On 29Si NMR relaxation as a structural criterion for studying paramagnetic supermicroporous silica-based materials: Silica-based materials incorporating Mn2+ ions into the silica matrix of SiO2-Al2O3-MnO

Supermicroporous paramagnetic materials SiO2-Al2O3-MnO and SiO2-MnO with different manganese concentrations have been probed by solid-state Si-29 NMR and magnetic susceptibility measurements. The Si-29 T-1 and T-2 experiments, performed in static and spinning samples, have resulted in determination of electron relaxation times, providing, in turn, quantitative interpretations of Si-29 T-1 times in terms of distances Mn-Si. The NMR relaxation data have revealed Si-29 T-1 time distributions, which are close to Gaussian and observed as different T-1 values obtained in MAS NMR experiments for isotropic Si-29 resonances and their sidebands. Such Si-29 T-1 distributions, being a common phenomenon in paramagnetic silica-based materials, can be however masked by the bulk magnetic susceptibility (BMS) effects increasing with concentrations of paramagnetic centers. The presence of T-1 distributions, in itself, is not a criterion for incorporation of paramagnetic ions into the silica matrix or its surface. However, a quantitative analysis of the experimentally-observed short Si-29 T-1 components, based on the well-determined electron relaxation times, can provide such a criterion. Published by Elsevier Inc.