Unusual chain length dependent adsorption of linear and branched alkanes on UiO-66

In this work, the zero coverage adsorption properties of C-5-C-10 n- and iso-alkanes on the UiO-66, UiO-66-Me and UiO-66-NO2 metal-organic frameworks are studied by gas phase pulse chromatography. Analysis of enthalpy values, entropy values, Gibbs free energies and Henry constants reveals unusual chain length dependent adsorption behaviour of linear and branched alkanes, caused by the complex structure of the zirconium metal-organic framework UiO-66. The UiO-66 structure consists of a small, tetrahedral and large, octahedral cage. It is shown that at specific carbon chain lengths (e.g. C-6-C-7 for n-alkanes), distinctive jumps in adsorption enthalpy, entropy values and Henry constants occur. This chain length dependent effect is even more pronounced for 2- and 3-methyl alkanes and double branched alkanes. This distinctive shift in adsorption behaviour occurs at a molecular size that coincides with the cavity dimensions of the smallest, tetrahedral cage. The resulting selective adsorption arises from confinement effects and is function of both the molecular shape and size.