27Al nuclear magnetic resonance spectroscopy investigation of thermal transformation sequences of alumina hydrates. Part 2.—Boehmite, γ-AlOOH

Abstract

High resolution ²⁷Al magic-angle spinning nuclear magnetic resonance (MAS NMR)(78.15 MHz) spectra are reported for boehmite and pseudoboehmite and for calcines occurring in their thermal transformation sequences (involving γ, δ, θ and α-aluminas). Spectra for boehmite, pseudoboehmite and corundum (α-alumina) are fully consistent with the known use of only octahedral aluminium (AlO₆) sites. Spectra for γ-aluminas are consistent with defect spinels, with the extent of disorder dependent on precursor material and calcination conditions. δ-Alumina either retains approximately cubic close packed (c.c.p.) oxygens (with aluminium atoms on both spinel and non-spinel octahedral and tetrahedral interstices) or may contain some five-co-ordinate Al (AlO₅) in addition to AlO₆ and AlO₄ environments (and hence have oxygen packing deviating from c.c.p.). Aluminiums in low-symmetry sites manifest themselves instrumentally for the most disordered systems as wings extending to either side of the central spectrum.