# Metal-Organic Frameworks Characterization via Inverse Pulse Gas Chromatography
## 3 Applications of IGC for the Characterization of MOFs
### MIL-Based MOFs  
MIL-47 is a rigid MOF composed of infinite chains of VO\({}_{6}\) octahedra connected via the dicarboxylate groups of terephthalate organic (H\({}_{2}\)BDC) ligands. In this structure, a microporous framework is created having one-dimensional diamond-shaped channels with pore apertures of 7.9 A \(\times\) 12.0 A (Figure 9) [90]. The adsorption properties of MIL-47(V) were evaluated by [PERSON] et al. [60] in a combined IGC experimental and Monte Carlo simulation study. Their report showed that both linear and branched alkanes can adsorb into the pores of MIL-47(V) without any steric hindrance. The dependency of adsorption enthalpy and the Henry constants on the carbon number was significantly correlated between the simulations and experiments. n-Hexane (50.6 kJ mol\({}^{-1}\)) was adsorbed more strongly than benzene (43.4 kJ mol\({}^{-1}\)) and cyclohexane (45.2 kJ mol\({}^{-1}\)) because it has more hydrogen atoms. The adsorption energies of the studied nonpolar molecules were governed by van der Waals interactions, and the adsorption of the benzene probe molecules was further affected by Coulombic interactions. Similar work on MIL-47(V) to evaluate its physicochemical properties via IGC was done on the adsorption of substituted aromatics in addition to other heterocyclic molecules [61]. Quantitative structure\(-\)property relationships (QSPRs) between the adsorption and molecular properties were established using principal component analysis and partial least-squares regression. With respect to heterofunctionalized probes, the adsorption properties tended to associate with the polarization and/or electronegativity of the substituent. The effect of the resulting dipole moment originating from the substituent placing was observed [61]. The experimental results indicated that the molecular polarizability of the halogen series perfectly fell on the alkyl aromatics series (Figure 10), suggesting that the observed  
Figure 9: Crystal structure of MIL-47(V) with a view along the chain axis, highlighting the 1D pores system. The blue polyhedra represent the V\({}^{4+}\)O\({}^{6}\) ocahedral shared in the chain. (Red: oxygen, and black: carbon). properties were likely due to increased polarization and not to higher electronegativity of the halogen atoms. The oxygen-containing functional groups like nitro, methyl ester, and carbonyl were removed from this pattern. Thus, the expanded \(\pi\)-system enabled further delocalization of \(\pi\)-electrons from the substituent and the aromatic ring. Consequently, the electron-rich substituent attached more tightly to the MOF material than the alkyl fragments. Flexible MOFs are a special class of MOF materials that can reversibly alter their framework when guest molecules are introduced or removed. This leads to unique properties such as the breathing effect and the gate-opening phenomenon, in which pores contract or extend during adsorption [91; 92; 93]. Examples of breathing materials are MIL-53 [94] and its functional derivatives [95; 96; 97; 91]. MIL-53 is constructed from MO\({}_{4}\)(OH)\({}_{2}\) octahedra (where M can be Fe\({}^{3+}\), Cr\({}^{3+}\), or Al\({}^{3+}\)) with 1,4-terephthalate (BDC) as the linker. In this way, MIL-53, which is also known under the commercial name Basolite A100, is a crystalline material having one-dimensional diamond-shaped pores with an 8.6 A free diameter [98; 91]. [PERSON] et al. [62] utilized IGC to study the adsorption properties of amino-MIL-53 (Al). Amino-MIL-53 (Al) also has one-dimensional channels, similar to the amino-free MIL-53 analog, with a pore diameter of nearly 7.5 A. The amino groups are directly inside the pores as a result of using 2-amino terephthalatic acid as a linker rather than terephthalatic acid [99], which leads to an enhanced affinity towards CO\({}_{2}\)[95]. [PERSON] et al. found that the adsorption enthalpy and Henry's adsorption constants of iso-alkanes were considerably smaller than those of linear alkanes, highlighting the shape selectivity features of amino-MIL-53.