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Structural changes of silica xerogels during low temperature dehydration
Journal
114
of Non-Crystalline
Solid 8X (1986) 114-130 North-Holland. Amsterdam
STRUCTURAL CHANGES OF SILICA XEROGELS DURING LOW TEMPERATURE DEHYDRATION
Rcccivcd Revised
10 Fcbruaw 1986 manuscript received
6 June
lYX6
In this work we investigated the tcmpcraturc evolution of the structure Fourier Transform Infrared (FTIR) Spectroscopy. The gels. prcparsd by dcnsation in the presence of an excess of water to tetramethoxysilanc. studied in transmission. It was found that the removal of u’atcr from the casicr when the gels are prepared in basic conditions. During the drying methanol oxidizes and produces formaldehyde. formic acid and formates. present in the gel at 300°C.
of silica thin films by hydrolysis and polyconwcrc thin enough to bc pores and gel surface is heat treatment. residual Thcsc spccics are still
1. Introduction Silica gels have been prepared by hydrolysis and polycondensation of metal alkoxides. Hypercritical conditions and natural evaporation, which produce aerogels and xerogels respectively, are the two main drying procedures. Crack free (monolithic) dried aerogels can be reliably prepared [l]. However, the problem of monolithicity has not been solved yet for xerogels. The variables investigated for production of monoliths include: nature of the electrolyte [2,3] (acidic or basic medium), the number of molecules of water for hydrolysis [4.5], processing temperature [6], or casting viscosity [7]. Monolithicity and purity of the final product are favored when the hydrolysis is carried out in a large excess of water and in acidic conditions [8]. However, minute additions of water may also lead to a material, close to an organic polymer. which retains its shape at a relatively low temperature due to its elastic character. Besides these possibilities for modifying the hydrolysis and polycondensation reactions of metalalkoxide compounds, it is now well known that an addition of a drying control chemical additive (DCCA) to a mixture of metalalkoxides, water and alcohol, improves the processing control of monolithic gels [9]. The influence of the formamide DCCA on the gelation process has been studied by ‘9Si NMR [lo], Raman spectroscopy, and N, adsorption-desorption isotherms [ll]. An addition of formamide DCCA decreases the hydrolysis rate * On sabbatical from: Cedex. France.
Laboratoire
de Science
des Materiaux
0022-3093/86/$03.50 st: Elsevier Science Publishers (North-Holland Physics Publishing Division)
R.V.
Vitrcux,
USTL,
34060
Montpcllier
and slightly increases the polycondensation rate. It does not significantly affect the specific surface area, but increases the pore volume and the average pore radius. Kinetics studies have been conducted in the wet state, i.e., before gelation or after gelation while the gel was still impregnated by pore liquor. In contrast. the studies on the structural properties of the silica gel monoliths have been performed on dried gels. For example, the N, desorption isotherms were obtained after outgassing the samples. Not much is known regarding the structural changes of xerogels during the low temperature drying stage. However, it is in the temperature interval. 25-300°C. that the structural evolution of gels is the most critical as far as maintaining monolithicity is concerned. The purpose of the present study is to follow the structural evolution of xerogels with and without formamide DCCA in the temperature range of drying which is critical for obtaining dried crack-free monolithic gels. The influence of the formamide DCCA on the dehydration of SiO, films will be discussed in another paper [12]. The samples are studied in transmission by Fourier Transform Infrared (FTIR) spectroscopy. During the formation of a gel from the reaction of tetramethylorthosilicate (TMOS) and Hz0 in alcoholic medium, IR bands appear at 950 and 1080 cm-’ from the outset due to the vibration of Si-OH and Si-0-Si bonds, respectively. These bands are very intense and correspond to the formation of the SiO, network. Their intensity increases very rapidly. Thus the gels will not be transparent in the spectral range of 1300-900 cm-‘. Consequently, the drying analysis is carried out primarily in the spectral range from 4800 to 1300 cm-‘. After heat treatment. the gels become more transparent in the 900-400 cm-’ region. However, it was not possible to obtain useful information in this spectral range.
2. Experimental
technique
FTIR has proven to be a very successful technique to follow the gelation process in silica and silicate systems [13]. The IR study was performed on a Nicolet MX-1 spectrometer with a 1200-S data analyzer. Five minute scans corresponding to an accumulation of 160 spectra, were used resulting in very good resolution. In order to study the gels in transmission the samples must be as thin as possible. All the gels had a thickness less than 0.01 cm. The samples were prepared by allowing a thin film of the desired solution to gel in a non-adhesive container. The gels were cast in Petri dishes previously sprayed with Teflon@. No organic residues could be detected by comparison with gels cast in non-sprayed containers. All the gels were cast approximately half an hour prior to gelation in order to avoid excessive evaporation of the solvent which could have resulted in structural changes. Under these conditions the films generally cracked while shrinking. However, it was relatively easy to find pieces of several cm’ suitable for analysis.
Table
1
Code
and composition
of the different
Code
I’
SW SWA
1.7 1.7
4 4
SWB SWR
1.7 1.7
4 10
R
solutions H?O
Ref.
DI
figs. 1. 4. 5 fig. 7
O.lN HCI O.OlN NH,OH O.OlN
HCI
fig. 8 fig. 3
For the dehydration heat treatments, the samples were maintained at the desired temperature for 10 min in a drying oven for temperatures below 220°C and in an air atmosphere furnace for higher temperatures. After the heat treatment, the gels were transferred to a dessicator which had been previously flushed with dry Nz for five minutes. The samples were mounted on the transmission cell “in situ,” under a dry N, atmosphere. In order to identify the dehydration products, spectra (not shown) of H,O, methanol, TMOS, formic acid and formaldehyde were recorded, using a cell with CdTe windows. The compositions of the different solutions are given in table 1, where: V = volume TMOS/volume MeOH R = #mol. H,O/#mol. TMOS Hz0 = deionized (DI), acidic (O.lN HCl), basic (O.OlN NH,OH).
3. Results
The spectrum of a SW silica gel, prepared with the following conditions using DI water for hydrolysis and no DCCA in the solvent and dried at room temperature, is given in fig. 1. The material is transparent in several frequency regions: (1) 4800-3600 cm- ‘. In this interval, the bands are mainly due to overtones or combinations of vibrations of Si-OH or HzO. However, overtones or combinations of vibrations of residual methyl groups can be present as well [14]. The broad absorption band between 3600 and 3000 cm-’ corresponds to fundamental stretching vibrations of different hydroxyl groups. It is generally composed of a superposition of the following stretching modes [15]: 3750 cm-’ -free Si-OH on the surface of the gel 3660 cm-’ - Pair of surface SiOH mutually linked by a hydrogen bond - internal Si-OH 3540 cm-’ - silanol groups linked to molecular water through hydrogen bonds 3500-3400 cm- i - absorbed water
117
SW
(Si 0,)
GEL - 25”~
P C-H
\ SIOH
1 Hz”
5600
4400
3200
2000
WAVENUMBERS Fig. 1. FTIR
spectrum
(CH,-OSi)
I
I
1400
BOO
2 10
(cm-l)
of a SW gel heat treated
at 25°C
(2) 3000-1350 cm-‘. The range (3000-2750 cm-‘) corresponds to the symmetric and antisymmetric fundamental stretching vibrations of CH, groups. Between 1800 and 1550 cm-‘. there are juxtaposed bands due to the vibrations of C=O groups, the deformation of molecular water, and combinations of vibration of the SiO, network [16,17]. The combination of the SiO, vibrational modes are generally located at 1960, 1870 and 1640 cm-‘. Most of the time, the band at 1960 cm-’ will appear as a shoulder of the 1870 cm-’ band, which is always well resolved, although not very intense. On the other hand, the band at 1640 cm-’ is often hidden by the band corresponding to the deformation of molecular water (1620 cm- ‘). The IR transmission spectra of a blown commercial Pyrex@ glass shows the above mentioned bands in the 3000-1350 cm-’ region (fig. 2). However. the broad band due to the hydroxyls, 3600-3000 cm-‘, is missing in the Pyrexm film. The absorption bands due to the silica network are located between 1100 and 800 cm-‘. Another band (deformation of the Si-0-Si bond) appears at 455 cm-‘. For microporous hydrated gels or glasses, a band at about 950 cm-’ can be observed. At room temperature, the IR spectrum of a gel is generally not well resolved, due to the high quantity of residual liquid within the pores (fig. 1). The large band between 4600 and 4300 cm- ’ is also poorly resolved. It is due to a combination of the stretching and deformation vibrations of the C-H bond [14], and the bands corresponding to the combinations of vibrations of
PYREX
I 5600
I 4400
/
I 3200
Fig. 2. FTIR
1
2000 WAVENUMBERS
spectrum
of a blown
I
FILM
I 1400
800
2 10
(cm-l) film of PyrcxK
glass.
