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

Abstract

Herein, a detailed analysis was carried out using high-field (19.9 T) ²⁷Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) on three specially prepared aluminum oxide samples where the γ-, δ-, and θ-Al₂O₃ phases are dominantly expressed through careful control of the synthesis conditions. Specifically, two-dimensional (2D) multiquantum (MQ) MAS ²⁷Al was used to obtain high spectral resolution, which provided a guide for analyzing quantitative 1D ²⁷Al 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-Al₂O₃ phase. The distinct line shapes related to the θ-Al₂O₃ phase provided an opportunity for effectively deconvoluting the more complex spectrum obtained from the δ-Al₂O₃ dominant sample, allowing the peaks/quadrupolar parameters related to the δ-Al₂O₃ phase to be extracted. The results show that the δ-Al₂O₃ 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-Al₂O₃ spectrum. It is found that the γ-Al₂O₃ phase consists of Al sites with local structures similar to those found in the δ-Al₂O₃ and θ-Al₂O₃ phases albeit with less ordering. Spin-lattice relaxation time measurement further confirms the disordering of the lattice. Collectively, this study uniquely assigns ²⁷Al features in transition aluminas, offering a simplified method to quantify complex mixtures of aluminum sites in transition alumina samples.