Encapsulation of a polyoxometalate into an organosilica microcapsule for highly active solid acid catalysis

Article abstract of DOI:10.1021/cs400923q

We have developed a highly active and water-tolerant solid acid catalyst by encapsulating a polyoxometalate into hydrophobic hollow polymethylsiloxane microspherical particles. 12-Tungstophosphoric acid and a cesium salt of a heteropoly acid (Cs_2.5H_0.5PW₁₂O₄₀, abbreviated as CsPW) were used for this purpose. The encapsulated catalysts were prepared by a sol-gel reaction of methyltrichlorosilane around water droplets containing heteropoly compounds in a water-in-oil emulsion. Both the catalysts were active in the hydrolysis of ethyl acetate in a large excess of water. While the 12-tungstophosphoric acid in the polymethylsiloxane microcapsules leached into the water during the repetition of the reaction, the activity of the CsPW encapsulated catalyst (0.86 mol·(acid-mol)^-1·min ^-1) was maintained. This activity was superior to that of bare cesium salt (0.34 mol·(acid-mol)^-1·min^-1). To our knowledge, the CsPW encapsulated catalyst exhibits the highest activity among solid acid catalysts with respect to reaction rate per acidic proton. The polymethylsiloxane shell plays several roles including shielding of CsPW and allowing penetration of reactants (ethyl acetate and water) and products (ethanol and acetic acid) through the microporous membrane.

Full text of DOI:10.1021/cs400923q

Research Article
pubs.acs.org/acscatalysis
Encapsulation of a Polyoxometalate into an Organosilica
Microcapsule for Highly Active Solid Acid Catalysis
Tomohiko Okada,* Kazuyoshi Miyamoto, Toshio Sakai, and Shozi Mishima
Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553,
Japan
*
S Supporting Information
ABSTRACT: We have developed a highly active and water-
tolerant solid acid catalyst by encapsulating a polyoxometalate
into hydrophobic hollow polymethylsiloxane microspherical
particles. 12-Tungstophosphoric acid and a cesium salt of a
heteropoly acid (Cs H PW O , abbreviated as CsPW) were
2
.5 0.5
12 40
used for this purpose. The encapsulated catalysts were prepared
by a sol−gel reaction of methyltrichlorosilane around water
droplets containing heteropoly compounds in a water-in-oil
emulsion. Both the catalysts were active in the hydrolysis of ethyl
acetate in a large excess of water. While the 12-tungstophos-
phoric acid in the polymethylsiloxane microcapsules leached into
−1
−1
the water during the repetition of the reaction, the activity of the CsPW encapsulated catalyst (0.86 mol·(acid-mol) ·min ) was
−1
−1
maintained. This activity was superior to that of bare cesium salt (0.34 mol·(acid-mol) ·min ). To our knowledge, the CsPW
encapsulated catalyst exhibits the highest activity among solid acid catalysts with respect to reaction rate per acidic proton. The
polymethylsiloxane shell plays several roles including shielding of CsPW and allowing penetration of reactants (ethyl acetate and
water) and products (ethanol and acetic acid) through the microporous membrane.
KEYWORDS: tungstophosphoric heteropoly acid, cesium salt, encapsulation, silica, water-tolerant catalyst
1
. INTRODUCTION
their supported heteropoly acid catalysts have been shown to
22−24
function as solid acids in water.
An acidic cesium salt of
The morphological design of catalysts is an important aspect for
improving their catalytic activity and selectivity. Well-defined
morphology, as exemplified by carbon fibers, layered solid
platelets, nanoporous solid films, and zeolite membranes,
has been recognized as being beneficial in the design of catalyst
supports. Hollow microspheres are also interesting as highly
designed catalysts with numerous advantages; the void space in
the particles can be used as a catalyst container, and the shell of
the particle can be regarded as a type of membrane. The shell of
the hollow microspheres has been reported to play a role in
1
2-tungstophosphoric acid (H PW O4 0 , HPW),
3
1 2
1
Cs H PW O (CsPW), is also highly active in acid-
2.5 0.5 12 40
2
−4
5,6
7
catalyzed reactions and is recognized as a water-tolerant
25
catalyst. However, leaching out and less sedimentation of
the polyoxometalates during the reactions are problematic.
8
,9
26
Inumaru et al. improved the catalytic activity of HPW
molecules by immobilizing them in the nanospaces of
27
organografted mesoporous silica. Horita et al. reported that
CsPW can be immobilized on organofunctionalized silica. A
magnetically collectable solid acid has been produced via
1
0
molecular sieving to reveal reaction selectivity and to act as a
retardant of supported metal agglomeration
ing the lifetime of a catalyst.
