Single- and triple-quantum 27Al MAS NMR study of the thermal transformation of kaolinite

The kaolinite-mullite reaction process has been studied by single- and triple-quantum Al-27 magic-angle spinning (MAS) NMR. All materials have also been characterized by Si-29 MAS NMR and powder X-ray diffraction. It has been shown that in order to record reliable Al-27 MAS NMR spectra it is important to use very fast sample spinning rates (which at 9.4 T are in excess of 30 kHz). The conflicting values previously reported in the literature for the four-, five- and six-coordinated Al metakaolinite populations are, at least in part, due to the relatively modest and different MAS rates and magnetic fields used by the authors. Triple-quantum Al-27 MAS NMR provides hard evidence for the presence in metakolinite of distorted (and distributed) four-, five-, and six-coordinated Al local environments. It also suggests that kaolinite calcined at 950 degrees C is a mixture of amorphous material, poorly crystalline mullite, and gamma-alumina and a small amount of unreacted metakaolinite.