Synthesis of group VI carbides and nitrides: application in catalytic hydrodechlorination

Abstract

Molybdenum and tungsten carbide (Mo2C, W2C) and nitride (Mo2N) were prepared from MoO3 and WO3 by temperature-programmed treatment with C2H6/H2 and N2/H2. We have examined the influence of synthesis procedure, applying single-step (direct carburisation) and two-step (nitridation-carburisation and partial reduction-carburisation) processes, on crystallographic phase and catalytic response in the gas phase hydrodechlorination (HDC) of 1,3-dichlorobenzene (1,3-DCB). Single-step carburisation generated (hexagonal close-packed) hcp-Mo2C and hcp-W2C. Two-step nitridation–carburisation yielded (face-centred cubic) fcc-Mo2C via fcc-Mo2N in a topotactic transformation where surface area was increased (to 145 m2 g−1) at higher gas hourly space velocity (from 4800 to 96000 h−1) and decreasing heating rate (from 0.6 to 0.3 K min−1). Partial reduction of MoO3 prior to carburisation produced a composite (fcc + hcp)-Mo2C. Negligible ambient temperature H2 chemisorption (≤0.3 μmol g−1) was recorded for the synthesised (nitride and carbide) materials, but temperature-programmed desorption (TPD) resulted in significant H2 release (up to 67 μmol g−1 for (fcc + hcp)-Mo2C) that was generated during temperature-programmed reduction. Under the same reaction conditions, hcp- and fcc-carbides delivered equivalent specific HDC activity that was appreciably lower (by a factor of up to 20) than that obtained with fcc-Mo2N. The composite (fcc + hcp)-Mo2C exhibited the highest HDC rate that can be correlated with H2 released during TPD and which approached the performance of a benchmark Ni/SiO2 catalyst. HDC selectivity (to chlorobenzene) as a function of 1,3-DCB conversion coincided for all the nitride and carbide systems but deviated from Ni/SiO2, which favoured concerted HDC to benzene.