Al Content Dependent Hydrothermal Stability of Directly Synthesized Aluminosilicate MCM-41

Aluminosilicate MCM-41 materials with Si/Al ratio between 10 and 50 were prepared via direct mixed-gel synthesis, and their hydrothermal stability in boiling water was investigated using a variety of techniques including powder XRD, N 2 sorption studies, and elemental analysis. The incorporation of Al markedly improved the hydrothermal stability of the aluminosilicate materials compared to their pure silica analogue. The improvement in hydrothermal stability was found to be dependent on the Al content. Small amounts of Al (Si/Al = 40.1) afforded much better protection of the mesoporous structure than larger amounts (S/Al = 8.5). A material with an intermediate amount of Al (Si/Al = 23.1) exhibited intermediate stability. The presence of Al in the MCM-41 framework, rather than as extraneous species, was found to be essential in the improvement of hydrothermal stability. The influence of Al content on the hydrothermal stability is explained with respect to the position occupied by the framework Al within the MCM-41 pore walls, and it is suggested that Al incorporated on the surface/near surface region of the pore walls provides the greatest protection.