Unusual CO 2 Adsorption in ZIF-7: Insight from Raman Spectroscopy and Computational Studies.

Here, we use Raman spectroscopy to investigate temperature-dependent changes in the atomic-scale structure of the zeolitic imidazolate framework ZIF-7 in a CO 2 atmosphere and uncover the mechanism of maximal CO 2 adsorption at 206 K. At 301 K, the Raman spectra of ZIF-7 at various CO 2 gas pressures reveal a narrow-pore (np) to large-pore (lp) phase transition commencing at 0.1 bar as a result of adsorption of CO 2 , as evident in the appearance of Fermi resonance bands of CO 2 at 1272 and 1376 cm -1 . Moreover, the Raman inactive bending mode of CO 2 becomes active due to geometrical distortion of adsorbed CO 2 . It further splits into two peaks due to hydrogen bonding interactions between CO 2 and the benzene ring of the benzimidazole linker ZIF-7, as supported by our computational studies. In addition, the interaction between CO 2 molecules plays a key role. Upon reducing the temperature at 1 bar CO 2 gas pressure, ZIF-7 exhibits softening of the imidazole puckering mode and the Fermi resonance CO 2 band due to interactions between CO 2 and the framework through hydrogen bonding. At 206 K, substantial modification in the lattice mode and disappearance of the Raman inactive CO 2 bending mode confirm the changes in the size of the pore cavity through structural rearrangements of CO 2 .