Effect of Bronsted acidity in propane oxidation over Cs2.5H1.5PV1Mo11-xWxO40 polyoxometallate compounds

Cs2.5H1.5PV2Mo11-xWxO40 Keggin-type polyoxometallate (POM) compounds have been synthesised and studied for selective propane oxidation in the 300-400 degrees C temperature range. Prior to reaction the samples were pre-treated at either 300 degrees C or 400 degrees C in order to change the concentration in Bronsted 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 NH3TPD of the H+ form and IR of the lattice vibrational modes (M=O and M-O-M 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 M-O 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, (C-3/O-2/He = 2/1/2, flow rate 15 cm(3) min(-1), 12 h on stream, reaction temperature in the 300-400 degrees 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 M-O bond (see supra and as shown by the higher reaction temperature necessary for the same conversion level). Compared to pre-treatment temperature at 300 degrees C, pre-treatment at 400 degrees C was observed to result in a higher extent of constitutional water removal i.e. a loss of Bronsted 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 Bronsted 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. (c) 2006 Elsevier B.V. All rights reserved.