Synthesis and properties of magnesium carbonate xerogels and aerogels [612604]

Synthesis and properties of magnesium carbonate xerogels and aerogels
Tobias Kornprobst, Johann Plank ⁎
Chair for Construction Chemicals, Technische Universität München, Lichtenbergstraße 4, 85747 Garching, Germany
abstract article info
Article history:
Received 2 August 2012Received in revised form 15 October 2012Available online 24 November 2012
Keywords:Magnesium carbonate;Methoxymagnesium methyl carbonate;Xerogel;Aerogel;Supercritical dryingA magnesium carbonate sol was synthesised by controlled hydrolysis of methoxymagnesium methyl carbonate
and then converted into an alcogel. From this, magnesium carbonate xerogels and aerogels were produced.SEM imaging, XRD and IR spectroscopy revealed that both xerogel and aerogel samples are composed of amor-
phous magnesium carbonate nanoparticles. Supercritical drying of the alcogel yields an opaque aerogel with
high surface area (~400 m
2/g) and pores in the low nanometer range which makes this aerogel interesting for
e.g. insulating applications.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Aerogels are highly interesting materials which can be used in a
wide range of applications. They consist of a porous network formed
by nanoparticles which is filled with air. The nanostructure provides
aerogels with unique properties such as extremely high speci fic surface
area (up to 1000 m2/g), porosity up to 99.8 vol.%, optical transparency,
ultra low weight ( b0.01 g/cm3) and excellent heat insulation properties
(λ=0.01 –0.02 W/mK) [1]. These properties qualify aerogels for appli-
cations such as in absorbents (e.g. NASA's stardust collector [2]), for
pharmaceuticals and cosmetics, optical and acoustical devices, ion
exchange materials, semipermeable membranes and heat insulating
materials [1,3–6]. The use of aerogels in heat insulating systems for
buildings such as e.g. insulating renders and plasters or insulating
panels is of utmost interest, because more energy ef ficient buildings
can contribute signi ficantly to the reduction of CO 2emissions and the
saving of natural resources, which are among the key challenges for
the next decades. The excellent heat insulating properties of aerogels
are owed to pore sizes in the low nanometer range, which signi ficantly
reduce the number of collisions between gas molecules. Through this
mechanism which is known as the Knudsen effect, heat transfer is effec-
tively reduced [7]. An additional bene fit is the optical transparency of
many aerogels. This allows the manufacture of translucent insulation
panels which can make use of natural daylight instead of using electriclight.
Typically, aerogels are produced by drying of a wet gel (aquagel or
alcogel) which are composed of a network of nanoparticles filled with
a solvent. Conventional drying methods involving simple evaporation
of the solvent normally lead to destruction of the gel network structure,which is owed to mechanical stress resulting from capillary forces on
liquid/gas interfaces in the pores. This way, so-called xerogels are
formed [8].
In 1931, Samuel Kistler first described supercritical drying as a
method to prepare highly porous silica aerogels under preservation
of the nanoporous structure [9,10] . In a supercritical fluid, no phase
boundaries exist and hence the network structure of nanoparticles
is preserved.
Most gels obtained from sol –gel processes contain an alcohol as sol-
vent. The handling of an alcohol under supercritical conditions however
is rather inconvenient and dangerous, especially due to high critical
temperatures (240 °C for methanol) and flammability. Carbon dioxide
is a more favourable solvent for supercritical drying, because its critical
constants (31.1 °C, 73 bar) are signi ficantly lower than those of alcohols.
Additionally, it is not flammable. Thus, exchange of the alcohol in the gel
with liquid CO
2and subsequent supercritical drying, the so-called “cold
supercritical drying ”is favoured compared to “hot supercritical drying ”
with an alcohol as solvent.
Because of this rather time consuming process, some methods for
ambient drying of silica aerogels were developed [11,12] . Still, up to
date supercritical drying is the method of choice for the production
of high quality monolithic aerogels in laboratory scale.
