Keggin-type mono lacunary silicotungstate supported onto zirconia: Synthesis, characterization, and esterification reaction

Article abstract of DOI:10.1080/00958972.2010.531274

A solid acid catalyst comprising of lacunary silicotungstate and zirconia was synthesized and characterized by physicochemical techniques. Surface morphologies of support and catalyst were studied by scanning electron microscopy. Catalytic properties were evaluated for the esterification of n-butanol with acetic acid. Esterifications of sec-butanol and isobutanol with acetic acid as well as the esterification of n-butanol with different acids such as formic acid and propionic acid were carried out under optimized conditions. Catalytic activities for calcined as well as recycled catalysts were also evaluated for the esterification under optimized condition.

Full text of DOI:10.1080/00958972.2010.531274

This article was downloaded by: [Australian National University]
On: 18 January 2015, At: 06:54
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Coordination Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gcoo20
Keggin-type mono lacunary
silicotungstate supported onto zirconia:
synthesis, characterization, and
esterification reaction
a
a
Soyeb Pathan & Anjali Patel
a
Department of Chemistry, Faculty of Science , M. S. University
of Baroda , Vadodara – 390002, Gujarat, India
Published online: 20 Nov 2010.
To cite this article: Soyeb Pathan & Anjali Patel (2010) Keggin-type mono lacunary silicotungstate
supported onto zirconia: synthesis, characterization, and esterification reaction, Journal of
Coordination Chemistry, 63:23, 4041-4049, DOI: 10.1080/00958972.2010.531274
To link to this article: http://dx.doi.org/10.1080/00958972.2010.531274
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“
Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Journal of Coordination Chemistry
Vol. 63, No. 23, 10 December 2010, 4041–4049
Keggin-type mono lacunary silicotungstate supported onto
zirconia: synthesis, characterization, and esterification reaction
SOYEB PATHAN and ANJALI PATEL*
Department of Chemistry, Faculty of Science, M. S. University of Baroda,
Vadodara – 390002, Gujarat, India
(
Received 19 June 2010; in final form 22 September 2010)
A solid acid catalyst comprising of lacunary silicotungstate and zirconia was synthesized and
characterized by physicochemical techniques. Surface morphologies of support and catalyst
were studied by scanning electron microscopy. Catalytic properties were evaluated for the
esterification of n-butanol with acetic acid. Esterifications of sec-butanol and isobutanol with
acetic acid as well as the esterification of n-butanol with different acids such as formic acid and
propionic acid were carried out under optimized conditions. Catalytic activities for calcined as
well as recycled catalysts were also evaluated for the esterification under optimized condition.
Keywords: Lacunary silicotungstate; Keggin; Heterogeneous catalysis; Esterification
1. Introduction
Lacunary polyoxometalates (LPOMs) represent an important class of compounds due
to their unique structural as well as chemical properties. LPOMs can be obtained by
removing one or two of the MO units from the fully occupied polyoxometalates
VI
nꢀ
VI
(nþ4)ꢀ
POMs), [XM O ] , giving rise to mono- and di-lacunary POMs, [XM O ]
11 39
(
1
2
40
(nþ5)ꢀ
36
VI
and [XM O ]
, respectively [1]. LPOMs can be used for applications in medicinal
1
0
chemistry [2], solid-state chemistry [3], electrocatalysis [4], and catalysis [5–7].
Among LPOMs, lacunary silicotungstate has been well explored by various groups
[8–14]. The catalytic activity of the LPOMs including monolacunary silicotungstate has
been reported by Hill and co-workers [5], who reported catalytic systems for the
effective and selective aerobic oxidation of H S to S. Detailed studies have been
2
carried out on oxidation reactions using dilacunary silicotungstates by a number of
groups [15–21]. Acid catalysis over divacant silicotungstates was also reported by
Yoshida et al. [20].
Few reports are available on the incorporated lacunary silicotungstates. Hu et al. [22]
reported the synthesis, characterization, and photochemical properties of ordered
*
Corresponding author. Email: aupatel_chem@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.531274

