Tailoring linker defects in MOF-808 to improve CO2 separation in mixed matrix membranes

Carbon capture is widely recognized as a pivotal technology for alleviating the accelerating impacts of global warming. Mixed matrix membranes (MMMs) that embed metal organic frameworks (MOFs) offer a compelling route to high-performance separations, and MOF-808 stands out among them because of its outstanding chemical stability and environmentally friendly synthesis. In this study, the defect density of MOF-808 was tuned by varying the formic acid modulator: Higher modulator loading produced MOF-808 with higher linker defect density. After synthesis, MOF-808 was washed with either water or ethanol and then incorporated into PEBAX matrix through a drying-free process that ensured uniform dispersion. Washing with ethanol altered the pore coordination environment of MOF-808, partially replacing formate with ethoxide. Introducing an appropriate amount of formic acid created additional open metal sites and increased both CO₂ permeability and CO₂/N₂ selectivity; however, excessive formic acid produced too many defects, causing severe pore blockage and consequently lowering the CO₂ permeability. The drying-free method, by preventing MOF aggregation and ensuring homogeneous dispersion, led to a 38 % increase in CO₂/N₂ selectivity compared with the conventional drying method. The ethoxide substitution induced by ethanol washing occupied part of the pore volume, leading to a reduction in CO₂ permeability. The optimized MOF-808/PEBAX MMM achieved a CO₂ permeability of 150 Barrer and a CO₂/N₂ selectivity of 76.4, both markedly higher than those of the pristine polymer. These results highlight a sustainable, scalable strategy for producing high-efficiency membranes for post-combustion CO₂ capture.