Tri(quaternary ammonium) surfactant with a benzene core as a novel template for synthesis of ordered porous silica

Article abstract of DOI:10.1246/cl.2010.236

A novel rigid-core surfactant where three trimethylammonium heads are attached to a benzene core via flexible hydrocarbon chains was examined as a template for synthesizing ordered porous silica. A well-ordered 2D hexagonal silica structure with relatively small pores was generated owing to the formation of the stable mesophase where the surfactants were stacked to form cylindrical assemblies.

Full text of DOI:10.1246/cl.2010.236

doi:10.1246/cl.2010.236
236
Published on the web February 6, 2010
Tri(quaternary ammonium) Surfactant with a Benzene Core as a Novel Template
for Synthesis of Ordered Porous Silica
Yuki Fukasawa, Ayae Sugawara, Hirotomo Hirahara, Atsushi Shimojima, and Tatsuya Okubo*
Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
(Received December 8, 2009; CL-091084; E-mail: okubo@chemsys.t.u-tokyo.ac.jp)
(a)
(b)
(c)
A novel rigid-core surfactant where three trimethylammo-
OR
nium heads are attached to a benzene core via flexible
hydrocarbon chains was examined as a template for synthesizing
ordered porous silica. A well-ordered 2D hexagonal silica
structure with relatively small pores was generated owing to the
formation of the stable mesophase where the surfactants were
stacked to form cylindrical assemblies.
TEOS
RO
OR
R = (CH2)10N+(CH3)3Br-
H2O
Porous silica with regularly arranged and uniform pores has
a wide range of potential applications such as catalysts and
adsorbents.1,2 Mesoporous silica materials are generally synthe-
sized through a surfactant-templating route, in which the size
and geometry of the pores can be widely tuned by varying the
type of surfactant as well as synthesis conditions. In addition to
conventional surfactants consisting of a hydrophobic tail and a
terminal hydrophilic head, other surfactant configurations, such
as gemini,3-5 bolaform,6,7 and disk-like,8-10 with two or more
head groups, have been investigated to expand the variety of
mesophases and to control the pore architecture precisely.
Surfactants with rigid hydrophobic segments have attracted
particular attention because they usually show more specific
self-assembly behavior than those with only flexible chains. For
example, bolaform surfactants where two ½-(trimethylammo-
nium)alkyl groups are connected by a biphenyl unit form
unusual two-dimensional mesostructures.6 Disk-like molecules
with rigid cores are also of interest because the mobility of side
chains are restricted so that structure and pore size of the
resulting mesoporous silica should be more unambiguously
determined. Phthalocyanine-,8 triphenylene-,9 and bipyridine
derivatives10 having amphiphilic side chains have been used to
produce 2D hexagonal silica-surfactant mesophases; however,
no unique structural feature of the mesoporous silica, as
compared to conventional ones, has been demonstrated.
Scheme 1. (a) Molecular structure of B-C10TMA, (b) polarizing
microscopic image showing a lyotropic liquid crystal of B-
C10TMA, and (c) structural model of the silica-surfactant
composite mesophase.
(a)
(a)
d10 = 2.7 nm
d = 3.0 nm
3
(b)
(b)
d10 = 2.7 nm
d = 3.4 nm
3
2
4
6
8
10
2
4
6
8
10
2θ (CuKα)/degree
2θ (CuKα)/degree
Figure 1. XRD patterns of the products prepared using B-
C10TMA (left) and C10TMA (right): (a) without and (b) with the
addition of TMB.
by reaction with trimethylamine (see Supporting Information for
details).12 A polarizing microscopic image of B-C10TMA in
water shows a fan-like texture characteristic of a columnar
lyotropic liquid crystal (Scheme 1b).
Ordered porous silica was synthesized using tetraethoxy-
silane (TEOS) as a silica source under standard basic con-
ditions.5 TEOS was added to the mixture of B-C10TMA,
sodium hydroxide, and water in the molar ratios of 1TEOS:
0.4NaOH:140H2O:0.02B-C10TMA. After 24 h of stirring at
room temperature, the mixture was aged at 60 °C for 24 h.
Precipitates were recovered by filtration, air-dried at 60 °C, and
finally calcined at 450 °C for 3 h to remove surfactant.
Powder X-ray diffraction (XRD) pattern of the calcined
sample (Figure 1, left, a) shows well-defined peaks indexed to
the (10), (11), and (20) reflections of a 2D hexagonal structure.
Periodic hexagonal and stripe patterns are clearly observed
by TEM (Supporting Information, Figure S2).12 The nitrogen
adsorption/desorption isotherm (Figure S3)12 of this sample is
In this letter, we report the synthesis of an ordered porous
silica using a novel tri(quaternary ammonium) surfactant con-
sisting of a simple benzene core and three ½-(trimethylammo-
¹
nium)alkyl side chains [C6H3(O(CH2)10N+(CH3)3Br )3, here-
after denoted as B-C10TMA] (Scheme 1a). B-C10TMA is a
new type of cationic and rigid-core surfactant that is quite
different from the aforementioned surfactants having multiple
aromatic rings as a core and nonionic alkyl polyoxyethylene
ether as side chains in common.8-10 A well-ordered 2D
hexagonal structure with relatively small pores compared to
those generated with typical alkyltrimethylammonium-type
surfactants was formed here, which was attributable to the
specific interactions between B-C10TMA molecules to form
stacked cylindrical assemblies.
B-C10TMA was synthesized by the reaction of 1,3,5-
trihydroxybenzene with an excess amount of 1,10-dibromo-
decane to form 1,3,5-tri(10-bromodecyloxy)benzene,11 followed
Chem. Lett. 2010, 39, 236-237
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

