Sepiolite, a hydrated magnesium silicate with tunnel-like structures, contains zeolitic water (H₂O), bound water (OH₂), and structural water (OH), which influence its thermal and structural properties. This study investigates the dehydration dynamics of sepiolite using in situ variable-temperature infrared (IR) spectroscopy (25–300 °C) and classical molecular dynamics (MD) simulations. The IR spectra reveal that most of the zeolitic water molecules ( v 3375 cm −1 and δ 1660 cm −1 ) are lost at ∼100 °C, and the residual zeolitic water molecules are lost at ∼200 °C. Confinement in tunnels makes zeolitic waters more thermodynamically stable than surface-adsorbed waters ( v 3252 cm −1 ), which are completely lost by 100 °C. The gradual evacuation of the tunnel results in a shift in the stretching vibration of structural waters from 3690 to 3675 cm −1 due to diminished repulsion from the hydrogen atoms of zeolitic waters. The bound waters ( v 3616, v 3565 and δ 1625 cm −1 ) are stable until 300 °C and subsequent loss induced the folding of layers. The MD simulations of a 4 a × 2 b × 10 c supercell (Sep-8H 2 O to Sep-0H 2 O zeolitic water hydration states) show a ∼ 1.2% tunnel height reduction upon dehydration, consistent with weakened Si–O–Si ribbons (1210 shifts to 1195 cm −1 ). The zeolitic water molecules form 4, 3, and 2 structured layers in Sep-8H₂O, Sep-4H₂O, and Sep-2H₂O, respectively, with diffusion coefficients increasing from 5.0 × 10 −10 m 2 /s to 7.5 × 10 −9 m 2 /s as hydrogen bonding decreases. These findings elucidate the coordinated interplay of water types and structural changes in sepiolite, offering insights into its thermal stability and potential applications in catalysis and adsorption.
Fashina et al. (Fri,) studied this question.