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 γ _s ^d of the surface energy of crystals of a Zr-based MOF, UiO-66 (Zr ₆ O ₄ (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 γ _s ^d 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 γ _s ^d 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 K _A =0.57, the base constant K _D =0.18 of the MOF particles and the ratio K _A /K _D  = 3.14 clearly proving a strong acid character of the UiO-66 surface.
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