High-flux mixed matrix membranes containing bimetallic zeolitic imidazole framework-8 for C3H6/C3H8 separation

Abstract

Membrane-based gas separation requires highly delicate engineering of molecular structures for the desired separation performance. Herein, a new efficient approach to fabricate high-flux mixed matrix membranes (MMMs) for C3H6/C3H8 separation is reported by utilizing ZnCo-mixed hybrid zeolitic imidazolate framework-8 (ZIF-8-67) with unexpectedly large aperture size. Our structural characterizations in molecular level reveal that the ZIF-8-67 exhibits a larger aperture size compared to ZIF-8. Such a large aperture size of ZIF-8-67 is attributed to the large angle between two neighboring ligands and the long metal-ligand bonding distance, which is confirmed by refining the computed XRD pattern until it matches the experimental one (Rietveld refinement). More importantly, the inclusion of ZIF-8-67 in the 6FDA-DAM matrix enhances the C3H6 permeability by up to 240% with the moderate C3H6/C3H8 selectivity improvement (70%), as compared to those of the polymeric counterpart. Furthermore, the temperature-dependent transport characterization verifies that the ZIF-containing MMMs improve the C3H6/C3H8 energetic selectivity at the sacrifice of entropic selectivity due to a unique molecular sieving behavior of the ZIFs, thereby enhancing the diffusivity selectivity. This study proposes a simple strategy to enable the fine-tuning of the aperture size of parent ZIFs towards the desired gas separation performance.