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, Sads = 590). [Cu­(bcppm)­H2O]·xS was synthesized in 82% yield by reaction of Cu­(NO3)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)­H2O]-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)­H2O]-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.