COMPARISON OF THE ACIDIC PROPERTIES OF H-[AL]ZSM-5, H-[FE]ZSM-5, AND H-[GA]ZSM-5 USING MICROCALORIMETRY, HEXANE CRACKING, AND PROPENE OLIGOMERIZATION

We have used a combination of microcalorimetry measurements of ammonia and pyridine and reactivity measurements for n-hexane cracking, propene oligomerization, and 2-methyl-2-propanol decomposition to compare the acid sites in H-[Fe]ZSM-5, H-[Ga]ZSM-5, and H-[Al]ZSM-5. On each of the molecular sieves, the differential heats of adsorption for both ammonia and pyridine were constant up to a coverage of one per Bronsted site. Within experimental uncertainty, the differential heats at coverages below 1:1 were identical on each of the materials, with values for ammonia of 145 +/- 5 kJ/mol on H-[Fe]ZSM-5, 150 +/- 5 kJ/mol on H-[Ga]ZSM-5, and 145 +/- 5 kJ/mol on H-[Al]ZSM-5 and for pyridine of 195 +/- 5 kJ/mol on H-[Fe]ZSM-5, 200 +/- 5 kJ/mol on H-[Ga]ZSM-5, and 200 +/- 5 kJ/mol on H-[Al]ZSM-5. However, while specific rates for n-hexane cracking at 700 and 750 K and propene oligomerization at room temperature were nearly identical for H-[Ga]ZSM-5 and H-[Al]ZSM-5, the reactivity of the Bronsted sites in H-[Fe]ZSM-5 for each of the reactions is much lower. The implications of these results for the characterization of acidity in solid acids are discussed.