Understanding the Role of Manganese Dioxide in the Oxidation of Phenolic Compounds by Aqueous Permanganate

Recent studies have shown that manganese dioxide (MnO₂) can significantly accelerate the oxidation kinetics of phenolic compounds such as triclosan and chlorophenols by potassium permanganate (Mn­(VII)) in slightly acidic solutions. However, the role of MnO₂ (i.e., as an oxidant vs catalyst) is still unclear. In this work, it was demonstrated that Mn­(VII) oxidized triclosan (i.e., trichloro-2-phenoxyphenol) and its analogue 2-phenoxyphenol, mainly generating ether bond cleavage products (i.e., 2,4-dichlorophenol and phenol, respectively), while MnO₂ reacted with them producing appreciable dimers as well as hydroxylated and quinone-like products. Using these two phenoxyphenols as mechanistic probes, it was interestingly found that MnO₂ formed in situ or prepared ex situ greatly accelerated the kinetics but negligibly affected the pathways of their oxidation by Mn­(VII) at acidic pH 5. The yields (<i>R</i>) of indicative products 2,4-dichlorophenol and phenol from their respective probes (i.e., molar ratios of product formed to probe lost) under various experimental conditions were quantified. Comparable <i>R</i> values were obtained during the treatment by Mn­(VII) in the absence vs presence of MnO₂. Meanwhile, it was confirmed that MnO₂ could accelerate the kinetics of Mn­(VII) oxidation of refractory nitrophenols (i.e., 2-nitrophenol and 4-nitrophenol), which otherwise showed negligible reactivity toward Mn­(VII) and MnO₂ individually, and the effect of MnO₂ was strongly dependent upon its concentration as well as solution pH. These results clearly rule out the role of MnO₂ as a mild co-oxidant and suggest a potential catalytic effect on Mn­(VII) oxidation of phenolic compounds regardless of their susceptibility to oxidation by MnO<sub>2