One-pot synthesis of bimodal dispersed mesoporous silica spheres

Article abstract of DOI:10.1246/cl.2005.780

The addition of polyethylene-glycol during the reaction led to the synthesis of bimodal dispersed mesoporous silica spheres in a one pot system. Copyright

Full text of DOI:10.1246/cl.2005.780

780
Chemistry Letters Vol.34, No.6 (2005)
One-pot Synthesis of Bimodal Dispersed Mesoporous Silica Spheres
Kazuhisa Yanoà and Yoshiaki Fukushima
Toyota Central R & D Laboratories, Inc., Nagakute, Aichi 480-1192
(Received January 27, 2005; CL-050125)
The addition of polyethylene glycol during the reaction led
to the synthesis of bimodal dispersed mesoporous silica spheres
in a one pot system.
added. After 8 h of continuous stirring the mixture was aged
overnight. The white powder was then filtered and washed with
distilled water at least three times, and dried at 318 K for 72 h.
The powder obtained was calcined in air at 823 K for 6 h to re-
move organic species.
The particle size of the mono-dispersed porous silica
spheres was influenced by the synthesis temperature, metha-
nol/water ratio, and silica source.19 It was also affected by the
addition of a polymer.21 It is assumed that the addition of a poly-
mer during the reaction brings to the formation of mono-dis-
persed spheres with different particle sizes if particles are gener-
ated successively. In order to confirm this assumption, polyeth-
ylene glycol (PEG) was added 4 min. after the experiment had
started, at which time the particle size had become maximum
but the scattering intensity was still increasing.20 Since the scat-
tering intensity at 4 min. was around half the maximum value, it
is expected that half of the particles generated are of different
sizes. Scanning electron micrographs of mono-dispersed meso-
porous silica spheres with or without the addition of PEG are
shown in Figure 1. The average particle size of the mono-dis-
persed mesoporous silica spheres without the addition of PEG
(Figure 1a) was 0.65 mm (standard deviation: 0.023 mm). When
Mesoporous silica has received much attention in the past
decade, and has been used for the catalyst, adsorbent and elec-
tronic devices because of its uniform mesopores and high specif-
ic surface area. The morphology of particles is usually dependent
on the synthesis conditions, and the particles with a variety of
shapes, for example, fibers,1–3 sponge-like membranes,4,5 rod-
like powders,6 films,7 polyhedral particles,8,9 and spheres10–12
have already been synthesized. Non-porous mono-dispersed
silica spheres in the micron-sized range were first synthesized
by Stober in 196813 using the water–alcohol–ammonia–tetra-
¨
alkoxysilane system. Later, this method was modified by the
addition of a cationic surfactant and/or other organic solvents
instead of alcohol, and the mesophases of silica spheres were
synthesized.14–17
In our laboratory, we have developed a method for the syn-
thesis of hexagonally ordered and well-defined mono-dispersed
mesoporous silica spheres from tetramethoxysilane (TMOS)
and n-alkyltrimethylammonium bromide.18–20 Mesoporous
mono-dispersed silica spheres in the submicron-sized range were
obtained under very controlled and specific conditions.
During the reaction, the clear solution turned opaque sud-
denly, a white precipitate appearing. Usually, it is reasonable
to consider that simultaneous generation, followed by propaga-
tion and subsequent termination of each and every particle leads
to the formation of mono-dispersed particles. In order to eluci-
date the formation mechanism, in situ particle size development
was measured.20 Just before three minutes after the experiment
had started, the particles grew immediately, reaching the final
size within 15 s. On the other hand, the scattering intensity,
which corresponds to the number of particles, increased gradual-
ly, becoming maximum 6 min. later. The results concerning the
scattering intensity well matched those of visual examination.
This suggested that the particles were generated successively
(not at once), growing immediately to the same maximum size
one after another for ca. 3 min.
(a)
(b)
Here, we describe the synthesis of bimodal dispersed meso-
porous silica spheres in a one pot system by utilizing the above
mentioned formation mechanism for mono-dispersed mesopo-
rous silica spheres.
In a typical synthesis procedure,20 3.52 g of hexadecyltri-
methylammonium chloride (C16TMACl) and 2.28 g of a 1 M so-
dium hydroxide solution were dissolved in 800 g of a methanol/
water (50/50 = w/w) solution. Then 1.32 g of TMOS was add-
ed to the solution with vigorous stirring at 298 K. After addition
of TMOS, the clear solution turned opaque suddenly, due to a
white precipitate. Just 4 min. after the experiment had started,
2 g of polyethylene glycol (Mw: 20,000) in 50 g of water was
1.1 µm
Figure 1. Scanning electron micrographs (SEMs) of samples
synthesized without (a) and with (b) the addition of polyethylene
glycol (PEG) (M.W. 20,000) after 4 min. The SEM pictures were
obtained with a SIGMA-V (Akashi Seisakusho). The surface of
the samples was coated with gold before the measurements.
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.6 (2005)
781
(100) plane as well as the other peaks assigned to the diffraction
of the (110) and (200) planes were observed, indicating that this
material had a highly hexagonal symmetry. A nitrogen adsorp-
tion isotherm is shown in Figure 3. Although a steep increase
on the high pressure side (P=P0 ꢀ 0:95) was observed because
of the co-aggregation of the small particles, this material had a
high specific surface area (1200 m2 gÀ1) and a large pore volume
(0.6 mL gÀ1). This indicated that both large and small particles
possessed mesopores inside.
10000
(100)
8000
6000
4000
In summary, we have developed the novel synthesis method
for bimodal dispersed mesoporous silica spheres in a one-pot
system. For densely packing, mixing smaller particles with
larger particles is considered. However, it is difficult to disperse
smaller particles because of aggregation. This finding would
bring close packing of monodispersed mesoporous silica spheres
with different average sizes.
2000
0
(200)
(110)
1 2 3 4 5 6 7 8 9 10
2
θ / deg, CuKα
Figure 2. X-ray diffraction pattern of the sample obtained with
the addition of PEG. X-ray measurement was performed with a
Rigaku Rint-2200 X-ray diffractometer using Cu Kꢀ radiation.
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Figure 3. Nitrogen adsorption isotherm of the sample obtained
with the addition of PEG. Desorption branch was identical to the
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Published on the web (Advance View) May 7, 2005; DOI 10.1246/cl.2005.780