Preparation of pyridinemonocarboxylic acids by catalytic vapour phase oxidation of alkylpyridines. I. Oxidation of 2- and 3-picoline and decarboxylation of 2- and 3-pyridinecarboxylic acid in the vapour phase

The vapour phase oxidation of 2-picoline with air using a singly promoted vanadium catalyst (V₂O₅/SnO₂, mole ratio 1/1.5), at a temperature of 375°C and a space velocity of 6130 h^?1, gave a 69% conversion with a selectivity of 64% for 2-pyridinealdehyde. No pyridoin formation was detected. In the vapour phase oxidation of 3-picoline using a doubly promoted vanadium catalyst (V₂O₅-TiO₂-Na₂O with V/Ti/Na=100/20/1.15) at a temperature of 400°C and a space velocity of 1450 h^?1, a 94% conversion of 3-picoline was obtained with a selectivity of 51% for niacin (3-pyridinecarboxylic acid) and 1% for 3-pyridinealdehyde. By varying the atom ratio V/Ti, the catalyst V/Ti = 100/25 was found to be the most active one. The decarboxylation of 2- and 3-pyridinecarboxylic acid in the vapour phase was also investigated under the conditions used in the oxidation of the picolines to the acids. At 350°C about 90% decarboxylation of picolinic acid occurs under these conditions, while only a few per cent of the niacin decarboxylated to pyridine between 300 and 500°C.