Liquid Recirculation Chromatographic Method

Abstract Attention should be devoted to the measurements of the adsorption properties of catalytic surfaces when they have to work in liquid-solid heterogeneous conditions. The mutual characteristics of the surface and the liquid affect the reagent interactions with the surface sites which could be engaged with the liquid interaction and then not-available for the reagent coordination. This leads to observe effective adsorption properties that could be different from the intrinsic properties of the surface. The possibility to quantitatively determine the effective acid properties of catalytic surfaces by base adsorption is here showed. The adsorption can proceed in any type of liquid of various characteristics (apolar, polar, aprotic, protic) with dynamic (pulse liquid chromatographic method) or equilibrium (liquid recirculation chromatographic method) methods. The measurements of effective acidity allows finding more sound relations with the catalytic activity for a better comprehension of the catalyst work and for a more correct determination of the turnover numbers in liquid-solid catalysis. 

Recently, some techniques have been presented in the literature making use of volumetric titrations of surface sites in liquids, of different polar and protic characteristics, to determine the amount of acid sites and relevant acid strength effective acidity). Two different methods will be here discussed the first one is based on a pulse liquid chromatographic method (a dynamic method) [8] and the second one on a liquid recirculation chromatographic method (a Masi-static method) [9]. When surface acidity studies are concerned, the measurements may be performed in apolar, aprotic liquid (like cyclohexane), for the determination of the intrinsic acidity, or in several other liquids with polar/protic characteristics, for the determination of the effective acidity. Basic probes of different basicity (following the pK scale) may be used (e.g., 2-phenylethylamine, PEA, aniline, AN, pyridine, PY, etc.). Titration temperatures may be varied from room temperature (r.t.) up to the normal boiling point of the liquid used in order to calculate, from the collected isotherms of adsorption, the isosteric heats of adsorption which can be related to the acid strength of the surface sites [10, 11]. 

The adsorption properties of solids with their various surface functionalities are governed in liquids by both the hydrophobic or hydrophilic properties of the surfaces and the properties of liquids (polarity, proticity, solvating ability). Experiments based on pulse liquid or recirculation chromatographic methods, here illustrated, allow... 

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