Using Porous Liquids to Perform Liquid-Liquid Separations

Porous liquids (PLs) are a new type of fluid sorbent investigated mainly for the separation of gas mixtures. Here, we explore their application to the separation of miscible liquids, using MEG/water (MEG=monoethylene glycol) and EtOH/water as proof-of-principle. Recovery of used MEG is industrially important but its extraction into conventional solvents from water is difficult. PLs ZIF-8@PDMS (PL1, PDMS=polydimethylsilicone) or ZIF-8@sesame oil (PL2) each consisting of 25 wt % of the hydrophobic microporous material ZIF-8 dispersed in PDMS or sesame oil respectively, were formulated and found to be exceedingly physically stable to sedimentation. A 5 nm PEEK membrane was used to provide a permeable barrier between the PL and the alcohol/water phase. MEG was selectively extracted through the membrane from approximately 50 : 50 wt % MEG/water mixtures into the PL phase and this procedure could be applied repeatedly. It was effective for MEG/water mixtures as dilute as 3 : 97 wt %. The PL could also be regenerated (80 °C at 0.01 bar) and re-used, suggesting its potential for continuous, cyclic extraction. Furthermore, PL3 (silicalite-1@PDMS) was effective in selective alcohol extraction from beverages. It shows potential for lowering the alcohol concentration in gin or wine due to its excellent chemical stability and nontoxicity. Overall, we show that the enhanced adsorption properties of PLs due the presence of empty pores, which provides unusually high gas solubilities, also makes them, in principle, applicable to liquid-liquid separations.