4
042
S. Pathan and A. Patel
macroporous hybrid silica materials based on mono-vacant silicotungstate. Guo et al.
[
divacant Keggin [(n-C H ) N] [ꢀ-SiW O (H O) ].
23] reported the catalytic oxidation of styrene with H O by the SBA-15-incorporated
2 2
4
9 4
4
10 34
2
2
No report is available on the study of mono-lacunary silicotungstate supported on
metal oxides. Recently, our group reported detailed study on supported mono-lacunary
phosphotungstate [24, 25]. The obtained results, especially for acid-catalyzed reactions,
are superior, encouraging us to extend our work for lacunary silicotungstate.
In this article we report the synthesis, characterization, and catalytic evaluation of
lacunary silicotungstates supported on to zirconia. A series of catalysts containing
10–40% loading of lacunary silicotungstates onto ZrO2 were synthesized and
characterized by various physicochemical techniques, such as chemical stability,
elemental analysis, thermo-gravimetric analysis (TGA), differential scanning calorim-
etry (DSC), Fourier transform infrared (FT-IR), diffuse reflectance spectroscopy
(DRS), and surface area measurement (BET method). Further, the surface morphol-
ogies of support and catalyst were studied by scanning electron microscopy (SEM).
Their catalytic properties were evaluated for the esterification of n-butanol with acetic
acid. The esterifications of sec-butanol and isobutanol with acetic acid as well as the
esterification of n-butanol with different acids were carried out under optimized
ꢁ
ꢁ
ꢁ
conditions. Further, the selected material was calcined at 200 C, 300 C, and 400 C.
Catalytic properties for calcined as well as recycled catalysts were evaluated for
esterification under optimized condition.
2
2
. Experimental
.1. Materials
All chemicals used were of A.R. grade. Sodium silicate (Loba chemie, Mumbai),
sodium tungstate (S.D. Fine Chem, Mumbai), and zirconium oxychloride (Loba
chemie, Mumbai) were used as received. Ethanol, n-butanol, sec-butanol, isobutanol,
formic acid, glacial acetic acid, propionic acid, and liquid ammonia were obtained from
Merck and used as received.
2
.2. Synthesis of the catalyst
Synthesis of the catalyst was carried out in three steps.
2.2.1. Synthesis of hydrous zirconia (ZrO2). The first step involves the synthesis of the
support, (ZrO ), synthesized by the method reported by Bhatt and Patel [26].
2
2
.2.2. Synthesis of lacunary silicotungstate (Na [SiW O ] 11H O). The second step
11 39 2
.
8
involves the synthesis of lacunary silicotungstate. Sodium tungstate (0.22 mol, 7.2 g) and
sodium silicate (0.02 mol, 0.56 g) were dissolved in 150–200 mL of doubly-distilled water
ꢁ
and heated to 80–90 C followed by the addition of concentrated nitric acid to adjust the
pH to 4.8. The volume was then reduced to half by evaporation and the heteropoly
anion was separated by liquid–liquid extraction with 80–100 mL of acetone.

Silicotungstate
4043
ꢀ
The extraction was repeated until the acetone extract showed the absence of NO3
(ferrous sulfate test). The extracted sodium salt was dried in air. The resulting material
is designated as SiW .
1
1
2.2.3. Supporting SiW11 onto ZrO2. The third step involves the supporting of SiW11
onto ZrO . A series of catalysts containing 10–40% SiW11 were synthesized by
2
impregnating 1 g of ZrO with an aqueous solution of SiW (0.1–0.4 g/10–40 mL of
2
11
ꢁ
doubly distilled water) and dried at 100 C for 10 h. The obtained materials were
designated as (SiW ) /ZrO , (SiW ) /ZrO , (SiW ) /ZrO , and (SiW ) /ZrO . The
1
1 1
2
11 2
2
11 3
2
11 4
2
ꢁ
best catalyst, [(SiW ) /ZrO ], was calcinated at 200, 300, and 400 C in air for 5 h and
1
1 2
2
designated as (SiW ) /ZrO , (SiW ) /ZrO , and (SiW ) /ZrO , respectively.
1
1 22
2
11 23
2
11 24
2
2
.3. Characterization
Different mineral acids and alcohols were used for checking the chemical stability of the
material. The estimation of tungsten [27] as well as silicon [28] was carried out
gravimetrically. TGA curve of the sample was obtained using the SEIKO Thermal
Analysis System SS 5100 TG/DTA 32. Different mineral acids and alcohols were used
for checking the chemical stability of the material. The DSC of the sample was
performed on a METTLER STAR SW 8.10. The FT-IR spectrum of the sample was
obtained using a KBr wafer on a Perkin-Elmer instrument. The UV-Vis-DR spectrum
was recorded at ambient temperature on a Perkin-Elmer 35 LAMDA instrument using
a 1 cm quartz cell. The adsorption–desorption isotherms of samples were recorded on
ꢁ
a micrometics ASAP 2010 surface area analyzer at ꢀ196 C. From the adsorption–
desorption isotherms, specific surface area was calculated using the BET method. The
surface morphologies of the support and catalyst were studied by SEM using a Jeol
SEM instrument (model-JSM-5610LV) with scanning electron electrode at 15 kV.
Scanning was done at 1 mm range.
2
.4. Catalytic reaction
The synthesized catalyst was evaluated for esterification reactions. The esterification
reaction of n-butanol with acetic acid (mole ratio 1 : 4.4, 9.16 : 25.2 mL) was carried out
in a 50 mL glass reactor provided with a double-walled air condenser, Dean–Stark
ꢁ
apparatus, magnetic stirrer, and a guard tube. The resultant mixture was heated at 80 C
for 4 h. Dean–Stark apparatus was attached to a round-bottom flask to separate the
water formed during the reaction. Similar reactions were carried out with different %
loading of SiW11 onto ZrO , different alcohol (n-butanol) to acid (acetic acid) ratio, and
2
different amount of catalyst. Under optimized conditions, esterification was carried out
for calcined as well as recycled catalysts. The esterification of sec-butanol (9.15 mL) and
isobutanol (9.27 mL) with acetic acid (25.2 mL) as well as the esterification of n-butanol
(9.16 mL) with formic acid (16.9 mL) and propionic acid (32.9 mL) were also carried out
under optimized conditions. The product ester was analyzed on a (Nucon-5700) gas
chromatograph using Carbowax 20 column.

