CHARACTERIZATION OF ACIDITY IN H-ZSM-5, H-ZSM-12, H-MORDENITE, AND H-Y USING MICROCALORIMETRY

Adsorption enthalpies have been measured for ammonia, pyridine, and isopropylamine, as a function of coverage at 480 K, on several high silica zeolites, including H-ZSM-5, H-ZSM-12, H-mordenite (H-M), and H-Y. Temperature-programmed desorption was used to determine the number of strong Bronsted acid sites in each material and estimate the mobilities. Except for isopropylamine in H-ZSM-5, each adsorbate exhibited a constant heat of adsorption up to the coverage of one molecule per Bronsted acid site, followed by a sharp drop in the measured heats. For isopropylamine in H-ZSM-5, the results were found to depend on Si/Al ratio and synthesis procedures, and it appears that hydrogen bonding between the adsorbate molecules affects the measured heats. For ammonia, the adsorption enthalpies were 150 kJ/mol for H-ZSM-5, H-ZSM-12, and H-Y and 160 kJ/mol for H-M. Adsorption enthalpies for pyridine were between approximately 200 and 210 kJ/mol for H-ZSM-5, H-ZSM- 12, and H-M but only - 180 kJ/mol on H-Y. Heats of adsorption for isopropylamine tended to be slightly higher than that for pyridine on each sample but appeared to depend on site concentrations. The implications of these results to acid catalysis are discussed.