Study of the acidity of ZSM-5 zeolite by microcalorimetry and infrared spectroscopy

Abstract

Microcalorimetry and i.r. spectroscopy were used to characterize the acid centres in ZSM-5 zeolite. Differential heats of adsorption of ammonia at 416 K showed that there exists a strong acidity in H–ZSM-5 zeolite and also in H–Z zeolite, which exceeds that present in H–Y zeolite. The energy distribution of strongly acid sites is more heterogeneous in H–ZSM-5 than in H–Z zeolite. A comparison of calorimetric and i.r. spectroscopic data for samples with different degrees of dehydroxylation has, moreover, shown that a fraction of Brönsted sites are weakly acidic whereas all Lewis sites are strongly acid. The spatial distribution of acidity in acid-treated ZSM-5 zeolite is not uniform; the most acidic sites, and particularly Lewis sites, are located within the channel structure of the material where they are not readily accessible to ammonia molecules at 416 K or mainly to pyridine molecules at room temperature.