Microporous acidic cesium salt of 12-tungstosilicic acid Cs 3HSiW12O40 as a size-selective solid acid catalyst

Article abstract of DOI:10.1246/cl.2010.881

An acidic Cs salt of H₄SiW₁₂O₄O ₄₀ (Cs₃HSiW₁₂O₄₀), which was prepared by titrating an aqueous solution of H₄SiW₁₂O ₄₀ with an aqueous solution of Cs₂CO₃, possessed only micropores and exhibited size-selective catalysis for acid-catalyzed reactions in liquid phase.

Full text of DOI:10.1246/cl.2010.881

doi:10.1246/cl.2010.881
Published on the web July 14, 2010
881
Microporous Acidic Cesium Salt of 12-Tungstosilicic Acid Cs HSiW O
3
12 40
as a Size-selective Solid Acid Catalyst
1
2
2
2
2
2
1
Yuichi Kamiya,* Shogo Sano, Yu-ki Miura, Yohei Uchida, Yuki Ogawa, Yukari Iwase, and Toshio Okuhara
1
Research Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Hokkaido 060-0810
2
Graduate School of Environmental Science, Hokkaido University, Sapporo, Hokkaido 060-0810
(
Received May 17, 2010; CL-100472; E-mail: kamiya@ees.hokudai.ac.jp)
An acidic Cs salt of H4SiW12O40 (Cs3HSiW12O40), which
was prepared by titrating an aqueous solution of H4SiW12O40
with an aqueous solution of Cs CO , possessed only micropores
We prepared acidic Cs salts, Cs3HSiW12O40, by titrating an
¹
3
aqueous solution of H4SiW12O40 (0.08 mol dm ) with an
¹
3
aqueous solution of Cs CO (0.10 mol dm ). In the preparation,
2
3
2
3
and exhibited size-selective catalysis for acid-catalyzed reactions
in liquid phase.
the addition of aqueous Cs2CO3 solution was carefully con-
3
trolled by using an automatic syringe pump: first, 2 cm was
3
¹1
added at a rate of 0.033 cm min , and then the remaining
3
¹1
solution was added at a rate of 0.2 cm min . The resulting
colloidal solution was allowed to stand for 48 h at room
temperature and then evaporated to obtain Cs3HSiW12O40 as a
white solid. If one added the aqueous Cs CO solution to
Crystalline microporous oxides, such as zeolites, are useful
materials for various applications, including catalysts, ion-
1
exchange materials, adsorbents, and molecular sieves. In
2
3
catalysis, microporous oxides are able to act as size-selective
catalysts due to their constrained pore size, and zeolites are
typical examples of size-selective solid acid catalysts.2 How-
ever, their applications are limited because the acid strengths are
relatively weak. Thus, a variety of microporous oxides other
than zeolites have been synthesized for potential use as solid
the aqueous solution of H4SiW12O40 at a constant rate of
3
¹1
0.2 cm min from the beginning of the titration, fine particles
with large external surface area were formed, which had adverse
effects in the size-selective catalysis. Thus, the careful control of
the addition of the aqueous Cs2CO3 solution is very important.
­4
2
1
Catalytic reactions (Figure S1 ) were performed in a three-
5­9
3
acid catalysts.
neck flask (100 cm ) after the catalysts were pretreated in a He
Heteropolyacids are oxide clusters with well-defined basic
structures. They are fascinating and important catalysts from a
practical standpoint because they are strong acids and are
flow at 473 K for 0.5 h. The reactions were carried out using
isopropyl actetate (8 mmol) in n-decane (140 mmol) at 373 K,
cyclohexyl acetate (14 mol) in n-nonane (112 mol) at 343 K, tert-
butyl acetate (20 mmol) in n-decane (125 mmol) at 313 K, ¡-
pinene (0.64 mmol) in toluene (97 mmol) at 273 K, and cyclo-
hexane (10 mmol) in 1,3,5-trimethylbenzene (216 mmol) at
363 K.
much more active than zeolites, such as H-ZSM-5, in
various acid-catalyzed reactions.1
0­13
Among heteropolyacids,
the Keggin-type heteropolyacid H PW O and its acidic Cs
3
12 40
salts have been extensively studied because of their high
stability, strong acidity, and high surface area.1
0,11,14
It has been
As shown in Figure 1, the changes in the surface area as a
function of Cs content in Cs H SiW O were remarkable.
