High-temperature acidity measurements and correlation with catalyst activity

Abstract

A method is described for the determination of catalyst acidity. It is based on the amount of oxygen used for the oxidation of ammonia retained on catalyst samples after they have been exposed to ammonia and outgassed at elevated temperatures. The method is versatile. It allows for different pretreatments, in the same apparatus, in order to establish conditions for the development and measurement of optimum acidity. It provides a means for detecting changes in the type of acid sites resulting from variations in the preparation and pretreatment procedures. One of the advantages of this method over the direct determination of adsorbed ammonia lies in its ability to distinguish between different types of acid sites. This distinction can be made on the basis of observed differences in the oxygen consumption pattern with varying adsorption and outgassing temperatures, and in the temperature required for oxidation of ammonia adsorbed on different sites. Good correlations were obtained between acidity as measured by this method and catalytic activity for fluorided aluminas of different fluoride content, and for silica-aluminas of varying silica-alumina ratios. The appearance, rise, and decline in one type of acidity with increasing pretreatment temperatures in NH 4-Y, rare earths-X, and rare earths-Y zeolites, coincides with the corresponding activity curve for ethylation of benzene. Another type of acidity, which increases steadily with increasing pretreatment temperatures, apparently has no pronounced influence on the activity. Therefore, any measure of “total” acidity would be misleading. Identification of these two respective types as Brönsted and Lewis acid sites is proposed.