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

Abstract

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