the hydroxyl groups. However some features of these bands have been interpreted differently by the various authors [l&20]. For the spectrum of the silica gel dried at room temperature (fig. l), we can observe on the high frequency side a large band centered around 3600 cm ’ and a small band at 3750 cm-’ due to free Si-OH groups. On the low frequency side of this broad band there is a weak shoulder, near 2860 cm- ’ which indicates the presence of methanol. The deformation band of the same CH, group will appear at 1400 cm- ‘. On the other hand, if the TMOS is not fully hydrolyzed, the IR spectrum will show absorption bands at 2958. 2857 and 1456 cm-’ [21]. The first two bands cannot be distinguished from those of CH,OH. On the other hand, the 1456 cm-’ band, can be easily seen in the spectrum. This indicates that at room temperature the gel contains Si-OCH, bonds, which come from an incomplete hydrolysis of TMOS rather than an esterification of SiOH groups. A gel prepared with a higher water content and in more acidic conditions should not display such a band. As an example the spectra of a SWR * gel are given in fig. 3. The 1456 cm-’ band is not present but a small hump can be seen at that wavenumber. The intensity of this hump does not seem to change significantly with temperature up to 170°C. The most intense bands of methanol (1050 cm-‘) and TMOS (1100 cm-‘) will always be located in a frequency range where the vibrations of the silica network are the most intense, and thus will not be observed. * SWR
gel corresponds
to R = 10 and HZ0
= O.OlN
HCI
. ..“-.._.
SWR ‘.
?
,
52 Fig. 3. FTIR
I
-,
4400
spectraof
I
3200 a SWR
2000 WAVENUMBERS km’) gel heat treated at: (a) 25°C.
1400
800
(b) XOOC. (c) 120°C.
2 ‘0 (d) 17O’C
The strong absorption band at about 1620 cm-’ indicates that the room temperature silica gel contains a large quantity of molecular water (fig. 1). The IR spectrum of the 60°C dried silica gel is given in fig. 4a. (It should be noted that for improving the reading of the figures, the spectra have been moved along the transmittance axis in figs. 3, 4. 5. 7 and 8.) After drying at 60°C the spectrum is better resolved. and the band at 4557 cm-’ can be observed. It corresponds to the combination of the stretching and deformation vibrations of free SiOH groups (3750 + 810 cm-‘) [20]. While the broad band corresponding to the vibrations of OH groups narrows after the 60°C drying, the band due to free Si-OH vibrations strongly increases. When a hydrogen bond is formed between surface silanols and any other compound, a new band appears at lower frequencies, and the intensity of the band at 3750 cm-’ decreases [22]. The band at 1620 cm-’ corresponding to molecular water decreases in intensity due to the 60°C drying and a new band appears at 1685 cm-‘. The location of this absorption band is characteristic of a C=O band. However, in
120
G. Orcel
er ul. / Struclurul
chmges
ojsilico
xerogels
SW
i !r’\
I
I 5600
4400 Fig. 4. FTIR
3200 spectra
I
I
2000 WAVENUMBERS
of a SW gel heat treated
“‘\
I
[C-H
(CH,-
I
,
1400
OSi)
I 800
I ; 10
(cm’) at: (a) 60°C.
(b) XO”C,
(c) 12O“C
carbonyl compounds the C=O group generally gives rise to a vibration whose wavenumber is larger than 1700 cm-‘. Only formaldehyde gives rise to a vibration band below 1700 cm-‘, more precisely between 1672 and 1685 cm-‘, depending on the degree of association. In this case the gel can be considered as a catalyst for formaldehyde, due to its high specific surface area. Formaldehyde can be synthesized by oxidation of methanol, according to the overall reaction [23]: CH,OH + +O, + CH,O + H,O, (1) After 80°C drying, the IR spectrum of the gel (fig. 4b) does not change much compared to the spectrum obtained after 60°C. However, a new absorption band corresponding to the symmetric stretching mode of Si-OCH, bonds, appears at 2958 cm-‘. The band due to molecular water decreases still further in intensity. Three new bands appear after 120°C drying. They are located respectively at 2990, 1390 and 1375 cm- ‘. One of the stretching modes of formaldhyde is at - 3000 cm-‘. On the other hand, the 1390 and 1375 cm -’ bands are more difficult to assign.
G. Orcel
et (II. / Srrucruml
changes
o/silicu
xerogels
The oxidation of methanol to produce formaldehyde, on concurrently with other reactions:
121
eq. (1) generally goes
CH,O + +O, + HCOOH,
(2)
CH,O + 40, + CO + H,O,
(3)
CHzO+CO+H,,
(4)
CH,O + 0, --, CO, + H,O.
(5)
Formaldehyde is often considered unstable so the presence of a catalyst is important. Most of the time, formaldehyde decomposes before all the alcohol is oxidized, and side reactions, such as the following ones, are possible: CH,O + CH,OH CH,OH
+ CH,OCH,OH,
+ HCOOH + HCOOCH,
(6) + HzO,
CH,O + H,O P CH,(OH),.
(7) (8)
However, reaction (8) is not likely to occur because CH,(OH)? is not stable at 80°C. The bands at 2990 cm-’ (stretching mode of C-H) and at 1390 cm-’ (deformation mode of C-H) in fig. 4b are associated. On the other hand, the band at 1375 cm-’ is assigned to a symmetric stretching vibration of a
-c, 40
group.
0-
Thus, the structure of the molecule should be: H O=C< o- ’ which means that increasing the temperature of gel dehydration leads to the formation of formates or esters [reaction (7)]. The bands at 2990, 1390 and 1375 cm-’ can thus be assigned to a chemical species obtained from the decomposition of methanol. This new molecule is probably linked to the surface of the gel. It is most probably a formate which would be produced according to the following overall reaction: SiCH,OH+O< SiE
0 ,’
SiE
H y-o< 0
+ +HzO. SiE
(9)
If this mechanism is valid, then all bands, identical to those observed for the absorption of methanol on an alumina substrate [24], should appear at higher temperatures. Indeed, the band at 1599 cm-’ which can be observed after 170°C drying (fig. 5a) can be assigned to formate groups. The other bands at 1472 and 1456 cm-’ are probably due to ESi-OCH, species, which are formed by a methylation of =Si-OH groups. The band previously located at 1685 cm-’ widens and increases in intensity. This illustrates the formation
G. Orcel el al. / StrucIurul
:
ii /
I -\;;,
I
I
Fig. 5. FTIR
I
4400 spectra
,
of silicu .uerogels
,i /;I / II,’ , ,, ,
i!
