Synthesis and formation of silica aerogel particles by a novel sol-gel route in supercritical carbon dioxide

Article abstract of DOI:10.1021/jp036973d

A new method to obtain silica aerogel particles using acetic acid as the condensation agent for silicon alkoxides in supercritical carbon dioxide (scCO₂) is proposed. The objective of this study was to determine the mechanism of silica aerogel formation in scCO₂ using in situ analysis techniques. The synthesis and formation of silica aerogel particles was carried out by a modified sol-gel route, based on the hydroxylation and condensation of silicon alkoxides in scCU₂; both submicron and micron-size aerogel spheres were obtained. By means of in situ Fourier Transform infrared spectroscopy (FTIR), the activity of acetic, formic, benzole, and chloroacetic acids were studied for the condensation of tetraethyl orthosilicate (TEOS). Formic and acetic acid gave slower rates than benzole and chloroacetic acids. Increasing the concentration of acid and addition of extra water led to an acceleration of the reaction. The reactions were also studied as a function of temperature and pressure. Higher rates of reaction were obtained at higher temperatures and lower pressures. Results from particle formation studies indicated that by slowing the rate of reaction, precipitation and agglomeration of particles could be minimized. A submicron particle size range was obtained by depressurization of the sol-gel solution inside the reaction vessel, while the rapid expansion of supercritical solutions (RESS) process was found to yield particles in the size range of approximately 100 nm.

Full text of DOI:10.1021/jp036973d

11886
J. Phys. Chem. B 2004, 108, 11886-11892
Synthesis and Formation of Silica Aerogel Particles By a Novel Sol-Gel Route in
Supercritical Carbon Dioxide
Ruohong Sui,† Amin S. Rizkalla,*,†,‡ and Paul A. Charpentier*,†
Department of Chemical and Biochemical Engineering, Faculty of Engineering, UniVersity of Western Ontario,
London, Ontario, Canada N6A 5B9, and DiVision of Biomaterials Science, UniVersity of Western Ontario,
London, Ontario, Canada N6A 5B9
ReceiVed: October 3, 2003; In Final Form: May 30, 2004
A new method to obtain silica aerogel particles using acetic acid as the condensation agent for silicon alkoxides
in supercritical carbon dioxide (scCO2) is proposed. The objective of this study was to determine the mechanism
of silica aerogel formation in scCO2 using in situ analysis techniques. The synthesis and formation of silica
aerogel particles was carried out by a modified sol-gel route, based on the hydroxylation and condensation
of silicon alkoxides in scCO2; both submicron and micron-size aerogel spheres were obtained. By means of
in situ Fourier Transform infrared spectroscopy (FTIR), the activity of acetic, formic, benzoic, and chloroacetic
acids were studied for the condensation of tetraethyl orthosilicate (TEOS). Formic and acetic acid gave slower
rates than benzoic and chloroacetic acids. Increasing the concentration of acid and addition of extra water led
to an acceleration of the reaction. The reactions were also studied as a function of temperature and pressure.
Higher rates of reaction were obtained at higher temperatures and lower pressures. Results from particle
formation studies indicated that by slowing the rate of reaction, precipitation and agglomeration of particles
could be minimized. A submicron particle size range was obtained by depressurization of the sol-gel solution
inside the reaction vessel, while the rapid expansion of supercritical solutions (RESS) process was found to
yield particles in the size range of approximately 100 nm.
Introduction
of the material in an organic solvent;14 while in the RESS
process,15-19 product molecules are solvated in the SCF that
nucleate and form clusters when the fluid rapidly goes through
a nozzle and is depressurized.
The use of supercritical fluids (SCFs) for the processing of
materials has attracted considerable interest in recent years.1-3
Of the supercritical fluids, the most exciting potential is provided
by the environmentally friendly solvent, supercritical carbon
dioxide (scCO2). It is generating a great deal of industrial interest
for several applications, including polymer synthesis, hydroge-
nation, drycleaning of clothes, silicon chip cleaning, microelec-
tronics, polymer foam extrusion, neutraceuticals, and particle
formation.4-10 This is because scCO2 is inexpensive, of low
toxicity, nonflammable, and environmentally and chemically
benign. The solubility of various compounds in scCO2 can also
be altered dramatically by relatively small changes in total
pressure and/or temperature, hence scCO2 is a “tunable sol-
vent”.11 In addition, supercritical fluids can be easily separated
from both organic cosolvents and solid products providing a
potential clean, recyclable, and environmentally friendly tech-
nology.
Designing new and improved silica aerogel particles has many
potential applications, such as catalyst matrixes, chromatography
resins, and precursors for ceramic powders for biomaterials.20-23
An aerogel is a gel that has a lower density than the fully
condensed form of the material forming it. Aerogels are typically
produced by replacing the liquid of a gel by air or another gas
without allowing complete collapse of the structure.24 Early work
led to aerogels through the use of supercritical fluids to extract
liquid, and it led to the hypothesis that the gel structure itself
can be preserved in the supercritical drying process.25 Silica
aerogels are conventionally prepared by hydrolyzing and
condensing silicon alkoxides in water and/or organic solvents
under ambient pressure with an inorganic acid or base as
catalyst, gelating for a long time, then drying in scCO2.26,27
A
route of silica aerogel particle synthesis was previously devel-
oped using formic acid to react with tetramethyl orthosilicate
(TMOS) or tetraethyl orthosilicate (TEOS) in a supercritical
fluid. The first monolithic silica aerogel was synthesized by
employing this methodology in 1997,28 and M. Moner-Girona
et al.29 in 2003 obtained micron-size spheres and fibers in
supercritical acetone, but found it difficult to have separated
particles in scCO2 because of agglomeration. Advantages of this
route include a shorter time for gel aging, obtaining dry product
directly after CO2 venting, and the ability of the size and
morphology of the silica aerogel to tailored by modifying the
temperature, pressure, relative amounts of reactants, and su-
percritical fluid venting rate. However, the mechanism and
kinetics of the reactions are not well understood.
High diffusivity and low viscosity in supercritical fluids is
considered highly effective for producing superior products of
fine and uniform particles.12 In particle formation, SCFs offer
many opportunities to form particles with improved properties.
From a processing aspect, producing particles by SCFs can be
classified into several major processes such as GAS, RESS,
PGSS, SEDS, etc.7,13 For example, the GAS process uses the
SCF as an anti-solvent to precipitate particles from a solution
* To whom correspondence should be addressed. E-mail: pcharpentier@
eng.uwo.ca, arizkalla@eng.uwo.ca. Phone: (519) 661-3466, (519) 661-2111
x 82212. Fax: (519) 661-3498.
† Department of Chemical and Biochemical Engineering, University of
Western Ontario.
‡ Division of Biomaterials Science, University of Western Ontario.
10.1021/jp036973d CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/16/2004

