Understanding the Effect of Ligands on C₂H₂ Storage and C₂H₂/CH₄, C₂H₂/CO₂ Separation in Metal–Organic\nFrameworks with Open Cu(II) Sites

Abstract

Safe\nand efficient storage and separation of acetylene pose a significant\nchallenge in industry. In this study, we investigated 11 open Cu­(II)\nsite (OCS)-based metal–organic frameworks (MOFs) formed by\nvarious organic ligands for their C₂H₂ adsorption\ncapacities and their C₂H₂/CO₂, C₂H₂/CH₄ separation performance using\nboth grand canonical Monte Carlo (GCMC) simulations and density functional\ntheory (DFT) calculations. Our simulations revealed that both OCSs\nand organic ligands of the MOFs play key roles in promoting C₂H₂ storage capacity and the separation of C₂H₂ over CH₄ and CO₂ under\n2 bar. Judicious selection of organic ligands with suitable dimensions\nand functional sites, such as methyl group, Lewis basic nitrogen site,\nand fluorine group, can facilitate C₂H₂ adsorption\nin addition to OCS and help distinguish C₂H₂ from CH₄ and CO₂. Short ligands presented\nin the MOFs, such as MOF-505 which gives the highest volumetric C₂H₂ storage capacity under 2 bar, not only increase\nthe density of OCSs but also create overlapped interaction regions\nfor guest molecules. GCMC simulation results suggested that NOTT-106\nhad the second highest volumetric C₂H₂ storage\ncapacity because of its methyl functionalized ligands. DFT calculations\nhowever suggested that the Lewis basic nitrogen functionalized ligand\nof ZJU-40 might have a stronger affinity with C₂H₂ than that of NOTT-106, which indicated that C₂H₂ storage capacity in ZJU-40 should be better than that in NOTT-106.\nIn contrast, MOF-505, ZJU-40, and NOTT-108 showed excellent C₂H₂/CH₄ separation performance as well\nas modest C₂H₂/CO₂ separation capability.