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C H APTER - Ill
THE ROLE OF THE CATALYST
IN SOL-GEL PROCESSING
OF SILICA GLASS
CHAPTER Ill
THE ROlE OF THE CATAlYST IN SOl-GEl
PROCESSING OF SiliCA GlASS
3.1 INTRODUCTION
The first recorded sol- gel synthesis of silica was conducted
in 1866, and since then sol-gel processing of inorganic solids has been the
subject of widespread among the material scientists (Friedel and Craft 1866,
Post 1943). Despite this fact, however, minimal understanding of the actual
mechanisms of hydrolysis and polymerization in the presence of different
catalysts has been developed. Unlike conventional glass and ceramic
synthesis, in which powders are reacted at high temperature, the sol- gel
process relies on a low temperature condensation reaction in liquid solution,
similar, to that utilized in the manufacture of some organic polymers.
In the sol- gel processing of silica, a silicon-containing raw
material, a solvent, water, and a catalyst are generally utilized. By accepted
definition, a constituent in a chemical reaction is considered a catalyst if :
1) The catalyst is unchanged chemically at the end of the reaction;
2) A small amount of catalyst is sufficient to bring about a
considerable extent of reaction, and ;
3) The catalyst does not affect the position of equilibrium in a
reversible reaction.(ller 1979)
A number of papers have been published on the effects of
pH on the properties of gels and the effect of rapid vs. slow hydrolysis, a
function of both pH and the water content of the solution.(Brinker et al
1982, Nogami and Moriya 1980, Yamane and Kojima 1981, Zarzycki 1982.
Klein and Garvey 1980, Mukherjee 1980, Rabinovich etal 1982, Sakka and
Kamiya 1980, Yamane and Okano 1979, Majumdar and Mahajan 1999,
Majumdar and Singh 1998). For example, Brinker reported that " under
comparable conditions, base- catalyzed hydrolysis proceeds much faster than
the acid- catalyzed reaction." (Brinker et al 1982). In a subsequent paper,
however, Brinker conducted another series of experiments from which he
postulated that base-catalyzed hydrolysis proceeds more slowly relative to the
polymerization reaction than acid- catalyzed hydrolysis. (Brinker et a! 1984).
Table 3.1 presents some examples of previous work in the authors'
conclusions under the column heading "comments".
No one has conducted a systematic study of the effects of a wide
variety of catalysts on sol-gel processing of silica under standardized
conditions. The gelation process of metal- alkoxides involves both hydrolysis
and polymerization reactions is complex, depending not only upon pH, but
also upon the reaction mechanism(s) of each catalytic agent. The aim of
this work is to systematically examine a range of catalysts to provide a
foundation upon which future work can be predicted. The objectives are
•
•
•
To determine how different catalysts affect gelation rate
To determine how different catalysts affect the properties of dried
and fired gels
To propose a tentative reaction mechanisms for the catalysts
considered.
3.2 GEL PREPARATION
In order to conduct a systematic study of the role of
catalysts, a standardized solution composition was utilized for all of the
results presented. The compositional ratio was four moles of enthanol and
four moles of water to one mole of tetraethoxysilane. Table 3.2 presents
this standard solution composition. The ethanol used in this study was
dehydrated, 200 proof supplied by Aldrich Chemicals Company U.S.A. The
tetraethoxysilane was 99.9 percent purity from Alfa Chemicals Ventuon
D1vision. The water was high purity, distilled, and double- deionized provided
by the Department of Chemistry, Government Science College, Raipur Figure
3.1 schematically shows the procedural steps in the preparation of the gel
samples produced in this study.
After mixing, solutions containing catalysts were allowed to
gel at 25°C. The time of gelation was measured by the "standardized
time of penetration method." It is important to determine exactly when a
solutions gels and, therefore, select some arbitrary viscosity value that can
be defined as the "gelled" state. In order to quantitatively determine when a
solutions gels, a viscosity of 10,000 poise was selected. A viscosity probe,
consisting of a thin glass rod was developed. A callibration curve for this
probe of viscosity vs. time penetration was prepared using Brookfield
viscosity standards, molten sucrose, and molten glucose at fixed
temperatures. A mark one inch from the tip of this probe represents the
fixed penetration distance. For a viscosity of 10,000 poise, the viscosity
probe requires about seventy seconds to reach the mark (Fig. 3.2).