Synthesis of Silica Aerogel Particles
J. Phys. Chem. B, Vol. 108, No. 32, 2004 11887
Figure 1. Schematic of experiment setup: (a) autoclave with FTIR and GC-MS, (b) view cell system, (c) particle collection device for the RESS
process. (T-1, T-3) CO2 cylinders, (T-2) liquid reactant tank, (Pump) syringe pump, (PT) pressure transducer, (TC) temperature controller.
Recent literature has shown that attenuated total reflection
Fourier transform infrared (ATR-FTIR) spectroscopy is techni-
cally feasible for in situ measurement of concentration, solubil-
ity, and supersaturation in crystallization processes for the
chemical and pharmaceutical industries.30,31 ATR spectroscopy
uses the phenomenon of total internal reflection. In this
technique, the radiation beam is directed onto an angled crystal
and reflected within the crystal until it emerges from the other
“end”, where it is collected. The depth of penetration, as the
beam of radiation penetrates a fraction of a wavelength beyond
the reflecting surface, distinctively produces (a) little to non-
disturbing interference from the solid particles already present
in the solution, or generated by crystallization, and (b) a less
intense solvent contribution to the overall infrared spectrum so
the solvent spectra can be easily subtracted from the sample
spectrum. These characteristics of small depth of penetration
with no sensitivity to solid particles constitute the technical
motivation for the employment of ATR-FTIR technology as an
online monitoring tool for supercritical crystallization.
BOC Canada and further purified by passage through columns
containing molecular sieves (Aldrich) and copper (II) oxide
supported on alumina (Aldrich) to remove excess water and
oxygen, respectively. Purified water was produced by means
of a 18.2 Mohm-cm Barnstead Easypure LFsystem.
Polymerization Apparatus. Figure 1a provides a schematic
of the experimental reactor used in this research. Carbon dioxide
and monomer were pumped using syringe pumps (Isco 260D).
All feed lines have check-valves (HIP) to prevent backflow and
rupture disks (HIP) for safety in case of overpressurization. The
reactor is a 100-mL stainless steel stirred autoclave (Parr
Instruments) with a turbine impeller to provide mixing of
ingredients. The reactor is heated by a furnace, has an installed
pressure transducer (Ashcroft K25F) and a thermowell contain-
ing a thermocouple (Parr-A472E2). Reactor temperature was
controlled by a digital controller (Parr 4842). Control of the
reactor temperature (T) and pressure (P) variation was excellent
during a given experiment run (T ) ( 0.5 °C and P ) ( 1
bar).
Combining sol-gel science with SCF technology provides
an opportunity to synthesize new and improved aerogel particles
from silicon alkoxides in scCO2. The goal of the present work
was (1) to investigate the technical potential and efficacy of
ATR-FTIR spectroscopy for the in situ monitoring of solute
concentration during the modified aerogel procedure in scCO2,
and (2) to study the particle formation by means of two
techniques of (a) destabilizing the sol-gel solution by pressure
reduction and (b) the RESS process.
Online Characterization. In situ FTIR monitoring of solution
concentration in the stirred autoclave was performed using a
high-pressure diamond immersion probe (Sentinel-ASI Applied
Systems). The probe is attached to an ATR-FTIR spectropho-
tometer (ASI Applied Systems ReactIR 4000), connected to a
microcomputer, supported by ReactIR software (ASI).
View Cell. An alternative for the autoclave is a 25-mL
stainless steel view cell with sapphire windows (Figure 1b). This
apparatus was similar in design to that reported on elsewhere.11
Polymerization Procedure. To determine the activity of the
various acids (acetic, formic, benzoic, and chloroacetic) for
catalyzing the condensation reaction, 20-mL acetone was added
to all acids to aid in dissolution (benzoic and chloroacetic acid
were not soluble in scCO2 without this cosolvent). The acid,
cosolvent, and orthosilicate monomer were added to the
autoclave before heating and addition of carbon dioxide. In the
second group of experiments, the acid (acetic or formic) was
added to the autoclave before heating and addition of carbon
dioxide to the desired pressure. Then TEOS (or TMOS) was
Materials and Methods
Materials. Reagent-grade tetraethyl orthosilicate (TEOS),
tetramethyl orthosilicate (TMOS), acetone (HPLC grade), acetic,
benzoic acid, and chloroacetic acid were obtained from the
Aldrich Chemical Co. (Ontario, Canada), and used without
further purification. Formic acid (Aldrich, reagent grade) was
further dehydrated by refluxing with phthalic anhydride (Sigma-
Aldrich, 99% reagent grade) for 6 h and then distilling.
Instrument-grade carbon dioxide (99.99%) was obtained from

