Cheng Lu, Cao Xiao-Ming
State Key Laboratory of Green Chemical Engineering and Industrial Catalysis, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China.
State Key Laboratory of Synergistic Chem-Bio Synthesis, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China.
Phys Chem Chem Phys. 2025 Jul 30;27(30):16113-16122. doi: 10.1039/d5cp01472g.
Conceptually, direct methane-to-methanol (DMTM) conversion represents an efficient approach for methane (CH) valorization, which is thermodynamically feasible at ambient temperature. However, this process consistently faces a conversion-selectivity trade-off. Particularly, when employing dioxygen (O) as the oxidizing agent, an additional compromise arises between O activation and the generation of reactive oxygen species necessary for CH activation. Enzyme-like iron-based zeolites are regarded as promising catalysts for DMTM. We investigated possible iron clusters in ZSM-5 based on thermodynamics analysis and identified the binuclear [Fe-(μ-O)-Fe] 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 CH and O, 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)-Fe] 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 O activation and methane conversion persists, methane C-H bond scission over [Fe-(O)(μ-O)-M] 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 (IE) of the secondary metal component as an effective descriptor for predicting methane conversion efficiency over these Fe-based binuclear catalysts. Remarkably, the elevated IE of Zn due to the fully occupied d orbital of Zn renders the hetero-binuclear Fe-Zn/ZSM-5 with the [Fe-(μ-O)-Zn] site to be a promising catalyst for enhancing methane conversion. Furthermore, the high IE 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.
从概念上讲，直接甲烷制甲醇（DMTM）转化是甲烷（CH）增值的一种有效方法，在环境温度下在热力学上是可行的。然而，这个过程一直面临着转化率与选择性之间的权衡。特别是，当使用二氧（O）作为氧化剂时，在O活化与CH活化所需的活性氧生成之间会出现额外的权衡。类酶铁基沸石被认为是DMTM有前景的催化剂。我们基于热力学分析研究了ZSM-5中可能的铁簇，并确定在报道的制备条件下，由Al对锚定的双核[Fe-(μ-O)-Fe]位点是Fe/ZSM-5催化剂中最稳定的构型。密度泛函理论计算揭示了双核铁位点上DMTM的类似Mars-van-Krevelen（MvK-like）机制，提供了一条规避同时活化CH和O挑战的途径，从而提高催化剂活性。然而，这种Fe/ZSM-5催化剂的活性仍然受到表面氧物种反应性的限制。此外，[Fe-(μ-O)-Fe]位点对甲醇O-H键断裂的偏好略高于甲烷C-H键断裂，这损害了甲烷转化与甲醇过度氧化之间的固有局限性。引入第二种金属组分可以通过电子方式调节表面氧反应性，有效地调整DMTM性能。虽然O活化与甲烷转化之间的基本权衡仍然存在，但对于铁基双核催化剂，总是发现甲烷在[Fe-(O)(μ-O)-M]上的C-H键断裂是速率决定步骤。基于甲烷C-H键的配体到金属电荷转移（LMCT）驱动的氢原子转移（HAT）机制，我们提出第二种金属组分的第三电离能（IE）作为预测这些铁基双核催化剂上甲烷转化效率的有效描述符。值得注意的是，由于Zn的d轨道完全占据，Zn的IE升高使得具有[Fe-(μ-O)-Zn]位点的异双核Fe-Zn/ZSM-5成为提高甲烷转化的有前景的催化剂。此外，Zn的高IE抑制了甲醇O-H键断裂，从而提高了甲醇选择性。这些机理见解为通过异双核金属中心的电子协同工程合理设计DMTM催化剂提供了指导。
