Hierarchically Porous Bimetallic Dual-Metal–Organic Frameworks for Synergistic and Selective Adsorption of Thiophenic Sulfur: A Density Functional Theory-Validated Metal–Sulfur Coordination Mechanism

Owing to their uniform microporous structure, monometallic MOFs often suffer from an inherent trade-off between mass transfer resistance and adsorption capacity, making it challenging to integrate both high adsorption kinetics and large capacity within a single adsorbent. To address this issue, this study proposes a MOF@MOF design strategy based on the coupling of bimetallic synergy and hierarchical porosity effects, through which a (MIL-101(Cr))@(Zn-BTC) composite with a well-defined "micro-meso-macroporous" hierarchical architecture is successfully constructed. Batch adsorption experiments demonstrate that the composite achieves a high saturated adsorption capacity of 161 mg/g for thiophene sulfur (Thiophene-S) under ambient temperature and pressure. The adsorption kinetics follow the pseudo-second-order model, and the isotherm data are well fitted by the Freundlich equation, suggesting a multilayer adsorption mechanism. Notably, owing to the macroporous cavity structure of Zn-BTC, the composite exhibits significantly enhanced diffusion kinetics─within the same adsorption period, its adsorption capacity reaches twice that of pure MIL-101(Cr). Moreover, the material effectively retains its adsorption performance even after water treatment, alleviating the capacity attenuation commonly observed in conventional MOFs under aqueous conditions. Mechanistic studies reveal that the high desulfurization performance of (MIL-101(Cr))@(Zn-BTC) stems from the synergistic contributions of high specific surface area, metal-sulfur coordination, π-π interactions, and acid-base cooperative effects. Further insights from DFT calculations and XPS characterization indicate that the bimetallic synergy significantly narrows the HOMO-LUMO energy gap, thereby reducing the adsorption activation energy, while XPS confirms electron transfer from sulfur atoms in Thiophene-S to metal centers. This study provides a theoretical foundation and a material design strategy for developing bimetallic hierarchical porous adsorbents for desulfurization.