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

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.