Porous organic cages (POCs) are discrete, covalently linked molecules with intrinsic cavities. The porous nature of POCs enables them to be nanoscale reaction vessels for catalysis, hosts for different guest molecules, and adsorbents for gas storage and separation. 

POCs materials exhibit solvatomorphs via altering their crystallographic packing in the solid state, but the investigation of real gas mixture separation by porous materials with such a behavior is still very rare. 

In a study published in ACS Appl. Mater. Interfaces, the 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 lantern-shaped calix[4]resorcinarene-based porous organic cage (POC, namely, CPOC-101) can exhibit eight distinct solid-state solvatomorphs via crystallization in different solvents, and this POC solvatomorphism strongly influences their gas sorption capacities and separation abilities. 

The researchers found that the apparent Brunauer-Emmett-Teller (BET) surface area determined by nitrogen gas sorption at 77 K for CPOC-101α crystallized from toluene/chloroform is up to 406 m² g^-1, which is much larger than that of the rest of CPOC-101 solvatomorphs with BET values less than 40 m² g^-1.  

They also found that C₂H₂ and CO₂ adsorbed capacities, in addition to the C₂H₂/CO₂ separation ability at room temperature for CPOC-101α, are superior to those of CPOC-101β crystalized from nitrobenzene, the representative of POC solvatomorphs with low BET surface areas.  

To understand the mechanism of the higher affinity toward C₂H₂ over CO₂ within CPOC-101, the researchers computed the interaction energies between the optimized cage host and gas guests by the first-principles dispersion-corrected density functional theory (DFT-D) calculations.  

The hydrogen-bond number (6 for C₂H₂ and 5 for CO₂), the average hydrogen-bond length (3.12 Å for C₂H₂ and 3.26 Å for CO₂), and the calculated interaction enthalpies highly indicated that the host-guest interaction between the C₂H₂ molecule and CPOC-101 is much stronger than that of CO₂. 

This study reveals the possibility of adjusting gas sorption and separation properties of POC materials by controlling their solvatomorphs. 

CPOC-101 for C₂H₂/CO₂ separation. (Image by Prof. YUAN's group)