Pillar iodination in functional boron cage hybrid supramolecular frameworks for high performance separation of light hydrocarbons

Abstract

A novel ultramicroporous boron cage pillared supramolecular metal–organic framework BSF-2 comprising functional cluster undecahydro-iodo-closo-dodecaborate [B₁₂H₁₁I]²⁻ pillars is designed and prepared by supramolecular assembly of CuB₁₂H₁₁I and 1,2-bis(4-pyridyl)acetylene (bpa). The structure and properties of BSF-2 are fully characterized by single crystal X-ray analysis, powder X-ray analysis, IR spectroscopy, EPR spectroscopy and TGA. The gas adsorption study of BSF-2 indicates substantially enhanced isosteric heats of adsorption (Q_st) for C₃H₈, C₂H₆, C₂H₂ and CO₂ but an almost identical Q_st for CH₄ as a result of adding an iodo group to the dodecaborate pillars. Moreover, the C₃H₈ capacity on BSF-2 is increased while the CH₄ uptake capacity is significantly reduced compared to parent BSF-1, leading to excellent separation selectivities for C₃H₈/CH₄, C₂H₆/CH₄, C₂H₂/CH₄ and CO₂/CH₄. The gas adsorption behavior of C₃H₈, C₂H₂ and CH₄ in BSF-2 is further revealed by modeling studies, indicating an enhanced affinity of BSF-2 towards C₃H₈ and C₂H₂ but a weak affinity towards CH₄, consistent with the experimental results. In addition, the practical separation of a ternary C₃H₈/C₂H₆/CH₄ gas mixture is investigated by recyclable dynamic breakthrough experiments. This work sheds light on designing high performance microporous materials for gas separation and provides new insights into understanding hydrocarbon adsorption/separation in supramolecular metal–organic frameworks.