Influence of the Organic Ligand Functionalization on the Breathing of the Porous Iron Terephthalate Metal Organic Framework Type Material upon Hydrocarbon Adsorption

A combination of manometry, X-ray powder diffraction (XRPD), and molecular modeling has been used to show that the functionalization of the flexible MIL-53(Fe)-X materials (MIL stands for Materials of the Lavoisier Institute; X = CH3, Cl, Br, NH2) modifies the adsorption process of normal alkanes, by facilitating pores filling as compared to that of the nonmodified MIL-53(Fe) analogue. The adsorption isotherms show that these materials undergo steps at pressures specific for each guest and functional group, associated with structural transitions as corroborated by the simulated isotherms and XRPD data. With the exception of methane, a transition from the closed pore form to the large pore form occurs through one intermediate pore form upon adsorption, thus differing from the case of the nonmodified MIL-53(Fe) where two intermediate pore forms are observed. The transitions are governed by a combination of factors, including energetic, kinetic, and steric aspects with, however, transition pressures becoming increasingly lower with increasing number of C atoms on the adsorbate. This can be rationalized in terms of increased host-guest interactions. In addition, molecular modeling gives insights into the adsorption mechanism, in terms of the arrangement of the molecules within the pores, allowing the rationalization of the different amounts of n-nonane adsorbed in relation to the other molecules. Finally, the arrangement of the molecules within the pores also plays a role in the kinetics of the adsorption process, which shows that the C6 (n-hexane) molecules require more time to reach a state of static equilibrium than the C9 (n-nonane) molecules, due to their greater degree of freedom resulting from their alignment along the pores.