Diffusive separation of propylene/propane with ZIF-8 membranes

The contribution of diffusive separation of propylene/propane was determined from the permeation properties of zeolitic-imidazolate framework-8 (ZIF-8) membranes. These membranes were prepared using a counter diffusion method, resulting in the formation of an 80 μm-thick ZIF-8 layer on the outermost section of a porous α-alumina capillary substrate. Single-component gas permeation properties were investigated using helium, hydrogen, carbon dioxide, oxygen, nitrogen, methane, propylene, and propane at temperatures ranging from 298 to 363 K. The molecular sieve gas permeation properties of the ZIF-8 membranes increased with reaction time, which is consistent with the evolution of ZIF-8 crystal formation. The ZIF-8 membrane permeances of hydrogen and propylene reached 9.1×10−8 and 2.5×10−9 mol m−2 s−1 Pa−1, respectively. The ideal separation factors for hydrogen/propane and propylene/propane at 298 K were found to be 2000 and 59, respectively. The diffusion separation factor increased with decreasing temperature, reaching 23, while the solution separation factor remained 2.7. Therefore, the separation of propylene/propane was attributed to be mainly governed by diffusive separation.