Silica constitutes the most widely studied aerogel material which
already has found some commercial applications. Additionally, aerogelsbased on TiO
2,F e 2O3,Z r O 2,A l 2O3,C r 2O3,V2O5,M o O 2,o rN b 2O5were
synthesised utilising sol –gel processes and metal oxides as precursors
[1]. Most recently, aerogels made from carbon [13]and carbon nanotubes
[14,15] as well as organic [16]and organic –inorganic hybrid aerogels [17]
were presented.
Aerogels consisting of alkaline earth metal compounds are of spe-
cial interest for construction applications because of their high com-
patibility and inertness with cement. Additionally, their nanoporousJournal of Non-Crystalline Solids 361 (2013) 100 –105
⁎Corresponding author. Tel.: +49 89 289 13151; fax: +49 89 289 13152.
E-mail address: sekretariat@bauchemie.ch.tum.de (J. Plank).
0022-3093/$ –see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jnoncrysol.2012.10.023
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structure quali fies them well for insulating renders and wall plasters
for buildings. They potentially can replace polystyrene beads and
boards which at present are the most popular materials for this appli-
cation. However, plasters consisting of an inorganic and an organic
material present a problem for disposal later on when the building
is modi fied or torn down. In 2009, we reported on the facile synthesis
of a CaCO 3aerogel from simple raw materials (CaO, CO 2and metha-
nol) [18]. Its preparation involves a calcium methanolate solution
which is purged with CO 2gas to yield calcium di(methylcarbonate)
as intermediate compound which is then subject to controlled hydro-
lysis to produce a CaCO 3sol containing particles in the low nanometer
size range. Under speci fic conditions, the sol converts to an alcogel
which upon supercritical drying yields a CaCO 3aerogel.
A direct transfer of this process starting with magnesium oxide
proved unsuccessful. For this reason, we developed a novel route for
a magnesium carbonate alcogel which is described in the following.
From the alcogel, xerogels and aerogels were prepared and characterised
with respect to their nanostructure utilising XRD and SEM analysis as
well as BET measurement.
2. Experimental2.1. Chemicals
Chemicals (analytical grade) were purchased from Merck KGaA,
Darmstadt/Germany. Methanol was dried and stored over molecular
sieve (mesh 0.3 nm) for a minimum of 24 hours prior to use. All
other chemicals were used without further puri fication. Carbon dioxide
(99.9% purity) was purchased from Westfalengas, Münster/Germany.
Gaseous CO
2was used for the synthesis of Mg(OCOOCH 3)(OCH 3)
while liquid CO 2from a riser pipe bottle was used for supercritical dry-
ing. Deionised (DI) water obtained from a Nanopure system (Barnstead
International, Dubuque, IA/USA) was used in all experiments.
2.2. Synthesis of magnesium methanolate (Mg(OCH 3)2)
Under vigorous stirring with a magnetic stir bar, 200 mL of methanol
w a sa d d e dt o1 0 . 0g( 0 . 4 1m o l )o fm a g n e s i u mt u r n i n g sp l a c e di na1L
three-neck round bottom flask equipped with a re flux condenser. The
suspension was heated to 65 °C. Excessive gas formation was observed,
indicating the release of hydrogen. Within 1 h, the magnesium turnings
were completely dissolved. Subsequent removal of the greyish residue
(identi fied as Mg(OCH 3)2contaminated mainly with Mg(OH) 2)b y
centrifugation (15 min, 10.000 g) yielded a slightly yellowish solution.
Methanol was removed in vacuo until an amorphous and very fine
colourless powder was obtained. IR spectrum (ATR technique, cm−1,
s=strong, m=medium, w=weak, b=broad): 3200 wb (atmospheric
H2O) 2927 s, 2865 s, 2802 s (CH stretching); 2600 w (combination
band), 1640 w (H 2O), 1456 s (CH 3asymmetric deformation); 1415 sb
(CH 3symmetric deformation); 1172 w (CH 3rocking); 1097 s, 1033 s
(C―O stretching); 852 m (unassigned); 536 s, 426 s, 406 s (MgO
vibrations) [19].
