CHARACTERIZATION OF THE BRONSTED ACID PROPERTIES OF H(NA)-BETA ZEOLITE BY INFRARED-SPECTROSCOPY AND THERMAL-ANALYSIS

Infrared spectroscopy and thermal analysis have been used to character ize the Bronsted acidity, the hydroxyl groups of H-Beta zeolite, and t he interaction of hydroxyls with benzene molecules. After pretreatment under dry oxygen and then under vacuum at 723 K, three hydroxyl group s at 3,789, 3,745, and 3,612 cm(-1), superimposed on a broader absorpt ion (3,800-3,200 cm(-1)), have been detected. These three peaks are as signed to Al-OH species near to one or more Si-OH groups generated whe n Al leaves the framework, terminal silanol groups, and framework brid ged Si-OH-Al species, respectively. The broader absorption band should be attributed to the internal silanol group. The present work shows t hat the hydroxyl group at 3,789 cm(-1) can be generated also after a m ild treatment. This observation is contrary to previously published re sults by other research groups. Furthermore, the present study shows t hat 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 hydro xyl group is evaluated by the shift Delta nu(OH) of the hydroxyls upon their interaction with benzene and is compared to other proton zeolit es such as HZSM-5, HY, and HEMT zeolites. It indicates that the framew ork-bridged Si-OH-Al groups of H(Na)-Beta zeolite have an intermediate Bronsted acidity. The amount of Bronsted acid sites as well as the to tal adsorption capacity for benzene have been determined from the stud y of the changes of the absorbance of the C-H out-of-plane vibrations with benzene coverage. The number of TEA species associated with tetra hedral aluminum and with SiO- structural defects or OH- ions has been also determined using thermobalance coupled with ammonia titration. (C ) Elsevier Science Inc. 1997.