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

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