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

Abstract

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/N-2 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 (similar to 12 nm) and micropores (similar to 0.5 nm), resulting in a high surface area and enhanced CO2 adsorption capacity. When incorporated into Pebax (R) 1074-based mixed-matrix membranes (MMMs) at 30 wt% loading, Meso@Micro-CaA achieved a CO2 permeability of 209 Barrer and a CO2/N-2 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.