Probing structural changes of sepiolite and zeolitic water dynamics with temperature-dependent infrared spectroscopy and molecular simulations

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.