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

Safe and efficient storage and separation of acetylene pose a significant challenge in industry. In this study, we investigated 11 open Cu(II) site (OCS)-based metal-organic frameworks (MOFs) formed by various organic ligands for their C₂H₂ adsorption capacities and their C₂H₂/CO₂, C₂H₂/CH₄ separation performance using both grand canonical Monte Carlo (GCMC) simulations and density functional theory (DFT) calculations. Our simulations revealed that both OCSs and organic ligands of the MOFs play key roles in promoting C₂H₂ storage capacity and the separation of C₂H₂ over CH₄ and CO₂ under 2 bar. Judicious selection of organic ligands with suitable dimensions and functional sites, such as methyl group, Lewis basic nitrogen site, and fluorine group, can facilitate C₂H₂ adsorption in addition to OCS and help distinguish C₂H₂ from CH₄ and CO₂. Short ligands presented in the MOFs, such as MOF-505 which gives the highest volumetric C₂H₂ storage capacity under 2 bar, not only increase the density of OCSs but also create overlapped interaction regions for guest molecules. GCMC simulation results suggested that NOTT-106 had the second highest volumetric C₂H₂ storage capacity because of its methyl functionalized ligands. DFT calculations however suggested that the Lewis basic nitrogen functionalized ligand of 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. In contrast, MOF-505, ZJU-40, and NOTT-108 showed excellent C₂H₂/CH₄ separation performance as well as modest C₂H₂/CO₂ separation capability.