Reactivity of Keggin-type heteropolycompounds in the oxidation of isobutane to methacrolein and methacrylic acid: Reaction mechanism

The oxidation of isobutane to methacrolein and methacrylic acid over a Keggin-type heteropolycompound of composition K-1(NH4)(2)PMo12O40/Fe1O1.5 was studied in both flow reactor studies and with FT-IR spectroscopy of species which develop at the surface. In both cases either isobutane or possible reaction intermediates were used as probe molecules. It is proposed that the activation of the isobutane at the tertiary C-H bond leads to the formation of an alkoxy species, which is then converted to an allylic alkoxy species, precursor for the formation of methacrolein and methacrylic acid. The mechanism proposed explains the experimental evidence relative to the absence of isobutene among the reaction products, and the parallel pathways to the two products. Competitive pathways are the formation of acetic acid, maleic anhydride and carbon oxides.