Tailoring molecular diffusion in core-shell zeolite imidazolate framework composites realizes efficient kinetic separation of xylene isomers.

The separation of xylene isomers is a critical and energy-intensive process in the petrochemical industry, primarily due to their closely similar molecular structures and boiling points. In this work, we report the synthesis and application of a novel core-shell zeolitic imidazolate framework (ZIF) composite, ZIF-65@ZIF-67, designed to significantly enhance the kinetic separation of xylene isomers through a synergistic "shell-gated diffusion and core-facilitated transport" strategy. The external ZIF-67 shell selectively restricts the diffusion of larger isomers (MX and OX), while the internal ZIF-65 core accelerates the diffusion of PX, thereby amplifying the diffusion differences among the isomers. This architecture yields remarkable improvements in both selectivity and diffusion rates, as demonstrated by vapor-phase adsorption studies and molecular dynamics simulations. The ZIF-65@ZIF-67 composite exhibits up to 12.5 times higher PX/OX selectivity in liquid-phase adsorption and 3.4 times higher dynamic selectivity in fixed-column breakthrough experiments compared to the individual ZIF components. Theoretical simulations further corroborate the heterogeneous diffusion control mechanism, revealing the time-dependent diffusion regulation within the core-shell architecture. This work underscores the great potential of core-shell MOF composites in optimizing molecular sieving processes for industrially significant separations and highlights a new route for enhancing kinetic separation efficiency in complex multicomponent systems.