High-Field One-Dimensional and Two-Dimensional 27Al Magic-Angle Spinning Nuclear Magnetic Resonance Study of θ-, δ-, and γ-Al2O3 Dominated Aluminum Oxides: Toward Understanding the Al Sites in γ-Al2O3

Herein, a detailed analysis was carried out using high-field (19.9 T) 27 Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) on three specially prepared aluminum oxide samples where the γ-, δ-, and θ-Al 2 O 3 phases are dominantly expressed through careful control of the synthesis conditions. Specifically, two-dimensional (2D) multiquantum (MQ) MAS 27 Al was used to obtain high spectral resolution, which provided a guide for analyzing quantitative 1D 27 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-Al 2 O 3 phase. The distinct line shapes related to the θ-Al 2 O 3 phase provided an opportunity for effectively deconvoluting the more complex spectrum obtained from the δ-Al 2 O 3 dominant sample, allowing the peaks/quadrupolar parameters related to the δ-Al 2 O 3 phase to be extracted. The results show that the δ-Al 2 O 3 phase contains three distinct Al O sites and three distinct Al T sites. This detailed Al site structural information offers a powerful way of analyzing the most complex γ-Al 2 O 3 spectrum. It is found that the γ-Al 2 O 3 phase consists of Al sites with local structures similar to those found in the δ-Al 2 O 3 and θ-Al 2 O 3 phases albeit with less ordering. Spin-lattice relaxation time measurement further confirms the disordering of the lattice. Collectively, this study uniquely assigns 27 Al features in transition aluminas, offering a simplified method to quantify complex mixtures of aluminum sites in transition alumina samples.