27Al multiple-quantum MAS and 27Al{1H} CPMAS NMR study of amorphous aluminosilicates

Two-dimensional Al-27 multiple-quantum magic angle spinning (MQMAS) NMR spectroscopy is used to extract the isotropic chemical shifts and quadrupolar parametsrs of five amorphous aluminosilicates, all of approximately mullite composition (3Al(2)O(3) . 2SiO(2)) 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 delta approximate to 30 ppm. remains a subject of debate. In some of the more anhydrous samples, two 6-coordinate AI sites al e observed. Significant distributions of isotropic chemical shifts and qundrupolar 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 Al-27{H-1} CPMAS Nh IR 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 delta approximate to 30 ppm site is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.