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

Abstract

Cs 2.5H 1.5PV 1Mo 11O 40 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 (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 (300–400 °C). The direct synthesis method was found more efficient than the classical cationic exchange technique for propane ODH.