Characterization of the Brønsted Acid Properties of H(Na)-Beta Zeolite by Infrared Spectroscopy and Thermal Analysis

Abstract

Infrared spectroscopy and thermal analysis have been used to characterize the Brønsted acidity, the hydroxyl groups of H-Beta zeolite, and the interaction of hydroxyls with benzene molecules. After pretreatment under dry oxygen and then under vacuum at 723 K, three hydroxyl groups at 3,789, 3,745, and 3,612 cm −1, superimposed on a broader absorption (3,800–3,200 cm −1), have been detected. These three peaks are assigned to AlOH species near to one or more SiOH groups generated when Al leaves the framework, terminal silanol groups, and framework bridged SiOHAl species, respectively. The broader absorption band should be attributed to the internal silanol group. The present work shows that the hydroxyl group at 3,789 cm −1 can be generated also after a mild treatment. This observation is contrary to previously published results by other research groups. Furthermore, the present study shows that all three hydroxyl groups situated at 3,789, 3,745, and 3,612 cm −1 can interact completely with benzene. This is in contrast with the case of pyridine. It seems that some framework protons (or Na + cations), located in the small cavities and being initially inaccessible, can be attracted toward the 12-R channels in the presence of benzene and finally become accessible for benzene. The acid strength of each hydroxyl group is evaluated by the shift Δν OH of the hydroxyls upon their interaction with benzene and is compared to other proton zeolites such as HZSM-5, HY, and HEMT zeolites. It indicates that the framework-bridged SiOHAl groups of H(Na)-Beta zeolite have an intermediate Brønsted acidity. The amount of Brønsted acid sites as well as the total adsorption capacity for benzene have been determined from the study of the changes of the absorbance of the CH out-of-plane vibrations with benzene coverage. The number of TEA species associated with tetrahedral aluminum and with SiO − structural defects or OH − ions has been also determined using thermobalance coupled with ammonia titration.