2.3. Preparation of Mg(OCOOCH 3)(OCH 3) and MgCO 3gels
Magnesium methanolate obtained from the synthesis as described
above was added to 50 mL of methanol in a 200 mL three-neck round
bottom flask. The amount of Mg(OCH 3)2precursor determines the
solid content of the MgCO 3gel formed later and the kinetics of the
gel formation. The higher the MgCO 3content, the faster gelation
occurs.
For preparation of Mg(OCOOCH 3)(OCH 3), a weak carbon dioxide
flow (1 mL/s) was bubbled through the Mg(OCH 3)2suspension from
above for 30 min while stirring. Subsequently, water was added in
order to initiate the gelation process. For gels up to 5 wt.% solid content,
a 5-fold molar excess of water compared to the amount of magnesiummethanolate achieved crack-free alcogels and reasonable gelation times,
while for gels containing up to 10 wt.% solids, a 2-fold excess was used.
After completion of water addition, solutions were immediately
transferred to glass petri dishes (5.3 cm×1.4 cm), covered and stored
at room temperature. After approx. 1 h, the solutions turned into
colourless clear gels.
2.4. MgCO 3xerogel
For the preparation of xerogels, the wet alcogel samples were
stored in open petri dishes at room temperature over night and sub-
sequently dried at 80 °C until all solvent was evaporated. Glass-like,
colourless and mostly translucent xerogel pieces were obtained.
2.5. MgCO 3aerogel
The alcogel samples were placed in a 1 L autoclave (Model 4621
equipped with an oblong silicate glass window from Parr Instruments,
Moline, IL/USA) which was cooled to 5 °C. After subjecting the samples
to an additional layer of methanol (~5 mm), the autoclave was closed
andfilled with liquid carbon dioxide. Within 15 min, pressure was
built up carefully to the equilibrium pressure of 40 bar in order to
keep mechanical stress on the samples as low as possible. After equili-
bration over 6 –10 h, the methanol-enriched carbon dioxide was re-
leased slowly from the autoclave and replaced with fresh carbon
dioxide. During the exchange process, the gel samples must always be
kept under liquid CO 2to prevent mechanical stress from phase bound-
aries within the gels. The solvent exchange was repeated until no more
evaporation of methanol was detectable during the CO 2release. This
was achieved after 6 –8 exchange cycles.
For supercritical drying, the autoclave was filled with liquid CO 2
again and heated to 40 °C within 1 h. This produces a pressure of
90 bar. Such conditions are clearly above the critical point of CO 2,a s
becomes evident from the disappearance of the phase boundary visi-
ble through the glass window on the autoclave. After equilibration
over 1 h, the pressure was slowly released (1 bar/s) at constant tem-
perature (40 °C).
The resulting aerogel samples were colourless opaque and brittle
monoliths which turned into an extremely fine powder when broken
mechanically.
2.6. Characterization of MgCO 3xerogels and aerogels
Powder x-ray diffraction (XRD) measurements were carried out
on a D8 Advance diffractometer (Bruker AXS, Karlsruhe/Germany)
with Bragg –Brentano geometry, equipped with a two-dimensional
Vantec-1® detector (Bruker AXS).
Scanning electron microscopy (SEM) imaging was performed on a
XL30 ESEM FEG microscope (Philips/FEI Company, Eindhoven/Neth-
erlands) equipped with an energy dispersive x-ray detector (EDX)
for elemental analysis (New XL30, EDAX Inc., Mahwah, NJ/USA).
Speci fic surface area and pore size distribution were determined
by nitrogen adsorption (Nova 4000e, Quantachrome Instruments,
Boynton Beach, FL/USA). BET method was used to measure the specif-
ic surface area, while pore size distribution was calculated by DFT
method. Calculations were performed with NovaWin software ver-
sion 9.0 (Quantachrome). Prior to measurement, all samples were
heated for 2 h to 200 °C.
