The influence of acetic acid on the pore structure and surface area of silica prepared by the sol-gel method was investigated. Experimental conditions of synthesis, such as gelation temperature and solvents, were also studied. N2 adsorption isotherms of the samples were type 1, typical of microporous materials, explaining the high surface area values (BET) observed. The simultaneous addition of acetic and hydrochloric acids as catalysts and of acetone as solvent, together with the use of a gelation temperature of 20 ºC, made it possible to prepare amorphous silica materials with surface area values up to 850 m² g-1. The high surface area value of these samples could be explained by the microporosity and the nanometric size of the particles.Nesse trabalho foi estudada a influência do ácido acético na estrutura de poros e na área superficial de sílicas preparadas pelo método sol-gel. Condições experimentais de síntese, tais como temperatura de policondensação e solventes, também foram estudadas. Isotermas de adsorção de N2 das amostras foram classificadas como do tipo 1, típicas de materiais microporosos, o que explica os altos valores de área superficial obtidos. A adição simultânea dos ácidos acético e clorídrico como catalisadores e de acetona como solvente, bem como o emprego de uma temperatura de policondensação de 20 ºC, possibilitaram a preparação de sílicas amorfas com valores de área superficial de até 850 m² g-1. O alto valor de área superficial dessas amostras pode ser explicado principalmente pela microporosidade e também pelo tamanho nanométrico das partículas.886890Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Arenas, Leliz T.

Benvenutti, Edilson V.

Costa, Tania M. H.

Dias, Silvio L. P.

Gushikem, Yoshitaka

Moro, Celso C.

Simm, Carolina W.

