Gas transport properties and propylene/propane separation characteristics of ZIF-8 membranes

High quality ZIF-8 membranes of about 2.5 μm in thickness were prepared on α-Al2O3 support by secondary growth synthesis in water solution. Single He, H2, CO2, N2, CH4, C3H6 and C3H8 permeation data for the ZIF-8 membranes were measured and analyzed by a permeation model to obtain diffusivity values of these gases in ZIF-8 crystals. The diffusivity values obtained from gas permeance tests were in the range of 1×10−6–1×10−4 cm2/s. Diffusivity value decreases with increasing molecular size in the order He>H2>CO2, >N2>CH4. Gas permeance for the above light gases remains constant while for C3H6 and C3H8 permeance decreased with increasing pressure due to the specific pressure dependency of the adsorption isotherm respective to each gas. The determined diffusivity for propylene and propane is 1.25×10−8 cm2/s and 3.99×10−10 cm2/s with activation energy for diffusion of 12.7 kJ/mol and 38.8 kJ/mol respectively. With equal-molar binary feed, the ZIF-8 membranes exhibit consistent C3H6/C3H8 selectivity of about 30 and C3H6 permeance of 1.1×10−8 mol/m2sPa. A month long stability test shows stable gas permeance and separation performance of the ZIF-8 membranes in both atmospheric conditions and C3H6/C3H8 mixture stream.