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 degreesC. 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 degreesC and employed as the catalyst for the selective oxidation of 3-picoline. During the calcination, a crystallization of monoclinic CrVO4-I phase of alpha-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 degreesC), 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 Bronsted acid assists the selective oxidation, and this was confirmed by infrared study of pyridine adsorption on the catalysts. The active catalysts, CrV1-xPxO4 (0 < x < 0.1), alone revealed a weak reduction peak of V around 350degreesC in the TPR, and the peak temperature was the lowest over the most active catalyst. The reduction-oxidation property was reversibly observed with the peak when the catalyst was treated by H-2 and O-2 atmosphere alternately. Moreover, activation energy calculated from 3-picoline consumption was also the lowest over the most active catalyst. All the V species are isolated as VO4 tetrahedra in the CrVO4-I structure and considered to work as the active sites via its reduction-oxidation assisted by both chromium and phosphorous atoms. It is concluded that 3-picoline is selectively oxidized on the V sites by the reduction-oxidation further assisted by the Bronsted-acid sites over the crystallized CrV0.95P0.05O4 catalyst. (C) 2003 Elsevier Inc. All rights reserved.