After gelation, samples were air dried at 25°C in semi- open
samples containers until they no longer exhibited weight loss due to
evaporation of residual water, catalyst, and solvent. The semi- open
containers were constructed such that one percent of the surface area of
the container lid was exposed to the air. After drying, physical properties of
the gels were measured. These included bulk density, apparent density,
porosity, volume percent shrinkage upon drying, and Vicker,s hardness.
Subsequent heat treatment was conducted at 600°C for 18 hours. after
which the physical properties of the fired gels were also measured.
3.3 THE EFFECT OF THE CATALYST ON GELATION TIME
AND PHYSICAL PROPERTIES
In the previous section, the preparation procedures were
described for the silica gels produced in this study. In this section, the
effect of varying the catalysts on gelation time and such physical properties
as porosity, bulk density, apparent density, volume shrinkage upon drying,
and Vicker's hardness are presented and discussed.
In table 3.3, gelation times and apparent initial pH of
solution values are presented for six different catalysts at equivalent mole
concentrations in solutions of the silica standard composition. Three of the
acid -catalyzed solutions exhibit low initial pH of solution values and long
gelation times. Acetic acid shows a significantly higher pH in solution which
could be attributed to a lower degree of dissociation for acetic acid. This
lower degree of dissociation has been documented in the C.R.C. Handbook
and basic chemistry texts for a wide variety of aqueous and alcohol
solutions. Another possible explanation for acetic acid will be discussed in
the next section.The higher pH for HF, on the other hand, cannot be
attributed to a lower degree of dissociation. It has been observed that for
solutions in which additional HF has been added to bring the pH below
0.5, gelation occurs in less than three minutes. The pH of solutions values
were measured with a glass electrode with a Systronix digital pH meter.
Figure 3.3 shows the pH of solution for four different catalysts during
the first hour after mixing. Subsequent pH measurements were conducted
near gelation and the pH was found not to vary significantly after the first
hour. The profiles for HN03 and H2S04 match almost identically the HCl
profile (Fig. 3.3). Gelation times do not appear to correlate with the pH of
solution for different catalysts, despites the fact that it is widely recognized
that for the same catalyst, decreasing the pH of solution also decreases the
time of gelation.(Aelion et al 1950) An example of this is presented by
Yamane and Kojima for the Sr0-Si02 system.(Yamane and Kojima 1981)
Table 3.4 presents some of the properties of the gels
examined in this study. The 25°C values are for gels dried in air with no
heat treatment., and the 600°C values are for gels heat treated at 600°C
for 18 hours. The apparent density is the density of the matrix including
closed pores and the bulk density is the total density including both open
and closed pores. The bulk densities of the HF and NH40H catalyzed gels
are significantly lower than those of the four other gels in both the heat
treated and unheat-treated samples.
In all properties measured, the HF catalyzed gels appear
similar to the NH40H catalyzed gels. The low porosities and high bulk
densities of the HCl, HN03 and H2S04, and acetic acid catalyzed gels
appear similar to separate results obtained by both Brinker and Zarzycki, for
HCl catalyzed gels, in what have been described as slow polycondensation
reactions. The high porosity and low bulk density of the NH40H catalyzed
gels has been attributed to hydrolysis due to nucleophilic substitution of OH
groups. (Brinker 1982) Despite the similarities in properties, the HF catalyzed
gels are transparent while the NH40H catalyzed gels are white/ opaque.
Figure 3.4 is a photograph of a highly transparent sample of HCl catalyzed
silica glass. Large plates, rods, and other more complex shapes have been
obtained from HCl catalyzed gels far more readily than any of the other
catalyzed silica gels so far examined. Unlike most typical gel samples
HF catalyzed gels can be heated directly from room temperature to
in a few hours with minimal possibility of cracking during the firing process.
In table 3.4, the Vicker's hardness was measured by the
micro- indentation method using a Tukon microhardness tester. The decrease
in hardness as a function of porosity seems to follow an inverse exponential
relationship similar to that proposed for the fracture strength of sintered
ceramics and metals produced by powder metallurgy.(Kingery et al 1976) In
fact, the data presented was used to develop the following quantitative
relationship between hardness and porosity.
H = H0 exp (-BP) ... ( 1)
where; H0 = 750 (measured for silica)
B 4.64
P volume fraction pores.
and H = hardness of heat- treated silica gel.
