Crystalline CrV0.95P0.05O4 catalyst for the vapor-phase oxidation of picolines

The catalytic behavior of CrV0.95P0.05O4 has been studied in the selective oxidation of picolines and the oxidation mechanism is discussed. Brönsted acid site was detected on the surface of CrV0.95P0.05O4, and the amount increased by the addition of steam in the reaction mixture, resulting in an enhanced activity for the selective oxidation. Picoline is adsorbed via the N atom coordination to Brönsted acid site, and the substituted methyl group is oxidized by surface oxide ion to produce aldehyde and then acid. Thus, Mars and van Krevelen mechanism is suggested for picoline oxidation based on the DRIFTS analysis. 2- and 4-Picolines were more quickly oxidized than 3-picoline due to the inductive hyperconjugative effect of nitrogen, resulting in an easy leaving of proton from the methyl group. 4-Picoline produced almost quantitatively isonicotinic acid, while 2-picoline afforded picoline-2-carbaldehyde as the main product due to the instability of the acid product, i.e., the decarboxylation of picolinic acid took place to form pyridine.