Cu-Based Metal–Organic Frameworks Characterized by V-Shaped Ligands and Application to Efficient Separation of C₂H₂/CO₂ and C₂H₂/CH₄

Two Cu-based metal–organic frameworks (Cu-MOFs) with new topologies were successfully synthesized under solvothermal conditions using V-shaped nitrogen-containing tetracarboxylic acid ligands: [Cu9O(PADDA)4(H2O)6]·xGuest (compound 1), [H3O][Cu6(PIDDA)3(H2O)4(HCOO)]·xGuest (compound 2), (H4PADDA = 5,5′-(pyrazine-2,6-diyl)diisophthalic acid; H4PIDDA = 5,5′-(pyrimidine-4,6-diyl)diisophthalic acid). Structural analysis reveals that both compounds are constructed from [Cu2(COO)4] characterized by classic paddlewheel building units. Notably, compound 1 features a rare structural unit of tetranuclear copper characterized by two paddlewheel-structured dinuclear copper units connected by a bridging oxygen atom. In contrast, compound 2 features a structural unit of tetranuclear copper formed by formic acid connecting two adjacent paddlewheel-structured dinuclear copper units. Both frameworks exhibit three distinct cage structures of different sizes. These compounds demonstrate high specific surface areas and excellent adsorption capabilities for small gas molecules, such as CO2, CH4, and C2H2, enabling effective purification of acetylene. At 100 kPa and 298 K, compound 1 show an ideal adsorbed solution theory (IAST) selectivity of 4.0 and 20.0 for equimolar C2H2/CO2 and C2H2/CH4 mixtures, respectively. The IAST selectivity corresponding to compound 2 increased to 7.1 and 57.6. Meanwhile, the actual separation performance was verified by breakthrough experiments, demonstrating the potential to separate C2H2/CO2 and C2H2/CH4 mixtures at room temperature.