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A method to predict the gas permeability of supported ionic liquid membranes (SILMs) was established, using as input the pore structure characteristics of asymmetric ceramic membrane supports and the physicochemical properties of the bulk ionic liquid (IL) phase. The method was applied to investigate the effect of IL nanoconfinement on the CO2 and N2 permeability/selectivity properties of novel SILMs developed on nanofiltration (NF) membranes employing for the first time the 1-ethyl-3-methylimidazolium and the 1-butyl-3-methylimidazolium tricyanomethanide ILs as pore modifiers. The selected ILs exhibit low viscosity, which allows for faster gas solvation rates and ease of synthesis/purification that makes them attractive for large-scale production. In parallel, the use of ceramic supports instead of polymeric ones presents the advantage of operation at elevated temperatures and pressures and offers the possibility to study the “real” permeability of the confined IL phase, avoiding additional contributions from the gas diffusion through the surrounding solid matrix. The developed SILMs exhibited enhanced CO2 permeability together with high CO2/N2 separation capacity, though with distinct variations depending on the alkyl chain length of the 1-alkyl-3-methylimidazolium cation. Application of the developed methodology allowed discriminating the contribution of the NF pore structural characteristics on the SILM performance and unveiled the subtle interplay of diverse IL confinement effects on the gas permeability stemming from the specific layering of ion pairs on the nanoporous surface and the phase transition of the IL at room temperature, dictated by small variations of the IL cation size.

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Zeolitic imidazolate framework ZIF-69 membranes were grown on porous α-alumina substrates via seeded secondary growth and further functionalized by a CO 2 -selective tricyanomethanide anion/alkylmethylimidazolium cation-based ionic liquid (IL) to plug the gaps between the ZIF crystals yet leave the framework pores open for gas diffusion. In this configuration, ZIF intergrain boundaries and defects were repaired by a medium that exhibits high selectivity for CO 2 . As a result, the selectivity of the hybrid membrane was significantly higher than that of as-grown ZIF membranes and, because of the existence of the ZIF channels, the permeability was higher than that corresponding to bulk IL. Specifically, CO 2 permeated 20 times faster than N 2 through the intact ZIF pores and 65 times faster than through the bulk IL phase. The developed membranes at room temperature and under a 2 bar transmembrane pressure exhibited CO 2 permeance of 5.6 × 10 −11 and 3.7 × 10 −11 mol m −2 s −1 Pa −1 and real CO 2 /N 2 selectivities of 44 and 64 for CO 2 /N 2 mixtures consisting of 44% and 75% (v/v) CO 2 , respectively. In addition, on the basis of the experimental evidence from the hybrid membranes, predictions were made on the expected performance of an ideal, crack-free, and homogeneous ZIF-69 membrane. This work provides a promising solution to the challenges associated with defect formation experienced during growth not only of ZIFs but also of other zeolite and inorganic membranes used for CO 2 separation.

Membranes consisting of ultrathin, oriented, single-wall carbon nanotube (SWCNT) micropores with a diameter of ∼4 Å were developed. c-Oriented AFI-type aluminophosphate (AlPO) films (AlPO4-5 and CoAPO-5), consisting of parallel channels 7.3 Å in diameter, were first fabricated by seeded growth on macroporous alumina supports, and used as templates for synthesis of CNTs inside the zeolitic channels by thermal treatment, utilizing the structure directing agent (amine) occluded in the channels as carbon source. Incorporation of CNTs inside the AFI channels altered the transport mechanism of all permeating gases tested, and imposed a substantial increase in their permeation rates, in comparison to the AlPO4-5 membrane, despite the pore size reduction due to nanotube growth. The enhancement of the permeation rates is attributed to repulsive potentials between gas molecules and occluded nanotubes, which limit adsorption strength and enhance diffusivity, coupled to the smooth SWCNT surface that enables fast diffusion through the nanotube interior. Separation ability, evaluated with respect to H2 and CO2 gases, was enhanced by using polysterene as defect-blocking medium on both AlPO and CNT/AlPO membranes and was preserved after CNT growth.