/-“I
5600
j ,I ‘..-. . .i
changes
!I I
I
3200
I
I
2000 WAVENUMBERS
of a SW gel heat treated
1
km-‘)
at: (a) 170°C.
1
1400 (b) 200°C.
I
I
800 (c) 22O”C,
I
2 10 (d) 240°C.
of a new band, due to the vibration of a C=O group, whose frequency is slightly higher. This C=O absorption band probably belongs to a formate group. However, it should not be overlooked that formic acid, which can be produced by oxidation of methanol, gives a band at the same location. Indeed, the bands at 1375 and 1700 cm-’ (which is assigned to C=O) vary in the same way. The shift of the band at 1685 cm-’ to 1700 cm-’ could be attributed to the formation of formic acid from formaldehyde. The bands at 1579 and 1492 cm-’ are difficult to assign. They could be produced by molecules similar to formates. The vibration bands of acetates are located at 1575 and 1466 cm-‘. .The decomposition of methanol can lead to different chemical species, depending on the nature of the substrate [25,26]. Some of these products of oxidation, such as dimethyl ether, are not very active in the IR. However, the presence of these compounds has been confirmed using NMR spectroscopy ~271.
G. Orcel er ul. / S0wturul
chmges
ojsilico
serogels
123
The spectra obtained for drying temperatures up to 220°C (figs. 5b, c) show the same absorption bands. However, the strong band at 1700 cm-’ is slightly shifted toward higher wavelengths, thus indicating a weaker association with the surface of the gel. Additional information on the dehydration of the gels can be obtained by plotting the band intensities as a function of temperature. However, such a plot may be inaccurate since even the sample will give a different IR throughput after being heat treated. This is primarily for two reasons: (1) It is not possible to reposition the film in the FTIR laser beam exactly in the same way after heat treatment. Since the film thickness is not completely uniform, the quantity of matter within the beam is not identical for each experiment. (2) The film shrinks during dehydration. When the sample thickness decreases the quantity of matter sampled by the laser beam increases. Thus it is difficult to obtain quantitative information by considering only the peak heights. In order to minimize these effects and thereby compare the various spectra taken after various heat treatments, and eventually compare different films, an internal standard is required. An absorption band corresponding to the vibration of a molecule with a constant concentration provides the best source of calibration. The band at 1867 cm-‘, which is a combination of various vibrations of SiOZ, fulfills all the requirements. It is present in all the spectra (figs. 1-8) and is well resolved for temperatures above 25°C. In addition, the concentration of SiO, is fairly constant since significant polymerization does not occur during the time of the FTIR scan [lo]. The esterification of silica does not occur to any significant extent according to the spectra obtained. Consequently, the quantity of SiOz in the films should not change appreciably during the dehydration thermal threatments. Thus, to compare the spectra in figs. l-8, we used an intensity number N, defined as follows N = 100 x Z/Z,,,,,
(10)
where I is the intensity of the band of interest and I,,,, is the intensity of the band at 1867 cm-‘. The results are reported in fig. 6. All the bands relative to the decomposition products of the solvent decrease in intensity (fig. 6). In contrast, the bands at 2958 and 2857 cm-’ (which were attributed to OCH, groups) do not follow this behavior. Thus the gels have to be heat treated to higher temperatures in order to oxidize the internal residual organic groups. The carbonization of the samples, observed as an incomplete oxidation of the films, is mainly due to non hydrolyzed alkoxy groups and not to the solvent. After 240°C drying, the absorption band of OH groups is still very broad (fig. 5d). It indicates that a large quantity of water is hydrogen bonded to the surface of the gel. The number of internal SiOH bonds is very large as well. The spectra of the silica gels prepared in a basic medium were studied for temperatures up to 170°C (figs. 7a, b, c, d). The general aspects of the spectra are similar to those obtained for gels synthesized with DI water (figs. l-6). However, it is worth noticing that: (1) The removal of molecular water occurs
124
G. Orcel
1
0
2990
cm”
XA0x
1700 2958 1466 2857
cm-l cm-’ cm4
er al. / Structural
chunges
of srlrca xerogels
Ff> % F. m %*-
100
I20
170 TEMPERATURE
Fig.
6. Variation
with
200
220
(“C)
temperature of the intensity number N of the bands 1700,1466 and 1375 cm-’ of a SW gel.
at 2990.
2958.
2857.
at higher temperatures compared with the previous DI water gels. The band assigned to molecular water can be seen at 120°C (fig. 7~). (2) The bands at 2990, 1700 and 1375 cm-’ which appear during the decomposition of the solvent can be observed at 170°C (fig. 7d). (3) The broad band due to the vibration of hydroxyl groups is poorly resolved, even at 170°C. Besides the silanol groups on the surface of the gel, there are SiOH groups linked by hydrogen bonds. The gels prepared in acidic condition have the simplest spectra, (figs. 8a, b, c, d) compared to the two previous gel systems. Only three major bands at 3800,3200, 1867 and 1640 cm-’ due to water, silica and water, respectively, can be observed. The evolution of the spectra with temperature is primarily a decrease in intensity of the bands attributed to water. However, it should be noted that: (1) No band due to free SiOH was resolved, even at 170°C. (2) No band due to alkoxy groups (from non hydrolyzed =SiOCH3, or from esterification of =SiOH), could be observed. (3) No products of decomposition or oxidation of the solvent gave rise to absorption bands. The high quantity of internal silanols can be attributed to the polymer type structure resulting from the acidic conditions [28]. Hydrochloric acid promotes the hydrolysis and it was shown that only a few minutes or less are needed for
G. Orcel
er ul. / S~ructurul
chatlges
o/silica
125
xerogels SWB
(BASIC)
:d
a 1 5600 Fig. 7. FTIR
I
I
4400 spectra
of a SWB
I
I 3200 2000 WAVENUMBERS gel heat treated
I
I 1400
000
i lo
(cm’) at: (a) 25’C,
(b) 8O’C.
(c) 120°C.
(d) 170°C.
the “total” hydrolysis of TMOS by water in presence of HCl [29]. It is more difficult to account for the fact that no bands characteristic of formates could be observed. One possible explanation is that the formates come from the oxidation of the remaining alkoxy groups. The second possibility is that the surface of the gel is not available to the solvent, and thus no catalytic effect occurs. A plot showing the temperature variation of the intensity of the bands due to formates and alkoxy groups (fig. 6) clearly shows that they vary independently. This allows us to conclude that =Si-OCHs groups are not responsible for the formation of formates at low temperature ( < 170°C). The unavailability of the gel surface to solvent molecules can be explained by the high quantity of adsorbed molecular water which covers the gel surface through hydrogen bonds. The methanol molecules have fewer SiOH groups available to react with, and the quantity of formates formed decreases. It is now well
..
SWA
(ACID)
0
Fig. 8. FTIR spectra of a SWA gel heat treated at: (a) 25OC.(b)
80°C.
(c) 110°C.
cd) 170°C.
known that HCl, which increases the hydrolysis rate, induces the formation of smaller particles and a smaller scale silica network [30]. As a result, the gel has a larger surface area, smaller pores and probably a larger degree of pore interconnection. Even though the reactive surface increases, it becomes more difficult for the MeOH molecules to reach the available SiOH groups to form formates. and the kinetics are slowed down.
