Investigation of Cu- and Fe-Doped CsH3PMo11VO40 Heteropoly Compounds for the Selective Oxidation of Methacrolein to Methacrylic Acid

Selective oxidation of methacrolein to methacrylic acid was carried out over Cu- and Fe-doped acidic cesium salts of molybdovanadophosphoric acids. The influences of preparation conditions on the structure and catalytic properties of the catalysts were investigated. Results showed that the addition method of NH4+ in the precursors greatly affected the structure of the calcined catalysts. When MoO3, V2O5, and H3PO4 were used to prepare the parent heteropoly acid, which further reacted with the mixed metal nitrates to form the Keggin-type precursor, the addition of ammonia promoted the formation of single-phase cubic secondary structure of the precursor. After being calcined at 360 °C for 12 h, ammonium ions totally decomposed, and vanadium ions were partially expelled from the primary Keggin units. The catalytically less active MoO3 phase, observed in the sample calcined in air, did not exist in the sample calcined in N2. When NH4VO3 was used to replace V2O5 and ammonia to prepare the precursor, the ammonium exhibited higher thermal stability, and the calcination atmosphere had almost no effect on the phase composition of the calcined catalysts. It is also found that Cu and Fe as counterions can improve the redox property and thereby enhance the performance of the catalyst.