Porosity of the xerogel was determined by mercury intru-
sion porosimetry (Poremaster60, Quantachrome GmbH & Co.
KG, Odelzhausen/Germany).
Infrared spectra were recorded on a Vertex 70 FT-IR spectrometer
(Bruker Optik GmbH, Ettlingen/Germany) equipped with a diamond
ATR cell.101 T. Kornprobst, J. Plank / Journal of Non-Crystalline Solids 361 (2013) 100 –105

3. Results and discussion
3.1. Reactions leading to MgCO 3gels
The sequence of reactions leading to the formation of the MgCO 3
sol was studied by isolation and characterization of the intermediate
products and is presented in Scheme 1 .
Atfirst, magnesium was dissolved in methanol which produces
magnesium methanolate. This reaction is well-known from literature
[19,20] . Characterization of the x-ray amorphous product by infrared
spectroscopy (spectrum Fig. 1 ) and elemental analysis proved that
highly pure magnesium methanolate was obtained.
In the second reaction step, magnesium methanolate was dis-
persed in methanol while CO 2was bubbled through the suspension,
which within 5 min turned into a clear solution. After completeness
of the reaction with CO 2(30 min), the product was isolated from
the solution by removal of methanol in vacuo, yielding a colourless,
x-ray amorphous powder. Comparison with literature data [21,22]
and earlier work of the authors [18]suggested that methoxymagnesium
methyl carbonate was obtained. This was con firmed by infrared spec-
troscopy which produced the spectrum ( Fig. 2 ) with characteristic
absorption bands as follows (cm−1): 3240 wb (atmospheric H 2O),
2950 m, 2895 w, 2822 m (CH stretc hing); 2615 w (combination
band), 1633 s (COO asymmetric stretching);1450 s (CH 3asymmet-
ric deformation); 1325 s (COO s ymmetric stretching); 1192 s
(CH 3rocking); 1097 s, 1052 s (CO stretching); 935 m (not assigned);
822 s, 713 m, 626 s (COO vibrations).
Quanti fication of the mass of dried product obtained, compared to
the amount of magnesium methanolate employed proved that CO 2
was inserted into one methylate group (theoretical mass increase fac-tor 1.51, found 1.58). This result differs from calcium methanolate
which can incorporate two molecules of CO
2[18]. Additionally, a certain
amount of CO 2is bound physically to the methoxymagnesium methyl
carbonate in solution [21]. During drying in vacuo, this amount of CO 2
is visibly released from the compound. Elemental analysis of the dried
product also con firmed the composition of Mg(OCOOCH 3)(OCH 3).
Reaction (III) represents the hydrolysis of methoxymagnesium
methyl carbonate in methanol. After addition of water to the
methoxymagnesium methyl carbonate solution, formation of gas bub-
bles was observed which indicates the release of physically boundCO
2. The amount of water added and the solid content of the sol are
the two critical parameters regarding the kinetics of the sol –gel trans-
formation. If only a stoichiometric amount of water is added, the gela-
tion time is in the range of several days to weeks, depending on the
solid content. However, when too much water is added (e.g. a 10-fold
excess compared to stoichiometric amount of methoxymagnesium
methyl carbonate), gelation occurs almost instantly, but the quality of
the resulting alcogel is poor, as CO 2bubbles are embedded in the gel.
Clear, homogeneous and crack-free alcogel samples are required to
yield high quality monolithic aerogels after supercritical drying.
Generally, higher solid contents lead to faster gelation. For gels
with a solid content of up to 5 wt.%, a 5-fold molar excess, and for
gels up to 10 wt.% solid, a 2-fold excess of water was found to be
ideal. They produce alcogels with the desired gel quality at moderate
gelation times of ~1 h. Attempts to produce alcogels with solid con-
tents signi ficantly higher than 10 wt.% always resulted in gels ofpoor quality while for solid contents lower than 3 wt.%, no gel forma-
tion could be observed.
