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.