Distinguishing surface versus buried cation sites in aluminosilicate mesoporous materials

Mesoporous MCM-41 aluminosilicates were prepared through direct synthesis and surface grafting resulting in the incorporation of aluminum into the pore walls and onto the wall surface, respectively. 7Li and 23Na NMR studies of ion-exchanged Li and Na-Al-MCM-41 were able to distinguish between cations in the surface region and those buried deeper in the pore walls. Thus it was demonstrated that most of the cations in the grafted Al-MCM-41 locate in the surface region, whereas the cations in the synthesized Al-MCM-41 are distributed throughout the pore walls. The NMR spectra of dehydrated Li- and Na-MCM-41 resemble those of glassy materials, reflecting the amorphous nature of this class of mesoporous materials. 7Li NMR studies of dehydrated Li-Al-MCM-41 prepared from direct synthesis in the presence of oxygen showed that most of the Li+ cations are not accessible to O2, while the Li+ cations in Al-grafted Li-Al-MCM-41 are accessible, which also confirms their locations. This study provides valuable insights for the understanding of the structure and properties of aluminosilicate mesoporous materials.