Formation of iron-doped mullite (Al4+2xSi2-2xO10-x) by temperature- and time-resolved X-ray powder diffraction and EXAFS, and by Al-27 MAS NMR spectroscopy

Temperature- and time-resolved X-ray powder diffraction and Fe K-edge EXAFS have been recorded simultaneously using synchrotron radiation during the c alcination of a gel prepared by hydrothermally processing a mixture of alum inium-, silicon- and iron-acetates. The results, which are supported by Al- 27 MAS NMR studies, show that iron is initially accommodated in the octahed ral sites of a poorly crystalline pseudoboehmite-type structure (gamma-AlOO H .(n) under bar H2O). At ca. 400 degrees C the material becomes amorphous and a small particle highly defective gamma-Al2O3-type structure develops a s the temperature is increased. No evidence for a discrete crystalline sili con-containing phase is observed, The first evidence of the formation of mu llite (Al-2(oct)((Al2+2xSi2-2xtet)-Si-tet) O10-x) in which iron is accommod ated in the octahedral sites is observed on reaching 1050 degrees C. The ph ase develops as a function of time at this temperature. rights reserved.