Synthesis and characterization of mesoporous silica AMS-10 with bicontinuous cubic Pn3m symmetry

Article abstract of DOI:10.1002/anie.200504114

(Figure Presented) Woven into place: By precisely controlling the neutralization degree of the anionic surfactant template, mesoporous silica with different structures was prepared, such as AMS-10. Detailed characterizations of AMS-10 show that it is a novel bicontinuous cubic Pn3m mesophase. The mesostructure is composed of an interwoven enantiomeric pair of 3D networks (see picture).

Full text of DOI:10.1002/anie.200504114

Angewandte
Chemie
Mesoporous Materials
conditions and careful structural studies are required to form
mesoporous crystals with the Pn3m space group.
¯
DOI: 10.1002/anie.200504114
Recently, we reported the preparation of ordered meso-
porous structures using anionic surfactants and co-structure-
directing agents (CSDAs).[17] The introduction of CSDAs into
the reaction system makes it feasible to produce electrostatic
interactions between anionic surfactants and inorganic spe-
cies and to control the packing parameter. These are key
factors in the formation of highly ordered mesophases and to
select a certain structure. Various mesophases, including
lamellar, 2D hexagonal p6mm, tetragonal P42/mnm, 3D
hexagonal P63/mmc, cubic Pm3n, cubic Fd3m, cubic Ia3d,
modulated structure, and a chiral mesostructure with helical
arrangement of the pores, were successfully prepared based
on this synthesis route,[4,9,17–19] though some of them have not
been observed even in liquid-crystal systems.
Synthesis and Characterization of Mesoporous
¯
Silica AMS-10 with Bicontinuous Cubic Pn3m
Symmetry**
Chuanbo Gao, Yasuhiro Sakamoto,*
Kazutami Sakamoto, Osamu Terasaki, and Shunai Che*
¯
¯
¯
It is well known that various mesoporous materials can be
synthesized based on the self-assembly of surfactant and
inorganic precursors. Mesoporous materials have attracted a
great deal of attention because of their controllable structures
and compositions, which make them suitable for wide
applications in catalysis, environmental clean-up, and in the
design of advanced materials. Since the first reports of
mesoporous materials (FSM-16[1] and M41S family[2]) by
Yanagisawa et al. and Mobil Research, respectively, a variety
of other phases have been reported. Until now, many well-
ordered mesoporous materials have been successfully synthe-
sized and it has been claimed that these have different
mesostructures such as orthorhombic (Pmmm[3] etc.), tetrag-
onal (P42/mnm,[4] P4/mmm[3]), three-dimensional (3D) hex-
Herein, we report a new mesoporous structure, AMS-10
(anionic surfactant templated mesoporous silica 10), which
¯
exhibits bicontinuous double diamond cubic Pn3m symmetry,
prepared with anionic surfactant N-myristoyl-l-glutamic acid
(C14GluA) as template and N-trimethoxylsilylpropyl-N,N,N-
trimethylammonium chloride (TMAPS) as CSDA. The
packing of the micelle was controlled by simply adjusting
the neutralization degree of the C14GluA surfactant.
