Journal of Porous Materials

https://doi.org/10.1007/s10934-018-0674-4

One-step eco-friendly fabrication of classically monolithic silica

aerogels via water solvent system and ambient pressure drying
Lin Yan1,2 · Hongbo Ren2 · Jiayi Zhu2 · Yutie Bi2 · Lin Zhang3

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Abstract
We developed an eco-friendly method to facilely prepare the good-formability methyltrimethoxysilane (MTMS)-based
silica aerogels via water solvent system and ambient pressure drying in one-step. In the whole procedure, water was used
not only as a reaction agent but also as a solvent, avoiding the expensive cost and environmental pollution caused by solvent
exchange. The structural properties of MTMS-based silica aerogels were characterized by the scanning electron microscopy,
transmission electron microscopy and Brunauer–Emmett–Teller methods. The results indicated that compared with other
silica aerogels prepared by traditional water solvent methods, this MTMS-based silica aerogels had the classic nanostructure
with the high specific surface area of 450 m2/g as well as hydrophobic property.

Keywords  Silica aerogels · Water solvent · Ambient pressure drying · Classic nanostructure

1 Introduction is often used [14–17], because the shrinkage of the porous

structure could be effectively prevented during the drying
The ­SiO2 aerogel is a classical type of nanoporous materials process. However, due to high equipment requirement, high
with many advantages, such as low density, high porosity, energy consumption and high operating risk, there comes
high specific surface area, low thermal conductivity, low a significant production cost by the supercritical drying
optical refractive index and low acoustic velocity, resulting method. Meanwhile, it is also difficult to achieve large-scale
in great potentials in many fields [1–13]. In the prepara- industrial production. In contrast, the ambient pressure dry-
tion of ­SiO2 aerogels, the supercritical drying technology ing with its advantages of simple operation, high safety and
large-scale application, attracts the widespread concern
[18, 19]. The conventional ambient pressure drying method
Electronic supplementary material  The online version of this needs tedious steps, such as solvent replacement and hydro-
article (https​://doi.org/10.1007/s1093​4-018-0674-4) contains phobic modification, which leads to the increase of cost
supplementary material, which is available to authorized users. and environmental pollution by the use of a large amount
* Hongbo Ren of organic solvents and modifiers. Thus, it is desirable to
Renhb@swust.edu.cn design a facile and eco-friendly method to accomplish the
* Lin Zhang good-formability silica aerogels by ambient pressure drying.
zhlmy@sina.com It is known that during the ambient pressure drying,
a large amount of organic solvents and modifiers are
1
State Key Laboratory of Environment‑friendly Energy used and this causes high cost and environmental pol-
Materials, School of Material Science and Engineering,
Southwest University of Science and Technology, lution [20]. Thus, the researches on the fabrication of
Mianyang 621010, People’s Republic of China ­S iO 2 aerogels by using water as the solvent are attrac-
2
State Key Laboratory of Environment‑friendly Energy tive and significant. However, to the best of our knowl-
Materials, School of Science, Southwest University edge, there still lacked the systemic study on the eco-
of Science and Technology, Mianyang 621010, friendly method by using water as the solvent. Although
People’s Republic of China some methods had been used in a few reports, the results
3
Research Center of Laser Fusion, China Academy were unsatisfactory. Kanamori et al. used the urea and
of Engineering Physics, Mianyang 621900, acetic acid as catalysts, combing with the surfactants
People’s Republic of China

13
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 Journal of Porous Materials

