In-situ IR monitoring of the acidic and structural variations of mordenite caused by pressure and temperature

Abstract

The effects of the surroundings (pressure and temperature) on the acidity and structure of mordenite zeolite have been investigated by monitoring the acidic bridging hydroxy groups, Si—OH—AI, by in-situ IR. Effects due to pressure have not been observed. However, severe effects due to temperature have been found. The frequencies of acidic bridging OH groups decreased and the acidity increased with increasing temperature. The weaker Brønsted acid sites could be turned into stronger Brønsted acid sites as the temperature was raised. In a series of mordenites, in the lower temperature region (300–473 K) the order of acidity depended on the conditions of treatment. In the higher temperature region (473–674 K), with increasing temperature the acidity of the sample decreased, reaching a minimum (Si : AI = 11.18) before increasing again (Si : AI > 11.18).

The effect of temperature on the structure of mordenite has shown that the more sensitive the IR frequencies of the acid OH groups to temperature, the greater the degree of distortion of the zeolite rings and, correspondingly, the less thermostable the structure of the dealuminated mordenite. Moreover, SiOHAI fragments tend to locate in the positions at which the ∠ TOT angles are minimised.