The upper temperature limit in cooperative assembly of ordered mesoporous materials

Article abstract of DOI:10.1246/cl.2003.660

Several examples for high temperature (95°C, higher than the cloud point of pure surfactant) synthesis of highly ordered hexagonal mesoporous silica structures by employing triblock copolymers such as P65 (EO ₂₀PO₃₀EO₂₀) a

Full text of DOI:10.1246/cl.2003.660

660
Chemistry Letters Vol.32, No.7 (2003)
The Upper Temperature Limit in Cooperative Assembly of Ordered Mesoporous Materials
Minjia Yuan, Jiawei Tang, Chengzhong Yu,ꢀ Yinghua Chen, Bo Tu,ꢀ and Dongyuan Zhao
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai
200433, P. R. China
(Received April 15, 2003; CL-030325)
Several examples for high temperature (95 ꢁC, higher than
the cloud point of pure surfactant) synthesis of highly ordered
hexagonal mesoporous silica structures by employing triblock
copolymers such as P65(EO 20PO30EO20) as structure-directing
agents have been presented. Such understanding may increase
the range of surfactants and the temperature regime under which
ordered mesoporous materials can be cooperatively assembled.
c
Recently, considerable research efforts have been made in
the field of mesoporous materials because of their emerging
uses in catalysis, adsorption, sensors, and separations.1{3 For
different applications, the fabrication of mesoporous materials
b
a
0
1
2
3
4
5
with controllable pore size, porosity, structure together with ad-
justable inorganic wall compositions is important. Recent ad-
vances in the formation mechanism of mesoporous materials re-
veal that such mechanism depends much on the surfactants
employed and the experimental conditions of synthesis.4 It is
noted that in the big family of surfactants, only a few have been
used as templates to synthesize highly ordered mesoporous ma-
terials. Recently, we proposed that the critical micelle concen-
tration (CMC) of surfactants may be used approximately to pre-
dict the quality of cooperatively assembled mesostructures:5;6
surfactants with low CMC values may result in high quality
mesoporous materials; therefore, the use of ‘‘salting-out’’ inor-
ganic salts7 may lead to highly ordered mesoporous materials.
Although increasing temperature may dramatically de-
crease the CMC especially for nonionic block copolymer
surfactants,8 the system may finally reach the cloud point
(CP) of surfactants, decreasing the association ability of surfac-
tant molecules. The potential of cooperative self-assembly of
mesostructures at high temperature is not extensively examined.
Kipkemboi et al. proposed that when the reaction temperature is
close to or reaches the surfactant CP, the silica mesostructure
formed was of bad quality.9 Synthesis of ordered mesoporous
materials above this limit (>CP) has not been studied.
Here we report the synthesis of highly ordered mesoporous
silica structures at very high temperature (95 ꢁC) by employing
triblock copolymer P65[EO 20PO30EO20, BASF, CP = 82 ꢁC in
1 wt% solution. EO is poly(ethylene oxide) and PO is poly(pro-
pylene oxide)] as a structure-directing agent. It is revealed that
high concentration of Hþ and a small amount of ethanol re-
leased by the hydrolysis of tetraethyloxysilane (TEOS) during
synthesis may greatly increase the CP of surfactant
(>100 ꢁC), thus the CP of surfactant in aqueous solutions can-
not be regarded as an upper limit for the synthesis of mesostruc-
tures at high temperatures. Moreover, our results suggest that
the cooperative assembly of highly ordered mesoporous materi-
als may also occur at very high temperatures, which may be
useful to synthesize new mesostructured materials whenever a
high temperature sol-gel process is available.
2 theta/degree
Figure 1. XRD patterns of calcined mesoporous silica
prepared by using P65template at different reaction tem-
peratures: a, b and c for temperature 55 ꢁC, 75 ꢁC and
95 ꢁC, respectively.
0.5
700
0.4
0.3
600
0.2
500
400
300
200
100
0.1
0.0
2
4
6
8
10
Pore size (nm)
a
b
c
0.0
0.2
0.4
0.6
0.8
1.0
p/p0
Figure 2. Nitrogen adsorption-desorption isotherm
plots and pore size distribution curves (inset) of cal-
cined mesoporous silica prepared by using P65tem-
plate at different reaction temperatures. a, b and c for
temperature 55 ꢁC, 75 ꢁC and 95 ꢁC, respectively.
In a typical preparation, 1 g of P65was dissolved in 30 g of
2 M HCl at desired temperature (55–95 ꢁC). To this solution,
2.08 g of TEOS was added with vigorous stirring for 24 h
(TEOS:P65:HCl:H2O=1:0.0003:6:166, molar ratio). The mix-
ture was aged at 100 ꢁC overnight without stirring. The solid
product was filtered and air-dried at RT. Calcination was carried
out at 550 ꢁC in air for 5h.
The powder X-ray diffraction (XRD) patterns of calcined
mesoporous silica prepared in the presence of P65at different
temperature (55–95ꢁC) are shown in Figure 1. With increasing
temperature, the first diffraction peak becomes much stronger
and much narrower; moreover, at the highest temperature em-
