Stay Together or Split Up? Investigating Selective Adsorption of Carbon Dioxide and Acetylene in Anion-Pillared Microporous MOFs

A major challenge in gas separation is designing porous materials with energy-efficient guest selectivity, primarily because molecular-level mechanisms underpinning adsorption and selectivity remain unclear from both the thermodynamic and kinetic perspectives. This work examines the selective adsorption of acetylene versus carbon dioxide in anion-pillared microporous metal-organic frameworks using solid-state nuclear magnetic resonance spectroscopy, quantum mechanical cluster models integrated with density functional theory calculations, and molecular dynamics simulations. The isostructural SIFSIX-1-Cu and SIFSIX-3- Cu MOFs share a pcu topology, are composed of the same Cu(II) metal node, and have an identical inorganic pillared ligand (SiF62-), but incorporate the different 4,4' bipyridine and pyrazine organic linkers. The variation in pore size and chemical composition between SIFSIX-1-Cu and SIFSIX- 3-Cu gives rise to distinct host-guest interactions. Multinuclear in situ variable temperature solid- state nuclear magnetic resonance experiments targeting single-component and binary mixtures of 13CO2 and C2D2, accompanied by ab initio calculations, reveal that SIFSIX-1-Cu is strongly selective toward acetylene adsorption while SIFSIX-3-Cu efficiently adsorbs both acetylene and carbon dioxide. The adsorption locations and guest dynamics of acetylene and carbon dioxide were determined from experimental and computational data. These findings provide important guidance for rational design of selective adsorption materials, including metal-organic frameworks.