Page 128 - Materials Chemistry, Second Edition

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2.4. The Amorphous State

as alumoxanes. Alumoxanes are important for applications in antiperspirants, cata-
lysts, and paint additives. Their structure is best described as Al(O)(OH) particles
with a core structure analogous to the minerals boehmite and diaspore (Figure 2.79),
and organic substituents on the surface.

Al(OSiR 3 Þ þ H 2 O ! Al(O)(OHÞ ðOSiR 3 Þ
3 x 1 x n ðgelÞ
D
ð44Þ ! ðAl 2 O 3 Þ ðSiO 2 Þ n
m
As one would expect from the similar electronegativities of Si and O, the hydrolysis
of silicon alkoxides are significantly slower than other metal analogues. For identi-
cal metal coordination spheres and reaction conditions, the general order or reactiv-
ity for some common alkoxides is: Si(OR) 4 Sn(OR) 4 ~ Al(OR) 3 < Zr
(OR) 4 < Ti(OR) 4 . That is, larger and more electropositive metals are more suscep-
tible to nucleophilic attack by water. As a result, the hydrolysis of most metal
alkoxides is too rapid, leading to uncontrolled precipitation. Although the ratio of
H 2 O/M(OR) n may be tuned to control the hydrolysis rate, the nature of the metal
alkoxide (e.g., altering the OR groups or metal coordination number) is the most
powerful way to control the rate of hydrolysis. It is also possible to control the
stepwise hydrolytic pathway, which governs the ultimate three-dimensional struc-
ture of the gel (Figure 2.80).
It should be noted that a sol-gel process may also take place through nonhydro-
lytic pathways. In these systems, a metal halide reacts with an oxygen donor such as
ethers, alkoxides, etc. to yield a crosslinked metal oxide product (Eq. 45).

ð45Þ M OR + MX ! [M O MŠ þ RX
n
In stark contrast to other metal alkoxides, the kinetics for the hydrolysis of Si(OR) 4
compounds often require several days for completion. As a result, acid (e.g.,
HCl, HF) or base (e.g., KOH, amines, NH 3 ) catalysts are generally added to the
mixture, which also greatly affects the physical properties of the final product.
Under most conditions, condensation reactions begin while the hydrolytic processes
are underway. However, altering the pH, [H 2 O/M(OR) n ] molar ratio, and catalyst
may force the completion of hydrolysis prior to condensation.
A likely mechanism for an acid-catalyzed system is shown in Figure 2.81.The
protonation of the alkoxide group causes electron density to be withdrawn from Si,
allowing the nucleophilic attack from water. In contrast, the base-catalyzed hydro-
lysis of silicon alkoxides proceeds through the attack of a nucleophilic deprotonated
silanol on a neutral silicic acid (Figure 2.82). In general, silicon oxide networks
obtained via acid-catalyzed conditions consist of linear or randomly branched
polymers; by contrast, base-catalyzed systems result in highly branched clusters
(Figure 2.83).
As condensation reactions progress, the sol will set into a rigid gel. Since the
reactions occur within a liquid alcoholic solvent, condensation reactions result in a
three-dimensional oxide network [M-O-M] n that contains solvent molecules within