Structural control of mesoporous silica by a bicontinuous microemulsion-aided process

Article abstract of DOI:10.1246/cl.2005.610

Mesoporous silica was successfully synthesized via a solgel process in a bicontinuous microemulsion of tetradecane/didodecyldimethylammonium bromide/aq HCl ternary system. The surface area, pore size and pore volume of silica varied dependent on the composition of bicontinuous microemulsion. Copyright

Full text of DOI:10.1246/cl.2005.610

610
Chemistry Letters Vol.34, No.4 (2005)
Structural Control of Mesoporous Silica by a Bicontinuous Microemulsion-aided Process
Isamu Moriguchi,ꢀy;yy Yuki Shono,y Hiromi Tachikawa,y Hirotoshi Yamada,y Tetsuichi Kudo,y
Yasutake Teraoka,yyy and Taisei Nishimiyyyy
yDepartment of Applied Chemistry, Faculty of Engineering, Nagasaki University, 1-14, Bunkyo-machi, Nagasaki, 852-8521
yyPRESTO JST
yyyDepartment of Molecular and Material Sciences, Graduate School of Engineering Sciences,
Kyushu University, Fukuoka 816-8580
yyyyFuji Photo Film Co. Ltd., 210 Nakanuma, Minami-Ashigara 250-0193
(Received February 8, 2005; CL-050184)
Table 1. The composition of bicontinuous microemulsion used
for the synthesis and parameters associated with porous structure
of silica
Mesoporous silica was successfully synthesized via a sol–
gel process in a bicontinuous microemulsion of tetradecane/di-
dodecyldimethylammonium bromide/aq HCl ternary system.
The surface area, pore size and pore volume of silica varied de-
pendent on the composition of bicontinuous microemulsion.
Composition of
Sa
V
D
Samples
TD/DDAB/aq HCla /m2 gꢂ1 b /cm3 gꢂ1 c /nmd
SiO2 [A]
SiO2 [B]
SiO2 [C]
SiO2 [D]
25/25/50
30/30/40
35/35/30
15/30/55
1290
903
652
1008
1.41
1.66
2.96
0.80
2.8
4.2
5.5, 20
2.0
Synthesis and application of porous materials are currently
the subjects of intensive research in materials science. A lot of
self-assembly systems of surfactants,1 amphiphilic block co-
polymers,2 and colloidal particles3 were employed as templates
for directing porous structures of silica, metal oxides, and car-
bons. From the viewpoint of controlling porous structure, the
use of bicontinuous microemulsion, which forms in a water/sur-
factant/oil ternary system with a balanced hydrophilicity and
lipophilicity, as a reaction medium for inorganic synthesis has
a potential to be a unique process for porous material synthesis.
The bicontinuous microemulsions are structured as compart-
mentalized liquid phases, in which the oil and water phases
are separated by surfactant layers into highly branched and
three-dimensionally interconnected nanometer-wide channel
networks.4 The volume and average width of each channel can
be easily controlled with components and composition of
water/surfactant/oil. Therefore, three-dimensional bicontinuous
porous structures of inorganic materials, the dimensions of pore
and wall thickness of which are tunable in wide range, are ex-
pected to be constructed if a mineralization reaction takes place
dominantly in the one compartmentalized phase. However, there
are only a few studies concerning the bicontinuous microemul-
sion-aided synthesis, and the reported studies are limited to a
preparation of macroporous frameworks of silica and calcium
phosphate.5,6 Although we have reported very recently a synthe-
sis of mesoporous TiO2 by the bicontinuous microemulsion-
aided process,7 a wide range control of porous structure expected
above is not yet accomplished due to a fast gelation of titanium
alkoxide. The present work succeeded for the first time a control
of pore size and pore volume of mesoporous silica dependent on
the composition of bicontinuous microemulsion. This would be
contrast to the micelle-templating process, in which the pore size
reflects the micelle size dependent on surfactant molecular
length and the amount of auxiliary additives such as mesitylene
if incorporated.8
awt %, bBET surface area, cmesopore volume, dmean pore size.
(a)
(b)
(c)
Figure 1. Feature change of a solution of bicontinuous micro-
emulsion during the sol–gel reaction; (a) a bicontinuous micro-
emulsion with composition [B], (b) passing 1 h and (c) passing 1
day after adding TMOS.
region of phase diagram reported by Ninham et al.,9 and it was
confirmed that the dilute HCl little affects on the phase diagram
by a preliminary experiment. The mixture solution of TD/
DDAB/aq HCl was stirred for 30 min at room temperature until
a transparent solution of bicontinuous microemulsion was ob-
tained (Figure 1a). Tetramethoxy silane (TMOS) was added
dropwise to the stirred bicontinuous microemulsion solution
with a constant weight ratio of DDAB:TMOS = 3.4:1.4. After
further stirring for 30 s, the solution was stored under static
condition at 25 ꢃC with a thermocontrolled bath for 1 day. The
solution became turbid with passing time (Figure 1b), and a
conservative and semitransparent gel was obtained after 1 day
(Figure 1c). The gel was washed thoroughly with hexane, dried
in an oven at 110 ꢃC for 12 h and finally calcined in air at 550 ꢃC
