Transition metal-doped heteropoly catalysts for the selective oxidation of methacrolein to methacrylic acid

Heteropoly compounds with the general formula Cs1M0.5x+H3‒0.5xP1.2Mo11VO40 (M = Fe, Co, Ni, Cu or Zn) and Cs1CuyH3‒2yP1.2Mo11VO40 (y = 0.1, 0.3 or 0.7) were synthesized and then used as catalysts for the selective oxidation of methacrolein to methacrylic acid. The effects of the transition metals on the structure and activity of the catalysts were investigated. FTIR spectra showed that the transition metal-doped catalysts maintained the Keggin structure of the undoped catalysts. X-ray diffraction results indicated that before calcination, the catalysts doped with Fe and Cu had cubic secondary structures, while the catalysts doped with Co, Ni or Zn had both triclinic and cubic phases and the Co-doped catalyst had the highest content of the triclinic form. Thermal treatment can decrease the content of the triclinic phase. NH3 temperature-programmed desorption and H2 temperature- programmed reduction results showed that the transition metals changed the acid and redox properties of the catalysts. The addition of Fe or Cu had positive effects on the activities of the catalyst which is due to the improvement of the electron transfer between the Fe or Cu and the Mo. The effects of the copper content on structure and catalytic activity were also investigated. The Cs1Cu0.3H2P1.2Mo11VO40 catalyst had the best performance for the selective oxidation of methacrolein to methacrylic acid.