MOBILITY OF ACIDITY - STUDY OF THE EFFECT OF TEMPERATURE ON ACID-BRIDGED GROUPS IN DEALUMINATED MORDENITE BY IN-SITU IR SPECTROSCOPY

The effect of temperature (300-673 K) on the acid-bridged OH groups of mordenite zeolite with Si/Al ratios between 5.56 and 23.60 was invest igated by in situ IR spectroscopy. With increasing temperature, the fr equencies of acid-bridged OH groups are shifted to lower values and sh ape of the spectrum becomes broad. The sensitivity of nu(OH) of the sa mples to the temperature shows differences (especially above 473 K). C orrespondingly, the acidity of the sample shows differences at differe nt temperatures and increases with increasing temperature. Moreover, s ome weaker acid sites can be made stronger as the temperature rises. I n a series of mordenite tested, in the lower temperature region (300-4 73 K), the order of acidity depends on the conditions of treatment. At higher temperatures (473-673 K), with increasing temperature the acid ity of the sample decreases, reaches a minimum (Si/Al = 11.18), and in creases again (Si/Al > 11.18).