In the synthesis of SBA-15 samples without Al, the pore size generally increases with hydrothermal duration at 100oC. However, for the direct synthesis of Al-SBA-15 samples, the pore size decreases as the hydrothermal duration is increased. The pH of my synthesis medium is about 1.8. As such I am puzzled why is there an decrease in pore size? Does it got to do anything with acid leaching of Al under acidic hydrothermal condition?
Although I am not specialist of this particular synthesis, I can say that hydrothermal conditions allow obtaining metastable structures ... providing that the synthesis time is not too long. Indeed, at longer times, the structures change and turn into others having lower free energy. As a consequence, more stable crystalline structures appear, porosity decrease, crystallites get bigger, etc., until the former metastable structures are replaced by more stable and denser ones.
Alain
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