Post-synthetic structural processing in a metal-organic framework material as a mechanism for exceptional CO₂/N₂ selectivity

Here we report the synthesis and ceramic-like processing of a new metal-organic framework (MOF) material, [Cu(bcppm)H₂O], that shows exceptionally selective separation for CO₂ over N₂ (ideal adsorbed solution theory, S_ads = 590). [Cu(bcppm)H ₂O]·xS was synthesized in 82% yield by reaction of Cu(NO ₃)₂·2.5H₂O with the link bis(4-(4-carboxyphenyl)-1H-pyrazolyl)methane (H₂bcppm) and shown to have a two-dimensional 4⁴-connected structure with an eclipsed arrangement of the layers. Activation of [Cu(bcppm)H₂O] generates a pore-constricted version of the material through concomitant trellis-type pore narrowing (b-axis expansion and c-axis contraction) and a 2D-to-3D transformation (a-axis contraction) to give the adsorbing form, [Cu(bcppm)H ₂O]-ac. The pore contraction process and 2D-to-3D transformation were probed by single-crystal and powder X-ray diffraction experiments. The 3D network and shorter hydrogen-bonding contacts do not allow [Cu(bcppm)H ₂O]-ac to expand under gas loading across the pressure ranges examined or following re-solvation. This exceptional separation performance is associated with a moderate adsorption enthalpy and therefore an expected low energy cost for regeneration.