Measurement of the acidity of various zeolites by temperature-programmed desorption of ammonia

The acidity of H-form zeolites which include mordenite, ZSM-5, Y-faujasite, and those modified by cation exchange or dealumination was measured by temperature-programmed desorption (TPD) of NH3. Ammonia was adsorbed at 373 K to eliminate the contribution of very weak acid sites and improve the spectrum resolution. TPD spectra of mordenites and ZSM-5 zeolites had two desorption peaks named l and h with peak maximum at around 420 and 680 K, respectively, while the spectra of Y-zeolites had a broad peak. From the peak maximum temperature of h-peak, the acid strength was found to be in the order of HM > HZSM-5 > HY. Acid amount was evaluated from the desorption amount of NH3, and correlated with the infrared intensity of the OH band at 3600 cm−1 and the ammonium band at 1447 cm−1, thus indicating that the amount as well as distribution of strength of Bronsted acid sites can be measured by the present method. Cation exchange poisoned the strong acid sites preferentially, whereas dealumination reduced the acid sites in the whole region.