Reverse Hierarchy of Alkane Adsorption in Metal-Organic Frameworks (MOFs) Revealed by Immersion Calorimetry

Immersion calorimetry into liquids of different dimensions is a powerful tool to learn about the pore size and shape in nanoporous solids. In general, in the absence of specific interactions with the solid surface, the accessibility of the liquid probe molecule to the inner porosity, and the associated enthalpy value decreases with an increase in its kinetic diameter (bulkier molecules have lower accessibility and packing density). Although this is true for majority of solids (e.g. activated carbons and zeolites), this study anticipates that this is not straightforward in the specific case of MOFs. The evaluation of different hydrocarbons and their derivatives reveals the presence of reverse selectivity for C6 isomers (2,2-dimethylbutane>2-methylpentane>n-hexane) in UiO-66 and HKUST-1, while size exclusion effects take place in ZIF-8. The immersion calorimetric findings have been compared with vapor adsorption isotherms and computational studies. Monte Carlo simulations suggest that the reverse selectivit...