27Al nuclear magnetic resonance spectroscopy investigation of thermal transformation sequences of alumina hydrates. Part 1.—Gibbsite, γ-Al(OH)3

Abstract

High-resolution ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR)(78.15 MHz) spectra are reported for commercial and natural gibbsites and materials occurring in the thermal transformation sequences of gibbsite (boehmite; χ, α and ρ-aluminas; calcines produced at 400, 700, 900 and 1000 °C). Separate peaks assignable to aluminiums in near-regular six- and four-co-ordination in oxygen are observed for the materials in which use of such sites is believed to occur. For ρ-alumina a third peak (at 27 ppm) is seen due to the presence of Al in five-co-ordination. κ-Alumina may retain approximately hexagonal-close-packed oxygens (with a defect structure leading to a grossly asymmetric contributory line from four-co-ordinate Al) or may contain some five-co-ordinate Al (AlO₅) in addition to AlO₆ and AlO₄ environments (and hence have oxygen packing deviating from h.c.p.). Distortions/disorders (when present) result in broader underlying signals and manifest themselves instrumentally for the most disordered systems as wings extending to either side of the central spectrum.