Polyoxometalate catalysts with co-substituted VO2+ and transition metals and their catalytic performance for the oxidation of isobutane†
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 Cs2.0V0.3Cu0.2PMo12O40 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.