Entropy-driven graded molecular selectivity in stacking-fault-programmed crystalline metallacycle membranes

Membrane-based molecular separations are fundamentally constrained by the lack of precisely defined transport pathways, limiting high-flux fractionation of species with subtle size differences. Here, we report a class of stacking-fault-programmed crystalline coordination metallacycle (CCM) membranes featuring programmable nanoarchitectures constructed via interfacial coordination self-assembly. By rationally tuning the angular geometry of metallacyclic precursors, we fabricate two structurally distinct membranes (CCM-1 and CCM-2) possessing ordered microporous frameworks with well-defined transport channels. Both membranes exhibit simultaneously high solvent permeance and sharp molecular sieving, surpassing the conventional permeability-selectivity trade-off. Molecular simulations coupled with thermodynamic analysis reveal that the high selectivity arises from an entropy-dominated transport mechanism, governed by stacking-fault-induced misalignment of metallacycles. This misaligned stacking generates graded steric confinement, enabling precise discrimination between molecules with minimal size differences. Furthermore, we construct a cascade separation system integrating CCM-1 and CCM-2, achieving stepwise fractionation of complex ternary mixtures, including high-value pharmaceutical compounds. The CCM membrane process demonstrates significantly lower costs, carbon emissions, and energy consumption (89% reduction) compared to traditional chromatography and distillation methods. This work establishes stacking-fault programming as a powerful design principle for crystalline membranes, providing a general strategy to engineer entropy-governed transport pathways for energy-efficient and precision separations in chemical and pharmaceutical processes. Membrane based separations can be constrained by the transport pathways in a system. Here the authors design a programmed crystalline coordination metallacycle which self-assembles to form well-defined transport channels.