4. Discussion The study of the dehydration of silica gels, prepared by hydrolysis and polycondensation of tetramethoxysilane in alcoholic medium, showed that intermediate compounds remain absorbed on the gel surface at temperatures higher than 300°C. Methanol successively decomposes into formaldehyde, then formic acid, producing formates which remain strongly bound to the surface of the gel. The residual organics, due to an incomplete hydrolysis of TMOS, or to a methylation reaction of SiOH groups, are oxidized at temperatures higher then those necessary for the removal of formates. To the best of our knowledge, it is the first time that direct proof of the absorption of these
G. Orcel
er (II. / Strucrurul
chunges
of siliw
xerogels
127
compounds on silica gels is given. The band assignment of the vibration of these molecules is recapitulated in table 2 where we add relevant information on IR bands in SiOz gels. In table 2 we adopted the following notation: H e.g. 2990 cm-’
Stretching C-H
>c=o (0 i H 1 which reads as: the band at 2290 cm-’ corresponds to the stretching mode of the C-H bond in the molecule H y=o. H An illustration of this structure is represented in fig. 9 by the sketch (f). In this figure we show schematically the surface of SiO, network which constitutes the gel. In order to improve the clarity of the figure, only a few SiO, tetrahedra are shown. For the same purpose only some of the Si atoms in the SiO, tetrahedra are represented. Transmission IR spectroscopy shows the essential differences in behavior of gels prepared in acidic and basic media. In an acidic medium, these structures contain a large quantity of internal silanols. IR studies show that the complete dehydration of this type of gel is difficult to achieve. In a basic medium the gel is more particle-like with SiOH groups situated mainly on the surface. The dehydration of this type of gel is relatively easy at low temperatures. Compared to the gels prepared in an acidic medium. at the same temperature basic gels contain less water but more surface silanols, free of hydrogen bonds. The variations in dehydration are due to the differences of gel structure: i.e., macroporous for the gels prepared in basic conditions versus microporous for the gels prepared in acidic conditions [9], and to the different types of structural water; i.e. surface silanols for the basic gels versus internal silanols for the acidic gels. These results are consistent with the behavior of the gels at higher temperature. It is well known that the gels prepared in acidic conditions densify at lower temperature than those prepared in basic conditions [32], which is related to the nature and quantity of SiOH species in the sample. When the concentration of silanol groups increases, the viscosity of the gel decreases and viscous flow, responsible for the densification of the gel, takes place at lower temperature.
5. Conclusions Transmission FTIR is a useful tool to follow the structural changes which occur at the surface of gels. In acidic conditions, the hydrolysis of TMOS is favored but more internal SiOH groups are retained in the gel at high temperature. On the other hand, for base catalyzed gels, the water is removed
128
G. Orcel
Table 2 Absorption
bands
of the water,
er al. / Slruclural
cltauges
methanol,
system
TMOS
oj silica
Ref Biblio.
Assignment
Wavenumber
cm-’ 3660 cm-’
stretching + deformation stretching+deformation stretching free surface stretching Si-O-H
C-H free Si-OH %-OH OH-Si
3540 cm-’
stretching stretching
internal Si-O-H
i-H
stretching symmetric stretching
2990 cm-’
stretching
2958 cm-’
symmetric stretching Si-d-CH, stretching C-H (CH,-OH) stretching C-H (CH,-OH, CH,-CHO CH, -0-Si) combination Si-0 combination Si-0
cm-’
4600-4300 4557 cm-’ 3750
cm-’ cm-’
3500-3400 3OOG2750
1960cm-’ 1867 cm-’
Fig.9
1141
PO1 1151 [I51
(a) (b)
I151
CC) cd)
adsorbed Hz0 and asymmetric CH1
1151
(e)
C-H
[341
(0
I211 [311
k9 (h) (h)
1151
Si-OH
,--
2857 cm-’
serogels
[21.31] 1161 U61
1700 cm-’
1311
(i)
1685 cm-’
1311
(0
1640 cm ’ 1620 cm-’ 1599cm-’
overtone Si-0 bending molecular H 2O asymmetric stretching
1161 P61 [241
ti) (k)
1579 cm-’
asymmetric
P61
(1)
1311
Cm)
[311
(0 00
1492 cm-
’
1472 cm-’
1466 cm-’
stretching
deformation
OH,
C
:, ( CH2-Co,)
bending C-H (CH,-0-Si) symmetric stretching -,B
/
‘$C-CH,\
o- l-0
1241
I
1456 cm-’ 1390 cm-’ 1375 cm-’
deformation deformation symmetric
C-H. (CHJ-0-Si) C-H. (CH,OH) stretching
1211
1080 950 800 455
stretching stretching stretching deformation
+ SiO + + Si Si-OH, Si-O+ Si-0-Si + Si-0-Si-0
~321 1321 [321 1321
cm- ’ cm-’ cm-’ cm-’
[311
~241
(0 0-d (k)
0-d (0) (P) (P)
G. Orcel
et ul. /
Slructurul
clunges
o/silicu
H \
tkl
129
serogels
C-0
0
I
I
I
H\--n
H-6
H
0 .H
Fig.
9. Schematic
molecular
representation
of the surface species.
of a gel, with
the different
adsorbed
more easily and less internal silanols remain in the sample. Thus, for the same experimental conditions, it is easier to prepare crack free base catalyzed samples. During drying residual methanol oxidizes and produces formaldehyde, or reacts with surface silanols to give formates and eventually formic acid. The removal of these molecules, which are still present at 300°C is essential when preparing glasses, free of cracks, porosity or impurities such as residual carbon.
The authors gratefully acknowledge Contract # F49620-83-C-0072.
