Effect of Brønsted acidity in propane oxidation over Cs2.5H1.5PV1Mo11−xWxO40 polyoxometallate compounds
Cs2.5H1.5PV1Mo11−xWxO40 Keggin-type polyoxometallate (POM) compounds have been synthesised and studied for selective propane oxidation in the 300–400 °C temperature range. Prior to reaction the samples were pre-treated at either 300 °C or 400 °C in order to change the concentration in Brønsted acid sites by decreasing the amount of constitutional water. Acid strength was enhanced by substituting increasing amounts of W6+ in the Keggin anion for Mo6+, between 0 and 6 per Keggin Unit (KU) as shown by NH3-TPD of the H+ form and IR of the lattice vibrational modes (MO and MOM with M = Mo6+ or W6+). As a matter of fact vibrational mode frequencies and thus the corresponding bond energies were observed to increase with W6+ substitution. This results in more covalent MO bonds and thus to freer protons. Chemical analysis, IR and DTA/TG data allowed us to determine the extent of W6+ substitution for Mo6+, the amount of constitutional water and any structural change in the samples. It was observed that under catalytic conditions, (C3/O2/He = 2/1/2, flow rate 15 cm3 min−1, 12 h on stream, reaction temperature in the 300–400 °C range) the catalyst structure was maintained, with only a very small part of the substituted elements (V5+, W6+ and Mo6+ atoms) being extracted from the Keggin anion. Catalytic data have shown that introduction of W6+ to replace Mo6+ led to lower propane activation, which may be due to a decrease of lattice oxygen anion mobility, related to the stronger MO bond (see supra and as shown by the higher reaction temperature necessary for the same conversion level). Compared to pre-treatment temperature at 300 °C, pre-treatment at 400 °C was observed to result in a higher extent of constitutional water removal i.e. a loss of Brønsted acid sites by a factor of 2–4.5 when W6+ amount varies from 0 to 6 per KU without destroying the primary Keggin structure, and to favour the formation of propene at the expense of acetic and acrylic acids and COx. This also shows that substituting W6+ for Mo6+, which enhances Brønsted acid strength is detrimental to propene formation and leads to higher selectivity to acetic acid and COx, i.e. the propane oxidation pathway via the isopropanol route and acetone rather at the expense of the usual main pathway forming acrylic acid via propylene.
Journal: Catalysis Communications - Volume 7, Issue 10, October 2006, Pages 811–818