How hexagonal mesoporous silica evolves in water on short and long term: Role of pore size and silica wall porosity

• We compare the alteration of two mesoporous silica materials at 60 °C.
• We followed short term alterations by in-situ small angle X-ray scattering.
• Long term alterations were studied ex situ by SAXS, 29Si NMR and N2 adsorption.
• Different alteration mechanisms were pointed out depending on the mesoporous silica.
• Modelling SAXS data allows determining a water diffusion coefficient in silica walls.

In this study, we have determined the evolution of the morphology and the structure of mesoporous silica MCM41 and SBA15 in saturation condition during short and long term alteration by water at 60 °C. These materials were characterized using in situ/ex situ Small Angle X-ray Scattering (SAXS), and ex-situ by nitrogen adsorption–desorption and 29Si Nuclear Magnetic Resonance (NMR). The results have shown that MCM41 lost its hexagonal order of pores. This phenomenon has been attributed to a change of the pore shape probably due to the dissolution of silica wall and to the recondensation of hydrolysed silica on the pore surface leading also to a partial pore clogging at high alteration progress. In the case of SBA15, as soon as the porous silica is in contact with water, an altered silica layer is formed at the pore surface and dissolved at a rate of 120 nm year−1 leading to a pore size increase. When an equilibrium between dissolution and recondensation of the silica is reached, the silica dissolution rate strongly decreases (7 nm year−1) and the altered layer growth follows a diffusive process with a diffusion coefficient of D = 1.4 × 10−24 m2 s−1. The differences of evolution between the two silica are explained by their different pore diameter and the presence of microporosity in the case of SBA15.

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