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

Two-dimensional 27Al multiple-quantum magic angle spinning (MQMAS) NMR spectroscopy is used to extract the isotropic chemical shifts and quadrupolar parameters of five amorphous aluminosilicates, all of approximately mullite composition (3Al2O3·2SiO2) 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.