Speciation evolution of iron species within ZSM-5 for selective methane oxidation: from redispersion to activation†

The heterogeneous nature of Fe species within Fe–zeolite catalysts presents a major hurdle for selective methane oxidation, primarily due to the presence of nonactive or detrimental species. In this study, we demonstrate that the ineffective Fe_xO_y particles within ZSM-5 can be transformed into active extra-framework isolated Fe³⁺ sites through high-temperature annealing in N₂ followed by activation in H₂O₂. This respeciation process maximizes the concentration of active Fe sites, thereby enabling exceptional catalytic performance for methane selective oxidation at 50 °C, achieving a liquid oxygenate yield of 419.1 mmol (g_cat h)⁻¹ with a selectivity of 90.3%. Detailed spectroscopic analysis reveals that during respeciation, the extra-framework isolated Fe³⁺ initially present in Fe/ZSM-5 seem to remain unchanged in both valence state and structural form; whereas Fe_xO_y particles in the initial Fe/ZSM-5 undergo a sequence of self-reduction, redispersion, and reoxidation, ultimately forming active Fe³⁺ species. Control experiments confirm that both high-temperature conditions and anaerobic environments are indispensable for enabling iron oxides to overcome the thermodynamic and kinetic barriers to redispersion.