On the hydrothermal stability of mesoporous aluminosilicate MCM-48 materials

Aluminosilicate MCM-48 materials with Si/Al ratio between 4 and 50 were prepared via direct mixed-gel synthesis, and their hydrothermal stability in boiling water and steam was investigated using a variety of techniques including powder XRD, N-2 sorption studies, and elemental analysis. The stability of Al-containing MCM-48 in boiling water was found in all cases to be higher than that of the pure silica material. The improvement in boiling water stability was dependent on the Al content and went through maxima as Al content increased. The optimum Al content for enhancing boiling water stability was found to be in the Si/Al range 8-15. The relationship between Al content and steam stability was somewhat the reverse of that observed for boiling water stability. Small amounts of Al (Si/Al ca. 50) afforded better protection of the mesoporous structure against steaming than larger amounts (Si/Al 4). The influence of Al content on the hydrothermal stability is explained with respect to the role played by the framework Al within the MCM-48 pore walls and its effect on the processes that occur when the Al-MCM-48 framework is in contact with water at low (refluxing) or high (steaming) temperature. The balance between the key processes, i.e., dehydroxylation, hydrolysis (dissolution of silica) and dealumination, determines the stability. Under refluxing conditions, hydrolysis predominates and therefore the presence of medium to large amounts of Al (and the formation of water resistance Si-O-Al bonds) helps protect the framework. Under steaming conditions, both hydrolysis and dehydroxylation (which have opposing effects) occur but dealumination predominates and therefore the presence of significant amounts of Al buried within the framework (which are removed during steaming) acts to diminish stability. It is proposed that, in general, Al positioned on the surface/near surface region of the pore walls provides the greatest protection against water attack under either refluxing or steaming conditions.