Direct Measurement of the Interaction Energy between Solids and Gases. XI. Calorimetric Measurements of Acidities of Aluminum Deficient H–Y Zeolites

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Abstract
Heats of adsorption of pyridine and ammonia were respectively measured on aluminum deficient H–Y zeolites using a microcalorimeter at 473 K. The calorimetrically determined acidity was discussed in relation to the positional difference between 3640 and 3550 cm−1-OH acidic sites and compared with the catalytic activity. Dealumination generally brings about a decrease in the acid site concentration. However, the extent of the decrease varied with the kind of titrated base and a significant change was observed in the ratio of acid site concentration when titrated with pyridine. This may be explained by the fact that the pore size of the zeolite becomes widely distributed as dealumination proceeds; 3550 cm−1-OH sites in supercages must become accessible to such a large molecule as pyridine. Also observed with dealumination was a slight increase in the heat of adsorption from initial to the middle stage of adsorption, which reflected the enhancement in acid strength of each OH site. The catalytic activity for cumene cracking increased with dealumination, which agreed with the acidity determined by the adsorption of pyridine rather than ammonia.