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Research Letters in Materials Science
Volume 2009, Article ID 138476, 3pages
doi:10.1155/2009/138476
Research Letter
Preparation and Characterization of
a Calcium Carbonate Aerogel
Johann Plank,1Heinz Hoffmann,2Joachim Sch¨
olkopf,3Wolfgang Seidl,1
Ingo Zeitler,2and Zheng Zhang3
1Chair for Construction Chemicals, Technische Universit¨
at M¨
unchen, Lichtenbergstraße 4, 85747 Garching bei M¨
unchen, Germany
2Bayreuth Center for Colloids and Interfaces, Universit¨
at Bayreuth, Universit¨
atsstraße 30, 95440 Bayreuth, Germany
3Omya Development AG, Baslerstraße 42, P.O. Box 335, 4665 Oftringen, Switzerland
Correspondence should be addressed to Johann Plank, sekretariat@bauchemie.ch.tum.de
Received 22 October 2008; Accepted 2 April 2009
Recommended by Manish U. Chhowalla
We report on a facile method for the preparation of a calcium carbonate aerogel consisting of aggregated secondary vaterite
particles with an approximate average diameter of 50nm. It was synthesized via a sol-gel process by reacting calcium oxide with
carbon dioxide in methanol and subsequent supercritical drying of the alcogel with carbon dioxide. The resulting monolith was
opaque, brittle and had overall dimensions of 6 ×2×1cm. It was characterized by X-ray powder diffraction, nitrogen adsorption
method (BET), and scanning electron microscopy.
Copyright © 2009 Johann Plank et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1. Introduction
The preparation of an aerogel was first described by Kistler
in the 1930s. He synthesized a silica-based aerogel via a
wet chemical sol-gel process and subsequent supercritical
solvent extraction [1,2]. The aerogel showed unique material
properties such as high optical transparency or opacity, heat
insulation capability, and high absorption capacity [3,4].
These properties led to the use of silica aerogels in ion
exchange materials, adsorbents, semipermeable membranes,
pharmaceuticals, cosmetics, optical and acoustical devices,
coating materials, and heat insulation applications for build-
ings [5–7]. Since then, various types of aerogels based on
carbon [8], alumina [9,10], transition metal oxides [11], or
main-group metal oxides [12] were synthesized.
In 2007, Horga et al. reported on the preparation of
a calcium carbonate aerogel for the first time [13]. Their
process involves the ionotropic gelling of alginate in a
calcium salt solution, followed by an exchange of the solvent
water against ethanol, and supercritical drying of the calcium
alginate alcogel. Calcination of the calcium alginate aerogel
finally leads to a calcium carbonate aerogel. Obviously, this
process requires many consecutive steps. In this communi-
cation we report on a more facile way to prepare a calcium
carbonate aerogel resulting from the condensation of vaterite
nanoparticles. These were prepared by controlled hydrolysis
of calciumdi(methylcarbonate) which was obtained by the
reaction of carbon dioxide with calcium oxide in absolute
methanol. The resulting CaCO3alcogel was subjected to
supercritical drying with CO2to produce the CaCO3aerogel.
2. Experimental Section
2.1. Preparation of CaCO3Sol. Buzagh’s method was used
to prepare the CaCO3sol [14]. Thus, 54.0 g (0.963 mol)
calcium oxide (calcined for 24 h at 950◦C) were suspended in
800 mL absolute methanol (dried and stored over molecular
sieve, 3 nm) and heated to 40◦C. After 90-minute stirring,
carbon dioxide was bubbled through the reaction vessel for
one hour at a flow rate of 1 L/min. The carbon dioxide was
fed into the reaction vessel through a metal tube close to a
stirrer (1000 rpm) to ensure a fast reaction in the methanolic
suspension. When all the calcium oxide was dissolved and
converted into a sol, the flow of carbon dioxide was stopped.
The resulting sol contained nanoparticles with particle sizes
ranging from 5 to 20 nm (dynamic light scattering mea-
surements, Zetasizer Nano-ZS, Malvern Instruments Ltd.).
2Research Letters in Materials Science
CaO + 2CH3OH
CaO + 2H
Ca(OH)2+2CH
3OH
Ca(OCH3)2+2CO
2
Ca(OCOOCH3)2+H
2O
Ca(OCH3)2+H
2O
Ca(OH)2
Ca(OCH3)2+2H
2O
Ca(OCOOCH3)2
CaCO3+ 2CH3OH + CO2
Sol/gel
(Ia)
(Ib)
(Ic)
(II)
(III)
2O
Scheme 1: Reaction steps involved in the formation of the CaCO3
sol from calcium oxide and gaseous carbon dioxide in methanol.
706050403020
0
2-θ[◦]
Relative intensity
Figure 1: X-ray powder diffraction pattern of the CaCO3aerogel.
