A new superior competitor for exceptional propylene/propane separations: ZIF-67 containing mixed matrix membranes

Recently, the ZIF-67 molecular sieve has emerged as an excellent substitute for the ZIF-8 counterpart due to its potentially high C3H6/C3H8 separation performance. Here, for the first time, we investigated the effect of ZIF-67 in mixed matrix membranes (MMMs) for C3H6/C3H8 separations by integrating them into 6FDA-DAM polymer matrix. The 6FDA-DAM/ZIF-67 (80/20 wt/wt) MMMs achieved an ideal selectivity of 29.9 and a separation factor of 19 under single and equimolar mixed gas conditions, respectively, at 35 degrees C. The intrinsic C3H6/C3H8 separation performance of ZIF-67 was estimated from our experimental transport results based on the Maxwell model. The calculation demonstrated its superior C3H6/C3H8 selectivity of 251, which is twice of that of ZIF-8, supporting the high molecular sieving capacity of ZIF-67 containing MMMs. Moreover, a thorough investigation on the temperature-dependent gas transports elucidated that the energetic selectivity is a major contributor for the high C3H6/C3H8 diffusivity selectivity in ZIF-67 containing MMMs. However, the minimum aperture size of ZIF-67 (4.5 A), derived by the cuboid dimensions of C3H6 and C3H8, substantially sacrificed the C3H6/C3H8 entropic selectivity of ZIF-67 containing MMMs. Lastly, the defect-free incorporation of ZIF-67 nanoparticles into 6FDA-DAM polymer matrix effectively retarded physical aging process, maintaining the excellent separation performance of the associated MMMs for 75 days.