β-Molybdenum nitride: synthesis mechanism and catalytic response in the gas phase hydrogenation of p-chloronitrobenzene

Abstract

A temperature programmed treatment of MoO3 in flowing N2 + H2 has been employed to prepare β-phase molybdenum nitride (β-Mo2N) which has been used to promote, for the first time, the catalytic hydrogenation of p-chloronitrobenzene. The reduction/nitridation synthesis steps have been monitored in situ and the starting oxide, reaction intermediates and nitride product have been identified and characterized by powder X-ray diffraction (XRD), diffuse reflectance UV-Vis (DRS UV-Vis), elemental analysis, scanning electron microscopy (SEM) and BET/pore volume measurements. Our results demonstrate that MoO3 → β-Mo2N is a kinetically controlled process where an initial reduction stage generates (sequentially) MoO2 and Mo as reaction intermediates with a subsequent incorporation of N to produce β-Mo2N. SEM analysis has established that the transformation is non-topotactic with a disruption to the platelet morphology that characterizes MoO3 and an increase in BET area (from 1 m2 g−1 to 17 m2 g−1). Moreover, temperature programmed desorption measurements have revealed a significant hydrogen uptake (0.71 μmol m−2) on β-Mo2N. This has been exploited in the hydrogenation of p-chloronitrobenzene where p-chloroaniline was generated as the sole product with an associated rate constant (k = 2.0 min−1) that is higher than values recorded for supported transition metals. Our study establishes the reaction mechanism involved in the synthesis of β-Mo2N and demonstrates its viability to promote selective –NO2group reduction as an alternative sustainable, high throughput route to commercially important haloamines.