Publication

Abstract : Aluminosilicates with varying Al2O3:SiO2 molar ratios (3 : 1{,} 3 : 2{,} 3 : 3 and 3 : 4) have been synthesized using a hybrid sol-gel route using boehmite sol as the precursor for alumina and tetraethyl orthosilicate (TEOS) as the precursor for silica. The synthesis of boehmite sol from aluminium nitrate{,} and its use as the alumina precursor{,} is cost effective compared to alkoxide precursors. Structural aspects{,} including bonding and coordination{,} are studied in detail for samples calcined in the temperature range 400-1400 [degree]C using both NMR and FTIR spectroscopy: the results are correlated with phase formation data (spinel and high temperature phases) obtained from XRD and thermal analysis. FTIR results show a broadening of peaks at 800 [degree]C indicating a disordered distribution of octahedral sites caused by crosslinking between AlO6 octahedral and SiO4 tetrahedral units prior to the formation of mullite. 27Al MAS NMR spectra are consistent with a progressive decrease in the number of AlO6 polyhedra with increasing temperature corresponding to Al in these units being forced to adopt a tetrahedral coordination due to the increasing presence of similarly coordinated Si species. XRD results confirm the formation of pure mullite at 1250 [degree]C for a 3Al2O3:2SiO2 system. At 1400 [degree]C{,} phase pure mullite is observed for all compositions except 3Al2O3:SiO2 where [small alpha]-Al2O3 is the major phase with traces of mullite. The synthesis of aluminosilicates through a hybrid sol-gel route and the detailed insight into structural features gained from spectroscopic and diffraction techniques contributes further to the development of these materials in applications ranging from nanocatalysts to high-temperature ceramics.