# Metal-Organic Frameworks Characterization via Inverse Pulse Gas Chromatography
## 3 Applications of IGC for the Characterization of MOFs
### Hkuet-1
Experimental setup: column: HKUST-1@SiO\({}_{2}\) (30 m \(\times\) 0.25 mm \(\times\) 0.15 \(\upmu\)m); carrier: He, flow: 4.37 mL min\({}^{-1}\), 76.2 cm s\({}^{-1}\); oven: 473.15 K, isotherm; injector: split, 11.7:1 at 250 \({}^{\circ}\)C; detector: FID at 250 \({}^{\circ}\)C. (**II**) Chromatographic separation of a mixture of methane, disisopropyl ether, diethyl ether, tetrahydrofuran, and di-n-propyl ether (black line) (a). Chromatograms of pure substances except methane. (b) Di-n-propyl ether. (c) Tetrahydrofuran. (d) Diethyl ether. (e) Diisopropyl ether. Experimental setup: column: HKUST-1@SiO\({}_{2}\) (30 m \(\times\) 0.25 mm \(\times\) 0.15 \(\upmu\)m); carrier: He, flow: 4.14 mL min\({}^{-1}\), 72.5 cm s\({}^{-1}\); oven: 523.15 K, isotherm; injector: split, 12:1 at 250 \({}^{\circ}\)C; detector: FID at 250 \({}^{\circ}\)C. (**—**impurity) [24]. Published by The Royal Society of Chemistry. interactions on the HKUST-1 material surface and more THF (basic) interactions on the Fe-BTC material surface. [PERSON] et al. [57] measured the \(\Delta H_{ads}\) on both Fe-BTC and HKUST-1 for n-alkanes, propane, propylene, benzene, toluene, and xylene. Dispersive interactions were the dominant adsorption mechanism for n-alkanes, and \(\Delta H_{ads}\) was found to increase with the number of carbon atoms. The electrostatic interaction of propylene and aromatics through its dipole and/or quadrupole moments via the guest-host \(\pi\)-\(\pi\) coupling led to a higher \(\Delta H_{ads}\) value for propylene than propane. Higher \(\Delta H_{ads}\) values were found for HKUST-1 than Fe-BTC, due to the ordered crystal structure and molecular ease of access to the porous structure of HKUST-1. In the case Fe-BTC, which has does not have a well-defined crystal structure, local deformations of its flexible organic ligand may account for the restricted accessibility of pocket-type pores, where small probe molecules were restrained in a small volume and thus contributed more to the adsorption forces of dispersive interactions. [PERSON] et al. [58] compared HKUST-1 and Fe-BTC in more depth by comparing the \(\Delta H_{ads}\) obtained from IGC and the \(\Delta H_{imm}\) (the enthalpy of immersion) obtained from immersion calorimetry. For the three studied adsorbates (n-heptane (HEP), methylcyclohexane (MCH), and toluene (TOL)), the interaction strength was higher for Fe-BTC than HKUST-1. This was a result of the better accessibility to the active sites in Fe-BTC, as it has a lower fraction of microporous surface area. Although Fe-BTC does not have a specified crystalline structure, the open metal sites can favor electrostatic interactions with the adsorbates, leading to an increase in the overall affinity of the interaction. The \(\Delta H_{imm}\) followed the same order as the enthalpy of adsorption, i.e., TOL > HEP > MCH, and was consistent with the \(\Delta H_{ads}\) measurements for HKUST-1 (Figure 6). However, unlike \(\Delta H_{ads}\), the \(\Delta H_{imm}\) values for Fe-BTC were in the inverse order with respect to the smallest size of the adsorbates: TOL (4.012) < HEP (4.014) < MCH (4.982) (Figure 7) [89]. This suggests that these electrostatic interactions are less crucial and the accessibility of the adsorbates to the pores is more relevant. The higher cross-section area of the adsorbent n-alkanes on the surface of both MOFs makes the dispersive interaction of the surface energy produced more prominent for HKUST-1 than for Fe-BTC. Additionally, the highest specificity of the interaction was also reported on HKUST-1, which is contrary to what was expected, as indicated by the specific interaction parameter (I\({}_{\text{Sp}}\)), which reflects on the surface acid-base nature. Figure 6: Enthalpy of adsorption for Fe-BTC and HKUST-1 for the three adsorbates. Reprinted with permission from [58]. Copyright (2015) Elsevier. Another research group [38] utilized IGC as a prescreening tool. They effectively examined the affinity of MOFs for targeted probes in the gas phase.