Different mesophases ranging from tetragonal P42/mnm
¯
(cage type, AMS-9), cubic Fd3m (cage type, AMS-8), to 2D
[5]
[6,7]
[8,9]
¯
¯
agonal (P63/mmc ), micellar cubic (Pm3n,
Fd3m,
hexagonal p6mm (cylindrical, AMS-3), and an unknown
mesophase (denoted as AMS-10) were obtained by decreas-
ing the amount of NaOH that was added into the reaction
system (Figure 1). The neutralization degree of the surfactant
increased with the amount of NaOH that was added into the
system. The first three types of mesostructure were charac-
terized by high-resolution transmission electron microscopy
(HRTEM; see Supporting Information). They gave many
reflections with similar magnitude of scattering vectors, and
these reflections could only be resolved by a single-crystal
experiment. Their electron diffractions were separately
observed as a single-crystal experiment and are consistent
with their XRD patterns. It is inferred that AMS-10 may
[7,10]
[11]
[10]
¯
¯
¯
Im3m,
Fm3m,
Pm3m,
etc.), bicontinuous cubic
[2,12]
[13]
and Pn3m[14]), rectangular (c2mm[15]),
¯
¯
¯
(Ia3d,
Im3m
two-dimensional (2D) hexagonal (p6mm[2,16]), and lamellar
phase.[2] Most structures have been proposed on the basis of a
combination of powder X-ray diffraction (XRD) studies and
an accumulated knowledge of liquid-crystalline phases, whilst
only a few have been precisely determined by electron
crystallography. Elaborate synthesis under well-controlled
[*] Dr. Y. Sakamoto, Prof. O. Terasaki
Structural Chemistry
Arrhenius Laboratory
Stockholm University
¯
exhibit a lower curvature close to bicontinuous cubic Ia3d
10691 Stockholm (Sweden)
Fax: (+46)816-3118
E-mail: yasuhiro@struc.su.se
from the sequence of the mesophases.
The XRD pattern of AMS-10 shows two intense and well-
resolved peaks in the range 18 < 2q < 28 with a d spacing ratio
pffiffiffi pffiffi
C. Gao, Prof. S. Che
of about 3/ 2, which is quite rare in the mesoporous silica
reported earlier. We can index the two peaks by any crystal
systems. If it is assumed as cubic, the two peaks can be
indexed to 110 and 111 reflections, or to 200 and 211
reflections, and so on. From a set of electron diffraction
patterns or HRTEM images, we can conclude that the crystal
belongs to the cubic system. The HRTEM images taken with
the [100], [110], and [111] incidences are shown in Figure 2.
The Fourier transform diffractogram of the image of [100]
incidence shows the reflection conditions 0kl: k + l = 2n and
00l: l = 2n, and that of the [110] incidence shows the
conditions hkl: none and hhl: none. From these results, two
School of Chemistry and Chemical Technology
State Key Laboratory of Composite Materials
Shanghai Jiao Tong University
800 Dongchuan Road, Shanghai 200240 (P.R. China)
Fax: (+86)21-5474-1297
E-mail: chesa@sjtu.edu.cn
Dr. K. Sakamoto
Research Center, Shiseido Co., LTD
2-2-1, Hayabuchi, Tsuzuki-ku, Yokohama 224-8558 (Japan)
[**] This work was supported by the National Natural Science
Foundation of China (grant no. 20425102 and 20521140450), the
China Ministry of Education, and the Shanghai Science Foundation
(0452nm061 and 05XD14010). O.T. thanks the Swedish Science
Research Council (VR) and Japan Science and Technology Agency
(JST) for financial support.
¯
¯
space groups are possible, Pn3 (201) or Pn3m (224). The
¯
space group Pn3m was chosen because of its high symmetry.
Thus, the first two peaks of the XRD pattern can be
concluded to be 110 and 111 reflections based on a cubic
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 3. Nitrogen adsorption isotherm and pore size distribution
(calculated by the BJH method based on the adsorption branch) of the
calcined mesoporous silica AMS-10.
Figure 1. XRD patterns of calcined mesoporous silica prepared with
C14GluA as surfactant and TMAPS as CSDA and by precisely adjusting
the acidity of the reaction system. The compositions of the gels are
C14GluA/TMAPS/TEOS/H2O/NaOH=1:1.5:15:1983:x; a) x=2 (P42/
mnm), b) x=1.5 (Fd3m), c) x=1 (p6mm), and d) x=0.75 (Pn3m;
AMS-10).
A 3D reconstruction of the structure was conducted on
the basis of the analysis of Fourier diffractograms taken from
TEM images along high-symmetry zone axes of AMS-10.
