PNNL

Abstract

Gamma-, sigma- and theta-Al2O3 are well known metastable “transitional” alumina structural polymorphs. Upon heating, Al2O3 transitions to the so-called and Al2O3 polymorphs and finally forms the thermally stable Al2O3. The poorly developed crystallinity and co-existence of the , , and Al2O3 prior to forming all Al2O3, making it difficult to characterize the structures as well as to quantify the various phases of the transition alumina. As a result, there are significant controversies in the literatures. In this work, a detailed NMR analysis was carried out at high magnetic field on three special aluminum oxide samples where the, , , Al2O3 phases are made dominant, respectively, by controlling the synthesis conditions. The goal is to simplify, including making unambiguous, spectral assignments in 27Al MAS NMR spectra of transition alumina that have not yet been commonly agreed previously. Specifically, quantitative 1D 27Al MAS NMR was used to quantify the ratios of the different alumina structural units, 2D MQMAS 27Al MAS was used for obtaining the highest spectral resolution to guide the analysis of the 1D spectrum, and a saturation pulse sequence was integrated into the 1D NMR to select the amorphous structures, including obtain spectra where the penta-coordinate sites are observed with enhanced relative intensity. Collectively, this study uniquely assigns Al-peaks (both octahedral and tetrahedral) to the Al2O3 and the Al2O3 phases and offers a new way of understanding, including quantifying, the different structural units and sites in transition alumina samples.