On the stability of Al13 Keggin cation in aqueous hydrogen peroxide solutions

Abstract

We report the first attempt to study the behavior of the [AlO4Al12(OH)25(H2O)11]6+ (Al13) Keggin cation (KC) in water–peroxide solutions. Addition of hydrogen peroxide into an aqueous solution containing the Al13 KC reduces pH due to the acidity of hydrogen peroxide. According to the 27Al NMR studies of water–peroxide solutions prepared just before the NMR experiment, with their pH adjusted to the initial value of 5.5 with aqueous NaOH, the Al13 KC concentration decreases immediately once hydrogen peroxide is added to the initial system. Addition of 18.2 wt % hydrogen peroxide to the initial 0.88 mM Al13 solution gives rise to a fourfold decline in Al13 polyoxo cation concentration to 0.22 mM. Then, the KC concentration in the test system remains unchanged for 1 week. Large hydrogen peroxide amounts (27.9 wt % or higher) added to the initial system almost completely degrade the KC. Sodium sulfate added to the initial water–peroxide solution of Al13 chloride where the hydrogen peroxide concentration is 5.5 wt % precipitates the earlier described Al13 sulfate [AlO4Al12(OH)25(H2O)11](SO4)3 · 16H2O, where the aluminum polyoxo cation does not contain coordinated hydrogen peroxide molecules, peroxo or hydroperoxo groups as shown by X-ray diffraction.