Preparation of mesoporous materials: approaching to
industrial viability
Santamaría. E, Maestro.A , Porras.M , Gutiérrez, J.M and González. C*.
Department of Chemical Engineering ,Faculty of Chemistry, University of Barcelona,
Barcelona 08028, Martí i Franqués 1-11, Spain
*e-mail: carme.gonzalez@ub.edu
1. INTRODUCTION
Industrial viability of the synthesis has been discussed during the last decade by
different authors. There are two critical points during the synthesis that led to the
authors to think that the process is not industrially viable. The first one is the
precursor used, usually TEOS or tetramethylorthosilicate (TMOS) as a silica source
that as other authors have reported [1-3] is quite expensive and therefore its use
represents a material cost disadvantage for producing mesostructured materials.
Another flaw for TEOS in front of sodium silicate is the production of ethanol as a
reaction sub product. Efforts have been devoted to reduce the cost in last decade by
using inexpensive sodium silicate as a silica source to replace TEOS.
Other critical point concerns to the surfactant lost during the synthesis. An alternative
proposed by other authors is the surfactant extraction by a dissolvent instead of the
calcination supplying a less destructive method [4-7]. The efficiency of the surfactant
extraction process depends basically on the strength of the interaction between
organic molecules and the material matrix. Moreover, surfactant recovery is not
complete and several extraction steps are needed increasing the extractor dissolvent
consumption and the energy used. These factors might trigger that the extraction will
not be a viable alternative for the calcination. Some authors have successfully
removed the surfactant used for the mesostructured silica synthesis by an extraction
with a HCl:Ethanol solution at 70ºC during 30 h. In some cases they need multiples
steps for the total template removal [4,8]. For the surfactant removal from the
mesoporous materials other strategies have been used as the extraction by a
supercritical fluid [6,9-10], photocalcination by vacuum [11], oxidation by H2O2 o
UV-H2O2 [12-13], with an extraction by methanol-improved with supercritical CO2
[14] or by ultrasounds [15].
Our approach consists in synthesize mesostructured materials by the CSA route using
sodium silicate solution as a precursor and recover the surfactant for its posterior
reuse by extraction from the material and evaporation of solvents: water, HCl, and
ethanol, in a rotatory evaporator. Mesoporous materials are synthesized again using
the recovered surfactant and properties of these materials are studied and compared
with that of materials obtained with fresh commercial surfactant, such specific area,
mesopore diameter, structure of the mesopores, etc.
Another significant improvement of the synthesis methods used until now is the
possibility to use ion exchange resins as a proton source in order to eliminate the Clfreed in the media as a consequence of the acid use to reach pH 2. Our approach is
based on the CSA method where sodium silicate and P84 cooperate to form the
mesostructure based on an electrically S0I0 assembly pathway, where S0 is a nonionic
surfactant (P84) and I0 represents the electrically neutral silicic acid. Silicic acid was
formed from sodium silicate through the use of an ion exchange resin, which was
reported before by Alexander et al. [16] who obtained monosilicic acid at pH 3 from
sodium metasilicate. Other studies shown that the use of ion exchange resins is an
easy method to produce colloidal silica with an homogeneous nucleation rate. [17-18].
However, the use of ion exchange resin has not been reported for the formation of
ordered mesoporous materials. The great advantage of the S0I0 pathway [19-20] is that
it relies on H-bonding between So and I0 and circumvents charge matching
constraints. Consequently the pathway affords more fully cross-linked framework
structures in comparison to mesoestructures formed through electrostatic S+I- and
S0(H+X-) pathways [21]. In CSA method, an acid is added to the solution, usually HCl
in order to reach pH≤2 [22-23] to improve the condensation reaction, because in the
range of pH 2-4 the silica polimerization is very slow [24]. The Na+ cations from the
sodium silicate remain free in the media so they must be removed washing the
material with ethanol: HCl mixtures. By means of an ion exchange resin the use of
HCl during the synthesis is not required because the resin is previously charged with
H+ and these cations are changed with the Na+ cations freed by the sodium silicate.
