Abstract
Chromium 12-molybdophosphate Cr[PMo12O40] was obtained with Cr3+ in cationic positions. On heating it transforms reversibly around 650K with the destruction of Keggin anions into an amorphous phase which is stable to about 750K, above which crystalline MoO3 appears. When after cooling the amorphous phase is exposed to water vapour, reconstructure of the initial phase takes place. XPS shows highly ionic bonding which results in hydroxylation of the surface and generation of strong acidic sites. Presence of chromium in cationic positions increases redox properties of Keggin anions rendering Cr[PMo] a highly active and selective catalyst for oxidation of methanol to formaldehyde, propane to propene and pentane to maleic anhydride. It is suggested that high activity in oxidative dehydrogenation of propane as opposed to high activity in insertion of oxygen in case of pentane may be due to the presence of amorphous phase.