Formation of iron-doped mullite (Al 4+2 xSi 2−2 xO 10− x) by temperature- and time-resolved X-ray powder diffraction and EXAFS, and by 27Al MAS NMR spectroscopy

Abstract

Temperature- and time-resolved X-ray powder diffraction and Fe K-edge EXAFS have been recorded simultaneously using synchrotron radiation during the calcination of a gel prepared by hydrothermally processing a mixture of aluminium-, silicon- and iron-acetates. The results, which are supported by 27Al MAS NMR studies, show that iron is initially accommodated in the octahedral sites of a poorly crystalline pseudoboehmite-type structure (γ-AlOOH· n H 2O). At ca. 400°C the material becomes amorphous and a small particle highly defective γ-Al 2O 3-type structure develops as the temperature is increased. No evidence for a discrete crystalline silicon-containing phase is observed. The first evidence of the formation of mullite (Al 2 oct (Al 2+2 x tet Si 2−2 x tet ) O 10− x ) in which iron is accommodated in the octahedral sites is observed on reaching 1050°C. The phase develops as a function of time at this temperature.