Liquid phase calorimetry and adsorption analyses of zeolite beta acidity

Zeolite beta acidity, activated at 550 degrees C, was characterized by calorimetric and adsorption studies of pyridine in cyclohexane (Cal-Ad). Two different acid sites were found: n(1) = 0.1612 mmol g(-1) with DeltaH(1) = -83.6 kJ mol(-1); and n(2) = 0.3820 mmol g(-1) with DeltaH(2) = -62.8 kJ mol(-1). Step calcinations followed by FTIR indicated n(1) as Lewis sites and n(2) as weak Brønsted sites. When activated at 450 degrees C, zeolite beta showed three different sites: n(1) = 0.0467 mmol g(-1) with DeltaH(1) = -227.3 kJ mol(-1); n(2) = 0.1163 mmol g(-1) with DeltaH(2) = -93.3 kJ mol(-1); and n(3) = 0.3322 mmol g(-1) with DeltaH(3) = -80.4 kJ mol(-1). Sites n(1) and n(2) were assigned as Brønsted centers, while n(3) as a combination of Brønsted and Lewis sites. These results suggest that the zeolite beta strongest Brønsted sites are converted to Lewis sites at 550 degrees C. These Lewis sites are not only related to extra-framework Al species, but also to aluminium atoms partially bonded to the zeolite framework, as evidenced by infrared bands at 3777 and 875 cm(-1). FTIR and thermogravimetric analyses of gas phase adsorbed pyridine were also used to investigate the materials acidity and a good agreement was found with Cal-Ad data. The results showed that the Cal-Ad method offers a more detailed description of the zeolite surface acidity by providing thermodynamic parameters not easily obtained by other techniques. A linear relationship between enthalpy and entropy values for several zeolites (ZSM-5, TS-1, MOR, Y and BEA) is also reported.