# Metal-Organic Frameworks Characterization via Inverse Pulse Gas Chromatography
## 3 Applications of IGC for the Characterization of MOFs
### UiO-66 Mof
at a N\({}_{2}\) flow rate of 1 mL min\({}^{-1}\) for the separation of: (**A**) hexane, heptane, octane, and nonane using a temperature program of 60 \({}^{\circ}\)C for 1 min, and then 40 \({}^{\circ}\)C min\({}^{-1}\) to 250 \({}^{\circ}\)C; (**B**) hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, and 2,2-dimethylbutane using a temperature program of 100 \({}^{\circ}\)C for 1 min, and then 10 \({}^{\circ}\)C min\({}^{-1}\) to 160 \({}^{\circ}\)C; (**C**) 1,3,5-trimethylbenzene, n-propylbenzene, and iso-propylbenzene using a temperature program of 140 \({}^{\circ}\)C for 1 min, and then 10 \({}^{\circ}\)C min\({}^{-1}\) to 180 \({}^{\circ}\)C; (**D**) m-, p-, o-xylene and ethylbenzene using a temperature program of 140 \({}^{\circ}\)C for 1 min, and then 10 \({}^{\circ}\)C min\({}^{-1}\) to 180 \({}^{\circ}\)C. Reprinted with permission from [73]. Copyright (2012) Elsevier. demonstrating that the bulkier cyclic molecule loses less freedom in the pores of UiO-66 than the linear n-hexane molecule. In comparison with linear alkanes, the cyclic alkane has both entropic and enthalpic advantages. The composition of the UiO-66 pore system may explain this. In tetrahedral cages of UiO-66, cyclohexane can maintain a relatively high degree of rotational freedom. The enhanced confinement in these small cages results in a great deal of energetic interaction. Seeking a deeper understanding of the reverse shape-selectivity performance of UiO-66, the same research group employed IGC at zero coverage for the separation of more than 70 molecules, including alkanes, alkenes, aromatics, and cyclic types on UiO-66 and its derived analogs, such as UiO-66-NO\({}_{2}\), UiO-66-Me, and UiO-66-Me\({}_{2}\)[75]. High adsorption selectivity indicators were found for cycloalkanes in comparison to n-alkanes, such as the ratio of Henry constants (up to 14). Additional functional groups within the linkers in the framework, such as methyl and nitro groups, markedly affected the adsorption properties, primarily by decreasing the effective pore aperture size. Although improved selectivity for UiO-66-Me was observed, the selectivity decreased when an additional CH\({}_{3}\) group was present in UiO-66-Me\({}_{2}\). This additional CH\({}_{3}\) group enabled confinement factors within the pores, and thus the material's adsorption properties, to be tuned. Selective adsorption was observed as a result of interactions in the smallest octahedral cages. However, intense confinement in the small tetrahedral cage led to stereoselective separation of cis/trans alkenes and disubstituted cycloalkanes. Monte Carlo simulations for pristine UiO-66 MOF and an analysis of the molecular positioning of adsorbates inside the framework supported the experimental results. In addition, the simulations illustrated contributions by entropic factors to the observed selectivity of adsorption. The effect of chain length on the adsorption of branched and linear alkanes on UiO-66 and its functionalized analogs has also been explored [76]. Characteristic enhancement in Henry constants, adsorption enthalpy, and adsorption entropy occurred at particular carbon chain lengths of n-alkanes for hexane and heptane In case of 2- and 3-methyl alkanes and double branched alkanes, the effect of chain length was even more pronounced. This recognizable change in adsorption took place at a molecular size that matched the pore size of the smallest tetrahedral cages. The resultant selective adsorption emerged from the confinement properties and was a function of the molecule's size and shape.