Vapor-phase oxidation of 3-picoline to nicotinic acid over Cr1−xAlxVO4 catalysts Original Research Article

Some metal vanadates were precipitated as the pure phases by adjusting the pH of aqueous solutions of the raw materials. The precipitates were calcined at 550 °C and tested for the vapor phase oxidation of 3-picoline to nicotinic acid. Chromium vanadate showed the highest activity among the vanadates tested; its structure belongs to monoclinic CrVO4-I. An addition of aluminum into CrVO4-I resulted in the formation of the solid solution of Cr1−xAlxVO4 still keeping CrVO4-I structure with high surface area in the range of x≤0.5. Upon further addition of aluminum, CrVO4-I structure changed to AlVO4 structure. The activity increased with increasing the Al content while keeping CrVO4-I structure; the highest activity was obtained at x=0.5. In the CrVO4-I structure, all the V species are isolated as VO4 tetrahedra and show the oxidation activity via its redox properties. NH3-TPD results showed that acidity increased by the replacement of Cr with Al in CrVO4-I. The addition of a large amount of water in the reaction mixture enhanced the yield of nicotinic acid. The highest activity was obtained over the Cr0.5Al0.5VO4 and the total yield of nicotinic acid and pyridine-3-carbaldehyde reached to 69% (selectivity, 86%) at 350 °C. Thus, 3-picoline can be effectively oxidized with O2 in the gas-phase to nicotinic acid on the Cr0.5Al0.5VO4 catalyst having Brønsted acid sites in addition to redox properties.