Mechanistic insights into electronic modulation of binuclear iron-based zeolites for selective oxidation of methane to methanol

Abstract

Conceptually, direct methane-to-methanol (DMTM) conversion represents an efficient approach for methane (CH₄) valorization, which is thermodynamically feasible at ambient temperature. However, this process consistently faces a conversion-selectivity trade-off. Particularly, when employing dioxygen (O₂) as the oxidizing agent, an additional compromise arises between O₂ activation and the generation of reactive oxygen species necessary for CH₄ activation. Enzyme-like iron-based zeolites are regarded as promising catalysts for DMTM. We investigated possible iron clusters in ZSM-5 based on ab initio thermodynamics analysis and identified the binuclear [Fe–(μ-O)₂–Fe]²⁺ site anchored by Al pairs as the most stable configuration in the Fe/ZSM-5 catalyst under the preparation conditions reported preparation conditions. Density functional theory calculations revealed a Mars-van-Krevelen-like (MvK-like) mechanism for DMTM over the binuclear iron sites, offering a pathway to circumvent the challenge of simultaneously activating CH₄ and O₂, thereby enhancing catalyst activity. Nevertheless, the activity of this Fe/ZSM-5 catalyst remains constrained by surface oxygen species reactivity. Moreover, the [Fe–(μ-O)₂–Fe]²⁺ site exhibits marginal preference for methanol O–H bond scission over methane C–H bond scission, compromising the intrinsic limitation between methane conversion and methanol overoxidation. The introduction of a second metal component could electronically regulate surface oxygen reactivity, effectively tuning DMTM performance. While the fundamental trade-off between O₂ activation and methane conversion persists, methane C–H bond scission over [Fe–(O₂)(μ-O)₂–M]²⁺ was always found to be the rate-determining step for Fe-based binuclear catalysts. Based on the ligand-to-metal charge transfer (LMCT)-enabled hydrogen atom transfer (HAT) mechanism for the methane C–H bond, we propose the third ionization energy (IE₃) of the secondary metal component as an effective descriptor for predicting methane conversion efficiency over these Fe-based binuclear catalysts. Remarkably, the elevated IE₃ of Zn due to the fully occupied d orbital of Zn²⁺ renders the hetero-binuclear Fe–Zn/ZSM-5 with the [Fe–(μ-O)₂–Zn]²⁺ site to be a promising catalyst for enhancing methane conversion. Furthermore, the high IE₃ of Zn suppresses methanol O–H bond scission, thereby improving methanol selectivity. These mechanistic insights provide a guide for the rational design of DMTM catalysts through the electronic synergy engineering of hetero-binuclear metal centers.