Concentrated-Solution Synthesis of the Metal–Organic Framework CALF-20 and Its Application as a Functional Filler in Pebax-Based Mixed Matrix Membranes for CO ₂ Separation

Mixed matrix membranes (MMMs) incorporating the metal−organic framework CALF-20 were developed for CO2 separation using Pebax® 2533 as the polymer matrix. CALF-20 prepared under concentrated synthesis conditions in a methanol−water mixed solvent was successfully dispersed into the polymer matrix with filler loadings of up to 90 wt% without mechanical brittleness. At moderate filler loadings, the MMMs exhibited significantly enhanced CO2 permeability and improved CO2/CH4 separation performance compared with pristine Pebax® 2533. Gas permeation analyses revealed that CO2 transport in the MMMs followed a dual-mode sorption behavior, indicating that the micropores of CALF-20 functioned as effective adsorption and diffusion pathways for CO2. The activation energy for CO2 permeation decreased upon CALF-20 incorporation, further supporting the contribution of the MOF micropores to gas transport. Performance prediction based on the Maxwell model suggested that CALF-20 possesses intrinsically high CO2 permeability and selectivity. At very high filler loadings, membrane selectivity deteriorated due to the formation of pinhole defects; however, these materials retained high CO2 adsorption capacity and fast adsorption kinetics. These results demonstrate that CALF-20-based MMMs are promising candidates for membrane-based CO2 separation, while also offering potential for adsorption-based CO2 capture applications.