Steaming of Zeolite BEA and Its Effect on Acidity: A Comparative NMR and IR Spectroscopic Study

Abstract

The kinetics and impact of steaming on the acid site concentration of zeolite beta were studied by 1H, 27Al, and 29Si MAS NMR spectroscopy as well as IR spectroscopy of adsorbed pyridine and temperature-programmed desorption of NH3. The main effects of steaming were the dealumination of the T3−T9 sites, the formation and migration of extra-framework Al species, and the healing of defect sites by condensation of silanol groups. These effects took place mainly in the first five hours of steaming, while after 14 h of steaming the system appeared to be stabilized. The concentration of framework Al atoms detected by 27Al MAS NMR spectroscopy is significantly higher than the concentration of Brønsted acid sites determined by 1H MAS NMR spectroscopy as well as by the sorption of basic probe molecules such as NH3 and pyridine. This shows conclusively that extra-framework Al oxide/hydroxide species act as cations balancing the framework charge. The concentration of extra-framework Al atoms matches the discrepancy between the concentration of framework Al atoms and the concentration of Brønsted acid sites indicating that each charge balancing entity contains only one aluminum.