Examination and standardisation of amine titration in surface acidity measurements of silicas, aluminas and synthetic zeolites

Abstract

AbstractThe amine titration method using Hammett and arylmethanol indicators has been examined on selected samples of porous silica, active alumina and synthetic zeolites. On the basis of the successive approximation technique the evaluation of surface acid strength distribution has been standardised. A new method has been developed to assess the total acidity without employing indicators. The distinction between Bronsted and Lewis acidity through measuring the Ho and HR acidity distribution function on aluminas and zeolites is shown to be erroneous. The acidity is strongly affected by the size of amine and by the density of surface acid centres—as demonstrated on a series of silicas and on synthetic zeolites. The size dependence can be utilised to detect the acidity of distinct pore size ranges within a given porous solid. Under certain conditions the acidity measurements can be correlated with those obtained from other techniques such as heterogeneous exchange and infrared spectroscopy.