New aspect of the mechanism of photocatalytic oxidation of organic compounds by polyoxometalates in aqueous solutions. The selective photooxidation of propan-2-ol to propanone: The role of OH radicals
The excited state of polyoxometalates, (POM) arising from absorption of light at the O→M charge transfer (CT) band (near-VIS and UV light), is a powerful oxidizing reagent. The oxidizing ability is manifested, mainly, through formation of OH radicals arising from the reaction of the excited POM with adsorbed water. The currently accepted mechanism of H-abstraction as the initial reaction of excited POM with organic substrates (mainly alcohols) is modified by addition of one more step that involves the formation of OH radicals which, as is well known, react with organic substrates, mainly alcohols, by H-abstraction. In view of the formation of OH radicals, and the high oxidizing ability of the excited POM, the photochemical selective oxidation of organic substrates, reported so far, in aqueous solutions, should be considered with reservation. Propan-2-ol is, indeed, selectively oxidized to propanone in the presence of PW12O403-. The selectivity is, however, a kinetic phenomenon rather than a thermodynamic one. Both photoreactions, as is the case with numerous other organic compounds, give, as final products, CO2 and H2O.