# Metal-Organic Frameworks Characterization via Inverse Pulse Gas Chromatography
## 3 Applications of IGC for the Characterization of MOFs
### UiO-66 Mof  
UiO-66 MOF (UiO = Universitetet i Oslo) is a zirconium-based MOF composed of hexamers of eight coordinated ZrO\({}_{6}\)(OH)\({}_{2}\) polyhedra and BDC linkers [124]. UiO-66 has  
Figure 14: Logarithm of Henry constants as a function of the logarithm of the vapor pressure at 423 K for linear (\(\blacksquare\)), monomethylbranched (\(\Diamond\)), dimethylbranched (\(\bullet\)), and trimethylbranched (\(\Delta\)) alkanes on ZIF-68. Reprinted with permission from [72]. Copyright (2015) American Chemical Society. an **fcu** topology, with a rigid cubic three-dimensional structure comprised of octahedral and tetrahedral cages with diameters of 11 and 8 A, respectively. The cages are accessed solely via the triangular pore windows of approximately 5-7 A (Figure 15). UiO-66 is considered one of the first MOFs that showed reverse shape-selectivity separation properties [125]. In a regular shape-selective material, a molecule having a more compact size with respect to its molecular weight will adsorb preferentially. Probes with higher boiling points and comparable chemical compositions will selectively adsorb on nonselective surfaces over lower-boiling molecules. [PERSON] et al. [73] demonstrated that a UiO-66-coated capillary column showed both a molecular sieving effect and a reverse shape selectivity. This capillary column favored adsorption of branched alkanes over their linear analogs, thus demonstrating reverse shape selectivity. This column also showed that branched hexane isomers had higher adsorption enthalpies than hexane, indicating stronger interaction with branched alkanes. However, the retention time of the bulkier 1,3,5-trimethylbenzene was smaller than that of n-propyl- and iso-propylbenzene, exhibiting a molecular sieving effect (Figure 16). Table 2 summarizes the author's attempt to investigate the effect of pore-window aperture and size on adsorption and separation properties of alkane molecules by comparing several MOFs with UiO-66. Figure 15: Crystal structure of UiO-66 MOF, where blue polyhedra represent the 12-coordinated Zr-based metal clusters that are interconnected by BDC linkers with **fcu** topology. (Red: oxygen, and carbon: grey; pink sphere represents the tetrahedral cage and brown sphere represents the octahedral cage). [PERSON] et al. [74] investigated the abovementioned effect of UiO-66 on cycloalkanes. The adsorption and separation of n-hexane and cyclohexane on UiO-66 MOF was studied via IGC. Cyclohexane, which is bulkier and has a higher boiling point than n-hexane, was retained more than n-hexane. The adsorption enthalpy of cyclohexane (\(-60\) kJ mol\({}^{-1}\)) was considerably greater than that of n-hexane (\(-53\) kJ mol\({}^{-1}\)), despite the slightly lower molecular weight. Furthermore, the adsorption entropy of n-hexane was more negative, \begin{table}
\begin{tabular}{c c c c} \hline \hline
\begin{tabular}{c} Accessible to n-alkanes or iso-alkanes \\ No size or shape selectivity \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The effect of pore size and pore window size on the adsorption and separation of alkane molecules on several MOFs. Figure 16: Gas chromatograms on the UiO-66 coated capillary column (20 m long \(\times\) 0.25 mm i.d.)