Designing Adsorptive Gating via Linker Side-Chain Functionalization in a Honeycomb-MOF

Metal-organic frameworks (MOFs) combine high guest-accessible porosity with high chemical versatility, which predestines tailor-making porous materials to overcome challenges in efficient next-generation gas separation processes. Adsorptive gating is an interesting material feature that enhances sorption selectivity, while its rational design is still beyond our knowledge. Herein, we report on a model system to explore the key factors of induced gating controlled by the implementation of conformationally flexible side chains at the linkers of a honeycomb-like, structurally rigid MOF, namely, Zn₂(2,5-difunctionalized-1,4-benzendicarboxylate)₂(4,4′-bipyridine). The sorption and separation properties of C₂H₂and CO₂were selected as the study case, and ideal adsorbed solution theory (IAST) selectivities, isosteric enthalpies of adsorption, Henry constants, breakthrough experiments, and simulations of dynamic properties were correlated with the linker functionalization. Two dominating factors that determine gating properties are identified: polarizability and sorption site accessibility. These factors are accessible through linker functionalization, and we show that adsorption strength differences can be influenced linearly within 1 order of magnitude, while breakthrough experiments show a selectivity increase toward C₂H₂, ranging from 4.1 to 10.9. The results suggest transfer to other MOFs toward extrinsic gating for sophisticated modulation of their sorption selectivity.