Highly hydrothermal stability of ordered mesoporous aluminosilicates Al-SBA-15 with high Si/Al ratio

The hydrothermal stability of ordered mesoporous aluminosilicate (Al-SBA-15) materials was systematically investigated under 100% steam at high temperature (600 – 800 °C). The characterization results clearly indicate that the incorporation of a little amount of aluminum in the framework, large amount of micropores and high calcination temperature can greatly improve the hydrothermal stability of mesoporous silicate materials. The two-dimensional (2-D) hexagonal mesostructure of the Al-SBA-15 materials calcined at 600 °C can be stable in the steam of 600 °C for more than 12 h. However, the mesostructures of Al-SBA-15 materials with a relatively low Si/Al ratio (molar ratios in the initial solution) of 10–30 completely collapse under the steam of 800 °C, but the samples with less Al content (a high Si/Al ratio of 40–50) are much stable and can withstand for 6 h in the 800 °C-steam. It is due to the presence of large amount of micropores and the formation of more stable Si–O–Al bonds on the surface of the frameworks. By elevating the calcination temperature to 800 °C, the polymerization degree of the frameworks is enhanced, and they can withstand the steam treatment of 800 °C for more than 12 h. In addition, it is observed that the mesoporous aluminosilicates (Al-SBA-15) are hydrothermally more stable than of the pristine silica SBA-15 in 100% steam. During the steam treatment, many four-coordinated Al species are changed into six-coordinated ones, which gives rise to a large decrease of their acidity.