Metal vanadate catalysts for the ammoxidation of 2-methylpyrazine to 2-cyanopyrazine

The ammoxidation of 2-methylpyrazine to 2-cyanopyrazine was carried out in a fixed bed metal reactor in the temperature range of 320-460 degrees C using a series of metal vanadate-containing solids (MV) as catalysts. These solids named as AIVO(4), FeVO4, CrVO4, NbVO5, LaVO4 and BiVO4 were prepared with a nominal V/M ratio = 1 always using the same synthesis procedure. Fresh and spent solids were characterized by X-ray diffraction, UV-vis DRS, XPS and pyridine-FTIR & ESR spectroscopy. The results revealed that the phase composition, near-surface-region behaviour and catalytic properties strongly depend on the nature of the metal used in MV solids. XRD showed the formation of crystalline MV phases in case of LaVO4 and BiVO4: whereas FeVO4 and CrVO4 exhibited poor crystallinity only. AIVO(4) sample revealed the clear formation of crystalline V2O5 whereas in NbVO5 only a small proportion of V2O5 was detected. XPS depicted that the enrichment of vanadium in the near-surface-region is clearly dependent on the type of MV. It can be concluded that high near-surface-region V/M molar ratios promote the selectivity to cyanopyrazine but reduce the catalytic activity and vice versa. NbVO5 showed the best catalytic performance compared to all other MVs. Almost 69% yield of 2-cyanopyrazine at total conversion could be successfully obtained. (C) 2012 Elsevier B.V. All rights reserved.