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

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.