Catalytic Oxidation of 4-Methylpyridine on Modified Vanadium Oxide Catalysts

Reaction of gas-phase oxidation of 4-methylpyridine on individual V2O5, binary and ternary vanadium-oxide catalysts was studied. These catalysts were modified by additives of SnO2 and TiO2. It was found that modifying V2O5 leads to increase activity of binary contacts. Upon transition from binary V2O5-SnO2 and V2O5-TiO2 catalysts to ternary system of V2O5-TiO2-SnO2, a higher increase in activity is observed. This extension in activity leads to increase conversion of initial substance and shifting the maximum yield of intermediate pyridine-4-carbaldehyde and isonicotinic acid to the low-temperature area. To research the mechanism of promotion, we used the quantum chemical method of Density Functional Theory. It was found that the promoting effect of SnO2 and TiO2 was caused by increasing a proton affinity of vanadyl oxygen (PAV=O). Upon transition from binary clusters to the ternary system of V2O5-TiO2-SnO2 the synergism effect is observed. It is shown that by transfer of a proton to vanadyl oxygen and formation of a new O−H bond the energy is emitted. This energy compensates a heterolytic C−H bond cleavage. It was found that the promoting effect of SnO2 and TiO2 causes the decrease of deprotonation enthalpy of the methyl substituent of the chemisorbed substrate. The results of the calculations agree with the experimental data on influence of oxide modifiers on activity and selectivity of the studied catalysts in 4-methylpyridine oxidation.