Lattice expansion and ligand twist during CO2 adsorption in flexible Cu bipyridine metal–organic frameworks

Abstract

Flexible metal–organic frameworks (MOFs) can show exceptional selectivity and capacity for adsorption of CO₂. The incorporation of CO₂ into flexible MOFs that have Cu²⁺ coordination centers and organic pillar ligands is accompanied by a distortion of the framework lattice arising from chemical interactions between these components and CO₂ molecules. CO₂ adsorption yields a reproducible lattice expansion that is enabled by the rotation of the pillar ligands. The structures of Cu₂(pzdc)₂(bpy) and Cu₂(pzdc)₂(bpe), CPL-2 and CPL-5, were evaluated using in situ synchrotron X-ray powder diffraction at room temperature at CO₂ gas pressures up to 50 atm. The structural parameters exhibit hysteresis between pressurization and depressurization. The pore volume within CPL-2 and CPL-5 increases at elevated CO₂ pressure due to a combination of the pillar ligand rotation and the overall expansion of the lattice. Volumetric CO₂ adsorption measurements up to 50 atm reveal adsorption behavior consistent with the structural results, including a rapid uptake of CO₂ at low pressure, saturation above 20 atm, and hysteresis evident as a retention of CO₂ during depressurization. A significantly greater CO₂ uptake is observed in CPL-5 in comparison with predictions based on CO₂ pressure-induced expansion of the pore volume available for adsorption, indicating that the flexibility of the CPL structures is a key factor in enhancing adsorption capacity.