In situ DRIFTS study of picoline oxidation over CrV0.95P0.05O4 catalyst

Abstract

The catalytic behaviour of CrV0.95P0.05O4 has been investigated in the selective oxidations of 2-, 3- and 4-picolines by in situ DRIFTS, and the model of picoline adsorption and the oxidation mechanism are proposed. Both Lewis and Bronsted acid sites were detected on the surface of CrV0.95P0.05O4, and the number of the latter increased on the addition of steam in the reaction mixture, resulting in enhanced activity for selective oxidations. The enhanced activity due to water addition is interpreted by the fact that Bronsted acid sites are produced by the hydrolysis of V–O–Cr 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 from the catalyst surface. Every 2-, 3- and 4-picoline was adsorbed on the catalyst surface via the N atom donating the electrons to the Bronsted acid sites, and the substituted methyl group was oxidized via hydrogen abstraction by surface oxide ion to form the radical intermediate, followed by oxygen insertion to produce the corresponding aldehyde and then acid. Even in the absence of gaseous oxygen, the oxygenated products were formed and observed over the catalyst surface by in situ DRIFTS. Thus, a Mars and van Krevelen mechanism was suggested for 2-, 3- and 4-picolines oxidations based on the spectral analysis. Both 2- and 4-picolines were more quickly oxidized than 3-picoline due to the inductive hyper-conjugative effect of nitrogen, resulting in an easy leaving of proton from the methyl group. 4-Picoline produced almost quantitatively isonocotinic acid, while 2-picoline afforded 2-picoline aldehyde as the main product due to the unstability of the acid product, i.e., the decarboxylation of picolinic acid took place to form pyridine.