A hierarchically porous MOF engineered via chelation-etching for high-capacity selective dye adsorption

Porous metal-organic frameworks (MOFs) are attractive candidates for selective adsorption of water pollutants due to abundant functional groups and tunable electrical properties, yet their microporous nature limits the maximum adsorption capacity. While larger-pore MOFs face synthetic, cost and stability constraints, hierarchically porous architectures offer promising solutions. This work reports a chelation-etching strategy to convert microporous MIL-53(Fe) into a hierarchically porous composite (MIL-53(Fe)@EDTA·2H) using 100 mM HCl/EDTA. The etched framework owns hierarchical porosity (5–50 Å), where mesopores coexist with preserved micropores. Compared to pristine MIL-53(Fe), the composite exhibits enhanced adsorption amount for cationic dyes in mixed systems, attributed to the electrostatic interactions and improved pore volumes. MIL-53(Fe)@EDTA·2H achieves a methylene blue (MB) adsorption capacity of 196.72 mg g⁻¹, which is approximately twice that of the parent MOF. To mitigate adsorbent leaching during recovery, polyvinylidene fluoride (PVDF) mixed matrix membranes (MIL-53(Fe)@EDTA·2H·M) were fabricated, effectively immobilizing particles while maintaining performance. This study establishes a controlled etching approach for engineering hierarchical porosity in MOFs and demonstrates their utility as ideal adsorbents of both high adsorption capacity and selectivity.