Multiple-Quantum and Cross-Polarized ²⁷Al MAS NMR of Mechanically-Treated Mixtures of Kaolinite and Gibbsite

Abstract Mixtures of the clay mineral kaolinite with gibbsite, when heated, form mullite, a high-temperature ceramic. The 27Al multiple-quantum magic angle spinning (MQMAS) NMR technique is used to study the local Al environment in mixtures of kaolinite and gibbsite that have been ground together for varying amounts of time. This mechanical treatment forms new chemical species which are amorphous in nature and, therefore, unsuited to analysis by X-ray diffraction. Both novel and established cross-polarization techniques are employed to characterise the starting materials and mixtures, confirm the spatial proximity of 1H and 27Al nuclei, and "edit" the two-dimensional 27Al MQMAS NMR spectra, thereby allowing overlapping peaks corresponding to octahedral Al sites to be studied.

(a) Two-dimensional 27Al triple-quantum MAS NMR spectrum of KG5 recorded with the z-filtered experiment. (b) and (c) Two-dimensional 27Al triple-quantum cross-polarized MAS NMR spectra of KG5 using a triple-quantum z-filtered cross-polarized MAS NMR experiment. (d) Two-dimensional 27Al triple-quantum MAS NMR spectrum of KG240 recorded with the z-filtered experiment. The centerband peaks are labelled 1,2, and 3; other peaks are spinning sidebands. (e) and (f) Two-dimensional 27Al triple-quantum cross-polarized MASNMR spectra of KG240 recorded with using a triple-quantum z-filtered cross-polarized MAS NMR experiment.