Investigation of the distribution of acidity in zeolites by temperature-programmed desorption of probe molecules. I. Dealuminated mordenites

Abstract

The acidity of dealuminated hydrogen mordenites (Si/Al = 12-39) is characterized by the temperature-programmed desorption of ammonia or pyridine which is monitored through a mass spectrometer. Four types of sites are indicated by ammonia desorption, viz., weak and strong sites of Broensted and Lewis type, whereas pyridine desorption reveals only the existence of three types of acidic sites which are tentatively assigned to one sort of Broensted site and weak and strong Lewis centers. Evaluation of the desorption spectra provides the temperatures of maximum rate of desorption, the population of the respective sites, the desorption rates as a function of the activation energies of desorption, and, finally, probability functions of the activation energies. The results are discussed in terms of coverage, Si/Al ratio, and accessibility.