4
044
. Results and discussion
S. Pathan and A. Patel
3
Leaching of the active species from the support makes the catalyst unattractive and
hence it is necessary to study the stability as well as leaching of SiW11 from the support.
HPA can be quantitatively characterized by the heteropoly blue color, which is
observed when it reacted with a mild reducing agent such as ascorbic acid. In the
present study, this method was used for determining the leaching of SiW11 from the
support. Standard samples amounting to 1–5% of SiW11 in water were prepared. To
1
0 mL of the above samples, 1 mL of 10% ascorbic acid was added. The mixture was
ꢀ1
diluted to 25 mL. The resultant solution was scanned at a ꢁmax of 785 cm for its
absorbance values. A standard calibration curve was obtained by plotting values of
absorbance with percentage solution. One gram of (SiW ) /ZrO with 10 mL n-butanol
1
1 2
2
was refluxed for 4 h. Then 1 mL of the supernatant solution was treated with 10%
ascorbic acid. No development of blue color indicates no leaching. The same procedure
was repeated with water, isobutanol, sec-butanol, formic acid, acetic acid, propionic
acid, and with the filtrate of the reaction mixture after reaction. The above procedure
was followed for all catalysts and no leaching was found. The study indicates the
presence of chemical interaction between the SiW11 and the ZrO , as well as the stability
2
of resulting catalysts under reaction conditions.
The chemical stability of the materials has been checked in different mineral acids
ꢀ
1
such as HCl, H SO , HNO and bases such as NaOH, Na CO up to 4 mol L
2
4
3
2
3
concentration. The present material shows no changes in color or form, indicating their
stability.
The sodium salt of lacunary silicotungstate has been analyzed for Si and W
gravimetrically. The results obtained are in good agreement with theoretical values.
Theoretical: W ¼ 65.4%; Si ¼ 0.91%; observed: W ¼ 63.8%; Si ¼ 0.89%. The number of
water molecules, determined from TGA curve (figure S1), was 11.
From the elemental as well as the thermal analysis, the chemical formula of the
isolated complex is proposed as Na [SiW O ]ꢂ11H O. TGA shows that SiW is less
8
11 39
2
11
ꢁ
ꢁ
stable (350 C) than (SiW ) /ZrO (475 C). This shows that supporting of SiW on the
1
1 2
2
11
surface of ZrO increases the thermal stability of SiW . An increase in thermal stability
11
2
indicates the presence of chemical interaction between SiW11 and ZrO2.
The DSC thermogram of SiW11 shows an endothermic peak in the range of 80–180 C
ꢁ
(figure S2), indicating loss of adsorbed water molecules. The DSC for the supported
catalyst [(SiW ) /ZrO ] shows an endothermic peak at 50–100 C (figure S2), indicating
ꢁ
1
1 2
2
ꢁ
loss of adsorbed water; the endothermic peak at 475 C indicates the decomposition of
ꢁ
the supported species. The above study indicates that SiW11 is thermally stable to 475 C
when supported onto ZrO2.
The FT-IR spectra of ZrO (figure S3) shows a broad band in the region of
2
ꢀ
1
3
400 cm , attributed to asymmetric hydroxo (–OH) and aquo (H O) stretches. Two
2
ꢀ
1
types of bending vibrations at 1600 and 1370 cm are observed indicating the presence
of (H–OH) and (O–H–O) bending, respectively. The FT-IR spectra of SiW (figure S3)
1
1
ꢀ
1
shows bands at 886, 795, and 727 cm corresponding to W–O–W stretches and at
ꢀ1
ꢀ1
9
87 cm to Si–O stretches. The stretching vibration at 948 cm corresponds to W¼O.
These bands are in good agreement with those reported in [15]. These vibrational bands
are also observed in the synthesized materials [(SiW ) /ZrO ] confirming the presence
of these groups in the synthesized materials.
1
1 2
2

