AL-27 NMR-SPECTROSCOPY OF IRON-BEARING MONTMORILLONITE CLAYS

The methods of Bloch decay and selective cross polarization 27Al NMR are applied to kaolin and several montmorillonite clays with varying levels of Fe3+. The Bloch decay experiments of the montmorillonite clays give rise two resonances at ∼3 ppm (Oh sites) and 70 ppm (Td sites). All chemical shifts are reported relative to Al(NO3)3·xH2O(liquid). The 27Al NMR Bloch decay results present evidence for a nonuniform distribution of Fe3+ in montmorillonite clays. The 27Al selective cross polarization method is demonstrated and applied to kaolin and some montmorillonite clays with low Fe3+ content. The clay systems provide an excellent opportunity to investigate the quantitative aspects of the 27Al cross polarization experiment. The quantization of the 27Al CP/MAS method is demonstrated and applied to the same samples. The consequences of MAS in these systems with respect to the CP experiment is discussed at length, e.g., measurement of T1p and Hartmann-Hahn match spectra. The surface Brønsted sites can be selectively enhanced via CP/MAS by reacting the surface hydroxyls with trimethylsilyl chloride. As a result of this procedure, one can distinguish three different classes of Al Oh sites in Fe-bearing montmorillonite clays. The combination of Bloch decay and cross polarization methods appears to an excellent method for the nondestructive characterization of the Al sites in natural clays. © 1990 American Chemical Society.