²⁷Al 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 (O_h sites) and 70 ppm (T_d 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 T_1p 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 O_h 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.