partial
financial
support of AFOSR
References [l] [Z] [3] [4] [5] [6] [7] [X] [9] [lo] [ll] [12] 1131 [14] [15] [16] [17] [1X] [lY] 1201 [21] [22] [23] [24] 1251 [26] [27] [2X] [29] [30]
(311 [32]
J. Zarrycki. in: Ultras~ructure Processing of Ceramics. Glasses and Composites. cds.. L.L. Hench and D.R. Ulrich (Wiley. New York, 19X4) p. 43. M. Yamane, S. Asot and T. Sakaino, J. Maler. Sci. 13 (197X) X65. M. Nogami and Y. Moriya. J. Non-Cryst. Solid5 37 (19X0) 191. C.J. Brinker, K.D. Keefer. D.W. Schaefer and C.S. Ashley. J. Non-Cryst. Solids 4X (19X2) 47. C.J. Brinkcr. K.D. Kecfer, D.W. Schaefer. R.A. Asaink. B.D. Kay and C.S. Ashley, J. Non-Cryst. Solids 63 (19X4) 45. L.C. Klein. T.A. Gallo and G.J. Garvcy, J. Non-Crysr. Solid.\ 63 (19X4) 23. L.C. Klein and G.J. Garvey. J. Non-Cryst. Solids 4X (19X2) 97. M. Yamane. S. Inoue and A. Yasumori. J. Non-Cryst. Solids 63 (19X3) 14 L.L. Hench and G. Orccl. J. Non-Cryst. Solids X2 (1986) 1. G. Orccl and L.L. Hench. J. Non-Cryst. Solids 79 (19X6) 177. I. Artaki. M. Bradley, T. Zerda. J. Jonas, G. Owl and L. Hcnch. Science of Ceramic Chemical Proccssing. eds.. L.L. Hen&. D.R. Ulrich (Wiley. New York, 19X6) p. 73. G. Orcel. J. Phalippou and L.L. Hench. 10 be submitted. M. Prassas and L.L. Hcnch. in: Ultrastruc[urc Processing of Ceramics. Glasses. and Composites, cds., L.L. Hench. D.R. Ulrich (John Wiley and Sons. New York. 19X4) p, 100. T. Woignicr and J. Phalippou. Riv. Staz. Sper. Vetro 5 (19X4) 47. R.S. McDonald. J. Phys. Chem. 62 (195X) 116X. H.A. Bcncai and A.C. Jones. J. Phys. Chcm. 63 (1959) 179. J.B. Peri. J. Phys. Chem. 70 (1966) 2937. V.A. Nikitin. A.N. Sidorov and A.V. Karyakin. Zh. Phys. Khim. 30 (1956) 117. J.H. Anderson and K.A. Wickersheim. Surface Sci. 2 (1964) 252. C.K. Wu. J. Non-Cryst. Solids 41 (1980) 525. M. Praasas. J. Phalippou and J. Zarzycki. XIII Int. Co&. on Glasses (July 19X5) Hamburg. Germany. G. Curthoys, V.Y. Davydov. A.V. Kiselev. S.A. Kiselev and B.V. Kuznctsov. J. Coil. Interf. Sci. 4X (1974) 59. J.F. Walker. in: Formaldehyde, 3rd cd. (Reinhold Publishing. New York. 1964). R.G. Greenler, J. Chem. Phys. 37 (1962) 2094. I. Yasumori. T. Nakamura and E. Miyazaki. Bull. Chem. Sot. Japan 40 (1967) 1372. E. Miyazaki and I. Yasumori. Bull. Chcm. Sot. Japan 40 (1967) 2010. G. Carturan. V. Gottardi and M. Graziani. J. Non-Cryst. Solids 29 (197X) 41. K.D. Keefer. in: Better Ceramics Through Chemistry. Vol. 32. eds.. C.J. Brinker. D.E. Clark. D.R. Ulrich, Materials Research Society (North-Holland, Amsterdam. 19X5) p. 15. I. Artaki. S. Sinha. A.D. Irwin and J. Jonas, J. Non-Cryst. Solids 72 (1985) 391. C.J. Brinker. W.D. Drotning and G.W. Scherer. in: Bctler Ceramics Through Chemistry. Vol. 32, eds.. C.J. Brinker. D.E. Clark, D.R. Ulrich. Materials Research Society (North-Holland. Amsterdam. 19X5) p. 25. C.J. Poucherl. in: The Aldrich Library of Infrared Spectra. cd.. III (Aldrich Chemical Co.. 1981). M. Prassas. PhD Thesis (1981) Montpellier. France.
114
of Non-Crystalline
Solid 8X (1986) 114-130 North-Holland. Amsterdam
STRUCTURAL CHANGES OF SILICA XEROGELS DURING LOW TEMPERATURE DEHYDRATION
Rcccivcd Revised
10 Fcbruaw 1986 manuscript received
6 June
lYX6
In this work we investigated the tcmpcraturc evolution of the structure Fourier Transform Infrared (FTIR) Spectroscopy. The gels. prcparsd by dcnsation in the presence of an excess of water to tetramethoxysilanc. studied in transmission. It was found that the removal of u’atcr from the casicr when the gels are prepared in basic conditions. During the drying methanol oxidizes and produces formaldehyde. formic acid and formates. present in the gel at 300°C.
of silica thin films by hydrolysis and polyconwcrc thin enough to bc pores and gel surface is heat treatment. residual Thcsc spccics are still
1. Introduction Silica gels have been prepared by hydrolysis and polycondensation of metal alkoxides. Hypercritical conditions and natural evaporation, which produce aerogels and xerogels respectively, are the two main drying procedures. Crack free (monolithic) dried aerogels can be reliably prepared [l]. However, the problem of monolithicity has not been solved yet for xerogels. The variables investigated for production of monoliths include: nature of the electrolyte [2,3] (acidic or basic medium), the number of molecules of water for hydrolysis [4.5], processing temperature [6], or casting viscosity [7]. Monolithicity and purity of the final product are favored when the hydrolysis is carried out in a large excess of water and in acidic conditions [8]. However, minute additions of water may also lead to a material, close to an organic polymer. which retains its shape at a relatively low temperature due to its elastic character. Besides these possibilities for modifying the hydrolysis and polycondensation reactions of metalalkoxide compounds, it is now well known that an addition of a drying control chemical additive (DCCA) to a mixture of metalalkoxides, water and alcohol, improves the processing control of monolithic gels [9]. The influence of the formamide DCCA on the gelation process has been studied by ‘9Si NMR [lo], Raman spectroscopy, and N, adsorption-desorption isotherms [ll]. An addition of formamide DCCA decreases the hydrolysis rate * On sabbatical from: Cedex. France.
Laboratoire
de Science
des Materiaux
0022-3093/86/$03.50 st: Elsevier Science Publishers (North-Holland Physics Publishing Division)
R.V.
Vitrcux,
USTL,
34060
Montpcllier
and slightly increases the polycondensation rate. It does not significantly affect the specific surface area, but increases the pore volume and the average pore radius. Kinetics studies have been conducted in the wet state, i.e., before gelation or after gelation while the gel was still impregnated by pore liquor. In contrast. the studies on the structural properties of the silica gel monoliths have been performed on dried gels. For example, the N, desorption isotherms were obtained after outgassing the samples. Not much is known regarding the structural changes of xerogels during the low temperature drying stage. However, it is in the temperature interval. 25-300°C. that the structural evolution of gels is the most critical as far as maintaining monolithicity is concerned. The purpose of the present study is to follow the structural evolution of xerogels with and without formamide DCCA in the temperature range of drying which is critical for obtaining dried crack-free monolithic gels. The influence of the formamide DCCA on the dehydration of SiO, films will be discussed in another paper [12]. The samples are studied in transmission by Fourier Transform Infrared (FTIR) spectroscopy. During the formation of a gel from the reaction of tetramethylorthosilicate (TMOS) and Hz0 in alcoholic medium, IR bands appear at 950 and 1080 cm-’ from the outset due to the vibration of Si-OH and Si-0-Si bonds, respectively. These bands are very intense and correspond to the formation of the SiO, network. Their intensity increases very rapidly. Thus the gels will not be transparent in the spectral range of 1300-900 cm-‘. Consequently, the drying analysis is carried out primarily in the spectral range from 4800 to 1300 cm-‘. After heat treatment. the gels become more transparent in the 900-400 cm-’ region. However, it was not possible to obtain useful information in this spectral range.
2. Experimental
technique
FTIR has proven to be a very successful technique to follow the gelation process in silica and silicate systems [13]. The IR study was performed on a Nicolet MX-1 spectrometer with a 1200-S data analyzer. Five minute scans corresponding to an accumulation of 160 spectra, were used resulting in very good resolution. In order to study the gels in transmission the samples must be as thin as possible. All the gels had a thickness less than 0.01 cm. The samples were prepared by allowing a thin film of the desired solution to gel in a non-adhesive container. The gels were cast in Petri dishes previously sprayed with Teflon@. No organic residues could be detected by comparison with gels cast in non-sprayed containers. All the gels were cast approximately half an hour prior to gelation in order to avoid excessive evaporation of the solvent which could have resulted in structural changes. Under these conditions the films generally cracked while shrinking. However, it was relatively easy to find pieces of several cm’ suitable for analysis.
Table
1
Code
and composition
of the different
Code
I’
SW SWA
1.7 1.7
4 4
SWB SWR
1.7 1.7
4 10
R
solutions H?O
Ref.