of cetyltrimethylammonium bromide (CTAB) and structure and surface wettability, were also discussed in
poly(ethylene oxide)-blockpoly(propyleneoxide)-block- details.
poly(ethylene oxide) triblockcopolymer (F-127) [21, 22],
to fabricate silica aerogels followed by the supercritical
drying or atmospheric pressure drying. However, the 2 Experimental
methylnonafluorobutylether, which was an expensive sol-
vent, was used to act as a solvent exchanger in the atmos- 2.1 Reagents and chemicals
pheric pressure drying, limiting its use in large-scale
industrial applications. Besides, Cheng et  al. reported Methyltrimethoxysilane (MTMS) and cetyltrimethylam-
that a complete water-solvent method was used to prepare monium bromide (CTAB, purity ≥ 99.0%) were supplied by
silica aerogels [23]. Although this method was simple and Tianjin Kermel Chemical Reagent Company. The ammo-
green, the skeletons and pores of the obtained silica aero- nium hydroxide ­(NH3·H2O, purity ≥ 99.5%) was supplied
gels with good-formability were on the order of microns, by Tianjin Kermel Chemical Reagent Company. All these
which caused low specific surface area and did not fit the reagents were used as received and all solutions were pre-
definition of a “classic aerogel”, which should have gel- pared with deionized water (< 18 M Ω, produced by UP
like structures normally with nanoscale coherent skeletons instruments).
and pores. Besides, in other similar reports, the skeletons
of nano-sized silica aerogels were always broken after 2.2 Synthesis of MTMS‑based silica aerogels
atmospheric pressure drying. Even so, according to the
above reports, it was known that the methyltrimethoxysi- The preparation of MTMS-based silica aerogels included the
lane (MTMS) molecule was a better silicon source rather sol–gel process and ambient pressure drying of hydrogels.
than tetraethoxysilicate (TEOS), because it had hydro- The MTMS, a commercially available silicon source, was
phobic methyl groups and did not require hydrophobic used as the precursor. At first, the 0.125 g CTAB was dis-
modification before ambient pressure drying. Besides, it solved into 25 mL ­H2O and stirred for 5 min, and then 10 mL
was also found that one-step preparation method of ­SiO2 MTMS (MTMS:H2O:CTAB molar ratio = 1:24.3:0.005) was
aerogels could avoid the use of a lot of solvent exchanger dropped into the above mixed solution. After the MTMS
resulting in the high cost and environmental pollution. In was added, the mixture was stirred for 30 min for suffi-
those methods, it was known that surfactants were the key cient hydrolysis. Finally, 1 mL N ­ H3·H2O aqueous solution
point. Surfactants could act as binders to avoid phase sepa- (1 mol/L) was used as the gelation agent. After the suffi-
ration between MTMS clusters and water. The type and cient aging, the obtained gel was soaked in distilled water
amount of surfactants had the significant impact on the at 60  °C for 12  h to remove the residual surfactant and
pore size of the aerogels. However, most of these resulting chemicals. After that, the gel was ambient pressure dried
aerogels either had the main pores larger than microns at at 60, 80, 100, 110 and 120 °C each for 2 h, respectively,
the expense of specific surface area, or had three-dimen- and the obtained MTMS-based silica aerogel was defined
sional nanoporous structure but brittle. In a word, all of as the sample A. Meanwhile, the sample B was obtained by
them could not be termed into “classical aerogels”, which the similar procedure with 35 mL water and 0.05 g CTAB
should have gel-like structures normally with nanoscale (MTMS:H2O:CTAB molar ratio = 1:27.9:0.002).
coherent skeletons and pores. Thus, it is still challeng-
ing to facilely achieve eco-friendly fabrication of classi- 2.3 Characterization
cally monolithic silica aerogels with nanoscale coherent
skeletons and pores via water solvent system and ambient Unless stated otherwise, all the tests were carried out in trip-
pressure drying. licate and the average values were given as the results. The
Herein, the MTMS was used as the silicon source and bulk density of the aerogels was measured by weighing an
the hydrophobic methyl groups in MTMS molecules could aerogel after it was shaped into a regular shape. The shrink-
achieve in situ hydrophobic modification. The water was age during drying was calculated from the diameters of the
used as the reaction agent and the solvent. The CTAB was wet gels and the corresponding aerogels. The surface mor-
added and formed micelles in solution, linking the MTMS to phology and microstructure of the aerogels were character-
the lipophilic segment and combining the MTMS with water ized by field emission scanning electron microscopy (SEM,
together. After that, classically monolithic silica aerogels SIRION 200, FEI) and transmission electron microscopy
with nanoscale coherent skeletons and pores were fabricated (TEM, JEOL JEM-2010). Water contact angles (WCAs)
via water solvent system and ambient pressure drying. The on the surface of the aerogel were measured at an ambi-
effects of the CTAB amount and the type of catalysts on ent temperature on a contact angle/interface system. The
the micelle number and aerogel parameters, such as porous specific surface area and average pore size were calculated

