Selective CO2 Adsorption on Metal-Organic Frameworks Based on Trinuclear Cu3‑Pyrazolato Complexes: An Experimental and Computational Study

Two related metal-organic frameworks (MOFs) based on trinuclear copper-pyrazolato structural building units (SBUs) connected by bipyridine linkers have been prepared and studied. A small chemical modification to the supporting pyrazolato ligands of a [Cu3]-SBU results in two vastly different (three-fold vs eight-fold interpenetrated) MOF structures. The gas sorption properties of the two MOFs have been determined experimentally in the 0 to 10 atm pressure range. Both MOFs sorb CO2 selectively over N2 and H2, showing hysteretic sorption–desorption profiles. The hysteretic behavior is attributed to the structural flexibility of the lattices, probed by powder X-ray diffraction before and after CO2 sorption. As the metal centers and hydroxy groups of the MOF surfaces are sterically hindered, the sorbed dipolar CO2 molecules are attached with low sorption enthalpy to the organic groups, as determined by theoretical calculations.