Specific behavior of β-zeolites upon the modification of the surface acidity by Cs and Li exchange

Abstract

A study on the modification of the surface acidity of beta zeolite exchanged with Cs and Li was carried out by means of X-ray diffraction, nitrogen adsorption (BET), ²⁷Al-MAS-NMR, (¹³³Cs, ⁶Li)-MAS-NMR, surface paramagnetic shift (SUPAS) with adsorbed O₂, FTIR of adsorbed pyridine and NH₃-TPD. An inverse correlation between the amount of cations exchanged and the number of acid sites was verified, as well as a gradual decrease of the micropore surface area, i.e. a 38% negative variation for the fully exchanged solids ZβCs2 and ZβLi3. The ¹³³Cs and ⁶Li (SUPAS)-MAS-NMR technique suggested that Cs cations were located in accessible sites, while Li cations were inaccessible and possibly located in cavities within the channels network. The crystallinity of the original beta zeolite was modified slightly after the ion exchange and no trace of amorphous material was detected by XRD and ²⁷Al-MAS-NMR techniques. By means of NH₃-TPD it was found that at similar cation molar concentrations the number of acid sites was about 4 times higher for Li-exchanged zeolites, therefore Cs showed a stronger neutralization power of the acid sites.