Revisiting kaolinite dehydroxylation: A 29Si and 27Al MAS NMR study

The 29Si and 27Al MAS NMR spectra of kaolinite are easily interpreted in terms of a Q3 environment for silicon and of an octahedral configuration of aluminum. Upon progressive dehydroxylation, these spectra become more complex. For a degree of dehydroxylation (α) between 1/10 and 9/10, three 27Al resonances are observed at +3, +28, and +55 ppm. The octahedral component decreases in intensity as α increases, whereas the other two become dominant. The line at +28 ppm is assigned to pentacoordinated Al, as suggested by Gilson et al. The +55-ppm line is attributed to tetrahedrally coordinated Al. Simultaneously, the -91-ppm (initial) component of the 29Si spectrum decreases in intensity, while at least two components corresponding to more shielded Si nuclei grow in intensity. As α increases beyond 9/10, further shifts are observed in the 29Si spectra as mentioned by Meinhold et al. Ultimately a line at -109 ppm suggests a significant proportion of the 29Si nuclei being in a Q4 environment, while the 27Al spectrum is reminiscent of that of a spinel-like material. The relationships between the 29Si and 27Al spectra are discussed and compared with IR data in the 1,200-400-cm-1 range and with earlier results on 1H (static) NMR