HIGH-FIELD MAS NMR, WITH SIMULATIONS OF THE EFFECTS OF DISORDER ON LINESHAPE, APPLIED TO THERMAL TRANSFORMATIONS OF ALUMINA HYDRATES

Samples of gibbsite, boehmite, pseudoboehmite and bayerite, calcined at temperatures up to 1000-degrees-C. were studied using Al-27 MAS NMR with spinning speeds up to 15 kHz in magnetic fields of 14.1, 11.8 and 7 T (156, 130 and 78 MHZ Al-27 frequencies). The high magnetic fields reduce the effects of the second order quadrupolar interactions, giving much better resolution of the peaks than previous studies at 7 T and allowing quantitation of the different species. The samples generally show only octahedral and tetrahedral species, with the latter increasing with calcination temperature. The exceptions are rho-alumina and chi-alumina which show a peak at 33 ppm (in the 130 MHz spectra) with about 20% and 7% respectively of the total intensity. The peaks show large upfield tails and a dependence on magnetic field which differs from that expected for a single electric field gradient. Spectra were simulated assuming a random distribution of electric field gradients, and the dependence of the spectra on the field strength, quadrupolar parameters and the range of electric field gradients is illustrated and empirical equations derived. The method provides a good fit to the observed spectra at all 3 field strengths. These materials show considerable disorder which can be categorized by the parameter used to describe the range of electric field gradients.