Table 1shows crystal structure factors, both amplitude and
phase, extracted from thin parts of crystals along three
different zone axes, namely the [100], [110], and [111]
directions. The origin of the symmetry was taken at the
¯
¯
¯
Pn3m space group, and it can be calculated from the 110 peak
¯
that the unit cell of AMS-10 is 9.6 nm. Also, the mesophase is
highly ordered as can be derived from the large uniform areas
of the HRTEM images.
inversion center of Pn3m (origin choice 2); that is, the phase
should be 0 or p. The electrostatic potential density map of
AMS-10 was obtained by taking inverse Fourier summation
of these crystal structure factors (not shown here). With a
silica density of about 2.2 gcmꢀ3 and a pore volume of
0.65 cm3 gꢀ1 from nitrogen adsorption data, the 3D pore
structure of AMS-10 was determined (Figure 4a). The pore
volume fraction therein corresponds to 58.8%. It can be
calculated that AMS-10 has a pore diameter of 4.6 nm and
wall thickness of 2.2 nm, values that are consistent with those
calculated from nitrogen adsorption measurements. As can be
Nitrogen adsorption and desorption isotherms of AMS-10
(Figure 3) show a typical type IV isotherm with an evident
hysteresis loop in the range 0.45 < P/P0 < 0.7. The specific
surface area and pore volume of the material are 493 m2 gꢀ1
and 0.65 cm3 gꢀ1, respectively. The pore diameter is about
4.7 nm calculated by the BJH method based on the adsorption
branch of the isotherm, which is larger than that of common
mesophases prepared with anionic surfactants.
Figure 2. HRTEM images and Fourier diffractograms of the calcined mesoporous silica AMS-10. The images were taken along the zone axes as
follows: a) [100], b) [110], and c) [111].
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Chemie
Table 1: Crystal structure factors (amplitudes and phases) extracted
from HRTEM images of calcined AMS-10 (a=9.6 nm).
the surfactant micelles, which increases with the amount of
NaOH added into the carboxylic acid surfactant solution. It is
useful to introduce the surfactant packing parameter, g = v/al,
where v is the chain volume, a is the effective hydrophobic/
hydrophilic interfacial area, and l is the chain length.[20] The
lower charge density contributes to a partial decrease in the
electrostatic repulsion between the charged surfactant head
groups and a decrease in the effective head group area of
surfactant, a, therefore resulting in an increase in the g value.
It is well-known that the g parameter of lyotropic liquid-
crystal phases increases in the order: micellar tetragonal P42/
h
k
l
h2 +k2 +l2
d [nm]
Amplitude
Phase
1
1
2
2
2
2
3
3
2
3
4
3
3
4
4
4
7
6
1
1
0
1
2
2
1
1
2
2
0
2
3
2
3
4
1
3
0
1
0
1
0
1
0
1
2
1
0
2
0
0
3
2
0
3
2
3
4
6
8
9
10
11
12
14
16
17
18
20
34
36
50
51
6.80
5.55
4.81
3.92
3.40
3.20
3.04
2.90
2.77
2.57
2.40
2.33
2.27
2.15
1.65
1.60
1.36
1.35
100.00
57.09
13.13
3.40
0.79
0.57
0.42
0.06
0.20
0.13
0.10
0.10
0.07
0.01
0.05
0.02
0.01
0.01
p
0
0
p
p
0
p
0
p
0
0
p
0
p
0
0
0
0
¯
mnm, micellar cubic Fd3m < cylindrical 2D hexagonal
[21]
¯
p6mm < bicontinuous cubic Pn3m. Thus, it is reasonable
that a lower charge density of the micelle surface facilitates
¯
the formation of the cubic Pn3m mesophase with a larger g
parameter.