for 6 h, to yield porous silica. It was confirmed by thermogravi-
metric measurements that all of organics were removed out from
the silica by heating in air upto 400 ꢃC (Seiko Instruments Inc.
Bicontinuous microemulsions were prepared by mixing di-
dodecyldimethyammonium bromide (DDAB), tetradecane
(TD) and aq HCl (4 ꢁ 10ꢂ3 mol dmꢂ3) with the composition
[A]–[D] indicated in Table 1. All of the composition of TD/
DDAB/aq HCl used here are in the bicontinuous microemulsion
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.4 (2005)
611
pore volume. It can be suggested that the sol–gel reaction of
TMOS occurs preferentially in the water phase or at the interface
of water/surfactant, and the space of oil phase would mainly
contribute to the porous space of mesoporous silica for the
bicontinuous microemulsion-aided process. This is supported
by the TEM observation of the formation of tube-like precursor
gels containing surfactant phase at the inner space for the report-
ed bicontinuous microemulsion-aided synthesis of mesoporous
TiO2.7 The increase in surface area and the decrease in pore
volume with the composition change of [B] ! [D], which is
the condition of constant surfactant amount in the bicontinuous
microemulsion, confirm again that the porous structure of silica
can be controlled with the composition ratio of water/oil.
Further study concerning an effect of surfactant amount in bicon-
tinuous microemulsion on the porous structure is now in prog-
ress.
20
(i)
(ii)
16
a
4
12
c
d
b
8
4
0
2
0
b
a
d
c
100
1
10
0
0.2 0.4 0.6 0.8
Relative pressure, p/p0
1
Pore diameter / nm
Figure 2. (i) Nitrogen adsorption–desorption isotherms and
(ii) BJH pore size distributions of porous silica; (a) SiO2[A],
(b) SiO2[B], (c) SiO2[C], (d) SiO2[D]. The closed and opened
symbols indicate adsorption and desorption processes, respec-
tively.
Since the controllable surface area, mean pore size and pore
volume of mesoporous silica were in the range of 652–
1290 m2gꢂ1, 2–20 nm and 0.8–2.96 cm3gꢂ1, respectively, only
in the present conditions, the microemulsion-aided process
would has a potential of controlling the porous structure in wide
range.
TG/DTA6200). In the following, the SiO2 samples after the cal-
cinations at 550 ꢃC are denoted as SiO2[A]–[D], where [A]–[D]
indicates the TD/DDAB/aq HCl composition of bicontinuous
microemulsion in Table 1.
This work was in part supported by a Grant-in-Aid for Sci-
entific Research from Ministry of Education, Culture, Science,
Sports and Technology of Japan. The study made use of instru-
ments (XRD) in the Center for Instruments Analysis of Nagasaki
University.
On X-ray diffraction (XRD) measurements (Rigaku
RINT2200), all the samples showed no distinct XRD peak, sug-
gesting that the silica did not have an ordered porous structure
like as in micelle-templated mesoporous silica. Figure 2i shows
N2 adsorption and desorption isotherms of the obtained SiO2
samples, which were measured at 77 K (Micromeritics, Gemini
2370). The pore volume and BJH pore size distribution were cal-
culated using their adsorption branches, and the specific surface
area was determined by the multipoint Braunauer–Emmett–
Teller (BET) method (Table 1, Figure 2ii). For SiO2[A], the
TD/DDAB/aq HCl composition of which was 25/25/50 by
weight, the inflection characteristics of capillary condensation
into mesopores is observed in the adsorption isotherm at
0:2 < p=p0 < 0:5, confirming the presence of mesopores in
SiO2[A]. The values of BET surface area and mesopore volume
of SiO2[A] were 1290 m2 gꢂ1 and 1.41 cm3 gꢂ1, respectively,
which are almost comparable to or larger than those of mesopo-
rous silica generally obtained by micelle-template process.10
With the change of TD/DDAB/aq HCl composition as
[A] ! [B] ! [C], in which the wt % of water decreases while
keeping the constant weight ratio of DDAB/TD in the bicontin-
uous microemulsion, the inflection characteristic of capillary
condensation into pores shifted to higher value of the relative
pressure. As indicated in Table 1 and Figure 2ii, BET surface
area of mesoporous silica decreases, and the pore size and pore
volume increase respectively with the composition change
[A] ! [B] ! [C], although the pore size-distribution became
broadened. The SiO2[C] seems to have a bimodal porous struc-
ture, which might be due to a formation of lamella phase at a
high surfactant concentration. On the other hand, the change
of [A] ! [D], which is the direction to decreasing oil fraction
at almost the same DDAB/aq HCl composition, resulted in the
decreasing pore size and pore volume of silica (Table 1,
Figure 2ii). These results mean that the ratio of water or oil con-
tent in the composition of bicontinuous microemulsion remark-
ably affects on the porous structure of mesoporous silica, that is,
the increase in oil fraction causes increasing silica pore size and
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Published on the web (Advance View) March 19, 2005; DOI 10.1246/cl.2005.610