Tailoring pore flexibility in mixed-ligand ZIF-8 membranes for exceptional propylene/propane selectivity

초록

Mixed-ligand ZIF-8 membranes were synthesized via an in-situ growth strategy to incorporate 2-aminobenzimidazole (2-abIm) into the framework without introducing macroscopic defects. Structural and spectroscopic analyses showed that replacing part of 2-methylimidazole (2-mIm) with 2-abIm maintains the sodalite topology, increases rigidity, and suppresses gate-opening. Nitrogen adsorption isotherms showed progressively diminished gate-opening transitions with increasing 2-abIm content, an observation corroborated by reduced propylene Maxwell-Stefan diffusivities above 14 mol% 2-abIm. Despite these local structural changes, global framework flexibility-such as lattice breathing motions-was retained, thereby maintaining the intrinsic molecular sieving cutoff for smaller gases (e.g., nitrogen and oxygen). In contrast, propane transport became markedly restricted, leading to a sixfold enhancement in propylene/propane selectivity-reaching 246 at 19 mol% 2-abIm. This improved hydrocarbon separation reflects a synergy between reduced ligand rotational freedom and partial preservation of the overall lattice flexibility, outperforming many existing polymeric, carbon-based, and MOF membranes. These findings underscore the promise of rational ligand design in ZIF-8 systems for optimizing pore environments and molecular sieving properties.

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