Highly efficient separation of CO2/N2 and CO2/CH4 via metal-ion regulation in ultra-microporous metal-organic frameworks

Abstract

Efficient separation of CO2 from CH4 and N2 is crucial for industrial processes. This study presents a novel approach to enhance CO2/N2 and CO2/CH4 separation using metal-ion regulation in ultra-microporous metal–organic frameworks (MOFs). Specifically, we synthesized ZSTU-8-M (M=Cd, Zn) by coordinating norfloxacin ligands with different metal ions, achieving precise modulation of the pore structure. Ideal adsorption solution theory (IAST) calculations demonstrated that ZSTU-8-Cd exhibited superior selectivity for CO2/N2 (952) and CO2/CH4 (369) mixtures compared to ZSTU-8-Zn. Grand canonical Monte Carlo (GCMC) simulations and in situ single-crystal X-ray diffraction (SCXRD) confirmed the presence of specific CO2 binding sites within the pore channels. The CO2 molecule shows a unique strong adsorption mechanism of hydrogen bonding sites in this structure, and thus achieves high adsorption capacity at low pressure. Dynamic breakthrough experiments further validated the practical separation efficiency of ZSTU-8-Cd, with high retention times and saturated adsorption capacities for CO2. These findings highlight the ability of metal-ion modulation to achieve simultaneous improvement in adsorption capacity and selectivity, demonstrating the potential of MOFs for advanced gas separation applications.