Addition of a large excess of water (e.g. 50-fold) to the
methoxymagnesium methyl carbonate solution leads to precipitation
of a colourless solid, and no gel formation occurs. Analysis of the crystal-
line precipitate by XRD and IR spectroscopy identi fied it as magnesium
carbonate.
3.2. Chemical composition of xerogels and aerogels
X-ray powder diffraction revealed an amorphous character of the
particles present in the xerogel and a erogel samples. Only very broad re-
flections which could not be assigned to a speci fic phase were detected.
Infrared spectroscopy however con firmed that the samples consist of
m a g n e s i u mc a r b o n a t e ,a si ss h o w ni n Fig. 3 . The normal vibrations
ν1–ν4of the carbonate ion were used as reference [23]. Additionally,
weak absorption bands characteristic for water were observed, most like-ly originating from atmospheric moisture. IR Data: 3292 wb, 1631 m
(H
2O); 1421 s ( ν3, CO asymmetric stretching); 1093 w, 1022 w ( ν1,C O
symmetric stretching); 856 m ( ν2,C O 3out-of-plane deformation); 650
sb (ν4, OCO in plane deformation). Elemental analysis performed by
EDX produced a magnesium content of 28.1 wt.% (theoretical MgCO 3:
28.8 wt.%).
No traces of intermediate products or impurities were found, proving
completeness of the reactions. No differences for the chemical character-
ization between samples obtained from alcogels possessing different
solid contents were found. Also, no differences between xerogel and
aerogel samples were detected.
3.3. Morphology of MgCO 3xerogel
Fig. 4 shows a photograph of a MgCO 3xerogel dried under ambi-
ent conditions. A glass-like, partially translucent and mechanically
stable solid was obtained which had shrunk to about 10 vol.% of its
initial volume.
Scheme 1. Reactions involved in the formation of a magnesium carbonate sol with magnesium methanolate and methoxymagnesium methyl carbonate as intermediate s; all reac-
tions were carried out in methanol as solvent.
Fig. 1. IR spectrum of Mg(OCH 3)2.102 T. Kornprobst, J. Plank / Journal of Non-Crystalline Solids 361 (2013) 100 –105

SEM imaging ( Fig. 5 ) revealed a porous structure made of dense,
agglomerated nanoparticles ( d=50 –100 nm).
3.4. Morphology of MgCO 3aerogel
Fig. 6 exhibits a photograph of a typical MgCO 3aerogel sample.
In all preparations, brittle, opaque monoliths were obtained which
maintained the dimensions of the preceding gel. Dimensions and stabil-
ity did not change with storage time. Opposite to this, alcogel samples
which were dried under ambient instead of supercritical conditions
showed complete collapse of the gel structure into a xerogel.
Fig. 7 shows SEM micrographs of a MgCO 3aerogel sample. There,
loosely connected, mostly spherical nanoparticles of 20 –50 nm in size
are visible. In some samples produced from gels possessing high solid
contents (~10 wt.%), also large agglomerates (diameter ~500 nm) were
found. Apart from this, no signi ficant differences between samples pos-
sessing varying solid contents were detected.
The samples exhibit poor mechanical stability, which is probably
owed to the fact that the magnesium carbonate particles present inthe gel possess no chemical groups which enable them to form cova-
lent inter-particle bonds as is the case e.g. for silanol groups in silica
aerogels.3.5. Porosity of MgCO
3xerogels and aerogels
Fig. 8 shows the microporosity of a typical magnesium carbonate
xerogel compared to that of an aerogel sample. The xerogel exhibits
a very dense packing of MgCO 3nanoparticles. No micropores are vis-
ible. Mercury intrusion porosimetry produced a value of only 2.5 vol.%
for the total porosity which re flects the dense packing observed by
SEM imaging.
Fig. 2. IR spectrum of Mg(OCOOCH 3)(OCH 3).
Fig. 3. IR spectra of MgCO 3aerogel and xerogel; ν1–ν4: normal vibrations of the
carbonate ion.
Fig. 4. Optical image of a MgCO 3xerogel sample after drying under ambient condi-
tions; grid 0.5 cm.
