(399x) ZIF-67 Containing Mixed Matrix Membranes for Exceptional C3H6/C3H8 Separations

Propylene/propane (C₃H₆/C₃H₈) separation is one of the most important, but challenging processes in the petrochemical industry due to their similar physical properties (i.e., T_b of C₃H₆ and C₃H₈ is -47.6 and -42.1 °C, respectively). Currently, C₃H₆/C₃H₈ separation is implemented by utilizing highly energy-intensive cryogenic distillation. If the inherent drawbacks of polymeric membranes are addressed, membrane-based separation technique can be a good auxiliary candidate in reducing energy consumption over the conventional cryogenic distillation process. In this work, a new promising molecular sieve, ZIF-67, for C₃H₆/C₃H₈ separations was successfully incorporated into 6FDA-DAM polymer matrix with a good dispersion as well as good adhesion with polymer matrix. 6FDA-DAM/ZIF-67 (80/20 wt/wt) MMMs (i.e., PZ67-20) achieved a C₃H₆/C₃H₈ separation factor of 19 under the equimolar binary mixture conditions at 35 °C. The higher C₃H₆/C₃H₈ selectivity of PZ67-20 MMMs over that of PZ8-20 counterparts was attributed to a higher diffusivity selectivity induced by a combination of much higher repulsion energy and more negative heat of sorption for C₃H₈ penetrants in ZIF-67. The Maxwell model was used to estimate the intrinsic separation performance of ZIF-67, showing a C₃H₆ permeability of 150.7 Barrer, which is 45% less than that of ZIF-8, but a C₃H₆/C₃H₈ selectivity of 251, which is 100% higher than that of ZIF-8. In addition to the excellent separation performance of ZIF-67 containing MMMs, the membranes showed substantially improved physical aging resistance due to a buttress role of ZIF-67 between polymer chains in the large free volume elements, further supporting their practical application for C₃H₆/C₃H₈ separations.