NMR STUDY OF EXCHANGE AND HYDRATION SITE IDENTIFICATION IN MCM-41

Abstract

Deuteron 1D and 2D NMR spectroscopy was used to study the dynamics of water molecules within the mesoporous material MCM-41. The deuteron spectra show three magnetization components for a sample hydrated to a 0.2 monolayer level. One component was assigned to the pore surface silanol group deuterons that exhibit a broad Gaussian line of 32.6 kHz FWHM and the other components were assigned to the water deuterons. At room temperature one water deuteron component has a powder pattern line shape (splitting of about 4.2 kHz and population of about 61%) and the other has a Lorentzian line shape (about 388 Hz FWHM and population of 39%). Magnetization exchange occurs between these components. An exchange model, based on multi-site exchange, was constructed and used to analyse the results for exchange. For the 0.2 monolayer sample the rate of magnetization exchange out of the hydration site where the water deuterons exhibit a Lorentzian line in the deuteron spectra is 1.3 ms. 2D measurements at 233 K and room temperature confirmed the magnetization exchange scenario for the two water deuteron sites. Combining the deuteron results with proton-silicon cross polarization magic angle spinning experiments together with heat treatment of the sample, definitive hydration site identification for MCM-41 was achieved. This study has shown that the water molecules bound to the hydrogen-bonded silanol groups produce the powder pattern while water molecules bound to the single silanol groups produce the Lorentzian line. This represents a necessary first step toward a meaningful modeling of NMR observables in terms of site-specific water molecule coordination and dynamics in MCM-41.