Solvent-induced structural rearrangement in hydrogen-bonded organic frameworks for efficient ethane/ethylene separation

Abstract

Polar solvent molecules often participate in the formation of hydrogen-bonded organic frameworks (HOFs) as building blocks, but the fragile nature of hydrogen bonds means that most HOFs collapse after activation, limiting their broad applications in gas separation. Herein, we report a strategy of solvent-induced structural rearrangement to eliminate the negative effects of polar solvent molecules for achieving efficient C₂H₆/C₂H₄ separation. Single-crystal X-ray diffraction (SCXRD) studies identified that a structural rearrangement was achieved from nonporous HOF-BPTC(MeOH) to porous ZJU-HOF-20 by soaking in low-polarity solvents such as n-hexane. This solvent-induced rearrangement not only removes the bonded methanol molecules from the framework, but also enables it to reversibly transform into a new threefold interpenetrated structure with highly enhanced framework stability. As a result, the activated ZJU-HOF-20a exhibits a largely improved BET surface area of 856 m² g⁻¹ compared to that of HOF-BPTC(MeOH), affording both high C₂H₆ uptake (2.35 mmol g⁻¹ at 298 K and 0.5 bar) and C₂H₆/C₂H₄ selectivity (2.0). The SCXRD studies on gas-loaded ZJU-HOF-20a reveal that the inherent nonpolar pore surfaces combined with suitable pore sizes provide much stronger multipoint interactions with C₂H₆ than C₂H₄, thus accounting for the preferential binding of C₂H₆ over C₂H₄. Breakthrough experiments confirm that ZJU-HOF-20a can efficiently separate actual 50/50 and 10/90 (v/v) C₂H₆/C₂H₄ mixtures to directly produce pure C₂H₄, affording high C₂H₄ productivities of 8.3 L kg⁻¹ and 15.7 L kg⁻¹, respectively.