Polyoxometalate catalysts with co-substituted VO2+ and transition metals and their catalytic performance for the oxidation of isobutane

Abstract

Cs-Salts of the Keggin-type phosphomolybdic acid with simultaneous doping of VO²⁺ and different transition metals are prepared, characterized and tested as catalysts for the oxidation of isobutane to methacrylic acid under isobutane-rich conditions. The results show that Cs_2.0V_0.3Cu_0.2PMo₁₂O₄₀ catalyst with co-substituted VO²⁺ and Cu²⁺ in its secondary structure exhibits higher catalytic performance than those with co-doped VO²⁺ and other transition metals such as Fe, Ni and Ce. This difference is ascribed to the synergistic effect between Cu²⁺ and VO²⁺ only in the secondary structure. This synergistic effect is related to the charge transfer between Mo⁶⁺ in the Keggin anions and Cu²⁺ counter cation, which promotes the re-oxidation of the catalyst and consequently its catalytic activity.