Researchers reveal robust ethane-trapping porous organic cage for efficient ethylene purification application

The removal of ethane (C₂H₆) from its analogous ethylene (C₂H₄) is of great importance in the petrochemical industry, and is highly challenging due to their similar physicochemical properties. The use of emerging porous organic cage (POC) materials for C₂H₆/C₂H₄ separation is still in its infancy.

Calix[4]resorcinarene, a kind of macrocyclic cavitand with an intrinsic cavity and eigh polar upper-rim phenolic groups, has been documented as excellent building block to construct cage compounds with tunable cavities for encapsulating various guest molecules.

In a study published in Nature Communications, the research group led by Prof. Yuan Daqiang from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences reported that a [6+12] octahedral calix[4]resorcinarene-based POC (CPOC-301) is an excellent C₂H₆ -selective material, and can be used as a robust absorbent to directly afford high-purity C₂H₄ from C₂H₆/C₂H₄ mixture.

The researchers prepared CPOC-301 via self-assembly of tetraformylresorcin[4]arene and p-phenylenediamine under mild condition.

Single-crystal X-ray diffraction revealed that CPOC-301 has a truncated octahedron structure, with eight trigonal ports having edge length reaching about 12 Å, and a large cavity with inner diameter as well as volume that respectively reach 16.8 Å and 4270 ų. The solid-state packing of CPOC-301 suggested that it possesses a one-dimensional channel, with a diameter of ~7 Å, viewed from [001] direction.

The nitrogen (N₂) gas adsorption isotherm of CPOC-301 showed a typical type I curve with a small fraction of a type IV adsorption behavior. The maximum N₂ adsorption is 670 cm³ g^-1, and the calculated Brunauer–Emmett–Teller (BET) of CPOC-301 is up to 1962 m² g^-1. Moreover, CPOC-301 exhibited preferential adsorption of C₂H₆ over C₂H₄ at room temperature.

The ideal adsorbed solution theory (IAST) calculation result demonstrated that C₂H₆/C₂H₄ selectivity range for CPOC-301 is from 1.3 to 1.4 at 293 K. The breakthrough curves further proved that CPOC-301 can efficiently realize the complete separation of C₂H₄ from C₂H₆/C₂H₄ mixtures, and its separation performance does not obviously change within seven continuous cycles.

Molecular modeling studies suggested the exceptional C₂H₆ selectivity is due to the suitable resorcin[4]arene cavities in CPOC-301, which form more multiple C–H···π hydrogen bonds with C₂H₆ than with C₂H₄ guests.

This study sheds light on the design and synthesis of POCs based on supramolecular cavitands as "porous additives" in column and membrane separation applications for industrially important gases in the future.