Silicotungstate
Table 1. Surface area for ZrO
4045
2
, (SiW11
)
1
/ZrO
2
, (SiW11
)
2
/ZrO
2
,
(
SiW11
Catalyst
ZrO
)
3
/ZrO
2
, and (SiW11
)
4
/ZrO
2
.
2
Surface area (m g
ꢀ1
)
2
170
269
325
341
240
(
(
(
(
SiW11
)
)
)
)
1
/ZrO
2
/ZrO
3
/ZrO
4
/ZrO
2
2
2
2
SiW11
SiW11
SiW11
The DRS gives information about the non-reduced heteropoly anion due to charge
transfer from oxygen to metal. DRS of SiW11 and (SiW ) /ZrO (figure S4) show
11 2
2
absorption maxima at 200–210 and 260–268 nm, attributed to oxygen–tungsten CT at
W¼O and W–O–W bonds, respectively. These values are in good agreement with values
reported in [15], confirming the presence of the undegraded SiW in the catalyst.
1
1
The values for BET surface area measurements are shown in table 1. The larger
surface area of all catalysts as compared to that of the support was because of the
supporting of SiW , as expected. Initially the value for surface area increases with
1
1
increase in loading from 10% to 30% while it decreases from 30% to 40%. This may be
due to the formation of multilayers of active species, SiW , onto ZrO , which may
1
1
2
cause blocking/stabilization of active sites on the monolayer.
The SEM of ZrO and (SiW ) /ZrO are shown in figure S5. The surface of the
2
11 2
2
support is distinctly altered, exhibiting considerable surface shining after supporting of
the SiW11 (figure S5c). Picture taken at higher magnification (figure S5d, 500ꢃ) clearly
shows the alternation and shining of the surface.
3
.1. Catalytic study
Esterification is a straightforward reaction subjected to general Bronsted acid catalysis.
The esterification reactions of different alcohols with acetic acid and different acids
with n-butanol have been carried out. The yields can be increased by increasing the
concentration of either alcohol or acid. In practical situation when one wants to prepare
an ester in maximum yield, the reactant that is less expensive of the two is taken
in excess.
3.1.1. Effect of % loading of SiW11 onto ZrO2. The esterification reaction was carried
out with unsupported SiW11 (active amount 25 mg) as well as with catalysts having
different % loading of SiW11 onto ZrO . Results are shown in figure 1. The % yield
2
increases with increase in the % loading of SiW11 from 10% to 30%. Further increase in
the % loading from 30% to 40% decreases the % yield. The decrease in % yield may be
correlated with the values of surface area (table 1). Hence, a decrease in % yield of
n-butyl acetate is as expected. The catalyst containing 20% loading [(SiW ) /ZrO ]
1
1 2
2
was used for detailed study.

