Surface thermodynamics and Lewis acid-base properties of metal-organic framework Crystals by Inverse gas chromatography at infinite dilution.

In this study, the surface thermodynamic properties and more particularly, the dispersive component γsd of the surface energy of crystals of a Zr-based MOF, UiO-66 (Zr6O4(OH)4(BDC)6; BDC = benzene 1,4-dicarboxylic acid), the specific interactions, and their acid-base constants were determined by using different molecular models and inverse gas chromatography methods. The determination of γsd of the UiO-66 surface was obtained by using several models such as Dorris-Gray and those based on the Fowkes relation by applying the various molecular models giving the surface areas of n-alkanes and polar organic molecules. Six models were used: Kiselev, spherical, geometric, Van der Waals, Redlich-Kwong, and cylindrical models. The obtained results were corrected by using our model taking into account the thermal effect on the surface areas of molecules. A linear equation was obtained between γsd and the temperature. The specific free energy, enthalpy and entropy of adsorption of polar molecules, as well as the acid and base constants of UiO-66 particles were determined with an excellent precision. It was also proved that the UiO-66 surface exhibited an amphoteric acid-base character with stronger acidity. The linear variations of the specific free energy of interaction as a function of the temperature allowed to obtain the specific surface enthalpy and entropy of adsorption, as well as the acid and base constants of UiO-66 by using ten different models and methods. The best results were obtained by using our model that gave the more precise values of the acid constant KA=0.57, the base constant KD=0.18 of the MOF particles and the ratio KA/KD = 3.14 clearly proving a strong acid character of the UiO-66 surface.