Chemical exchange and spectral coalescence in low-hydration MCM-41 studied by proton NMR

Abstract

In Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) experiments, involving silica-based porous media, containing surface OH groups and water, it is generally observed that the water proton and OH group proton resonances tend to coalesce with increasing moisture content. It has been suggested that chemical exchange between protons of surface OH groups and water may not be the only mechanism producing the coalescence in the mesoporous material MCM-41. [Chem. Eur. J. 10 (2004) 5689] Here the importance of such exchange was investigated in MCM-41, hydrated to one water molecule per OH group, using low-field, static NMR relaxation techniques and high-field MAS NMR techniques. First the surface-water chemical exchange rate was estimated from two dimensional time domain NMR selective inversion recovery experiments. Second the exchange rate required to produce coalescence was estimated by modeling the exchange spectra using expected water proton NMR spectral positions and surface OH proton NMR spectral positions obtained from MAS experiments in dry MCM-41. The results suggest that the surface-water chemical exchange rate is too small to produce substantive proton NMR spectral coalescence in the MCM-41 material at this hydration level.