Thermal behaviour of mixed alumina–silica gels obtained from alkoxides: Phase formation and morphology of powders

The SiO2–Al2O3 powders of various chemical composition were synthesized via the alkoxide sol–gel route from solutions containing aluminium tri-sec-butoxide (Al(OC4H9)3; TBA) and tetraethyl orthosilicate (Si(OC2H5)4; TEOS) as the precursors. The composition and morphology of the obtained gels were examined by TG/DTA, XRD, FTIR, SEM and isothermal nitrogen adsorption (BET). In this research, all the gels after heating at 500 °C were amorphous and became crystalline after sintering at 1200 °C. The results show that the crystallinity and structure of the phases formed depend on the molar ratio of the TEOS/TBA precursors, i.e. the atomic ratio Si:Al. In the powder obtained from the sol in which the atomic ratio Si:Al = 1:3 (as in mullite) only one crystalline phase was observed — an orthorhombic mullite. In both the powders poorer and richer in silica (in comparison with the stoichiometric ratio of mullite), the crystallization of the mullite phase is less efficient and is preceded by spinel crystallization. The obtained powders show a low value of specific surface development. The specific surface area of the silica–alumina powder obtained from the sol with stoichiometry 2SiO2 ·3Al2O3 after thermal treatment (1000 °C; 3 h) is the highest (10.5 m2g−1) while that of the other powders is considerably smaller and almost identical (~ 1.1 m2g−1).