Solid-state porosity is very common in the world around us, from the construction of buildings to biology and of course the bathroom sponge. In chemistry, solid frameworks such as Metal Organic Frameworks (MOFs), Covalent Organic Frameworks (COFs) and Zeolites have been a focus for research for many years now. These frameworks normally form a cavity into which a small molecule could be absorbed. This internal cavity makes these materials good candidates for research into gas separation and storage. While solids are likely better for gas absorption they still have a distinct disadvantage, they do not move. While at first glance this immobility appears to be a mute point, if you want to form a flow process this could be a sticking point as MOFs, COFs and zeolites are solid until high temperatures.
In 2007, O’Reilly et al. published a concept article outlining their basis of what a porous liquid would be and how it could be defined. In this article they defined three categories of porous liquid; neat (Type 1), an empty host in a molecular solvent (Type 2) and a framework dispersed within a sterically hindered solvent (Type 3). A caroon schematic of this can be seen in figure 1. A few years later Melaugh and Giri in a collaborative computational/experimental study at Queen’s University Belfast studied the potential of a COF first reported by Cooper et al. to form a porous liquid. The computational part of the study showed medium length alkyl tails on the Cooper cages gave an acceptable balance of cage penetration and gas absorption. In the real world the alkyl cages although synthetically possible, proved to be near impossible to dissolve in traditional solvents and were only liquid above room temperature. This leadto the development of cages with crown ether functionality in place of the alkyl groups allowing studies of the cages to be carried out in crown ether solvents. For methane it was shown that the ethereal type 2 system formed by the alkylated cage and 15-crown-5 ether, increased gas absorption by ten times over the neat crown ether as seen in figure 2 below.
While further work is required upon these non-ionic systems both experimentally and computationally, thoughts have turned towards utilising ionic liquids. Ionic liquids already have an extensive body of research into their ability to absorb and separate gases, notably carbon dioxide. While a truly ionic cage would be both an ideal and a true type one porous ionic liquid, synthetically it is a daunting task. Therefore to start with it would be desirable to take advantage of the wide range of ionic liquids available and their well-documented solvent properties. These solvent properties can be exploited to dissolve these porous cages inside that are insoluble in traditional solvents to form a type 2 porous ionic liquid. This study is a collaborative work between the Atomistic Simulation Centre and the School of Chemistry and Chemical Engineering at Queen’s University Belfast, and will consist of both experimental and computation studies to form the first porous ionic liquids.