Effect of amino functional groups on the surface properties and Lewis's acid base parameters of UiO-66(NH2) by inverse gas chromatography

Amino-functionalized metal organic frameworks (MOFs) have attracted much attention for various applications such as carbon dioxide capture, water remediation and catalysis. The focus of this study is to determine the surface and Lewis's acid-base properties of UiO-66(NH₂) crystals by the inverse gas chromatography (IGC) technique at infinite dilution. The latter was applied to evaluate the dispersive component of the surface energy γsd(T) by using thermal model and several molecular models. The obtained results proved that γsd(T) decreases when the temperature increases. The best results were achieved by using the thermal model that takes into account the effect of the temperature on the surface areas of the organic molecules. We also observed a decrease of the Gibbs surface free energy of adsorption by increasing the temperature of the different organic solvents. The polar interactions of UiO-66(NH₂) were obtained by using the methods of Saint-Flour Papirer, Donnet et al., Brendlé-Papirer and the different molecular models. The Lewis's acid base constants KA and KD were further calculated by determining the different variables of adsorption of the probes on the solid surface and the obtained values were 1.07 and 0.45 for KA and KD respectively, with an acid-base ratio (K_A/K_D) of 2.38. These values showed the high acidic surface of the solid substrate; whereas, the values of the entropic acid base parameters, ωA, ωD and ωA/ωD respectively equal to 1.0×10-3, 3.8×10-4 and 2.73, also highlighted the important acidity of UiO-66-(NH₂) surface. These important findings suggest that the surface defects (missing linkers and/or clusters) in UiO-66(NH₂) are the main determining factor of the acid-base properties of UiO-66 based structures.