Studies on the hydrothermal stability of natural and modified STI zeolite

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 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 diffraction (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.

Mesopore-rich STI zeolite with FSAR of 21.3, prepared by de-alumination of NH4-STI, still keeps the framework in steam at 750 °C for 3 h, which contain tetrahedral ([AlO4], 55 ppm), distorted tetrahedral or pentahedral ([AlO5], 28 ppm) and octahedral ([AlO6], 0 ppm) on 27Al MAS NMR spectrum.Figure optionsDownload as PowerPoint slideHighlights
► The thermal and hydrothermal stability of natural STI zeolite is very weak.
► The order of hydrothermal stability is followed as NH4-STI > H-STI > Ca-form STI.
► The framework SiO2/Al2O3 ratio increases linearly as temperature from 6.5 to 21.3.
► The surface area and pore volume increase from 350 to 550 °C, and decrease from 550 to 750 °C.