Journal of the Brazilian Chemical Society

English

Repositorio da Producao Cientifica e Intelectual da Unicamp

J. Braz. Chem. Soc., Vol. 18, No. 5, 886-890, 2007.
Printed in Brazil - ©2007  Sociedade Brasileira de Química
0103 - 5053  $6.00+0.00
Ar
ti
cl
e
*e-mail: taniaha@iq.ufrgs.br; benvenutti@iq.ufrgs.br
Synthesis of Silica Xerogels with High Surface Area using Acetic Acid as Catalyst
Leliz T. Arenas,a Carolina W. Simm,a Yoshitaka Gushikem,b Silvio L. P. Dias,a Celso C. Moro,a
Tania M. H. Costa*,a and Edilson V. Benvenutti*,a
aInstituto de Química, Universidade Federal do Rio Grande do Sul, CP 15003,
91501-970 Porto Alegre-RS, Brazil
bInstituto de Química, Universidade Estadual de Campinas, CP 6154, 13084-971 Campinas-SP, Brazil
Nesse trabalho foi estudada a influência do ácido acético na estrutura de poros e na área superficial
de sílicas preparadas pelo método sol-gel. Condições experimentais de síntese, tais como temperatura
de policondensação e solventes, também foram estudadas. Isotermas de adsorção de N2 das amostras
foram classificadas como do tipo 1, típicas de materiais microporosos, o que explica os altos valores
de área superficial obtidos. A adição simultânea dos ácidos acético e clorídrico como catalisadores
e de acetona como solvente, bem como o emprego de uma temperatura de policondensação de
20 °C, possibilitaram a preparação de sílicas amorfas com valores de área superficial de até 850
m2 g-1. O alto valor de área superficial dessas amostras pode ser explicado principalmente pela
microporosidade e também pelo tamanho nanométrico das partículas.
The influence of acetic acid on the pore structure and surface area of silica prepared by the
sol-gel method was investigated. Experimental conditions of synthesis, such as gelation
temperature and solvents, were also studied. N2 adsorption isotherms of the samples were type
1, typical of microporous materials, explaining the high surface area values (BET) observed.
The simultaneous addition of acetic and hydrochloric acids as catalysts and of acetone as solvent,
together with the use of a gelation temperature of 20 °C, made it possible to prepare amorphous
silica materials with surface area values up to 850 m2 g-1. The high surface area value of these
samples could be explained by the microporosity and the nanometric size of the particles.
Keywords: silica, microporous materials, high surface area, acetic acid
Introduction
Inorganic materials possessing high surface area have
been largely studied due to their possible application as
sorbents, catalysts, sensors, molecular sieves etc.1-4 In
general, these materials are formed mainly by silicon and
aluminum oxides, with ordered structures like those
presented by zeolites and clays. Since the 1990s, after the
appearance of a new class of highly ordered silica
mesoporous materials called MCMs (Mesoporous
Crystalline Materials), the interest in high surface area
silicas was renewed.5-8 Several synthetic methods aiming
at the preparation of materials with high surface area and
tailored pore size in the micro, meso and macropore range
have been proposed. Among the proposed methods, the
sol-gel synthesis appears as an interesting alternative
process to obtain these materials.9-11 This method, that is
based on hydrolysis and polycondensation of silicon or
metal alkoxides, allows the preparation of materials with
different morphological characteristics by varying
experimental synthetic conditions such as pH, solvent,
temperature and catalyst.9,12,13 The reaction medium can
be acid, neutral or basic, depending on the inorganic
catalyst added, as, for example, HCl, HF or NH3.14 Organic
catalysts such as citric and acetic acids have been also
used.15-17 Anionic and cationic surfactants, as well as water
soluble polymers, have also been added to sol-gel systems
in order to produce materials with controlled porosity.18,19
The use of acetic acid as catalyst has been reported
for several sol-gel systems, as in the synthesis of high
purity dense silica glass microspheres,20,21 photo-
luminescent lanthanide-doped silica microspheres with
controllable size,22 silica-based hybrid materials17,23 and
titania-based systems.24,25 Although the mechanism of
acid acetic action is not completely known, it has
already been well studied in several works as follows.
887Arenas et al.Vol. 18, No. 5, 2007
It was first proposed by Pope and Mackenzie26 that the
acetic acid solvolysis occurs by a two-step mechanism
involving the formation of ester as intermediate. In the
sequence, a consensus was established on the formation
of intermediate metal acetate species.15,17,24 For pure
silica systems, besides acting as acid catalyst, acetic
acid can influence the kinetics of polycondensation,
decreasing the speed of reaction and consequently
influencing the primary particle growth of the gel, or
causing a narrow particle diameter distribution.15,16 In
a previous work by our research group,27 we had
observed that in silica/cellulose sol-gel systems, the
presence of acetic acid produced high surface area
hybrid materials and, as far as we know, this fact has
not been reported yet. Thus, we believe that acetic acid
can be better explored as an important factor in the
sol-gel synthesis of controlled pore size and high
surface area materials.
In this work, the effects of the presence of acetic acid
on the pore structure and surface area of silica xerogels
were investigated. The influence of experimental
conditions such as the solvent used and the gelation
temperature were also studied.
Experimental
Synthesis of silica xerogels
Two sample series of silica xerogels were prepared
by the sol-gel method based on hydrolysis and
polycondensation of tetraethylorthosilicate (TEOS) in
acid medium. Table 1 summarizes the synthetic
conditions for the silica xerogel samples of the first
