Studies on the acidity of mordenite and ZSM-5. 2. Loss of Brønsted acidity by dehydroxylation and dealumination

Quantitative determination of Brønsted acid site concentrations in calcined H-mordenite and H-ZSM-5 by means of conductometric titration and infrared spectroscopy reveals that the concentration of Brønsted acid sites present is lower than would be expected on the basis of the aluminum content. This discrepancy is attributed to dealumination and dehydroxylation processes occurring during calcination. A model is proposed according to which the degree of these processes strongly depends on the concentration of vicinal aluminum T sites in the zeolite: Brønsted acid sites associated with these T sites are lost, and the remaining Brønsted acidity is related to the concentration of the (remaining) isolated aluminum T sites. This model quantitatively correlates the experimentally determined concentration of Brønsted acid sites with the concentration of isolated aluminum T sites.