29Si spin-lattice NMR relaxation in microporous silica-based materials with high Mn2+ concentrations.
The 29Si T1 time measurements have been performed under magic angle spinning for supermicroporous Si/Mn materials 1-6 containing Mn2+ in concentrations of 0 (1), 1.2 (2), 2.5 (3), 3.8 (4), 15 (5) and 19.8 wt% (6). It has been found that the central lines, corresponding to isotropic chemical shifts, and their sidebands relax differently in samples 2-4 with relatively small Mn2+ contents. Because the relaxation curves for 1-6 are nonexponential and identical for 6 at spinning rates of 5, 10 and 12 kHz, dipolar relaxation via the paramagnetic centers is dominant. The relaxation data have been interpreted as a function of Mn2+ concentrations in terms of incorporation of the Mn2+ ions into the silica lattice when their concentration is small (<or=2.5 wt%). At higher concentrations, most of the manganese is located on the surface of the silica.
Bakhmutov
Vladimir I
VI
Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, USA. bakhmoutov@mail.chem.tamu.edu
Shpeizer
Boris G
BG
Clearfield
Abraham
A