Vertical dotted lines correspond to JCPDS entry 33-0268 (vaterite).
The particle size distribution curve showed a maximum at
approximately 11 nm.
2.2. Sol-Gel Conversion. The sol turns into a translucent
alcogel when stored for a short time (<1 hour). The gelation
time depends on the amount of water present in the reaction
vessel and the temperature. When more water was present
during the synthesis (e.g., because of not using absolute
methanol), gelation already took place in the reaction vessel.
Higher temperature also leads to faster sol-gel conversion.
The alcogels obtained were stable for days when stored under
refrigeration. In air and at room temperature, they dried
to an opaque powder which, when being freshly prepared,
consisted of pure vaterite particles.
2.3. Preparation of CaCO3Aerogel. A part of the calcium
carbonate alcogel was subjected to supercritical drying with
carbon dioxide. The supercritical drying was carried out in a
1 L pressure autoclave (Parr Company, Germany). Approx-
imately 18 g of alcogel were placed in the autoclave, and
absolute methanol was added until the gel was completely
immersed in the solvent. The autoclave was then sealed,
pressurized slowly with carbon dioxide to 6.0 MPa and
brought to a temperature of 281 K. When equilibrium was
achieved between the methanol in the gel and the carbon
dioxide surrounding the gel, the pressure was reduced to
4.0 MPa and then repressurized with carbon dioxide to
6.0 MPa. This procedure was repeated several times until
the methanol was completely removed from the system.
Subsequently, the autoclave was heated to 316K which is
above the critical temperature of carbon dioxide and kept
there for at least one hour. After slow depressurization to
atmospheric pressure, an opaque and brittle aerogel with
Acc.V
20.00 kV
Spot
3.0
Magn
50000×
Det
GSE WD
7.9 1.0 mBar 500 nm
200 nm
Figure 2: SEM pictures of the CaCO3aerogel; magnifications:
50,000×(large picture) and 100,000×(insert).
Aggregation Aggregation
Secondary particles:
spherical or fibre like
aggregates
< 10 nm
Primary
CaCO
particles
3
~ 50 nm
Mesopores
~ 50 nm
Tertiary st ructure:
mesopores and
macropores
Macropores
0.1–100 μm
Figure 3: Schematic drawing of the reaction steps involved in the
formation of the CaCO3gel.
dimensions of 6 ×2×1 cm was obtained. The size of the
aerogel was limited by the dimensions of the autoclave.
3. Results and Discussion
The sequence of reactions involved in the formation of the
CaCO3alcogel which is the precursor for the preparation of
the CaCO3aerogel is shown in Scheme 1.Theaerogelwas
obtained by displacing methanol present in the alcogel with
CO2.
The phase composition of the aerogel was determined
by X-ray powder diffraction (Figure 1). The XRD pattern
observed was identical with vaterite, a polymorph of calcium
carbonate which is metastable at ambient temperature. The
relatively large half width and the low intensity of the Bragg
reflections indicate small particle sizes and only a moderate
crystallinity.
Aspecificsurfacearea(BET)of45m
2/g was found for the
calcium carbonate aerogel by nitrogen absorption. This value
corresponds to an average particle diameter of approximately
50 nm, provided the particles in the aerogel are discrete,
monosized, and spherical.
In Figure 2, SEM pictures of the aerogel are shown.
The secondary vaterite particles in the aerogel exhibit a
spherical and/or fibre-like shape with an average diameter
of approximately 50nm. This value corresponds quite well
Research Letters in Materials Science 3
with the diameter calculated from the nitrogen adsorption
measurement. As can be seen in the SEM pictures, the
individual particles are not strongly connected with each
other. In fact, they are merely aggregated, which explains
the brittle character of the calcium carbonate aerogel when
mechanical stress is applied.
The formation of the calcium carbonate aerogel occurs
in a three-step process which is illustrated in Figure 3. First,
calcium di(methylcarbonate) is hydrolyzed by water to form
an intermediate sol containing primary CaCO3nanoparticles
showing a size of approximately 5 to 20 nm. Existence of
these primary particles was also confirmed by TEM pictures
(not shown here). In a second step, the primary particles
grow to spherical or fibre-like secondary particles which were
observed under the SEM. In a third step, these secondary
particles finally aggregate to the gel.
Because water is the starting reagent for seed formation
in this system, the amount of water present during the
reaction greatly influences the number, morphology, and size
of the primary and secondary particles and therefore also the
bulk properties of the aerogel.
4. Conclusion
Through the simple synthesis described here, calcium
carbonate aerogels are readily available from inexpensive
starting materials. Aerogels with different surface areas,
specific densities, and pore sizes are accessible. Our process
allows to produce monoliths with a volume of 20–30cm3.
Potential applications include heat insulating materials and
fillers for plastics.
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