2
8
1
1−14
incorporation with a magnetic compound. Acid−base
interactions of heteropoly compounds with 3-aminopropyl
groups in organosilicas have been adopted to prevent leaching
of the heteropoly compounds into water. Encapsulation of
for maintain-
Solid acid catalysts are well-known materials employed in a
15
wide variety of industrial applications. Although liquid acids
such as H SO and AlCl are still utilized in the chemical
industry, the replacement of the liquid catalysts with solid acids
is desirable for the development of environmentally benign
2
9
heteropoly compounds into mesoporous molecular sieves,
a
2
4
3
30
metal−organic framework (MOF), and spherical silica
3
1,32
microspheres
has been proposed as another approach.
1
6,17
Here, we report an alternative method to immobilize CsPW;
we encapsulated the CsPW fine particles in a hollow
polymethylsiloxane microsphere with a thin shell (Scheme 1).
The synthetic approach was based on the deposition of an
processes.
Water-tolerant solid acid catalysts have been
developed to prevent the degradation of the catalytic activities
of solid acids in aqueous solutions. The highly active solid acids
in water, as reported in the literature, include H-ZSM-5
zeolite,
and Amberlyst-15), and sulfated amorphous carbons.
Heteropoly acids (polyoxometalates) are water-soluble acid
1
8,19
20
niobium oxide, ion exchange resins (e.g., Nafion
21
Received: October 14, 2013
Revised: December 1, 2013
Published: December 3, 2013
17
catalysts employed in various acid-catalyzed reactions, and
©
2013 American Chemical Society
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ACS Catalysis
Research Article
Scheme 1. Schematic Drawing of the Preparation of Polyoxometalate-Encapsulated HMS Catalysts
organosilica shell on water droplets in a water-in-oil (W/O)
mixture was allowed to stand for 5 min at room temperature.
The ultrasonic agitation was repeated five times. MTCS (1.35
g) in isooctane (30 mL) was poured into the W/O emulsion
under magnetic stirring. The mixture was stirred at room
temperature for more than 3 h to form a polymethylsiloxane
shell around aqueous droplets. During the reaction, air with
saturated water vapor (ca. 0.1 L/min) was continuously
supplied. After centrifugation, the product was washed with
isooctane and then dried at 323 K for 1 day. The dried product
33
emulsion through a sol−gel reaction of alkylsilyl trichlorides.
3
4
When a specific compound (e.g., an acid catalyst or
35,36
precursors of magnetic compounds
) is included in the
water droplets, vaporization of the water leads to encapsulation
by the organosilica shell. In the present system, droplets of an
aqueous CsPW suspension were dispersed in viscous liquid
paraffin to create a W/O emulsion with good dispersion
stability, and methyltrichlorosilane (MTCS) was subsequently
hydrolyzed on the droplets to form CsPW−polymethylsilane
microcapsules. As a result of water vaporization, CsPW fine
particles were successfully immobilized into the capsules
without the application of definite acid−base interactions.
The polymethylsiloxane shell allowed the penetration of
reactants (i.e., ethyl acetate and water) to promote the acid-
catalyzed hydrolysis reactions in water and prevented the
deactivation due to leaching of heteropoly compounds into the
aqueous phase. Water-soluble HPW was also encapsulated to
compare its reusability as a water-tolerant solid acid catalyst.
(
abbreviated as HPW-HMS) was heated at 393 K in air for 1
day. Rather than using aqueous HPW solution as the water
phase, a 0.04 M CsPW aqueous suspension (0.75 mL) was
added to enclose CsPW in HMS (abbreviated as CsPW-HMS).
HMS particles that did not contain these polyoxometalates
were also prepared by the same procedure, except that
deionized water was used as the water phase for the W/O
emulsion. The amount of heteropoly compounds in the
products was estimated from the tungsten contents in water
by inductively coupled plasma atomic emission spectroscopy
(
ICP-AES) after it was converted to aqueous form by alkali
2. EXPERIMENTAL SECTION
fusion.
2
-4. Hydrolysis of Ethyl Acetate in Aqueous Solution.
2
-1. Materials. Octyltrichlorosilane (OTCS) and MTCS
The hydrolysis of ethyl acetate was conducted in a two-neck
Pyrex flask equipped with a refluxing condenser and a
thermometer. The reaction was performed using an aqueous
solution of ethyl acetate (30 mL, 18 mmol of ethyl acetate) at
were purchased from Aldrich Chemical Co., Ltd., and Shin-Etsu
Chemical Co., Ltd., respectively. HPW, cesium carbonate,
isooctane (2,2,4-trimethylpentane), and liquid paraffin were
purchased from Wako Chemical Co., Ltd.. These chemicals
were used without further purification.
