Zeolite Imidazolate Framework–Ionic Liquid Hybrid Membranes for Highly Selective CO₂ Separation

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Abstract

Zeolitic imidazolate framework ZIF-69 membranes were grown on porous a-alumina substrates via seeded secondary growth and further functionalized by a CO2-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 CO2. 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, CO2 permeated 20 times faster than N2 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 CO2 permeance of 5.6 × 10–11 and 3.7 × 10–11 mol m–2 s–1 Pa–1 and real CO2/N2 selectivities of 44 and 64 for CO2/N2 mixtures consisting of 44% and 75% (v/v) CO2, 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 CO2 separation.

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