Oxidation of azo dyes in oil-in-water microemulsions catalyzed by metalloporphyrins in presence of lipophilic acids

Hydrogen peroxide oxidation of two aqueous azo dyes, methyl orange and amaranth, catalyzed by manganese porphyrins, have been performed in an oil-in-water microemulsion based on a nonionic surfactant, C12E8. A lipophilic acid was used as cocatalyst in a variable amount. The oxidation was followed by UV spectroscopy. It was shown that the reactions performed in the microemulsion were rapid for both dyes and the reaction rate increased strongly with increasing amount of lipophilic acid added to the reaction mixture. As reference, the same reactions were performed in a two-phase system without surfactant. The reaction profiles in the two-phase system were similar to those in the microemulsion, but the rate was considerably lower. The effect of addition of small amounts of ionic surfactant was also investigated. Addition of anionic surfactant gave a pronounced decrease in reaction rate, whereas a cationic surfactant gave a small reduction in rate. We propose that the rate-limiting step is formation of a metallo-acylperoxy complex at the boundary between oil and water domains. The metallo-acylperoxy complex oxidizes the azo dye in a subsequent step.