FTIR study of the role of surface complexes in a transformation of picoline isomers on vanadium-titanium oxide catalysts

Surface complexes of 2-, 3- and 4-picoline on the VTi oxide catalysts were studied by FTIR spectroscopy. The sequence of its transformations has been studied in the temperature range 100–260 °C. In the first step, isomers of picoline interact with the strong Brönsted acid sites of the catalysts to form the corresponding methylpyridinium ions. Further, these complexes are transformed into the corresponding surface complexes of pyridinecarbaldehyde. The latter are transformed further into the corresponding carboxylates. The carboxylates are the direct precursors of picolinic, nicotinic, and isonicotinic acids. 2-Pyridine carbaldehyde complex are transformed also into α-pyridoin complex via the condensation. This fact explains the low yield of picolinic acid. Published data on the order of activity in oxidation and ammonolysis of the picoline isomers (2 ≈ 4 > 3) can be explained by the values of the basicity of isomeric picolines and by values of pKa of the methyl group deprotonation.