Hydrothermal stability of zeolites: Determination of extra-framework species of H-Y faujasite-type steamed zeolite

Zeolite H-Y of faujasite type (Si/Al=2.70) was hydrothermally treated at different temperatures in the range between 403 and 473 K for 72 h in liquid water (=saturated steam), by use of a water/H-Y zeolite ratio of 100. The batch was treated in Teflon-coated autoclaves at different temperatures and thus different steam pressures. Treated samples show growth of decomposition with increasing temperature. The parent H-Y zeolite and the hydrothermally treated H-Y zeolite samples, as well as obtained transformation products, were characterized by IR spectroscopy, water sorption uptake and XRD methods. The hydrothermal destruction process of H-Y zeolite into extra framework aluminum and silicon species is visible at 423 K and is followed by growth of kaolinite and amorphous substances such as silica and probably metakaolinite. These processes are slow at 423 K and accelerate between 443 and 473 K. The appearance of kaolinite as transformation product is experimentally identified for the first time. The structural transformation mechanism of H-Y zeolite to kaolinite, silica gel and metakaolinite was suggested and discussed.