The stability of natural STI zeolite in saturated steam at high temperatures (defined as hydrothermal sta- bility as well) is first studied. The stability of parent STI (denoted as R-STI) in steam is better than that in
air. The NH4 type STI zeolite possesses the best hydrothermal stability. The structure, composition and
adsorption properties of NH4-STI treated in steam from 350 to 750 。C are characterized with X-ray dif- fraction (XRD), 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), X-ray fluorescence scattering spectroscopy (XRF), and N2 adsorption and desorption, respectively. High-temperature steam can make aluminum atoms escape from the zeolite
framework to become non-framework aluminum. The NH4 cations in NH4-STI, which can block the pores
as well as the non-framework aluminum, can be fully decomposed to get H-STI at 550 。C. The framework SiO2/Al2O3 ratio can enhance from 6.5 to 21.3 though the chemical SiO2/Al2O3 ratio is almost unchanged
