Hierarchical core–shell CaA zeolite fillers via impregnation-assisted synthesis for enhanced CO2/N2 separation in mixed-matrix membranes

In this study, we report a novel strategy for synthesizing hierarchical LTA-type core–shell zeolites (Meso@Micro-CaA) via an impregnation-assisted dry-gel conversion method. The resulting structure features a mesoporous CaA core to promote rapid gas diffusion, encased in a microporous CaA shell that provides molecular sieving functionality and prevents polymer chain intrusion. This architecture enables high CO₂/N₂ selectivity without sacrificing permeability, owing to minimized diffusion resistance from the thin microporous shell. The dry-gel conversion process offers key advantages over conventional hydrothermal synthesis, including simplified operation, suppression of unwanted nucleation, uniform shell formation, reduced water consumption, and shorter synthesis time. The synthesized Meso@Micro-CaA exhibits a well-defined hierarchical pore structure with interconnected mesopores (∼12 nm) and micropores (∼0.5 nm), resulting in a high surface area and enhanced CO₂ adsorption capacity. When incorporated into Pebax® 1074-based mixed-matrix membranes (MMMs) at 30 wt% loading, Meso@Micro-CaA achieved a CO₂ permeability of 209 Barrer and a CO₂/N₂ selectivity of 121.3, exceeding the 2019 upper bound. These improvements are attributed to the synergistic effects of hierarchical porosity and the core–shell design. This work demonstrates the potential of hierarchical core–shell zeolites as next-generation fillers for high-performance membrane-based gas separation and presents a versatile synthesis platform applicable to other zeolite topologies.