Selective Oxidation of Isobutane to Methacrylic Acid by Metal-Substituted Ammonium Salts of Molybdovanadophosphoric Acid

A series of ammonium salts of molybdovanadophosphoric acid with different metal substitution and x values (abbreviated as (NH4)1.85Mx, M=Cs, Cu and Fe) were synthesized to catalyze the partial oxidation of isobutane to methacrylic acid. By XRD, TG/DTG, XRF, FT-IR, Raman spectroscopy, H2-TPR, NH3-TPD, it can be found that, compared to the un-doped (NH4)1.85H2.15PMo11VO40 catalyst, the specific surface area, amount of acid sites, and immigrating amount of V atom in Keggin unit into the secondary structure were strongly dependent on the substituted metal ions and their content. The optimum activity was obtained over (NH4)1.85Cs0.5 catalyst, which could provide a larger surface area of 37.72 m2 g−1, a higher amount of acid sites (82.47 μmol g−1), and more VO2+ species and V2O5 clusters from the immigration of V atom in Keggin structure. Furthermore, Cu-substituted catalyst accelerated the catalytic cycle of isobutane oxidation due to the higher electron-delivering efficiency, and the MAA desorption became faster and the catalysts exhibited the excellent selectivity to MAA.