Modification of redox and catalytic properties of keggin-type, Sb-doped P/Mo polyoxometalates in the selective oxidation of isobutane to methacrylic acid: control of preparation conditions

Keggin-type phosphomolybdates (POM's) were tested as catalysts for the selective oxidation of isobutane to methacrylic acid. Doping the POM with antimony improves the catalytic performance especially at isobutane-lean conditions, since a redox reaction between Sb 3+ and Mo 6+ leads to the development of a reduced POM which is stable even in an oxidizing environment, and which is more selective to methacrylic acid. Another important factor to control the reactivity is the pH of precipitation of the POM. When the preparation of the catalyst is carried out via the formation of a lacunary precursor, the time necessary to reach steady catalytic performance (“equilibration time”) is considerably less than that for POM's prepared conventionally at strongly acid pH. An hypothesis about the nature of the active sites is formulated.