Improvement of catalytic performance in isobutane oxidation to methacrylicacid 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 <degrees>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, (N-H-4)(3)PMo12O40. Even so, the catalytic performanc e was different for the catalysts prepared. In particular, the differences mainly concerned the equilibration stage necessary to reach a steady cataly tic performance. Based on comparison of the fresh and unloaded catalysts, a hypothesis is formulated concerning the nature of the active sites for iso butane selective oxidation. It is proposed that the active sites are genera ted as a consequence of the incipient structural decomposition of the POM a nd migration of Mo from the Keggin anion into the cationic position of the framework. This phenomenon occurs either during the calcination treatment o f the lacunary precursor, or during the equilibration stage under reaction conditions for the catalyst prepared starting from the Keggin precursor.