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 72h, at a higher temperature of 300°C, the membrane performance started deteriorating after ~2h and ~10h 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.