4
046
S. Pathan and A. Patel
%
Yield of BA
Effect of % loading
100
8
0
0
0
0
6
4
2
0
/
ZrO
/ZrO
/ZrO
)
SiW
/ZrO
(
)
)
)
SiW
(SiW
(SiW
(SiW
%
Loading
Figure 1. Effect of % loading of SiW11 onto ZrO
ꢁ
2
on the esterification of n-butanol; reaction time: 4 h;
temperature: 80 C; amount of catalyst: 0.15 g; 1 : 4.4 mole ratio.
2 2
Table 2. Effect of mole ratio on % yield using (SiW11) /ZrO
as catalyst.
Mole ratio of alcohol to acid
% Yield
1
1
: 2
: 4.4
63
83
ꢁ
Amount of catalyst: 0.15 g; reaction time: 4 h; temperature: 80 C; 1 : 4.4
mole ratio.
3
.1.2. Effect of mole ratio of alcohol and acid. The reaction was carried out by varying
ꢁ
mole ratio of n-butanol to acetic acid, with 0.15 g of the catalyst for 4 h at 80 C. It is
observed from the table 2 that with the increase in the concentration of acid the %
yields of products increase. So all reactions are carried out in 1 : 4.4 molar ratio of
reactants.
3
.1.3. Effect of amount of catalyst. The reaction was carried out by varying the
amount of catalyst and the % yields are shown in figure 2. The activity increases with
an increase in the amount of catalyst from 0.10 to 0.25 g. For further increase in the
amount of catalyst, the % yield decreases and then remains almost the same. This may
also be due to the blocking of the active sites as after achieving a certain amount, the
catalyst may behave as a bulk.
The optimum conditions for maximum yield of butyl acetate were with the mole ratio
ꢁ
of n-butanol to acetic acid as 1 : 4.4, with 0.15 g of catalyst and 4 h reaction time at 80 C.
ꢁ
ꢁ
3.1.4. Esterification of 1 and 2 alcohols with acetic acid and esterification of various
acids with n-butanol using (SiW ) /ZrO as catalyst. The reactions were carried out
under optimized conditions described earlier and the % yields of various esters, n-butyl
1
1 2
2

Silicotungstate
4047
Effect of amount of catalyst
100
%
Yield of BA
8
0
0
0
0
6
4
2
0
0.1
0.15
0.25
0.5
0.6
0.7
Amount of catalyst (g)
Figure 2. Effect of the amount of catalyst on the esterification of n-butanol; reaction time: 4 h; temperature:
ꢁ
80 C; 1 : 4.4 mole ratio.
100
8
6
4
2
0
0
0
0
0
n-BF
n-BA
n-BP
sec-BA
iso-BA
Different esters
ꢁ
Figure 3. % Yields of different esters; amount of catalyst: 0.15 g; reaction time: 4 h; temperature: 80 C;
: 4.4 mole ratio.
1
formate (n-BF), n-butyl acetate (n-BA), n-butyl propionate (n-BP), sec-butyl acetate
sec-BA), and isobutyl acetate (iso-BA), are shown in figure 3.
(
3
.2. Effect of calcination temperature on [(SiW ) /ZrO ]
11 2 2
The esterification of n-butanol was carried out with different calcinated samples under
optimized conditions. The results are shown in table S1. On calcination, there was no
appreciable change in % yield of butyl acetate, indicating that the catalyst is stable up
ꢁ
to 400 C. From this, it can be concluded that the catalyst was stable and its catalytic
ꢁ
activity is unaltered even on calcinations. The catalyst can be used up to 400 C.
3
.3. Heterogeneity test
For rigorous proof of heterogeneity, a test [29] was carried out by filtering catalyst from
ꢁ
the reaction mixture at 80 C, after 1 h, and allowing the filtrate to react up to 4 h, that

4
048
S. Pathan and A. Patel
Table 3. Heterogeneity test.
Catalyst
% Yield (n-Butyl acetate)
(
Filtrate (4 h)
SiW11
)
2
/ZrO
2
(1 h)
19.4
19.0
ꢁ
Amount of catalyst: 0.15 g; temperature: 80 C; 1 : 4.4 mole ratio.
Table 4. % yield of n-butyl acetate with recycled catalyst.
Cycle
% Yield (n-Butyl acetate)
TON
Fresh
1
2
83
79
79
10246
9752
9752
ꢁ
Amount of catalyst: 0.15 g; reaction time: 4 h; temperature: 80 C; 1 : 4.4
mole ratio.
is, completion of the reaction. The reaction mixture of 1 h and filtrate were analyzed by
gas chromatogram. No change in the % yield was found (table 3), indicating that the
present catalyst falls into category C [29]. On the basis of these results, it can be
concluded that there is no leaching of SiW11 from the support and the present catalysts
are truly heterogeneous.
3
.4. Regeneration/recycling of the catalyst
In order to investigate the details of the deactivation, repeated use of the catalyst was
examined. The catalyst was separated from the reaction mixture by simple filtration,
ꢁ
washed with conductivity water till filtrate is free from acid, dried at 100 C in an oven
for 5 h and the recovered catalyst was charged for further run. The results obtained are
shown in table 4; in the regenerated sample, the yield decreased by 4% and becomes
constant on further regeneration.
4
. Conclusion
FT-IR and DRS spectra show that lacunary silicotungstate keeps its Keggin-type
structure when supported on ZrO . Thermal studies show that catalyst is stable up to
2
ꢁ
4
ZrO . The catalysts are successful in the esterification of different alcohols with acetic
75 C. A SEM study shows uniform dispersion of heteropoly anion onto the surface of
2
acid as well as the esterification of n-butanol with different acids under mild conditions
as compared with traditional liquid acid catalysts. In all reactions, removal of the
catalyst consists of single filtration and catalyst can be re-used after a simple work-up.
No structural change takes place in reused catalysts, indicating stability. Thus the
present catalysts are promising alternatives to the traditional acid catalysts for liquid-
phase-acid-catalyzed reactions.

