π-Quadrupole-driven gate-opening in ZIFs for selective VOC capture

Abstract

The selective capture of volatile organic compounds (VOCs) is a significant challenge in environmental remediation. In this study, we explore how MOF structural flexibility and the electronic properties of VOCs influence their adsorption by comparing two functionalized zeolitic imidazolate frameworks: the flexible ZIF-8_CH3 and the rigid ZIF-8_Br. Using benzene and hexafluorobenzene as probe molecules with contrasting quadrupole moments, we demonstrate that ligand functionalization significantly impacts both structural dynamics and adsorption/separation performance. ZIF-8_CH3 exhibits higher overall uptake capacities, reaching up to 7.3 mmol/g for hexafluorobenzene. In contrast, ZIF-8_Br shows superior separation capabilities, with IAST selectivity values reaching 17.1 for benzene/hexafluorobenzene mixtures at low pressures. Our experimental and computational analyses reveal that aromatics with negative quadrupole moments more readily appear to trigger the gate-opening phenomenon, establishing a potential correlation direct correlation between electron density distribution and molecular sieving efficiency. These findings offer new insights into the rational design of functionalized frameworks for selective VOC capture, highlighting the crucial role of electronic effects in determining host-guest interactions and separation performance.