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Journal of Porous Materials

using the Brunauer–Emmett–Teller (BET) method and the due to the large amount of CTAB, and so maybe the higher
pore size distribution (PSD). Thermal stability analysis was capillary pressure gave it a slightly larger shrinkage. Mean-
conducted by using thermogravimetry analyzer coupled dif- while, sample B had a water contact angle of 160° (Fig. 2b),
ferential scanning calorimeter (TG-DSC, SDT-600) under a because a small amount of CTAB contained in sample B was
heating rate of 10 °C/min under an air flow. completely removed during the washing process, resulting
in almost no residual hydrophilic groups on the surface of
sample B. Therefore, sample B had a higher water contact
3 Results and discussion angle respected to sample A.
Moreover, Fig.  3 showed the SEM and TEM images
As shown in Fig. 1a, a good-formability MTMS-based silica of sample A and B, respectively. Figure 3a, c showed that
aerogel (sample A) was obtained by one-step eco-friendly sample A had a typical silica cluster texture with three-
method via water solvent system and ambient pressure dry- dimensional cobweb-like networks with numerous voids.
ing. The aerogel surface was smooth without broken, and For sample A, its pore size as well as the skeleton size were
as seen from Fig. 2, the water contact angle on the surface both on the nanometer scale, which would be further dem-
of sample A was 120.1°, showing its native hydrophobicity onstrated by the subsequent nitrogen adsorption/desorption
resulted by in situ hydrophobic modification from hydropho- measurements. Besides, due to the reduction of the CTAB
bic methyl groups. Due to the hydrophobicity, the shrink- amount, sample B had the different structure with sample A.
age of sample A was 35% and its density was 220 mg/cm3. As shown in Fig. 3b, d, the skeleton and pore size of sample
To verify the effect of the CTAB amount on the morphol- B were on the order of microns, but they were still less than
ogy and microstructure of the silica aerogels, the sample those of previously reported silica aerogels (~ hundreds of
B was prepared and its surface began to become slightly microns) [23, 24]. Compared with the sample A, the local
rough (Fig. 1b) and the depression on the surface appeared. aggregation in sample B occurred and its pore size was not as
In fact, sample B had a smaller shrinkage of 16% relative uniform as that of sample A. Moreover, nitrogen adsorption/
to sample A, which was attributed that sample B was more desorption measurements were employed to characterize the
like a foam than aerogel and had the better structure stabil- specific surface area and pore structure of the MTMS-based
ity when water molecules were separated from the skeleton. silica aerogels (Fig. 4). The specific surface areas of aerogels
In sample A, the skeleton of the aerogel was more compact were calculated by using the BET method (Fig. 4a, c). All
the aerogels showed the typical IV isotherm according to
the IUPAC classification [25, 26], which is characteristic
of the mesoporous structure. The desorption cycles of the
isotherms all showed a hysteresis loop, which was generally
attributed to the capillary condensation that occurred in the
mesopores. As shown in Table 1, sample A had the higher
specific surface area and smaller pore size than sample B,
and it had a specific surface area as high as 450 m2/g and
a pore volume of 1.61 cm3/g. Figure 4b, d plotted the pore
size distributions derived from the BJH method. Within the
detectable range of pore sizes (1.78–69.84 nm), there was
a broad distribution and the greatest volume fraction was
Fig. 1  The photo images of as-prepared MTMS-based silica aerogels attributed to pores of 2.59 nm in diameter for sample A. All
with good-formability. a sample A and b sample B the above analysis well revealed the nanoscale pore structure

Fig. 2  The contact angle value

of a sample A and b sample B

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 Journal of Porous Materials

Fig. 3  SEM and TEM images of

a, c sample A and b, d sample
B, respectively

of sample A and was consistent with SEM and TEM images. in methylgroups [24]. Besides, the absorption bands at
Meanwhile, due to the reduction of the CTAB amount, the 790 cm−1 and 1030 cm−1 were corresponded to the symmet-
sample B had a specific surface area of 149 m2/g and the ric and antisymmetric Si–O–Si stretching vibrations, respec-
greatest volume fraction was attributed to pores of 2.52 nm tively, and the absorption band at 1148 cm−1 was ascribed to
in diameter, which were caused by its stronger skeleton and the bending vibration of Si–CH3 groups.
bigger pore size on the order of microns. Therefore, it could Furthermore, according to the above analysis, it was
be demonstrated that, to the best of our knowledge, com- seen that the structure of the MTMS-based silica aerogels
pared with these traditional preparation methods (Table S1), largely depended on the CTAB amount. CTAB is a cationic
this was the first report that the classically monolithic silica surfactant which is stable in acid solution. When the sur-
aerogel with both nanoscale coherent skeletons and pores face adsorption is saturated, the surfactant molecules can
was achieved via eco-friendly water solvent system and not continue enrichment on the surface. Nevertheless, the
ambient pressure drying in one-step. hydrophobic groups of surfactant molecules are still try-
Figure 5 showed the TG-DSC of sample A and sample ing to make the surfactant molecules escape from the water
B, respectively. In Fig. 5a, for sample A, a small weight environment. Thus, the surfactant molecules become self-
loss occurred at 200–250 °C due to the removal of surface- polycondensate to the micelles, i.e., the hydrophobic groups
adsorbed and the lattice water. As seen from the weight form an inner core together and the hydrophilic groups
loss curve, the CTAB in the sample had been substantially contact water outward. In present work, the CTAB added
washed. The main weight loss during 450–550 °C was con- amount for MTMS-based silica aerogels was 17 times of its
sidered as the oxidation of Si–CH3 groups, which was con- critical micelle concentration (CMC). As shown in Fig. 7,
sistent with previous reports [23]. Sample B had a similar due to high CTAB molecule accumulation, the MTMS mol-
TG curve (Fig. 5b), which had two endothermic peaks on the ecule was surrounded by the hydrophobic groups in CTAB,
DSC curve because CTAB might remain in the large pores. while the hydrophilic groups in CTAB attached to water
In order to investigate chemical compositions of aero- molecules. The micelle forming way determined the skel-
gels, FTIR analyses of sample A and sample B were shown eton structure of the aerogels. After the addition of aqueous
in Fig. 6. It was seen that the two samples had the similar ammonia, the micelles were opened to release MTMS mol-
FTIR spectrum. The absorption bands at 2983 cm−1 and ecules, which rapidly reacted with water under the catalysis
1278 cm−1 could be assigned to the absorption of C-H bond of hydroxyl ion to form a gel. Thus, it could be demonstrated