237
close to IUPAC type I classification13 but has a slight shoulder
below the relative pressure of 0.15, suggesting that the pore size
is in an intermediate range between micropore and mesopore
regions. The average pore sizes calculated based on Barrett-
Joyner-Halenda (BJH) theory13 and nonlinear density functional
theory (NLDFT)14 are 1.5 and 2.2 nm, respectively (Table S1).12
Note that BJH theory tends to underestimate the pore size in the
miropore region.15 The BET surface area and pore volume are
increase of pore size is relatively facile, e.g., by using swelling
agent or block copolymers, the generation of well-regulated
small pores is still difficult.16,17 In this context, the tricationic
surfactant used in this study provides a promising method.
Further decrease of the pore size can be achieved by reducing the
length of the alkylene spacers. When B-C6TMA having C6
spacers was used as template, only a broad peak was observed
by XRD (data not shown); however, the pore size was greatly
decreased to 1.1 nm, as evaluated by NLDFT (BJH theory is no
more applicable for such a small pore).
In conclusion, a novel rigid-core surfactant consisting of
three trimethylammonium heads attached to a benzene core via
flexible hydrocarbon chains was examined as a template for
synthesizing ordered porous silica. This surfactant was effective
for generating small and regularly arranged pores, which is
attributed to its strong tendency to form cylindrical assemblies.
The synthesis of such ordered porous silica with small
mesopores is of both scientific and practical importance.
¹1
calculated to be 870 m2 g and 0.5 cm3 g¹1, respectively.
To clarify the specific properties of B-C10TMA, decyltri-
methylammonium bromide (C10TMA) was also examined as
template. A control experiment was performed at the molar
ratios of 1TEOS:0.4NaOH:140H2O:0.06C10TMA under other-
wise identical conditions. In contrast to the highly ordered pore
structure achieved with B-C10TMA, the sample prepared with
C10TMA is less ordered, exhibiting broader peaks in the
XRD pattern (Figure 1, right, a). A lower BET surface area
(360 m2 g¹1) is also confirmed. These results are consistent with
the fact that it is generally difficult to obtain highly ordered
porous silica using alkyltrimethylammonium surfactants with
short alkyl chains containing less than 12 carbons, due to
hydrophobic interaction poorer than the electric repulsion
between cationic head groups.16,17 Thus, the rigid benzene core
of B-C10TMA should play a crucial role in the formation of
highly ordered structure.
UV-vis spectroscopy indicates ³-³ stacking of benzene
cores in the silica-B-C10TMA composites18 (Figure S4). The
absorption bands of the composites are red-shifted compared to
those of the simple mixture of silica powder and B-C10TMA.
Such ³-³ stacking in addition to the hydrophobic interactions
of B-C10TMA should stabilize the silica-surfactant composite
mesophase. It is plausible that B-C10TMA molecules are
stacked to form cylindrical micelles where the side chains
radiate outward from the central benzene cores (Scheme 1c).
This model is supported by the experimental result that the
expansion of the micelle diameter hardly occurred even by
adding TMB, a typical swelling agent. In general, the d10
spacing is increased by the addition of hydrophobic swelling
agents5 when conventional surfactants like C10TMA are used
as template; however, such a behavior was not observed for
B-C10TMA (Figure 1b).
In the silica-B-C10TMA mesophase, a trisubstituted ben-
zene core should be rotated to some degree from the adjacent
core to minimize the steric repulsion between alkylene spacers.
It appears that the alkylene spacers are not in a fully extended
state, because the pore size of the calcined sample is much
smaller than the molecular size of B-C10TMA with all-trans
alkylene spacers (ca. 3.8 nm). Note that cationic head groups
do not thrust into silica walls, unlike nonionic surfactants
which penetrate silica walls and produce micropores. It is also
interesting that despite the presence of the benzene core (ca.
0.7 nm in diameter, including ether oxygen atoms), the pore size
is slightly smaller than that templated with C10TMA (1.6 nm).12
This can be explained by the low number density of trimethyl-
ammonium(alkyl) groups in the silica-B-C10TMA composite.
In fact, the N/Si ratio calculated by combination of CHN
analysis and thermogravimetry was 0.18, which is lower than for
the sample prepared with C10TMA (0.20).
This work was supported in part by a Grant-in-Aid for
Scientific Research (B) from the Japan Society for the Promotion
of Science. This work was partly conducted in the Center for
Nano Lithography & Analysis, The University of Tokyo,
supported by the Ministry of Education, Culture, Sports, Science
and Technology, Japan. We thank Prof. Kazunari Domen at the
University of Tokyo for UV-vis measurements.
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Chem. Lett. 2010, 39, 236-237
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/