Insights into Fe Species Structure-Performance Relationship for Direct Methane Conversion toward Oxygenates over Fe-MOR Catalysts

Abstract

Direct oxidation of methane to value-added products under mild conditions remains a grand challenge due to the problem of selective CH₄ activation. Here we report a Fe-MOR-F catalyst prepared via freeze-drying method, indicating excellent catalytic performance for the direct partial oxidation of methane using H₂O₂ under mild conditions. Various characterizations including XRD, NH₃-TPD, H₂-TPR, UV-Vis, XPS, and EPR studies revealed that Fe-MOR-F catalyst possesses more highly dispersed monomeric and dimeric iron species, which is strongly correlated to its high catalytic activity for direct methane oxidation. The DFT calculations indicate the highly dispersed monomeric and dimeric iron species can effectively homogenize H₂O₂ into active hydroxyl groups, while iron clusters can decompose H₂O₂ into inactive O₂. The EPR experiments show more ⋅OH was generated on Fe-MOR-F catalyst, which is in line with DFT calculation results. These structural characteristics endow Fe-MOR-F catalyst with superior catalytic performance for direct methane oxidation in terms of oxygenates yield (9.8 mol kg_cat⁻¹ h⁻¹) and selectivity (91.8 %) at 80 °C. This work not only highlights the excellent catalytic performance of methane activation on the highly dispersed monomeric and dimeric iron species, but also promotes the potential application by freeze-drying method for the preparation of the highly dispersed active species.