Influence of the Method of Aqueous Synthesis and the Nature of the Silicon Precursor on the Physicochemical Properties of Porous Alumina.

The aqueous sol-gel synthesis of γ-Al2O3 modified with different silicon precursors {trimethoxysilane, triethoxysilane and N-[3-(trimethoxysilyl)propyl]ethylene diamine} has been investigated. Two parameters have been studied: the silicon addition step and the type of silicon alkoxide. A first observation was that adding a silicon alkoxide either before or just after the precipitation step influences both the crystallinity and structure, whereas adding the alkoxide after a long agitation time only slightly modifies the support properties. It was also highlighted that due to their higher reactivity, silicon precursors with methoxy groups interact more strongly with the bulk alumina than the silicon precursors with ethoxy groups. This difference of Si location in the alumina structure permitted not only their resistance toward phase transition to be increased at high temperature, but also to stabilize them more efficiently against sintering under normal and steam conditions. Among the different silicon alkoxides tested, samples prepared with N-[3-(trimethoxysilyl)propyl]ethylene diamine showed very unusual properties. The alumina supports modified with this precursor showed grain-shaped nanocrystallites, whereas all other samples were platelet-like. As a result, this sample presented a very narrow pore diameter distribution of around 5 nm and a remarkably high specific surface area (530 m2/g). [ABSTRACT FROM AUTHOR]