series. They were obtained from 22 mmol of TEOS
dissolved in the solvent specified in Table 1. Afterwards,
the catalyst and water, in stoichiometric ratio with Si r =
4/1 (1.6 mL), were added to the precursor solutions,
under stirring, except in the case of sample A, that was
synthesized without water. The samples were covered
and left for gelation and solvent evaporation, without
sealing, for two weeks at 20 ± 5 ºC. The xerogels obtained
were comminuted in an agate mortar, washed with 10
mL of water and ethanol and oven-dried at 100 ºC. Nine
samples were prepared, varying the solvent and the
catalyst employed, either glacial acetic acid or HCl
solution 0.1 mol L-1, or both, according to Table 1.
The second series was obtained using the experimental
conditions of sample E (Table 1) and varying the gelation
temperature. Temperatures of 5, 15, 20, 25, 30 and 50 ±
1 ºC were employed. The precise temperatures used in
this sample series were attained with the use of a water-
containing thermostated system or of an oil bath. The same
gelation and washing procedures described for the first
series were used in this case.
N2 adsorption-desorption isotherms
The nitrogen adsorption-desorption isotherms of xerogels
previously degassed at 150 °C were determined at liquid
nitrogen boiling point in a homemade volumetric apparatus,
connected to a vacuum line system employing a turbo
molecular Edward vacuum pump. The pressure measurements
were made using a capillary Hg barometer. The specific surface
areas of hybrid materials were determined from the BET
(Brunauer, Emmett and Teller) multipoint method28 and the
pore size distribution was obtained using the BJH (Barret,
Joyner and Halenda) method.29
SAXS analyses
Small angle X-ray scattering (SAXS) data were
obtained on a Shimadzu XD3A equipment, using the
Cu K
α
 radiation source. The average pore radii of the
samples were obtained by using the Guinier law.30,31
This law establishes that the scattering intensities I (q)
are described as ln I(q) = ln I
o
 – (RG2 q2) / 3, where RG
is the gyration radii and q is defined as q = (4π senθ) /
λ, where θ is the scattering angle and λ the wavelength
of the X-ray used. The gyration radii could be calculated
by the equation RG = (3p)1/2, where p is the slope of
the Guinier plot, (ln (q) vs. q2). Considering spherical
pores, the pore radii were obtained by the equation Rp =
(5/3)1/2 RG.
TEM analyses
Transmission Electron Microscopy images were
obtained on a JEOL JEM 2010 apparatus, with an
Table 1. Experimental conditions for the synthesis of the first silica
xerogel series
Sample Solvent Total solvent Catalyst Catalyst
volume/mL  volume/mL
A a acetone 5 acetic acid 5
B acetone 5 acetic acid 5
C acetone:ethanol (1:1) 5 acetic acid 5
D ethanol 5 acetic acid 5
E acetone 5 acetic acid / HCl 5 / 0.1
F ethanol 5 acetic acid / HCl 5 / 0.1
G acetone 10 HCl 0.2
H ethanol 5 HCl 0.2
I acetone:ethanol (1:1) 10 HCl 0.2
a
 without water.
888 Synthesis of Silica Xerogels with High Surface Area using Acetic Acid as Catalyst J. Braz. Chem. Soc.
acceleration voltage of 200 kV, using powdered samples.
The magnification used was up to 500,000 times. Samples
were ground to very fine powders in an agate mortar,
dispersed in isopropylic alcohol and ultrasonicated for 20
minutes. The samples were then settled over a copper grid
for analysis.
Results and Discussion
The results of surface area, pore volume and gelation
time of the first sample series are shown in Table 2. In a
general way, in the present work, samples prepared with
acetic acid as the only catalyst (samples B, C and D in
Table 1), present higher surface area values, near twice as
high as those prepared only with hydrochloric acid (samples
G, H and I, ca. 370 m2 g-1). One exception is sample A,
which was synthesized with acetic acid but without initial
water addition, for which the surface area obtained was
440 m2 g-1. The highest surface area value was obtained for
sample E, when both acid catalysts were used at the same
time, and acetone as solvent, being this value 825 m2 g-1.
Additionally, it can be observed in Table 1 that, for samples
E and F, where hydrochloric and acetic acids were
simultaneously used, the gelation time was lower than those
observed for reactions catalyzed by acetic acid only.
 It has already been reported that the use of acetic acid
in silica sol-gel systems can produce changes in the
microstructure and final characteristics of the materials,
allowing the control of particle size distribution.15,16 The
silica sol-gel films obtained are crack-free and show
optical transparency.16,17 Although the mechanisms which
govern the formation of these materials are not well
known, there is a general consensus about the formation
of the species ≡Si-OOCCH3 and CH3COOC2H5 in TEOS/
ethanol systems. The ≡Si-OOCCH3 species were
identified by infrared spectroscopy.15,20 It was proposed
that, in the presence of acetic acid, several reaction steps
could occur, which are shown in Scheme 1.15,17
It has been observed that the addition of acetic acid to
sol-gel silica systems produces a reduction in gelation
rates in comparison with the addition of inorganic acids.
This effect can be overcome by the addition of water;
however, water also influences the microstructure of the
materials.13,17,32 Based on results obtained in previous
works, the water/TEOS molar ratio used was 4:1, since
the use of this molar ratio resulted in hybrid or pure silica
xerogels having high surface area.5,13,32
Figure 1 shows the N2 adsorption isotherms for the
first sample series. It can be seen that all curves are type
1 isotherms, typical of microporous materials.33 This result
can explain the high surface area values observed for this
sample series.
Considering that the highest surface area value of the
