Improvement of Catalytic Performance in Isobutane Oxidation to Methacrylic Acid of Keggin-Type Phosphomolybdates by Preparation via Lacunary Precursors: Nature of the Active Sites

Heterogeneous catalysts for the oxidation of isobutane to methacrylic acid were prepared by precipitation of ammonium salts of P/Mo polyoxometalates (POMs) at pH ranging from strongly acidic (pH<1) to mildly acidic (pH 4.0), followed by thermal treatment in air at 350°C. Depending on the pH of precipitation, different types of POMs were obtained (Keggin-type and lacunary-type), the calcination of which, however, in all cases gave rise to a Keggin-type POM, (NH4)3PMo12O40. Even so, the catalytic performance was different for the catalysts prepared. In particular, the differences mainly concerned the equilibration stage necessary to reach a steady catalytic performance. Based on comparison of the fresh and unloaded catalysts, a hypothesis is formulated concerning the nature of the active sites for isobutane selective oxidation. It is proposed that the active sites are generated as a consequence of the incipient structural decomposition of the POM and migration of Mo from the Keggin anion into the cationic position of the framework. This phenomenon occurs either during the calcination treatment of the lacunary precursor, or during the equilibration stage under reaction conditions for the catalyst prepared starting from the Keggin precursor.