Understanding the preferential adsorption of CO₂ over N ₂ in a flexible metal-organic framework

The unusual uptake behavior and preferential adsorption of CO₂ over N₂ are investigated in a flexible metal-organic framework system, Zn₂(bdc)₂(bpee), where bpdc = 4,4′-biphenyl dicarboxylate and bpee = 1,2-bis(4-pyridyl)ethylene, using Raman and IR spectroscopy. The results indicate that the interaction of CO₂ with the framework induces a twisting of one of its ligands, which is possible because of the type of connectivity of the carboxylate end group of the ligand to the metal center and the specific interaction of CO₂ with the framework. The flexibility of the bpee pillars allows the structure to respond to the twisting, fostering the adsorption of more CO₂. DFT calculations support the qualitative picture derived from the experimental analysis. The adsorption sites at higher loading have been identified using a modified van der Waals- Density Functional Theory method, showing that the more energetically favorable positions for the CO₂ molecules are closer to the C=C bond of the bpee and the C-C bond of the bpdc ligands instead of the benzene and pyridine rings of these ligands. These findings are consistent with changes observed using Raman spectroscopy, which is useful for detecting both specific guest-host interactions and structural changes in the framework.