Thermodynamics and Kinetics of Gas Storage in Porous Liquids

The recent synthesis of organic molecular liquids with permanent porosity opens up exciting new avenues for gas capture, storage, and separation. Using molecular simulations, we study the thermodynamics and kinetics for the storage of CH₄, CO₂, and N₂ molecules in porous liquids consisting of crown-ether-substituted cage molecules in a 15-crown-5 solvent. It is found that the intrinsic gas storage capacity per cage molecule follows the order CH₄ > CO₂ > N₂, which does not correlate simply with the size of gas molecules. Different gas molecules are stored inside the cage differently; e.g., CO₂ molecules prefer the cage's core whereas CH₄ molecules favor both the core and the branch regions. All gas molecules considered can enter the cage essentially without energy barriers and leave the cage on a nanosecond time scale by overcoming a modest energy penalty. The molecular mechanisms of these observations are clarified.