Liquid phase calorimetry and adsorption analyses of zeolite beta acidity

Zeolite beta acidity, activated at 550 °C, was characterized by calorimetric and adsorption studies of pyridine in cyclohexane (Cal–Ad). Two different acid sites were found: n1 = 0.1612 mmol g−1 with ΔH1 = −83.6 kJ mol−1; and n2 = 0.3820 mmol g−1 with ΔH2 = −62.8 kJ mol−1. Step calcinations followed by FTIR indicated n1 as Lewis sites and n2 as weak Brønsted sites. When activated at 450 °C, zeolite beta showed three different sites: n1 = 0.0467 mmol g−1 with ΔH1 = −227.3 kJ mol−1; n2 = 0.1163 mmol g−1 with ΔH2 = −93.3 kJ mol−1; and n3 = 0.3322 mmol g−1 with ΔH3 = −80.4 kJ mol−1. Sites n1 and n2 were assigned as Brønsted centers, while n3 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 °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.