Improved catalytic performance of Keggin-type polyoxometalates in the oxidation of isobutane to methacrylic acid under hydrocarbon-lean conditions using antimony-doped catalysts

Ammonium salt of 12-molybdophosphoric acid (NH4)(3)PMo12O40, having the Keg gin structure (polyoxometalate, POM) was prepared and tested as heterogeneo us catalyst for the selective oxidation of isobutane to methacrylic acid, u nder both hydrocarbon-rich and hydrocarbon-lean conditions. The same compou nd was doped with Sb3+ ions, in an amount corresponding to 0.23 Sb atom per Keggin unit. Doping with antimony caused a decrease in the oxidation state of molybdenum, due to the electron exchange occurring between Sb3+ and Mo6 + during the calcination treatment: Sb3+ + 2Mo(6+) <----> Sb5+ + 2Mo(5+). T his reduced state for molybdenum in the POM was thus stabilized even under oxidizing conditions. This effect modified the reactivity of the POM-based catalyst in the title reaction with respect to the undoped catalyst, especially under isobutane-l ean conditions (i.e. 1% isobutane, 13% oxygen), thus under conditions which are considerably more oxidizing than those at 26% isobutane (isobutane-ric h conditions), typically used for the selective oxidation of isobutane cata lyzed by POMs. In fact, while in the absence of antimony, the oxidized cata lyst was completely unselective to methacrylic acid under isobutane-lean re action conditions, the Sb-doped catalyst reached a selectivity as high as 4 0-45% to methacrylic acid. On the contrary, under intrinsically reducing co nditions (i.e. under isobutane-rich conditions), a reduced catalyst also de veloped in the reaction medium in the case of the undoped system. In this c ase, the advantage in selectivity gained by using a Sb-doped catalyst was m uch less than that obtained at low concentration of the hydrocarbon in the feed. (C) 2001 Elsevier Science B.V. All rights reserved.