Crystalline CrV1−xPxO4 catalysts for the vapor-phase oxidation of 3-picoline

The heterogeneously catalyzed vapor-phase oxidation of 3-picoline to nicotinic acid over a series of mixed oxides, CrV1−xPxO4, was investigated at 300–400 °C. Characterizations of the catalysts were carried out using X-ray diffraction, FT-IR, TG-DTA, BET, NH3-TPD, TPR, and pyridine adsorption diffuse reflectance infrared Fourier-transform spectra (DRIFTS). The mixed oxides, CrV1−xPxO4 (x = 0–1.0), were precipitated by adjusting the pH from an aqueous solution of the mixture of the raw materials. The precipitates were calcined at 550–700 °C and employed as the catalyst for the selective oxidation of 3-picoline. During the calcination, a crystallization of monoclinic CrVO4-I phase of α-MnMoO4 structure was clearly observed by TG-DTA, FT-IR, and XRD analyses of the CrV1−xPxO4, in the range of x<0.1. In these CrV1−xPxO4, P atoms replace V atoms in the VO4 tetrahedra still keeping a monoclinic CrVO4-I structure. This structure changed to amorphous by further addition of P (x>0.25) or to orthorhombic CrVO4-III by further calcination at high temperature (>700 °C), resulting in the catalyst deactivation. It was found that CrVO4-I was originally active and the addition of a small amount of P resulted in a high enhancement in the catalytic activity; CrV0.95P0.05O4 showed the highest activity among the catalysts tested. NH3-TPD showed an increase in the acid site by replacing V with a small amount of P in CrVO4-I. A favorable effect of water addition suggests that Brönsted acid assists the selective oxidation, and this was confirmed by infrared study of pyridine adsorption on the catalysts. The active catalysts, CrV1−xPxO4 (0