The Mechanism of Formation of Active Fe-TAMLs Using HClO Enlightens Design for Maximizing Catalytic Activity at Environmentally Optimal, Circumneutral pH

Abstract

Fe-TAML/peroxide catalysis provides simple, powerful, ultradilute approaches for removing micropollutants from water. The typically rate-determining interactions of H2O2 with Fe-TAMLs (rate constant k I) are sharply pH-sensitive with rate maxima in the pH 9–10 window. Fe-TAML design or process design that shifts the maximum rates to the pH 6–8 window of most wastewaters would make micropollutant eliminations even more powerful. Here, we show how the different pH dependencies of the interactions of Fe-TAMLs with peroxide or hypochlorite to form active Fe-TAMLs (k I step) illuminate why moving from H2O2 (pK a, ca. 11.6) to hypochlorite (pK a, 7.5) shifts the pH of the fastest catalysis to as low as 8.2. At pH 7, hypochlorite catalysis is 100–1000 times faster than H2O2 catalysis. The pH of maximum catalytic activity is also moderated by the pK a’s of the Fe-TAML axial water ligands, 8.8, 9.3, and 10.3, respectively, for [Fe{4-NO2C6H3-1,2-(NCOCMe2NSO2)2CHMe}(H2O) n ]− (2) [n = 1–2], [Fe{4-NO2C6H3-1,2-(NCOCMe2NCO)2CF2}(H2O) n ]− (1b), and [Fe{C6H4-1,2-(NCOCMe2NCO)2CMe2}(H2O) n ]− (1a). The new bis(sulfonamido)-bis(carbonamido)-ligated 2 exhibits the lowest pK a and delivers the largest hypochlorite over peroxide catalytic rate advantage. The fast Fe-TAML/hypochlorite catalysis is accompanied by slow noncatalytic oxidations of Orange II.

Two Major Pathways in TAML-Catalyzed Degradation of Carbendazim by H2O2: Elimination of a Nitrogen-Rich Benzimidazole Fungicide and Its Parallel Nitration

“…The catalysts have demonstrated utility for urban wastewater treatment, as well as for treatment of specific pollutants, including dyes, organophosphorus pesticides, imidacloprid, naphthenic acids, and many others. Recently, we focused our attention on bis-amide bis-sulfonamide activators similar to 1 due to their several advantageous properties, including high catalytic activity with a pH optimum closer to the neutral region than earlier variants. ,, This work highlights an additional feature of the bis-sulfonamide activator 1 , which has an ability to oxidatively degrade organic chemicals rapidly, when earlier TAMLs, of which 2b being the overall most useful example, display little catalytic activity. We demonstrate here the following.…”