It is interesting to note that for strength vs. porosity, the
constant "B" ranges between 4 and 7 (Kingery et a! 1976). The constant
calculated for the hardness of silica gels falls within this range. The acetic
acid catalyzed gel, possessing only 1.9 percent porosity after heat treatment,
has a Vicker's hardness of 667, which is considerably harder than window
glass and almost as hard as melt cast fused silica.
In the present chapter, the effects of varying the catalyst on
gels have been presented. It is evident that the effect of the catalyst on the
gelation process dramatically affects the properties of the resultant gel.
Possible mechanisms and further evidence is presented in the following
sections.
3.4 MECANISMS Of CATALYSIS
In !hl' JHt'VHHI.., '>t't l1o11, tfu• qt"ll1tlon ltttll'' .1nd JHOJU'tllt'" ot
,tlu" W<'l<' Jll<''<'IJII•cl '"' '"lut1"11' 111 wluch only till' ',,t,1Jv,r w." '''"'"'"
Tlw 'i'JIIIfico~nt v.lll<llion '>f ql'l.1t1on tunes ,md propl'tlll'' '.umot lw t•xpl.lllll'cl
"''''''! on till' h."" of pi I <IIlli till' wl<ltiv<' dt•qr<•t• of d1"'"'l•1t1on of <'·H·h
co~t.IIV't Tlw p.u-twul.u 1<'<11'11<>11 ""'' h.mis111 <'tllploved l>v ''""" , •lt.th)'t 11111't
"'"' lw con'"'''rl'd. l.1k<'ly tllod<•l'. based upon informaiJon ,,!n•.Hly in till'
litPr<lture, 111 conJutKtlon w1th tlw t•xperimental rl'sults lwrein cont.litwd c.1n
be proposed
Many authors have already proposed il pH dependency that
has been typically associated with general acid and general base hydrolysis.
(Klein and GaiVey 1980, Mukherjee 1980, Rabinovich eta! 1982. Sakka and
Kamiya 1980, Yamane and Okano 1979, Majumdar and Mahajan 1999.
Majumdar and Singh 1998). Acid catalyzed hydrolysis is and electrophilic
reaction that can be expressed by equations 2(a) and 2(b).
(R0) 3Si0R + Hp+ -------- (R0) 3SiOH + H+ + HOR . (2a)
... (2b)
In this kind of hydrolysis reaction, the reaction rate is
governed by the concentration of hydronium in solution. In base catalyzed
hydrolysis, a nucleophilic substitution of hydroxyl ions for OR groups occurs.
This reaction is presented in equations 3a and 3b.
(R0)3SiOH + H30+ -------- (R0)3SiOH + OR
OK + H20 -------- HOR + OH-
... (3a)
... (3b)
Analogous to the acid catalyzed reaction, the rate of the
base catalyzed hydrolysis is a function of the hydroxyl concentration in
solution. (Aelion et al 1950)
The mechanisms proposed in equations 2 and 3 appear to
apply when one only considers HCl vs. NH40H catalyzed reactions, as has
been the case in most of the silica sol-gel literature. Expanding the study
to include more catalysts reveals that other factors besides pH alone
contribute to the gelation times and properties of silica gels prepared under
comparable conditions.
In the case of HF catalyzed reactions, it is evident that the
anion must play a significant role in the gelation process. In order to
confirm this observation, silica gels were prepared using equivalent molar
concentrations of potassium halide salts (Table 3.5). In only one of these
cases, using potassium chloride, did the salt appear not to dissociate fully in
solution, as evidenced by the salt remaining at the bottom of the beaker.
This is also the only instance in which solution immiscibility was observed
for this standard silica solution composition. The gelation times for the KBr
and KI catalyzed gels were three orders of magnitude greater than for the
case of KF catalyzed solution. The KF catalyzed gel was white and opaque
while the KBr and Kl gels were transparent. In addition to having the most
rapid gelation time, the KF solution exhibited a slightly basic character while
the KBr and KI solutions were mildly acidic.
Further confirmation of this trend can be seen in table 3.4,
comparing the gelation times for solutions catalyzed by four different acid
halides. Once again, the solution containing fluorine gelled most rapidly.
The hydrolysis and polymerization reactions presented in figures 3.6 and 3.7
are responsible for the rapid gelation of fluorine catalyzed gels. In the
hydrolysis reaction, a fluorine anion approaches a molecule of
tetraethoxysilane in solution forming a highly unstable pentacovalent activated
intermediate. This complex rapidly decomposes to form a partially fluorinated
silicon alkoxide plus by- products of water and alcohol. Another
pentacovalent complex is formed in the presence of water that decomposes
in to a partially hydrated silicon alkoxide plus regenerated fluorine and
hydronium. This process can continue until nearly all of the ethoxide bonds
are replaced by OH. More than likely, however, the polymerization process
begins before all of the tetraethoxysilane is hydrated.