DI
figs. 1. 4. 5 fig. 7
O.lN HCI O.OlN NH,OH O.OlN
HCI
fig. 8 fig. 3
For the dehydration heat treatments, the samples were maintained at the desired temperature for 10 min in a drying oven for temperatures below 220°C and in an air atmosphere furnace for higher temperatures. After the heat treatment, the gels were transferred to a dessicator which had been previously flushed with dry Nz for five minutes. The samples were mounted on the transmission cell “in situ,” under a dry N, atmosphere. In order to identify the dehydration products, spectra (not shown) of H,O, methanol, TMOS, formic acid and formaldehyde were recorded, using a cell with CdTe windows. The compositions of the different solutions are given in table 1, where: V = volume TMOS/volume MeOH R = #mol. H,O/#mol. TMOS Hz0 = deionized (DI), acidic (O.lN HCl), basic (O.OlN NH,OH).
3. Results
The spectrum of a SW silica gel, prepared with the following conditions using DI water for hydrolysis and no DCCA in the solvent and dried at room temperature, is given in fig. 1. The material is transparent in several frequency regions: (1) 4800-3600 cm- ‘. In this interval, the bands are mainly due to overtones or combinations of vibrations of Si-OH or HzO. However, overtones or combinations of vibrations of residual methyl groups can be present as well [14]. The broad absorption band between 3600 and 3000 cm-’ corresponds to fundamental stretching vibrations of different hydroxyl groups. It is generally composed of a superposition of the following stretching modes [15]: 3750 cm-’ -free Si-OH on the surface of the gel 3660 cm-’ - Pair of surface SiOH mutually linked by a hydrogen bond - internal Si-OH 3540 cm-’ - silanol groups linked to molecular water through hydrogen bonds 3500-3400 cm- i - absorbed water
117
SW
(Si 0,)
GEL - 25”~
P C-H
\ SIOH
1 Hz”
5600
4400
3200
2000
WAVENUMBERS Fig. 1. FTIR
spectrum
(CH,-OSi)
I
I
1400
BOO
2 10
(cm-l)
of a SW gel heat treated
at 25°C
(2) 3000-1350 cm-‘. The range (3000-2750 cm-‘) corresponds to the symmetric and antisymmetric fundamental stretching vibrations of CH, groups. Between 1800 and 1550 cm-‘. there are juxtaposed bands due to the vibrations of C=O groups, the deformation of molecular water, and combinations of vibration of the SiO, network [16,17]. The combination of the SiO, vibrational modes are generally located at 1960, 1870 and 1640 cm-‘. Most of the time, the band at 1960 cm-’ will appear as a shoulder of the 1870 cm-’ band, which is always well resolved, although not very intense. On the other hand, the band at 1640 cm-’ is often hidden by the band corresponding to the deformation of molecular water (1620 cm- ‘). The IR transmission spectra of a blown commercial Pyrex@ glass shows the above mentioned bands in the 3000-1350 cm-’ region (fig. 2). However. the broad band due to the hydroxyls, 3600-3000 cm-‘, is missing in the Pyrexm film. The absorption bands due to the silica network are located between 1100 and 800 cm-‘. Another band (deformation of the Si-0-Si bond) appears at 455 cm-‘. For microporous hydrated gels or glasses, a band at about 950 cm-’ can be observed. At room temperature, the IR spectrum of a gel is generally not well resolved, due to the high quantity of residual liquid within the pores (fig. 1). The large band between 4600 and 4300 cm- ’ is also poorly resolved. It is due to a combination of the stretching and deformation vibrations of the C-H bond [14], and the bands corresponding to the combinations of vibrations of
PYREX
I 5600
I 4400
/
I 3200
Fig. 2. FTIR
1
2000 WAVENUMBERS
spectrum
of a blown
I
FILM
I 1400
800
2 10
(cm-l) film of PyrcxK
glass.
the hydroxyl groups. However some features of these bands have been interpreted differently by the various authors [l&20]. For the spectrum of the silica gel dried at room temperature (fig. l), we can observe on the high frequency side a large band centered around 3600 cm ’ and a small band at 3750 cm-’ due to free Si-OH groups. On the low frequency side of this broad band there is a weak shoulder, near 2860 cm- ’ which indicates the presence of methanol. The deformation band of the same CH, group will appear at 1400 cm- ‘. On the other hand, if the TMOS is not fully hydrolyzed, the IR spectrum will show absorption bands at 2958. 2857 and 1456 cm-’ [21]. The first two bands cannot be distinguished from those of CH,OH. On the other hand, the 1456 cm-’ band, can be easily seen in the spectrum. This indicates that at room temperature the gel contains Si-OCH, bonds, which come from an incomplete hydrolysis of TMOS rather than an esterification of SiOH groups. A gel prepared with a higher water content and in more acidic conditions should not display such a band. As an example the spectra of a SWR * gel are given in fig. 3. The 1456 cm-’ band is not present but a small hump can be seen at that wavenumber. The intensity of this hump does not seem to change significantly with temperature up to 170°C. The most intense bands of methanol (1050 cm-‘) and TMOS (1100 cm-‘) will always be located in a frequency range where the vibrations of the silica network are the most intense, and thus will not be observed. * SWR
gel corresponds
to R = 10 and HZ0
= O.OlN
HCI
. ..“-.._.
SWR ‘.
?
,
52 Fig. 3. FTIR
I
-,
4400
spectraof
I
3200 a SWR
2000 WAVENUMBERS km’) gel heat treated at: (a) 25°C.
1400
800
(b) XOOC. (c) 120°C.
2 ‘0 (d) 17O’C
The strong absorption band at about 1620 cm-’ indicates that the room temperature silica gel contains a large quantity of molecular water (fig. 1). The IR spectrum of the 60°C dried silica gel is given in fig. 4a. (It should be noted that for improving the reading of the figures, the spectra have been moved along the transmittance axis in figs. 3, 4. 5. 7 and 8.) After drying at 60°C the spectrum is better resolved. and the band at 4557 cm-’ can be observed. It corresponds to the combination of the stretching and deformation vibrations of free SiOH groups (3750 + 810 cm-‘) [20]. While the broad band corresponding to the vibrations of OH groups narrows after the 60°C drying, the band due to free Si-OH vibrations strongly increases. When a hydrogen bond is formed between surface silanols and any other compound, a new band appears at lower frequencies, and the intensity of the band at 3750 cm-’ decreases [22]. The band at 1620 cm-’ corresponding to molecular water decreases in intensity due to the 60°C drying and a new band appears at 1685 cm-‘. The location of this absorption band is characteristic of a C=O band. However, in
120
G. Orcel
er ul. / Struclurul
chmges
ojsilico
xerogels
SW
i !r’\
I
I 5600
4400 Fig. 4. FTIR
3200 spectra
I
I
2000 WAVENUMBERS
of a SW gel heat treated
“‘\
I
[C-H
(CH,-
I
,
1400
OSi)
I 800
I ; 10
(cm’) at: (a) 60°C.
(b) XO”C,
(c) 12O“C
carbonyl compounds the C=O group generally gives rise to a vibration whose wavenumber is larger than 1700 cm-‘. Only formaldehyde gives rise to a vibration band below 1700 cm-‘, more precisely between 1672 and 1685 cm-‘, depending on the degree of association. In this case the gel can be considered as a catalyst for formaldehyde, due to its high specific surface area. Formaldehyde can be synthesized by oxidation of methanol, according to the overall reaction [23]: CH,OH + +O, + CH,O + H,O, (1) After 80°C drying, the IR spectrum of the gel (fig. 4b) does not change much compared to the spectrum obtained after 60°C. However, a new absorption band corresponding to the symmetric stretching mode of Si-OCH, bonds, appears at 2958 cm-‘. The band due to molecular water decreases still further in intensity. Three new bands appear after 120°C drying. They are located respectively at 2990, 1390 and 1375 cm- ‘. One of the stretching modes of formaldhyde is at - 3000 cm-‘. On the other hand, the 1390 and 1375 cm -’ bands are more difficult to assign.
