ACIDITY OF ZEOLITES-BETA DEALUMINATED BY ACID LEACHING - AN FTIR STUDY USING DIFFERENT PROBE MOLECULES (PYRIDINE, CARBON-MONOXIDE)

Two series of H-beta zeolites were prepared by an acid-leaching treatment (HCl 0.5 and 1 N) of two parent samples synthesized either in alkaline (Hbeta(OH-)) Or fluoride (Hbeta(F-)) medium. Study of the Si/Al global ratio and the wavenumber of framework vibrations showed that dealumination occurred. Acidity was measured by infrared spectroscopy using pyridine and carbon monoxide, adsorbed, respectively, at 423 and 100 K. It appears that the H-beta(OH-) parent sample presented a greater amount of aluminum cationic species and amorphous extraframework debris than did H-beta(F-). The former species were eliminated by acid leaching, whereas the latter explained the greater number of Lewis acid sites in the H-beta(OH-) series. Two types of Bronsted acid sites were revealed by CO adsorption, whatever the samples. Dealumination increased their strength and decreased their number, in particular that of the weakest ones. The presence of the latter was related to sites located in the framework of non-well-crystallized areas of the samples.