The polymerization process, which for simplicity will be
modeled with a completely hydrated silicon, required a hexacovalent
intermediate. This hexacovalency is required in order for the neighboring
monomer species to approach the silicon. Jler postulates that the
effectiveness of the fluorine in th epolymerization reaction, whichhe proposed
for silicic acid, is due to the smaller ionic radius of the fluorine anion
versus that of the hydroxyl, which performs the same function of temporarily
increasing the coordination of silicon.(Iler 1979)
It has been observed that nucleophilic catalysis of the gelation
reaction tends to microstructures of large spherical particles of silica with
large porosities and pore diameters. In as much as the proposed fluorine
catalysis mechanism is also a nucleophilic substitution reaction, it is not
unreasonable to expect that a similar structure might also be observed,
Figure 3.8 is a scanning electron micrograph of an HF catalyzed gel fired
to 700°C for 18 hours. This photograph shows a well polished surface with
a void in the center. Inside this void, many spherical particles of
approximately 1000 A in diameter are present. The polished surface also
reveals these particles but with mush less clarity.
In table 3.3, the apparent initial pH of solution and gelation
times were presented for six different catalysts. In the case of acetic acid,
it was pointed out that the pH of solution was significantly higher than for
HCl, HN03 , and H2S04 catalyzed solutions (3. 70 vs. 0.05). One obvious
explanation for this difference might be the lower degree of dissociation
documented for acetic acid.
The fact that the gelation time was shorter than these other
three catalysts would suggest that another possible explanation is necessary.
In fact, anionic substitution of the acetyl radial has been documented for
hydrolysis involving silicon alkoxides, including tetraethoxylilane (Andrianov
1965). The difference between this and other catalytic reactions is that true
catalysis, as defined in the introduction, would not be occuring (Figure 3. 9).
In this reaction, the acetyl groups are being consumed in the reaction to
form ethyl acetate as a by- product. This mechanism would also explain the
significantly higher pH observed for acetic acid catalyzed solutions.
Although traditional acid and base catalyzed gelation
reactions, typified by HCI and NH40H, have been discussed in the literature,
these reaction mechanisms cannot be dismissed. The total rate of gelation is
a functions of both the hydrolysis and polymerization reactions. The slowest
of these will be the rate determining step. Aelion, et a!., have shown that
the concentration of HCI and base (Aelion 1950). As a consequence, the
rate of hydrolysis without a catalyst and, therefore, the rate of gelation, is
extremely slow (Table 3.3). Iler demonstrated that the polymerization rate of
silicic acid, a fully hydrolyzed silicon, in an aqueous solution exhibits a
complex sinusoidal shape due to the interaction of several competing
processes. (Jler 1979)
In Iler's model for the gelation of silicic acid, three main
factors were considered to explain the roughly sinusoidal rate of gelation (i.e.
polymerization). These three factors are :
•
•
•
The isoelectric point (lEP) for silica in a stable solution is about
pH=2
At low pH, trace impurities of fluorine, introduced with the acid
catalysts, will greatly accelerate the polymerization reaction, and
As the OH concentration in solution is increased, the
electronegative surface charge on particles suspended in solution
will increase, thereby, retarding gelation.
Neglecting the differences in solvent medium between the
;stems studied by Aelion, ller, and the standardized solution herein
iscussed, the generalized inverse rate of reaction vs. pH of solution model
f Figure 3.10 can be proposed. This model applies for systems in which
eneral acid and general base catalysis are predominate.
Moreover, the relative rates of these reactions to one another
tnd, hence, the total rate of gelation, will be affected by such factors as
olution concentration, water content, and the type of silicon-containing raw
naterial utilized. The dashed lines in Fig. 3.10 represent the rate of reaction
iSsuming no trace impurities of fluorine introduced into the solution. Since
Jnly a few parts per million fluorine can affect the rate of reaction
:lramatically, the "with impurity" reaction rate has been drawn solid
(ller 1979).
In total, twelve different catslysts and their effect on gelation
have been examined in this study. These catalysts fall into four distinct
categories:
• general acid catalysts
•
•·
•
general base catalysts
salts
fluorine containing compounds .
