Preparation of Crystalline CrVO<sub>4</sub> Catalyst by Soft Chemistry Technique and Application for Vapor-phase Oxidation of Picolines

Abstract

A series of Cr–V mixed oxide catalysts (CrVO4, Cr1−xAlxVO4 and CrV1−xPxO4) have been prepared by "soft chemistry" technique. By mixing the aqueous solution of the raw materials and controlling the pH value, chromium vanadates (CrVO4-I and III) were prepared as the pure orthovanadate crystalline form. Prepared chromium vanadates were employed in the vapor phase oxidation of 2-, 3-, and 4-picolines in order to investigate the relationship between the catalysis and structure of chromium vanadate. CrVO4-I (monoclinic) showed much higher activity than CrVO4-III (orthorhombic), suggesting that the bridging oxygen in the V–O–Cr bond is responsible for the catalytic activity. When a part of Cr or V was replaced with Al or P without changing the CrVO4-I type structure, the activities were enhanced. The acidity increased by the replacement of Cr with Al and V with P in CrVO4-I, resulting in the accelerating desorption of products. A partial replacement of V with P significantly improved activity and this is probably due to the modification of the redox properties of V–O–Cr site in addition to the accelerating desorption of products. An addition of large amount of steam in the reactants improved the yield of desired products. Both Lewis and Brønsted acid sites were detected on the surface of CrV0.95P0.05O4, and the number of the latter increased by the addition of steam. The enhanced activity by the addition of steam is interpreted by the fact that Brønsted acid sites are produced by the hydrolysis of V–O–Cr bond on the surface and activate picoline molecules by withdrawing the electrons of the pyridine ring, and at the same time, enable to accelerate the desorption of the acid products. Even in the absence of gaseous oxygen, the oxygenated products were formed, thus, a Mars and van Krevelen mechanism was suggested.