Catalytic Oxidation of 4-Methylpyridine on Modified Vanadium Oxide Catalysts

Abstract

<em>The 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 the activity of binary contacts. Upon transition from binary V₂O₅-SnO₂ and V₂O₅-TiO₂ catalysts to the ternary system of V₂O₅-TiO₂-SnO₂, a higher increase in activity is observed. This extension of 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 (PA_V=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 TiO₂causes the decrease of deprotonation enthalpy of the methyl substituent of the chemisorbed substrate. The results of the calculations agree with the experimental data on the influence of oxide modifiers on activity and selectivity of the studied catalysts in 4-methylpyridine oxidation.</em>