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 C 2 H 6 /C 2 H 4 separation, but there are few examples of one-step acquisition of C 2 H 4 from C 2 H 6 /C 2 H 4 because it is still difficult to achieve the reverse-order adsorption of C 2 H 6 and C 2 H 4 . In this work, we boost the C 2 H 6 /C 2 H 4 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 C 2 H 6 . The difference in the capacities for C 2 H 6 and C 2 H 4 is 23.4 cm 3  g -1 for HOF-NBDA, and the C 2 H 6 /C 2 H 4 uptake ratio is 136 %, which are much higher than those for HOF-NBDA(DMA) (5.0 cm 3  g -1 and 108 % respectively). Practical breakthrough experiments demonstrate HOF-NBDA could produce polymer-grade C 2 H 4 from C 2 H 6 /C 2 H 4 (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 C 2 H 6 and thus boosts selective separation of C 2 H 6 /C 2 H 4 .