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

Abstract

A tetragonal molybdenum nitride (β-Mo 2N) has been prepared by temperature programmed treatment of MoO 3 in flowing N 2 + H 2 and for the first time shown to catalyze the liquid phase selective hydrogenation ( T = 423 K; P H 2 = 11 bar ) of a series of para-substituted (–H, –OH, –O–CH 3, –CH 3, –Cl, –I and –NO 2) nitrobenzenes to give the corresponding aromatic amine. Reaction over Pd/Al 2O 3, 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. β-Mo 2N has been characterized in terms of temperature-programmed reduction (TPR), H 2 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 β-Mo 2N, characterized by an agglomeration of flake-like crystallites. Post-synthesis, the nitride was passivated by contact with 1% (v/v) O 2/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 β-Mo 2N. 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 β-Mo 2N to promote selective nitroarene hydrogenation.