G. Orcel
et (II. / Srrucruml
changes
o/silicu
xerogels
The oxidation of methanol to produce formaldehyde, on concurrently with other reactions:
121
eq. (1) generally goes
CH,O + +O, + HCOOH,
(2)
CH,O + 40, + CO + H,O,
(3)
CHzO+CO+H,,
(4)
CH,O + 0, --, CO, + H,O.
(5)
Formaldehyde is often considered unstable so the presence of a catalyst is important. Most of the time, formaldehyde decomposes before all the alcohol is oxidized, and side reactions, such as the following ones, are possible: CH,O + CH,OH CH,OH
+ CH,OCH,OH,
+ HCOOH + HCOOCH,
(6) + HzO,
CH,O + H,O P CH,(OH),.
(7) (8)
However, reaction (8) is not likely to occur because CH,(OH)? is not stable at 80°C. The bands at 2990 cm-’ (stretching mode of C-H) and at 1390 cm-’ (deformation mode of C-H) in fig. 4b are associated. On the other hand, the band at 1375 cm-’ is assigned to a symmetric stretching vibration of a
-c, 40
group.
0-
Thus, the structure of the molecule should be: H O=C< o- ’ which means that increasing the temperature of gel dehydration leads to the formation of formates or esters [reaction (7)]. The bands at 2990, 1390 and 1375 cm-’ can thus be assigned to a chemical species obtained from the decomposition of methanol. This new molecule is probably linked to the surface of the gel. It is most probably a formate which would be produced according to the following overall reaction: SiCH,OH+O< SiE
0 ,’
SiE
H y-o< 0
+ +HzO. SiE
(9)
If this mechanism is valid, then all bands, identical to those observed for the absorption of methanol on an alumina substrate [24], should appear at higher temperatures. Indeed, the band at 1599 cm-’ which can be observed after 170°C drying (fig. 5a) can be assigned to formate groups. The other bands at 1472 and 1456 cm-’ are probably due to ESi-OCH, species, which are formed by a methylation of =Si-OH groups. The band previously located at 1685 cm-’ widens and increases in intensity. This illustrates the formation
G. Orcel el al. / StrucIurul
:
ii /
I -\;;,
I
I
Fig. 5. FTIR
I
4400 spectra
,
of silicu .uerogels
,i /;I / II,’ , ,, ,
i!
/-“I
5600
j ,I ‘..-. . .i
changes
!I I
I
3200
I
I
2000 WAVENUMBERS
of a SW gel heat treated
1
km-‘)
at: (a) 170°C.
1
1400 (b) 200°C.
I
I
800 (c) 22O”C,
I
2 10 (d) 240°C.
of a new band, due to the vibration of a C=O group, whose frequency is slightly higher. This C=O absorption band probably belongs to a formate group. However, it should not be overlooked that formic acid, which can be produced by oxidation of methanol, gives a band at the same location. Indeed, the bands at 1375 and 1700 cm-’ (which is assigned to C=O) vary in the same way. The shift of the band at 1685 cm-’ to 1700 cm-’ could be attributed to the formation of formic acid from formaldehyde. The bands at 1579 and 1492 cm-’ are difficult to assign. They could be produced by molecules similar to formates. The vibration bands of acetates are located at 1575 and 1466 cm-‘. .The decomposition of methanol can lead to different chemical species, depending on the nature of the substrate [25,26]. Some of these products of oxidation, such as dimethyl ether, are not very active in the IR. However, the presence of these compounds has been confirmed using NMR spectroscopy ~271.
G. Orcel er ul. / S0wturul
chmges
ojsilico
serogels
123
The spectra obtained for drying temperatures up to 220°C (figs. 5b, c) show the same absorption bands. However, the strong band at 1700 cm-’ is slightly shifted toward higher wavelengths, thus indicating a weaker association with the surface of the gel. Additional information on the dehydration of the gels can be obtained by plotting the band intensities as a function of temperature. However, such a plot may be inaccurate since even the sample will give a different IR throughput after being heat treated. This is primarily for two reasons: (1) It is not possible to reposition the film in the FTIR laser beam exactly in the same way after heat treatment. Since the film thickness is not completely uniform, the quantity of matter within the beam is not identical for each experiment. (2) The film shrinks during dehydration. When the sample thickness decreases the quantity of matter sampled by the laser beam increases. Thus it is difficult to obtain quantitative information by considering only the peak heights. In order to minimize these effects and thereby compare the various spectra taken after various heat treatments, and eventually compare different films, an internal standard is required. An absorption band corresponding to the vibration of a molecule with a constant concentration provides the best source of calibration. The band at 1867 cm-‘, which is a combination of various vibrations of SiOZ, fulfills all the requirements. It is present in all the spectra (figs. 1-8) and is well resolved for temperatures above 25°C. In addition, the concentration of SiO, is fairly constant since significant polymerization does not occur during the time of the FTIR scan [lo]. The esterification of silica does not occur to any significant extent according to the spectra obtained. Consequently, the quantity of SiOz in the films should not change appreciably during the dehydration thermal threatments. Thus, to compare the spectra in figs. l-8, we used an intensity number N, defined as follows N = 100 x Z/Z,,,,,
(10)
where I is the intensity of the band of interest and I,,,, is the intensity of the band at 1867 cm-‘. The results are reported in fig. 6. All the bands relative to the decomposition products of the solvent decrease in intensity (fig. 6). In contrast, the bands at 2958 and 2857 cm-’ (which were attributed to OCH, groups) do not follow this behavior. Thus the gels have to be heat treated to higher temperatures in order to oxidize the internal residual organic groups. The carbonization of the samples, observed as an incomplete oxidation of the films, is mainly due to non hydrolyzed alkoxy groups and not to the solvent. After 240°C drying, the absorption band of OH groups is still very broad (fig. 5d). It indicates that a large quantity of water is hydrogen bonded to the surface of the gel. The number of internal SiOH bonds is very large as well. The spectra of the silica gels prepared in a basic medium were studied for temperatures up to 170°C (figs. 7a, b, c, d). The general aspects of the spectra are similar to those obtained for gels synthesized with DI water (figs. l-6). However, it is worth noticing that: (1) The removal of molecular water occurs
124
G. Orcel
1
0
2990
cm”
XA0x
1700 2958 1466 2857
cm-l cm-’ cm4
er al. / Structural
chunges
of srlrca xerogels
Ff> % F. m %*-
100
I20
170 TEMPERATURE
Fig.
6. Variation
with
200
220
(“C)
temperature of the intensity number N of the bands 1700,1466 and 1375 cm-’ of a SW gel.
at 2990.
2958.
2857.
at higher temperatures compared with the previous DI water gels. The band assigned to molecular water can be seen at 120°C (fig. 7~). (2) The bands at 2990, 1700 and 1375 cm-’ which appear during the decomposition of the solvent can be observed at 170°C (fig. 7d). (3) The broad band due to the vibration of hydroxyl groups is poorly resolved, even at 170°C. Besides the silanol groups on the surface of the gel, there are SiOH groups linked by hydrogen bonds. The gels prepared in acidic condition have the simplest spectra, (figs. 8a, b, c, d) compared to the two previous gel systems. Only three major bands at 3800,3200, 1867 and 1640 cm-’ due to water, silica and water, respectively, can be observed. The evolution of the spectra with temperature is primarily a decrease in intensity of the bands attributed to water. However, it should be noted that: (1) No band due to free SiOH was resolved, even at 170°C. (2) No band due to alkoxy groups (from non hydrolyzed =SiOCH3, or from esterification of =SiOH), could be observed. (3) No products of decomposition or oxidation of the solvent gave rise to absorption bands. The high quantity of internal silanols can be attributed to the polymer type structure resulting from the acidic conditions [28]. Hydrochloric acid promotes the hydrolysis and it was shown that only a few minutes or less are needed for
G. Orcel
er ul. / S~ructurul
chatlges
o/silica
125
xerogels SWB
(BASIC)
:d
a 1 5600 Fig. 7. FTIR
I
I
4400 spectra
of a SWB
I
I 3200 2000 WAVENUMBERS gel heat treated
I
I 1400
000
i lo
(cm’) at: (a) 25’C,
(b) 8O’C.