11888 J. Phys. Chem. B, Vol. 108, No. 32, 2004
Sui et al.
Figure 2. ATR-FTIR plot of the progress of an aerogel reaction in supercritical carbon dioxide, where the lines are experimental data: 4.4 mmol
TEOS + 22 mmol HAc + 4.4 mmol H2O at 3000 psig and 60 °C from 1 to 7 h at 1-h intervals.
pumped in with a separate syringe pump. FTIR spectra were
obtained in situ, and GC-MS spectra were obtained by venting
sample gas through a tube that connected the autoclave and the
GC-MS. After reactions were completed, excess carboxylic acid
was washed with fresh scCO2 at a fixed venting rate (≈20 mL/
min). The results provided were the mean of three separate
experiments, and the error bars represent one standard deviation.
Particle Formation and Collection Techniques. Two
techniques for particle formation were employed. One was direct
formation of particles in the stirred reactor during depressuriza-
tion with a fixed CO2 venting rate. For the other one, the RESS
process was used (see Figure 1c), where particles were sprayed
through a variable-volume nozzle (HIP) into a closed stainless
steel chamber with heating jacket (Parr Instruments) to collect
particles sprayed at a constant depressurizing rate. The inlet of
the particle collection device was connected to the stirred reactor
and the outlet went through a filter (HIP) and was vented to a
high velocity fume hood.
Characterization Techniques. Scanning electron microscopy
(SEM) measurements were used to determine the size and
morphology of particles. These measurements were made at
either Surface Science Western on a Hitachi S-4500 FE
instrument using an accelerating voltage of 5kV, or at the UWO
Photonics and Nanotechnology Laboratory using a LEO 1530
Field Emission Scanning Electron Microscope. Solid aerogel
powders were characterized by a Bruker Vector 22 FTIR
instrument using a MIRacle Single Reflection HATR (Pike
Technologies).
correspond to the stretching frequency of the Si-O-Si bond
from the product,32,33 whose concentration increases as the
reaction proceeds. The produced silica aerogel powder was
further analyzed by a separate offline FTIR instrument, which
is designed for measuring solid materials. Figure 3 shows the
results of this analysis, which confirm the peaks at 1065, 928,
and 797 cm-1 belong to the aerogel product. The in situ FTIR
analysis showed that the product peaks were found to increase
during the course of the polymerization and then would remain
constant if the reaction approached equilibrium, or the saturated
concentration of polymer was reached. As mentioned in the
Introduction, ATR-FTIR analysis is mainly sensitive to material
dissolved in solution. This reaction was repeated in an agitated
view-cell reactor, and the solution remained clear during the
course of the condensation reaction. As the condensation product
is most likely polydisperse with a broad range of molecular
weights, it is not possible to directly determine concentration
of the molecules by FTIR, only of the actual bond concentration.
Analysis of the FTIR spectrum also shows a product peak at
3344 cm-1, which increases during the course of the reaction
from the production of the condensate molecule. The condensate
molecule is a mixture of H2O and CH3CH2OH. Both the alcohol
and water will merge into an indistinguishable broad peak in
this range.
Activity of Various Acids. To compare the relative activity
of the various acids studied in the condensation of orthosilicates,
the main absorbance from the product at 1066 cm-1 was plotted
versus reaction time (see Figure 4). The results showed that
among the four acids, benzoic and chloroacetic acid were the
most active in promoting conversion to product. However,
several problems make these acids poor candidates for the
polymerization agent. First, as we will see later, slower
condensation of orthosilicates is preferred to form discrete
particles. Second, as both of these acids are solid at room
temperature, and as they have poor solubility in scCO2, this
Results and Discussion
FTIR Analysis of Polymerization Reaction. Figure 2
provides a series of traces for a typical experiment in scCO2,
taken at fixed time intervals, from the in situ ATR-FTIR analysis
of the condensation of TEOS using acetic acid. Analysis of the
spectra shows that the absorptions at 1065, 928, and 797 cm-1