And excess of resin is used in order to ensure that all the Na+ are removed from the
media. Therefore the resulting material does not need to be washed.
The chance of using ion exchange resins to produce mesostructured silica opens the
door to a new industrial fabrication process which seems more viable than the
proposed synthesis proposed until now.
Meso-macroporous materials were successfully obtained with a mesostructured pore
arrangement even using 75 % of dispersed phase.
2. METHODS.
Mesoporous silica preparation. First of all, 60 g of water and 3 g of surfactant were
mixed and kept under stirring at 50ºC to ease the melt of the surfactant until a clear
solution was obtained. Then 8.8 g of sodium silicate solution (Na2O ∼10.6% and SiO2
∼26.5%) was added drop by drop. To this reaction mixture 17.7 g of HCl(c) was
quickly added with vigorous magnetic stirring as reported Stucky et al. [25]. The
resulting mixture was placed in an oven at 100 ºC during 24 h in order to allow the
silicate condensation producing a white precipitate that corresponds to the ordered
mesoporous material. The solid product was filtered off and dried at room
temperature. The product was then slurried in ethanol:HCl (1M) 1:1 mixture (50 g),
filtered off, dried up and calcined at 550 ºC during 5 h in order to eliminate the
residual surfactant. The final look of the material was a white colored powder.
Surfactant recovery. During the synthesis the material was filtered twice. In the first
one, water and remaining surfactant were separated from the mesoporous material. In
the second one, the ethanol:HCl mixture 1:1 serves to wash the material and extract
some of the surfactant used. In both, the filtrate liquids were introduced in a rotatory
evaporator in order to recover the ethanol in a first stage. Ethanol is separated from
the water:surfactant mixture. While the evaporation was going on the water starts to
evaporate and the residue remaining corresponds to the surfactant.
Material synthesized by the ion exchange resin. First of all, water and surfactant were
mixed and kept under stirring at 50ºC to ease the melt of the surfactant until a clear
solution was obtained. Then 100 g of an ion exchange resin were added to the water
and surfactant solution. Later on, a sodium silicate solution (Na2O ∼10.6% and SiO2
∼26.5%) was added drop by drop. The resulting slurry was stirred, so the Na+ of the
sodium silicate solution substituted the H+ on the exchange sites of cation resin. The
100 g of resin provided an excess of exchange sites in order to assure the complete
removing of Na+ from the solution. The pH was measured and it was stable after 15
min approximately, indicating that the ion-exchange was complete. The slurry was
filtered in order to separate the ion exchange resin from the liquid. The filtered liquid,
which did not have condensated silica yet, was placed in an oven at 100 ºC during 24
h in order to allow the silicate condensation producing a white precipitate that
corresponds to the ordered mesoporous material.
3. RESULTS AND DISCUSSION.
A series of experiments have been carried out (chart 1) in order to study the influence
of the use of recovered surfactant on the specific surface of the obtained materials.
The materials synthesized with recovered surfactants were obtained maintaining the
water/surfactant/sodium silicate solution ratio used for the fresh materials as can be
observed in chart 1.
Water
Surfactant
(g)
(g)
P84
60
3.0
P84_rec
30
P84_res
30
Experiment
Sodium
Surfactant
HCl (c)
SBET
(g)
(m2/g)
8.8
17.7
600
7.3
2.23
1.5
4.4
8.8
635
5.9
-
0.5
2
-
485
5.4
silicate
(g)
φ (nm)
recovery
(g)
Chart 1. List of the carried out experiments: Specific surface area (SBET), and pore diameter (φ) for the
experiments with fresh surfactant and recovered surfactant.
Recovered surfactants were analyzed in order to determine the purity of the
surfactant. Chart 2 shows the elemental analysis for the surfactants. The Cl- content
was 2.09 %. The relationship between C/O and C/H are very similar giving us an idea
that the chemical structure of the surfactant has been maintained. Mainly the
recovered surfactant has the same properties than the fresh one, and the specific
surface and properties seem very similar for all the experiments (chart 1)
Experiment
C (%)
H (%)
O (%)
Cl (%)
% total
COratio
C/H ratio
P84
59.15
11.18
30.06
-
100.39
1.97
5.29
P84_rec
53.78
9.53
28.12
2.09
93.52
1.91
5.64
Chart 2. Elemental analysis results for commercial P84 and the recovered one.
