Geometrical pore engineering via ligand racemization in metal–organic frameworks for enhanced Xe capture and separation

Abstract

Achieving high-performance Xe adsorption and Xe/Kr separation on porous solids is still a daunting challenge due to the similarities in the physicochemical properties, molecular size, and shape between Xe and Kr. Most top-performing materials feature narrow one-dimensional channels for strong Xe-framework interaction, providing a high separation selectivity but also suffering from low adsorption capacity. Herein, we tackled this issue by modifying the 1D channel pore structure into divided pore cavities via the ligand racemization strategy to increase the utilization of pore space for efficient Xe capture. The substitution of enantiopure L-malic acid with racemate DL-malic acid as the carboxylate ligand constructed a racemic framework with reticulated channels formed by small cavities connected by narrow windows (MOF-OH-DL), instead of the homochiral material featuring 1D channel pores (MOF-OH-L). The divided pore space in MOF-OH-DL offered more adsorption regions for Xe capture, enabling a significantly higher Xe packing density (2.7 g cm⁻³) and uptake capacity (2.23 mmol g⁻¹) in MOF-OH-DL compared to those of MOF-OH-L (2.1 g cm⁻³ and 1.6 mmol g⁻¹) and other top-performing materials. In situ single-crystal X-ray diffraction studies on Xe-loaded samples unveiled the dense packing mechanisms of Xe in MOF-OH-DL. Breakthrough experiments further validated the superior separation performance of MOF-OH-DL for the binary and dilute Xe/Kr gas mixtures under dynamic conditions. The efficient Xe capture and separation in MOF-OH-DL demonstrated the effectiveness of the ligand racemization strategy in maximizing the utilization of pore space for efficient gas separation and highlighted the considerable potential of chiral ligands in structural modulation of MOFs.