3
43 K for 2 h with stirring. The amount of polyoxometalates
−
5
used was 1.25 × 10 mol. A recycling reaction was performed
with the CsPW and HPW-HMS catalysts. After the first run,
the catalyst in the solution was recovered by filtration
2
-2. Preparation of an Acidic Cs Salt of 12-
Tungstophosphoric Acid, Cs2.5H0.5PW O (CsPW).
CsPW was prepared by titration of an aqueous solution of
HPW with an aqueous solution of Cs CO , according to the
reported procedure. The molar concentrations of the aqueous
HPW and Cs CO solutions were 0.08 and 0.10 M,
respectively. We obtained an aqueous suspension of CsPW
0.04 M) by aging the resulting mixture for at least 1 day after
the titration.
12
40
(
Millipore, Omnipore PTFE membrane, 0.2 μm). After being
2
3
37
dried at room temperature, the catalysts were used for the next
run without any activation. This reaction was repeated three
times. The reactant and product (ethanol and acetic acid) were
commercially guaranteed grade reactants (Wako Pure Chem-
icals) and were used as received. All the solutions were
prepared from Elix UV pure water (Millipore). The products
were analyzed with a gas chromatograph using a DB-WAX
capillary column (Agilent J&W).
2
3
(
2-3. Preparation of Catalysts by Encapsulating CsPW
and HPW into Hollow Polymethylsiloxane Microspher-
ical (HMS) Particles. A W/O emulsion was created by mixing
0
.01 M aqueous HPW solution (0.75 mL) and liquid paraffin
2-5. Instrument. X-ray powder diffraction (XRD) patterns
were obtained on a Rigaku RINT 2200 V/PC diffractometer
(monochromatic Cu Kα radiation), operated at 20 mA and 40
kV. Fourier transform infrared (FTIR) spectra were recorded
(
50 mL) by ultrasonic agitation. After 5 min of ultrasonic
2
irradiation (19.5 kHz, 15 W/mm , ultrasonic homogenizer, US-
600T produced by Nihon Seiki Seisakujo Co., Ltd.), the
7
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on a JASCO FT/IR-4200 spectrophotometer by the KBr pellet
method, and nitrogen adsorption−desorption isotherms were
measured at 77 K on a Belsorp-mini (BEL Japan, Inc.). Before
the adsorption experiments, the samples were heat-treated at
3
93 K under reduced pressure. Scanning electron microscopy
(
SEM) images were captured on a Hitachi S-4100 field-
emission scanning electron microscope, operated at 15 kV.
Scanning transmission electron microscopy (STEM) observa-
tions were conducted on a Hitachi High-Tech HD-2300A
scanning transmission electron microscope and a JEOL JEM-
2
010 transmission electron microscope, operated at an
acceleration voltage of 200 kV. ICP-AES was performed on a
Shimadzu ICPS-7500 spectrometer. Elemental mapping images
were obtained using a Shimadzu EPMA-1610 electron probe
microanalyzer. Gas chromatography was performed on a
Hewlett-Packard 5890 gas chromatograph equipped with a
flame ionization detector.
Figure 2. XRD patterns of the polyoxometalate-enclosed catalyst,
HMS, and polyoxometalates.
3. RESULTS AND DISCUSSION
3-1. Preparation of Catalysts. When an HPW aqueous
solution or a CsPW aqueous suspension (0.75 mL) was
Figure 1. SEM images of HPW-HMS and CsPW-HMS catalysts.
Table 1. BET Surface Areas and Pore Volumes of the
Samples
pore volume/10− cm ·g
2
3
−1
Figure 3. SEM images, bright-field TEM images, and dark-field STEM
images of (top) HPW-HMS and (bottom) CsPW-HMS.
2
−1
a
b
sample
BET surface area/m ·g
total
7.0
micropore mesopore
CsPW-
HMS
117
0.4
6.6
observed (Supporting Information, Figure S1). Thus, hydro-
lyzed MTCS was polymerized around the droplets in liquid
paraffin to produce polymethylsiloxane particles. The specific
surface areas derived from Brunauer−Emmett−Teller (BET)
HPW-
HMS
17
2.6
0.4
2.2
CsPW
HMS
157
28
16.9
0.057
10.3
0.3
6.6
5.4
a
b
plots of N adsorption isotherms (Supporting Information,
2
t-plot. Barrett−Joyner−Halenda (BJH) method.
