NATURE AND STRENGTH OF ACID SITES IN HY ZEOLITES - A MULTITECHNICAL APPROACH

The combination of several physico-chemical techniques such as tempera ture programmed desorption, microcalorimetry, infrared spectroscopy an d photoelectron spectroscopy, together with structural data (by XRD an d NMR), has allowed a detailed study of the acidity of commercial HY f aujasites, both non-dealuminated and dealuminated by hydrothermal trea tment. These techniques have been used with different basic probe mole cules 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 betwe en 70 and 130 kJ mol(-1) are of Lewis and Bronsted types, the former b eing predominent in the dealuminated zeolites. The sites of intermedia ry strength are essentially the framework bridged hydroxyl groups (hea ts 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 Bronsted sites. The strength of these sites seems to be due to mutual interactions between Lewis and Bronsted sites. The us e of 2,6-lutidine has furthermore allowed us to differentiate between two different populations of Lewis sites. (C) 1997 Elsevier Science B. V.