Fig. 5. SEM micrograph of a MgCO 3xerogel sample; magni fication: 40,000×.
Fig. 6. Optical image of a MgCO 3aerogel sample obtained after supercritical drying.103 T. Kornprobst, J. Plank / Journal of Non-Crystalline Solids 361 (2013) 100 –105

In contrast to this, the aerogel structure shows a much higher poros-
ity with pore diameters in the range of several hundred nanometers.
The differences in the microstructure explain the different appear-
ances of xerogels and aerogels. While the densely packed xerogel net-
work is translucent, the irregular pore structure of the aerogel leads
to more light scattering and thus, the macroscopic appearance is
opaque.
For three aerogel samples produced from alcogels possessing
3.5 wt.%, 6 wt.% and 10 wt.% solid contents, the total porosities were cal-
culated by comparing their bulk densities ( Table 1 ) with their skeletal
density (density of magnesium carbonate, 2.96 g/cm). Using this method,values of >97 vol.% for all samples were obtained. They con firm the re-
sults obtained from the SEM images.
Additionally, speci fic surface areas of the aerogel samples were
determined using nitrogen adsorption. All samples show a high spe-
cific surface area (~100 –400 m2/g), con firming the nanoscale struc-
ture ( Table 1 ).
The theoretical particle sizes calculated from the speci fics u r f a c ea r e a
are 15 nm for sample 1 and 5 –6 nm for samples 2 and 3, respectively.
This is less than the particle sizes observed on the SEM micrographs. It
suggests the presence of substructures or surface roughness which are
not resolved by the SEM.
For sample 2, nanoporosity was studied. Nanoscale pore size dis-
tribution was calculated from nitrogen adsorption data by DFT calcu-
lation. The result is shown in Fig. 9 .
The graph exhibits a maximum at a pore size of 12 nm, with a first
shoulder at a pore size around 5 nm. This is in the range of pore sizes
where the Knudsen effect plays a signi ficant role.
4. Conclusion
Magnesium carbonate alcogels can be prepared utilising a simple
route which involves magnesium metal, methanol and CO 2.F i r s t ,m a g –
nesium methanolate is synthesised as a precursor. By reaction with CO 2
in methanol, methoxymagnesium methyl carbonate is formed. Con-trolled hydrolysis of this solution yields a MgCO
3sol. The sol transforms
into an alcogel, whereby the solid content can be varied between 3 and
10 wt.%.
Ambient drying of the alcogel yields a xerogel. MgCO 3aerogels were
produced by subjecting the alcogel to solvent exchange with liquid CO 2
and subsequent supercritical drying. Both xerogel and aerogel consist of
a network made of amorphous magnesium carbonate nanoparticles.
While the nanostructures of xerogel and aerogel are similar, the
aerogel exhibits much higher microporosity than the xerogel (total po-
rosity aerogel ~99 vol.%, xerogel 2.5 vol.%).
Fig. 7. SEM micrographs of a MgCO 3aerogel sample exhibiting a nanoporous structure;
large image: magni fication 40,000×, insert: 80,000×.
Fig. 8. Comparison of the microporosity of magnesium carbonate xerogel (a) and
aerogel (b) by SEM imaging; magni fication 5,000×.Table 1
Bulk densities, speci fic surface areas (BET, N 2) and total porosities of MgCO 3aerogel
samples made from alcogels possessing 3.5 wt.%, 6 wt.% and 10 wt.% solid contents.
Sample Solid content of gel
precursor [wt.%]Bulk density
[g/cm3]Speci fic surface
area [m2/g]Total porosity
(calculated) [vol.%]
1 3.5 0.024 134 99.2
2 6.0 0.041 417 98.63 10.0 0.079 362 97.3
Fig. 9. Pore size distribution of a MgCO 3aerogel sample produced from an alcogel with
6 wt.% solid content.104 T. Kornprobst, J. Plank / Journal of Non-Crystalline Solids 361 (2013) 100 –105

Speci fic surface areas up to ~400 m2/g and nanopores in the range of
10–15 nm render magnesium carbonate aerogels interesting for a vari-
ety of applications such as thermal insulation in building materials.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.jnoncrysol.2012.10.023 .
