Formation of ZIF-8 membranes inside porous supports for improving both their H-2/CO2 separation performance and thermal/mechanical stability

Abstract

Zeolitic imidazolate framework-8 (ZIF-8) membranes are highly suitable as H-2-selective walls in membranes reactors used for water gas shift reactions because of their efficient molecular sieving properties that favor H-2 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 gamma-Al2O3 layer-coated alpha-Al2O3 disc (gamma-/alpha-Al2O3 disc), was used in addition to an intact alpha-Al2O3 disc. ZIF-8 membranes in the alpha-Al2O3 disc (membrane ZIF-8_alpha) were primarily formed on top similar to a conventional supported-membrane, whereas those in the gamma-/alpha-Al2O3 disc (membrane ZIF-8_gamma alpha) were produced inside the support. As desired, membrane ZIF-8_gamma alpha showed marked H-2 separation performance with a maximum (max) H-2/CO2 separation factor (SF) of similar to 9.9 +/- 1.2 at 250 degrees C (vs. a max H-2/CO2 SF of similar to 7.5 +/- 0.2 for membrane ZIF-8_alpha). Although both type membranes persisted at 200 and 250 degrees C for up to 72 h, at a higher temperature of 300 degrees C, the membrane performance started deteriorating after similar to 2 h and similar to 10 h for membranes ZIF-8_alpha and ZIF-8_gamma alpha, respectively. This indicates that the gamma-Al2O3 layer served as a protective layer for preserving the performance of the ZIF-8 membrane. The performance at 300 degrees C was completely degraded due to the eventual conversion of ZIF-8 into ZnO phases.