Distribution of acid-site strengths in hydrogen zeolites and relationship between acidity and catalytic activity

Abstract

Adsorption microcalorimetry was applied to determine heats of adsorption of NH 3 over hydrogen forms of zeolites with different framework topologies. Differential heats of adsorption of ammonia indicate the presence of acid sites with different energies that characterize the pattern of distribution of acid-site strengths in zeolites. The calorimetric values suggest that in hydrogen faujasites the number of the strong sites with q>110 kJ/mol is negligibly low, whereas in hydrogen mordenites such sites predominate among the acid centers. Dealumination of faujasites results in the appearance of the very strong acid sites with q>120 kJ/mol, and in dealuminized mordenites the sites with q>150 kJ/mol are formed. The plots of the number of very strong sites as a function of Al content are described by the curves with the maximum both for Y zeolites and mordenites. The maximum typically corresponds to composition dealuminized to 50%. The behavior of zeolites in the cracking of octanes, isomerization of o-xylene, aromatization of ethylene, disproportionation of ethylbenzene and dehydration of 3-methylbutanol-2 is discussed. These results clearly show that catalytic activity of zeolites in the conversions of hydrocarbons is related to protonic acid sites. The most active acid sites appear to associate with the “isolated” AlO 4 tetrahedra.