silica xerogels was obtained for sample E, where acetone
was used as solvent and acetic and hydrochloric acids as
catalysts, we decided to study this xerogel sample
preparation in a more detailed way. In previous works with
silica-based hybrid systems, it has been observed that the
gelation temperature is an important parameter in
determining surface area values.12,34 In those papers, a
decrease in surface area values with increasing temperature
(from room temperature to 50 °C) was reported, being this
fact accompanied by an increase in organic incorporation
of the xerogels. In the present paper, a second series of
Table 2. Surface area and pore volume of the first xerogel series
Sample Surface area/ Pore volume/ Gelation
(± 25 m2 g-1)  (± 0.03 cm3 g-1) time/day
A 440 0.24 3
B 750 0.18 2
C 680 0.18 4
D 785 0.31 3
E 825 0.19 2
F 620 0.11 2
G 335 0.10 1
H 425 0.12 < 1
I 355 0.10 < 1
Figure 1. N2 adsorption isotherms for the first sample series.
0.0 0.2 0.4 0.6 0.8 1.0
50
100
150
200
250
300
A
m
o
u
n
t
a
d
so
rb
e
d
/
(c
m
3
g
-1
)
P/Po
A D G
B E H
C F I
Scheme 1.
≡Si-OC2H5 + CH3COOH → ≡Si-OOCCH3 + C2H5OH
CH3COOH + C2H5OH → CH3COOC2H5 + H2O
≡Si-OOCCH3 + C2H5OH → ≡Si-OH + CH3COOC2H5
≡Si-OC2H5 + ≡Si-OOCCH3 → ≡Si-O-Si≡ + CH3COOC2H5
889Arenas et al.Vol. 18, No. 5, 2007
pure silica was prepared, maintaining the experimental
conditions of the sample E and varying the gelation
temperature in a more controlled way. The used values were
5, 15, 20, 25, 30 and 50 ± 1 °C. For this new sample series,
the samples obtained were named E5, E15, E20, E25, E30
and E50, respectively. The surface area values obtained for
these samples are presented in Table 3. The highest surface
area value attained was of 850 m2 g-1 for the sample
submitted to a gelation temperature of 20 °C (sample E20).
For lower temperatures, a decrease in surface area occurred
and, for higher temperatures, there was also a slight decrease
according to the results cited above, already reported for
silica-based hybrid materials.12,34
For the sample E20, that presents the highest surface
area value, we decided to improve the characterization.
The pore size was determined by SAXS analysis. From
the Guinier plot, it was possible to adjust the curve with a
linear fitting presenting slope of -0.2598. The radii value
obtained was 1.1 nm. Transmission electron microscopy
images were also obtained (Figure 2). It is possible to see
that, at the magnification employed, the silica xerogel
sample E20 presents particles with diameter lower than
50 nm.
Although the high specific surface area value for this
sample is mainly due to the microporosity contribution,
as observed from the N2 isotherms (Figure 1 and Tables 2
and 3), there is also a contribution of the external area of
the nanometric-sized xerogel particles, observed on TEM
images. Additionally, these observed nanostructures could
be agglomerates presenting internal structures, and the
surface area contribution of these presumably primary
particles cannot be discarded.
Considering morphological aspects, the main papers
published about the silica/acetic acid system report the
preparation of microspheres with controlled size and
shape.15,20-22 In our case, the materials presented different
characteristics not observed up to now, such as very high
surface area and nanometric particle size with diameter
between 20 and 50 nm. Although the water / acetic acid /
TEOS ratios used were the same, 4:4:1, there are important
experimental differences in the synthetic method as
follows: in the cited papers solvents were not used and
the reaction was performed under vigorous stirring. On
the other hand, in the present work ethanol and acetone
were employed as solvents to promote homogeneity,
considering that water and TEOS are immiscible.
Conclusions
The use of acetic acid in sol-gel silica systems was
satisfactory to produce high surface area microporous
materials without addition of any template. The
simultaneous addition of acetic and hydrochloric acids as
catalysts and acetone as solvent made possible to prepare
amorphous silica materials with surface area values up to
850 m2 g-1, taking a gelation time of two days. The gelation
temperature also influences the surface area, being the
highest value attained at gelation temperature of 20 °C.
For this material, the estimated micropore radius, obtained
by SAXS, was 1.1 nm, and the observed particle size was
below 50 nm. The high surface area value of this sample
could be explained mainly by the microporosity and also
by the nanometric size of particles.
Table 3. Surface area and pore volume of the second xerogel series
Sample Temperature/ Surface area/ Pore volume/ Gelation
°C (± 25 m2 g-1)  (± 0.03 cm3 g-1) time/day
E0 0 520 0.13 14
E15 15 700 0.16 3
E20 20 850 0.24 2
E25 25 670 0.17 2
E30 30 630 0.15 1
E50 50 630 0.15 < 1
Figure 2. Transmission electron microscopy images of the sample E20.
890 Synthesis of Silica Xerogels with High Surface Area using Acetic Acid as Catalyst J. Braz. Chem. Soc.
Acknowledgments
We thank FAPERGS (Fundação de Amparo à Pesquisa
no Estado do Rio Grande do Sul), FAPESP (Fundação de
Amparo à Pesquisa no Estado de São Paulo)  and CNPq
(Conselho Nacional de Desenvolvimento Científico e
Tecnológico-Brazil), for financial support and grants. We
also thank CME-Centro de Microscopia Eletrônica-
UFRGS for the use of the TEM equipment.
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Received: September 27, 2006
Web Release Date: July 20, 2007
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Synthesis of silica xerogels with high surface area using acetic acid as catalyst

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Synthesis of silica xerogels with high surface area using acetic acid as catalyst

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