Direct measurement of the interaction energy between solids and gases. X. Acidic properties of hydroxyl sites of H-Y zeolite determined by high-temperature calorimetry.

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Differential heats of adsorption of bases on H–Y zeolite were measured at 473 K and separated into those evolved at 3640 and 3550 cm−1-OH sites respectively by a combined use of calorimetric and IR measurements, and then the acid strength of two OH sites was compared each other. In case of pyridine, heats of adsorption were larger on OH sites at 3640 cm−1 than 3550 cm−1, which was in agreement with the results obtained by many authors by IR measurements. However, heats of adsorption on 3640 cm−1-OH sites were only slightly higher than those on 3550 cm−1-OH sites in case of ammonia, and almost the same in case of piperidine. The difference in heats of adsorption between the OH sites was explained by positional difference between the two OH sites, molecular size of the titrated bases and proton mobility induced by a strong base. The order of the strength of bases was determined by replacing a base molecule with another one on H–Y zeolite as follows; piperidine>pyridine>ammonia. This differs from the order expected from their pKa values in solution.
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