Design of Pore Properties of an Al-Based Metal-Organic Framework for the Separation of an Ethane/Ethylene Gas Mixture via Ethane-Selective Adsorption.

A series of Al-based isomorphs (CAU-10H, MIL-160, KMF-1, and CAU-10pydc) were synthesized using isophthalic acid (ipa), 2,5-furandicarboxylic acid (fdc), 2,5-pyrrole dicarboxylic acid (pyrdc), and 3,5-pyridinedicarboxylic acid (pydc), respectively. These isomorphs were systematically investigated to identify the best adsorbent for effectively separating CH/CH. All CAU-10 isomorphs exhibited preferential adsorption of CH over that of CH in mixture. CAU-10pydc exhibited the best CH/CH selectivity (1.68) and the highest CH uptake (3.97 mmol g) at 298 K and 1 bar. In the breakthrough experiment using CAU-10pydc, 1/1 (v/v) and 1/15 (v/v) CH/CH gas mixtures were successfully separated into high-purity CH (>99.95%), with remarkable productivities of 14.0 L kg and 32.0 L kg, respectively, at 298 K. Molecular simulations revealed that the exceptional separation performance of CAU-10pydc originated from the increased porosity and reduced electron density of the pyridine ring of pydc, leading to a relatively larger decrease in π-π interactions with CH than in the C-H···π interactions with CH. This study demonstrates that the pore size and geometry of the CAU-10 platform are modulated by the inclusion of heteroatom-containing benzene dicarboxylate or heterocyclic rings of dicarboxylate-based organic linkers, thereby fine-tuning the CH/CH separation ability. CAU-10pydc was determined to be an optimum adsorbent for this challenging separation.