ployed (95 ꢁC), two additional peaks are observed, suggesting
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.7 (2003)
661
successful synthesis of highly ordered mesoporous silica struc-
tures at high temperature (ꢂ95 ꢁC) may increase the range of
surfactants and the temperature regime under which ordered
mesoporous materials can be cooperatively assembled. While
the use of inorganic salts may decrease the synthesis tempera-
ture at the low end,7 the use of strong acidic condition and a cer-
tain amount of organic solvents may increase the temperature at
the other end. Such understanding may be useful to fabricate
new mesostructured materials once a high temperature sol-gel
process is employed and combined with the self-assembly po-
tential of surfactants.
Figure 3. TEM images of calcined hexagonal mesoporous
silica synthesized by using P65as a template at 95 ꢁC along
(a) [100] direction and (b) [001] direction.
Similar results have also been obtained for P85
(EO26PO39EO26) and B40-2500 [EO17BO14EO17, where BO re-
presents poly(butylene oxide)]. Highly ordered hexagonal
mesoporous silica materials are obtained at high temperatures
(>90 ꢁC) by utilizing these templates (see Table 1 for the CP
of surfactants, synthesis temperature and the physicochemical
properties of the resultant calcined silica structures.)
that the mesoscale order is much improved with increasing tem-
perature. The silica material synthesized by P65at 95 ꢁC can be
attributed to a two-dimensional hexagonal structure (p6mm)
ꢀ
with a cell parameter of 86.4 and 77.7 A for as-synthesized
and calcined materials, respectively. Representative N2 adsorp-
tion-desorption isotherms and the corresponding pore size dis-
tribution curves (analyzed by using BdB model)10 of calcined
mesoporous materials synthesized at different temperatures
are shown in Figure 2. All calcined materials yield type IV iso-
therms with type-H1 hysteresis loops. From the pore size distri-
bution curves, it is calculated that the full wave at half maxi-
Table 1. The CP of block copolymers, synthesis temperature
and the physicochemical properties of the resultant calcined sil-
ica structures
Block
copolymers
CPa CPb
/ꢁC /ꢁC /ꢁC /cm3gꢄ1 /m2gꢄ1 /nm
683 4.7
EO26PO39EO26 ꢂ85 >95 951.09 87 5 6.8
4.8
T
V
S
D
EO20PO30EO20 ꢂ82 >95 950.92
ꢀ
mum (FWHM) is 9.5, 8.9, and 6.5 A for materials synthesized
at temperature 55, 75, and 95 ꢁC, respectively, indicating that
the pore size distributions become more uniform at higher tem-
peratures, in good accordance with the XRD results. The pore
EO17BO14EO17 ꢂ80 >95 950.95 787
aIn 1 wt% aqueous solution. bIn a system similar to our synthe-
sis conditions; T: reaction temperature; V: pore volume; S: sur-
face area; D: pore diameter.
ꢀ
size increases a little at high temperature (37.4, 43.2, 47.3 A
for temperature 55, 75, and 95 ꢁC, respectively).11 The trans-
mission electron micrograph (TEM) results confirm that meso-
porous materials prepared by P65template at 95 ꢁC are highly
ordered hexagonal structures (Figure 3).
The Support by the National Science Foundation of China
(29925309 and 20233030), State Key Research Program
(G2000048001 and 2002AA321010), Shanghai Science Com-
mittee (0212nm043) and Shanghai Education Committee
(02SG01) is greatly acknowledged.
The above results reveal that the quality of mesoporous ma-
terials templated by P65at high temperature is much better than
that at low temperature. This can be explained by the tempera-
ture-CMC-mesostructure correlation as we proposed before.7
However, the structure-directing agent P65utilized in our syn-
thesis has a CP of 82 ꢁC in 1 wt% water solution.12 Obviously,
the fact that highly ordered mesoporous materials can be
synthesized by using P65templates at 95 ꢁC suggests that the
CP of surfactant in water cannot be simply regarded as an upper
limit in the synthesis of mesoporous materials.9 Cooperative as-
sembly of silica mesostructures involves a complex system. In
our synthesis, strong acidic conditions and the ethanol released
by the hydrolysis of TEOS are found to greatly increase the CP
of P65. At the same concentration of 3 wt% (the reactant ratio in
synthesis), the CP of P65is found to be ꢂ65 and ꢂ85 ꢁC in
water and 2 M HCl solutions (The measurement of CP is ac-
cording to reference13), respectively, suggesting that the strong
protonation of EO groups may greatly decrease the ability of
liquid-liquid phase separation at high temperatures. Moreover,
after adding 1.84 g (0.04 mol, similar to the amount of ethanol
released by TEOS in synthesis) ethanol into 30 g of 3 wt%
P65water solution, the CP of P65in the final solution is found
to be ꢂ75 ꢁC. Finally, the CP of P65in a system similar to our
synthesis condition is found to be larger than 95 ꢁC (ꢃ30 ꢁC
higher than normal CP in water solutions) because of the com-
bination of the Hþ and ethanol effect. More importantly, the
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Published on the web (Advance View) June 27, 2003; DOI 10.1246/cl.2003.660