AL-27 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY INVESTIGATION OF THERMAL TRANSFORMATION SEQUENCES OF ALUMINA HYDRATES .1. GIBBSITE, GAMMA-AL(OH)3

High-resolution Al-27 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; chi, alpha and rho-aluminas; calcines produced at 400, 700, 900 and 1000-degrees-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 rho-alumina a third peak (at 27 ppm) is seen due to the presence of Al in five-co-ordination. kappa-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.