Environmental Engineering Reference
In-Depth Information
MOH 2 + MOR M(OH)M + ROH (olation)
(Eq. 2.4)
where M denotes a metal or Si, and R is an alkyl group. Both forms of reactions are
catalyzed by acids and bases; however, they may also proceed under neutral conditions.
The rate of hydrolysis and gelation are strongly pH dependent. Table 2.1 lists common
alkoxides for sol-gel processing.
Table 2.1 Common alkoxides for sol-gel processing.
Name
Formula
Weight (g)
Density at
20 °C (g/cm 3 )
Formula
TEOS
Si(OC 2 H 5 ) 4
208
0.936
Trimethyl Borate
B(OCH 3 ) 3
104
0.915
Aluminum Sec -Butoxide
Al(OC 4 H 9 ) 3
246
0.967
Titanium Isopropoxide
Ti(OC 3 H 7 ) 4
284
0.955
Zirconium Isopropoxide
Zr(OC 3 H 7 ) 4
327
1.05
Normally, the alkoxide is dissolved in its parent alcohol and hydrolyzed by the
addition of water, in the case of more electronegative metals and metalloids, an acid or
base catalyst is prerequisite. Through the hydrolysis process, alkoxide groups are
replaced with a reactive M-OH hydroxo group, and subsequent condensation reactions
involving the M-OH hydroxo group produce oligomers or polymers composed of M-O-
M or M-(OH)-M bonds by alcoxolation, oxolation and olation. Alcoxolation and
oxolation reaction form a bridging oxo group through the elimination of an alcohol
molecules and water molecules, respectably. In case of olation reaction, bridging
hydroxo groups are formed through the elimination of solvent molecules.
Both hydrolysis and condensation process lead to formation of a gel. Then, these
are undergone the drying process and optionally subjected to thermal treatment.
Depending on the drying process, two types of gels can be generated; aerogels (dried
supercritically) and xerogels (dried without supercritical fluid). In general, aerogels have
lower densities, higher porosities, and higher surface areas than xerogels, but both
materials contain an architecture that includes interconnected particles and pores with
nanoscopic dimensions (Hench and West, 1990).
Through careful control of sol preparation and the processing, various oxide
nanoparticles have been synthesized, such as Fe 2 O 3 , SnO 2 Al 2 O 3 , TiO 2 and ZnO (Bruni
et al., 1999; Manorama et al., 1999; Joo, et al., 2005; Shojaie-Bahaabad and Taheri-
Nassaj, 2008). For example, Shojaie-Bahaabad and Taheri-Nassaj (2008) synthesized -
 
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