Based upon the information in the literature in combination
with the experimental evidence and models in this study, a summary of
catalysts and their principle catalytic mechanisms in the hydrolysis and
polymerization of tetraethoxysilane are presented in table VII. lt is evident
that catalytic mechanisms based upon pH alone are inadequate to explain
the gelation process.
3.5 EFFECT OF THE CATALYST ON PORE AND CAPILLARY
EFFECTS DURING DRYING
It has been obsered during the drying of gels, that the
surface tension and vapor pressure of the solvent and pore diameter of the
gel have a pronounced effect upon the volume shrinkage and porosity of the
resultant gel. (Zaezycki 1982) It has also been observed that the choice of
the catalyst produces a wide variation in these same properties. The
purpose of this particular section is to explain these observations based upon
the effect of the catalyst and pH on the gelation process, as developed in
the preceding section.
The final shrinkage and porosity of a gel can be viewed as
the result of two opposing forces; the capillary pressure produced by the
volitilization of the liquid residuals pulling the gel matrix towards collapse
versus the ability of the matrix to resist deformation due to its rigidity, as
determined by its extent of crosslinking and mcrosturctural stiffness.
Fig. 3.11 shows these interrelated processes, and the main factors that
affect them.
Fig. 3.12 presents an idealized schematic of a fluid inside a
cylilndrical pore. The capillary pressure inside the pore is described by
equation 3.4.
where
2y,1 cos e P=----
' r
Y = liquid-vapor surface tension vi
r = the pore radius
and e = the wetting angle.
.... (3.5)
The welting angel can be determinde knowing the liquid
vapor surface tension, the liquid-gel surface tension, and the vapor-gel
surface tension using equation 3.5.
cosO=---- . ... (3.6)
In chemical handbooks, however, only the liquid-vapor surface
tension is given. Values of the other two surface tensions for silica gels and
different solvents do not currently exist in the literature. An approximation
suggested by Zarzycki is to assume complete wetting, acondition for which
only the liquid- vapor surface tension os required (Zarzycki 1982). For
Zarzycki's approximation, equation 3.4 reduces to a simplified form given by
equation 3.6.
2 Yvg
P,= --r
.... (3. 7)
ln actual cases, however, it is not realistic to expect the wetting angle to
approach zero for all liquids.
It is appropriate, at this point, to relate the catalytic
mechanisms of the gelation process and tje resulting microstructures with
the properties of the dried and fired silica gels. Many authors have
postulated a fine network structure of linear chains for acid catalyzed gels
and more dense spherical particles with large interstices between them for
base catalyzed gels. (Brinker et al 1982, Nogami and Moriya 1980, Yamane
and Kojima 1981, Zarzycki 1982, Klein and Garvey 1980, Mukherjee 1980,
Rabinovich eta! 1982, Sakka and Kamiya 1980, Yamane and Okano 1979,
Majumdar and Mahajan 1999, Majumdar and Singh 1998).
In view of Fig. 3.10, which combines ller's results on the
effect of pH on the polymerization reaction with Aelion's relationship on the
effect of catalyst concentration on hydrolysis, the microstructural differences
between acid and base catalyzed gels can be explained. For pH values
above the isoelectric point of the solution, which is approximately 2 for
silica, increasing the OH concentration increases the rate of
polymerization.This would be expected to produce a continuous decline in
gelation time with increasing OH concentration. This decrease in gelation
time,however, reaches a minima due to repulsive charge build- up on the
particulate clusters. As pH is increased beyond this minima point, the size
of the sol particles must corrospondingly increase in order to overcome this
repulsive surface charge effect (Her 1979). This results in gels with
microstructures consisting of large spherical particles for high pH catalyzed
solutions. In the extreme case, precipitation may occur before gelation.
In the low pH regime, linear chain growth seems to be
prefered. This may be the result of higher reactivity at chain ends
promoting continued linear growth due to a lower mass density that allows
for easier approach of reactive groups. Simply stated, the chain ends are
less crowded. A more sophistcated approach would be to suggest that the
electron charge density at the chain ends is perturbated, resultion in a
higher electropotential for reaction.
In Fig. 3.13, idealized drawings of acid and base catalyzed
microstructures are presented with comments. For acid catalyzed gels. a
network structure of linear chains with low cross-linking and, therefore, low
stiffness is formed. These thin chains have a greater degree of freedom to
bend, rotate, and plastically deform due to capillary forces, as discussed
earlier, than the interconnected spheres obtained for base catalyzed gels.