In summary, mesoporous silica AMS-10 with a novel
¯
bicontinuous cubic mesophase Pn3m was synthesized by
using an anionic surfactant as template. Detailed character-
izations were performed through XRD, HRTEM, and nitro-
gen adsorption studies, and a 3D structure was derived from
these results and those obtained through electron crystallog-
raphy. Precise control of the neutralization degree of the
anionic surfactant is key to the synthesis of AMS-10 and also
proves a feasible and simple means to achieve mesophase
control of anionic surfactant templated mesoporous silica
(AMS). Because of its characteristic mesoporous structure,
this bicontinuous cubic mesoporous material could be useful,
for example, as biocompatible materials for the encapsula-
tion, controlled release, and delivery of drugs and biomole-
cules.
seen from Figure 4a,b, the silica wall of the AMS-10 follows a
typical diamond minimal surface (D surface) in analogy to the
gyroid minimal surface (G surface) as observed for the silica
wall of MCM-48.[12] From this result, we conclude that AMS-
10 has a bicontinuous structure composed of an enantiomeric
pair of 3D mesoporous networks that are interwoven as
shown in Figure 4c. Each network, which is divided by a
D surface, consists of tetrahedral connection of pores at the
¯
43m position, 2a site (Wyckoff notation), although in the case
¯
of a bicontinuous cubic Ia3d structure each network that is
divided by a G-surface consists of three connected pores at
the 32 position, 16b site (Wyckoff notation).
The surfactants CnGluA (n = 12–18) with different chain
lengths also gave rise to the ordered bicontinuous cubic Pn3m
Experimental Section
¯
TMAPS (Azmax) and tetraethyl orthosilicate (TEOS; TCI) were
purchased and used as received. The synthetic method of glutamic
acid derived surfactant C14GluA is described in the Supporting
Information.
mesostructure in combination with either TMAPS or 3-
aminopropyltrimethoxylsilane (APS). The neutralization
degree of the surfactant is crucial to form the specific
mesophase in both synthesis systems. The effect of the
neutralization degree on the formation of mesostructures
can be explained in terms of the surface charge density (s) of
In a typical synthesis of AMS-10, NaOH (7.5 g of 0.1 molLꢀ1
solution) was added to a solution of C14GluA (0.357 g, 1mmol) in
deionized water (28.2 g) stirring at 608C. After the solution became
homogeneous a mixture of TMAPS (0.773 g; 50% in methanol,
¯
Figure 4. 3D structure of a) the bicontinuous cubic Pn3m mesoporous silica AMS-10 (as derived from electron crystallography), b) the D surface,
and c) the 3D networks of double diamond structure divided by the D surface. They show a 2”2”2 unit cell.
Angew. Chem. Int. Ed. 2006, 45, 4295 –4298
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Communications
1.5 mmol) and TEOS (3.12 g, 15 mmol) was added, and the mixture
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was stirred at 608C for 10 min. The molar composition of the final gel
was C14GluA/TMAPS/TEOS/H2O/NaOH= 1:1.5:15:1983:0.75. The
reaction system was then kept static for 2 days at 608C to allow the
product to hydrolyze, condense, and age. To remove the surfactant,
the as-synthesized mesoporous silica was calcined at 5508C for 6 h or
extracted using a mixture of HCl (37 wt%) and ethanol (90% w/w)
for 12 h.
XRD patterns were recorded on a Rigaku X-ray diffractometer
D/MAX-2200/PC with CuKa radiation (40 kV, 20 mA) at a rate of
1.0 degminꢀ1 over the range of 18 < 2q < 68. HRTEM was performed
with a JEOL JEM-3010 microscope operating at 300 kV (Cs =
0.6 mm, point resolution 1.7 Š). Images were recorded with a CCD
camera (MultiScan model 794, Gatan, 1024 ” 1024 pixels, pixel size
24 ” 24 mm2) at 50000–80000 magnification under low-dose condi-
tions. The adsorption/desorption isotherms were measured with an
ASAP2100 using N2 as adsorbate at 77 K. The specific surface area
was calculated by the BET method, and the pore size was obtained
from the maxima of the pore size distribution curve calculated by the
BJH method by using the adsorption branch of the isotherm.
Received: November 18, 2005
Revised: February 11, 2006
Published online: May 26, 2006
Keywords: electron diffraction · electron microscopy ·
.
mesoporous materials · surfactants · template synthesis
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