On the hydrothermal stability of MCM-41. Evidence of capillary tension-induced effects

• The hydrothermal stability of various types of MCM-41 materials was investigated.
• The interpretation considers hydrolysis and capillary stress-induced effects.
• Capillary stress is relevant when the structure is not mechanically robust.
• Capillary stress can produce loss of ordering and shrinkage.
• The aged TEOS-based material has a good hydrothermal stability.

MCM-41's limited hydrothermal stability has been often related to the hydrolysis of Si–O–Si bonds due to the low degree of condensation, its thin walls or a combination of them. In this work, evidence for an additional factor is provided; a physical effect that occurs during the drying of the hydrothermally treated calcined material due to the intense capillary stress exerted in water. Depending on both physical (i.e. mechanical) and chemical (i.e. hydrolysis) resistances, the structure undergoes differently. Three MCM-41 samples with different degree of condensation were investigated. The most remarkable results are found with un-aged TEOS based material, which gets fully disordered and shrunk for all applied hydrothermal temperatures in water. Comparison between water and a low-surface-tension-solvent drying revealed that capillarity is responsible for the loss of ordering (and shrinkage) at moderate hydrothermal temperatures. The material's structure is hexagonal and shrinkage-free under the low-surface-tension-solvent route. At a high hydrothermal temperature, hydrolysis is extensive and responsible for the loss of ordering. The other remarkable finding regards the aged MCM-41 mesostructure that maintains the hexagonal features at all applied temperatures in water, and it is more stable against capillarity at high temperature. The Na-metasilicate based material is mechanically very stable and gets disordered at high temperature due to hydrolysis.

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