Fe2+-Triggered Unexpected Room Temperature H2S Catalytic Oxidation Activity in MIL-100(Fe)

The abundant Fe³⁺ Lewis acidic sites and well-developed pore structure endow the Fe-MOFs with great potential for H₂S catalytic oxidation at room temperature, but their performance is inferior owing to the insufficient redox ability of the Fe³⁺ species inherent in frameworks. Herein, an effective protocol was proposed for the first time to remarkably improve the catalytic oxidation performance of MIL-100(Fe) under ambient conditions through the introduction of Fe²⁺ coordinatively unsaturated site (CUS) species into the framework. As expected, the sample after modification had a superior H₂S breakthrough capacity of 267.3 mg S/g, an 18.4-fold increase compared with that without Fe²⁺ CUS (14.5 mg S/g). The introduced Fe²⁺ species triggered the Fe³⁺/Fe²⁺ redox cycle for H₂S catalytic oxidation by enhancing the oxidative ability of the Fe³⁺ CUS intrinsic to the framework. Moreover, the Fe²⁺ CUS was also found act as an active site for O₂ activation into O₂^∙–, which not only provides an additional pathway for H₂S catalytic oxidation but also accelerates the redox cycling of Fe³⁺/Fe²⁺ during the catalytic oxidation process. On the basis of the experimental findings, a plausible catalytic mechanism involving mixed-valence iron sites in MIL-100(Fe) was proposed.