Investigation of the surfactants in CTAB-templated mesoporous silica by 1H HRMAS NMR

Abstract

High resolution magic angle spinning (HRMAS) leads to nearly liquid-state quality NMR spectra of molecules with restrained mobility. We show here how 1H HRMAS can be applied to organic molecules encapsulated inside mesoporous materials. We investigated an uncalcined surfactant-templated mesoporous silica synthesized from a mixture of cationic and anionic surfactants, CTAB and HPMSP. The pyrazolone HPMSP is adding cation-extracting properties to the silica, which contains 60% of organic compounds in weight. MAS NMR at moderate spinning speeds allows to resolve proton spectra on samples where a small amount of methanol is added to the dried as-synthesized silica. NMR experiments allow to distinguish between solvated surfactants involved in ion pairs and less mobile templating surfactants. Liquid state NMR experiments like 2D NOESY can be performed in these conditions, but suffer from spin diffusion. 1D and 2D solid-state NMR experiments, like Rotational Resonance, which take advantage of the partly solid-state behavior of the surfactant system, are proposed as alternative experiments to get information about spatial connectivity.