Tuning Pore Polarization to Boost Ethane/Ethylene Separation Performance in Hydrogen-Bonded Organic Frameworks.

Hydrogen-bonded organic frameworks (HOFs) show great potential in energy-saving C2H6/C2H4 separation, but there are few examples of one-step acquisition of C2H4 from C2H6/C2H4 because it is still difficult to achieve the reverse-order adsorption of C2H6 and C2H4. In this work, we boost the C2H6/C2H4 separation performance in two graphene-sheet-like HOFs by tuning pore polarization. Upon heating, an in situ solid phase transformation can be observed from HOF-NBDA(DMA) (DMA = dimethylamine cation) to HOF-NBDA, accompanied with transformation of the electronegative skeleton into neutral one. As a result, the pore surface of HOF-NBDA has become nonpolar, which is beneficial to selectively adsorbing C2H6. The difference in the capacities for C2H6 and C2H4 is 23.4 cm3∙g-1 for HOF-NBDA, and the C2H6/C2H4 uptake ratio is 136%, which are much higher than those for HOF-NBDA(DMA) (5.0 cm3∙g-1 and 108% respectively). Practical breakthrough experiments demonstrate HOF-NBDA could produce polymer-grade C2H4 from C2H6/C2H4 (1/99, v/v) mixture with a high productivity of 29.2 L∙kg-1 at 298 K, which is about five times as high as HOF-NBDA(DMA) (5.4 L∙kg-1). In addition, in situ breakthrough experiments and theoretical calculations indicate the pore surface of HOF-NBDA is beneficial to preferentially capture C2H6 and thus boosts selective separation of C2H6/C2H4.