# Metal-Organic Frameworks Characterization via Inverse Pulse Gas Chromatography
## 3 Applications of IGC for the Characterization of MOFs
### Zeolitic Imidazole Frameworks (ZIFs)  
ZIFs have isomorphic topologically with zeolites. They are composed of tetrahedrally linked transition metal ions like zinc, cobalt, etc., that are coordinated by imidazolate-based ligands. The metal-imidazole-metal angle is 145\({}^{\circ}\), which is analogous to the Si-O-Si angle present in zeolites. ZIF-8 is one of the most utilized members of this family, consisting of 2-methylimidazolole ligands connected with Zn\({}^{2+}\) ions to form eight sodalite (**sod**) cages that  
Figure 12: Correlation between a VOC deformation polarizability and the free energy of adsorption (retention volume) at 200 \({}^{\circ}\)C as measured by IGC shown for (**a**) IRMOF-1 sample 1; **(b**) IRMOF-1 sample 2, and (c) IRMOF-1 sample 3. In these plots, the solid circles represent the series of n-alkanes, and the triangles represent other measured analytes. Reprinted with permission from [67]. Copyright (2010) American Chemical Society. construct an overall cubic structure (Figure 13). ZIF-8 has been commercialized under the commercial name Basolite Z1200. [PERSON] et al. [69] applied IGC to investigate the adsorption behavior of many gases and VOCs on ZIF-8. Such highly halogenated compounds, branched alkanes, and aromatic compounds eluted rapidly with small tailing peaks and retention times, demonstrating a molecular sieving behavior that inhibited them from accessing the narrow windows of the MOF structure. On the other hand, the retention behavior of many of the studied compounds, especially n-alkanes, implied an unexpected flexibility of the ZIF-8 structure, even with n-alkanes with kinetic diameters similar or larger than 0.43 nm [112], significantly larger than the recorded ZIF-8 pore window size of 0.34 nm [113; 114]. Owing to the mainly organic character of the reachable binding sites, ZIF-8 exhibited a lower surface energy than several other MOFs and microporous materials [113]. H-bond interactions were considered a dominant factor, as in the case of acids, amines, and alcohols that bind too strongly to be adequately studied in this method. Nevertheless, the adsorption of other probes, such as ketones, tends to be far less effective owing to the absence of such strong acid sites. Additionally, electrostatic interactions were essential, as the definite component of the adsorption free energy was observed to be significantly linked to the dipole moments of the adsorbates. In addition, a major increase in the adsorption of alkenes over alkanes was found to originate from the polarizability of the \(\pi\)-electrons, in addition to the interactions resulting from the induced dipole with the ZIF-8 structure. The molecular sieving behavior and adsorption properties of ZIF-8 were also highlighted by [PERSON] et al. [71; 115]. They examined the impact of integrating various percentages of ZIF-8 into a BuMA-co-EDMA (butyl methacrylate-co-ethylene dimethacrylate) monolithic capillary column. The adsorption enthalpy values of both the neat monolith and the ZIF-8@(BuMA-co-EDMA) composite monolith did not reach the limit of physical and chemical adsorption (62.8 kJ mol\({}^{-1}\)) [116], suggesting the presence of only physical alkane interactions. The proximity to the heat of liquefaction for n-alkanes [117] and its change in enthalpy of adsorption showed that secondary weak intermolecular forces dominate the interactions through the monolith-n-alkane interface [118]. The addition of ZIF-8 particles to the monolithic organic material increased the dispersive element of the surface energy  
Figure 13: Crystal structure of ZIF-8 with the underlying **sod** topology, where blue tetrahedra indicate the ZnN 4 SBUs. (Blue: nitrogen, and black: carbon). (\(\gamma_{5}^{D}\)) values, revealing a more energetic surface. A Lewis basic character with \(K_{\text{B}}\)/\(K_{A}>1\) (but with a higher basicity for the neat monolith) was reported based on the results of the \(K_{\text{B}}\)/\(K_{A}\) ratios for both materials and suggests an acidic character for ZIF-8 [71].