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Chemistry Letters Vol.38, No.1 (2009)
One-pot Synthesis of Inorganic–Organic Hollow Microsphere Solid-Acid Catalysts
in a W/O Microemulsion System
Ã
Tomohiko Okada, Shozi Mishima, and Shingo Yoshihara
Department of Chemistry and Material Engineering, Shinshu University, Nagano 380-8553
(Received September 30, 2008; CL-080943; E-mail: tomohiko@shinshu-u.ac.jp)
Acid-supporting hollow microspheres were successfully
(molar ratio of octyltrichlorosilane to Span 60 was 19) (abbrevi-
ated as S-TPA-HMS). The resulting solids were washed with tol-
uene and subsequently dried at 393 K. The dehydration of etha-
nol was carried out under N2 flow by using a fixed-bed flow re-
prepared through a novel synthetic procedure on the introduction
of a catalytic active species in the void space; one-pot synthesis
was achieved by addition of 12-tungstophosphoric acid to the
aqueous phase in a water-in-oil emulsion and simultaneous for-
mation of a polyorganosiloxane shell (precursors are in oil
phase) through sol–gel reactions. The resulting solid was porous
and was active as solid-acid catalyst as shown by vapor-phase
dehydration of ethanol.
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0
actor. All of the sample catalysts were activated in air at 673 K
for 3 h (Method of the sample preparation was summarized in
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4
Scheme ).
FE-SEM observation, as exemplified in Figure 1, revealed
that all of the samples are microsphere with a cavity in each of
the particles. In some cases, the shape of the particles is not a
good sphere but deformed. The particle sizes are in a range of
1–4 mm (thickness of the shell: a few hundred nm) for TPA-
HMS. When Span 60 was used as additive, the particle size
has decreased to a range of 0.4–1 mm (thickness of the shell: be-
low one hundred nm). It is thought that Span 60 acts as a strong
emulsifier to give relatively stable microemulsion compared to
that without Span 60. By crushing the particles by mortar and
pestle, a cavity is observed in each particle, showing hollow par-
Structural design of catalysts is an important aspect for the
improvement of catalytic activity and selectivity. Carbon nano-
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3
tubes, mesoporous materials, and zeolite membranes, for ex-
ample, possess characteristic shapes and are noted as highly de-
4
signed catalysts. Hollow microspheres are also interesting as
highly designed catalysts with many advantages. We have re-
ported the successful preparation of hollow microspheres from
alkyltrichlorosilanes by using W/O emulsion and that the pro-
duced particles composed of polyorganosiloxane are micropo-
À2
ticles. The supported quantities of TPA are 7{9 Â 10 and
À1
2{3 Â 10 mmol/g for TPA-HMS and S-TPA-HMS, respec-
5
11
rous and hydrophobic. The void space in the particle can be
tively. In the XRD patterns (see Supporting Information,
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used as a catalyst container, and the shell of the particle can
be regarded as a kind of membrane. It was recently reported that
the shell of the hollow microspheres has played a role in molecu-
Figure S1), TPA exists in both hollow particles as crystals
with a particle size of ca. 20 nm (estimated from the Scherrer
equation). From IR spectra of samples heat-treated at 673 K
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lar sieving to reveal reaction selectivity and inhibition of sup-
7
(see Supporting Information Figure S2), absorption bands
due to alkyl groups (ꢀC–H) and Si–C bonds (ꢁC–Si–C) are
observed at around 2900 and 1400 cm , respectively, showing
ported metal agglomeration to maintain catalyst lifetime.
À1
Here we apply hollow microspheres as a catalyst container,
namely, introduction of 12-tungstophosphoric acid (TPA) in the
void space of hollow particles as a new supported acid catalyst.
There are many reports on the preparation of heteropoly-acid-
that prepared microspheres are on organic–inorganic hybrid
material showing hydrophobicity and are stable below 673 K.
Porosity has been shown only when the products are heat-
8
mounted catalysts because heteropoly acids themselves act as
9
treated at 673 K (N adsorption/desorption isotherms are shown
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5
strong acids without corrosion of equipment. While most of
in Figure S3). In our previous comnunication, specific surface
the supported catalysts have been prepared by impregnation,
one-pot synthesis can be achieved here by directly addition of
TPA to W/O emulsion and simultaneous formation of a polyor-
ganosiloxane shell through sol–gel reactions. In addition, hydro-
phobic characteristics of hollow particles derived from polyor-
ganosiloxane is a merit when using for catalytic reactions such
as oxidation of a hydrophobic reactant and dehydration to pro-
tect active sites from poisoning by water, improving catalytic ac-
tivities. In the present study, vapor-phase dehydration of ethanol
has been examined as a test reaction to elucidate the catalytic
properties of the hollow microsphere supported acid catalysts.
A W/O microemulsion was formed by ultrasonic agitation
of a mixture of octyltrichlorosilane (12 mmol) dissolved in tol-
uene (50 mL) with aqueous TPA solution (50 mM, 0.79 mL).
Then methyltrichlorosilane (9.0 mmol) dissolved in toluene
(a)
(
b)
4
.0 µm
4.0 µm
crushed (a)
crushed (b)
(
10 mL) was added to the emulsion to form hollow microsphere
0
.7 µm
1
.0 µm
under magnetic stirring for 3 h with air flow (abbreviated as
TPA-HMS). In the other case, a small amount of sorbitan mono-
stearate (Span 60) was added in addition to octyltrichlorosilane
Figure 1. SEM images of (a) TPA-HMS and (b) S-TPA-HMS.
Copyright ꢀ 2009 The Chemical Society of Japan