Figure S2) are listed in Table 1. Among the tested solids,
2
CsPW-HMS exhibited a higher surface area (117 m /g),
dispersed in liquid paraffin (50 mL) by ultrasonic agitation, a
stable suspension was obtained as a W/O emulsion. The
addition of MTCS induced precipitation in both cases where
polyoxometalates were employed. SEM observations of the
dried samples (CsPW-HMS and HPW-HMS), as exemplified in
Figure 1, revealed that the samples contained spherical particles
and that a certain portion of the samples exhibited a cup-shaped
morphology. The sizes of the particles range from 0.1−3 and
indicating the formation of a microporous polymethylsiloxane
shell.
XRD patterns of the products are shown in Figure 2 together
with those of CsPW, HPW, and HMS (without the
polyoxometalates). HMS was shown to be amorphous. In the
cases of CsPW-HMS and HPW-HMS, diffraction peaks due to
the polyoxometalates were observed in their XRD patterns, and
the positions of the peaks were identical to those of bare CsPW
and HPW crystals, respectively; these results indicate that each
heteropoly compound was included in HMS as a crystal. The
amounts of HPW and CsPW included in the dried samples
0
.1−2 μm for CsPW-HMS and HPW-HMS, respectively. The
−1
FTIR absorption bands of C−H (2960 and 1450 cm ), Si−C
−1
−1
(
1270 cm ), and Si−O−Si (1100−1000 cm ) bonds were
7
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Figure 4. (a) Time courses of hydrolysis of ethyl acetate over CsPW-HMS and bare CsPW (amount of CsPW: 1.25 × 10−5 mol), (b) changes in
conversion with repeated reactions (2 h for each run), (c) change in the XRD patterns of the polyoxometalate encapsulated by HMS after the third
run of the hydrolysis reaction, and (d) TEM images captured after the third run.
Table 2. Catalytic Activity in the Hydrolysis of Ethyl Acetate in Water
reaction rate
state of the catalyst acid amount/mmol·g− per catalyst weight/μmol·g−1·min−1 per acidic proton/mol·(acid-mol)−1·min−1 recovery (%)
1
b
catalyst
CsPW-HMS
CsPW
solid
solid
solid
liquid
solid
solid
liquid
solid
0.021
0.15
0.010
1.0
18
51
0.86
0.34
0.092
0.084
0.070
0
94
0
HPW-HMS
HPW
0.92
97
0
84
28
0
a
H-ZSM-5
0.39
0.47
a
γ-Al O3
2
a
H SO4
19.8
992
0
0.046
0
2
HMS
93
a
b
Reference 25. Ratio of the mass of catalyst collected by filtration (Millipore, Omnipore PTFE membrane, 0.2 μm) to the mass of catalyst added
w/w).
(
were 3 and 7 mass%, respectively. Figure 3 shows SEM, bright-
field transmission electron microscopy (TEM), and dark-field
STEM images of both samples. The dark-field STEM images
confirmed that the core and shell were composed of
polyoxometalates and organosilica, respectively, because the
contrast of the core is greater than that of the shell. Notably,
rattle-type core−shell structures were readily observed in the
bright-field TEM images because of the thin polymethylsilox-
ane shell with a thickness of 20−30 nm. HPW existed as
relatively large crystals within the hollow particles, whereas
CsPW existed as minute particles (ca. 10 nm) on the internal
surface of HMS. The crystal sizes of HPW and CsPW in the
capsules were estimated, by Scherrer’s equation from the XRD
patterns, to be 44 and 11 nm, respectively. A majority of the
CsPW was dispersed as nanoparticles inside HMS rather than
being agglomerated. All the particles contained CsPW as
confirmed by the elemental map of tungsten (Supporting
Information, Figure S3).
3-2. Hydrolysis of Ethyl Acetate in Water. Figure 4a
shows the time course of the hydrolysis of ethyl acetate over
CsPW-HMS together with that of the hydrolysis over pristine
CsPW. Both reactions proceeded in a catalytic manner under
7
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the present conditions, as shown in this figure. The products
were only acetic acid and ethanol. Interestingly, the turnover
frequency for CsPW-HMS per unit acid site was much higher
The enfolding of CsPW into the polymethylsiloxane shell
during the MTCS polymerization may cause the production of
leaching-free CsPW catalyst in the aqueous media.
