Pore-Space-Partitioned Metal-Organic Frameworks with Acyclic Diene Linkers for C₂H₂/CO₂ Separation

Isoreticular tuning of pore-space-partitioned metal-organic frameworks (MOFs) provides a powerful strategy to decouple the adsorption of acetylene (C₂H₂) and carbon dioxide (CO₂). Conventional isoreticular methods typically rely on substitution or bioisosteric replacement, which incrementally increases framework bulkiness. Herein, we report an alternative isoreticular tuning pathway that instead reduces the framework bulkiness through the incorporation of acyclic diene linkers. Using an in situ ligand transformation from trans-butene-1,4-dicarboxylic acid (tbdc), we obtained the metastable s-cis-trans, trans-muconate (scMA) linker and constructed a new series of pacs frameworks, scMA-pacs. In contrast to their conformational isomer stMA-pacs and the length-matched bdc-pacs, the scMA-based frameworks exhibit a markedly reduced CO₂ affinity while maintaining high C₂H₂ capacity. At 298 K and 1 bar, InNi-scMA-pacs achieves a C₂H₂/CO₂ capacity ratio of 3.2, an ideal adsorbed solution theory (IAST) selectivity of 5.80 (50:50 mixture), and a breakthrough separation window of 44 min/g, outperforming its stMA and bdc analogues. These results establish acyclic diene replacement as a distinct and effective isoreticular tuning strategy to optimize pore environments and sharpen the gas separation performance in partitioned MOFs.