A zeolite in room temperature catalysis
MFI-confined single-atom iron enables efficient ambient peroxidation catalysis
image: (a) UV-Vis absorbance at 420 nm. Steady-state kinetic assay for (b) Fe@MFI and (c) Fe/MFI. Insets show double-reciprocal plots of activities. (d) Intracellular ROS levels in CT26 cells. (e) Cell viability of CT26 cells following incubation with Fe3O4 NPs, Fe/MFI, and Fe@MFI at different concentrations (n = 6). (f) Calcein-AM/PI co-staining images of CT26 cells. (g) Scheme of the experiment schedule. (h) Tumor volume growth curves for different treatment groups (n = 5). Photographs of (i) resected tumors and (j) tumor weights after different treatments (n = 5). (k) Body weight curves of mice after different treatments (n = 5). Data are presented as mean ± SD. The P-values were calculated using one-way ANOVA. *P < 0.05, ***P < 0.001, ****P < 0.0001.
Credit: ©Science Bulletin
Peroxidation reactions are widely applied in environmental remediation, biosensing, and tumor therapy. Traditional metal nanoparticles often suffer from issues such as low atomic utilization and aggregation of active components. Although carbon-supported single-atom catalysts enhance metal utilization, they are susceptible to etching and decomposition in highly oxidative environments, and the adsorption capacity of carbon for polar molecules is limited. The siliceous framework of MFI zeolites exhibits excellent stability and oxidation resistance. Its regular sub-nanometer channels can not only isolate and stabilize metal atoms but also enrich small substrate molecules and optimize reaction selectivity. If single-atom metal could be "locked" into the MFI framework, it would be promising to obtain a peroxidation catalyst with superior performance. However, the challenges lie in: how do metals enter the framework? What is their specific coordination structure? What interactions exist between metals and acid sites of zeolites? And how do these factors influence catalytic performance?
The research team developed a hydrothermal synthesis strategy to prepare Fe@MFI. Through cross-validation by multiple characterization techniques, including electron microscopy, infrared spectroscopy, and nuclear magnetic resonance, it was confirmed that single iron atoms entered the MFI framework via Si-O-Fe bonding, existing in the +3 oxidation state and forming a tetrahedral Fe(OSi)3(O(H)Si) coordination structure. Compared with commercial Fe3O4 nanoparticles and the Fe2O3/MFI control sample, Fe@MFI exhibited significantly faster peroxidation catalytic rates. Steady-state kinetics followed a typical Michaelis–Menten relationship, with the Kₘ value approaching the substrate affinity level of cytochrome c. Mechanistic studies revealed that the outstanding activity did not arise solely from a single iron site. The Fe-O4 center activated H2O2 via a Fenton-like pathway to generate hydroxyl radicals, while the adjacent Si(OH)Fe-type Brønsted acid site adsorbed o-phenylenediamine through acid-base affinity, bringing the substrate close to the radical generation region, thereby enhancing the conversion of o-phenylenediamine. Fe@MFI significantly elevated the intracellular reactive oxygen species (ROS) level in CT26 colon cancer cells, achieving efficient tumor-cell killing. In vivo experiments further verified that Fe@MFI could effectively suppress tumor growth.