Ionic-Liquid-Based Engineering of MOF Membranes: Tuning from H2- to CO2-Selective

Reprogramming the transport preference of crystalline membranes offers a useful strategy for adapting a single membrane platform to different gas-separation tasks. Herein, we report ionic liquid (IL) engineering of pure ZIF-8 membranes to switch H2/CO2 transport from H2- to CO2-preferential behavior. A continuous ZIF-8 membrane was first fabricated on an anodic aluminum oxide support by cathodic deposition, followed by the loading of an IL of type 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]). The compact, well-intergrown, and crystalline ZIF-8 membrane was preserved after IL incorporation, and vibrational spectroscopy provided evidence for molecular interactions between confined [BMIM][TFSI] and the ZIF-8 framework. Gas-permeation measurements revealed that increasing IL loading decreased both H2 and CO2 permeances, indicating an increased transport resistance caused by partial pore occupation. However, the H2 permeance decreased significantly more rapidly than the CO2 permeance. The H2/CO2 selectivity decreased from 3.6 for the pristine ZIF-8 membrane to approximately 0.3 at 15 vol % ionic-liquid loading, corresponding to a CO2/H2 selectivity close to 3. The separation reversal originates from the combined effects of IL-induced pore narrowing and preferential CO2 affinity. This work demonstrates that confined ILs can reprogram adsorption-diffusion coupling in MOF membranes and provides a route toward process-adaptive gas-separation membranes.