References
[1] N. Hüsing, U. Schubert, Angew. Chem. Int. Edit. 37 (1998) 23 –45.
[2] P. Tsou, J. Non-Cryst. Solids 186 (1995) 415 –427.
[3] A.S. Dorcheh, M.H. Abbasi, J. Mater. Process. Technol. 199 (2008) 10 –26.
[4] J. Fricke, A. Emmerling, J. Am. Ceram. Soc. 75 (1992) 2027 –2036.
[5] M. Schmidt, F. Schwertfeger, J. Non-Cryst. Solids 225 (1998) 364 –368.
[6] R. Baetens, B.P. Jelle, A. Gustavsen, Energy Build. 43 (2011) 761 –769.
[7] K. Raed, U. Gross, Int. J. Thermophys. 30 (2009) 1343 –1356.
[8] C.J. Brinker, R. Sehgal, S.L. Hietala, R. Deshpande, D.M. Smith, D. Loy, C.S. Ashley,
J. Membr. Sci. 94 (1994) 85 –102.
[9] S.S. Kistler, Nature 127 (1931) 741.[10] S.S. Kistler, J. Phys. Chem. 36 (1932) 52 –64.
[11] M.A. Einarsrud, L.E. Farbrodt, S. Haereid, in: first ed., L.L. Hench, J.K. West (Eds.),
Chemical Processing of Advanced MaterialsWiley, Chichester, 1992, pp. 355 –361.
[12] D.M. Smith, R. Deshpande, C.J. Brinker, in: first ed., K. Ishizaki, L. Sheppard, S.
Okada, T. Hamasaki, B. Huybrechts (Eds.), Porous Materials, Volume 31, American
Ceramic Society, Westerville, 1993, pp. 71 –80.
[13] J. Wang, L. Angnes, H. Tobias, R.A. Roesner, K.C. Hong, R.S. Glass, F.M. Kong, R.W.
Pekala, Anal. Chem. 65 (1993) 2300 –2303.
[14] A.E. Aliev, J. Oh, M.E. Kozlov, A.A. Kuznetsov, S. Fang, A.F. Fonseca, R. Ovalle, M.D.
Lima, M.H. Haque, Y.N. Gartstein, M. Zhang, A.A. Zakhidov, R.H. Baughman,
Science 323 (2009) 1575 –1578.
[15] J. Zou, J. Liu, A.S. Karakoti, A. Kumar, D. Joung, Q. Li, S.I. Khondaker, S. Seal, L. Zhai,
ACS Nano 4 (2010) 7293 –7302.
[16] R.W. Pekala, F.M. Kong, J. Physiol. Paris 50 (1989) C433 –C440.
[17] U. Schubert, N. Husing, A. Lorenz, Chem. Mater. 7 (1995) 2010 –2027.
[18] J. Plank, H. Hoffmann, J. Schoellkopf, W. Seidl, I. Zeitler, Z. Zhang, Res. Lett. Mater.
Sci. (2009) 3 p. (Article ID 138476), http://dx.doi.org/10.1155/2009/138476 .
[19] H.D. Lutz, Z. Anorg. Allg. Chem. 353 (1967) 207 –215.
[20] H. Thoms, M. Epple, H. Viebrock, A. Reller, J. Mater. Chem. 5 (1995) 589 –594.
[21] H.L. Finkbeiner, M. Stiles, J. Am. Chem. Soc. 85 (1963) 616 –622.
[22] A. Adam, V. Cirpus, Z. Anorg. Allg. Chem. 620 (1994) 1702 –1706.
[23] F.A. Andersen, L. Brecevic, Acta Chem. Scand. 45 (1991) 1018 –1024.105 T. Kornprobst, J. Plank / Journal of Non-Crystalline Solids 361 (2013) 100 –105