On the Shelf Life and Aging Stability of Mesoporous Silica: Insights on Thermodynamically Stable MCM-41 Structure from Assessment of 12-Year-Old Samples

The aging characteristics and stability of decade (12 years) old mesoporous silica MCM-41 samples of varying pore size and wall thickness has been tracked and compared with freshly synthesized samples. It was found that calcined forms of hydrothermally synthesized MCM-41 samples are, depending on their initial wall thickness, generally stable to storage under ambient conditions over the 12 year period. The calcined samples retain all (i.e., 96–100%) of their surface area and virtually all (87–96%) their pore volume. Rather surprisingly, calcined MCM-41 with wall thickness of ca. 11 Å exhibit greater apparent aging stability with respect to retention of unit-cell parameter compared to thicker walled (22 Å) analogues. After 12 years of storage, both types of samples exhibit pore wall thickness of 11–12 Å representing much greater unit cell contraction for the 22 Å sample. Despite varying levels of initial silica condensation and silanol concentration, both types of aged calcined samples exhibit similar 29 Si NMR spectra with similar silanol concentration (ca. 6.5 mmol/g) and silanol density (4.5/nm 2 ). The wall thickness, level of silica condensation, silanol loading, and silanol density of the calcined aged samples closely match values that have been shown by molecular dynamics studies to have the highest stabilization energy for MCM-41 structures. The aged calcined samples are thermally stable to further calcination at 800 °C, wherein they retain 90% of their surface area. Aged as-synthesized forms of MCM-41 also have high stability and remain largely unchanged, and on calcination, behave in a manner similar to that of freshly prepared MCM-41. For aged as-synthesized samples, there is an apparent increase in silica condensation and phase separation of template molecules from the silica, and this forms the basis of their aging stability and stability to subsequent calcination.