Synthesis of Silica Aerogel Particles
J. Phys. Chem. B, Vol. 108, No. 32, 2004 11889
Figure 3. FTIR plot of the silica aerogel powder, where the line is experimental data. The spectrum was taken at ambient conditions.
zation rates. The rate of condensation clearly increases with
temperature, as one would expect.35 Figure 7 shows an interest-
ing effect from experiments varying the reactor pressure.
Increasing the reactor CO2 pressure caused lower initial reaction
rates, but led to higher final concentrations. Similar results were
found when the agitation rate was tripled from 200 to 640 rpm,
indicating the reactions are not likely mass-transfer limited for
these conditions. At lower pressures, such as the case for the
experiment performed at 1300 psig, the low density of CO2 near
the critical point causes increased relative concentrations of
orthosilicate and acid in certain “clusters”. This clustering can
increase the rate of reaction.36 The higher absorbance values
observed at higher pressures may be explained by increased
solubility of the reaction products.
Experimental Phase Behavior. Using the view-cell system
Figure 4. Effect of acid type on TEOS condensation activity in
for the experimental conditions studied, it was found that TMOS,
supercritical CO2. The points are experimental data from the ATR-
FTIR absorbance at 1066 cm-1 (n ) 3) and the lines are best-fit
smoothed lines through the data points: acetic acid (s), formic acid
(9), benzoic acid (2), and chloroacetic acid (b). The experimental
conditions are 60 °C and 3000 psig, 0.0044 mol TEOS, 0.0176 mol
acid, and 0.275 mol acetone.
TEOS, acetone, acetic acid, and formic acid were miscible with
CO2. Benzoic and chloroacetic acid had poor solubility, but were
soluble in CO2 with added acetone cosolvent. When the FTIR
experimental conditions were further examined by means of the
view cell reactor, it was found that precipitates appeared
spontaneously after the silicon alkoxide and 96% formic acid
(≈4% water) were pressurized with scCO2 and heated. Figure
8 shows an example of the typical agglomerated micron size
spheres that were produced. The situation could be improved
by adding excess formic acid as cosolvent in scCO2, and by
decreasing the concentration of reactants (Figure 9), but
precipitation of the products was still inevitable. However, when
using anhydrous acetic or formic acid and small amounts of
water (water/alkoxysilane ) less than 2/1), the fluid remained
clear, indicating that the products remained in solution.
makes separation from the final product difficult. Both anhy-
drous acetic and formic acids were less active than the other
two acids. At 10 h of reaction at 60 °C and 3000 psig, the
absorbance at 1066 cm-1 was still increasing, indicating that
the equilibrium of the polymerization had not yet been reached.
Figure 5 shows that production of the condensation product
could rise remarkably by addition of (a) a small amount of water
(Alkoxysilane: H2O ) 1 ∼ 2) or (b) more acid. NMR studies
in aqueous sol-gel polymerizations show similar accelerations
with increased concentrations of water.34 However, too much
water was found to make the reaction rate too high, causing
precipitation and agglomeration of aerogel particles. Also, too
much acid is not economically feasible and causes downstream
separation problems.
Particle Formation. Particles were formed either by depres-
surizing the reactor vessel or expanding the particles, using a
modified RESS process, into a heated collection chamber. When
depressurizing the reactor vessel, particles would form at
pressures of approximately 900 ∼ 1000 psig. Results from the
view cell observation and SEM showed that separated micro-
Effect of Temperature and Pressure. Figure 6 shows that
higher temperatures resulted in significantly higher polymeri-

