Liquid phase calorimetry and adsorption analyses of zeolite beta acidity

Abstract

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.