Ammoxidation of 2-methylpyrazine to 2-cyanopyrazine over Nb–V oxides: marked effect of the Nb/V ratio on the catalytic performance

Abstract

A series of bulk Nb–V-containing mixed oxide catalysts with varying Nb/V ratios were synthesized and studied by various solid-state characterization methods. Their catalytic performance was evaluated for the gas phase ammoxidation of 2-methylpyrazine (MP) to 2-cyanopyrazine (CP) that has received growing interest in the chemical industry, recently. The catalysts were characterised by BET-SA, XRD, UV-vis DRS, FTIR, XPS, and TEM. The BET surface area decreased continuously with increase in vanadia content. XRD data confirmed the changes in the crystalline phases with altering Nb/V ratios. UV-vis DRS and FTIR spectroscopic results showed the formation of various kinds of V-oxide species in the catalysts with change in V content. An increase in the concentration of vanadium changes the nature of VO_x species from isolated vanadia species to polymeric vanadia species and then to crystalline vanadia species. Among all the catalysts, the Nb–V–O catalyst with a Nb/V ratio of 1 exhibited the best performance in the ammoxidation reaction (i.e. X-MP ~100% and selectivity to 2-CP ~70%). Additionally, a very high space–time yield of CP (>440 g_CP kg_cat⁻¹ h⁻¹) could be successfully achieved. This best catalyst sample revealed two-dimensional polymeric V-oxide species. TEM and SEM showed the formation of a rod-shaped nanoparticle morphology. XPS data revealed that the vanadium is present in two oxidation states (V⁵⁺ and V⁴⁺) in the fresh catalyst (Nb/V = 1) and only one oxidation state (V⁵⁺) in the spent catalyst.