Influences of the aqueous synthesis way and organosilane nature on the physico-chemical 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).