reported that Cs2.1H0.9PW O (abbreviated as Cs2.1-P) is
12
40
x
4¹x
12 40
1
5­17
microporous
and that this acts as a size-selective catalyst
The surface areas were estimated on the basis of a Langmuir plot
obtained from an N2 adsorption isotherm at 77 K because all of
for acid-catalyzed reactions.15 In addition, its platinum-modi-
fied form shows size-selective catalytic activity toward the
hydrogenation of alkenes and the complete oxidation of
3
00
8
0
0
0
1
8
alkanes.
In contrast, there are only a few reports involving Cs salts of
H SiW O , though H SiW O is a Keggin-type heteropoly-
4
4
12 40
4
12 40
19
acid similar to H3PW12O40. Moffat et al. have reported that
a neutral Cs salt of H4SiW12O40, Cs4SiW12O40, is microporous
with a high surface area (150 m g ), but the size of its
micropores is uncertain. Blanco and co-workers have reported a
synthesis of acidic Cs salts of H4SiW12O40, Cs3.4H0.6SiW12O40
and Cs3.8H0.2SiW O , and their acid-catalytic activity toward
phenol tetrahydropyranylation. However, the salts that they
prepared have mesopores as well as micropores, and thus, size-
selective catalytic reactions over them have not been attempted.
In this study, we synthesized microporous Cs HSiW O with
200
0
0.5
1
2
¹1
P/P0
12
40
100
0
20
3
12 40
little mesopores and a high surface area. Here we report that
Cs3HSiW12O40 exhibits size-selective catalytic activity toward
acid-catalyzed reactions in a liquid­solid reaction system. Since
the formation mechanism of the micropores in Cs3HSiW12O40 is
completely different from that in Cs2.1-P (discussed later), the
pore sizes are different between the two, leading to different
size-selective catalytic properties.
0
1
2
3
4
x in Cs H SiW O
12 40
x
4-x
Figure 1. Surface area of Cs H SiW O . The surface area
x
4¹x
12 40
was determined from an N adsorption isotherm at 77 K after
2
pretreatment of the catalyst at 473 K in a flow of N . Inset: N
2
2
adsorption and desorption isotherm for Cs3HSiW12O40 at 77 K.
Chem. Lett. 2010, 39, 881­883
© 2010 The Chemical Society of Japan www.csj.jp/journals/chem-lett/

8
82
Decomposition:
10
10
(3.8)
OCOCH3
(
4.2)
(
14)
OCOCH3
(
22)
(
12)
OCOCH3
(
30)
Isomerisation:
(
54)
0.4 0.6 0.8 1.0
2
4
6
8
10
(
530)
Pore diameter/nm
Alkylation:
(
5)
Figure 2. Micropore and mesopore size distributions for
Cs3HSiW12O40 (solid line) and micropore size distribution for
Cs2.1-P (dotted line). Micropore size distribution was deter-
mined by using the Saito­Foley method on an Ar adsorption
isotherm, and mesopore size distribution was determined by
using the Dollimore­Heal method on an N2 desorption isotherm.
(
47)
+
0.0
0.2
0.4
0.6
0.8
1.0
Relative activity
Figure 3. Relative activities of Cs3HSiW12O40 toward various
kinds of reactions to those of Cs2.5H0.5PW O (Cs2.5-P) in the
the Cs salts were microporous. The surface area increased
significantly when the Cs content (x) exceeded 1.5 and reached
maximum at x = 3 (239 m g ), although it decreased slightly
when x > 3.
12
40
liquid­solid reaction system. The catalytic activity of Cs2.5-P
for each reaction is set to unity. The figures in the parentheses
2
¹1
¹
1 ¹1
are the reaction rates in the unit of mmol g
h .
As shown in the inset of Figure 1, Cs3HSiW12O40 which
had the highest surface area gave a types I isotherm of N2
adsorption­desorption at 77 K, indicating that this material was
microporous. The total and external surface area were estimated
+
+
(Cs and H ) ratio must be one third, while the actual ratio was
one fourth due to the charge balance. Thus it would be expected
that quarter heteropoly anions were defective, in other words,
anion vacancies were present in the crystallites. Therefore, these
anion vacancies formed the micropores of Cs3HSiW12O40. In
contrast to Cs3HSiW12O40, the crystallites of Cs2.1-P were
2
¹1
from the slopes of two lines in a t-plot to be 191 and 7 m g
respectively (Figure S2 ). The external surface area corre-
sponded to less than 4% of the total surface area.
