Pore Space Partition Enabled by Lithium(I) Chelation of a Metal-Organic Framework for Benchmark C2H2/CO2 Separation.

Adsorptive separation of acetylene (C2H2) from carbon dioxide (CO2) offers a promising approach to purify C2H2 with low-energy footprints. However, the development of ideal adsorbents with simultaneous high C2H2 adsorption and selectivity remains a great challenge due to their very small molecular sizes and physical properties. Herein, we report a lithium(I)-chelation strategy for pore space partition (PSP) in a microporous MOF (Li+@NOTT-101-(COOH)2) to achieve simultaneous high C2H2 uptake and selectivity. The chelation model of Li+ ions within the framework was visually identified by single-crystal X-ray diffraction studies. The immobilized Li+ ions were found to have two functions: (1) partitioning large pore cages into smaller ones while maintaining high surface area and (2) providing specific binding sites to selectively take up C2H2 over CO2. The resulting Li+@NOTT-101-(COOH)2 exhibits a rare combination of a simultaneous high C2H2 capture capacity (205 cm3 g-1) and C2H2/CO2 selectivity (13) at ambient conditions, far surpassing that of NOTT-101-(COOH)2 (148 cm3 g-1 and 3.8, respectively) and most top-tier materials reported. Theoretical calculations and gas-loaded SCXRD studies reveal that the chelated Li+ ions combined with the segmented small cages can selectively bind with a large amount of C2H2 through the unique π-complexation, accounting for the improved C2H2 uptake and selectivity. Breakthrough experiments validated its excellent separation capacity for actual C2H2/CO2 mixtures, providing one of the highest C2H2 productivities of 118.9 L kg-1 (>99.5% purity) in a single adsorption-desorption cycle.