Porous Ionic Liquids: Structure, Stability, and Gas Absorption Mechanisms

Porous ionic liquids prepared from phosphonium‐based ionic liquids and metal‐organic frameworks (MOFs) are fluid in large ranges of temperature including ambient. It is shown that the ion pairs are too voluminous to enter the pores of the MOF, so the porous liquids remain several months as suspensions with permanent free volume, capable of absorbing large quantities of gases. The increase in gas absorption, when compared with the pure ionic liquids, is proportional to the amount of porous solid in suspension. Structural features of the MOFs and of the ionic liquids are maintained in the suspensions. Thermodynamic analysis and molecular simulations show that the driving force for gas absorption by the porous ionic liquids is energetic as well as structural being controlled by gas‐solid affinity or by the porous liquid free volume. The enthalpy of gas absorption allows easy regeneration of the porous liquid in all cases. The dissolved gases fluidify the porous ionic liquids, different gases having distinct effects on mass transport. The molecular mechanisms that explain the stability of the suspensions and their capacity for gas absorption are identified and point toward easy design rules that will enable numerous applications of these innovative materials as reaction or separation media. Porous ionic liquids, prepared as stable suspensions of metal organic frameworks in voluminous salts, are liquids at room temperature and combine permanent porosity with fluidity in large ranges of temperature. The size of the ion pairs and of the pores in the solid are at the basis of design rules that enable applications of these materials.