Enhanced selective nitroarene hydrogenation over Au supported on β-Mo2C and β-Mo2C/Al2O3

Abstract

β-Mo2C and β-Mo2C/Al2O3 have been synthesised via temperature programmed carburisation and employed, for the first time, as supports for gold catalysts. 1%w/w Au/Mo2C and Au/Mo2C/Al2O3 were prepared by deposition–precipitation with urea and used to promote the gas phase hydrogenation of para-chloronitrobenzene (p-CNB) and meta-dinitrobenzene (m-DNB) where 1% w/w Au/Al2O3 served as a reference catalyst. The supports and supported Au catalysts have been characterised in terms of point of zero charge, temperature programmed reduction (TPR), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. Both Mo2C and Mo2C/Al2O3 exhibited hydrogenation activity that was significantly enhanced with the incorporation of Au. XPS and elemental analysis of Mo2C/Al2O3 revealed a lesser carbidic character and the presence of free surface carbon. Moreover, preparation of Au/Mo2C/Al2O3 was accompanied by a leaching of the Mo component into solution. The carbide-based catalysts were characterised by a broader distribution of larger Au particles compared with the reference Au/Al2O3, which can be attributed to weaker Au/carbide interactions resulting in Au agglomeration during synthesis and activation. Nevertheless, the carbide systems delivered higher hydrogenation rates relative to Au/Al2O3. All the Au catalysts tested exhibited 100% selectivity to the target p-chloroaniline product in p-CNB hydrogenation. In the case of m-DNB, both nitro groups were hydrogenated to generate m-phenylenediamine as principal product for reaction over Au/Al2O3 whereas Au/Mo2C promoted the exclusive production of m-nitroaniline at low conversions. The results demonstrate a synergistic effect between Au and Mo2C that can be exploited in the cleaner production of commercially important aromatic amines.