Cu- and Fe-substituted ZSM-5 zeolite as an effective catalyst for wet peroxide oxidation of Rhodamine 6 G dye

Several Cu-exchanged and Fe-exchanged ZSM-5 catalysts were prepared and studied for the decoloration, degradation and mineralization of Rhodamine 6 G in catalytic wet peroxide oxidation (CWPO). The Cu-ZSM-5 catalysts were shown to be more effective than the parent H-ZSM-5 zeolites (containing Fe-ions as impurity) and Fe-exchanged ZSM-5 to Rhodamine 6 G oxidation by hydrogen peroxide. Effects of copper loadings, Si/Al ratio, structure of copper species, and initial pH were ascertained on catalytic behavior of Cu-ZSM-5 and copper leaching under CWPO conditions. At acid pH, the highest decoloration of Rhodamine 6 G was typical of Cu-ZSM5 with Si/Al equal to 30 and Cu content of ca. 0.5 - 0.65 wt% having isolated Cu2+ ions, whereas the maximal conversions of total organic carbon (TOC) were exhibited by catalysts with 1.5 - 2.8 wt% Cu and CuO-like clusters on the external zeolite surface. At alkali pH, the over-exchanged 2.8% Cu-ZSM-5 with CuO-like clusters was more active than under-exchanged 0.5% Cu-ZSM-5 to decoloration of Rhodimine 6 G, while both catalysts provided identical TOC conversions. Catalyst with 0.5 wt% copper loading showed reproducible activity up to 7th cycle. According to UV-Vis DR and EPR studies, the high efficiency of Cu-ZSM-5 to Rhodamine 6 G oxidation in acid solution is provided by the isolated Cu2+ ions, while the nanostructured square-planar CuO-like clusters (on the zeolite surface) ensure activity at alkali pH. Two mechanisms of organic substrate oxidation, i.e. radical (Fenton-like) mechanism activated by isolated Cu2+ ions and nucleophilic substitution via copperhydroperoxide complex generation, were discussed.