DISTRIBUTION OF ACID SITES AND DIFFERENTIAL HEAT OF NH3 CHEMISORPTION ON SOME ALUMINAS AND ZEOLITES

The distribution and the strength of acid sites in some aluminas and zeolites were obtained from microcalorimetry measurements of ammonia adsorption. They were compared to the results of FTIR of adsorbed CO and of H-1 --> 27 Al cross-polarization (CP) MAS NMR using chemisorbed ammonia as proton reservoir. FTIR of CO adsorption yields the numbers of Lewis sites and CP MAS NMR gives the coordination of surface aluminum. In spite of the variety of catalysts and of acidity encountered, interesting correlations were found with both the energy of adsorption and singularities in the microcalorimetric titration. In particular, in zeolites a clear difference exists in the distribution of the Lewis and Bronsted acid site strengths with respect to the amount of irreversibly adsorbed NH3.