IN-SITU IR MONITORING OF THE ACIDIC AND STRUCTURAL VARIATIONS OF MORDENITE CAUSED BY PRESSURE AND TEMPERATURE

The effects of the surroundings (pressure and temperature) on the acid ity and structure of mordenite zeolite have been investigated by monit oring the acidic bridging hydroxy groups, =Si-OH-Al=, by insitu IR. Ef fects 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 tempera ture. The weaker Bronsted acid sites could be turned into stronger Bro nsted acid sites as the temperature was raised. In a series of mordeni tes, in the lower temperature region (300-473 K) the order of acidity depended on the conditions of treatment. In the higher temperature reg ion (473-674 K), with increasing temperature the acidity of the sample decreased, reaching a minimum (Si :Al = 11.18) before increasing agai n (Si:Al > 11.18). The effect of temperature on the structure of morde nite 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 structur e of the dealuminated mordenite. Moreover, =SiOHAl= fragments tend to locate in the positions at which the angle TOT angles are minimised.