Influence of vanadium location in titania supported vanadomolybdophosphoric acid catalysts and its effect on the oxidation and ammoxidation functionalities

Titania supported vanadomolybdophosphoric acid (VMPA) catalysts with vanadium in different positions, incorporated into the molybdophosphoric acid structure by substituting for molybdenum and cation exchanged on the molybdophosphoric acid surface, were synthesized. The catalysts were characterized by XRD, FT-IR, Raman spectroscopy, TPR, XPS, and ESR techniques. The FT-IR and Raman spectral analyses confirm that the molybdophosphoric acid is present with the Keggin ion structure during modifications. The incorporation of vanadium into the primary or secondary structures,of the Keggin heteropoly acid was confirmed by ESR, Raman, and XPS analysis. These catalysts were investigated for the vapor phase ammoxidation of 2-methyl pyrazine and methanol oxidation. The specific location of vanadium in the 12-molybdophosphoric acid was found to affect the catalytic redox performance. The catalyst with vanadium present in the primary structure of the Keggin ion possesses enhanced catalytic redox activity and selectivity compared to when vanadium is present on the Keggin surface for both the ammoxidation and oxidation reactions.