LAXS and [formula omitted] MAS NMR studies on the temperature-induced changes of non-crystalline single phase type mullite precursors

Abstract

The structural short-range-order of heat-treated non-crystalline precursors with stoichiometric mullite composition (72 wt% Al 2O 3, 28 wt% SiO 2) has been analyzed by large angle X-ray spectroscopy (LAXS), and by 27Al nuclear magnetic resonance (NMR) spectroscopy under magic angle spinning (MAS) conditions. The precursors were prepared from tetraethoxysilane (TEOS) and aluminium- sec-butoxide (Al(OBu s ) 3) by a special slow hydrolysis method. 27Al MAS NMR analyses reveal that the heat-treated mullite precursors have three different Al sites, which, according to literature data are assigned to tetrahedral (fourfold coordinated) [4]Al , pentahedral (fivefold coordinated) [5]Al , and octahedral (sixfold coordinated) [6]Al . The fraction of [5]Al increases with the calcination temperature, whereas the [4]Al and [6]Al percentage decrease. The results of X-ray radial distribution function (RDF) calculations are consistent with the 27Al NMR data. The RDF indicates a gradually increasing degree of polymerization of TO 4 (T=Al and Si), AlO 5, and AlO 6 polyhedra with the calcination temperature.