−1
(
40 molecules h ), as indicated by the slope of the line in this
figure; this turnover frequency was greater than that for the
bare CsPW by 3-fold, even when the CsPW crystals were
encapsulated by the polymethylsiloxane shell. The catalytic
activities of various solid and liquid acids in the hydrolysis of
ethyl acetate are summarized in Table 2. The activity was
estimated from the amount of acetic acid produced after 2 h of
the reaction and was normalized by unit time and amount of
4. CONCLUSIONS
CsPW nanocrystals were successfully incorporated into
polymethylsiloxane hollow microspheres without the use of
definite acid−base interactions to produce a highly active and
readily collectable heterogeneous solid acid catalyst in water.
This incorporation was achieved by the hydrolysis and
polymerization of MTCS in a W/O emulsion system composed
of liquid paraffin (oil phase) and an aqueous suspension of
CsPW (water phase). The produced microcapsules hydrolyzed
ethyl acetate in water, and the activity was superior to that over
bare CsPW with respect to the reaction rate per acidic proton.
The activity did not decrease when the reaction tests were
repeated. The polymethylsiloxane thin shell shielded the CsPW
nanoparticles against deactivation due to leaching heteropoly
compounds and acted as a microporous membrane that allowed
the penetration of the reactants (ethyl acetate and water) and
products (ethanol and acetic acid).
catalyst. Most inorganic solid acid catalysts (e.g., γ-Al O , HY
2
3
zeolite, and SiO −Al O ) have been reported to be inactive in
2
2
3
25
water with the exception of H-ZSM-5 and Nb O . HMS itself
2
5
was also inactive in the hydrolysis reaction. The activity of the
present CsPW-HMS catalyst exceeded those of other catalysts
including H SO , HPW, and HPW-HMS with respect to
2
4
reaction rate per acidic proton.
Figure 4b shows the changes in the catalytic activities of
CsPW-HMS and HPW-HMS when the reaction was repeated
three times. After the first run, the activity of HPW-HMS
gradually decreased when it was filtered and reused during the
reaction period. After the third run, HPW crystals were not
observed in the XRD pattern (Figure 4c) and the TEM image
ASSOCIATED CONTENT
Supporting Information
■
*
S
(
Figure 4d) of the catalyst. ICP analysis showed that HPW-
FTIR spectra (HMS and heteropoly compounds encapsulated
by HMS, Figure S1), nitrogen adsorption−desorption iso-
therms of HMS and heteropoly compound-enclosed HMS and
their pore size distributions (Figure S2), BEI and elemental
mapping images of Si and W for CsPW-HMS particles (Figure
S3), and SEM, bright-field TEM, and dark-field STEM images
of CsPW-mounted HMS particles (prepared by an impregna-
tion method) captured before the hydrolysis of ethyl acetate in
water and after the first run (Figure S4). This material is
available free of charge via the Internet at http://pubs.acs.org.
HMS contained 40% of the total HPW. The deactivation is
explained by leaching of HPW into the aqueous solution.
However, the activity of CsPW-HMS was almost unchanged. In
addition, the CsPW-HMS catalyst was readily recovered by
filtration from aqueous media because of the occlusion of
CsPW by hollow polymethylsiloxane microspherical particles,
although CsPW passed through the filter paper (Table 2). XRD
and TEM results (Figures 4c−d) reveal that CsPW crystals
remained in the microcapsules of HMS after the third run.
Although a half amount of HPW (5 μmol) was lost by leaching
after the first run, as confirmed by the ICP analysis, 3% of the
encapsulated heteropoly compounds in CsPW-HMS (5 μmol)
AUTHOR INFORMATION
Corresponding Author
T. Okada. Tel: +81-26-269-5414. Fax: +81-26-269-5424. E-
mail: tomohiko@shinshu-u.ac.jp.
■
*
3
8−40
was lost, probably due to leaching HPW in CsPW.
In the
case where CsPW was mounted on the external surface of HMS
by an impregnation method, no activity was observed after the
first run due to the lack of CsPW on HMS (results of STEM
observations are summarized in the Supporting Information,
Figure S4). Thus, the polymethylsiloxane microcapsules serve
as good reaction containers for solid acid catalysis by enclosing
water-tolerant ultrafine CsPW particles.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by JSPS (Grant-in-Aid for
Scientific Research, Challenging Exploratory Research
#23655143). Shinshu University also supported us financially
(Funding Program for Green Innovation).
To our knowledge, the catalytic activity in the CsPW-HMS
−1
−1
system (0.86 mol·(acid-mol) ·min ) is the highest among
known solid acid catalysts. Because the polymethylsiloxane shell
formed around the aqueous droplets of the CsPW suspension,
the CsPW crystals may enfold around the aqueous droplets
during the polymerization of the hydrolyzed MTCS. The larger
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dx.doi.org/10.1021/cs400923q | ACS Catal. 2014, 4, 73−78