TEM images (figure 1) show the presence of ordered mesopores in all the cases,
either for the fresh materials or the recovered ones and for the material synthesized
with the ion exchange resin. The fact that using recovered surfactant could obtain
materials with the same structure of the materials produced by commercial surfactants
is a goal for the possibility to approach to an industrial process. In figure 1 SAXS
patterns for the obtained materials are shown. Figure 1.a shows the SAXS patter for
the fresh material and figure 4.1b for the recovered surfactant synthesized material.
For all the experiments the SAXS patterns confirm the observations from TEM
images, as the all the materials present an ordered mesopore arrangement. In all cases
SAXS peaks clearly showed three peaks indexed as [100], [110], and [200] which can
be associated with well-ordered two-dimension (2D) hexagonal mesostructure, in
accordance with the original reports [26-28]. The use of ion exchange resin provides a
well structured material in its mesostructure.
Figure 1. SAXS pattern (left) and TEM images (right) for materials synthesized with (a) commercial
P48, (b) recovered P84 and (c) ion exchange resin.
The ordered meso-macroporous materials have been obtained through a co-templated
approach combining a cooperative templating mechanism [29-30]. In this mechanism
(figure 2) the emulsion is formed thanks to placing the surfactant in the interphase of
the drops (figure 2a), and the exceeding surfactant forms spherical or cylindrical
micelles. The silica precursor interacts with these isolated micelles and leads to the
formation of an organic-inorganic mesophase (figure 2b). The condensation of the
inorganic precursor takes place in the external surface of the micelles (figure 2c). The
ordered mesophase is got after the intermicellar condensation. Finally the assembly
and the polymerization of the source of silica are completed with a hydrothermal
treatment at high temperatures. The material is washed and calcined in order to
eliminate oil and surfactant that could remain in the material. The drops of the
emulsion origin the macropores due the co-templated mechanism while the micelles
of the exceeding surfactant form an ordered net of mesopores in cooperation with
precursor (cooperative templating mechanism) as it is shown in the figures 2d, 2e and
2f.
Figure 2. Co-templating approach combining a cooperative templating mechanism. (a) Emulsion
formation (b) Self-assembly of the free molecules of surfactant (c) Polymerization of the silica
precursor in the hydrophilic region (d) Elimination of the disperse phase and the surfactant template (e)
SEM images of the macropores of the material (f) Ordered net of mesopores that links the net of
macropores.
A series of experiments was carried out (chart 3) in order to study the influence of the
different composition variables on the specific surface of the obtained materials, as
well as its influence on the pore diameter and their pore volume.
Experiment
Water/g
P84
/g
HCl
/g
Decane
/g
Dispersed
phase
SBET
/m2/g
φ /nm
Vp /
cm3/g
1
Sodium
silicate
/g
3
P84_Emulsion1
20
6
7,5
0.20
412
4.14
0.63
P84_Emulsion2
P84_Emulsion3
20
1
3
6
20
0.40
395
4.17
0.57
20
1
3
6
30
0.50
330
4.19
0.51
P84_Emulsion4
20
1
3
6
40
0.57
214
5.59
0.42
P84_Emulsion5
20
1
3
6
90
0.75
154
8.47
0.30
Chart 3. Relation of the carried out experiments: Specific surface area (SBET), pore diameter (φ) and
pore volume (Vp) as a function of the oil fraction (weight), surfactant and sodium silicate
concentrations.
SAXS was used to determine the type of structure of the obtained material. The
intensity of the obtained peaks is low due to the fact that the materials possess a large
quantity of macropores. In figure 3 SAXS patterns for three samples with different
content in dispersed phase are shown. In the first one, with 0.20 of dispersed phase
fraction, three peaks are detected on the SAXS pattern. The three reflections at q
ratios 1:√3:2, show a possible hexagonal symmetry. According to the Bragg’s law, the
unit cell dimension (a0=2d100/√3), which corresponds to the sum of the pore diameter
and the thickness of the pore wall, could be calculated and its value is 9.2 nm.
