Measurement of Bronsted Acidity of Silica-Alumina Solid Catalyst by Base Exchange Method

Silica-Alumina catalysts having three different Si/Al ratios were prepared. Surface acidity of these catalyst samples was measured by the base exchange method using aqueous ammonium acetate, CH₃COONH₄ solution and the results were compared with those measured by the base exchange method using KOH and NH₄OH as adsorbates from aqueous solution. The amount of K⁴ and NH₄⁺ adsorbed was determined titrimetrically and spectrophotometrically, respectively for measuring the concentration of K⁺ and NH₄⁺ in the solution before and after adsorption. It was found that the surface acidity was more pronounced when ammonium acetate solution was used as an adsorbate. With the use of NH₄OH and KOH as bases considerable dealumination takes place with a consequent decrease of surface acidity of the catalyst samples. In all cases, the surface acidity increased with decrease of sample size. It is seen that surface acidity measured with NH₄OH becomes closer to that measured with KOH at the same(?) Si/Al ratio, but contrary to this, surface acidity measured with NH₄OH becomes closer to that measured with CH₃COONH₄ at high Si/Al ratio. This phenomenon is attributed to dealumination of silica-alumina catalyst samples by the strong base.