Regulating Mono/Binuclear Fe Species in Framework Al-Rich Zeolites for Efficient Low-Temperature Alkane Oxidation

Abstract

Zeolite-encapsulated extra-framework mono/binuclear Fe³⁺ species present higher catalytic activities compared to clusters and nanoparticles for direct low-temperature alkane oxidation. However, the fine control of mono/binuclear Fe³⁺ in zeolites is challenging and the reaction mechanism of low-temperature alkane oxidation remains not clearly identified. Different from previous impregnation and ion-exchange methods generally generating clusters/nanoparticles, here we developed an efficient amino acid-assisted one-pot hydrothermal synthesis strategy for in situ incorporating mono/binuclear Fe³⁼ species into framework Al-rich ZSM-5 zeolites. The high framework Al content (Si/Al=9) provided sufficient negatively-charged sites to anchor mono/binuclear Fe³⁺ (Fe loading=0.44~0.90 wt%). The as-prepared 0.44Fe@Z-L_0.3H₆-9 catalyst exhibited superior catalytic properties for selective oxidation of both methane and ethane in the H₂O₂ solution at 50 °C, presenting a top-level catalytic performance among various heterogeneous/homogeneous catalysts. Combining advanced characterizations and density functional theory calculations, the complex reaction networks for methane and ethane conversions into C1/C2 oxygenates over mononuclear and binuclear Fe³⁺, for the first time, were mapped out. The mononuclear Fe³⁺ was found more active than binuclear Fe³⁺ for both methane and ethane conversions. This work not only provides a whole picture on low-temperature alkane oxidation mechanisms but also guides the rational design of high-performance catalysts for C−H bond activation and beyond.

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