Mobile Constituent-Boosted Dynamic Separation of C₂H₂/C₂H₄/CO₂ Ternary Mixtures in Metal–Organic Frameworks

The separation of acetylene (C₂H₂), ethylene (C₂H₄), and carbon dioxide (CO₂) is critical in the chemical industry, driven by the increasing demand for high-purity C₂H₂ and C₂H₄. While metal-organic frameworks (MOFs) offer an energy-efficient approach for adsorptive gas separation, achieving sub-angstrom precision in pore size adjustment remains challenging. In this work, we leverage two synergistic mechanisms in a double-interpenetrated framework: (1) global structural flexibility, arising from dynamic displacement of subnetworks to tailor pore dimensions, and (2) local flexibility, enabled by counterion and ligand rotation, to modulate the aperture binding affinity for precise molecular discrimination. A series of isostructural MOFs, NUS-33-CF₃SO₃ and NUS-34-BF₄, were designed to enable one-step purification of C₂H₄ and concurrent recovery of C₂H₂ from ternary gas mixtures. Within pores of optimal dimensions, the synergistic interplay between counterion-mediated host-guest interactions and local framework adaptability enables precise and simultaneous regulation of static and kinetic gas adsorption properties. Notably, NUS-34-BF₄ achieves a dynamic C₂H₄ productivity of 2.62 mmol/g and a C₂H₂ uptake of 1.26 mmol/g. This study highlights the pivotal yet underexplored role of counterions as dynamic gatekeepers, offering a tunable strategy to engineer pore environments in flexible MOFs for advanced gas separations.