Abstract
Two-dimensional ²⁷Al MQMAS NMR spectroscopy is used to extract the isotropic chemical shifts and quadrupolar parameters of five amorphous aluminosilicates, all of approximately mullite composition (3Al₂O₂.2SiO₂) but of widely differing synthetic origin. Three principal types of Al site are apparent in each sample: two of these are conventionally assigned to 4- and 6-coordinate Al, while the nature of the third site, observed at a shift of δ = 30 ppm, remains a subject of debate. In some of the more anhydrous samples, two 6-coordinate Al sites are observed. Significant distributions of isotropic chemical shifts and quadrupolar parameters are evident in each of the Al sites resolved in the two-dimensional spectra and lineshape fitting is used to estimate the means and widths of these. Additional data are obtained from ²⁷Al {¹H} CPMAS NMR experiments and suggest that the protons in the samples are most closely associated with particular 6-coordinate Al sites. The NMR results from the five samples are compiled and compared with those reported for other amorphous and crystalline aluminosilicates and the possible nature of the δ = 30 ppm site is discussed.

Computer-simulated spin I=5/2 lineshapes in two-dimensional triple-quantum MAS NMR spectra: (a) single ridge lineshape with CQ = 5 MHz, η = 1 and δiso = 0 ppm; (b) includes a 'box-car' distribution of δiso from -8 to +8 ppm; (c) includes a box-car distribution in CQ from 3 to 7 MHz; (d) includes uncorrelated box-car distributions of both CQ (from 3 to 7 MHz) and δiso (from -8 to +8 ppm).