For a 0.50 dispersed phase fraction the first peak found corresponds to 12.0 nm cell
dimension. This result confirms the data reported by Du et al. [31] and Blin et al. [32],
concluding that the more disperse phase used, the bigger the cell dimension of the
obtained material.
Figure 3. SAXS pattern of obtained material with (a) 0.20 oil fraction (b) 0.50 oil fraction and (c) 0.75
oil fraction.
Figure 4 shows several representative scanning electron micrographs (SEM) of the
synthesized silica. It can be observed how the macropores density increases when
increasing the dispersed phase fraction. When more macropores exist, the real
thickness of the material is smaller and, therefore, the SAXS peaks present less
intensity. These materials present porosity at nanometric and micrometric scale.
Figure 4. SEM images (left) and TEM images (right) for materials synthesized with a fraction oil of (a)
0.20 (b) 0.40 (c) 0.50 (d) 0.57 and (e) 0.75.
In the sample with a 0.75 of disperse phase (figure 4 (left) e) it can be observed the
typical image of macroporous material, where the macropores take up a large part of
the material volume and are separated by a thin layer of material. The macropores
have the shape of the drops in highly concentrated emulsions. TEM images (figure 4)
show how the meso-macroporous materials are clearly ordered in its mesostructure,
with well oriented channels. It can be observed in the figure 4a the characteristic
honey-comb arrangement. From these observations it can be concluded that the walls
surrounding the macropores have a structured net of mesopores. In the images 4b-e it
is shown that some polyhedron that correspond to macropores of the material (more
than 50 nm), some of them can be appreciated through TEM, surrounded by thin
walls of meso sized pores. The bigger macropores have been observed through SEM.
When increasing the dispersed phase fraction from 0.20 to 0.75 it is noticeable a
decrease of the specific area from 412 to 154 m2/g (chart 3). This has been observed
by other authors [32] and could be explained by the fact that when increasing the
quantity of oil in the emulsion the interphase oil-water increases too, and the
surfactant locates preferably in this interphase stabilizing the emulsion. This causes a
decrease of free surfactant in the aqueous continuous phase where mesostructured
material is formed. Therefore the more disperse phase, the less available surfactant in
the medium to form mesostructure, and less density of ordered mesopores, deriving
into a smaller specific area, because the SBET only considers the contribution of the
meso sized pores. The silica tends to polymerize and accumulate in the interphases,
where do not present mesoporous structure, so as more disperse phase less structure
and consequently, less specific surface.
4. CONCLUSIONS
The use of ion exchange resin and the possibility to recover the surfactants used open
a door to industrial availability of the process. This novel approach may allow the
obtaining of materials in an industrial way making them less expensive than now.
Meso-macroporous materials were successfully obtained with an ordered pore
structure in its mesophase even using highly concentrated emulsions. So, these
materials provides both benefits, a mesostructured net of pores surrounded by a
structure of macroporores which can ease the molecule diffusion to the mesopore
network.
5. REFERENCES
[1] D. Pan, L. Tan, K. Qian, L. Zhou, Y. Fan, C. Yu, X. Bao, Materials Letters 64 (2010) 1543–1545
[2] J. Kim, G. D. Stucky, Chem. Commun (2000) 1159–1160
[3] E. Santamaria, M. Cortes, A. Maestro, M. Porras, J.M. Gutierrez, C. Gonzalez, Chem Lett, 41
(2012) 1041-1043
[4] S. Hitz, R. Prins, J. Catal. 168 (1997) 194–206
[5] T. Linssen, K. Cassiers, P. Cool, E.F. Vansant, Adv. Colloid Interface Sci. 103 (2003) 121–147
[6] W.A. Gomes Jr., L.A.M. Cardoso, A.R.E. Gonzaga, L.G. Aguiar, H.M.C. Andrade, Mater. Chem.
