Structural Differences in Mullite Xerogels from Different Precursors Characterized by 27Al and 29Si MASNMR

Abstract

Mullite (3Al2O3-2SiO2) was prepared from different precursors by multicomponent metal alkoxide and metal alkoxide-metal nitrate methods. Extreme hydrolysis conditions were used to prepare mullite gels of various degree of homogeneities. Thermal treatment of the mullite gels prepared by the multicomponent metal alkoxide method led to the crystallization of pure mullite powder at lower temperatures (≤ 1200°C) via a cubic spinel phase, while the gel prepared by the metal alkoxide-metal nitrate method crystallized to mullite completely only around 1300°C, via a cubic spinel and α-Al2O3. The nearest neighbor environments of aluminum and silicon in the gels themselves and after they were heated to various temperatures were studied by 27Al and 29Si magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. The sequence of VI, V, and IV coordination changes in aluminum are clearly identified. The results show that there are clear differences in structure at the nearest neighbor level between nitrate-organic and purely organic derived gels. The difference between gels derived from tetraethylorthosilicate (TEOS) and tetramethylorthosilicate (TMOS) are also clearly shown. The exotherm appearing between 970 and 995°C in the differential thermal analysis (DTA) curve is discussed based on the X-ray diffraction (XRD) and MASNMR data. 29Si MASNMR results indicated the presence of silica-rich regions with some Si(Al) or residual-OH groups attached to silicon in the partially crystallized samples containing the cubic spinel.