Acidity of dealuminated mordenites studied by infrared spectroscopy

Abstract

The hydroxy groups of dealuminated mordenites having SiO₂/Al₂O₃ molar ratios 14–44 have been studied by means of infrared spectroscopy. The intensity of the low-frequency (3609 cm^–1) band decreases with increasing the SiO₂/Al₂O₃ ratio and the intensity of the high-frequency (3738 cm^–1) band increases. Using pyridine as adsorbate the acidity of this mordenite series has been determined and it was found that on dealumination the number of strong acid sites increases with a simultaneous decrease of the total number of acid sites. The SiOH groups which are formed during dealumination increase the strength of the remaining acid sites. A band at 1462 cm^–1 is observed after pyridine desorption at ca. 673 K and is assigned as a characteristic band of pyridine adsorbed on stronger Lewis sites. Increased dealumination renders the Brönsted sites more accessible to pyridine at lower temperatures.