(c) 120°C.
(d) 170°C.
the “total” hydrolysis of TMOS by water in presence of HCl [29]. It is more difficult to account for the fact that no bands characteristic of formates could be observed. One possible explanation is that the formates come from the oxidation of the remaining alkoxy groups. The second possibility is that the surface of the gel is not available to the solvent, and thus no catalytic effect occurs. A plot showing the temperature variation of the intensity of the bands due to formates and alkoxy groups (fig. 6) clearly shows that they vary independently. This allows us to conclude that =Si-OCHs groups are not responsible for the formation of formates at low temperature ( < 170°C). The unavailability of the gel surface to solvent molecules can be explained by the high quantity of adsorbed molecular water which covers the gel surface through hydrogen bonds. The methanol molecules have fewer SiOH groups available to react with, and the quantity of formates formed decreases. It is now well
..
SWA
(ACID)
0
Fig. 8. FTIR spectra of a SWA gel heat treated at: (a) 25OC.(b)
80°C.
(c) 110°C.
cd) 170°C.
known that HCl, which increases the hydrolysis rate, induces the formation of smaller particles and a smaller scale silica network [30]. As a result, the gel has a larger surface area, smaller pores and probably a larger degree of pore interconnection. Even though the reactive surface increases, it becomes more difficult for the MeOH molecules to reach the available SiOH groups to form formates. and the kinetics are slowed down.
4. Discussion The study of the dehydration of silica gels, prepared by hydrolysis and polycondensation of tetramethoxysilane in alcoholic medium, showed that intermediate compounds remain absorbed on the gel surface at temperatures higher than 300°C. Methanol successively decomposes into formaldehyde, then formic acid, producing formates which remain strongly bound to the surface of the gel. The residual organics, due to an incomplete hydrolysis of TMOS, or to a methylation reaction of SiOH groups, are oxidized at temperatures higher then those necessary for the removal of formates. To the best of our knowledge, it is the first time that direct proof of the absorption of these
G. Orcel
er (II. / Strucrurul
chunges
of siliw
xerogels
127
compounds on silica gels is given. The band assignment of the vibration of these molecules is recapitulated in table 2 where we add relevant information on IR bands in SiOz gels. In table 2 we adopted the following notation: H e.g. 2990 cm-’
Stretching C-H
>c=o (0 i H 1 which reads as: the band at 2290 cm-’ corresponds to the stretching mode of the C-H bond in the molecule H y=o. H An illustration of this structure is represented in fig. 9 by the sketch (f). In this figure we show schematically the surface of SiO, network which constitutes the gel. In order to improve the clarity of the figure, only a few SiO, tetrahedra are shown. For the same purpose only some of the Si atoms in the SiO, tetrahedra are represented. Transmission IR spectroscopy shows the essential differences in behavior of gels prepared in acidic and basic media. In an acidic medium, these structures contain a large quantity of internal silanols. IR studies show that the complete dehydration of this type of gel is difficult to achieve. In a basic medium the gel is more particle-like with SiOH groups situated mainly on the surface. The dehydration of this type of gel is relatively easy at low temperatures. Compared to the gels prepared in an acidic medium. at the same temperature basic gels contain less water but more surface silanols, free of hydrogen bonds. The variations in dehydration are due to the differences of gel structure: i.e., macroporous for the gels prepared in basic conditions versus microporous for the gels prepared in acidic conditions [9], and to the different types of structural water; i.e. surface silanols for the basic gels versus internal silanols for the acidic gels. These results are consistent with the behavior of the gels at higher temperature. It is well known that the gels prepared in acidic conditions densify at lower temperature than those prepared in basic conditions [32], which is related to the nature and quantity of SiOH species in the sample. When the concentration of silanol groups increases, the viscosity of the gel decreases and viscous flow, responsible for the densification of the gel, takes place at lower temperature.
5. Conclusions Transmission FTIR is a useful tool to follow the structural changes which occur at the surface of gels. In acidic conditions, the hydrolysis of TMOS is favored but more internal SiOH groups are retained in the gel at high temperature. On the other hand, for base catalyzed gels, the water is removed
128
G. Orcel
Table 2 Absorption
bands
of the water,
er al. / Slruclural
cltauges
methanol,
system
TMOS
oj silica
Ref Biblio.
Assignment
Wavenumber
cm-’ 3660 cm-’
stretching + deformation stretching+deformation stretching free surface stretching Si-O-H
C-H free Si-OH %-OH OH-Si
3540 cm-’
stretching stretching
internal Si-O-H
i-H
stretching symmetric stretching
2990 cm-’
stretching
2958 cm-’
symmetric stretching Si-d-CH, stretching C-H (CH,-OH) stretching C-H (CH,-OH, CH,-CHO CH, -0-Si) combination Si-0 combination Si-0
cm-’
4600-4300 4557 cm-’ 3750
cm-’ cm-’
3500-3400 3OOG2750
1960cm-’ 1867 cm-’
Fig.9
1141
PO1 1151 [I51
(a) (b)
I151
CC) cd)
adsorbed Hz0 and asymmetric CH1
1151
(e)
C-H
[341
(0
I211 [311
k9 (h) (h)
1151
Si-OH
,--
2857 cm-’
serogels
[21.31] 1161 U61
1700 cm-’
1311
(i)
1685 cm-’
1311
(0
1640 cm ’ 1620 cm-’ 1599cm-’
overtone Si-0 bending molecular H 2O asymmetric stretching
1161 P61 [241
ti) (k)
1579 cm-’
asymmetric
P61
(1)
1311
Cm)
[311
(0 00
1492 cm-
’
1472 cm-’
1466 cm-’
stretching
deformation
OH,
C
:, ( CH2-Co,)
bending C-H (CH,-0-Si) symmetric stretching -,B
/
‘$C-CH,\
o- l-0
1241
I
1456 cm-’ 1390 cm-’ 1375 cm-’
deformation deformation symmetric
C-H. (CHJ-0-Si) C-H. (CH,OH) stretching
1211
1080 950 800 455
stretching stretching stretching deformation
+ SiO + + Si Si-OH, Si-O+ Si-0-Si + Si-0-Si-0
~321 1321 [321 1321
cm- ’ cm-’ cm-’ cm-’
[311
~241
(0 0-d (k)
0-d (0) (P) (P)
G. Orcel
et ul. /
Slructurul
clunges
o/silicu
H \
tkl
129
serogels
C-0
0
I
I
I
H\--n
H-6
H
0 .H
Fig.
9. Schematic
molecular
representation
of the surface species.
of a gel, with
the different
adsorbed
more easily and less internal silanols remain in the sample. Thus, for the same experimental conditions, it is easier to prepare crack free base catalyzed samples. During drying residual methanol oxidizes and produces formaldehyde, or reacts with surface silanols to give formates and eventually formic acid. The removal of these molecules, which are still present at 300°C is essential when preparing glasses, free of cracks, porosity or impurities such as residual carbon.
The authors gratefully acknowledge Contract # F49620-83-C-0072.
partial
financial
support of AFOSR
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(311 [32]
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