The 2025 Eli Ruckenstein Memorial Lecture

There is an urgent need to reduce CO2 emissions, which are driving rising global temperatures. Among the various streams from which CO2 could be removed, air is the most dilute, and direct air capture (DAC) requires the most energy. Most emerging technologies rely on solid or liquid chemical sorbents that generally have poor thermal stability, which is needed for regeneration.  For DAC to be deployed at scale, new technologies must emerge that offer exceptional CO2 selectivity, potentially lower operating costs, and the ability to regenerate/operate at lower temperatures. If they have exceptionally high selectivity and permeability, membrane technologies for DAC (m-DAC) may offer this alternative. While m-DAC systems require compression and/or vacuum, they do not require thermal regeneration. We incorporated a series of N – heterocyclic anion ionic liquids (AHA ILs), a class of ILs originally developed in our laboratory for post-combustion CO2 capture, in porous ceramic and polymer supports to make supported ionic liquid membranes (SILMs). Using both pure and mixed gas permeation experiments, both in dry and humidified environments, we have explored the performance of these SILMs at CO2 concentrations down to 420 ppm.  Through measurements of water uptake by the AHA ILs as a function of relative humidity, the effect of water on viscosity, and the effect of humidity on CO2 capacity at low partial pressures of CO2, we have been able to identify appropriate AHA ILs with the potential for m-DAC applications. The best-performing SILM achieved a CO2 permeability of 49,100 Barrer and CO2/N2 permselectivity of 13,200 at 420 ppm CO2 and 40% relative humidity (i.e., DAC conditions)––the highest reported permeability/selectivity combination reported to date. 

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