The large interstices formed by the packing of these spheres provide an
easy path by which volitiles can be removed.
In the present section we have made an altern! to explain,
the process by which volatilization of residual liquids in the gel results in
shrinkage. A better understanding of how the wet gel microstructure, as
determined by the catalyst employed in the hydrolysis and polymerization
reactions, affects the volume shrinkage during drying has been discussed.
The role of the catalyst in the sol- gel processing of silica
has been examined in a systematic approach. The catalyst affects both the
hydrolysis and polymerization reactions. The slowest of these reactions is the
rate determining step. Different catalysts produce significant variations in the
gelation time and properties of gels. Gelation times and properties do not
solely depend upon the pH of solution, but also depend upon the catalytic
mechanism in both the hydrolysis and polymerization reactions.
In the case of fluorine compounds, nucheophilic substitution of
fluorine catalyzes both the hydrolysis and polymerization reactions. For
general acid and general base catalyzed reactions, the rate of gelation vs.
pH of solution is complex, depending upon the relative rate of hydrolysis,
the isoelectric point of solution, electrorepulsive surface charge build- up on
particles with increasing OH concentrations, and trace impurities of fluorine
as introduced by the acid catalyst. It has been demonstrated that the
catalyst and pH of solution affect the microstructure of the wet gel and,
therefore, the volume shrinkage and drying properties of the gel.
PROCEDURAL STEPS IN PREPARATION OF GELS
MIX SOLUTION OF WATER,
ETHANOL, AND CATALYST
DECANT SOLUTION INTO
MEASURED QUANTITY
OF TEOS
MONITOR VISCOSITY BY
"STANDARDIZED TIME OF
PENETRATION METHOD"
AIR DRY AT 28°C
HEAT TREATMENT AT 700°C
FOR 24 HOURS
MEASURE PROPERTIES
FIG 3.1 SCHEMATIC REPRESENTATION OF STANDARDIZED GEL PREPARATION PROCEDURE.
1 X 106
5 X t05 VISCOSITY TIME (SEC)
t23 p 2
600 p 5
1000 p 9
J X 105 6600 p 40
28000 p 130 s x to• 500000 p 1980
&i' [JJ
0 !::-;... t x to• t: [JJ
0 u :!2
5 X t03
>
I X t03
s x to'
• lxt02 L-~------~---L--------~--_.---------L----L---~ l 5 10 50 100 500 1000
TIME OF PENETRATION (LOG SCALE)
FIG3.2 VISCOSITY VS. TIME OF PENETRATION STANDARDIZATION CURVE FOR VISCOSITY PROBE.
10.0
9.0
8.0
7.0
;.: 0
6.0 -F-;:, ...l 0 "' "" 5.0 0 :I: HOAC Q.
4.0 ~ ..--- • • •
3.0 HF
2.0 v 1.0
HCL ... 0.0 J
05 10 30 60
TIME (MINS.)
FIG. 3.3 APPARANT PH VS. TIME FOR SELECTED CATALYSTS.
I
FIG. 3.4 PHOTOGRAPH OF HCI CATALYZED PLANE
-----~~~~~~~~------
800 -(7)
(I) NH40H
(2) HF 700
(6) (3) H,so,
1:1: (4) HN03
"' 600 (5) HCI ~
:E ~ (6) HOAC z <Fl 500 (7) Fused Silica <Fl • (4)
,lj "' z • ~ 1:1: 400 < :t <Fl 1:1:
"' 300 ::.:: u -> • (3)
200
100
0 10 20 30 40 50 60 70 80
PERCENT POROSITY
FIG 3.5 \"ICKERS HARDNESS VS. POROSITY FOR 700°C HEAT TREATED SILICA GELS.
'
p 0
F "').I_ RO -S1- OR
I 0 R
Silicon Alkoxide Plus Fluorine
- F -H2 o H - ' I
' .
.,..
