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

Two-dimensional Al-27 multiple-quantum magic angle spinning (MQMAS) NMR spe ctroscopy 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 origi n. Three principal types of Al site are apparent in each sample: two of the se 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. rema ins a subject of debate. In some of the more anhydrous samples, two 6-coord inate AI sites al e observed. Significant distributions of isotropic chemic al shifts and qundrupolar parameters are evident in each of the Al sites re solved in the two-dimensional spectra and lineshape fitting is used to esti mate 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 a re most closely associated with particular 6-coordinate Al sites. The NMR r esults from the five samples are compiled and compared with those reported for other amorphous and crystalline aluminosilicates and the possible natur e of the delta approximate to 30 ppm site is discussed. (C) 2001 Elsevier S cience B.V. All rights reserved.