Silicotungstate
4049
References
[
[
[
[
[
[
[
[
[
1] M.T. Pope. Heteropoly and Isopoly Oxometalates, Springer-Verlag, Berlin (1983).
2] J.T. Rhule, C.L. Hill, D.A. Judd, R.F. Schinazi. Chem. Rev., 98, 327 (1998).
3] D.E. Katsoulis. Chem. Rev., 98, 359 (1998).
4] M. Sadakane, E. Steckhan. Chem. Rev., 98, 219 (1998).
5] M.K. Harrup, C.L. Hill. Inorg. Chem., 33, 5448 (1994).
6] C.L. Hill, M.P. Mc-Cartha. Coord. Chem. Rev., 143, 407 (1995).
7] D.S. Rozner, P. Witte, P.L. Alsters, R. Neumann. Adv. Synth. Catal., 346, 339 (2004).
8] C.M. Tourne, G.F. Tournd. Bull. Soc., Chim. Fr., 1124 (1969).
9] A. Teze, G. Herve. J. Inorg. Nucl. Chem., 39, 999 (1977).
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
10] A. Teze, G. Herve. Inorg. Synth., 27, 85 (1990).
11] R. Contant. Inorg. Synth., 27, 104 (1990).
12] N. Haraguchi, Y. Okaue, T. Isobe, Y. Matsuda. Inorg. Chem., 33, 1015 (1994).
13] D. Chartier, L. Donnet, J.M. Adnet. Radiochim. Acta, 83, 129 (1998).
14] D. Chartier, L. Donnet, J.M. Adnet. Radiochim. Acta, 85, 25 (1999).
15] C. Nozaki, I. Kiyoto, Y. Minai, M. Misono, N. Mizuno. Inorg. Chem., 38, 5724 (1999).
16] K. Kamata, K. Yonehara, Ya. Sumida, K. Yamaguchi, S. Hikichi, N. Mizuno. Science, 300, 964 (2003).
17] K. Kamata, Y. Nakagawa, K. Yamaguchi, N. Mizuno. J. Catal., 224, 224 (2004).
18] N. Mizuno, K. Yamaguchi, K. Kamata. Coord. Chem. Rev., 249, 1944 (2005).
19] T.D. Phan, M.A. Kinch, J.E. Barker, T. Ren. Tetrahedron Lett., 46, 397 (2005).
20] A. Yoshida, S. Hikichi, N. Mizuno. J. Organomet. Chem., 692, 455 (2007).
21] K. Kamata, M. Kotani, K. Yamaguchi, S. Hikichi, N. Mizuno. Chem.-Eur. J., 13, 639 (2007).
22] Y. Guo, Y. Yang, C. Hu, C. Guo, E. Wang, Y. Zou, S. Feng. J. Mater. Chem., 12, 3046 (2002).
23] X. Yu, L. Xu, X. Yang, Y. Guo, K. Li, J. Hu, W. Li, F. Ma, Y. Guo. Appl. Surf. Sci., 254, 4444 (2008).
24] P. Shringarpure, A. Patel. Dalton Trans., 3953 (2008).
25] P. Shringarpure, A. Patel. Dalton Trans., 2615 (2010).
26] N. Bhatt, A. Patel. J. Mol. Catal. A: Chem., 238, 223 (2005).
27] R. Neumann, C.A. Gnim. J. Chem. Soc., Chem. Commun., 1324 (1989).
28] G.H. Jeffery. J. Bassett, J. Mendham, R.C. Denney, Vogel’s Textbook of Quantitative Chemical
Analysis, 5th Edn, p. 486, Longman Scientific & Technical, Wiley, Harlow, Essex, England (1989).
29] A. Sheldon, M. Walau, I.W.C.E. Arends, U. Schuchurdt. Acc. Chem. Res., 31, 485 (1998).
[