Investigation of pyridine sorption in USY and Ce/USY zeolites by liquid phase microcalorimetry and thermogravimetry studies

USY (ultrastabilized Y) and Ce/USY (5 wt.% supported) zeolite acidities were characterized by microcalorimetric and adsorption studies of pyridine using liquid phase (Cal-Ad), thermogravimetry, and infrared analysis. The average adsorption enthalpies determined by microcalorimetry were −125.0 kJ mol−1 for USY and −97.2 kJ mol−1 for Ce/USY. A heterogeneous distribution of acid sites with heats of adsorption ranging from −134.0 (maximum heat value for USY) to −73.5 (minimum heat value for Ce/USY) kJ mol−1 was found for both zeolites. A two-site model was best fitted by the Cal-Ad method for HUSY (n1 = 0.1385 mmol g−1 with ΔH1 = −134.0 kJ mol−1, and n2 = 0.7365 mmol g−1 with ΔH2 = −101.5 kJ mol−1) and Ce/HUSY (n1 = 0.0615 mmol g−1 with ΔH1 = −117.6 kJ mol−1, and n2 = 0.7908 mmol g−1 with ΔH2 = −83.6 kJ mol−1). DRIFTS measurements after pyridine adsorption showed that USY zeolite possesses only Bronsted acidity and that cerium impregnation leads to the appearance of Lewis sites. Based on these results, three families of acid strength were distinguished: (i) strong Bronsted sites (ΔH > −130 kJ mol−1); (ii) Bronsted sites with intermediate strength (−100