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Nanomaterials Fabrication 55

Figure 3.15 Structural models proposed (a and b) and observed (c) for
aluminum oxide nanoparticles formed from the hydrolysis of aluminum
compounds.

proposed that alumoxanes could be prepared directly from the mineral
boehmite. Such a “top-down” approach represented a departure from the
traditional synthetic methodologies.
Assuming that hydrolytically stable alumoxanes have the boehmite-like
core structure (Figure 3.15c), it would seem logical that they could be pre-
pared directly from the mineral boehmite. The type of capping ligand
used in such a process must be able to abstract and stabilize a small frag-
ment of the solid-state material. In the siloxy-alumoxanes it was demon-
strated that the “organic” unit itself contains aluminum, as shown in
Figure 3.16a. Thus, in order to prepare the siloxy-alumoxane the “ligand”

[Al(OH) 2 (OSiR 3 ) 2 ] , would be required as a bridging group; adding this
unit clearly presents a significant synthetic challenge. However, the car-

boxylate anion, [RCO 2 ] , is an isoelectronic and structural analog of the
organic periphery found in our siloxy-alumoxanes (Figure 3.16).
Thus, it has been shown that carboxylic acids (RCO 2 H) react with
boehmite, [Al(O)(OH)] n , to yield the appropriate carboxy-alumoxane:
HO 2 CR
⎯⎯⎯⎯⎯→ [Al(O) (OH) (O CR) ]
[Al(O)(OH)] n x y 2 z n
Initial syntheses were carried out using the acid as the solvent or xylene
[80, 81], however, subsequent research demonstrated the use of water as
a solvent and acetic acid as the most convenient capping agent [82]. A
solventless synthesis has also been developed [83]. Thus, the synthesis

Figure 3.16 Structural relation-
ship of the capping ligand for
(a) siloxy and (b) carboxylate alu-
moxane nanoparticles.