Investigation of the structural order and stability of mesoporous silicas under a humid atmosphere (original) (raw)
The low stability of Si-MCM-41 limits its usage in the presence of water. The loss of structural order is mainly due to H 2 O physisorption and hydrolysis of the ≡Si-O-Si≡ bonds. In this work, it was demonstrated that water molecules could cause destruction of the pore walls. X-ray diffraction experiments revealed the effect of physisorbed water on the intensity of the diffraction peaks. In addition, partial regeneration of the hydrated samples was observed after thermal treatment. Evaluation was made of the structural stability of mesoporous silicas (Si-MCM-41) synthesized at different temperatures (30, 60, and 90°C), as a function of time, when placed in a controlled humidity atmosphere. The materials were characterized by X-ray diffraction, which confirmed formation of the hexagonal phase at all the temperatures used. Furthermore, loss of long-range structural organization was observed when the calcined mesoporous silicas were conditioned in humid environments. In situ investigation was made of the thermal treatment of the hydrated silicas, with thermogravimetric quantification of silanols after the hydration and heating process. Transmission electron microscopy analysis revealed partial loss and/or reduction of the walls of some pores, caused by hydrolysis of the siloxane bonds (≡Si-O-Si≡). This resulted in decrease of the mean diameter of the pores that were resistant to wall degradation. X-ray diffraction analyses, as a function of temperature, showed that there was recovery of peak intensity and a shift of the diffraction peaks towards greater angles, as the duration of exposure to the heating increased, evidencing a decrease in the interplanar spacing.
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