonstrated the formation of a superficial passivating oxide overlayer without bulk oxidation. Pre-reaction, activation by TPR to 673 K was necessary to remove the passivating film. Hydrogen TPD has revealed significant hydrogen uptake (0.7 µmol m^-2) associated with ß-Mo₂N. Nitro group reduction kinetics have been subjected to a Hammett treatment where the reaction constant (p = 0 4) is diagnostic of an increase in rate due to the presence of electron-withdrawing substituents on the aromatic ring, consistent with a nucleophilic mechanism. The results presented in this study establish the viability of ß-Mo₂N to promote selective nitroarene hydrogenation. © 2011.