- --~ o---si-oo I
H I \
I \
0 0 R R
Pentacovalent Complex (SNi-Si Reaction)
( - )
+
R 0 I ( + H3o J
R 0
I F---Si--- OR
II 0 0 R R
Pentacovalent Activeted Complex (SN2-Si Inversion Reaction)
<+H2o J ~
R 0 I
HO- Si- OR I 0 R
1-
F- Si- OR-tHOR+ H20 I 0 R
Partially Fluorinated Silicon Alkoxide Plus Alcohol and Water
+ H30 + F
Partially Hydrated Silicon Alkoxide Plus Regenerated Fluorine Anion
and Hydronium
FIG 3.6 MECHANISM OF FLUORINE CATALYZED HYDROLYSIS
-H F
~r HO- Si -OH
~\ H H HO 0
" / Si
H 0/ " OH
Monomer (Silicic Acid) Plus Fluorine and Another Monomer
--+
H H 0 0 ( ~)
' ,
' / HO- -Si- -F
/ ' H()" 'oH /OH
" Si
H 0/ 'o
H
Partially Activated Intermediate Dimer Ion
FIG 3.7 FLUORINE CATALYZED POLYMERIZATION
H H 0 0 I I
HO-Si-0-Si-OH I I 0 0 H H
.,.F + H20
Disilicic Acid Plus Regenreted Fluorine Anion and Water
(-HORl
Pentacovalent Compound Formation (S,i-Si reaction)
(R ~ C2H
5-)
R CH3 I I
0- -C = 0 I I R-- o
I RO- Si -OR
I 0 R
Reaction of Ethanol with Triethoxyacetoxysilane
0 • RO 0-C-CHJ " / Si
RO/ "oR
Formation of Triethoxyacetoxysilane
H 0
I
( +HORJ
RO- Si -OR + CH3 COOC2H5 I 0 H
Partially Hydrated Monomer Pins by Product
of Ethyl Acetate
FIG 3.9 POSSIBLE CATALYTIC MECHANISM OF ACETIC ACID.
0
I I I I I I I~ I
Total Rate of Reaction / E (Hydrolysis+ Polymerization)/ ~ I
I ~ I 1/ ~ J ~
= Without Impurities ~ ~
'/ ~
1
Rate of Hydrolysis
Rate of Polymerization
\
"
pH OF SOLUTIONS
14
FIG 3.10 IN\'ERSE REACTION RATES FOR GENERAL ACID AND GENERAL BASE CATALYZED HYDROLYSIS AND POLYMERIZATION.
I
FORMATION OF RIGID STIUICTliRE
Factors
• Extent of cross-linking during
gelation.
• development of particulate vs.
linear cham structure.
• amount of cross-linking within
network structure.
II
SHRINKAGE DUE TO
VOLATILIZATION OF
RESIDUAL LIQUIDS
Factors
• vapor pressure of solvent.
• evaporation rate as affected
by external environment.
• pore size effect on capillary
pressure.
• residual liquid surface
tension effect on capillary
pressure.
FIGURE 3.11 TWO INTERRELATED PROCESSES AFFECTING
SHRINKAGE DURING DRYING.
Y,, Vapor - Liquid Surface Tension
Y,, Vapor (v) Cos 9 Yvg y,, + "(,.,
2 Y,, Cos 9 Y,, P, r
'Yvl r Pore Radius
e Wetting Angle Gel (g) Liquid (I)
Pore Wall
FIG 3.12 CAPILLARY EFFECT IN GELS.
ACID CATALYZED
• network structure of linear chains with low functionality and low stiffness.
• greater freedom to bend, rotate and plastically deform.
• fine pore structure that resists flow of volatiles.
BASE CATALYZED
• spherical particles of high functionality and stiffness.
• large interastices between spheres that allow volatiles to escape during drying.
• freedom to deform primarily limited to abilioty of spheres to compress.
FIG 3.13 COMPARISON BETWEEN THE MICROSTRUCTURES OF ACID AND BASE CATALYZED GELS.
TABLE 3.1
SURVEY OF PREVIOUS WORK
AUTHOR(S) SYSTEM CATALYSTS COMMENTS
Brinker Si02 HCl Slow hydrolysis relative to the (J.N.C.S.) condensation reaction. Small 1982 pores.
NHpH Rapid hydrolysis relative to the condensation reaction. Large pores.
Nogami Si02 HCl High bulk density. No particles Moriya observed. (J.N.C.S.) 1980 NH40H Low bulk density. Spherical
particles observed.
Zarzycki Si02 Catalyst The rate of gelation is (J.M.S.) used but not proportional to the OH 1982 specified concentration above pH=2,
and proportional to the H+ concentration belowpH=2.
