Abstract
The acid strength distribution of silica–alumina catalyst (alumina content 13 wt%) evacuated at 400 °C for 5 h was determined by the measurement of heat of adsorption of ammonia, and compared with that determined by amine titration by use of a series of Hammett indicators. The heats of adsorption of ammonia on silica–alumina poisoned with Hammett indicators were also measured in order to obtain the relation between the dissociation constant of the acid (pKa) and the heat of adsorption. When the dissociation constants of acid were equal to −5.6 and +3.3, the differential heats of adsorption of ammonia were observed to be 76.1 and 55.1 kJ/mol, respectively. The number of acid sites calculated from the heat vs. adsorbed amount curve was found to be about twice as large as that determined by amine titration. The discrepancy is discussed.
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