Role of acid and redox properties on propane oxidative dehydrogenation over polyoxometallates

Cs_2.5H_1.5PV₁Mo₁₁O₄₀ heteropolyoxometallate compounds have been studied for propane oxidative dehydrogenation (ODH) in the 340–400 °C temperature range. Their redox and Brønsted acid properties have been tuned by introducing a redox metal element M such as Co^II, Fe^III, Ga^III, Ni^II, Sb^III or Zn^II in a V:M atom ratio equal to 1:1. This introduction was carried out either directly in the synthesis solution or by usual aqueous cationic exchange of protons of the solid Cs salt. TGA and FT-IR analyses allowed us to determine the extent of metal M substitution for Mo^VI in the Keggin anion and proton replacement by the M cation. It was observed that, under catalytic conditions (C₃:O₂:He=2:1:2, flow rate 15 cm³ min⁻¹, 12 h on stream), the catalysts were stable, with only a small part of the substituted elements (V and/or M) being extracted from the Keggin anion during the reaction. The presence of these metal M cations enabled us to tune the redox and acid properties of the material and to get high selectivity for propene (60–80% at 5 and 10% propane conversion) at a relatively low temperature (300–400 °C). The direct synthesis method was found more efficient than the classical cationic exchange technique for propane ODH.