Thermal stability of ZIF-8 membranes for gas separations

The thermal stability of ZIF membranes is important for high temperature separation applications but has not been systematically studied. This work highlights the results of a thermal stability study of ZIF-8 membranes in terms of material structure, H₂/CO₂ gas permeation and separation characteristics. During binary and single gas temperature dependent permeance tests conducted from 25 to 250 °C, both H₂ and CO₂ permeances decrease as a function of temperature. In the binary test, H₂/CO₂ selectivity increases between 25 and 225 °C, and then decreases as temperature is further increased between 225–275 °C. The results can be explained by the adsorption/diffusion mechanism. Beyond 275 °C, H₂/CO₂ permeance and selectivity drastically increase with respect to temperature and is indicative of ZIF-8 membrane partial carbonization during the dynamic 30 h temperature dependent test. The time/temperature dependency of the onset of ZIF-8 thin film structural change was deconvoluted in isothermal transient permeation experiments. Transient tests performed at 50, 100, 150 and 300 °C for 24 h indicate that ZIF-8 thin films maintain their crystallinity and structural integrity below 150 °C. However, at temperatures of 150 °C and greater the framework undergoes increased magnitudes of thermally induced carbonization as a function of temperature. Thermomechanically induced stresses between the ZIF-8 membrane thin film and α–alumina support may account for differences in static thermal stability observed when comparing ZIF-8 membranes and ZIF-8 crystalline powders.