FORMATION OF MULLITE FROM HYDROTHERMALLY PROCESSED SOLS - AN X-RAY-POWDER DIFFRACTION AND MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE INVESTIGATION

The formation of mullite by the drying and sintering of hydrothermally processed sols has been examined by X-ray powder diffraction (XRD) and magic-angle spinning (MAS) Al-27 and Si-29 nuclear magnetic resonance spectroscopy (NMR). The solid formed by drying the separately hydrothermally processed aluminium acetate was characterised as pseudoboehmite, gamma-AlOOH.nH2O. The X-ray amorphous dry solid formed from hydrothermally processed silicon acetate was shown by Si-29 CP/MAS to be silica gel. Simultaneous processing of aluminium and silicon acetate gave a sol which, when dried, gave an XRD pattern similar to that of pseudoboehmite but with shifts in the (020) and (120) line positions. The MAS NMR results showed that the hydrothermal co-processing of aluminium and silicon acetate does not give a simple mixture of pseudoboehmite and silica gel. We suggest a model in which there is intimate mixing of particulate pseudoboehmite-related and silica gel phases such that interfacial interactions have a marked influence on the structure of the silicon-rich phase. Such a model is not inconsistent with a biphasic mixture on the nanometre scale. Sintering of the dried solid from the simultaneously processed aluminium and silicon acetates at ca. 1250-degrees-C gave monophasic 3/2-type Mullite. The influence of sintering on the formation of mullite is discussed.