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Journal of Porous Materials

Fig. 4  N2 adsorption/desorption isotherms and pore size distributions of a, b sample A and c, d sample B, respectively

Table 1  BET data and Sample ID SaBET ­(m2/g) VbT ­(cm3/g) DcA (nm) Shrinkage (%) Bulk den-
physiochemical characterization sity (mg/
of silica aerogels cm3)

Sample A 450 1.61 4.56 35 ± 1 220 ± 3

Sample B 149 1.07 16.50 16 ± 1 130 ± 4
a
 BET surface area
b
 Total pore volume
c
 BJH desorption major pore width

that the more CTAB added, the more micelles formed, and gel process was extremely rapid and the gel network rapidly
the smaller coherent skeletons and pores of aerogels formed. formed as a whole. In the process of using ammonia as the
Moreover, it was found that the CTAB amount could not catalyst, the higher ammonia added amount could make the
increase indefinitely because the superfine skeleton would gel process faster and form a stronger solid network struc-
be collapsed during the ambient pressure drying. Thus, to ture, which could also offset the frangibility caused by high
obtain well-formed aerogels at high CTAB added amount, CTAB added amount to a certain extent. Therefore, it was
an one-step catalytic procedure by ammonia was used and seen that compared with the previous reports, the precise
after adding ammonia directly into the solution, the whole control of CTAB amountand and one-step base catalysis

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 Journal of Porous Materials

Fig. 5  TG-DSC results of a sample A and b sample B, respectively

OH- OH-
OH-

N+
OH-
N+ N+ OH-

N+ N+
OH-

N+ Si N+

OH- OH-

N+
N+

OH- N+
N+
N+ OH-

OH-
OH-

-H2O
Fig. 6  FTIR spectra of sample A and sample B, respectively
-R

-OCH 3
were the key point to achieve the eco-friendly fabrication of
classically monolithic silica aerogels with both nanoscale -CH3

coherent skeletons and pores via water solvent system and

ambient pressure drying. Fig. 7  Schematic illustration of MTMS molecule dispersing state in
aqueous CTAB solution

4 Conclusion eco-friendly water solvent system and ambient pressure dry-

ing in one-step. In the whole procedure, MTMS was mainly
We developed an eco-friendly method to facilely prepare used as a silicon source, water was mainly used as a solvent,
the good-formability MTMS-based silica aerogels via water and CTAB served as a binder to prevent the macroscopic
solvent system and ambient pressure drying in one-step. The phase separation between MTMS and water. Besides, CTAB
MTMS-based silica aerogels with both nanoscale coherent was dispersed to form micelles in solution, and the skel-
skeletons and pores had bulk density (~ 220 mg/cm3), mod- eton and microstructure of the aerogels were controlled by
erate shrinkage, high specific surface area (~ 450 m2/g) and micelles. It was revealed that the precise control of CTAB
native hydrophobicity (~ 120.1°). This was the first report amount and one-step base catalysis were the key point to
that the classically monolithic silica aerogel with both achieve classically monolithic silica aerogels with both
nanoscale coherent skeletons and pores was achieved via nanoscale coherent skeletons and pores. In a word, the

13
Journal of Porous Materials

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