,
21
1
6
In order to determine the micropore-size distribution of
Cs3HSiW12O40, we acquired an Ar adsorption isotherm at 87 K.
Figure 2 shows pore-size distributions of the micropores as well
as mesopores of Cs HSiW O . The micropore and mesopore
nonporous. Thus its micropores are considered to be inter-
particle voids between the crystallites. The difference in the
formation mechanism of the micropores results in micropore
sizes between the two, though the size of heteropoly anions are
very similar each other. Schlögl and co-workers have proposed a
similar micropore structure for Cs4PMo11VO40 to Cs3HSiW12-
3
12 40
size distributions were determined from the Ar adsorption
isotherm by using the Saito­Foley method22 and from an N2
2
3
24
adsorption isotherm by using the Dollimore­Heal method,
O40. However, at present, there is no direct evidence for the
respectively. As a reference, micropore-size distribution of
Cs2.1-P is put in the figure. In the micropore-size distribution
for Cs3HSiW12O40, an intense peak at 0.53 nm and a shoulder at
around 0.69 nm were observed. On the other hand, only a small
amount of the mesopores was present.
structure of the micropores in the crystallites of Cs HSiW O .
Thus, further investigations are needed.
3
12 40
Figure 3 shows the relative catalytic activity of Cs3HSiW12-
O40 for each reaction in relation to those of Cs2.5H0.5PW O
12
40
(abbreviated as Cs2.5-P). Cs2.5-P has been shown to have
1
7
The peak position for Cs2.1-P was similar to that for
Cs HSiW O , but the pore-size distribution was narrow and
micropores with sizes of more than 0.75 nm and mesopores
1
6
with sizes of ca. 4 nm and does not exhibit any size-selective
3
12 40
1
5
the larger-size micropores were nonexistence in it.
The size of the crystallites of Cs3HSiW12O40 was estimated
to be about 49 nm from the line width in the X-ray diffraction
catalytic activity due to its large pore size. As seen in Figure 3,
Cs2.5-P catalyzed all of the reactions with considerable
activities (the reaction rates are shown in the parentheses in
Figure 3). On the other hand, although Cs3HSiW12O40 was as
active as Cs2.5-P toward the decomposition of isopropyl acetate,
the relative activity decreased in the following order: decom-
position of isopropyl acetate > decomposition of cyclohexyl
acetate > decomposition of tert-butyl acetate > isomerization of
¡-pinene µ alkylation of 1,3,5-trimethylbenzene. Cs3HSiW12-
O40 showed very low relative activity for the isomerization of ¡-
pinene and alkylation of 1,3,5-trimethylbenzene. After the
reactions of the decomposition of isopropyl acetate, Cs3HSiW12-
O40 was recovered by a simple filtration from the mixture and
dried at 373 K. The recovered Cs3HSiW12O40 showed similar
(XRD) pattern. Assuming the external surface is only the surface
of Cs3HSiW12O40, the surface area was calculated from the
2
¹1
density and the particle size to be 19 m g . However, this value
was significantly smaller than the actual surface area (239
2
¹1
m g ). This great difference can be explained if micropores are
present inside the crystallites of Cs3HSiW12O40. Cs3HSiW12O40
showed a XRD pattern due to body-centered cubic (bcc)
21
structure (Figure S3 ), in which the locations of heteropoly
4¹
anion (SiW12O40 ) are allowable in the center and each corner
+
+
of the unit cell, and that of Cs or H is between two heteropoly
anions. To construct the bcc structure, heteropoly anion to cation
Chem. Lett. 2010, 39, 881­883
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

8
83
activitiy to the fresh one for the reactions. Critical sizes of the
molecules were estimated from van der Waals radii and a
molecular model obtained by using MM2 calculations to be
the following: isopropyl acetate (0.52 nm) < cyclohexyl
acetate (0.53 nm) < tert-butyl acetate (0.54 nm) < ¡-pinene
York, 1989.
