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)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 4(4)-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.