Studies on the hydrothermal stability of natural and modified STI zeolite

Abstract The stability of natural STI zeolite in saturated steam at high temperatures (defined as hydrothermal stability as well) is first studied. The stability of parent STI (denoted as R-STI) in steam is better than that in air. The NH 4 + type STI zeolite possesses the best hydrothermal stability. The structure, composition and adsorption properties of NH 4 -STI treated in steam from 350 to 750 °C are characterized with X-ray diffraction (XRD), 27 Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), X-ray fluorescence scattering spectroscopy (XRF), and N 2 adsorption and desorption, respectively. High-temperature steam can make aluminum atoms escape from the zeolite framework to become non-framework aluminum. The NH 4 + cations in NH 4 -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 SiO 2 /Al 2 O 3 ratio can enhance from 6.5 to 21.3 though the chemical SiO 2 /Al 2 O 3 ratio is almost unchanged.