Development of heterogeneous catalysts for automatic dishwashing

Bleaching in automatic dishwashing products currently uses sodium percarbonate and tetraacetylethylenediamine (TAED) to form peracetic acid, which is the primary bleaching agent. Premium products utilise a homogeneous catalyst, MnTACN, to enhance the bleaching performance through oxygen transfer to the stain. The formation of peracetic acid was investigated over a range of reagent concentrations and conditions to ascertain if a catalyst was required to increase the formation of peracetic acid or aid oxidation of the stain. Peracetic acid was facile under a range of reagent concentrations; however, at low temperatures and neutral or acidic pH, peracetic acid formation was reduced or not observed. Under these conditions a catalyst could be used to increase the formation of peracetic acid and improve bleaching; however, these conditions were not relevant to the automatic dishwashing application. The amount of peracetic acid formed was linked to the performance in both tea stain bleaching tests and morin oxidation, increasing peracetic acid amount in solution increases tea stain bleaching and morin oxidation. MnTACN was tested for tea stain oxidation and was found to be highly active. For morin oxidation it was demonstrated that MnTACN can bleach morin with peracetic acid, hydrogen peroxide and O2 as the oxidant. Demonstrating the highly active nature of MnTACN for automatic dishwashing. Manganese, copper, iron and zinc supported on ZrO2 were tested for tea stain bleaching and copper and zinc were active for heterogeneous stain removal. Copper catalysts were investigated and an acid treatment with HNO3 removed large copper iii oxide particles, improving the tea stain bleaching activity of the copper catalysts. Characterisation using X-ray diffraction (XRD) and X-ray photon spectroscopy (XPS) showed that, after the acid treatment, ZrO2 under went a phase change to intimately mixed CuZrO3 and the presence of copper hydroxide species on the surface. Copper catalysts were tested for morin oxidation and the rate of morin oxidation increased over time, indicating an induction phase for the catalyst, which was further investigated with a radical scavenger, mannitol. Addition of mannitol to the morin oxidation reaction in the presence of copper catalysts reduced the conversion of morin over time, showing that copper catalysts generate radicals. Overall, a heterogeneous catalyst has been found to be active for heterogeneous stain bleaching.