Thermal Behavior of Alumina—Silica Xerogels During Calcination

Xerogels of 3Al 2 O 3 ·2SiO 2 mullite were prepared by hydrolyzing Al(NO 3 ) 3 ·9H 2 O and Si(OC 2 H 5 ) 4 solutions with pH values of 8.3, 9.4, 10.1, and 10.4; the xerogels were composed of a combination of singlephase and diphasic materials. A strong alkaline solution enhanced bayerite formation in the gels. Mullite from the diphasic xerogels was produced by reacting θ‐Al 2 O 3 with amorphous SiO 2 , whereas mullite from the single‐phase xerogels was transformed from Al‐Si spinel. For the single‐phase xerogel, the DTA curve closely resembled the kaolinite‐to‐ mullite reaction. For the diphasic xerogels, the Al 3+ ‐containing solution gelled to pseudoboehmite, which transformed to bayerite in solution. The bayerite then decomposed to η‐Al 2 O 3 and to θ‐Al 2 O 3 sequentially on heating.