Klein Si02 HCl Accelerated hydrolysis and Garvey retarded polymerization. (Soluble Silicates) NHpH "Limited hydrolysis"
1982 NH4Cl "Postponed hydrolysis"
followed by rapid gelation
Yoldas Al20 3 16organic Fvaluated peptizing effects of
(Ceram. and inorganic acids on slurry of AI( OHh
Bull.) acids
1975
I
TABLE 3.2
STANDARD SILICA GEL SOLUTION COMPOSITION
CONSTITUENT CONCENTRATION WEIGHT (Mole : TEOS) PERCENT
Tctracthoxysilanc (TEOS) I 44.8
Ethanol 4 39.6
Water 4 15.6
Catalyst 0.05 ------
TABLE 3.3
GELATION TIMES AND APPAREMT INITIAL pH OF SOLUTION FOR SIX CATALYSTS
CATALYST CONCENTRATION APPARENT GELATION (Mole : TEOS) INITIAL pH TIME(HR)
OF SOLUTION
HF 0.05 1.90 12
HCI 0.05 0.05* 92
HN03 0.05 0.05* 100
H2S04 0.05 0.05* 106
HOAC 0.05 3.70 72
NH40H 0.05 9.95 107
No Catalyst ----- 5.00 1000
*Between 0.00 and 0.05
TABLE- 3.4
PROPERTIES OF GELS DRIED AT 25°C AND 600°C FOR DIFFERENT CATALYSTS
25°C PROPERTIES 25°C PROPERTIES
S.No. Catalyst Volume Bulk Apparent Percent Volume Bulk Apparent Percent Vickers
Shrikage Density Density Porosity Shrikage Density Density Porosi~· Hardness
(%) (grnlcc) (grn!cc) (%) (grn!cc) (gmfcc)
L HOAC 84 132 133 0.7 ----- 2.08 2.12 L9 666.5
2. HCI 81.3 ----- ----- ----- 85.2 2.06 2.12 2.8 429
3. HN03 79.9 1.14 1.16 !.7 85.2 !.82 2.02 10.0 470
4. H2S04 7!.6 ----- ----- ----- 80.0 !.46 2.12 3!.0 224
5. HF 78.4 0.54 !.24 56.7 82.7 0.71 2.13 67.0 75
6. NH40H 67.8 0.49 L13 57.0 7L7 0.70 2.21 68.0 28
7. No 87.5 0.95 2.09 54.6 ----- !.25 2.21 43A -----
Catalyst - -
TABLE 3.5
GELATION TIMES AND INITIAL pH OF SOLUTION VALUES FOR SILICA GELS CATALYZED BY POTASSIUM HALIDES
CATALYST CONCENTRATION APPARENT GELATION COMMENTS (Mole : TEOS) pH TIME
KF 0.05 8.50 6mins. Salt fully dissolved in solution
KCI 0.05 4.70 48 hrs. Immiscible solution
KBr 0.05 3.50 I 00 hrs. Clear solution, salt fully dissolved
KI 0.05 3.40 ISO hrs. Clear solution, salt fully dissolved
- -
TABLE 3.6
GELATION TIMES AND INITIAL pH OF SOLUTION VALUES FOR SILICA GELS CATALYZED BY ACID HALIDES
CATALYST CONCENTRATIOINI INITIAL GEJ,A TIOINI (Mole : TEOS) pH TIME
HF 0.05 1.90 12 hrs.
HCI 0.05 0.05 92 hrs.
HBr 0.05 0.20 285 hrs.
HI 0.05 0.30 400hrs.
TABLE 3.7
CATALYSTS AND THEIR PRINCIPLE CATALYTIC MECHANISM IN THE HYDROLYSIS AND POLYMERIZATION REACTIONS OF T.E.O.S
CATALYST HYDROLYSIS POLYMERIZATION (CONDITIONS) REACTION REACTION
HF, KF Nucleophilic substitution ofFiuorine Anion Nucleophilic substitution ofFiuorine (all pH) (SN2-Si) Anion (SN2-Si)
HCI, HBr, HI and Electrophilic reaction ofHydronium Ion Nucleophilic reaction ofHydronium NH03 (above pH=2) Ion
H2S04, HOAC Possible SNi-Si reaction in addition to " (abovepH=2) electrophilic substitution ofHydronium Ion !
NH40H Nucleophilic substitution ofHydroxyl Ion " I KCI,KBr,KI Nucleophilic substitution of respective anion " (abovepH=2) (SN2-Si)
HCI, HBr, HI, KCI, Nucleophilic substitution of fluorine introduced " KBr, KI, HOAC, tbrough trace impurities in addition to HN03, H2S04 mechanisms postulated for higher pH values. (below pH=2)
I
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