Accelerating Fe(Ⅲ)/Fe(Ⅱ) cycle via Fe(Ⅱ) substitution for enhancing Fenton-like performance of Fe-MOFs

Metal-organic frameworks (MOFs), especially iron-based MOFs (Fe-MOFs), are deemed as promising Fenton-like catalysts due to their well-developed pores and accessible active sites. However, the Fenton-like performance of Fe-MOFs is limited by the relatively low redox rate of Fe(Ⅲ)/Fe(Ⅱ) couples and density of coordinatively unsaturated iron centers (CUICs), which depend on the intrinsic structure of iron-oxo nodes in Fe-MOFs. Herein, we manipulated the structure of iron-oxo nodes in MIL-53(Fe) via a Fe(Ⅱ) substitution method, affording a mixed-valence (i.e., Fe(Ⅱ)/Fe(Ⅲ)) material (denoted as FeⅡ-MIL-53(Fe)) with highly improved Fenton-like performance. The substituted Fe(Ⅱ) centers could serve as stronger active sites, where Fe(Ⅱ)→Fe(Ⅲ) half-reaction occurs, over original Fe(Ⅲ) centers to rapidly activate H2O2 for efficient destruction of 4-nitrophenol (4-NP). Meanwhile, Fe(Ⅱ) substitution induced the formation of a larger amount of Fe(Ⅲ) CUICs, which was proven by the increased Lewis acidity of MIL-53(Fe). The increased density of Fe(Ⅲ) CUICs promoted the interaction between Fe(Ⅲ) centers and H2O2 and thus rendered another half-reaction, Fe(Ⅲ)→Fe(Ⅱ), with a greater rate. Accordingly, the accelerated cycle of Fe(Ⅲ)/Fe(Ⅱ) couples favored OH generation for improving 4-NP degradation. As a result, the 4-NP degradation and rate on FeⅡ-MIL-53(Fe) were 1.39 and 9.48 times higher than that on pristine MIL-53(Fe). Moreover, FeⅡ-MIL-53(Fe) showed a good stability and reusability over multiple cycles. Our work provides insights into the rational design of Fe-MOFs as promising Fenton-like catalysts for advanced water/wastewater treatment.