INVESTIGATION OF THE DISTRIBUTION OF ACIDITY STRENGTH IN ZEOLITES BY TEMPERATURE-PROGRAMMED DESORPTION OF PROBE MOLECULES .2. DEALUMINATED Y-TYPE ZEOLITES

The acidity of dealuminated hydrogen forms of Y-type zeolites (Si/Al = 2.4-8.6) is determined by temperature-programmed desorption of ammonia or pyridine, which is monitored through a mass spectrometer. Four types of acidic sites are indicated by ammonia, viz., weak Bronsted and/or Lewis centers and medium and strong Bronsted and strong Lewis sites. In contrast, pyridine, after sample activation at 675 K, probed only two types of sites, i.e., medium and strong Bronsted sites. This difference is ascribed to different accessibility of sites for the two probe molecules. From the desorption spectra (i) the fractional coverage of the various sites, (ii) the most frequent energies of activation, E(d), for desorption, and (iii) the probability functions of the activation energies are derived by using a previously described method of evaluation.