Determination of catalyst surface acidity in liquid by a pulse liquid-chromatographic technique

A pulse liquid-chromatographic technique, set up by the use of a HPLC apparatus, is here proposed for the characterization of surface acidity of solid acids in liquid in view of their use as catalysts in liquid-solid reactions. Dosed amount of solutions of two different basic probe molecules (2-phenylethylamine, pKa = 9.84, and aniline, pKa = 4.63) in solvents of different polarity were injected onto solid samples (acidic resins, Amberlites, and a mixed oxide, silica-zirconia) at a constant flow rate of the solvent. The weakest basic probe, aniline, was generally able to titrate a lower amount of acidic sites than 2-phenylethylamine. The amount of acidic sites determined at the surface was greatly affected by the solvent. For silica-zirconia, a higher amount of acidic sites was titrated in apolar than in polar solvent. The results were compared with those obtained by conventional methods: ionic exchange carried out employing salts and, for silica-zirconia, Hammett titration. The quite satisfactory results obtained are of some importance for the aim of establishing valuable relationships between catalyst acidity and catalytic activity in various solvents.