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

Cs2.5H1.5PV1Mo11O40 heteropolyoxometallate compounds have been studied for propane oxidative dehydrogenation (ODH) in the 340-400degreesC temperature range. Their redox and Bronsted 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 (C-3:O-2:He = 2:1:2, flow rate 15 cm(3) 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 (300-400degreesC). The direct synthesis method was found more efficient than the classical cationic exchange technique for propane ODH. (C) 2003 Elsevier B.V. All rights reserved.