11890 J. Phys. Chem. B, Vol. 108, No. 32, 2004
Sui et al.
Figure 5. Effect of water and acid concentration on TEOS condensation activity in supercritical CO2. The points are experimental data from the
ATR-FTIR absorbance at 1066 cm-1 (n ) 3) and the lines are best-fit smoothed lines through the data points. 0.0088 mol TEOS + 0.0352 mol
HAc (b); 0.0088 mol TEOS + 0.0352 mol HAc + 0.0088 mol H2O (9); 0.0088 mol TEOS + 0.0704 mol HAc (2). The experimental conditions
are 50 °C and 3000 psig.
Figure 6. Effect of temperature on TEOS condensation activity in supercritical CO2. The points are experimental data from the ATR-FTIR absorbance
at 1066 cm-1 (n ) 3) and the lines are best-fit smoothed lines through the data points: 40 °C (b); 50 °C (9); 60 °C (2). The experimental
conditions are 3000 psig, 0.0088 mol TEOS, and 0.0352 mol acetic acid.
spheres were formed when the pressure dropped to about 900
∼ 1000 psig. When using the modified RESS process, the
particle size was found to decrease with increasing rate of
depressurization and the particles were significantly smaller than

Synthesis of Silica Aerogel Particles
J. Phys. Chem. B, Vol. 108, No. 32, 2004 11891
Figure 7. Effect of pressure on TEOS condensation activity in supercritical CO2. The points are experimental data from the ATR-FTIR absorbance
at 1066 cm-1 (n ) 3) and the lines are best-fit smoothed lines through the data points: 3000 psig (b); 2000 psig (9); 1300 psig (2). The experimental
conditions are 50 °C, 0.0088 mol TEOS, and 0.0352 mol acetic acid.
Figure 9. SEM of silica aerogel powder. The experimental conditions
are 0.176 mmol TEOS + 2.64 mmol 96% HCOOH in the 25-mL view
cell, at 2000 psig, 40 °C.
Figure 8. SEM of silica aerogel powder. The experimental conditions
are 1.1 mmol TEOS + 7.7 mmol 96% HCOOH in the 25-mL view
cell, at 2000 psig, 40 °C.
Conclusions
those formed from depressurizing the reactor vessel. Generally,
a higher venting rate made smaller particles with narrower
distributions and lower levels of agglomeration. The particles
gathered in the autoclave, spheres in the range between
submicron and several microns (Figure 10), were larger than
those formed in the device outside the autoclave by the RESS
process (with approximate diameters of 100 nm) (Figure 11).
Future work will be directed at a better understanding of the
kinetics of the sol-gel polymerization mechanism in scCO2,
and the mechanism of particle formation. A knowledge of these
can provide for a one-step mechanism to produce nanoceramic
particles from SCFs.
This study of the particle formation process for aerogels in
supercritical carbon dioxide demonstrated that in situ ATR-FTIR
spectroscopy is a valuable technique for studying this high-
pressure process. Both anhydrous formic and acetic acid were
found to be mild and controllable agents for sol-gel route
preparation of silica aerogel particles in scCO2. By modifying
the ratio of silicon alkoxide, acetic acid, and water, the
polymerization rate is tunable and precipitation can be prevented,
and therefore agglomeration minimized. Increasing the reaction
temperature led to higher rates of reaction and increasing the
pressure decreased the rate of reaction. Submicron particle sizes

11892 J. Phys. Chem. B, Vol. 108, No. 32, 2004
Sui et al.
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Acknowledgment. We would like to thank Dr. Xingsheng
Li for his help on GC-MS. We thank Mr. Brad Kobe and Surface
Science Western, along with Dr. Ian Mitchell and the UWO
Photonics and Nanotechnology Laboratory, for use of the
Scanning Electron Microscope. This work was financially
supported by the Canadian Natural Science and Engineering
Research Council (NSERC) and by the Materials and Manu-
facturing Ontario EMK program.