# Metal-Organic Frameworks Characterization via Inverse Pulse Gas Chromatography
## 3 Applications of IGC for the Characterization of MOFs
### Hkuet-1
This suggests that HKUST-1 cannot be efficiently used for separating linear alkanes from 2-methyl-branched alkanes based on their shape, but it can separate linear alkanes from 3-methyl and dimethyl-branched alkanes. [PERSON] and [PERSON] [24] applied the IGC method to evaluate the donor properties of the HKUST-1 linker BTC and the acceptor properties of the CUS sites. These experiments were executed using a GC wall-coated silica capillary column of 30 m length and 0.25 mm inner diameter, unlike the majority of IGC studies, which typically utilize a short-packed column (Table 1) [24]. The chromatogram indicates that benzene showed a longer retention time than cyclohexane (Figure 5), but a lower boiling point. This suggests the existence of specific interactions, for example, \(\pi\)-\(\pi\) interactions between the benzene ring \(\pi\)-electrons of the BTC linker in HKUST-1 and the \(\pi\)-electrons in the benzene moiety of the analyte. Benzene also showed a higher value of \(\Delta H_{ads}\) than cyclohexane, implying a stronger interaction. A thermodynamic investigation of a group of ethers with different branches, including THF (Figure 5), revealed a significant effect of the geometry of the analytes on adsorption. It was concluded that there was a combination of two factors: open metal site accessibility, which allowed interaction with oxygen free electron pairs; and an entropic effect caused by the degrees of freedom of the alkyl chains [24]. The Gutmann donor and acceptor numbers of a particular probe analyte permitted a quantifiable assessment of the Lewis acidity and basicity sites in the HKUST-1 structure. The HKUST-1 acceptor properties had a value of 1.6 for \(K_{A}\), and the surface donor properties had a value of 0.4 for \(K_{B}\). From these numbers, it was concluded that HKUST-1 can be categorized as primarily Lewis acidic, which is attributed to the prevalence of CUS [24]. Figure 4: Logarithm of the Henry constants of linear (\(\lozenge\)), 2-methylbranched (\(\square\)), 3-methyl-branched (\(\Delta\)) and dimethyl-branched alkanes (o), calculated from the measured chromatograms at 160 \({}^{\circ}\)C, versus the logarithm of their saturated vapor pressure at 160 \({}^{\circ}\)C. Reprinted with permission from [49]. Copyright (2007) Elsevier. In another study by [PERSON] et al. [59], n-butylamine thermodesorption analyses were performed using IGC to compare HKUST-1 to its analog Fe-BTC with respect to their acid-base properties. Samples of the proposed materials were saturated with n-butylamine to quantify both sites (acidic and basic) from the desorption curves of n-butylamine using the n-butylamine thermodesorption technique. The IGC measurements yielded 0.81 and 0.60 as the acidity constants (\(K_{A}\)) and 0.09 and 0.28 as the basicity constants (\(K_{B}\)) for Fe-BTC and HKUST-1, respectively. Moreover, the n-butylamine thermodesorption study indicated a total acidity of 2.06 mmol g\({}^{-1}\) for Fe-BTC and 0.49 mmol g\({}^{-1}\) for HKUST-1. This behavior may be attributed to the superior polarizing rule of Fe\({}^{3+}\) compared to Cu\({}^{2+}\) due to the higher charge. As a result, both MOFs, HKUST-1 and Fe-BTC, were verified as being predominantly Lewis acidic. Specific interaction contribution values of 4.1 and 3.2 kJ mol\({}^{-1}\) at 200 \({}^{\circ}\)C were acquired for the benzene moiety in HKUST-1 and Fe-BTC, respectively. Furthermore, the contributions of the specific interactions regarding THF at 200 \({}^{\circ}\)C were found to be 14.5 kJ mol\({}^{-1}\) for HKUST-1 and 17.9 kJ mol\({}^{-1}\) for Fe-BTC. HKUST-1 has a mildly basic surface character, whereas Fe-BTC has a more prominent acidic character, resulting in more benzene (acidic)  
Figure 5: (**I**) Chromatographic separation of a mixture of methane (for the determination of the dead time in all chromatograms), cyclohexane, and benzene (a) pure benzene (b), and of pure cyclohexane (c).