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 are non-volatile,v ersatile materials that associate porosity and fluidity.N ew porous ionic liquids,b ased on the ZIF-8 metal-organic framework and on phosphonium acetate or levulinate salts,were prepared and showa ni ncreased capacity to absorb carbon dioxide at low pressures.P orous suspensions based on phosphonium levulinate ionic liquid absorb reversibly 103 %m ore carbon dioxide per mass than pure ZIF-8 at 1bar and 303 K. We show howthe rational combination of MOFs with ionic liquids can greatly enhance low pressure CO 2 absorption, paving the way towards an ew generation of high-performance,r eadily available liquid materials for effective lowp ressure carbon capture.