Mobility and separation of linear and branched C₅ alkanes in UiO-66 (Zr) probed by ²H NMR and MD simulations

The UiO-66 (Zr) metal-organic framework (MOF) is of notable interest due to its facile synthesis, robustness under a wide range of chemical and physical conditions and its capability to separate industrially relevant hydrocarbons mixtures. However, the knowledge of the molecular mechanisms behind these process remains limited. Here, we present a combined experimental (H-2 NMR) and computational study of the molecular mobility, transport and adsorption of C-5 alkanes isomers in a dehydroxylated UiO-66 (Zr) MOF. We show that the tetrahedral cages of the MOF are the preferred adsorption location for both n-pentane and isopentane. In a binary mixture of the isomers, isopentane interacts more strongly with the material leading it to occupy more of the tetrahedral cages than n-pentane, resulting in an isopentane/n-pentane adsorption selectivity of alpha(ads) = 2 (at 373 K). At the same time, the microscopic diffusivity for n-pentane, D-n (E-n = 18 kJ mol(-1)), is significantly lower than for isopentane, D-iso (E-iso = 28 kJ mol(-1)), which results in a high separation selectivity for a n-pentane/isopentane mixture of alpha approximate to 13 (at 300 K). This shows that the UiO-66 MOF is indeed a promising active material for use in light hydrocarbon separation processes.