Nature and strength of acid sites in HY zeolites: a multitechnical approach

The combination of several physico-chemical techniques such as temperature programmed desorption, microcalorimetry, infrared spectroscopy and photoelectron spectroscopy, together with structural data (by XRD and NMR), has allowed a detailed study of the acidity of commercial HY faujasites, both non-dealuminated and dealuminated by hydrothermal treatment. These techniques have been used with different basic probe molecules such as ammonia, pyridine and 2,6-lutidine (2,6-dimethylpyridine). They have demonstrated the existence of three different regions of acid strength. The weak sites associated to NH₃ adsorption heats between 70 and 130 kJ mol⁻¹ are of Lewis and Brönsted types, the former being predominent in the dealuminated zeolites. The sites of intermediary strength are essentially the framework bridged hydroxyl groups (heats between 130 and 150 kJ mol⁻¹). The strong sites (heats above 160 kJ mol⁻¹) contain a significant number of extra-framework Lewis-type sites, but also Brönsted sites. The strength of these sites seems to be due to mutual interactions between Lewis and Brönsted sites. The use of 2,6-lutidine has furthermore allowed us to differentiate between two different populations of Lewis sites.