Solid-state NMR study of MCM-41-type mesoporous silica nanoparticles.

A systematic study of the surface of MCM-41-type mesoporous silica nanoparticles prepared under low surfactant concentration was carried out using high-resolution solid-state nuclear magnetic resonance spectroscopy. The structures and concentrations of various species present during dehydration and rehydration of mesoporous silicas between -25 and 500 degrees C were detailed by employing one-dimensional and two-dimensional (1)H, (13)C, and (29)Si NMR, including (1)H signal intensity measurements, (1)H-(1)H homonuclear correlation experiments (double quantum, exchange, and RFDR), and (1)H-(29)Si heteronuclear correlation NMR. These experiments employed high MAS rates of up to 45 kHz. The study shows that the surfactant (CTAB) was almost completely removed by acid extraction. The residual molecules assumed prone positions along the pores, with the tailgroup being most mobile. The weakly adsorbed water was hydrogen bonded to the silanol groups, all of which were involved in such bonds under ambient humidity. Specific structures involving water and silanol groups were proposed for various stages of thermal treatment, which included dehydration, dehydroxylation, and subsequent rehydration.