Phys. 93 (2005) 133–137
[7] M. Kruk, M. Jaroniec, C.H. Ko, R. Ryoo, Chem. Mater. 12 (2000) 1961–1968
[8] C.Y. Chen, H.-X. Li, M.E. Davis, Micropor. Mater. 2 (1993) 17–26
[9] Kawi, M.W. Lai, Chem. Commun. (1998) 1407–1408
[10] Z. Huang, L. Huang, S.C. Shen, C.C. Poh, K. Hidajat, S. Kawi, S.C. Ng, Micropor. Mesopor.
Mater. 80 (2005) 157–163
[11] A. Hozumi, H. Sugimura, K. Hiraku, T. Kameyama, O. Takai, Chem. Mater. 12 (2000) 3842–
3847
[12] J. Kecht, T. Bein, Micropor. Mesopor. Mater. 116 (2008) 123–130
[13] L. Xiao, J. Li, H. Jin, R. Xu, Micropor. Mesopor. Mater. 96 (2006) 413–418.
[14] Z. Huang, L. Xu, J.-H. Li, S. Kawi, A.H. Goh, Sep. Purif. Technol. 77 (2011) 112–119.
[15] Shaghayegh Jabariyan, Mohammad A. Zanjanchi, Ultrasonics Sonochemistry 19 (2012) 1087–
1093
[16] G.B Alexander, J. Am. Chem. Soc, (1953) 2887–2888
[17] A. Yoshida , The colloidal chemical of silica, advance in chemistry series 234, oxford university
press, oxford, (1994), pp 51-62
[18] M. Tsai, Materials Science and Engineering B106 (2004) 52-55
[19] P. T. Tanev, T. J. Pinnavaia, Chem. Mater, 8 (1996) 2068–2079
[20] T. R. Pauly, T. J. Pinnavaia,, Chem. Mater, 13 (2001) 987–993
[21] Q. S. Hue, D. I. Margolese, U. Ciesla, P. Y. Feng, T. E. Gier, P. Sieger, R. Leon, P. M. Petroff, F.
Schuth, G. D. Stucky, Nature 368 (1994) 317–321
[22] D. Y. Zhao, J.L. Feng, Q.S. Huo, N. Melosh, G.H. Fredrickson, B.F, Chmelka,, G.D. Stucky,
Science 279 (1998) 548-552
[23] Zhao,D.Y, Huo,Q.S, Feng,J.L, Chmelka,B.F, Stucky,G.D, J. Am. Chem. Soc. 120 (1998) 60246036
[24] V. N. Romannikov,
, A. N. Shmakov, M. E. Malyshev. A N. Vodennikov, V. B.
Fenelonov, Russian Chemical Bulletin, International Edition, 57 (2008) 29-35
[25] J. Kim, G. D. Stucky, Chem. Commun (2000) 1159–1160
[26] C.Z. Yu, J. Fan, B.Z. Tian, D.Y. Zhao, G.D. Stucky, Adv. Mater. 14 (2002) 1742
[27] H.F. Yang, Q.H. Shi, B.Z. Tian, S.H. Xie, F.Q. Zhang, Y. Yan, B. Tu, D.Y. Zhao, Chem. Mater.
15 (2003) 536
[28] D.Y. Zhao, P.D. Yang, B.F. Chmelka, G.D. Stucky, Chem. Mater. 11 (1999) 1174
[29] A. Firouzi, D.Kumar, L.M. Bull, T.Beiser, P. Sieger, Q. Huo, S.S. Walker, J.A. Zasadzinski, C.
Glinka, G. D. Stucky, Science 267, (1995), 1138-1143
[30] Y.S. Lee, D. Sujardi, J.F. Rathman, Langmuir 12, (1996), 6202-6010
[31] N. Du, M.J. Stébé, R. Bleta, J.L. Blin. Colloids and Surfaces A: Physicochem. Eng. Aspects 357,
(2010), 116-127
[32] J.L. Blin, R.Bleta, J.Ghanbaja, M.J.Stébé. Microporous and mesoporous materials 94, (2006), 7480