# Metal-Organic Frameworks Characterization via Inverse Pulse Gas Chromatography
## 3 Applications of IGC for the Characterization of MOFs
### Other MOFs  
Generally, the adsorption properties of MOFs can be modified via chemical functionalization of the MOF pores. [PERSON] et al. [77] examined SO\({}_{2}\)-DUT-5 and SO\({}_{2}\)-(Al(OH)(bpdc) MOF (bpdc = 4,4\({}^{\prime}\)-biphenyl dicarboxylate)), which is a sulfone-functionalized DUT-5 MOF [126]. The compensation plot (Figure 17) shows the relationship between adsorption entropy and adsorption enthalpy, where the logarithm of pre-exponential factors is presented against the adsorption enthalpies. Generally, a linear connection was witnessed for a group of linear alkanes. The more strongly the probed molecule was adsorbed, the less freedom it had, which was identified as the compensation effect [2; 127; 128]. This proportional linear connection amongst linear alkanes was perceived for both pristine and SO\({}_{2}\)-functionalized DUT-5 MOFs. The slopes of the compensation lines, which showed an increase in adsorption entropy with respect to the adsorption enthalpy, were similar for both MOFs, which indicated that they had dissimilar adsorption enthalpies for identical molecules. Thus, the loss of freedom for a given interaction was identical in both MOFs. Thus, the shift in pre-exponential factors between the two MOFs resulted from their different adsorption sites and not from their different entropies of adsorption. Based on that, it was determined that the SO\({}_{2}\) functionalization offered better adsorption sites for n-alkane molecules that showed a 4-5 kJ mol\({}^{-1}\) higher adsorption enthalpy compared to pristine DUT-5. SO\({}_{2}\)-DUT-5 demonstrated shape-selective activity in the adsorption of branched and linear alkanes at low coverage because of the reduction in pore size. In contrast, due its larger pore size, pristine DUT-5 was not shape selective. The ratio of Henry constants of linear and branched alkanes (i.e., separation factors) for all isoalkane/n-alkane pair separations on SO\({}_{2}\)-DUT-5 were larger than 1.7. DUT-5, in contrast, did not show a marked difference at all for the branched molecules [77]. Alkanes and alkenes adsorbed on SO\({}_{2}\)-DUT-5 to a similar degree. Although DUT-5 did not have a specific interaction with molecules possessing unsaturated bonds, SO\({}_{2}\)-DUT-5 demonstrated an improved interaction with aromatic molecules, although this interaction is not very strong and led to only moderate low-pressure separation. Breakthrough studies have shown that at higher pressures and loading, mixtures of aromatics with alkanes or alkenes can be separated, with both pristine and SO\({}_{2}\)-functionalized DUT-5 MOFs [77]. This distinction is a result of the molecular packing effects rather than specific interactions at a high level of pore filling. Such an effect has already been described several times with other MOFs, for example MIL-47 and MIL-53 [129; 130], where separation occurred above a critical level of pore filling, in which the shift occurs from a single alignment of molecules in the one-dimensional pores to a double alignment of molecules. In conclusion, SO\({}_{2}\)-DUT-5 showed a slight tendency to adsorb \(\pi\)-bond-containing molecules oversaturated molecules, as indicated by the marginally higher Henry constants for unsaturated compounds. Steric effects and specific adsorption played crucial roles and were correlated to the separation selectivity of alkanes/ aromatics, also leading to shape selectivity in isoalkane adsorption. Van der Waals interactions dominated the adsorption properties of DUT-5. Consequently, aromatics, cyclo-alkanes, and alkenes exhibited lower values for both Henry constants and adsorption enthalpies than n-alkanes [77]. Due to its ultramicroporous structure with respect to its molecular dimensions, Cu(Qc)\({}_{2}\) [Cu(quinoline-5-carboxylate)\({}_{2}\)] is a rather attractive MOF [131]. IGC studies were performed on Cu(Qc)\({}_{2}\) to determine its surface free energy of adsorption in order to evaluate the affinity between the O\({}_{2}\)/N\({}_{2}\) mixture [78].