Formation of ZIF-8 membranes inside porous supports for improving both their H₂/CO₂ separation performance and thermal/mechanical stability

Zeolitic imidazolate framework-8 (ZIF-8) membranes are highly suitable as H₂-selective walls in membranes reactors used for water gas shift reactions because of their efficient molecular sieving properties that favor H₂ transport, coupled with their high thermal stability and processability. In this study, we adopted an in-situ counter diffusion method to fabricate a ZIF-8 membrane; Zn sources, already placed inside a porous support, were allowed to diffuse out and react with the 2-methylimidazole (mim) molecules in the bulk phase. Because the reaction rates between the Zn source and the mim molecule were very high, their diffusion rates played a key role in determining the final properties of the membranes. To control the diffusion rate, a hierarchically structured support, i.e., a γ-Al₂O₃ layer-coated α-Al₂O₃ disc (γ-/α-Al₂O₃ disc), was used in addition to an intact α-Al₂O₃ disc. ZIF-8 membranes in the α-Al₂O₃ disc (membrane ZIF-8_α) were primarily formed on top similar to a conventional supported-membrane, whereas those in the γ-/α-Al₂O₃ disc (membrane ZIF-8_γα) were produced inside the support. As desired, membrane ZIF-8_γα showed marked H₂ separation performance with a maximum (max) H₂/CO₂ separation factor (SF) of ~9.9 ± 1.2 at 250 °C (vs. a max H₂/CO₂ SF of ~7.5 ± 0.2 for membrane ZIF-8_α). Although both type membranes persisted at 200 and 250 °C for up to 72 h, at a higher temperature of 300 °C, the membrane performance started deteriorating after ~2 h and ~10 h for membranes ZIF-8_α and ZIF-8_γα, respectively. This indicates that the γ-Al₂O₃ layer served as a protective layer for preserving the performance of the ZIF-8 membrane. The performance at 300 °C was completely degraded due to the eventual conversion of ZIF-8 into ZnO phases.