High-Field One-Dimensional and Two-Dimensional Al Magic-Angle Spinning Nuclear Magnetic Resonance Study of θ-, δ-, and γ-AlO Dominated Aluminum Oxides: Toward Understanding the Al Sites in γ-AlO.

Herein, a detailed analysis was carried out using high-field (19.9 T) Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) on three specially prepared aluminum oxide samples where the γ-, δ-, and θ-AlO phases are dominantly expressed through careful control of the synthesis conditions. Specifically, two-dimensional (2D) multiquantum (MQ) MAS Al was used to obtain high spectral resolution, which provided a guide for analyzing quantitative 1D Al NMR spectra. Six aluminum sites were resolved in the 2D MQ MAS NMR spectra, and seven aluminum sites were required to fit the 1D spectra. A set of octahedral and tetrahedral peaks with well-defined quadrupolar line shapes was observed in the θ-phase dominant sample and was unambiguously assigned to the θ-AlO phase. The distinct line shapes related to the θ-AlO phase provided an opportunity for effectively deconvoluting the more complex spectrum obtained from the δ-AlO dominant sample, allowing the peaks/quadrupolar parameters related to the δ-AlO phase to be extracted. The results show that the δ-AlO phase contains three distinct Al sites and three distinct Al sites. This detailed Al site structural information offers a powerful way of analyzing the most complex γ-AlO spectrum. It is found that the γ-AlO phase consists of Al sites with local structures similar to those found in the δ-AlO and θ-AlO phases albeit with less ordering. Spin-lattice relaxation time measurement further confirms the disordering of the lattice. Collectively, this study uniquely assigns Al features in transition aluminas, offering a simplified method to quantify complex mixtures of aluminum sites in transition alumina samples.