Roles of Acidic Proton for Fe-Containing Zeolite in Direct Oxidation of Methane

Abstract

Fe-containing zeolite catalysts are active in N₂O decomposition and direct oxidation of unreactive methane. Except for the well-known ability that acid sites realize the subsequent reaction of methanol to hydrocarbon, the roles of acidic protons in the direct oxidation of methane have not been studied regarding the formation of active Fe components and the reactivity in the reaction. Herein, on the premise of the comparable total Fe and Al contents, the acidity of one-pot synthesized Fe-AEI and ion-exchanged Fe/AEI zeolites was adjusted by various Na contents, and the catalytic activity in methane oxidation reactions was compared under different conditions. Ultraviolet–visible (UV–vis) spectra at 25–500 °C of the as-synthesized Fe-AEI and the reaction performance at the corresponding conditions were combined to clarify the formation of potential active Fe species. Acidic proton was favorable for the formation of potential active Fe species for the one-pot synthesized Fe-AEI zeolite. However, for the H-type Fe/AEI zeolite, thermal treatment at high temperatures was prone to dealuminate, reduced the number of anchors for Fe³⁺ attachment, and resulted in inactive Fe_xO_y. Furthermore, in contrast to the robust N₂O adsorption capacity of Na⁺, the acidic proton exhibited weak competition with Fe³⁺ for N₂O adsorption and thus contributed to the higher activity in the methane oxidation reaction. Our findings highlighted the importance of the acidic proton for the formation of potential active Fe species, the frail competition of adsorption N₂O with Fe species, and the feasibility of the tandem reaction of methane to methanol and methanol to hydrocarbon.