Carboxylic Group Rotation and Lattice Expansion in a Co₂(Pyrazine-2,3-Dicarboxylate)₂(4,4′-Bipyridine) Porous Coordination Polymer Induced by CO₂ Adsorption at Ambient Temperature

A Co2(pzdc)2(bpy)(H2O)m (pzdc: pyrazine-2,3dicarboxylate; bpy: 4,4′-bipyridine) porous coordination polymer (PCP) was studied for CO2 uptake and concomitant structural changes at ambient temperature. Extended structural characterization included evaluation of lattice parameter changes upon CO2 adsorption and in situ synchrotron X-ray powder diffraction data. The PCP effective pore size increased by ∼2% with gas uptake over the pressure range of 1–50 atm, allowing the adsorption capacity to double. Furthermore, the hysteretic behaviors seen during CO2 adsorption at moderate pressures are commensurate with the structural changes from synchrotron data. The adsorption and hysteresis occur with rotation of the linking carboxylate groups, and yet only minor changes in unit cell volume (ΔV ≈ 6 Å3) are observed. This contrasts the findings for [Cu2(pzdc)2(bpy)]n, where a combination of pillar bpy rotations and significant lattice expansion (ΔV ≈ 68 Å3) takes place upon hysteretic adsorption of CO2. In situ high-temperature X-ray diffraction revealed that the Co(II)-based material has good thermal stability up to ca. 200 °C. Finally, the CO2 uptake also appears to be at a physisorption level, with adsorbent–adsorbate interactions that are ca. 30% stronger than what has been reported for CO2 adsorption onto [Cu2(pzdc)2(bpy)]n.