Rechargeable and Reusable Catalysts for Advanced Oxidation of Organic Dyes: Fe and Mn Cross-Bridged Tetraazamacrocycle Complexes Electrostatically Bound to an Ion Exchange Resin

Mn and Fe complexes of ethylene cross-bridged tetraazamacrocycles, known to retain their reactive metal ions even under harsh aqueous conditions due to the rigidity and topological constraint of the ligands, with methyl, allyl and benzyl pendant arms were electrostatically fixed to a commercial cation exchange resin, and their effectiveness and recyclability in the bleaching of organic dyes, namely, methyl orange, methylene blue and rhodamine B, were determined. These molecular Mn and Fe tetraazamacrocycle catalysts have been previously reported as homogeneous solution phase catalysts for bleaching organic dyes, but could not be feasibly recovered for multiple cycles of dye bleaching. Herein, we report the potential of these resin-fixed solid-state metal complex catalysts in the degradation of organic dyes. The Mn catalysts were faster in the first cycle of dye bleaching, but their bleaching rate slowed considerably (to about 10% on average) in additional cycles. In contrast, the Fe catalysts were slower in the first cycle of dye bleaching, but were able to bleach dyes for up to five cycles while retaining more of their original reactivity (30–50% on average), suggesting their potential recyclability. Thus, these resin-fixed tetraazamacrocyclic metal complex catalysts, when coupled with the demonstrated rechargeability of the resin with fresh catalyst, may one day be effectively used for water purification applications. SEM and optical microscopy images demonstrated the robustness of the resin bead across multiple cycles, while EDS experiments confirmed the continued presence of Mn and Fe on the bead surface after multiple cycles.