# Preparation and Characterization of a Calcium Carbonate Aerogel

Authors: Johann Plank; Heinz Hoffmann; Joachim Schölkopf; Wolfgang Seidl; Ingo Zeitler; Zheng Zhang
Journal: Research Letters in Materials Science
 (2009)

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## Abstract

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 50 nm. 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 of6×2×1 cm. It was characterized by X-ray powder diffraction, nitrogen adsorption method (BET), and scanning electron microscopy.


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## Body

## 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
buildings [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 communication 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 CaCO3 alcogel
was subjected to supercritical drying with CO2 to produce the CaCO3 aerogel.


## 2. Experimental Section

### 2.1. Preparation of CaCO3 Sol

Buzagh’s method was used to
prepare the CaCO3 sol [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 measurements, Zetasizer Nano-ZS, Malvern
Instruments Ltd.). 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 CaCO3 Aerogel

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).
Approximately 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 316 K 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 dimensions of6×2×1 cm was obtained. The size of the aerogel
was limited by the dimensions of the autoclave.


## 2.1. Preparation of CaCO3 Sol

Buzagh’s method was used to
prepare the CaCO3 sol [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 measurements, Zetasizer Nano-ZS, Malvern
Instruments Ltd.). 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 CaCO3 Aerogel

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).
Approximately 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 316 K 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 dimensions of6×2×1 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 CaCO3 alcogel which is the
precursor for the preparation of the CaCO3 aerogel is shown in
Scheme 1. The aerogel was obtained by displacing methanol present in the
alcogel with CO2.Scheme 1
Reaction steps involved in the
formation of the CaCO3 sol from calcium oxide and gaseous carbon
dioxide in methanol.The phase composition of the
aerogel was determined by X-ray powder diffraction (Figure1). 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.Figure 1
X-ray powder diffraction pattern of
the CaCO3 aerogel. Vertical dotted lines correspond to JCPDS entry
33-0268 (vaterite).A specific surface area
(BET) of 45m2/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 Figure2, 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 50 nm.
This value corresponds quite well 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.Figure 2
SEM pictures of the CaCO3 aerogel; magnifications: 50,000× (large picture) and 100,000× (insert).The formation of the calcium
carbonate aerogel occurs in a three-step process which is illustrated in Figure3. First, calcium di(methylcarbonate) is hydrolyzed by water to form an
intermediate sol containing primary CaCO3 nanoparticles 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.Figure 3
Schematic drawing of the reaction
steps involved in the formation of the CaCO3 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 20–30 cm3. Potential applications include
heat insulating materials and fillers for plastics.



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*Source: 138476-2009-05-03.xml*