# Metal-Organic Frameworks Characterization via Inverse Pulse Gas Chromatography
## 3 Applications of IGC for the Characterization of MOFs
### Isoreticular Metal-Organic Frameworks (IRMOF)  
The IRMOF structure consists of Zn\({}_{4}\)O tetranuclear clusters linked by various rigid linear ditopic linkers, giving the obtained frameworks different pore aperture sizes and forming a cubic structure with connected channels (Figure 11) [101]. The diffusion and adsorption of xylene isomers as well as ethylbenzene (EB) on IRMOF-1 (MOF-5) (Zn\({}_{4}\)O(BDC)\({}_{3}\))\({}_{\text{n}}\)-mDMF) [102] and MOF-monoclinic ([Zn\({}_{3}\)(BDC)\({}_{3}\)(H\({}_{2}\)O)\({}_{3}\)(DMF)\({}_{4}\)]\({}_{\text{n}}\)) [103] were studied using IGC, breakthrough adsorption, and static vapor-phase adsorption [66]. Figure 11: Crystal structure of IRMOF-1 (MOF-5), with the formula Zn\({}_{4}\)O(BDC)\({}_{3}\), where BDC = 1,4-benzodicarboxylate, and blue tetrahedrons represent the Zn\({}_{4}\)O(-CO\({}_{2}\))\({}_{6}\) SBU from coordination of BDC to the zinc center. (Oxygen; red, and black: carbon). IRMOF-1 and MOF-monoclinic have been shown to have different selectivity for the separation of the studied probes. On IRMOF-1, EB was found to be eluted first; however, xylene isomers were eluted similarly. The equilibrium constants controlled the elution order of the xylenes and EB on IRMOF-1. On the other hand, MOF-monoclinic exhibited a preferable adsorption for the p-xylene isomer, which was believed to result from some hindrance due to pore size that governed the diffusion of the isomers in the MOF micropores. The McReynolds constants of IRMOF-1 showed that the nonpolar nature of that material was advantageous for the thermodynamic separation of the xylene and EB isomers. The intermediate polarity of MOF-monoclinic was attributed to the higher accessibility of its SBUs to isomers, and its pore shape was found to play a more significant role in separation than in IRMOF-1. The properties of IRMOF-1 can change markedly based on its synthesis, activation method, and contact with atmospheric humidity [104; 105; 106; 107]. Different IRMOF-1 samples prepared with different synthetic schemes were used to examine the impact of structural decomposition on adsorption behavior [67]. The infinite-dilution assumption in the adsorption isotherm linear segment was confirmed as valid through the independence of the chromatographic peak retention times for nonpolar n-alkanes with respect to concentration. On the other hand, the compounds having considerable hydrogen-bond basicity groups, such as diethyl ether, acetone, ethyl acetates and THF, were observed to vary from this ideal behavior. The independence of the amended retention volume from the flow rate showed that the mass transfer did not affect the retention time. This implies that the amended retention volumes and all related calculated values indicate equilibrium behavior. The calculated values for enthalpy and entropy were more significant than those predicted by Monte Carlo simulations [108]. The presence of a nonporous phase in IRMOF-1 appeared to result in active sites with higher energies than those existing in standard IRMOF-1. The obtained linear relationships amongst the carbon number, the enthalpy of adsorption, and the free energy of adsorption were attributed to the additive nature of dispersive interactions. The dispersive component of the surface energy (\(\gamma_{S}^{D}\)) values for IRMOF-1 samples was calculated to be between 48 and 91 mJ m\({}^{-2}\), which is noticeably lower than the values for zeolites and activated carbons, which have been frequently recorded in the range of 200-500 mJ m\({}^{-2}\)[51; 109; 110; 111]. This indicated a comparatively inactive surface for IRMOF-1, which was believed to be due to its large cages. Most of the interactions between the n-alkanes and the organic part of the framework were due to steric constraints near the metallic corners and the size of the n-alkanes. The deformation polarizability was much better suited to measuring negative values for free adsorption energy than for vapor pressure (Figure 12).