Stability of Zeolites in Hot Liquid Water

Zeolites Y and ZSM-5 with varying Si/Al ratios are treated in liquid water at 150 and 200 degrees C under autogenic pressure to assess their hydrothermal stability. The changes in the structure are characterized by atomic absorption spectroscopy, X-ray diffraction, scanning electron microscopy, argon physisorption, (27)Al and (29)Si MAS NMR spectroscopy, temperature-programmed desorption of ammonia, and pyridine adsorption followed by IR spectroscopy. During treatment in hot water, zeolite Y with a Si/Al ratio of 14 or higher is transformed into an amorphous material, and the rate of this degradation increases with increasing Si/Al ratio. In contrast, ZSM-5 is not modified under the same conditions. The main degradation mechanism is suggested to be hydrolysis of the siloxane bonds (Si-O-Si) as opposed to dealumination, which dominates under steaming conditions. In the resulting amorphous phase, Al remains tetrahedrally coordinated, but the micropore volume and concentration of accessible acid sites is reduced dramatically. The results demonstrate that potential structural changes of zeolites have to be considered when these materials are used as catalysts for aqueous phase conversion of biomass.