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

Abstract

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.