¹⁷O and ²⁷Al MAS and 3QMAS NMR Study of Synthetic and Natural Layer Silicates

We report 17 O and 27 Al NMR results for 17 O-enriched synthetic and natural layer silicates with significantly improved resolution of oxygen sites including basal and apical oxygens using 17 O 3QMAS NMR spectroscopy. These oxygen sites have well-defined ranges of 17 O isotropic chemical shifts (δ iso ) and quadrupolar coupling constants ( C q ). The C q of basal oxygen ( [4] Si−O− [4] Si) range from 4.1 to 5.2 MHz for these phases, and δ iso varies from 39 to 55 ppm. The δ iso value of the basal oxygen sites increases with increasing C q . The C q and δ iso values of apical oxygens ( [4] Si−O−2 [6] Al) are about 3.4 MHz and about 64 ppm, respectively, and are similar among the phases examined. The hydroxyl group (2 [6] Al−OH) has the largest C q (about 7 MHz) among the oxygen sites, as well as the most shielded chemical shift of about 37−44 ppm. Analysis of the 27 Al MAS NMR spinning sideband manifolds yields precise NMR parameters for the [6] Al sites. 27 Al MAS and 3QMAS NMR spectroscopies at 14.1 T yield excellent quantification of the [6] Al/ [4] Al ratio of 2 for natural muscovite. The methods described here, in particular 17 O 3QMAS at high H 0 field (14.1 T), are effective probes of crystallographically distinct sites with similar C q and δ iso value in layer silicates and are thus potentially useful for characterizing the complex and heterogeneous natural layer silicates and other nanoscale earth materials, such as crystalline and amorphous oxyhydoxides and oxides.