T. F. Degnan, Jr., J. Catal. 2003, 216, 32.
M. Sadakane, K. Kodato, T. Kuranishi, Y. Nodasaka, K.
Sugawara, N. Sakaguchi, T. Nagai, Y. Matsui, W. Ueda,
Angew. Chem., Int. Ed. 2008, 47, 2493.
4
5
(
0.61 nm) < 1,3,5-trimethylbenzene (0.77 nm). Although the
6
X.-F. Shen, Y.-S. Ding, J. Liu, J. Cai, K. Laubernds, R. P.
Zerger, A. Vasiliev, M. Aindow, S. L. Suib, Adv. Mater.
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critical size of tert-butyl acetate was similar to that of cyclohexyl
acetate, the cross-sectional area of the former (0.54 nm ©
0
0
.46 nm) was much larger than that of the latter (0.53 nm ©
.37 nm). Therefore, the catalytic activity of Cs HSiW O can
7
8
Q. Feng, H. Kanoh, K. Ooi, J. Mater. Chem. 1999, 9, 319.
K. Inumaru, H. Nakajima, T. Ito, M. Misono, Chem. Lett.
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M. Fisch, T. Armbruster, B. Kolesov, J. Solid State Chem.
2008, 181, 423.
3
12 40
be understood if the catalyst differentiates the reactant molecules
according to their sizes; that is, it is size-selective.
9
In the case of the Csx-P system, microporous Cs2.1-P
1
5
showed reactant size selectivity in the acid-catalyzed reactions.
10 T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 1996, 41,
113.
11 I. V. Kozhevnikov, Catal. Rev., Sci. Eng. 1995, 37, 311.
12 C. L. Hill, C. M. Prosser-McCartha, Coord. Chem. Rev.
1995, 143, 407.
Cs2.1-P exhibited an activity for the dehydration of 2-hexanol
but was inactive for the decomposition of isopropyl acetate due
to the pore-size limitation.15 On the other hand, as seen in
Figure 3, Cs3HSiW12O40 showed reasonably high activity for
the decomposition of isopropyl acetate. This indicates that the
pore size of Cs HSiW O is larger than that of Cs2.1-P, which
13 N. Mizuno, M. Misono, Curr. Opin. Solid State Mater. Sci.
1997, 2, 84.
3
12 40
is consistent with the pore-size distributions shown in Figure 2.
Therefore, Cs3HSiW12O40 is a novel example of a heteropoly-
acid catalyst showing size selectivity in reactions with larger
sized reactants.
14 Y. Kamiya, T. Okuhara, M. Misono, A. Miyaji, K. Tsuji, T.
Nakajo, Catal. Surv. Asia 2008, 12, 101.
15 T. Okuhara, T. Nishimura, M. Misono, Chem. Lett. 1995,
155.
Typical 12-membered oxygen ring zeolites, such as H-¢,
showed only one tenth of the activity of Cs3HSiW12O40 for the
decomposition of isopropyl acetate under the present reaction
conditions. Since the pore size of H-¢ (0.76 nm © 0.64 nm) is
larger than the critical size of isopropyl acetate, the low activity
was due to the weaker acid strength of the zeolites. Therefore,
the strong acidity of Cs3HSiW12O40 greatly contributes to the
high activity toward the decomposition of isopropyl acetate and
cyclohexyl acetate. We wish to emphasize that the much larger
micropores and strong acidity of Cs3HSiW12O40 expand the
possible applications of microporous solid acids.
16 T. Yamada, Y. Yoshinaga, T. Okuhara, Bull. Chem. Soc. Jpn.
1998, 71, 2727.
17 T. Yamada, K. Johkan, T. Okuhara, Microporous Mesopo-
rous Mater. 1998, 26, 109.
18 Y. Yoshinaga, K. Seki, T. Nakato, T. Okuhara, Angew.
Chem., Int. Ed. Engl. 1997, 36, 2833.
19 D. B. Taylor, J. B. McMonagle, J. B. Moffat, J. Colloid
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20 L. R. Pizzio, M. N. Blanco, Microporous Mesoporous Mater.
2007, 103, 40.
21 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
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2
3
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Chem. Lett. 2010, 39, 881­883
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/