Solvent-Free Synthesis Enables Encapsulation of Subnanometric FeO_<i>x</i> Clusters in Pure Siliceous Zeolites for Efficient Catalytic Oxidation Reactions

Metal/metal oxide clusters possess a higher count of unsaturated coordination sites than nanoparticles, providing multiatomic sites that single atoms do not. Encapsulating metal/metal oxide clusters within zeolites is a promising approach for synthesizing and stabilizing these clusters. The unique feature endows the metal clusters with an exceptional catalytic performance in a broad range of catalytic reactions. However, the encapsulation of stable FeO_<i>x</i> clusters in zeolite is still challenging, which limits the application of zeolite-encapsulated FeO_<i>x</i> clusters in catalysis. Herein, we design a modified solvent-free method to encapsulate FeO_<i>x</i> clusters in pure siliceous MFI zeolites (Fe@MFI). It is revealed that the 0.3–0.4 nm subnanometric FeO_<i>x</i> clusters are stably encapsulated in the 5/6-membered rings intersectional voids of the pure siliceous MFI zeolites. The encapsulated Fe@MFI catalyst with a Fe loading of 1.4 wt % demonstrates remarkable catalytic activity and recycle stability in the direct oxidation of methane, while also promoting the direct oxidation of cyclohexane, surpassing the performance of conventional zeolite-supported Fe catalysts.